Rheomolded polystyrene

Mecastability has been induced in polystyrene by applying static pressure or "Rheomolding ." These treatments are accompanied by a spectacular decrease in the critical temperature. This evolution is indicative of low temperature mobility. 

TSC Spectra. Figure 17 shows the TSC current liberated from the rheomolded polystyrene (solid line spectra) in comparison with that from the reference polystyrene (dashed line spectra) after polarization at 130°C. Above the glass transition, we note the liberation of molecular movements that remain frozen in the reference material. A more systematic study of the polarization recovery is presented in Figures 18 and 19 for polarization temperatures lower than and greater than 150 C, respectively. For Tp = 105°C (solid line spectrum. Fig. 18), only the TSC peak at 100 C is observed when Tp increases (dotted and dashed lines, Hg. 18), a new peak at 132 C progressively appears. For higher values of polarization temperatures (curve bb. Fig. 19), a shoulder appears at 155 C. This component becomes predominant for Tp = 170°C and 180 C (spectra a and c . Fig. 19). 

Relaxation Map. The Arrhenius diagram of rheomolded polystyrene represented in Figure 20 is qualitatively analogous with the one of hydrostatic polystyrene (see Fig. 16). The relaxation times isolated in the glass transition mode follow a compensation law with = 145 C and = 0.19... 

Discussion. In hydrostatic and rheomolded polystyrene, the TSC peak associated with the glass transition is situated, like in reference polystyrene, in the temperature range 90-100 C. Its increasing magnitude has been attributed to conformational changes favoring microbrownian motion. The width of the distribution function of the relaxation time and also the order parameter is... 

Fig. 17. Comparison of TSC spectra of rheomolded (solid line) and reference (dashed line) polystyrenes after polarization at temperatures indicated by the arrows.

In hydrostatic and rheomolded polystyrenes, the TSC peak associated with the liquid-liquid transition is shifted towards lower temperatures. Figure 21 illustrates this shift for rheomolded polystyrene (solid line spectra) in comparison with reference polystyrene (dashed line spectra). This evolution reflects a strong decrease in the critical temperature T down to 16 C and -22°C for hydrostatic and iheomolded polystyrenes, respectively. The spectacular decrease of T in rheomolded polystyrene, illustrated in the Arrhenius diagram of Fig. 21, is due to the liberation of molecular movements frozen in reference polystyrene. 

The short time mode corresponds to the glass transition. In polymers like polystyrene, a narrow distribution is observed. Ihe width of the distribution reflects the width of the distribution of the order parameter it is increased after mechanical orientation by addition of a dopant or additive, or under special glass forming conditions (hydrostatic pressure or rheomolding). The distributed relaxation times obey a compensation law, they are reduced to a single time at the compensation temperature T. The departure of from the glass transition is related to the kinetic aspect of the transition. Thermodynamic models are based on the linear relationship between the activation enthalpy and the activation entropy. 

Fig. 20. Arrhenius diagram of dielectric relaxation times of rheomolded polystyrene. The arrows indicate the temperatures of the TSC maxima.

The purpose of the present communication is to provide experimental support for the idea that Tp (TTg) are all issued from a unique relaxation kinetics which is primary responsible for the Tg effect The suggestion that Tp is a precursor of Tg is not new 2.4 the idea that T// behaves like a classical kinetic transition, i.e., like a sort of Tg is not new eitfaer. > Frenkel in the USSR suggests that the 7 transition merges with the Tg transition on a log frequency vs. 1/7 plot at a temperature which corresponds to 7//. These observations all concur separately with our own attempt to reconcile into a unique relaxation mechanism the 7 Tg, and Tn "transitions." Atactic polystyrene is chosen here for the experimental data because the physical properties relevant to our discussion are well-documented in the literature for this particular polymer, representative of the class of amorphous polymers. Furthermore, TSC (Thermally Stimulated Current) and DSC results on rheomolded polystyrene samples can be systematically compared since the influence of the rheomolding parameters (frequency of vibration during molding, amplitude of vibration, and post treatment armealing effects) are simultaneously studied and analyzed by both techniques. " ... 

Fig. 2. Effect of the initial temperature, Tq, at constant frequency and amplitude of vibration, on the DSC trace of Rheomolded polystyrene specimens.

We turn our attention now to the effect of annealing above Tg. We will use the Thermally Stimulated Current data of Bemes et al. for the same Rheomolded polystyrene specimen as that of Fig. 8. Details on the TSC technique and spectrum analysis can be found in another article in this... 

Fig. 9. Tg and Tg peak positions on reheating (TSC) as a function of the polarization temperature for Rheomolded polystyrene specimens.

In the introduction of this communication we pointed out the similarities between the pressure dependence, the activation energy, and the annealing effects of the (Tp< Tg) and the (T/ > Tg) transitions, which strongly suggests that these transitions are the manifestation of the same relaxation process, probably issued from the complexity of the kinetic mechanism responsible for the glass transition temperature itself. In the second part of this communication we present and reanalyze DSC studies of atactic Rheomolded polystyrene specimens treated with various thermal-mechanical histories. The effect of frequency, amplitude of vibration, and annealing time... 

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