POLYMER CHARACTERIZATION PROGRAM

The Polymer Characterization Program provides measurement methods, data and standard reference materials needed by U.S. industry, research laboratories, and other federal agencies to characterize polymers for processibility, rheological and mechanical properties, and performance. Molecular mass and its distribution have significant effects on the processibility of polymers through dramatic effects on their rheological properties. Mechanical properties and performance are significantly affected by the solid state structure formed during processing. Importantly, unlike many other common engineering materials, polymers exhibit mechanical properties that exhibit time dependent viscoelastic and aging behaviors, even at low temperatures. As a result, the focus of the program is on techniques that measure molecular mass and its distribution, solid state structure, mechanical properties, and rheological behavior of polymeric materials.

Primary methods employed for molecular mass characterization are dilute solution light scattering and osmometry. Chromatographic techniques, which require calibration by standards of known molecular mass, provide information on molecular mass distribution. Recent activities exploit advances in mass spectrometry using matrix assisted laser desorption ionization (MALDI) to develop the method as a primary tool for the determination of the molecular masses of synthetic polymers. Solid state structure of polymers is elucidated using small and wide angle x-ray scattering, atomic force microscopy, electron microscopy, as well as spectroscopic methods such as solid state nuclear magnetic resonance (NMR) and infrared spectroscopy.

Complete characterization of the time-dependent viscoelastic and aging behaviors of polymers requires large amounts of testing. For this reason, methods are developed that reduce the number of tests required to describe the nonlinear thermo-viscoelastic properties of polymers. The approach applies the frameworks of solid and fluid continuum mechanics to interrelate mechanical responses under different geometries of deformation and in varied deformation histories. Phenomenological models and underlying physical theories are tested using the NIST torsional dilatometer, conventional rotational rheometry, and servo-hydraulic mechanical testing machines.

The polymer industry and standards organizations assist in the identification of current needs for standard reference materials. Based on these needs, research on characterization methods and measurements are conducted leading to the certification of standard reference materials. Molecular standards are used primarily for calibration of gel permeation chromatographs, the principal method employed by industry for assessing molecular mass and molecular mass distributions. Melt flow standards are used in the calibration of instruments used to determine processing conditions for thermoplastics. Rheological standards are developed for secondary calibrations of rheological instruments in industry and academia.

Significant Accomplishments

• Overcame a major obstacle to the use of mass spectrometry to determine the molecular mass distribution of synthetic polymers through improvements in sample preparation methods that reduced variation of signal intensity from 42% on samples prepared by conventional techniques to below 5% on samples prepared by the new procedures.
• Showed that mass spectrometry is capable of determining the three dimensional molecular structure of polysilsesquioxanes and its dependence on molecular mass. For polymethacryloxypropyltrimethoxysilane it was found that the number of closed intramolecular loops (ladder steps) scaled directly with the number of repeat units.
• Determined by optical and electron microscopy that crystallization into spherulites can either distort or completely destroy microphase separated morphologies exhibited by tri- and diblock polymers containing one crystallizable component.
• Chemical shift calculations for the ethylene defect in isotactic polypropylene confirmed assignments of experimental NMR data used to establish relationships between molecular defects and crystallinity.
• Completed research to identify material for a new Nonlinear Fluid Standard and organized an inter-laboratory comparison of fluid involving 35 researchers from industry (instrument makers and material suppliers) and academe.
• Recommendations for standards for estimation of statistical errors in SAS data sets were made to the Commission on Small-Angle Scattering of the International Union of Crystallography
• Successful application of a model for time dependent surface concentration to the non-Fickian moisture uptake in a commercial epoxy adhesive. The model accurately described both absorption and reabsorption behaviors.
• Confirmed, with a 95 % confidence limit, the existence of an expansion gap in volume recovery data from polymer glasses through analysis of literature data and unpublished data from the Institut Charles Sadron in Strasbourg, France.
• In a collaboration with Eastman Kodak Company, poly(ethylene naphthalate) (PEN) was shown to be "thermo-rheologically complex" because the alpha and beta relaxations showed different temperature and aging time dependencies in classical aging time experiments.
• Successfully described volumetric and mechanical responses of an epoxy glass to jumps in relative humidity.
• In a collaboration with General Electric Company and the University of Pittsburgh, the evolution of the viscoelastic response of a commercial epoxy thermoset during cure was successfully modeled.
• Showed that an approximate method proposed in the literature for the determination of the thermal conductivity using Temperature Modulated DSC can lead to significant (»30 %) errors in the determination of the thermal conductivity.
• Successfully described, within approximately 20%, the stress response of polyurethane elastomers to multiple step and trapezoidal deformation histories through application of a hybrid VL-BKZ model developed in this laboratory.
• Showed that the glass transition temperature of semi-dilute polystyrene solutions, when confined to small pores having sizes from approximately 10 nm to 50 nm, decreases below that of the bulk.
• Demonstrated that structural recovery in polymeric glasses can dramatically affect the heat flow signal in temperature modulated DSC measurements. This leads to nonlinear responses that cannot be analyzed by simple linear response theory.

Standard Reference Materials

Charles Guttman, William Blair, Bruno Fanconi and John Maurey

Objective
The objective is to provide the U.S. polymer industry with standards for calibration of instruments used in the control of the synthesis and processing of polymers. Principal polymer standards are certified for molecular mass and melt flow rate; the former are used to calibrate gel permeation chromatographs and the latter to calibrate melt flow indexers.

Technical Description
New Polyethylene Molecular Mass Standards
Polyethylene is the dominant commercial polymer in the United States and worldwide markets. High temperature (150 °C) Size Exclusion Chromatography (SEC), although a relative method requiring calibration, is the most commonly used method to establish the molecular mass of these polymers. Yet, few SEC calibration standards are available on the market for the calibration of high temperature SEC. Molecular mass fractions with M_w/M_n of less than 1.2 are the most useful materials for calibrating SEC. The NIST polyethylene standard reference materials are the only available narrow fractions of polyethylene. These SRM's will soon be out of stock.

The lack of commercial standards arises out of the difficulties obtaining sharp molecular mass fractions of polyethylenes and in measuring absolute molecular masses by light scattering or osmometry at temperatures as high as 150 °C.

The original fractionation of a broad distribution polyethylene that provided the fractions previously certified also yielded other fractions including ones with molecular masses around (5,000, 75,000 and 160,000) g/mole in sufficient quantity to produce three additional narrow fraction polyethylene SRM's.

These new materials along with current SRM 1482 and 1483 would provide a set of polyethylene fractions covering the molecular masss range from 5,000 g/mol to 160,000 g/mol. This will provide the polymer industry with an adequate set of molecular mass calibrants for polyethylene. This work is supported by the Standard Reference Materials Program (SRMP) at NIST.

Planned Outcomes
The three new polyethylene standards, together with existing standards will improve calibration of size exclusion chromatographs by providing the polymer industry with molecular mass standards covering the range 5,000 g/mol to 160,000 g/mol.

Accomplishments
New Polyethylene Molecular Mass Standards: SRM 2885, the lowest polyethylene molecular mass standard, and SRM 2886, the middle polyethylene molecular mass standard, have been blended and bottled by SRMP. The bottled materials have been evaluated for homogeneity. No bottle-to-bottle variation was found. A glass rod was calibrated against benzene at room temperature and then used to obtain the V_v , the absolute scattering intensity, of 1,4 trichlorobenzene at 130 °C. As a check on the alignment and the calibration, the M_w of SRM 1483 was measured and found to be 33.4 ku in close agreement with the certificate value. Calibration measurements have commenced on SRM 2886.

Mass Spectrometry of Polymers

Charles Guttman, William Blair, William Wallace and Bruno Fanconi

Objective
The objective is to improve reliability of calibration standards for size exclusion chromatography (SEC), widely used by the polymer industry for characterizing the molecular mass distribution (MMD) and the moments of the MMD of synthetic polymers. Mass spectrometry (MS) is developed as a reliable method to measure the MMD and the moments of the MMD of polymers that will become the next generation calibration materials for SEC.

Technical Description
Recent advances in Matrix Assisted Laser Desorption Ionization (MALDI) Time of Flight (TOF) Mass Spectroscopy (MS) allow determination of the mass spectrum of whole undegraded polymer molecules with molecular masses up to 300,000 g/mol. As part of an effort to explore the usefulness of this technique for the production of molecular mass SRM's , research was initiated to determine the limits of applicability for the measurement of the absolute molecular mass of a polymer molecule. Research in FY 98 were focused on the following three activities.

i. Sample Preparation Methods: Development of sample preparation techniques that yield consistent repeatable signals in the MS was undertaken to improve on the conventional random crystallization of a matrix and polymer sample on a MALDI target that produces an inhomogeneous surface and large fluctuations in polymer signal intensity. For SRM certification of polymer properties, valid statistical sampling of a target surface does not permit searching for regions that produce the most intense polymer signals. Statistically random sampling must be used. New sample preparation techniques are investigated in an effort to produce a homogenous sample surface with a uniform signal intensity.

ii. Data Analysis methodology: Methodology is developed to analyze the mass spectral data of synthetic polymers in collaboration with staff in the NIST Statistical Engineering Division. Other absolute methods (light scattering or membrane osmometry) yield only one moment of the molecular mass distribution (MMD) of synthetic polymers. To obtain the MMD from the SEC is fraught with broadening problems and interpretation problems. MALDI-TOF-MS of the whole synthetic polymer can, in principle, yield the overall MMD in which (in the best case) at least 20 different n-mer peaks can be analyzed. In addition, autocorrelation function analysis was applied to mass spectra to resolve polymer architecture and copolymer composition in some samples.

iii. Molecular Structure of Polysilsequioxanes: Polysilsesquioxanes have silicon coordinated with three bridging oxygen atoms in the form of [RSiO3/2] and as a result are generally complex three-dimensional structures. The typically highly-branched silsesquioxane reacts with itself to form intramolecular, closed topographic loops. Specifically, two -SiOH groups within the molecule may react to eliminate water (mass of 18 u) and form a new Si-O-Si bridge. The number of -Si-O-Si- bridges has a profound effect of the solubility and rheology of the polymer. Counting the number of closed loops in the molecule (or, conversely, determining the number of -SiOH groups) is not possible by techniques other than high-resolution time-of-flight mass spectrometry.

iv. Polymer Fragmentation: Under MALDI TOF MS, some polymers will fragment, breakup as a result of the MALDI process. Fragmentation results in a degradation of the signal of the MS and thus possibly an incorrect MMD. Fragmentation of a polyethylene glycol was investigated by post source decay to understand how this fragmentation may affect the measured MMD.

