A polymer by definition is made up of many repeating units (monomers). However, it is exceptionally rare that a polymer is comprised of replicates of same exact molecule – where each molecule consists of the same specific shape, molecular size and weight, etc. It is quite typical that two batches of the “same” polymer have slightly differing molecular polymeric profiles – reflecting slight differences in molecular weight distributions, among other possible chemical differences. Depending upon the polymer and it usage, these slight differences may have little to no significant impact, or it may have a dramatic impact on the final product. For this reason, appropriate polymer characterization can be critical to ensuring suitable ultimate product quality, performance and even safety.
Polymer characterization is performed as a function of chemical / physio-chemical, thermal and mechanical analysis. However, if properly qualified and performed, the chemical analysis is the basis for the ultimate critical properties of the polymer. The average molecular weight can impact polymer melting point or even strength. The molecular weight distribution can impact the melting range or other mechanical properties. Chemical functional groups within a polymer can impact the conformation and even the behavior of a polymer under certain conditions.
More typical polymer characterization is performed using the following list of analytical techniques. Beyond these, are certain more specialized approaches to suitable characterization.
Functional Group Analysis – FTIR, Raman, chemical analysis
These are techniques that can identify and measure the type and presence of the functional chemical groups making up or contained with the the monomers helping to provide certain polymeric properties.
Molecular Weight / Size – NMR (end group analysis), GPC / SEC, light scattering
Methods of determining overall size and molecular weight of the polymer distribution – with NMR generally restricted to relatively small polymers (> 6 kDa), gel permeation or size exclusion chromatography is used to create a distributed molecular profile and help elucidate the impact of the molecular distribution. Light scattering provides a true “absolute” measure of the molecular weight responses.
Conformation – SEC/light scattering (MALLS), SEC / viscometry, Field Flow Fractionation
These techniques can be used to determine and quantify a polymeric conformation and determine its impact on overall properties. An example of measuring polymeric branching is provided.
Certain Mechanical Properties – DSC, TGA, viscometry, etc.