Development of orientation in polystyrene under isothermal and non-isothermal conditions

Highly oriented polystyrene was prepared by pulling fiber-like samples in the rubbery state and quenching them rapidly to the glassy state. A primary objective of the work was to produce samples of high orientation (frozen-in stress) and to measure the resulting mechanical properties. A second objective was to theoretically analyze the rheological behavior during the elongational flow above the glass transition temperature (T_g). Isothermal and non-isothermal temperatures histories were imposed before quenching.

Under isothermal conditions it eas found that one can maximize orientation by using temperatures about 10°c above the equilibrium T_g. Under non-isothermal conditions (cooling at linear rates of 2.5 - 4.5°c/sec.) it was possible to induce somewhat higher orientations, although it was not possible to take the very highest stress levels into the glassy state because of rapid relaxation.

The birefringence (orientation) in the quenched sample correlated quite well with the tensile stress developed in the rubbery state. The mechanical properties in the glassy state in turn correlated quite well with the birefringence, independent of how the stress was developed (isothermally or non-isothermally). The only significant effect of non-isothermal histories was to allow one to produce somewhat higher stresses before failure in the rubbery state.

Non-isothermal viscoelastic theory for flow above T_g is already available in the literature. Using that theory one finds that the predicted stresses are lower than the experimental ones. At moderate cooling rates (2.5°c/sec.) introduction of a cooling rate-dependent T_g brought the curves into good agreement; but at higher cooling rates (4.5°c/sec.) this modification alone was not adequate to resolve the difference.