External Collaborations
A collaboration with S. Samal of Revenshaw College, Cuttack, India , seeks to develop matrix materials that are compatible with synthetic polymers.
In collaboration with Professor J. Girard at American University methodology is developed to compare SEC and MALDI-TOF-MS.
In collaboration with Paul Kowalski of Bruker Instruments fragmentation processes that occur in MALDI-TOF- MS are investigated.

Accomplishments

• Variation in signal intensity was reduced from 42 % on samples prepared by conventional techniques to below 5 % by using a combination of electrospray sample deposition, specially synthesized matrix compounds and very thick deposits of matrix/polymer material.
• Of the approximately 12 polysilsesquioxanes studied, polymethacryloxypropyltrimethoxy-silane was investigated in greatest detail. This polymer when synthesized in the bulk was found by mass spectrometry to occur mainly as a ladder-like structure. Such structural information could not be inferred from other characterization studies, such as NMR or FTIR. It was found for this polymer that the number of closed intramolecular loops (ladder steps) scaled directly with the number of repeat units. This indicates that as the molecule grows the likelihood of it reacting with itself does not increase, or that locally the molecule looks the same and can be said to be self-affine.
• Fragmentation studies on polyethylene oxide revealed that preferential cleavage occurs at the carbon-oxygen bond.

Outputs

Publications
C.M. Guttman, W.R. Blair, and P.O. Danis, Comparison of Molecular Weight Moments from MALDI-TOF-MS with other Absolute Methods on Standard Reference Polymer, Conference Proceedings at ANTEC '98, Atlanta, GA, April 1998, p. 2109.

W.E. Wallace, C.M. Guttman and J.M. Antonucci, MALDI-TOF Mass Spectrometry of Polysilsesquioxanes, Proceedings of the 46th Annual Conference On Mass Spectrometry and Allied Topics, 1998, p. 1076.

W.E. Wallace, C.M. Guttman and J.M. Antonucci, Molecular Structure of Silsesquioxanes Determined by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry, Journal of the American Society for Mass Spectrometry, submitted.

W.R. Blair, C.M. Guttman and W.E. Wallace, Comparison Investigation of Chemical and Mechanical Modifications of MALDI Sample Preparation techniques for Synthetic Polymers, Proceedings of the 46th ASMS Conference on Mass Spectrometry and Allied Topics, 1998, p. 1059.

P. Kowalski , W.E. Wallace, and C.M. Guttman, PSD analysis of Polymers by MALDI TOF MS, Proceeding of the 46th ASMS Conference on Mass Spectrometry and Allied Topics, 1998, p. 1060.

Presentations
C.M. Guttman, W.R. Blair, and P.O. Danis, Comparison of Moments from MALDI-TOF-MS with other Absolute Methods on a Standard Reference Polymer, ANTEC ?98, Atlanta, GA, April, 1998.

C.M. Guttman, W.E. Wallace, and W.R. Blair, Statistical Analysis of Polymer Molecular Weight Distribution and its Moments from MALDI TOF MS , The 45th ASMS Conference on Mass Spectrometry and Allied Topics,Orlando, FL, June, 1998.

W.E. Wallace, C.M. Guttman, and J.M. Antonucci, MALDI-TOF Mass Spectrometry of Polysilsequioxanes, The 46th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, FL, June, 1998.

W.R. Blair, C.M. Guttman and W.E. Wallace, Comparison Investigation of Chemical and Mechanical Modifications of MALDI Sample Preparation techniques for Synthetic Polymers, The 46th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, FL, June, 1998.

P. Kowalski , W.E. Wallace, and C.M. Guttman, PSD analysis of Polymers by MALDI TOF MS, The 46th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, FL, June, 1998.

Characterization of Polymer Morphology by Microscopy Techniques

C. L. Jackson, G. Kim^1
1 Stevens Institute of Technology

Objectives
The objectives are to use optical, electron and scanning force microscopy to study the morphology of polymeric materials and to investigate the potential of new microscopy techniques, such as cryo-TEM and TEM with image processing, to solve problems in polymer microscopy. Cryo-TEM, a relatively new technique in which polymers such as colloids, polymer micelles, gels and dendrimers in solvent are vitrified and imaged at low temperature, is especially useful because the native structure can be imaged without drying or staining.

Technical Description
The characterization of polymer morphology is important to better understand the relationship between structure and properties in many materials. Techniques used include transmission electron microscopy (TEM), scanning electron microscopy (SEM), optical and atomic force microscopy. The combination of microscopy techniques with other methods such as scattering, diffraction or spectroscopic measurements gives a more complete description of the structure of complex blends, composites or other multiphase materials. Many types of materials have been studied to date, including polymer blends, block copolymers, liquid crystalline polymers, dendrimers and organic-inorganic hybrid materials.

External Collaborations
Dr. Henri Chanzy of CERMAV and C NRS in Grenoble, France, morphology of dendrimers.
Dr. Frank Booy of the Laboratory of Structural Biology, NIAMS, NIH, Betheda, MD, morphology of dendrimers.
Dr. Kenncorwin Gardner, DuPont Company, Wilmington, DE, morphology of spider silk.
Dr. Jonathan Coddington, National Museum of Natural History, Smithsonian, Washington D. C., morphology of spider silk.
Professor Reimund Stadler, University of Bayreuth, Bayreuth, Germany, morphology of semi-crystalline block copolymers.
Dr. Donald Tomalia, Michigan Molecular Institute, Midland, MI, morphology of dendrimers.
Professor Matthew Libera, Stevens Institute of Technology, morphology of semi-crystalline block copolymers.

Accomplishments

• Various thermal conditions were used to prepare thick films (5-10 µm) and bulk specimens of polystyrene(S)-polybutadiene(B)-polycaprolactone(C) (SBC) triblock and SC and BC diblock copolymers with or without spherulitic crystalline morphology. A comparison of the macroscopic crystalline superstructure (optical range 4-1000 µm) and microphase separated morphology (TEM range 5-100 nm) showed that crystallization into spherulites can either distort or completely destroy the microphase separated morphology. Long range crystallization of the C block can be suppressed in the SBC triblock, however, by appropriate annealing conditions. In this case, the microphase separated structure resembles that seen for ABC type amorphous block copolymers; for S₃₅B₁₅C₅₀ a lamellar microstructure with B cylinders at the interphase boundaries is observed. The competition between microphase separation and crystallization of the C block depends strongly on the rubbery or glassy character of the amorphous blocks and the overall composition.
• Previously, the characterization of dendrimer molecules of poly(amidoamine) (PAMAM) was achieved for generations ten (G-10) down to G-5, ranging in size from 14.7 nm to 4.5 nm, respectively, using biological-type TEM staining methods. For G-10 dendrimer, cryoelectron microscopy was also possible, where the molecules were imaged in vitrified solvent at very low temperature. More recent results on dendrimer blends show that PAMAM dendrimers (G-11) can be uniformly dispersed in interpenetrating polymer networks (IPN?s) of hydroxyethylmethacrylate (B. Bauer, D. Liu) at 10 % by mass. Characterization by small angle x-ray scattering (Prosa et al.) measures the R_g to be 6 nm, corresponding to a diameter of 16 nm, assuming a uniform dense sphere. The TEM of thin sections showed that the dendrimers were well dispersed and visible as individual molecules, in good agreement with the SAXS results.
• A small amount of fumed silica (1 %) added to blends of polystyrene/ polybutadiene (PS/PB) (2K/3K) shifts the upper critical solution temperature (UCST) upward about 5 °C (Nakatani et al.). Optical microscopy shows that the fumed silica is aggregated and suggests that the PB preferentially wets the filler particles during the phase separation process (Hobbie et al.). TEM has been used to show that the silica is preferentially wetted by the PB phase, which is stained with OsO₄. The destabilization of the blend by filler is unexpected, and suggests interactions between the filler and PB phase are strong.

Outputs

Publications
C. L. Jackson, H. D. Chanzy, F. P. Booy, B. Drake, D. A. Tomalia, B. J. Bauer and E. J. Amis, Visualization of Dendrimer Molecules by Transmission Electron Microscopy (TEM): Staining Methods and Cryo-TEM of Vitrified Solutions, Macromolecules, in press.

G. Kim, C. L. Jackson, F. V. Gyldenfeldt, V. Balsamo, M. Libera, R. Stadler and C. C. Han, Morphology Study of Polystyrene-Polybutadiene-Polycaprolactone (PS-b-PB-b-PCL) and Polybutadiene-Polycaprolactone (PB-b-PCL) and Polystyrene-Polycaprolactone (PS-b-PCL) Semicrystalline Block Copolymers, MSA Proc., 806 (1998).

B. J. Bauer, A. Topp, T. J. Prosa, D-W. Liu, C. L. Jackson and E. J. Amis, Small Angle Scattering Studies of Dendrimer Blends and Interpenetrating Polymer Networks, SPE ANTEC, 2065, (1998).

Presentations
B. J. Bauer, A. Topp, T. J. Prosa, D-W. Liu, C. L. Jackson and E. J. Amis, Small Angle Scattering Studies of Dendrimer Blends and Interpenetrating Polymer Networks, Society of Plastics Engineers, Atlanta, GA, April, 1998.

