Film Thickness Dependence of the Growth Rate Ratio of the β- to α-Crystal of Isotactic Polypropylene in Ultrathin Films

Previous research studies on isotactic polypropylene (iPP) have shown that the ratio of growth rate β to α phase (G_β/G_α) exceeds 1 when the crystallization temperature (T_c) is between the lower and upper critical temperature limits (T_c^** ∼ 100 °C and T_c^* ∼ 141 °C, respectively), due to the faster surface kinetics of β-iPP. However, in this work, we observed that when the film thickness (d) decreases from 24 to 9 nm, the temperature interval of G_β/G_α > 1 narrows and even disappears, as T_c^* gradually shifts to lower temperatures while T_c^** remains constant. This is presumably because, as d decreases, the width (w_d) of the depletion zone of β-iPP expands more significantly than that of α-iPP, resulting in slower diffusion toward the growth front of β-iPP. Consequently, G_β decreases more than G_α as d diminishes, leading to a reduction in the value of G_β/G_α, especially at higher T_c, where G_β/G_α becomes less than 1. Moreover, we found that the diffusion activation energy (E_a^β) of β-iPP is consistently greater than that of α-iPP (E_a^α) at a given d, and the gap between E_a^α and E_a^β increases as d decreases. This further highlights that the slower diffusion process for β-iPP progressively becomes the main factor affecting the difference between G_α and G_β as d diminishes.