Self-Nucleation of β-Form Isotactic Polypropylene Lamellar Crystals in Thin Films

Abstract

The self-nucleation behavior of β-form isotactic polypropylene (β-iPP) crystals in ultrathin films has been probed by atomic force microscopy (AFM) and optical microscopy (OM). The hexagonal β-iPP single crystals can be obtained via a self-seeding method at much higher crystallization temperatures (e.g., 156 °C). We found that the average number nucleation density (N) of β-iPP lamellar crystals deceases with increasing self-nucleation temperature (T s ) and heating rate (V h ). Upon heating the polymorphic structures induced by shear flow to a temperature between 143 and 156 °C, since α-iPP grows about 20−2400% faster than β-iPP, the former overtakes and engulfs the growth front of the later when crystallized for a sufficiently long time. However, the morphological observations show that both α-iPP and β-iPP were grown by homoepitaxy on their own crystals, and no trace of the epitaxial overgrowth of α-iPP lamellae on the entire growth front of β-iPP was observed, that is, a high-temperature βα growth transition appears to be absent, which probably indicates that the free energy barrier for secondary αα nucleation and ββ nucleation is much lower than that of βα nucleation in ultrathin films.

Recrystallization of Quenched β-Form Isotactic Polypropylene Lamellar Crystals in Thin Films

“…Upon heating a quenched β-iPP lamella, since there are latent α-iPP nuclei formed on its fold surface, materials produced by the melting of the β-iPP phase can be used for ββ-recrystallization as well as βα-recrystallization. Our previous study showed that no β-iPP crystals via self-seeding were obtained if T s exceeds 157 °C, which is consistent with the previous reports . Thus, no ββ-recrystallization occurs when T s is above 157 °C and neither does βα-recrystallization.…”

“…From Figure c, one can see that a number of cross-hatched α-iPP crystals were formed at the periphery of the initial β-iPP dendrite. We have demonstrated that there is no α-iPP nucleation on the growth front of β-iPP in a temperature range of 140–156 °C . Therefore, these results are probably attributed to the thinner thickness at the grown front of lamellae, which leads to the lower melting temperature in these regions, and then latent α-iPP nuclei in such regions grow into edge-on α-iPP crystals with the help of the materials from the melting of β phase.…”

“…For the nascent β-iPP lamella, hundreds of faceted β-iPP single crystals appeared in the range of the starting crystal after self-seeding (see Figure a). Details of the morphology in Figure b,c show that no α-iPP crystals appear in the region of the initial β-iPP lamella during this process . For the quenched β-iPP sample, plenty of the cross-hatched α-iPP crystals and a few hexagonal β-iPP crystals were formed within the area of the initial lamella after crystallization at T c2 = 148 °C (see Figure d–f).…”

“…Plots of average number density on a logarithmic scale vs partial melting temperature ( T s ) for α-iPP crystals generated in zone II ( N α , black squares), α-iPP crystals created in zone III ( N αα , red triangles), and β-iPP single crystals ( N ββ , blue circles) formed within the region of initial β-iPP lamellar crystals obtained by performing route A. The data points for N ββ are from our previous work . The lines represent linear fits to the data.…”

“…Subsequently, ββ-recrystallization seems to disappear, and βα-recrystallization is dominant within the T s range of 153–157 °C. It has been found that the crystal growth rate of α-iPP is faster than that of β-iPP when the temperature is above 141 °C; ,, thus, the dominance of βα-recrystallization in the range of 151–157 °C depends on growth kinetics. On further raising T s to above 157 °C, βα-recrystallization is absent and only αα-recrystallization remains.…”