Research Highlights of Polymers Division

Polyolefins: Serving the Largest Market

-Standard Reference Materials (SRMs) for calibration of Gel Permeation Chromotography (GPC).
-Mass Spectrometry for absolute mass distribution and molecular structure.

Molecular Characterization

The increasing number of distinct polyolefin molecules that have entered the marketplace and the larger number that can now be synthesized in the laboratory drive the need for characterization techniques, in particular with respect to molecular mass and distribution. These quantities determine the processability as well as the final properties of the material.

Materials Science and Engineering Laboratory research and services in molecular characterization provide industry with the tools to properly ascertain molecular mass and distribution. Calibration of instruments with National Institute of Standards and Technology standard materials allows suppliers and processors to trade with confidence. Research into next generation calibration techniques ensures that U.S. industry will maintain its competitiveness as increasingly stringent demands are placed upon materials producers.

SRMs for Molecular Characterization

Fig. 1: Six narrow polyethylene fractions, with the molecular mass moments obtained by absolute methods, are used for calibration of instrumentation and for research. Plotted is the differential refractive index (DRI) versus elution volume in mL.

Mass Spectrometry of Polyethylene

Polyethylene is not amenable to analysis by the usual mass spectrometric methods owing to the lack of chemical functionality to yield intact macromolecular ions.

In the last year the polyolefin mass spectrometry team produced intact, gas-phase polyethylene single chains more than six times greater in mass than previously reported by any research group. Matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and the covalent cationization method, developed last year in the Polymers Division, were each optimized by a multidisciplinary team of Division scientists. A palette of spectroscopic techniques was used to find the optimal conditions for the necessary conversion of polyethylene into its ionizable phosphonium salt.

Figure 2. Mass spectrum of SRM 1482 obtained by the covalent cationization method. The insert shows details of the spectrum above 15,00 g/mol showing that the technique sees higher molecular mass species but not in the correct proportion. Plotted is signal intensity in arbitrary units (au) versus mass in u.

he molecular mass moments determined from mass spectrometry are about two-thirds as large as those found classically. This discrepancy may be due, in part, to the highly crystalline nature of the polyethylenes examined that may reject the MALDI matrix during target preparation. In order to prevent this occurrence a heated sample stage will be fitted to a TOF mass spectrometer allowing experiments to be performed above the melting temperature of polyethylene. In addition, experiments on the covalent cationization of atactic polystyrene are being undertaken in order to rule out effects solely derived from the ionization process. This project enjoys keen interest from industrial producers who have offered to synthesize specialized materials to further the Division's work.

Microscopic Structure

Complex microscopic structures due to crystallization, phase separation, and particulate dispersion directly determine physical properties such as permeability, impact strength, and toughness. The various microstructures are developed during processing operations and reflect the molecular characteristics of the polyolefin such as monomer regularity and chain branching.

During the past year, extensive experimental work has been carried out to understand the effect of blending and thermal history on structure development in polyolefin blends. This represents the first comprehensive investigation of blends that exhibits both liquid phase separation and crystallization. Among many interesting findings, we mention just two. First an abnormally large effect of co-crystallization in this blend, which is not expected if truly random copolymerization, is assumed. Second, the effect of simple shear flow on crystal suspensions in polyolefin blends has been investigated using light scattering (see figure below) and microscopy. The rich complexity of structure that we observed demonstrates the potential for directed control of microstructure through thermal and shear history.

Figure 3. Structural information regarding polymer blends under shear flow is obtained via in-situ light scattering, where HH indicates horizontal incident and scattered light polarization, and VV is vertical light polarization.

Influence of Monomer Structure on Polymer Blend

Many commercially important polymer materials are "alloys" of polymers having different monomer structures. Flory-Huggins theory has provided the main theoretical framework for describing the miscibility, phase separation and scattering properties of polymer blends, but this theory completely neglects the dissimilarity of monomer structure. This fundamental issue was taken up in collaboration with the University of Chicago. A generalization of the Flory-Huggins theory, accounting for monomer structure for high molecular weight (incompressible) blends, indicates that monomer structure has profound effects on blend properties and miscibility. We discovered a new classification scheme, featuring four distinct classes of blends. A re-analysis of earlier experimental studies from the Polymers Division provides strong evidence for this new theoretical framework.

