Molecular motion during physical aging in polystyrene: Investigation usingprobe reorientation

A photobleaching method has been used to observe the reorientation of tetra cene and rubrene in polystyrene during physical aging. Rotation times chang e more than an order of magnitude during isothermal aging after a temperatu re quench from above T-g. Down- and up-jumps of the temperature show the ex pected asymmetry due to the nonlinearity of the aging process. The rotation times of tetracene and rubrene require the same amount of time to reach eq uilibrium after a temperature change (10(3) - 10(5) s in the range 93-99 de grees C). These equilibration times are the same order of magnitude as equi libration times for volume and enthalpy relaxation, but have a somewhat wea ker temperature dependence. Very near equilibrium, the rates of aging are d ifferent for the two probes, with rubrene approaching equilibrium more rapi dly at very long times. This may be understood if the aging process is spat ially heterogeneous, that is, if aging occurs more rapidly in some small re gions of the sample than in others. (C) 2000 John Wiley & Sons, Inc.