The
crystallization behavior of a butene-1/ethylene random copolymer with
9.88 mol % ethylene counits was investigated by means of differential
scanning calorimetry, wide- and small-angle X-ray scattering, and
polarized optical microscopy. Unlike in its homopolymer counterpart
which crystallizes always into a metastable form II from the melt
state, the random copolymer was found to crystallize either into form
II or stable form I′ directly from its melt state after being
cooled down. The occurrence of either crystalline form only depended
on the temperature where the crystalline material was molten before
cooling down. Even though the material was brought to temperatures
higher than the equilibrium melting temperature, heterogeneities of
segmental segregation character were preserved which promoted massive
nucleation of form I′ crystallites, which makes it possible
that the material is able to crystallize into pure form I′.
Only if when the melt temperature was high enough where all heterogeneities
of the above-mentioned character were erased can the material be crystallized
into pure form II. The effect is found independent of the previous
crystalline form, meaning that the helical conformation of chains
in the heterogeneous melt does not affect the nucleation of stable
form I′.
Stretching-induced structural changes in polybutene-1 with stable crystalline modification of form I at elevated temperature was investigated by means of the in-situ synchrotron wide-angle X-ray diffraction technique. It was found that oriented metastable form II crystallites with the polymer chain aligned along the stretching direction gradually appear during tensile deformation. Based on the fact that a solid state I to II phase transition cannot take place due to the restriction in chain conformations and lattice dimensions in both phases, the observed occurrence of transition from form I to form II must proceed via a two-step process. First, those form I crystallites with their polymer chain direction tilted with respect to the stretching direction undergo a stress-induced melting process because they experience larger shear stress than the rest. Second, the freed polymer chain segments which have lost their conformational memory in stable form I recrystallize into metastable form II crystallites with their chain direction preferentially aligned along the stretching direction. This result is considered to provide a direct evidence for the stress-induced melting−recrystallization mechanism during tensile deformation of semicrystalline polymers.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.