Evaluation of the reversible contribution to the reversing heat capacity in isotactic polystyrene

We report the results of time dependent analysis of the reversing heat capacity peaks in isotactic polystyrene (iPS). Solution grown crystals (SGCs) and bulk films of iPS were examined using quasi-isothermal temperature modulated differential scanning calorimetry (QI-TMDSC). Both types of iPS samples showed two reversing heat capacity peaks as reported before [H. Xu, P. Cebe, Macromolecules 37 (2004) 2797; H. Xu, P. Cebe, Macromolecules 38 (2005) 770], and the two peaks are associated with melting of different portions of crystal lamellae with different degree of thermal stability [T. Liu, J. Petermann, Polymer 42 (2001) 6453]. Here, the behavior of the reversing heat capacity is studied as a function of holding time from the establishment of steady state, until the end of the quasi-isothermal holding period. The reversing heat capacity decreases with time at each of the two peak positions. For both types of iPS (SGC and bulk film) the time dependence of each of the reversing heat capacity peaks is fitted using Cp(t) = Cp∞ + A1 exp(−t/τ1) + A2 exp(−t/τ2). The fit parameters (equilibrium reversible heat capacity, Cp∞; prefactors, A1 and A2; relaxation times, τ1 and τ2) provide a means to compare the decay of the dual reversing heat capacity peaks of iPS solution grown crystals with bulk cold-crystallized film. Results show that iPS bulk films have longer relaxation times (τ1 and τ2) and smaller Cp∞ value than the iPS SGCs. Both results suggest that the iPS bulk films have higher degree of crystal perfection than SGCs.