Regulating Two Distinct Formation Pathways of the Thermodynamically Stable Phase to Tune Crystal Polymorphism: The Case of Butene/Pentene Copolymers

Abstract

Macromolecules with unique long-chain architecture can not only crystallize into a thermodynamically stable crystal but also form a kinetically favored crystal, where the latter may subsequently transform into the former to further lower the free energy. Therefore, to obtain the thermodynamically stable crystal, there are in principle two distinct formation pathways including the direct crystallization from the amorphous melt and the indirect phase transition from the initially generated modification, of which both are crucial to the crystal polymorphism. In the present work, a series of butene/pentene copolymers with the broad co-unit range of 4.0−36.1 mol % were synthesized to explore the correlation of crystal polymorphism with the molecular factor and external stimuli employing in situ wide-angle X-ray diffraction. The results show that different from the highly isotactic homopolymer, the incorporation of pentene co-units is able to not only induce the formations of both the thermodynamically stable trigonal phase and the kinetically favored tetragonal phase from the amorphous melt but also accelerate their solid II-I phase transition. As the concentration of pentene co-units reaches 17.6 mol % and higher, the thermodynamically stable phase has two distinct formation pathways, where those trigonal crystallites obtained from the direct melt crystallization and the indirect phase transition were referred to as forms I′ and I, respectively. It is also unexpected to find that different from the quiescent case where cooling is required to generate the thermal stress for triggering form I nucleation, both pathways can occur at the same temperature with the crystallization of the kinetically favored tetragonal phase, which can be facilitated by the increase in pentene incorporation. The elevation of temperature is beneficial to the formation of form I′, while the decrease in temperature facilitates the solid II-I phase transition into form I. Furthermore, flow-induced formation of the trigonal phase was also investigated by examining the correlation between the formation pathways and flow strength. It is interesting to find that the relatively weak flow accelerates the crystallization of both forms II and I′, while the severe flow induces the amorphous melt to completely crystallize into tetragonal crystallites and simultaneously trigger them to quickly transform into the ultimately stable form I.

Thermally Induced Phase Transition of Polybutene-1 from Form I′ to Form II through Melt Recrystallization: Crucial Role of Chain Entanglement

“…Figure 2 shows that all the freeze-dried samples present strong scatterings at q = 7.1, 12.3, and 14.4 nm −1 , assigning to the (110), (300), and (220 + 211) lattice planes of form I′ or I. 18 Figure 3 shows the DSC curves of freeze-dried PB-1 collected upon heating at different rates (1−20 °C/min). All the freeze-dried samples with different MWs but the same c (FD-5) exhibit two melting regions at 85−100 °C and 110−125 °C upon heating.…”

“…Considering the facility of processing and the better stability of end-use products, it is highly desired to directly obtain form I′ in melt processing without passing through the intermediate form II phase. Typical methods developed to directly obtain form I′ include self-seeding, nanoscale confinement, copolymerization, and CO 2 treatment . Nevertheless, form I′ is unstable at high temperatures and may transform back into form II (designated as I′-to-II transition) upon further heating or stretching through a melt-recrystallization pathway. , This thermally induced phase transition can impact the thermal stability and thermomechanical properties of form-I′ PB-1 materials used at high temperatures.…”

Crystallization and Phase Transition of 1‐Butene Copolymers with Distinct Cyclic Co‐Units

“…In the 1‐butene copolymers incorporated with linear α‐olefin co‐monomers, the relatively small co‐units (including ethylene, propylene, and 1‐pentene) effectively accelerate the kinetics of phase transition, whereas the longer co‐units with more than 5 carbon atoms greatly slow down the process of phase transition. [ 45‐50 ] Zheng et al . [ 51 ] synthesized 1‐butene/4‐methyl‐1‐pentene copolymers to investigate the influence of branched co‐units on phase transition behaviors.…”

Investigation of the synergistic effect of melt-extension and nanofiller on the crystal–crystal phase transition from form II to I of isotactic polybutene-1

“…It has been reported that lots of factors would affect the phase transition kinetics, such as the blending with other polymers, [25][26][27][28] copolymerization, [29][30][31][32][33][34] molecular weight, [35][36][37] temperature, 13,38,39 pressure, 40,41 extension, 11,12,42,43 nanofillers, [44][45][46] crystallization conditions, [47][48][49][50][51][52] and so on. Among all these factors, adding nanofillers and changing the crystallization conditions of form II are of significant value which can be used in the practical processing process to promote the phase transition process.…”