G. Kim et al., Morphologies of Semicrystalline Block Copolymers Based on Polycaprolactone (PCL) (poster) and N. Eidelman, C. Zimba and C. L. Jackson, Measuring and Mapping the Calcium in Calcified Bovine Pericardium Implants Retrieved from Rats by Scanning Transmission X-ray Microscopy (STXM), X-ray Microscopy Workshop, NIST, Gaithersburg, MD, May, 1998.

J. A. Coddington, H. Chanzy, C. L. Jackson, G. Raty, K. H. Gardner, Structure and Morphology of Major Ampullate Silk System From Recluse Spiders, University of Virginia, VA, June, 1998.

C. L. Jackson, H. D. Chanzy and F. P. Booy, Characterization of Polymer Micellar Suspensions and Dendrimer Solutions by Cryo-electron Microscopy (poster), Workshop on Properties and Applications of Dendritic Polymers, NIST, Gaithersburg, MD, July, 1998.

G. Kim, C. L. Jackson, F. V. Gyldenfeldt, V. Balsamo, M. Libera, R. Stadler and C. C. Han, Morphology Study of Polystyrene-Polybutadiene-Polycaprolactone (PS-b-PB-b-PCL) and Polybutadiene-Polycaprolactone (PB-b-PCL) and Polystyrene-Polycaprolactone (PS-b-PCL) Semicrystalline Block Copolymers, Microscopy Society of America, Atlanta, GA, July, 1998.

C. L. Jackson, Characterization of the Structure of Dendrimer Molecules by Transmission Electron Microscopy and Scattering Techniques, North Carolina State University, Materials Science and Engineering Department, Raleigh, NC, October, 1997.

Characterization of Polymers by Spectroscopic Techniques

David VanderHart, Bruno Fanconi and Hanqiao Feng^1
1Chinese Academy of Sciences

Objectives
The objectives are to: i. develop and use Nuclear Magnetic Resonance (NMR) techniques for characterization of molecular and microstructural level features that control properties of polymers, ii. establish capability to employ rapid phase shifts, amplitude changes and frequency shifts within pulse sequences, iii. quantify two-pulse phase modulation techniques for improving resolution in strongly coupled spin systems, iv. develop solid state NMR characterization methods at 7.05 T, particularly for the morphological locating of defect structures in olefins synthesized by metallocene catalysts and v. develop a modest capability for solution-state microstructural characterization of polymers.

Technical Description
Various NMR phenomena and experimental approaches to solid-state characterization are employed; the most useful being:

• Proton spin diffusion is characteristic of an extended network of dipolar-coupled protons. Spin diffusion is evidenced by the flow of proton polarization in the presence of an imposed polarization gradient and in the direction that restores spin equilibrium. Thus, properly designed proton spin diffusion experiments yield information about spatial relationships and domain size (typically for the 3 - 50 nm range) in organic polymers.
• Multiple pulse proton techniques offer a way to quench spin diffusion and simultaneously, especially in the presence of magic angle sample spinning (MAS), achieve some chemical shift resolution (hence the opportunity for chemical identification) for protons in solids.
• ¹³C spectra, obtained using MAS, offer the best chemical resolution for organic solids. Also, NMR signals arising from the same chemical species in different states of order, e.g. crystal or non-crystal, can be distinguished. There is a continuing effort to develop strategies for improving resolution in order to provide a greater ability to distinguish not only chemically different species but also chemically similar species in physically different sites.

Collaborations
Yi Feng, Goodyear Tire and Rubber Co., Akron, OH (ionomer blends)
Professor Robert Weiss of Univ. of Connecticut, Storrs, CT (ionomer blends)
Professor Norritsu Terashima of Nagoya, Japan (lignin synthesis)
Satish Kumar of Georgia Institute of Technology, Atlanta, Ga. (modified PBZT)
R. Alamo and L. Mandelkern of Florida State University, Tallahassee, FL (iPP's)
J.A. Randall of Exxon Research, Baytown, TX (iPP's)
E. Pérez of the Institute for Polymer Science in Madrid, Spain (iPP's, branched vinyl polymers and copolymers)
M. Pursch (working with Lane Sander),Analytical Chemistry Division, NIST (Silica gels with organic coatings for high pressure chromatography)

Planned Outcomes

• Establish mechanisms whereby different molecular defect structures in isotactic polypropylene (iPP) dictate properties such as crystallinity, modulus and melting point. Here, "defect" refers to propylene insertions having stereo or regio irregularities or to the presence of various 1-alkene comonomers at low concentrations. The role of the catalyst, e.g. Ziegler-Natta or metallocene type, in determining defect distribution is also expected to reveal itself qualitatively in these studies.
• Develop a better understanding of morphology in polymer blends when one blend component is decorated with a few groups which strongly interact with the other component.
• Develop a better understanding of crystallization and its relation to tacticity in 1-alkene homopolymers, like poly(octadecene) or poly(dodecene), having 16- or 18-carbon branches on every other backbone carbon.
• Develop a better understanding of what controls spectral resolution in the ¹³C MAS spectra of strongly-dipolar-coupled, crystalline polymers to aid NMR spectroscopists improve spectral resolution in organic solids.
• In connection with derivatized (coated) silica gels that are used for inverse-phase, high-pressure, liquid chromatography (HPLC), develop insight into the relationship between molecular order in the organic coating and molecular-shape selectivity for component molecules in the analyzed mixture.
• Assess the effectiveness of strategies whereby high-strength, high-temperature-stable fibers like poly (benzo-bis-thiazole) are chemically modified so that their compressive strengths can be improved via lateral crosslinking.

Accomplishments

• In iPP, the most common stereo and regio (2,1) defects are incorporated into the crystalline regions at 40 % and 30 %, respectively, of the overall content. Comonomer defects, like ethylene, have corresponding concentration levels near 37 %. Other 1-alkene defects have lower concentrations. Theoretical chemical shift calculations for the ethylene defect have successfully matched experimental spectral positions under the reasonable assumption that the backbone 3₁ helix is preserved.
• Several blend systems were characterized using proton spin diffusion and proton longitudinal relaxation (T₁^H) measurements. The systems include: I: a 10/90 PAMAM-dendrimer/poly(hydroxyethylmethacrylate) (PHEMA); II: blends of OH-decorated polystyrene (PSOH) with poly(methylmethacrylate) (PMMA) (both isotactic (i-) and atactic (a-) PMMA) or with poly(butylmethacrylate) (PBMA).
• Backbone crystallization was correlated with iso-tacticity in poly(dodecene) and inferred the backbone conformation of the crystal.
• A comprehensive paper was published in the Journal of Magnetic Resonance on spectral resolution in ¹³C spectra with magic angle spinning.
• Derivatized silica gels (candidates for HPLC) coated with mixtures of silane-bonded benzene and n-C-30-alkanes were characterized.

Outputs

Publications
D.L. VanderHart and G.C. Campbell Off-Resonance Proton Decoupling On-Resonance and Near Resonance: A Close Look at ¹³C CPMAS Linewidths in Solids for Rigid, Strongly-Coupled Carbons Under CW Proton Decoupling, Journal of Magnetic Resonance, in press.

E. Pérez, M.L. Cerrada and D.L. VanderHart, Rapid Determination of Comonomer Content, Crystallinity, and Long Spacing by Multiple Pulse Proton NMR in Ethylene-Vinyl-Alcohol Copolymers, J. Polym. Sci. B, Polym. Physics, 36, 2103, (1998).

X. Hu, S. Kumar, M.B. Polk and D.L. VanderHart, Tetramethylbiphenyl Substituted Poly(benzobisthiazole) 2. Synthesis and Characterization of Polymers, J. Polym. Sci. A, Polymer Chemistry, 36, 1407 (1998).

Presentations
D.L. VanderHart, Morphological Study of an Ionomer/poly(amide) Blend Using Proton Spin Diffusion, Federation of Analytical Chemistry and Spectroscopy Societies, Providence, RI, October 26, 1997.

D.L. VanderHart, Partitioning of Various Defects Within the Semicrystalline Morphology of Isotactic Poly(propylene), American Physical Society, Los Angeles, CA, March 17, 1998.

D.L. VanderHart, C¹³ NMR Study of the Inclusion of Stereo and Regio Defects in the Crystalline Region of Isotactic Poly(propylene), Rocky Mountain Conference on Analytical Chemistry, Denver, CO, July 26, 1998.

Structural Characterization of Polymers by Small Angle X-Ray Scattering

J.D. Barnes and T.J. Prosa^1
1Kutztown University of Pennsylvania

Objectives
The objectives are: i. to maintain an up-to-date, state-of-the-art Small Angle X-ray Scattering (SAXS) Facility that incorporates improvements in analysis methods and instrumentation to expand the use of SAXS as a technique for characterizing materials and processes, and ii. to use the SAXS Facility as a user facility for outside industrial clients and to support research within the Polymers Division.

Technical Description
By maintaining a fully capable SAXS facility, Polymers Division researchers are able to perform in-house research relevant to ongoing programs as well as collaborative research with both industrial and academic scientists. The facility has a two-dimensional area detector capability and flexible sample handling for oblique incidence scattering, elevated temperature measurements, liquid samples, and uniaxial deformation. As a result, the NIST SAXS camera is an attractive research tool. Test and analysis capabilities are constantly upgraded and implemented as the need or opportunity arise. Projects using the facility are either external, with outside collaborators, or internal with NIST staff.

Internal NIST Research
Past work on matrix effects in semicrystalline polymers has been extended to studies of poly(4-methylpentene-1) (P4MP), polychlorotrifluorethylene, and a copolymer of tetrafluroethylene and hexafluoropropylene. P4MP, in particular, has demonstrated a striking temperature effect that parallels the one seen earlier in syndiotactic polystyrene. The formalism for resolving scattering patterns into lamellar and matrix components involves using both the temperature effect and the anisotropy in the scattering pattern from oriented materials. A variety of polymers are investigated to elucidate the role of molecular architecture. Studies of crystallization kinetics and the effect of crystallization temperature on long period provide background data needed to control the processing of these materials.

