Research Highlights of Polymers Division

Standard reference materials and reference materials are issued to address needs of the producers, processors and users of polymers for calibration and for performance evaluation of instruments used in the control of the synthesis and processing of polymers as well as benchmarks for comparisons of measurement methods and development of new materials. Recently produced standard reference materials include polyethylene of narrow mass distribution certified for mass average molecular mass and intrinsic viscosity and a nonlinear fluid standard for rheological measurements. In addition, the first reference biomaterial, an orthopedic grade ultrahigh molecular weight polyethylene, was issued for use in development of improved test methods for wear and as a benchmark for development of improved materials.

New Polyethylene Molecular Mass Standards Three new polyethylene standard reference materials were certified for mass average molecular mass by light scattering and intrinsic viscosity in 1,2,4 trichlorobenzene. The three narrow mass fractions, Mw/Mn=1.2, together with the three narrow fraction polyethylene standard reference materials that NIST already provides, make a total of six narrow fraction polyethylene SRMs certified for molecular mass averages. The mass standards cover the mass range from 6000 u to 200,000 u. The polymer industry, analytical laboratories and university researchers use these unique standards primarily for calibration of gel permeation chromatographs, the principal instrument used for determination of the mass distribution of polymers. The figure displays the chromatograms of these six standards. The starting materials for all six of these standards were produced by fractionation of a whole linear polyethylene that is also available from NIST, SRM 1475.

Polyethylene is the dominant commercial polymer in the United States and worldwide markets. High temperature (130 °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 commercially available for calibration of high temperature SEC. Molecular mass fractions of low polydispersity, Mw/Mn, of less than 1.2 (Mw is the mass average molecular mass and Mn is the number average molecular mass), are the most useful materials for calibrating SEC. The NIST polyethylene standard reference materials, SRMs, are the only available narrow fractions of polyethylene. Prior to the availability of these standards, crude approximations had to be used in the calibration of gel permeation chromatographs for mass distribution measurements.

Virtually all U.S. manufacturers of polyethylene purchase NIST molecular mass standards for measurement assurance

Non-linear Fluid Standard for Rheological Measurements

Non-Newtonian rheological standards are developed to exhibit the typical polymeric behaviors of shear thinning and normal stresses; these standards are also used for calibration of rheological instruments and for research into improved measurement methods.

Polymer fluids, such as polymer melts and solutions, often do not follow the simple Newtonian ideal in their flow behavior, demonstrating shear-rate dependent viscosities and normal stresses. Such fluids see wide application in everyday life (injection molding, paints and coatings, food products, etc.), and the ability to measure and characterize their behavior accurately 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 way for comparing the performance of different instruments, as well as providing tools for research into better methods of measuring the rheological properties of polymeric fluids. SRM 2490 is certified for the shear-rate dependence of viscosity and first normal stress difference at temperatures of 0 °C, 25 °C and 50 °C. The linear viscoelastic responses are also certified, along with the temperature dependence of the shift factors. The new polymer solution, SRM 2490, supercedes the previous SRM 1490 Nonlinear Fluid Standard, which was composed of a high-molecular mass polyisobutylene dissolved in normal hexadecane. This standard fluid was found to have two problems: first, it showed evidence of loss of homogeneity on storage, and second, the working range of the fluid was limited by the crystallization of the normal hexadecane, which has a melting point of 18 °C (64 °F). SRM 2490, on the other hand, consists of polyisobutylene (molecular mass of approximately 10 6 g/mol) dissolved in 2,6,10,14- tetramethylpentadecane. This solvent is a branched alkane of a slightly higher molecular mass than normal hexadecane; the branching prevents crystallization or vitrification down to at least –60 °C, and the higher molecular mass reduces the rate of evaporation as compared to normal hexadecane. SRM 2490 will be used in an interlaboratory comparison involving over 20 laboratories, representing instrument manufacturers and users in industry and at universities. The interlaboratory comparison will provide information regarding the laboratory-to-laboratory variability in the measurements; the variability in the measurement of the first normal stress difference is of particular interest.

Reference Biomaterial for Orthopedic Research

Reference Material (RM) 8456, an orthopedic grade Ultra High Molecular Weight Polyethylene (UHMWPE), became available in October 2000. RM 8456 is intended primarily for use in mechanical characterization of material properties and laboratory-simulated performance of orthopedic joint replacement implants. The availability of this reference polyethylene is expected to aid in development of improved test methods and materials by providing a benchmark for comparisons. The need for this reference biomaterial was identified at a workshop on reference biomaterials held at NIST and its development was the result of collaboration among a materials supplier, the orthopedic research community and NIST. The material used to prepare RM 8456 was donated by Poly Hi Solidur, Inc., MediTECH Division, Fort Wayne, IN in a form similar to that from which many orthopedic components are machined: a cylindrical bar with nominal dimensions of 7.62 cm (3 in) in diameter. Reference properties, reported as mean values with their expanded uncertainties, are Young's modulus, tensile yield strength, tensile ultimate strength, and tensile elongation-to-failure. These properties characterize the bar across the center 5.62 cm (2.21 in) of its diameter and down the entire bar length. Material beyond the central 5.62 cm was found to differ significantly from that within.

For more information on this topic

Charles Guttman – Polymer molecular mass standards

Carl Schultheisz – Nonlinear fluid standard

John Tesk – Reference biomaterials

Certification of the Relative Molecular Mass and the Limiting Viscosity of SRM 2885-7, NISTIR 6454, 6456, 6487.

Standard Reference Materials: Non-Newtonian Fluids for Rheological Measurements, Proc. SPE ANTEC, Vol. 1, pp. 1042-1046, Orlando, Fl, January 5, 2000.

Biomaterials FORUM, Vol.22, #6, Nov-Dec, 2000.