The βα growth transition of isotactic polypropylene during stepwise crystallization at elevated temperature

Growth transition from β-isotactic polypropylene (iPP) to its α phase during the stepwise crystallization process is investigated. β-iPP hexagonites and spherulites are prepared first at 135 °C by using pimelic calcium as a nucleation agent, and transformation of them to their α-counterparts is realized by continuous isothermal crystallization at elevated temperatures, e.g., 151 °C. The β to α growth transition regions are investigated by optical and scanning electron microscopies. The results show that β-iPP hexagonites and spherulites grow in thin and thick films, respectively, while α-iPP spherulites are always observed in both thin and thick films. After abrupt temperature jump from 135 to 151 °C, α-iPP spherulites grow continuously with edge-on lamellae extending smoothly from the region crystallized at 135 °C to the area grown at 151 °C. On the contrary, the growth of the β-iPP crystals stops. At the growth front of β-iPP crystals, α-iPP crystallization takes place via nucleation and subsequent crystal growth, leading to sporadically dispersed α-iPP crystals around the growth front of β-iPP crystals. The similar structure of the transformed α-iPP crystals with that produced directly by isothermal crystallization at 151 °C also supports this conclusion. Moreover, the same cross-hatched lamellar structure of α-iPP in contact with both the flat-on and edge-on β-iPP lamellae demonstrates that there is no fixed structure relationship between the newly formed α- and the original β-iPP crystals.