Specific heat of synthetic high polymers. X. Isotactic and atactic polypropylene

Abstract

Abstract The specific heats of isotactic and atactic polypropylene have been measured over the ranges −14 to +211°C. and −55 to +204°C., respectively. In both cases, the samples were annealed at about 140°C. and the measurements repeated, but a slight thermal degradation prevented the results from the second heating from having additional significance. Isotactic polypropylene shows no glass transition in the range studied; the only transition observed is the melting transition. From the enthalpy—temperature curve for a measured fraction of crystallinity of 0.60, the heat of fusion of completely crystalline isotactic polypropylene was calculated to be 35 cal./g. This value is intermediate between previously published values of 15 and 62 cal./g. and in fair agreement with the value of 44 cal./g. calculated from pressure—melting point data. Atactic polypropylene shows a marked glass transition at −12°C. and two small first‐order transitions at 70°C. and at 155°C. The value for the Δ c p per mole of chain atoms at the glass transition of 2.4 is in reasonable agreement with the theoretical value of 2.97 derived by Wunderlich from the Hirai‐Eyring theory of liquids. The small transition at 155°C. is attributed to an isotactic or syndiotactic content, either as impurity or as isotactic or syndiotactic segments in the atactic chains. The transition at 70°C. is tentatively attributed to the melting of heterotactic sequences. Infrared measurements indicate that the atactic polypropylene contained a few per cent of isotactic material while from the heat of melting about 1% was estimated.