Formation of Periodically Modulated Polymer Crystals

We report novel morphologies with periodic height modulations of isotactic polystyrene (iPS) crystals, resulting from alternating stacks of correlated lamellae. Systematic experiments were performed on iPS films of several thicknesses (h) for varying degrees of undercooling, ΔT = T_m^∞ – T_C, where T_m^∞ and T_C are the equilibrium melting temperature and the crystallization temperature, respectively. We demonstrate that the spatial period (λ), i.e., the mean distance between neighboring stacks of lamellae, exhibits a power-law dependence on h and an exponential dependence on 1/ΔT. We propose that self-induced nucleation of stacked layers caused periodic deviation in the growth rate of iPS crystals, yielding periodic height modulations.