S.V. Orski, W.S. Farrell, K.L. Beers
Keywords: standards, size-exclusion chromatography, polyolefins
Summary:Advanced synthetic processes and novel metal-based catalysts have revolutionized manufacturers’ ability to tailor molar mass, polydispersity, and degree of short chain branching into polyolefin plastics synthesized from petrochemical feedstocks. These capabilities have resulted in polymers with more finely-tuned, advanced material properties. These materials have also fueled the development of in-line infrared detectors for size exclusion chromatography (SEC) to quantify average chemical and branching composition of a polymer across its molecular mass distribution curve. This analytical technology can be used to study not only these advanced polymers, but also enable the study chemical and structural heterogeneity of crude oil byproducts, such as asphaltenes. Standards representative of these complex chemistries and architectures are required to properly quantify the relationships between molecular structure, processing, and performance of these materials. The polymers in NIST’s SRM portfolio is comprised predominately of thermoplastics developed in the 1970s to quantify molar mass and melt properties of polymers, especially for linear polyethylene. SRM 1475A, a broad distribution linear polyethylene (Mw = 52,000 g/mol) and standards derived from its fractions are widely used NIST standards for molecular mass and molecular mass distributions. These linear standards have provided calibration and measurement traceability to stakeholders in manufacturing, industrial R&D, and analytical laboratories. While these standards still provide some value for core polymer manufacturing activities, the existing NIST polyethylene standards have become less representative of advanced commercial materials, and the traditional SEC methods they were designed to enable have limited ability to quantify the critical values of interest in these new classes of polymeric materials. NIST’s approach to modernizing its synthetic polymer reference materials entails both the development of new polymer standards while evolving foundational standards, like polyethylene, using the same synthetic processes. This is necessary to be able to directly quantify chemical functionality and degree of branching relative to a linear standard of comparable molar mass and polydispersity. These precision polymers minimize uncertainty due to material heterogeneity and still provide needed molar mass standards to the broader materials community. A candidate standard material has been developed using metallocene catalysts to generate branched polyolefins with low polydispersity and sequence control, where the degree of alkyl branching and alkyl branching distribution are known. This process is adaptable to many branch lengths and varied chemistries as it only requires replacement of the monomer feedstock with the desired chemical functionality. This material and its linear analog were measured by SEC with tetra-detection to quantify molecular mass distribution and degree of alkyl branching and were compared to existing NIST polyethylene standards. Furthermore, these materials are currently being applied to study the effect of branching on polymer chain thermodynamics near surfaces and comparing experimental results to established theoretical models.