Improved tools for analyzing SAXS data, culminating in a set of recommendations for the proper treatment of noise and systematic errors, resulted from a detailed study of the statistical properties of the SAXS area detector. Data reduction algorithms should be designed to propagate properly both kinds of errors through the entire process and to present standard deviations arising from both sources of noise in the output data sets. These insights are being applied to standardization activities currently under way within the worldwide SAS community.

A lack of methods for describing the morphology of anisotropic scatterers has been a major difficulty for industrial users of SAS. This project has attempted to address this need by carrying out Monte Carlo simulations combined with SAXS experiments on track-etched membranes. The analysis of these results has shown how to construct basis functions to jointly describe the orientation texture and the distribution of particle size parameters in this model system. Work is currently underway to devise procedures for using these basis functions in fitting procedures designed to provide numerical descriptions of the morphology of the model system. Future work is expected to extend these methods to the description of morphology in semicrystalline polymers that are important for commercial applications.

External Collaborations
The SAXS facility was used during the year for projects with outside users that fall into three categories. Collaborative research that is expected to lead to technical publications are currently underway with Air Products and Chemicals Company (microstructure of high performance polyurethanes) and Exxon Corp (characterization of matrix effects in lamellar microstructures). Proprietary measurements in which the industrial client is interested in protecting sensitive data were performed by Zimmer Corporation. A continuing collaboration with the University of Maryland examines processing methods for producing oriented block copolymers as starting materials for nano-engineered polymer products.

Planned Outcomes
NIST researchers and external users from industry, government and academic laboratories will have available a state-of-the-art small angle x-ray facility operated by experts who can assist in collection and analysis of data.

Accomplishments

• Proprietary usage beam time was provided to Zimmer Corporation.
• In collaboration with University of Maryland researchers, it was demonstrated that shear using a channel die when crystallizing EtP/Et/EtP triblock copolymers (EtP = ethylene -propylene copolymer, E = ethylene) produces two distinct kinds of orientation depending on cooling history.
• Recommended standards for inclusion of statistical errors in SAS data sets were provided to task groups of the Commission on Small-Angle Scattering of the International Union of Crystallography.

Outputs

Publications
J.D. Barnes, Small-Angle Scattering for the Industrial Plastics Laboratory - An Overview, ANTEC 98, Proceedings of the Society of Plastics Engineers, Annual Technical Conference, II, 2048-2051 (1998).

S. Musselmman, T. Santusosso, J. D. Barnes, and L. E. Sperling, Domain Structure and Interphase Dimensions in Poly(urethaneurea) Elastomers using DSC and SAXS, J. Polym Sci, Physics Edition, submitted.

Presentations
J.D. Barnes and T. J. Prosa Statistics in Area Detector Data Analysis, New opportunities for Better User Group Software workshop, Argonne Natl. Lab, Argonne, IL, November, 1997.

J.D. Barnes and T. J. Prosa, Statistics in Area Detector Data Analysis, Collective Aid for Nomadic Small-Angle Scatterers workshop, ILL/ESREF, Grenoble, France, Februar,y 1998.

J. D. Barnes, Small Angle Scattering in the NIST Polymers Division, University of Hamburg/EMBL, Risoe Natl Lab, ESRF, Hamburg, Germany, January, 1998.

J. D. Barnes, SAS Characterization of Polymers - An Overview, University of Maryland Materials and Nuclear Engineering Departmental Seminar, College Park, MD, March, 1998.

J. D. Barnes, Small-Angle Scattering for the Industrial Plastics Laboratory - An Overview, Society of Plastics Engineers ANTEC 1998, Atlanta, GA, April, 1998.

J. D. Barnes, Data handling for Small-Angle Scattering, American Crystallographic Assn workshop, Arlington, VA, July 18, 1998.

T. J. Prosa and J. D. Barnes, Lamellar vs.. Supramolecular Structure in Semicrystalline Polymers, American Crystallographic Assn, Arlington, VA, July 19, 1998.

J. D. Barnes, Small Angle Scattering in Polymer Processing and Characterization, American Chemical Society Division of Polymeric Materials Science and Engineering Workshop, Boston, MA, August, 1998.

J. D. Barnes, Lamellar vs. Supramolecular Structure in Semicrystalline Polymers, ACS PMSE Symposium on Scattering in Polymers, Boston, MA, August, 1998.

T. J. Prosa and J. D. Barnes, SAXS Characterization of Anisotropic Microstructures: Polycarbonate Track-Etched Films, American Crystallographic Assn, Arlington, VA, July 19, 1998.

Standardization and Organizational Activities
J. D. Barnes - Chair, Commission on Small-Angle Scattering of the International Union of Crystallography

J. D. Barnes - Member, International Advisory Committee for XIth World Congress on Small-Angle Scattering, Brookhaven Natl Lab, May 16-20 1999.

Nonlinear Viscoelasticity of Solid Polymers

G. B. McKenna and P. A. O'Connell^1
1University of Leeds, U.K.

Objectives
The objectives are: i. develop measurement methods to characterize the nonlinear viscoelastic response of solid polymers and ii. evaluate the range of validity of material clock models for the prediction of the behavior of solid polymers.

Planned Outcomes

• A comprehensive materials data base for the nonlinear viscoelastic properties of a single glassy polymer in multiple deformation geometries (tension, torsion, compression) in stress relaxation conditions is to be created. The data are to be available for researchers to evaluate different constitutive models of polymer glasses.
• The range of validity of reduced time based nonlinear viscoelastic constitutive laws to the behavior of solid, glassy polymers will be established.
• Fundamental knowledge of the nonlinear viscoelastic response of glassy polymers will be provided.

Task 1. Time-Temperature, Time-Aging Time and Time-Strain Superposition Below the Glass Transition

G.B. McKenna, P.A. O'Connell

Objective
The objective is to test the applicability of time-temperature, time-aging time and time-strain superposition principles to polymer glasses below the glass transition temperature.

Technical Description
Reduced time or material clock models of polymer behavior have been proposed as potentially powerful methods of introducing nonlinear behavior into the constitutive description of polymeric solids. Such models are a conceptual extension of the time-temperature and time-aging time superposition principles to the mechanical domain. However, there are few systematic studies that evaluate the validity of such concepts. Torsional stress relaxation measurements are used to characterize and compare time-temperature, time-aging time and time-strain superposition responses of a polycarbonate glass in the temperature range from 30 °C to 135 °C; that is, below the glass transition temperature.

Accomplishments

• Time-temperature superposition was shown to be a valid descriptor for the viscoelastic response below the glass transition temperature in polycarbonate. The KWW (Kohlrausch-Williams-Watts) stretched exponential function often used in liquid and glassy state relaxation, however, does not successfully describe the master curve.
• The first ever comparison between time-strain and time-temperature superposition master curves was made. Time-strain superposition at each temperature appears to be valid. However, it is found that the stress relaxation master curves obtained from time-strain superposition are not the same as those obtained from time-temperature superposition.
• Aging of polycarbonate into equilibrium below the glass transition temperature results in the appearance of a non-WLF regime in the shift factor-temperature relationship. This result is in agreement with the recent DiMarzio-Yang theory expectation of a transition from Vogel to Arrhenius behavior in the transport properties; hence no singularity in viscosity is expected near the Kauzmann temperature.

Outputs

Publications
J.-J. Pesce and G. B. McKenna, Prediction of the Sub-Yield Extension and Compression Responses of Glassy Polycarbonate from Torsional Measurements, J. Rheology 41, 929 (1997).

P.A. O'Connell and G.B. McKenna, Large Deformation Response of Polycarbonate: Time-temperature and Time-Aging Time Superposition, in Polycarbonate Science and Technology, ed. by D.G. LeGrand and J.T. Bendler, Marcel Dekker, New York, in press.

D.M. Colucci, P.A O'Connell and G.B. McKenna, Stress Relaxation Experiments in Polycarbonate: A Comparison of Volume Changes for Two Commercial Grades, Polymer Engineering and Science 37, 1469 (1997).

P.A. O'Connell and G.B. McKenna, Large Deformation Response of Polycarbonate: Time-Temperature, Time-Aging Time and Time-Strain Superposition, Polymer Engineering and Science 37, 1485 (1997).

P. A. O'Connell, C.R. Schultheisz, and G.B. McKenna, The Physics of Glassy Polycarbonate: Superposability and Volume Recovery, in The Physics of Glassy Polymers, ed. by A. Hill and M. Tant, ACS books, 1998, in press.

P.A. O'Connell and G.B. McKenna, Time-Temperature Superposition at Equilibrium in Polycarbonate below T_g, Proc. NATAS 25th Annual Conference, September, 1997, p. 420.

P.A. O'Connell and G.B. McKenna, Temperature Shift Factors for the Segmental Relaxation of Polycarbonate below the Glass Transition, Society of Plastics Engineers, ANTEC 98 II, 2152-2157 (1998).

Presentations
P.A. O'Connell and G.B. McKenna, Sub T_g Response of Polycarbonate: Deviations of the Equilibrium Response from WLF Expectations, 69th Annual Meeting of the Society of Rheology, Columbus, OH, October, 1997.

G.B. McKenna, Physical Aging in Engineering Plastics, "and" Time Dependent Properties of Polymers: Long Term Performance and A Framework for Accelerated Testing, GE Plastics, Mount Vernon, IN, November, 1997.

P.A. O'Connell and G.B. McKenna, Sub-Glass Temperature Dependence of the Segmental Shift Factor for Polycarbonate: Evidence for Arrhenius Behavior, American Physical Society, Los Angeles, CA, March, 1998.

G.B. McKenna, Torque and Normal Force Measurements to Characterize the Non-Linear Viscoelastic Response of Solid Polymers, 2nd International Conference on Mechanics of Time Dependent Materials, Pasadena, CA, March, 1998.

