Nucleation Mechanism for Form II to I Polymorphic Transformation in Polybutene-1

Abstract

By means of in situ small-/wide-angle X-ray scattering (SAXS/WAXS) techniques, nucleation mechanism for Form I crystals of polybutene-1 (PB-1) has been investigated by heating the early-stage polymorphic transformation specimens (from Form II to Form I) above the nominal melting temperature of Form II crystals. Upon selective melting of Form II crystals at 124 °C, the SAXS curves of the few surviving Form I nanocrystals (volume content about 4.6%) in the melt can be fitted using a form factor for polydisperse cylinders. The geometrical size of Form I cylindrical nanocrystals was around height ≈40 ± 2 nm and radius ≈ 23 ± 1 nm when the aging time reached 12 h. Surprisingly, the height of Form I nanocrystals not only increased with lamellar thickness (d c) of initial Form II crystals but also was always larger than the d c of both initial Form II and pure Form I (i.e., complete transformation). We estimated that the Form I folded-chain nanocrystals consist of two parts, namely, the nuclei of Form I and the epitaxial growth of Form I crystals. Furthermore, our findings suggested that nucleation of Form I has a higher chance to occur at the thicker part of initial Form II crystals. This is probably because the longer parallel-chain stems give rise to a reduction of conformational entropy of the chains, allowing a smaller energy barrier for nucleation of Form I.

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

“…A typical nonequilibrium and nonlinear physics concept, polymer crystallization, is required to explain how a randomly coiled state can transform into a crystalline metastable state. , The process of crystallization may be further attractive in polymorphic polymers due to the transition of various modifications in different metastable states, which is far from being fully understood . Competition in the kinetics of nucleation and crystallization among different modifications plays a crucial role in the evolution of the complex crystalline morphology and structures. − Motivated by industrial as well as scientific interests, isotactic polypropylene (iPP), a widely used semicrystalline polymorphic polymer, has been intensively investigated. , Thermodynamically, the most stable phase at ambient temperature and pressure is the α-iPP. , Since the nucleation of metastable β-iPP occurs much more rarely than that of the predominant α-iPP, it can only be obtained through special procedures, such as in a thermal gradient, in an oriented melt, − with special β-iPP nucleating agents, and so on. − A unique feature of iPP is the existence of two crossover points at around 100 and 140 °C in the growth rate curves of two forms. , …”

Microstructural Change of Poly(1-butene) during Crystallization, Phase Transition, and Melting Revealed by Synchrotron Small-Angle X-ray Scattering

“…The thick form II lamellar crystal has a lower folded-surface free energy in an unit volume, which favors a form II to form I phase transition. Therefore, a form II to form I phase transition most likely initials from thick lamellar crystal and then occurs in thin lamellar crystals, as proposed by Zhang recently …”

“…Therefore, a form II to form I phase transition most likely initials from thick lamellar crystal and then occurs in thin lamellar crystals, as proposed by Zhang recently. 55 The lateral sizes during phase transition were also determined with our new methods. Parts a−c of Figure 16 show the determination processes at 0, 90, and 233 min, respectively.…”

Self-evolving materials based on metastable-to-stable crystal transition of a polymorphic polyolefin

“…For example, the PB film (thickness ∼0.4 mm) in the M phase has a higher transmittance (64%) than that in the S phase (44%). This can be explained by the fact that the density of the M phase is similar to that of the amorphous phase, 25 thus reducing the structural heterogeneity of the material. During the M-to-S phase transition, the transmittance of the film initially decreases from 64% to 10% at t a = 36 h, then increases to 44% and finally remains constant after 168 h (Fig.…”