The effects of temperature on the nonlinear mechanical behaviors of hard-elastic isotactic polypropylene films are systematically studied with in-situ ultrafast synchrotron radiation small-and wide-angle X-ray scattering techniques (SAXS/ WAXS) during uniaxial tensile deformation at temperatures from 30 to 160 °C. Based on the mechanical behaviors and structural evolutions in strain−temperature two-dimensional space, three temperature regions (I, II, and III) are clearly defined with the α relaxation temperature (T α ≈ 80 °C) and the onset of melting temperature (T onset ≈ 135 °C) as boundaries, where different mechanisms dominate the nonlinear deformations after yield. In region I, microstrain in lamellar stacks ε m obtains an accelerated increase after yield and reaches a value significantly larger than corresponding macrostrain ε, during which neither slipping, melting, nor cavitation occurs. We propose stress-induced microphase separation of interlamellar amorphous to be responsible to the hyperelastic behavior in region I. Above T α in region II, due to reduced cohesive strength and enhanced chain mobility, the irreversible reduction of crystallinity and the formation of slender cavities suggest that crystal slipping overwhelms microphase separation and plays the major role in nonlinear deformation, during which chains in lamellar crystals are pulled out and recrystallize into nanofibrillar bridges. In region III above T onset , melting−recrystallization dictates the nonlinear deformation. A schematic roadmap for structural evolution is constructed in strain−temperature space, which may guide the processing of microporous membranes for lithium battery separators as well as other high performance polymer fibers and films.
Stretch-induced crystallization
(SIC) and phase transitions of
poly(dimethylsiloxane) (PDMS) have been studied with the in
situ synchrotron radiation wide-angle X-ray scattering technique
(WAXS) during tensile deformation at temperatures ranging from −45
to −65 °C. The phase transitions during tensile deformation
go through different processes at different temperature regions, where
four phases are involved in namely oriented amorphous (OA), mesophase,
α form, and β form crystals. We found that SIC of the
α form can proceed via two different multistage ordering processes
with either the mesophase or β form as the structural intermediate.
Further cyclic tensile experiments demonstrate that the transition
from the β to α form is a reversible process controlled
by stress, which is attributed to the different helical pitches in
β and α forms. A nonequilibrium phase diagram of SIC and
phase transitions are constructed in strain–temperature space,
which is of great significance for practical applications of PDMS
at low temperature.
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