G.B. McKenna and P.A. O'Connell, Temperature Shift Factors For The Segmental Relaxation of Polycarbonate Below The Glass Transition, SPE Annual Technical Conference, Atlanta, GA, May, 1998.

Physical Aging and Structural Recovery in Polymers

G.B. McKenna, C.R. Schultheisz, P.A. O'Connell, M. L. Cerrada¹, S.L. Simon², M. Vangel³, A. Rukhin³, S.D. Leigh³, B. Lotz⁴, C. Straupe^4
1Instituto de Ciencia Y Technologia de Polimieros, Madrid Spain
²University of Pittsburgh, Pittsburgh, Pennsylvania
³Statistical Engineering Division, NIST
⁴Institute Charles Sadron, Strasbourg, France

Objectives
The objectives are to develop new methods and incorporate known measurement methods to determine the interrelationships between the thermodynamics and kinetics of glass formation and to relate these to the underlying microstructure of the glass through appropriate physical models.

Planned Outcomes

• Provide the scientific and industrial communities with fundamental data and model parameters that describe the physical aging and structural recovery responses of polymeric glasses.

Task 1. Dilatometric Investigation of the Structural Recovery of Polymer Glasses

G.B. McKenna, C.R. Schultheisz, M. Vangel, S.D. Leigh, A. Rukhin, C. Straupe and B. Lotz

Objectives
The objectives are: i. to analyze the Kovacs volume recovery data for poly(vinyl acetate) using both his published and unpublished data to establish whether or not the Struik criticisms of the data as insufficiently accurate to support the existence of the tau-effective paradox and expansion gap. ii. to determine the time-scales for mechanical and volume responses in the model epoxy being studied in the NIST torsional dilatometer at temperatures further below the glass transition than obtained previously and iii. to create an international round robin with the same polycarbonate to investigate the glassy and approach to glassy behavior using multiple measurement techniques for glassy dynamics.

Technical Description
i. Kovacs' volume recovery data are fundamental to our understanding of the dynamics of glass formation and polymeric material behavior for considerations of dimensional stability and long term performance. The original Kovacs (1964) dilatometric data were re-analyzed using modern correlation of errors methods to determine if the expansion gap in asymmetry of approach experiments is supported by the data. The existence of the expansion gap is fundamental to the development of models of glassy kinetics.

ii. For an amorphous polymer below its glass transition temperature, rapid temperature changes result in a material that is not in thermodynamic equilibrium, but that evolves (slowly) into equilibrium. This equilibration process can be followed through measurements of a number of properties of the material, such as the volume, enthalpy, birefringence or mechanical stiffness. Previous research in this laboratory and elsewhere suggested different time scales for equilibration of the different properties. If true, it suggests that different properties depend on different aspects of the structure of the polymer, and it also means that mathematical models of the material must be fairly complex (requiring a different "clock" for each property). Direct comparisons of these properties are difficult, because the measurements are typically made on separate samples, leading to questions of homogeneity and equality of thermal histories between samples. The NIST Torsional Dilatometer instrument allows simultaneous measurement of volume recovery and mechanical response to a torsional deformation on the same sample, making direct comparisons possible. The torsional deformation also induces small volume changes and forces along the axis of the cylinder, which are also recorded by the torsional dilatometer.

iii. The time-scale issue for different material response functions has been identified as an important problem in glass dynamics at an international meeting held in Vigo, Spain in the summer of 1997. It was decided at that time to create a research material from the polycarbonate available to NIST through the GM/GE Thermoplastic Engineering Design Project supported by the ATP. NIST agreed to share responsibility for the program with Dr. J.M. Hutchinson of Aberdeen University (UK). The material is to be machined into standard shapes and distributed to laboratories around the world.

Accomplishments

• It was shown that the Kovacs volume recovery data in T-jump experiments to 40 °C exhibit the expansion gap, as originally claimed by Kovacs. The analysis provided a 95 % confidence limit for the values of volume at which the expansion gap exists. However, the data taken at 35 °C were shown to have been taken at time intervals that were larger than at 40 °C. This resulted in very weak correlation of errors for the 35 °C data and a larger uncertainty in the data for the expansion gap. Hence, the 35 °C data could not be used to support the claim for the existence of an expansion gap.
• Replication of previous measurements on a model epoxy system indicates that the volume recovery and the mechanical response to the torsional deformation equilibrate at roughly the same time, as does the axial force on the cylinder. Interestingly, the volumetric response to the torsion is somewhat different from the relaxational behavior of either the torsional or the axial stresses. These experiments have also shown chemical aging of the sample over its 3-year lifetime, as reflected by an order-of-magnitude increase in the time scale for stress relaxation. The chemical aging may be caused by additional curing or by a decrease in the moisture content of the material.
• Polycarbonate material was sent to Aberdeen for sample preparation (Dielectric and calorimetric geometries). NIST samples were successfully prepared (mechanical testing geometries, reserve material). As a back-up, NIST is now preparing the sample geometries originally tasked to Aberdeen.

Outputs

Publications
G.B. McKenna, M.G. Vangel, A.L. Rukhin, S.D. Leigh, B. Lotz and C. Straupe, The tau-Effective Paradox Revisited: An Extended Analysis of Kovacs' Volume Recovery Data on Poly(vinyl Acetate), Polymer, in press.

C.R. Schultheisz and G.B. McKenna, Volume Recovery, Physical Aging and the Tau-Effective Paradox in Glassy Polycarbonate Following Temperature Jumps, Proc. NATAS 25th Annual Conference, September, 1997, p. 366.

P. A. O'Connell, C.R. Schultheisz, and G.B. McKenna, The Physics of Glassy Polycarbonate: Superposability and Volume Recovery, in The Physics of Glassy Polymers, ed. by A. Hill and M. Tant, ACS books, 1998, in press.

Presentations
G.B. McKenna, M.G. Vangel, A.L. Rukhin, S.D. Leigh, B. Lotz and C. Straupe, The Expansion Gap in Volume Recovery of PVAc Glass: Kovacs? Data Revisited, American Physical Society March Meeting, Los Angeles, CA, March, 1998.

C.R. Schultheisz and G.B. McKenna, Investigations of Nonlinear Material Behavior Using Simultaneous Measurements of Volume Recovery and Physical Aging, Symposium on Time-Dependent and Nonlinear Effects in Polymers and Composites, American Society for Testing and Materials, Atlanta, GA, May 4-5, 1998.

C.R. Schultheisz and G.B. McKenna, Investigations of Nonlinear Material Behavior Using Simultaneous Measurements of Volume Recovery and Physical Aging, 70th Annual Meeting of the Society of Rheology, Monterey, CA, October 4-8, 1998.

G.B. McKenna, New Perspectives on Glass Forming Systems: Isochoric Behavior,? Mini-Symposium on Spin Glass and Structural Glass Dynamics, University of California, Riverside, Riverside, CA, March, 1998.

D.M. Colucci and G.B. McKenna, Path Dependence of the Viscosity-Temperature Relationship in Polymer Melts: Fragility and Time-Pressure-Temperature Equivalence, 69th Annual Meeting of the Society of Rheology, Columbus, OH, October, 1997.

Task 2. The Viscoelastic Response of Industrial Materials

G.B. McKenna, M.L. Cerrada, W.H. Han¹, J.M. O'Reilly², J. Greener², J. R. Gilmore², O. Sindt³, S.L. Simon³, E. Liang^4
1Korean Government Fellowship for Oversas Doctoral Studies
²Eastman Kodak, Rochester, New York
³University of Pittsburgh
⁴General Electric Corporation Research and Development

Objectives
The objectives are i. to develop methods of analysis of the aging response of industrial materials that do not follow classical time-aging time and time-temperature superposition, ii. to perform experiments to characterize the aging response of industrial polymers, and iii. to investigate the impact of state of cure and moisture on the viscoelastic response of thermosets.

Technical Approach
Creep and stress relaxation measurements are been used to study the physical aging response of industrial materials subjected to temperature-jumps and moisture jumps. Experimental techniques are developed to perform experiments on films as thin as 50 µm to assure that diffusion times are short relative to the aging times of interest. Experiments are also performed in torsion for studies of an epoxy during cure and in the fully cured state. The Tool-Narayanaswamy-Moynihan model of structural recovery is extended to consider humidity or moisture impact on aging. Classical time-aging time superposition is used to characterize thermal histories. A model extending time-temperature superposition principles to the case of curing epoxies are also developed.

Accomplishments

• In a collaboration with Eastman Kodak Company, Poly(ethylene naphthalate) (PEN) was shown to be "thermo-rheologically complex" because the alpha and beta relaxations showed different temperature and aging time dependencies in classical aging time experiments. This was true for both amorphous and semi-crystalline materials.
• In conjunction with the collaborative work with Eastman Kodak Company, a new experimental protocol to perform physical aging experiments in "iso-structural" conditions was used to study PEN. It was discovered that, in these conditions, time-temperature superposition is applicable to describe the viscoelastic response of amorphous PEN. Different "structures", however, exhibit different time-temperature master curves.
• The Tool-Narayanaswamy-Moynihan model was successfully extended to describe relative humidity jump experiments in an epoxy glass. Both volumetric and mechanical responses could be described with the model.
• In a collaboration with General Electric Company and the University of Pittsburgh, the evolution of the viscoelastic response of a commercial epoxy thermoset during cure was successfully modeled.

Outputs

Publications
M.L. Cerrada and G.B. McKenna, Viscoelastic and Physical Aging Behavior in Semi-Crystalline PEN, Proc. NATAS 25th Annual Conference, September, 1997, pp. 404-410.

W.H. Han and G.B. McKenna, Polymers and Small Molecules: Glass Transition and Aging Effects, Proc. NATAS 25th Annual Conference, September, 1997, pp. 382-389.

