Characterizing polymer degradation during processing using multi-detector GPC and capillary rheometry

Polystyrene (PS) and poly(methyl methacrylate) (PMMA) are two of the most widely used synthetic polymers. Polystyrene is used as a protective packaging for anything from foods to CD cases and disposable cutlery, while PMMA is regularly used as a polymeric alternative to glass and even in some medical technologies, and countless other consumer products.

In both cases, melting and molding are regular processes that these polymers must endure.  It is therefore important to understand how they will respond to this kind of treatment.  It is well recognized that the bulk properties of a polymer such as strength, toughness, flexibility, etc. are strongly dependent on molecular properties such as molecular weight and branching.  If the molecular properties of these polymers change as a consequence of the processing, then it is likely that the final properties of the molded product will vary from those expected of the virgin polymer. 

A study was therefore undertaken to explore how these polymers respond to the stresses of molding.  This application note describes the molecular and rheological changes that occurred as samples of PS and PMMA were repeatedly extruded through a capillary rheometer, simulating molding.  The capillary rheometer was used to measure changes in melt viscosity, while multi-detector GPC (Gel Permeation Chromatography) was used to characterize changes in molecular weight and structure in the samples after each cycle through the rheometer.

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