A DSC Study of α- and β-Relaxations in a PS−Clay System

Abstract

Differential scanning calorimetry has been used to study the α-relaxation (glass transition) as well as β-relaxation (sub-T g relaxation) in both virgin polystyrene (PS) and PS−clay nanocomposite. The β-relaxations have been measured as a small endothermic peak that appears on heating of the samples previously annealed at temperatures from 30 to 80 °C. The relaxations have been characterized by determining the effective activation energies (E) and evaluating the sizes of cooperatively rearranging regions at the glass transition (V g). The values of V g have been determined from the heat capacity data. The β-relaxation in both systems shows comparable values of E that increase with annealing temperature from ∼90 to 170 kJ mol-1. In the PS−clay system the increase occurs slower, suggesting that this process is more decoupled from the α-relaxation than in virgin PS. The α-relaxation demonstrates markedly larger values of E (∼340 vs ∼270 kJ mol-1) for the PS−clay system than for virgin PS. The PS−clay system has also been found to have a significantly larger value of V g, 36.7 nm3 as compared to 20.9 nm3 for PS. The increase in E and V g reflects an increase in molecular cooperativity that is introduced by the mutually dependent motion of the polymer chains anchored to the surface of clay sheets.

Physical Processes

“…The nonlinearity can be quite obvious when T g is measured in a wide range of the heating rates as illustrated in Fig. 3.16 for the glass transition in PS [30]. From this plot, we can see again that the activation energy of the glass transition decreases with increasing temperature.…”

“…On the other hand, on heating, the DSC signal is complicated by a peculiar feature associated with enthalpy overshoot, which manifests itself as a relatively small endothermic peak at the end of the glass transition step. Figure 3.10 displays an example of this feature observed in the glass transition of polystyrene (PS) and PS-clay nanocomposite [30].…”

“…In Eq. 3.21, z * is inversely proportional to the configurational entropy that increases with T so that both z * and the effective activation energy (i.e., z * Δμ) decrease with T. Note that even before the first applications [30,46] of isoconversional methods to the glass transition kinetics, the trend for the activation energy to decrease with increasing temperature was observed in other studies [47][48][49]. In them, the activation energy was determined from the shift in the value of T g with the heating rate in accord with the equation proposed by Moynihan et al: [42,43] (3.…”

“…The E values of 290 and 170 kJ mol −1 are found respectively from the three slowest and the three fastest heating rates. (Adapted from Vyazovkin and Dranca [30] with permission of ACS)…”

Assesment of the usability of Mg(OH)2 obtained from the solution after sphalerite leaching for the winning of polyetylene composition

“…Char residue is increasing also with an increase of the Mg(OH) 2 content which is a good promise for lower flammability of the composite 9 . Char reduces the diffusion of volatile products of polymer degradation out of polymer bulk; therefore the reduction in the rate of mass loss is observed.…”