M. L. Cerrada, G.B. McKenna, J.M. O'Reilly, J. Greener and J.R. Gillmor, Physical Aging Behavior in Amorphous PEN as Measured by Creep, Society of Plastics Engineers, ANTEC 98 II, 2198 (1998).

M.L. Cerrada and G.B. McKenna (1998) Creep Behavior in Amorphous and Semi-crystalline PEN, in Time Dependent and Nonlinear Effects in Polymers and Composites, ed. by R.A. Schapery, American Society of Testing and Materials, Special Technical Publication, STP 1357 submitted.

O. Sindt, S.L. Simon, G.B. McKenna, and E. Liang, Cure, Shrinkage and Properties of an Epoxy Material, Society of Plastics Engineers, ANTEC 98 II, 1658-1662 (1998).

Presentations
W.H. Han and G.B. McKenna, Structural Recovery in Plasticized Epoxy: First Evidence of a Moisture Induced Memory Effect, 69th Annual Meeting of the Society of Rheology, Columbus, OH, October, 1997.

M.L. Cerrada and G.B. McKenna, Viscoelastic and Physical Aging Responses in Amorphous PEN Films, 69th Annual Meeting of the Society of Rheology, Columbus, OH, October, 1997.

M.L. Cerrada and G.B. McKenna, J.M. O'Reilly, J. Greener and J.R. Gillmor, Physical Aging Behavior in Amorphous PEN as Measured by Creep, Society of Plastics Engineers, ANTEC, Atlanta, GA, May, 1998.

M.L. Cerrada and G.B. McKenna, Creep Behavior in Amorphous and Semi-crystalline PEN, Special Symposium on "Time Dependent and Nonlinear Effects in Polymers and Composites," American Society for Testing and Materials, Atlanta, GA, May, 1998.

O. Sindt, S.L. Simon, G.B. McKenna, and E. Liang Cure, Shrinkage and Properties of an Epoxy Material, Society of Plastics Engineers, ANTEC, Atlanta, GA, May, 1998.

G.B. McKenna, Kinetics in Glass-Forming Liquids, Phase Transitions in Polymers: Special Symposium Honoring E.A. DiMarzio, NIST, Gaithersburg, MD, November, 1997.

G.B. McKenna, Kinetic and Viscoelastic Behaviors of Glass Forming Systems, Polytechnic University of New York, Department of Chemical Engineering, Brooklyn, NY, November, 1997.

G.B. McKenna, Kinetic and Viscoelastic Behaviors of Glass Forming Systems, University of Massachusetts Amherst, Department of Polymer Science and Engineering, Amherst, MA, December, 1997.

G.B. McKenna, Kinetic and Viscoelastic Behaviors of Polymers and Other Glass Forming Systems, American Chemical Society, Pittsburgh Polymer Group, Pittsburgh, PA, February, 1998.

G.B. McKenna, Time Dependent Properties of Polymers: Physical Aging, Viscoelasticity, and Their Implications for Long Term Use, Gemfire Corporation, Palo Alto, CA, March, 1998.

G.B. McKenna, Kinetic and Viscoelastic Behaviors of Glass Forming Systems, University of California at Berkeley, Department of Mechanical Engineering, Berkeley, CA, March, 1998.

G.B. McKenna, Glass Forming Materials: Fundamental Physics and Mechanical Properties, California Institute of Technology, Materials Science Department, Pasadena, CA, May, 1998.

G.B. McKenna, The Physics of Nonlinear Viscoelasticity and other Time Dependent Properties of Polymeric Glasses, Xerox Research Center, Missassagua, CN, July, 1998.

G.B. McKenna, Time Dependent Properties in Polymeric Glass Formers: Structural Recovery and Its Effect on Mechanical Performance, University of Guelph, Department of Physics, Guelph, CN, July, 1998.

Task 3. Effects of Structural Recovery and Thermal Lag on Modulated and Dynamic Thermal Measurements

G.B. McKenna and S.L. Simon

Objectives
The objectives are to model the response of polymers to oscillatory thermal signals and to investigate the impact of physical parameters, such as structural recovery, thermal lag, etc. on the interpretation of signals from instruments in which such measurements are made.

Technical Approach
The Tool-Narayanaswamy-Moynihan (TNM) model of structural recovery has been solved for oscillatory thermal histories to address problems in the thermal analysis of glass forming materials. One issue is the impact of structural recovery on the thermal signal obtained in modulated DSC experiments. This problem is important because increasing numbers of laboratories are purchasing modulated DSC calorimeters for thermal analysis and one needs to fully understand this new technique. Hence, the TNM model was solved for typical thermal histories that would be encountered in the thermal analysis laboratory using material parameters for polystyrene (PS) and poly(vinyl chloride) (PVC). By using a Lissajous loop analysis in which heat flow is plotted against the time derivative of temperature (HF vs dT/dt) it was shown that the response can be tremendously influenced by structural recovery and that a linear analysis will give erroneous results in these cases. Hence it is recommended that users of the modulated DSC avoid thermal histories in which there is a great deal of structural recovery (enthalpy overshoot). It was further shown in the analysis that thermal lag in the samples due to simple thermal conductivity can have a significant effect on the data analysis, even for relatively small samples (thickness»0.2 cm).

The boundary value problem for heat flow between a sample pan and the sample, the important parameter being the heat transfer coefficient, was solved. The impact of a non-infinite heat transfer coefficient on estimates of the thermal conductivity of large samples was then investigated using an approximate solution published in the literature.

Accomplishments

• Model calculations demonstrated that structural recovery in polymeric glasses can dramatically affect the heat flow signal in modulated DSC measurements leading to nonlinear responses that cannot be analyzed by simple linear response theory.
• The Tool-Narayanaswamy-Moynihan model of strucutural recovery was used with values for the material parameters obtained from the literature to predict quantitatively the dynamic heat spectroscopy results on glycerol and poly(vinyl acetate). This demonstrates that the dynamic heat spectroscopy results are a simple manifestation of structural recovery in a high fictive temperature glass.
• It was shown that a published approximate method for the determination of the thermal conductivity using Temperature Modulated DSC can lead to significant (>30 %) errors in the determination of the thermal conductivity.

Outputs

Publications
S.L. Simon and G.B. McKenna, Interpretation of the Dynamic Heat Capacity Observed in Glass-Forming Liquids, J. Chem. Phys. 107, 8678 (1997).

S.L. Simon and G.B. McKenna, The Effects of Structural Recovery and Thermal Lag in MDSC, Thermochimica Acta 307, 1-10 (1997).

S.L. Simon and G.B. McKenna, The Effects of Structural Recovery and Thermal Lag in Modulated DSC Measurements, Proc. NATAS 25th Annual Conference, September, 1997, pp. 358-365.

G.B. McKenna and S.L. Simon, Interpretation of the Dynamic Heat Capacity Observed in Glass-Forming Liquids, Proc. NATAS 25th Annual Conference, September, 1997, pp. 677-684.

S.L. Simon and G.B. McKenna, Measurement of Thermal Conductivity using Temperature-Modulated Differential Scanning Calorimetry: Solution to the Heat Flow Problem, Society of Plastics Engineers, ANTEC 98 II, 2042 (1998).

S.L. Simon and G.B. McKenna, Measurement of Thermal Conductivity using Temperature-Modulated Differential Scanning Calorimetry: Solution to the Heat Flow Problem, J. Reinforced Plastics and Composites, submitted.

G.B. McKenna and S.L. Simon, Time Dependent Volume and Enthalpy Responses in Polymers, in Time Dependent and Nonlinear Effects in Polymers and Composites, ed. by R.A. Schapery, American Society of Testing and Materials, Special Technical Publication, STP 1357, submitted.

G.B. McKenna and S.L. Simon, The Glass Transition: Its Measurement and Underlying Physics,? in Handbook of Thermal Analysis and Calorimetry, Vol. 3, ed. by S.Z.D. Cheng, Elsevier, submitted.

Presentations
G.B. McKenna and S.L. Simon, Time Dependent Volume and Enthalpy Responses in Polymers, ASTM Symposium on "Time Dependent and Nonlinear Effects in Polymers and Composites," Atlanta, GA, May, 1998.

G.B. McKenna, Everything (Or Almost) You Always Wanted to Know about the Glass Transition, but Were Afraid to Ask, Carnegie-Mellon University/University of Pittsburgh Polymer Mini-Symposium, Pittsburgh, PA, March, 1998.

Failure in Polymers and Adhesives

M.Y.M. Chiang, H. Chai¹ and M. Fernandez-Garcia
¹Tel Aviv University, Israel

Objectives
Develop numerical models of adhesive behavior and validate these with experimental observation. Investigate the effects of aging and environment on the failure and pre-failure behavior of polymers and adhesives.

Planned Outcomes

• Experiments and finite element analyses conducted to demonstrate that mode II fracture can be described by a criterion in which the local strain at the crack tip determines failure.
• Methods of testing and evaluation developed that impact on long term performance evaluation and predictability in polymers, composites and adhesives.
• Correlations established between long term moisture exposure and the mechanical response of commercial adhesives to support ATP focus program on Composites.
• Theoretical models of craze initiation and growth tested by using structural recovery (physical aging) to vary the material's viscoelastic response.

Task 1. The Effect of Strain Rate on the Mechanical Properties of Adhesive Bonds

M.Y.M. Chiang, H. Chai

Objectives
The objectives are to study the effect of strain rates on yielding and failure processes in a commercial adhesive and to correlate the results of this study to those of previous work from this laboratory in which strain rate effects on the crack propagation and interfacial fracture were analyzed within a fracture mechanics framework.

