Kinetics of Crystal Nucleation in Molten Isotactic Polypropylene

Abstract

The crystallization of polypropylene droplets was investigated with the optical microscope. It is found that the droplets will undercool approximately 100 deg (T_m⁰=178°C). The undercooling attained for most of the droplets is increased by 1.4₃ deg when the cooling rate is increased by a factor of 10. An isothermal experiment indicates that the droplets crystallize sporadically in time, and nucleation frequency of 1.5 (10⁷) cm⁻³ sec⁻¹ is found at 79.9°C. Conventional analysis of this data yields a value of K_v, the pre‐exponential factor of the nucleation frequency, of the order of 10⁵⁰, which is much higher than expected from the paraffin data. Analysis which treats the free energy of formation of the nucleus as a discontinuous function of the nucleus size yields the more reasonable value of K_v of the order of 10³⁰, and a value of the surface energy perpendicular to the chain axes, δ_e, between 60 and 95 erg/cm².