Doctoral Dissertations

Date of Award

8-1985

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

John F. Fellers

Committee Members

J. E. Spruiell, E. S. Clark, J. S. Lin, W. F. Jones

Abstract

Two aspects of the mechanical deformation of polymers in the glassy state are investigated in this dissertation. The glassy polymers investigated in this work include polystyrene, polycarbonate, poly(methyl methacrylate) and polysulfone, with several different molecular weights of the polycarbonate and polysulfone.

First, stress relaxation is modelled using a nonlinear viscoelastic constitutive equation which is based upon the Williams and Watts relaxation element. The resulting four-constant model is shown to fit the relaxation behavior of these materials over several orders of magnitude in time. Thus the stress-strain behavior of glassy polymers can be described with constant material parameters for several deformation kinematics. The material parameters which govern the time dependent character of the mechanical deformation are tau and n. The classical interpretation for tau, being a relaxation time, is valid in this analysis. However, n is an exponent acting on the time variable and accounts for what has previously been treated as a spectrum of relaxation times. The magnitude of log(tau[minutes]) is shown to be in the range of six to thirty, and generally increases with molecular weight parameters and decreases with temperature. Conversely, the magnitude of n is shown to be in the range of 0.02 to 0.2, and generally decreases with molecular weight parameters and increases with temperature. The behavior of n can be correlated to some behavioral aspects of the glass. For example materials with relatively high impact resistance display lower values of n, while materials which exhibit more active creep behavior have higher values for n. Thus an understanding of n from a conceptual viewpoint may provide an important link between structure and properties of a deformed glass.

Second, a small-angle x-ray scattering study of the crazing phenomenon in glassy polymers is performed. It is reported that the crazed matter of polycarbonates contains comparatively fewer crack-like defects and less void fraction than the crazed matter of polystyrene. Polysulfone and poly(methyl methacrylate) show intermediate behavior. Therefore the polycarbonate crazes contain more stress supporting microfibrils. These observations are proposed to account for the observed superior ultimate mechanical properties exhibited by the polycarbonates.

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