Technical Description
Based on past experience in this laboratory, the "napkin ring" shear test was chosen for the purpose of this task. The specimen for this test is composed of two tubular adherends butted coaxially and joined with the adhesive. Loading is applied with a remote torque. By making the tube wall sufficiently thin, radial variations are minimized. This leads to a state of virtually simple shear within the bond layer. The experimental procedure includes the preparation of napkin ring specimens with controlled bond thicknesses, testing, and then interpretation of the results. Once the adherents are bonded with the adhesive, the specimens are milled in the area of bond. The milled surfaces are then polished, and bond thicknesses measured with an optical microscope. Specimens are tested under torsion at room temperature in dry condition. The adherends were 5086 aluminum alloy while the adhesive was a toughened epoxy (BP-907). The bondline thickness varied from 25 micrometers up to 120 micrometers, with most of the bonds being fairly close to 60 micrometers. The straining rate (loading rate) was varied from 0.001/sec. to 0.25/sec. The specimens were tested in a MTS servo hydraulic testing machine. The shear stress and the shear strain were recorded during the tests.

Accomplishments

• A test fixture to hold the test specimens was prepared. This includes a specimen housing unit, a loading arm and an attachment to adapt the extensometer which is used for establishing a truly controlled shear strain in the bond.
• The preliminary results showed that the stress-strain behavior of the material greatly depends on the loading rate, both in the small and in the large deformation regimes.

Outputs

Publications
Chiang, M.Y.M. and Chai, H., Nonlinear Computational Analysis of Experimentally Measured Crack Growth Along a Polymer-Metal Interface, the Proceedings of Sixth Pan American Congress of Applied Mechanics ( PACAM VI), Rio de Janeiro, Brazil, January, 1999.

Chiang, M.Y.M., Kuo, A. and Chen,W., Stress Intensity Solutions for Plastic IC Package Delamination, ASME Journal of Electronic Packaging, submitted.

Presentations
M.Y.M. Chiang and H. Chai, Finite Element Analysis and Experimental Monitoring of Strain Localization and Failure Processing of Thin Adhesive Bonds, International Conference on Polymer Characterization, Denton, TX, January, 1998.

M.Y.M. Chiang and H. Chai, Stable Interfacial Crack Extension in Polymeric Adhesive Bonds Under Shear, AIChE 1997 Annual Meeting, Los Angeles, CA, November, 1997.

Task 2. Hygrothermal Aging Studies of a Commercial Structural Adhesive

M.Y.M. Chiang, M. Fernandez-Garcia

Objectives
The objectives are to characterize the hygrothermal aging of a commercial epoxy-based adhesive in adhesive joints with different bond thicknesses and to identify the failure mechanisms induced by the hygrothermal aging of the adhesive.

Technical Description
Increased use of adhesive bonding in structural components has renewed interest in the analysis of adhesive joints, taking into account the diffusion of moisture in the adhesive layer. Diffusion of moisture or solvent into the adhesive can significantly change the stress field and hence the reliability of the bond joints. Therefore, this research involves the disciplines of polymer science and applied mechanics, where the former addresses effect on the micro-level and latter is related to macro-response. A further goal of this project is to examine the effect of moisture on the failure behavior of highly constrained thin adhesive layers.

The adhesive used in this study is an epoxy-based structural adhesive supplied by 3M Corporation (3M 5024). Free-standing films of adhesive, 300 µm thick, have been produced by curing the epoxy between Teflon sticks in a pressure vessel at 130 °C and 1.38 MPa (200 psi) for 13 h. DSC experiments have demonstrated that the curing time, being much longer than recommended by the manufacturer, does not cause any alteration such as degradation or aging. After curing, the film is cut to dimensions of ca. 40 mm x 6 mm x 0.3 mm. Specimens are hygrothermally exposed in distilled water baths at constant temperatures of 30 °C, 35 °C, 40 °C, 45 °C, 50 °C and 70 °C. The specimens are taken from the baths at different times, water on the surface of specimens is removed by using a dry, clean paper towel, specimens are weighed and the mass uptake is determined.

Accomplishments

• Preliminary results of absorption and reabsorption at the temperatures studied showed that the water uptake did not follow Fickian diffusion which can be represented by a time-dependent mathematical model. The time scale for the mechanical relaxation at higher temperature is similar to that for water uptake. Consequently, their interaction significantly extends the time scale to reach the equilibrium.
• The observed increase in the rate of sorption in the previously absorbed specimens (reabsorption process) compared with dry specimens (absorption process) reflects the effect of this time-dependent reconfiguration of the polymeric molecules.

Outputs

Publications
M. Fernandez-Garcia and M.Y.M. Chiang, Hygrothermal Aging Studies of A Commercial Structural Adhesive, Proceedings of the Adhesion Society 21st Annual Meeting, February, 1998.

Presentations
M.Y.M. Chiang and M. Fernandez-Garcia, Water Absorption and Desorption in a Structural Adhesive, The Adhesion Society's 21st Annual Meeting, Savannah, GA, February, 1998.

Task 3. Measurement of Craze Initiation and Growth in Polymers

G.B. McKenna, M. Delin^1
1Chalmers University of Technology, Goteborg, Sweden

Objectives
The objectives are i. to develop instrumentation and methods to investigate the craze initiation and growth responses during physical aging and ii. to determine the impact of physical aging on crazing in polymeric glasses.

Technical Approach
This project is based on the premise that crazing is a process that depends very strongly on the viscoelastic response of the glassy polymer under study. It is already known that temperature can have large effects on craze initiation and growth. Here the viscoelastic response will be altered by changing both temperature and aging time. Altering the aging time permits isothermal changes in the "glassy structure". Craze initiation and growth can be studied using special fixtures to provide equibiaxial and uniaxial loading in creep conditions and uniaxial loading in stress relaxation conditions.

Accomplishments

• A transition in craze growth rate (in logarithmic time) was discovered in stress relaxation conditions for a styrene-acrylonitrile copolymer. The logarithmic craze growth rate was found to decrease dramatically over a very narrow range of aging times. The magnitude of the logarithmic growth rate decreases by approximately a factor of five in going from below to above the transition aging time. It was also found that the aging time at the transition decreases as the temperature increases.
• The transition in craze growth rates was predicted, approximately, using the modified meniscus instability model developed originally by Kramer.

Outputs

Publications
G.M. Gusler and G.B. McKenna, The Craze Initiation Response of A Polystyrene and a Styrene-Acrylonitrile Copolymer During Physical Aging, Polymer Engineering and Science 37, 1442 (1997).

M. Delin and G.B. McKenna, Impact of Physical Aging on Craze Growth in a Styrene-Acrylonitrile Co-Polymer, Proc. NATAS 25th Annual Conference, September, 1997, p. 436.

M. Delin and G.B. McKenna, Craze Growth in Stress Relaxation Conditions: Effects of Physical Aging, SPE ANTEC, 1668 (1998).

Presentations

M. Delin and G.B. McKenna, Craze Growth in Stress Relaxation Conditions: Effects of Physical Aging, Society of Plastics Engineers ANTEC, Atlanta, GA, April, 1998.

M. Delin and G.B. McKenna, Impact of Physical Aging on Craze Growth under Stress Relaxation Conditions, 69th Annual Meeting of the Society of Rheology, Columbus, OH, October, 1997.

M. Delin and G.B. McKenna, The Craze Growth Response in Stress Relaxation Conditions for a Styrene-Acrylonitrile Copolymer During Physical Aging, ASTM Symposium on "Time Dependent and Nonlinear Effects in Polymers and Composites," Atlanta, GA, May, 1998.

Rheological Characterization of Polymer Dynamics

G.B. McKenna, C.R. Schultheisz, V. Rouiller¹, and V. Soulivong^2
1NIST Guest Researchers from Houdemont, France
²Universite Louis Pasteur, France

Objectives
The objectives are to develop rheological descriptions for polymeric materials that provide standard reference materials, to produce critical data and to provide defining tests of existing physical and phenomenological models for polymer dynamics.

Task 1. Develop a Nonlinear Fluid Standard

G.B. McKenna and C.R. Schultheisz

Objective
The objective is to develop and thoroughly characterize a non-Newtonian fluid that can be used as a benchmark to measure the performance of rheometers in situations involving complex fluids.

Technical Description
Polymeric fluids, such as polymer melts and solutions, often do not follow the simple Newtonian ideal in their flow behavior. Such fluids are found in numerous applications in everyday life (injection molding, paints and coatings, food products, etc.), and the ability to measure accurately and characterize their behavior is very important to optimizing their processing conditions. Since there are a number of commonly used methods to measure the flow behavior of polymers, the new Standard Reference Material (SRM 2490) will provide a means for comparing the performance of different instruments, as well as a tool for research into better methods of measuring the rheological properties of polymeric fluids. The new fluid will be certified for the shear-rate dependence of viscosity and first normal force difference over a temperature range of approximately 50 °C. The linear viscoelastic responses will also be certified along with the temperature dependence of the shift factors and the zero-shear-rate viscosity.

The new fluid supersedes the previous Standard Reference Material 1490 Nonlinear Fluid Standard, which was composed of a high-molecular- mass polyisobutylene dissolved in normal hexadecane. This fluid was found to have two problems: first, it showed evidence of a loss of homogeneity on storage, and second, the working range of the fluid was limited by the crystallization of the hexadecane, which has a melting point of 18 °C (64 °F). The new fluid will consist of a high-molecular- mass polyisobutylene dissolved in 2,6,10,14-tetramethylpentadecane (pristane). The pristane is a branched alkane of a slightly higher molecular mass than the normal hexadecane; the branching prevents crystallization or vitrification down to -60 °C, while the higher molecular mass reduces the rate of evaporation of the pristane as compared to the normal hexadecane.

This project will also include a round robin test of the candidate Standard Reference Material involving instrument manufacturers and users in industry and at universities. The round robin will ensure that the chosen material properties are acceptable to the community, and will provide information regarding the laboratory-to-laboratory variability in the measurements. The Statistical Engineering Division at NIST will be involved in evaluating the results of the round robin.