Macroscopic Processing

One primary reason for the size of the polyolefin market is that numerous processing techniques have been developed which provide the bridge between the raw polymer and the final product. Processing considerations force strong constraints upon new polymer development, for example, frequently a new polymer molecule or formulation must be processable with existing equipment in order to succeed in the market. We frequently work closely with the polyolefin industry on well-focused topics to ensure the commercial relevance of our work.

One frequent concern in the processing of polyethylene is its susceptibility to "sharkskin melt fracture" during extrusion. We have utilized and developed instrumentation to conduct precise measurements of the cause and location of this processing defect. Current work includes the construction of a Total Internal Reflection Coating Instrument, which will be able to quantitate the buildup of fluoropolymer additives, which is one strategy to eliminate sharkskin.

A second critical issue in polyolefin processing is the shear induced temperature excursions, which can significantly alter the viscosity and hence the processing. Previously, we have developed fluorescent based techniques to conduct in-situ measurements of polymer temperature during extrusion. Recently, we have utilized this methodology to carry out simultaneous measurements of polyethylene rheology and temperature (see figure below). We find that the during a standard ASTM test method, the resin temperature can increase uncontrollably by 20 oC, meaning that great care must be taken in the application of the generated data.

A third area concerns molecular orientation during polypropylene film production. Using fluorescent probes that can orient with the extensional flow, we make fiber optic based sensors, which determine the anisotropy of the material.

SRMs for Processing

Virtually all polyolefins are specified by their melt flow index (MFI). NIST produces melt flow rate standards that are used for checking an ASTM method (ASTM D1238) commonly used to determine processability of polyethylenes.

Physical Properties

Physical properties are a result of molecular structure, microstructure and processing history. Here we describe two activities intended to improve the final properties

Flammability of Polypropylene- Nanotube Composites

There is much current interest in the enhancement of physical properties of polymeric materials by incorporation of nano-scale additives, such as carbon nanotubes and clay. In conjunction with the Fire Research Division, we have recently shown that carbon nanotubes can significantly reduce the heat release rate of polypropylene under controlled burning conditions. This work was carried out with multi-walled nanotubes that were well dispersed at a 1 % volume fraction, without the addition of compatiblizers or organic treatments of the nanotubes. Preliminary measurements indicate that the fire suppression effectiveness is better than achieved for clay filler nanocomposites at much higher loadings (order 10 % to 20 % volume fraction), where surfactant is required for effective clay dispersal. The rapidly decreasing cost of the multi-walled carbon nanotubes and the advantages of these composites for recycling make this approach to fire suppression potentially attractive for commercial development in the future. Fire suppression in polyolefin materials, in particular, is a problem of crucial importance to society because of the widespread use of these materials and the pollution associated with existing fire retardant agents.

SRMs for Orthopedic Implants

Concerns about long-term durability of polyethylene hip implants have created a need for a standard material to test the properties of implant materials. In response, the first reference biomaterial, an orthopedic grade ultrahigh molecular weight polyethylene, UHMWPE, was issued for use in development of improved test methods for wear and as a benchmark for development of improved materials. Properties reported are the Tensile: Young's Modulus, Yield Strength, Ultimate Strength, and Elongation to Failure. In anticipation of continuing need for reference material UHMWPE for radiation studies, NIST issues orthopedic grade UHMWPE cubes for radiation cross-linking tests and analysis

For more information on this topic:

Molecular Characterization: Bruno Fanconi, Charles Guttman, William Wallace

Microscopic Structure: Jack Douglas, Charles Han, Howard Wang

Macroscopic Processing: Anthony Bur, Kalman Migler

Physical Properties: Jack Douglas, John Tesk