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

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