A study of the crystallization kinetics of isotactic polystyrene
The spherulitic growth rate data for a molecular weight series of isotactic polystyrene are analyzed in context of the Lauritzen-Hoffman kinetic theory of polymer crystallization. The primary objectives of the study are to critically test the Lauritzen-Hoffinan theory under conditions not rigorously investigated before and to gain a better understanding of the molecular weight dependence of crystal growth rate for isotactic polystyrene.
The analyses yield values for fundamental kinetic and thermodynamic quantities associated with polymer crystallization. The physical meaning of the resulting parameters is assessed by comparing these results to those obtained from methods independent of crystal growth rate or crystallization theory altogether. This study differs from others reported in the literature in a number of ways, such as, the narrow molecular weight distribution and the molecular weight range of polystyrenes investigated. Also, growth rate measurements were extended to higher temperatures and a more appropriate kinetic equation for crystal growth rate analysis was applied. The majority of published studies that have used the Lauritzen-Hoffman theory applied an approximated form of the kinetic equation which does not fully describe the temperature dependence of polymer crystallization.
The results of the study show that a transition from molecular weight dependent to independent crystal growth rate occurs at a molecular weight of about 250,000 g/mole for isotactic polystyrene. Also, comparison of viscoelastic and crystal growth rate data indicate that the Vogel form of the transport term in the Lauritzen Hoffman kinetic growth rate equation correctly describes the temperature dependence of molecular transport for the crystallization process of isotactic polystyrene. Furthermore, the study suggests that the equilibrium melting temperature for the polymer is significantly higher than the value that has been generally accepted for the past 25 years.
The study also provided the opportunity to investigate various other factors and theories associated with polymer crystallization. For example, the theoretical relationship between the crystal's lateral surface free energy, σ, and the characteristic ratio, C∞, was evaluated. Also, the spherulitic morphology as a function of molecular weight and temperature was examined by scanning electron microscopy, SEM.