Rheological response of amorphous polymers under rapid cooling conditions

Author

Vikas S. Joshi

Date of Award

8-1989

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

Donald C. Bogue

Committee Members

J. E. Spruiell, Edward S. Clark

Abstract

The rheological behavior of polymers under conditions of simultaneous deformation and temperature change ("nonisothermal rheology") is not well understood. Since most polymer processing operations involve rapid changes of temperature while the material is being shaped or molded, the practical importance of nonisothermal rheology is evident.

The experimental portion of the present work involved determining the Theological response of an amorphous polymer, polystyrene, as it was cooled rapidly across the glass transition while it was constrained and not allowed to shrink. It was observed that the higher the cooling rate, the higher was the stress induced in the samples. The rheological parameters of polystyrene required for the analysis were obtained by independent rheological characterization experiments. In the glassy and transition states dynamic mechanical measurements were done using the Autovibron. The viscosity and dynamic properties data in the rubbery state were available in the literature. Rate dependent specific volume-temperature data were estimated from dilatometric data in the literature.

The nonisothermal analysis consisted of modeling two different sets of experimental data, one being the shrinkage stress data of the present work and the other being prior stress build-up data for melts undergoing simultaneous elongation and cooling. The traditional theory, based on thermorheological simplicity and comprising a classical constitutive equation and a time dependent PVT equation, predicts lower stresses than observed. Also approaches based on time-temperature superposition or a cooling rate dependent relaxation time give unreasonable results. Finally a phenomenologically motivated but empirical equation relating modulus to the cooling rate seems plausible. This new equation, coupled with an upper convected Maxwell model and a rate dependent PVT equation, fits all the data obtained under different operating conditions for both of the very different kinds of experiments. The final model has potential for a wide range of applications in polymer processing operations.

Recommended Citation

Joshi, Vikas S., "Rheological response of amorphous polymers under rapid cooling conditions. " Master's Thesis, University of Tennessee, 1989.
https://trace.tennessee.edu/utk_gradthes/12982