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 (Gβ/Gα) exceeds 1 when the crystallization temperature (Tc) is between the lower and upper critical temperature limits (Tc** ∼ 100 °C and Tc* ∼ 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 Gβ/Gα > 1 narrows and even disappears, as Tc* gradually shifts to lower temperatures while Tc** remains constant. This is presumably because, as d decreases, the width (wd) 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, Gβ decreases more than Gα as d diminishes, leading to a reduction in the value of Gβ/Gα, especially at higher Tc, where Gβ/Gα becomes less than 1. Moreover, we found that the diffusion activation energy (Eaβ) of β-iPP is consistently greater than that of α-iPP (Eaα) at a given d, and the gap between Eaα and Eaβ increases as d decreases. This further highlights that the slower diffusion process for β-iPP progressively becomes the main factor affecting the difference between Gα and Gβ as d diminishes.