Modeling the glass formation phenomena in amorphous polymers using a thermoviscoelastic constitutive model

Prashant Vishwanath Shrikhande, Purdue University

Abstract

In continuation of the development of a thermoviscoelastic constitutive model for amorphous polymers, for the first time, both volume and enthalpy relaxation phenomena of poly (vinyl acetate) are described by using a single set of model parameters. The thermoviscoelastic constitutive equations based on the Rational thermodynamics framework are derived from a non-equilibrium Helmholtz free energy functional, which incorporates the coupling of arbitrary thermal and deformation histories. The thermoviscoelastic model is also applied to the enthalpy relaxation of LS2-polycarbonate. The thermoviscoelastic contributions from the Frechet expansions in all the constitutive functionals are truncated at second-order integral terms. The time-independent property functions are determined from experimental data by using a rigorous methodology, which allows the thermodynamic properties to be related to an equilibrium stability criterion. As a reduced-time model, the thermoviscoelastic model employs a shift-factor based on the Adam and Gibbs configurational entropy theory. The ability of the configurational entropy and energy shift-factor models to describe the pressure dependence of T g is critically examined for five different amorphous polymers and ortho-terphenyl. The thermoviscoelastic model is applied to the dynamic relaxation behavior of poly (vinyl acetate) including: (1) volume relaxation experiments (Kovacs, 1963), (2) the enthalpy relaxation experiments (Sasabe and Moynihan, 1978), and (3) the temperature and pressure dependencies of mobility data. The model quantitatively describes the features of the glass transition phenomena fairly well; however, a modified entropy approach enabling a higher activation energy associated with short-time processes would be required to improve the description of crossover peaks in Kovacs' memory experiments and overshoot peaks in enthalpy relaxation experiments. The predictions of the model for enthalpy relaxation experiments of LS2-polycarbonate are comparable with the Tool-Narayanaswamy-Moynihan model (O'Reilly, 2000). The thermoviscoelastic model is the only model derived from the simplest assumptions of continuum physics with the unique capability of unifying a variety of mechanical, structural, and enthalpy relaxation behavior as well as glass transition phenomena observed in amorphous polymers.

Degree

Ph.D.

Advisors

Caruthers, Purdue University.

Subject Area

Chemical engineering|Plastics

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