Polymer segmental dynamics and solvent thermal transitions in Poly(ethyl acrylate)/p-xylene mixtures

Abstract

A poly(ethyl acrylate) polymer network was swollen with different concentrations of the nonpolar solvent p-xylene, cpx, from xerogel until saturation (0 cpx 0.85). Differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) techniques were employed to study the polymer segmental dynamics and the solvent thermal transitions in homogeneous (cpx < 0.20) and partially crystallized (cpx 0.20) PEA/p-xylene mixtures. Our DSC measurements indicate that p-xylene undergoes cold crystallization for intermediate solvent concentrations, 0.20 cpx 0.30 while for higher cpx values crystallization takes place during cooling. The results show that for cpx 0.30 the Tg decreases with increasing cpx (plasticization effect) obeying the respective Fox equation. For the same cpx range we found that both the dielectric strength and the heat capacity increment of the segmental (a) relaxation process increase gradually with cpx whereas the distribution of relaxation times for the underlying molecular relaxations does not change. For cpx > 0.30 the partially crystallized mixtures exhibit a constant Tg corresponding to the gel phase of PEA with an amount of p-xylene which is not able to crystallize under any conditions. The concentration of this noncrystallized p-xylene, cUCpx, has been estimated to be between 0.12 and 0.15, independent of the total p-xylene concentration in the mixtures. When a separate p-xylene crystal phase is formed (for cpx > 0.30) the segmental dielectric strength and heat capacity increment decrease significantly exhibiting values significantly lower than those measured for the homogeneous gels. In addition, we found that the presence of p-xylene crystals may induce marginal spatial heterogeneity of polymer (or p-xylene) concentration within the gel phase affecting thus slightly the breath of the segmental relaxation of PEA. We attribute these results to restrictions of polymer segmental configurations due to constraints imposed by the p-xylene crystals and/or to the immobilization of a part of the polymer chains. VC 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 455¿466, 2011