Deformation induced crystal–crystal transition of polybutene-1 (PB-1) from forms II to I at different temperatures is studied with in situ synchrotron radiation wide-angle X-ray scattering (WAXS). Analyses on the evolution of crystallinity and orientations of forms II and I during tensile deformation show that stretch accelerates the transformation from forms II to I, which is interpreted based on either a direct crystal–crystal transition or an indirect approach via an intermediate state of melt, namely a melting recrystallization process. A three-stage mechanical deformation including linear deformation, stress plateau, and strain hardening is observed in the engineering stress–strain curves, which corresponds to a process of incubation, nucleation, and gelation of form I crystals. It establishes a nice correlation between phase transition and mechanical behavior in this study.

Load-bearing biological tissues, such as muscles, are highly fatigue-resistant, but how the exquisite hierarchical structures of biological tissues contribute to their excellent fatigue resistance is not well understood. In this work, we study antifatigue properties of soft materials with hierarchical structures using polyampholyte hydrogels (PA gels) as a simple model system. PA gels are tough and self-healing, consisting of reversible ionic bonds at the 1-nm scale, a cross-linked polymer network at the 10-nm scale, and bicontinuous hard/soft phase networks at the 100-nm scale. We find that the polymer network at the 10-nm scale determines the threshold of energy release rateG0above which the crack grows, while the bicontinuous phase networks at the 100-nm scale significantly decelerate the crack advance until a transitionGtranfar aboveG0. In situ small-angle X-ray scattering analysis reveals that the hard phase network suppresses the crack advance to show decelerated fatigue fracture, andGtrancorresponds to the rupture of the hard phase network.

Extension flow induced crystallization of isotatic polypropylene (iPP) has been studied with a combination of extension rheological and in situ small-angle X-ray scattering (SAXS) measurements at 140 °C. Rheological data of step extension on iPP melt are divided into before and beyond fracture strain zones in strain–strain rate space, where intermediate strains between them lead to fracture of samples. Coincidently, weak and strong accelerations of nucleation are observed in the before and beyond fracture strain zones respectively, where distinctly different features of crystallization kinetics and nucleation form occur in these two zones. The microrheological model explains the acceleration of nucleation in the “before fracture strain zone” well, while a “ghost nucleation” mechanism is proposed to interpret the strong acceleration of nucleation in the “beyond fracture strain zone”. The “ghost nucleation” is due to the displacement of initial parent point nuclei, where daughter nuclei are induced along the trails. This new mechanism explains well the acceleration of nucleation in orders of magnitude and the formation of shish in iPP melt.

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.

Combining a homemade film blowing machine and an in situ synchrotron radiation source with small- and wide-angle X-ray scattering (SAXS and WAXS) capability, an investigation of film blowing of polyethylene (PE) has been studied. From the die exit to the positions above the frost line, four zones defined with different structural features are observed with SAXS and WAXS measurements. In zone I, precursor and crystal structures emerge from the polymer entanglement network during cooling and extension, which lead to the formation of a deformable crystal-cross-linked network at the boundary between zones I and II. The occurrence of the crystal-cross-linked network enhances the effective chain stretching during further deformation in zone II. Crystallization is largely accelerated, which generates crystals with high orientation. Further increasing the crystallinity results in the deformable crystal-cross-linked network transforming into a nondeformable crystal scaffold at the frost line (the boundary between zones II and III), which stabilizes the bubble and prevents further deformation. In zones III and IV, the scaffold and the entire sample are gradually filled up by crystals, respectively. Interestingly, increasing the take-up ratio (TUR) does not influence the critical crystallinity (χI–II) for the formation of the deformable crystal-cross-linked network, while the crystallinity (χf) at the frost line or for the formation of nondeformable scaffold does vary with TUR. This suggests that the former (χI–II) is mainly controlled by molecular parameters, while the latter (χf) is determined by both processing and molecular parameters of PE material.

The role of long chains in extension flow-induced crystallization was studied with a combination of extension rheological and in situ small-angle X-ray scattering (SAXS) measurements at 52 °C. To elucidate the effects of long chains, bidisperse blends of poly(ethylene oxide) (PEO) with the long-chain concentration above the overlap concentration were prepared, constructing long-chain entanglement network in short-chain matrix. Rheological data of step extension on PEO melt are divided into two regions with fracture strain of pure short-chain sample as a boundary. Distinctly different features of crystallization kinetics and crystal morphologies are observed in these two regions, exactly corresponding to rheological behavior. A new mechanism based on entanglement network perspective is proposed, in which the second entanglement network constructed by long chains has three effects: (i) helping flow to change the free energy of polymer melt more effectively; (ii) ensuring the specific work can impose on the system; (iii) favoring the formation of precursors. This mechanism captures both rheological observation and crystallization behavior successfully and offers a new viewpoint for FIC study.

The sequence and coupling of intra- and interchain orderings in flow-induced crystallization (FIC) of partially cross-linked isotactic polypropylene (iPP) is studied with in situ Fourier transform infrared spectroscopy (FTIR) and synchrotron radiation X-ray scattering techniques, which reveal that multiscale structural intermediates emerge prior to the onset of crystallization. Upon imposing flow, intrachain conformational ordering or coil–helix transition (CHT) occurs first, which is directly correlated with external stress. As helical content is built up at large strain, density fluctuation happens, and sufficient long helices may result in orientation ordering before FIC. The results demonstrate that stress induced intrachain CHT is the essential structural intermediate in FIC, which can be further coupled with interchain orientation and density providing either helical content or length meets the criterions for the phase transitions. We propose that coupling among external stress, intrachain conformational, and interchain orientation and density orderings to be the molecular mechanism for FIC of polymer forming helical structures.

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.