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 (α) 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. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Hydration studies in polymer hydrogels

“…Dielectric studies on such systems do not suffer from solvent relaxations, dc conductivity and interfacial polarization masking the polymeric dipolar relaxations in hydrogels. In agreement with the phase diagram of the system, constructed on the basis of DSC and TSDC data,131 the mixtures can be classified into three qualitatively different groups, each one corresponding to different solvent content, c px, region. More specifically, for p ‐xylene contents c px < 0.20, the mixtures are homogeneous in the whole temperature range studied and the polymer is monotonously and strongly plasticized.…”

“…Of particular interest are partially crystallized mixtures, including as a prominent category the polymer/water mixtures, that is, polymer/solvent mixtures, where the polymer component does not spontaneously crystallize, whereas the thermodynamics allows the phase separation and crystallization of the solvent. Interestingly, DSC results indicated that the phase diagram of a polymer mixture with a non‐polar solvent can be similar to those of hydrogels 131. Figure 16 shows results from a work in progress, the Arrhenius plot of the α process in cross‐linked PEA swollen in p ‐xylene in wide ranges of composition, based on DRS and TDSC data.…”

“…It was concluded that cold crystallization might be generated by caging the water molecules in a small space by the polymer chain with a small hydration area and that cold crystallisable water is different than intermediate water and non‐freezable water. For the highest p ‐xylene concentration region studied, 0.30 < c px ≤ 0.85, the mixtures exhibit phase separation during cooling and a fraction of p ‐xylene crystallizes 131. In addition, the glass transition of the gel is increased and stabilized at a higher value (regulation effect26, 28, 131) corresponding to the reduced solvent content in polymer/solvent phase.…”

“…For the highest p ‐xylene concentration region studied, 0.30 < c px ≤ 0.85, the mixtures exhibit phase separation during cooling and a fraction of p ‐xylene crystallizes 131. In addition, the glass transition of the gel is increased and stabilized at a higher value (regulation effect26, 28, 131) corresponding to the reduced solvent content in polymer/solvent phase. In consistency with that, the time scale of the α process becomes independent of composition and, moreover, similar to the time scale of mixtures in the range of original p ‐xylene contents between c px = 0.11 and 0.15 (Fig.…”

“…More specifically, for p ‐xylene contents c px < 0.20, the mixtures are homogeneous in the whole temperature range studied and the polymer is monotonously and strongly plasticized. For intermediate p ‐xylene contents, 0.20 ≤ c px ≤ 0.30 the mixtures undergo phase separation at relatively low temperatures and exhibit crystallization (cold crystallization) only on heating at temperatures higher than the respective polymer glass transition temperature 131. The polymer component is still plasticized; however, the temperature dependence of the time scale becomes specifically intriguing reflecting the microphase separation.…”