Study On Enthalpy Relaxation Of Polymer Using The Free Volume Theory

Structure relaxation and physical aging of the polymers have been important topics inpolymer science. It had the direct relationship with polymer mechanical properties and thefundamental understanding of the nature of the glass transitio n. Many methods were used tostudy the nature of the relaxation of polymer. Because the study on enthalpy relaxation caneasily examined the nonlinear and the non-exponential characteristics of polymer, it waswidely used by scholars.The enthalpy relaxation of polystyrene (PS) was restudied by using Vogel model inthis paper. The heat capacity data obtained from Tools-Narayanaswamy-Moynihan (TNM)and Adam-Gibbs-Scherer (AGS) model consistent with the experimental data excellently.The model parameters of Vogel model had chear physical meaing and small dependence onthe thermal history. Vogel model shows its superiority while it still has defects.We considered the influence of the volume thermal expansion. It showed that thevolume thermal expansion has slight effect on the study of the enthalpy relaxation and thethermal history dependence of the parameters were not significantly improved.The enthalpy relaxation of polymer was restudied by using the Free-Volume theory.Two models for describing the thermodynamic properties of the equilibrium state ofpolymer at low temperature were proposed. The free volume was assumed to be zero and tobe frozed at the values of the equilibrium state of the polymer at temperature TgFin modelsM1and M2, respectively. The relaxation time was calculated using Vogel model. It wasfound that the fitting results of CpNusing model M2are better than that of M1. Meanwhile,the obtained Tf′and Tmaxvalues are more close to the experimental results. The thermalhistory dependence of the Vogel model parameters decreases greatly and a more reasonablethermal history dependence of β was obtained when M1is instead by M2.The enthalpy relaxation of polymer was studied by using the Macedo model. The heatcapacity data obtained from Macedo model consistent with the experimental dataexcellently. The Kohlrausch parameter β which was obtained by using Macedo model wasfound to be changed more reasonable.