Melting temperature and enthalpy of isotactic polypropylene

The fusion process, melting temperature and heat of fusion of carefully crystallized fractions of isotactic polypropylene (molecular weights from 5·4 × 10⁴ to 2·2 × 10⁵) have been investigated. The equilibrium melting temperature, obtained by the extrapolation method in experiments involving very low degrees of crystallinity, corresponds to 208°. The fusion enthalpy was found to be 1·386 cal/mole. This low value and the high depression of Tm^o suggest a relatively high interfacial energy; a value of 285 erg/cm² is estimated for σ_ec.