External Collaborations

Instrument Manufacturers

ATS RheoSystems	Paar Physica
Bohlin Instruments	Rheometric Scientific
Brookfield Engineering Laboratories	TA Instruments
Capillary Rheometer	TherMold Partners
Goettfert	Vilastic Scientific
Haake

Industrial Users

DuPont	National Starch and Chemical
Fluid Dynamics	Procter and Gamble

Universities

David Boger	University of Melbourne
Ralph Colby	Pennsylvania State University
Francis Gadala-Maria	University of South Carolina
William Graessley	Princeton University
Chris Macosko	University of Minnesota
Jaye Magda	University of Utah
Gareth McKinley	Harvard University
Skip Rochefort	Oregon State University

Accomplishments

• A candidate material has been produced, and the process to produce and package the material for sale is underway. A new rheometer, with improved normal force measurement capability, has been acquired. Industry interest in the new fluid and expressed interest in participation in the round robin has been high.

Outputs

Publications
C.R. Schultheisz and G.B. McKenna, (1998) A Nonlinear Fluid Standard Reference Material: Progress Report, Society of Plastics Engineers, ANTEC 98 II, 2162-2166, (1998).

Presentations
C.R. Schultheisz and G.B. McKenna, Nonlinear Fluid Standard Reference Material: Progress Report, 69th Annual Meeting of the Society of Rheology, Columbus, OH, October, 1997.

C.R. Schultheisz and G.B. McKenna, A Nonlinear Fluid Standard Reference Material: Progress Report, SPE ANTEC, Atlanta, GA, April, 1998.

C.R. Schultheisz and G.B. McKenna, A Nonlinear Fluid Standard Reference Material: Progress Report, 70th Annual Meeting of the Society of Rheology, Monterey, CA, October, 1998.

C.R. Schultheisz and G.B. McKenna, Thermal Expansion of the Force Rebalance Transducer in a Rheometric Scientific ARES Rotary Rheometer, 70th Annual Meeting of the Society of Rheology, Monterey, CA, October, 1998.

Task 2. Viscoelastic Constitutive Model for the Creep Behavior of Polyurethane Foams

G.B. McKenna and V. Rouiller

Objectives
The objectives are to apply the hybrid-VL-BKZ theory to the viscoelastic response of polyurethane foams, and to measure and model the behavior of the polyurethane materials as a function of temperature and moisture content.

Technical Approach
In the past, a hybrid VL-BKZ model has been successfully used in our laboratory to describe the nonlinear viscoelastic response of polymeric melts. The same approach is applied to investigate and model the nonlinear response of polyurethane elastomers.

Accomplishments

• The hybrid VL-BKZ model has been implemented such that the stress response to multiple step and trapezoidal deformation histories can be described within approximately 20 %. In addition, a computer program has been developed to calculate creep loading histories. Comparison with experimental results provides good agreement for complicated loading histories.

Outputs

Publications
V. Rouiller and G.B. McKenna, A Viscoelastic Constitutive Model for the Creep Response of Polyurethane Rubber. (Progress Report for the Period September, 1996 to December, 1997), NISTIR 6177, August, 1998.

V. Rouiller and G.B. McKenna, A Hybrid Nonlinear Constitutive Model: Comparisons with Multiple Step Data for a Polyurethane Rubber, Society of Plastics Engineers, ANTEC 98 II, 2138-2143, (1998).

J.M. Niemiec, V. Rouiller, G.B. McKenna, J. Sweeney, and I.M. Ward, Prediction of Uniaxial Extension, Pure Shear, and Equibiaxial Responses in Constant Deformation Rate Experiments from Single Step Stress Relaxation Experiments in Uniaxial Extension, J. Rheology, submitted.

Presentations
V. Rouiller and G.B. McKenna, A Hybrid Nonlinear Constitutive Model: Comparisons with Multiple Step Data for a Polyurethane Rubber, Society of Plastics Engineers, ANTEC, Atlanta, GA, May, 1998.

V. Rouiller and G.B. McKenna, Viscoelastic Response of Polyurethane Elastomers, 69th Annual Meeting of the Society of Rheology, Columbus, OH, October, 1997.

G.B. McKenna, A Hybrid Valanis-Landel/BKZ Constitutive Model for Rubber and Polymer Melts, Carnegie Mellon University, Department of Chemical Engineering, Pittsburgh, PA, February, 1998.

Task 3. Melt and Solution Nonlinear Rheology

G.B. McKenna, C.R. Schultheisz, V. Soulivong

Objective
The objective is to characterize the dependences of viscosity and first normal stress difference for model polyisobutylene and polystyrene systems as functions of concentration and temperature.

Technical Approach
Molecular rheology has been well investigated over the years in the domain of the linear viscoelastic response. In particular, the reptation theory has proven highly successful in dealing with many aspects of polymer melt dynamics. On the other hand, the nonlinear response of materials and, especially, the concentration dependence of the polymer solute is less well investigated. Here soluitions of polyisobutylene dissolved in 2,6,10,14-tetramethylpentadecane (the material to be used as the nonlinear fluid standard) will be made and the effects of concentration on the material functions will be examined. Second, solutions of monodisperse polystyrene in ortho-terphenyl will be used to investigate the impact of concentration and molecular mass on the time-dependent strain potential function (BKZ theory) and the damping function (Doi-Edwards theory).

Accomplishments

• Methods have been established for producing solutions of polystyrene in ortho-terphenyl and preliminary tests have been performed.
• The rheometer to be used for the polyisobutylene solutions has been shown to have a vibrational problem when high viscosity samples are placed in the cone and plate and narrow gap parallel plate geometries. The problem is being fixed by the manufacturer.

General Outputs in Rheology

Publications
F. Horkay, W.H. Han, G. B. McKenna, A Comparison of Some Rubber Network Models with Stress-Strain Data on Dry and Swollen Polymer Rubbers, J. Mathematics and Mechanics of Materials, in press.

R. Zorn, F.I. Mopsik, G.B. McKenna, L. Willner, D. Richter, Dynamics of Polybutadienes with Different Microstructures: 2. Dielectric Response and Comparisons with Rheological Behavior, J. Chem. Phys. 107, 3645-3655 (1997).

G.B. McKenna, F.I. Mopsik, R. Zorn, L. Willner, and D. Richter, Mechanical and Dielectric Measurements in Polymers: Differences in Time-temperature Superposition Behavior, SPE ANTEC II, 1027 (1997).

Calorimetric Measurement of the Glass Transition in Nano-scale Confined Geometries

McKenna, G.B. and Park, J.Y.^1
1Korea Advanced Institute of Science and Technology

Objectives
The objective is to make the first calorimetric measurements of the effect of size and confinement on the glass transition of polymer chains imbibed into nano-meter scale porous glasses.

Technical Approach
The glass transition in confined geometries in films and porous materials is currently of great interest both scientifically and industrially. The approach adapted follows that developed previously in this laboratory; fluids were imbibed into control pore glasses to investigate the effect of size and confinement on phase transitions. The prior work was performed with organic, small molecules. The large size of polymer chains and their high viscosity makes it difficult to get the chains into the (10-50) nm size pores of interest. Consequently, the experiments are performed in solution where known concentrations of polystyrene in ortho-terphenyl are imbibed into the porous glasses. The glass transition is measured using conventional differential scanning calorimetric approaches.

Planned Outcomes

• Measurements will show the impact of size and confinement at the nano-meter scale on the calorimetric glass transition of polymer solutions.
• Concentration and molecular architecture effects on the glass transition of polymer solutions in nano-meter size pores will be established.
• The effect of pore size on the structural (enthalpy) recovery of vitrified polymer solutions will be established.

Accomplishments

• Measurements of the glass transition for solutions of a 100,000 molecular mass polystyrene in ortho-terphenyl imbibed into pores having sizes from approximately 10 nm to 50 nm have been made. Concentrations ranged from c* to 4 c*.

Outputs

Presentations
G.B. McKenna, Kinetics in the Vicinity of the Glass Transition: Some Old Results and Some New Perspectives, Canadian Association of Physicists, Annual Meeting, Waterloo, CN, July, 1998.

Theory and Modeling of the Ten Polymer Phase Transitions

E. A. Di Marzio

Objectives
The objectives are i. to discover, classify and understand all the polymer phase transitions, ii. to determine how these couple to one another, iii. to use polymer phase transitions as models for self-assembly, and iv. to investigate the coupled phase transitions as technology opportunities.

Technical Description
A classification scheme is developed for coupled pairs, triplets, etc. of polymer phase transitions. An attempt is being made to collect, catalogue, and classify the various examples of coupled polymeric phase transitions in order to determine what technology opportunities exist in the area of self-assembly.

Planned Outcomes

• Methodology will be developed for classifying phase transitions in polymers.

Accomplishments

• A tenth polymer phase transition, a polymer molecule threading a membrane separating two solutions, has been discovered. The transition is first-order. In addition, an important insight was obtained which shows why linear macromolecules display phase transitions in abundance. Other transitions are: The polymer adsorption transition (2nd-order); the helix to random coil transition (diffuse-order for polypeptides, 2nd-order for DNA, 1st-order for Collagen); the collapse transition (1st and 2nd-order); equilibrium polymerization (2nd-order).
• The number of coupled phase transitions was determined. The 10 classes of polymeric phase transitions result in 1013 a priori possible coupled phase transitions.
• The extent to which self-assembly in polymeric systems can be viewed as thermodynamic phase transition phenomena was investigated . It is concluded that self-assembly in polymers is essentially nothing more than the equilibrium, kinetic and pattern formation aspects of polymeric phase transitions.

Outputs

Publications
E. A. Di Marzio and A. J-M Yang, Configurational Entropy Approach to the Kinetics of Glasses, J. Research, NIST 102, 135 (1997).

E. A. Di Marzio, The Use of Configurational Entropy to Derive the Kinetic Properties of Polymer Glasses, ACS Books, Symposium on Polymer Glasses, San Francisco, CA, April, 1997, in press.

E. A. Di Marzio and A. Mandell, Phase Transition Behavior of a Linear Macromolecule Threading a Membrane, J. Chemical Physics, in press.