EQUILIBRIUM MELTING TEMPERATURE AS A RESULT OF INDUCTION TIME MEASUREMENTS - ISOTACTIC POLYPROPYLENE

A method for the determination of equilibrium melting temperature from induction time measurements is suggested. Theory of the induction tim e, t(i) (most probable period from the beginning of isothermal crystal lization to the instant when a stable crystal nucleus starts growing) involves parameters that influence the nucleation-crystallization proc ess, such as specific interfacial free-energy parameter, specific surf ace energies of a growing nucleus, enthalpy of crystal melting, diffus ion activation energy, undercooling and the equilibrium melting temper ature, T-m(o). An extrapolation method exploiting the aspect of the in duction time that it increases to infinity, that is, 1/t(i) decreases to zero at the equilibrium melting temperature, cannot be used to calc ulate the equilibrium melting temperature. High- or low-temperature ap proximations of the basic equation yield some simplifications that mak e it possible to find its parameters via the best fit of the equation with experimental data. This procedure can yield also the value of the equilibrium melting temperature if the measured data are sufficiently precise. Applying that procedure to crystallization data of isotactic polypropylene, we obtained the values of the equilibrium melting temp eratures 199.5 degrees C (high-temperature approximation) and 212.7 de grees C (low-temperature approximation). A more detailed discussion of the procedure suggests that from both these reasonable values, the hi gher one is more justified. This result agrees well with higher T-m(o) data reported in the literature.