Mechanical properties and residual stresses in non-equilibrium glasses

The effect of thermal history on the glassy state properties of amorphous polystyrene was investigated, giving separate consideration to homogeneous and non-homogeneous samples. The work on homogeneous samples dealt with how the degree of non-equilibrium (free volume) frozen into the quenched material affects the tensile properties of small, fiber-like samples. Some exploratory work was also done on the tensile properties of standard dumbbell specimens (1/8” thick) cooled under identical conditions as the small samples. The work in non-homogeneous samples was a continuation of recent work at the University of Tennessee on the measurement and prediction of residual birefringences related to residual stresses frozen into large slab-like samples.

The extent of non-equilibrium (i.e., the severity of the quench) was found to affect the ductility and other mechanical properties of the final, presumably homogeneous) fiber-like samples. Dramatic changes in ductility (elongations-to-break as high as 100% in some cases) were observed in quenched polystyrene specimens at a testing temperature as low as 70°C, whereas the literature indicates the brittle-to-ductile transition for this polymer to occur at about 90°C. The ductility measurements scattered considerably, however, (from a low of about 10% up to 140%) , presumably due to random irregularities in the test pieces. In contrast, quenching larger dumbbell specimens resulted in no appreciable changes in ductility.

Residual birefringence profiles were measured on slab-like samples quenched from 120°C and 160°C into ice water. In agreement with the existing literature both quenching cases exhibited positive birefringences at the surface, sind negative birefringences in the center of the slab. Although similar in shape, these profiles were nonetheless significantly different in detail. Quenching from a relatively low temperature (120°C) produced an approximate balancing of the positive and negative, birefringences, whereas a high initial temperature resulted in highly negative, unbalanced profiles. The simplified theory of Aggarwala and Saibel (2) used in earlier work at the University of Tennessee, was not succesful in explaining this difference. Therefore, the more general theory of Lee, Rogers, and Woo (47) was brought into the analysis. Lacking basic data on a number of functions and parameters (relaxation functions for both the mechanical and optical behavior and a function for the thermal coefficient of expansion), certain arbitrary decisions were necessary. This more general theory was able to predict the observed result qualitatively, and also quantitatively, if a certain (although arbitrary) choice of parameters was made.