Depression of the Zero Growth Temperature by Counits Observed for s-Polypropylene in Light Attenuation Experiments

Abstract

The growth rate of polymer crystals decreases exponentially with rising temperature, thus indicating a growing activation barrier. The barrier is proportional to the inverse of the supercooling below the “zero growth temperature” T_zg, which is always located below the equilibrium melting point. In order to see the effect of counits on T_zg, we studied with time-dependent light attenuation measurements the growth kinetics of s-polypropylene and a derived statistical copolymer with 15% of octene counits. Using a home-built device, we achieved a sensitivity superior to DSC or X-ray experiments. An accurate extrapolation procedure yielded for both samples the zero growth temperature. The counits lead to a downward shift of T_zg by 40 K, which is larger than the downward shift of the melting point (30 K). We interpret the results in the framework of our multistage model which assumes participation of a transient mesophase in the growth process of polymer crystals. T_zg is identified with the equilibrium melting point of the mesophase. Knowledge of T_zg allows to establish for both samples a size−temperature nanophase diagram. Data evaluation yields the heat of melting and the surface free energy of mesomorphic layers as well as the activation barrier per monomer.