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

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.