Anomalous Temperature Dependence of Isotactic Polypropylene α-on-β Cross-Nucleation Kinetics

Abstract

A particular kind of heterogeneous nucleation, i.e., cross-nucleation, is sometimes observed in polymorphic substances, when a new crystal structure nucleates on the surface of a crystal of a different modification. Here we show a unique and apparently incongruous nucleation behaviour in polymorphic isotactic polypropylene (i-PP). The rate of cross-nucleation of the monoclinic -phase on the trigonal -phase crystals increases with increasing temperature. This behaviour is contrary to that of the heterogeneous nucleation kinetics of the same crystal on various solid substrates, and also to the previously reported cases of cross-nucleation rate of other polymorphic systems, which exhibit the expected decrease with temperature. i-PP -on-cross-nucleation apparently eludes the nucleation theory. The results are explained as a manifestation of a kinetic competition between -on-cross-nucleation and growth of -crystalline seeds, and finally reconciled with the theoretical understanding of nucleation. These new finding indicates that further theoretical efforts are needed to include the cross-nucleation phenomenon in the framework of a comprehensive understanding of polymorphic crystallization. Incidentally, this study highlights the intrinsic limits of the, industrially desirable, promotion of -phase formation in polypropylene.

Differential scanning calorimetry study of cross‐nucleation between polymorphs in isotactic poly(1‐butene)

“…it can be considered that the concentration of heterogeneous nuclei is constant at a given temperature within a certain time interval, cross-nucleation on Form I spherulites is 'pseudo-homogeneous', with the surface density of Form II nuclei linearly increasing with time. 24,27,28 Moreover, the amount of seed surface available for cross-nucleation is similarly time-dependent, being progressively occupied by Form II morphologies growing on the substrate or impinging from neighboring seeds. Therefore, the application of a simple crystallization model, such as the Avrami, is meaningless.…”

“…19,20 Cavallo and Alfonso have shown that many more overlooked examples exist in the polymer crystallization literature. [21][22][23] Moreover, systematic extension of a quantitative approach to cross-nucleation to different macromolecules (isotactic poly(1-butene), polypivalolactone and isotactic polypropylene) [23][24][25][26][27][28] allowed us to understand that the nucleation of one polymorph on another can be regarded as a special case of heterogeneous nucleation, unless the parent phase grows at rates comparable to those of the daughter polymorph. 24,28 All the wealth of information so far obtained on cross-nucleation has been exclusively gathered by optical microscopy analysis of the developing morphology.…”

Cross‐nucleation of polybutene‐1 Form II on Form I seeds with different morphology

“…It was found that nucleation rate at constant undercooling increased by 60 times with decreasing Δ σ/σ from 0.23 to 0.13, which confirms that Δ σ is extremely sensitive for detecting the nucleating ability of a substrate . For what concerns cross‐nucleation, although the kinetics of the process has already been studied for several systems, a detailed description based on Δ σ calculation was not extensively applied.…”

Enhanced beta to alpha recrystallization in beta isotactic polypropylene with different thermal histories

“…Incidentally this can be considered a case of hetero‐polymorphic self‐nucleation, similar to the case of the enhanced form II crystallization kinetics produced by self‐nuclei obtained from the melting of trigonal form I crystals in poly(1‐butene) . The recrystallization of the α‐phase from β‐phase self‐nuclei is favored by the larger growth rate of the former polymorph above 140°C, and possibly by the extensive occurrence the of α‐on‐β cross‐nucleation, which presents an overwhelming rate at these temperatures …”