The T_ll (>T_g) transition of atactic polystyrene

Abstract

Abstract 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, M̄ n = 600–2×10 6 , M̄ w /M̄ n ≃ 1.1; (b) fractionated thermal series, M̄ n = 2,000–1.1×10 5 , M̄ w /M̄ 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 M̄ n = 110,000. The common dependence of T g and T ll on M̄ 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 10 4 –10 5 poises. The employment of narrow molecular weight polymers is presumably responsible for both the linear variation of the T ll transition with M̄ 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 M̄ 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.