Studies in nonisothermal rheology: Behavior near the glass transition temperature and in the oriented glassy state

摘要

AbstractAs part of a continuing study of nonisothermal rheology (meaning the simultaneous application of strain and temperature changes), we here consider the behavior of polystyrene near the glass transition temperatureTg. In particular, we measured the increase of the apparentTgas the cooling rate is increased from 0.003 to 4.5°C/sec. This change (up to 16°C increase) has both practical and theoretical implications. For enhancing the mechanical properties of a glassy product, one desires maximum orientation (stress) just prior to quenching; the optimum deformation/temperature strategy for maximizing stress is affected by the level ofTg. By using a nonisothermal strategy we were able to produce higher frozen‐in orientations, and thus higher mechanical properties, than have been previously reported. For a theoretical understanding of the rubbery state just prior to quenching, we used the generalized time‐temperature superposition of our prior work; we found that a modified shift factor of the formaT(T,T Rg), whereT Rgrefers to a rate‐dependentTg, gives an improved fit to data but is not by itse