posted on 2021-10-29, 16:05authored byYahui Lou, Long Liu, Wei Li, Ruijun Zhao, Zhe Ma
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.