The Tll (>Tg) transition of atactic polystyrene

Torsional braid analysis (TBA) (～0.3 Hz) and differential thermal analysis (DTA) data are presented for the temperature for the region 0–200°C for two series of atactic polystyrenes with narrow molecular weight distributions: (a) anionic series, M?_n = 600–2×10⁶, M?_w/M?_n ? 1.1; (b) fractionated thermal series, M?_n = 2,000–1.1×10⁵, M?_w/M?_n < 1.25. Preliminary results on bimodal blends are also reported. Heating and cooling cycles were employed with TBA; only the heating mode was used with DTA. In addition to a dynamic mechanical loss peak at T_g, a higher temperature loss peak was also found. Designated the T_ll or liquid–liquid transition (relaxation), its temperature is 1.1 to 1.2 T_g (°K) for polymers with molecular weight below the critical molecular weight (M_c) for chain entanglements. Above M_c ? 35,000, it rises steeply, being ?200°C for M?_n = 110,000. The common dependence of T_g and T_ll on M?_n^?1 below M_c suggests a common molecular origin. The two facts, (a) that T_ll > T_g and (b) that T_ll reflects chain entanglements, further suggest that T_ll involves a longer chain segment length and possibly the entire molecule. Comparison of T_ll versus log M plots with T versus log M isoviscous state plots based on zero-shear melt viscosity data from the literature implies that T_ll measured by the TBA technique corresponds to an isoviscous state of 10⁴–10⁵ poises. The employment of narrow molecular weight polymers is presumably responsible for both the linear variation of the T_ll transition with M?_n^?1 (which suggests a free volume basis for the relaxation) and the form of the variation of the T_ll transition with log M (which suggests an isoviscous basis for the relaxation). The sharpness of the T_ll loss peak by TBA decreases with increasing molecular weight and dispersity. The DTA endothermic event corresponding to T_ll is clearly related to the occurrence of flow since the fused films which result from heating granules to 200°C and cooling to R.T. do not reveal a T_ll on reheating. If a fused film is crushed, a T_ll event is observed on heating. For bimodal blends with M?_n < M_c for both components, the T_ll transition was averaged; with one component less than and one greater than M_c, the T_ll transitions of the components appeared to occur independently at temperatures corresponding to those of the isolated components. In accordance with Ueberreiter and Orthmann, T_g appears to separate a glassy state from a fixed liquid state, whereas T_ll separates the fixed liquid from a true liquid state. Possible molecular interpretations for the T_ll process are discussed. Systematic bodies of data from the literature which indicate the presence of the T_ll process in other polymers are summarized.