Spontaneous Form II to I Transition in Low Molar Mass Polybutene-1 at Crystallization Temperature Reveals Stabilization Role of Intercrystalline Links and Entanglements for Metastable Form II Crystals

Abstract

As for the solid−solid transformation from form II to form I in polybutene-1, the intercrystalline links in amorphous phase and the temperature difference between crystallization and phase transition were thought to be essential in previous studies for generating the internal stress to promote nucleation and growth of the phase transition. In this work, we report experimental evidence showing that such a transition can be accomplished without supercooling at the temperature of crystallization directly in a low molar mass sample where there are almost no intercrystalline links. This result reveals two significant aspects of the form II to I transition in polybutene-1 against conventional wisdom: (1) the nucleation barrier of the transition is very low and can be overcome by thermal fluctuation; (2) entanglements and intercrystalline links in high molar mass systems play a role of stabilizing the metastable form II via hindering the translational movements of chains in crystalline phase and slowing the relaxation of amorphous chains during phase transition. Direct formation of form I′ from melt begins in the early stage of the phase transition and proceeds during the phase transition. The melting temperatures of these form I′ crystals increase with the aging temperature. Moreover, the structure and melting point of transformed form I highly depend on the annealing temperature.

In situ POM and FTIR investigation on quiescent phase transition of polybutene‐1 from form II to form I

“…In the early stage, it promotes form I nucleation; however, it becomes a retardance factor in the later stage. It is not as claimed either by classical view that the intercrystalline links always promoted form I nucleation or by recent view that the intercrystalline links only stabilize the metastable form II …”

“…Owing to special long chain conformation, polymer often forms different crystalline structure under different crystalline condition, and kinetically favored phases will transform to thermodynamically stable phase, for example, orthorhombic‐to‐hexagonal phase transition of polyethylene, mesomorphic‐to‐monoclinic phase transition of isotactic polypropylene, form II‐to‐form I phase transition of polybutene‐1 (PB‐1), disorder‐to‐order phase transition of poly( l ‐lactide), β‐to‐α phase transition of poly(butylene adipate), and so on. Among these phase transitions, form II‐to‐form I phase transition is an important kind of phase transition, receiving considerable attention in recent years . An important reason is that the production of PB‐1 was increased greatly in the past decades; however, the constraint of the phase transition on the melt processing has not been well addressed.…”

“…Nevertheless, the role of intercrystalline links was questioned recently . Qiao et al found that intercrystalline links might retard phase transition in the later stage .…”

“…Low molecular weight and high crystallization temperature always contributed to a greater degree of transition. Even, they found that without annealing at a lower temperature, form II could also transform to form I rapidly in a low molecular weight of PB‐1 synthesized by themselves, which chain was too short to form intercrystalline links . Moreover, Ma et al found that the stress dominating the phase transition was not the stress parallel to the normal direction of the lamellar stack but the stress perpendicular to the normal direction .…”

Thermal Analysis of Crystallization and Phase Transition in Novel Polyethylene Glycol Grafted Butene-1 Copolymers

“…The molecular structure can be designed by varying molecular weight, as well as regularity, or incorporating the extra structural co-units. Recently, Qiao et al studied the correlation of the II-I phase transition with molecular weights in a polybutene-1 homopolymer [12,33,34]. It was found that high molecular weight facilitates the formation of the intercrystallite links because of the relative large radius of gyration, with respect to the stacking size of crystalline and amorphous domains, which enhances stress transfer into the lamellae.…”