Segmental α-Relaxation for the First Step and Sub-Rouse Modes for the Second Step in Enthalpy Recovery in the Glassy State of Polystyrene

Abstract

Cangialosi and co-workers found two steps in the enthalpy recovery deep in the glassy state of high and low molecular weight bulk polystyrene (PS). We attribute the first step to the segmental α-relaxation and the second step to the sub-Rouse modes and explain the two-step enthalpy recovery by the dynamic properties of the two processes in bulk PS by the coupling model (CM). The two-step enthalpy recovery was found in nanometer thin films of polystyrene (PS). On decreasing film thickness h, the first step was shifted to much lower temperature than the second step, and the effect is explained by the segmental α-relaxation shifting to lower temperature much more than the sub-Rouse modes, as predicted by the CM. Furthermore, the dependences of the faster and slower processes of the two-step enthalpy relaxation on film thickness are analogues of two effects observed on decreasing h of freestanding polystyrene films. One effect is the observation of two transitions by ellipsometry. We associated the lower transition at Tgl(h) with the segmental α-relaxation and the upper transition at Tgu(h) with the sub-Rouse modes and explained the experimentally observation of Tgl(h) decreasing more rapidly than Tgu(h) on decreasing h also by the CM. The other effect is the rubbery stiffening observed by creep compliance measurements on decreasing h of freestanding films in polymers including PS. This effect was explained by the more rapid decrease of the segmental α-relaxation time τα(T,h) with decreasing h than the sub-Rouse relaxation time τsR(T,h), again using the CM. These two analogues of the two-step enthalpy recovery in bulk and nanometer PS thin films strengthen the common CM explanation of the first step by the segmental α-relaxation and the second step by the sub-Rouse modes in enthalpy recovery.