Crystallization and melting behavior of i-PP: a perspective from Flory's thermodynamic equilibrium theory and DSC experiment

Abstract

Crystallization and melting are integral parts of the isotactic polypropylene (i-PP) end-product fabrication process. Therefore, the nonisothermal crystallization kinetics and melting behavior of an un-nucleated commercial i-PP have been pursued by integrating a new nonisothermal crystallization model, Flory's thermodynamic equilibrium theory, Gibbs–Thompson equation, and DSC experiments. By applying this simple conceptual integration, the relative crystallinity α, temperature-dependent instantaneous crystallinity χ, the crystallization kinetic triplet, and the lamellar thickness distribution have been duly correlated, as appropriate, to the level of undercooling θ, crystal surface free energy D, and critical stable crystallite sequence number n*. Consequently, new insightful results, interpretations, and explanations have been concluded. The nonisothermal primary and secondary crystallizations of i-PP occur isokinetically with constant (temperature-, entropy-, and cooling rate-invariant) apparent kinetic energy E_a, which is also unaffected by θ, D, and the lamella-inclusive pendant –CH₃ group of the propylene repeat unit. The crystal dimension n_d varies, irrespective of θ and D, from cylinder to sphere, depending on the system entropic disorder. Instantaneous (athermal/heterogeneous) and sporadic (thermal/homogeneous) nucleation processes may co-prevail. The occurrence of lamellar thickening with increasing melting temperature has been discussed, considering the chain sliding diffusion theory proposed by Hikosaka et al., and the variation of θ and D. The present approach also applies to the evaluation of the influence of catalyst structure, backbone defect types, and their distribution on the crystallization and melt behavior of i-PP.