Now you see me and now you don’t: GPC analysis of polysiloxanes

Introduction

Gel permeation chromatography (GPC) is at the heart of polymer characterization. This characterization technique is implemented throughout the life of the material.This can include after initial polymerization, before and after purification, after post-polymerization modification, in the final formulation, after aging, failure and/or degradation. The primary focus of analyzing polymers using GPC is to characterize their molecular weights and molar mass distributions. There are many motivations behind determining the molecular weight including predicting how the products' properties will be affected and determining how well a reaction went. In addition to molecular weight, scientists also want to know about changes to the sample’s structure, branching, size, solution viscosity and compositional changes; the OMNISEC multi-detection GPC system provides this information with the highest level of sensitivity. The OMNISEC uses a combination of refractive index, light scattering, UV/vis spectrometer and viscometer detectors to provide a far more complete characterization than the relative molecular weight results provided by using only RI detection. 

Polysiloxanes are an interesting material to analyze by GPC, as if one tries to analyze the most common polysiloxane, polydimethylsiloxane (PDMS), in the standard organic solvent for GPC, THF, the peaks in the RI are extremely small and none existent in the light scattering detectors. However, when analyzed in toluene the PDMS signals in both of these detectors are intense. This is due to the refractive index increment (dn/dc) of the material (for further details on the dn/dc value please read this interesting blog post)

Polysiloxanes are widely used for many applications as they are very chemically stable, unreactive, hydrophobic, have good electrical insulation and can form a variety of material types, such as elastomers, gels, lubricants, foams, and adhesives. The chemical functionality incorporated into a polysiloxane can be chosen to convey specific properties to the polysiloxane. 

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