Deformation-induced phase transition of polyamide 12 (PA 12) was studied with in situ wideangle X-ray scattering (WAXS) at temperatures below and above glass transition temperature. Irrespective to the testing temperature, a transient R 00 phase occurred in the early plastic deformation stage, whose lifetime is decreasing with the increase of temperature. At temperature below glass transition temperature, the transient R 00 phase further transforms into a mesomorphic state with increasing the strain, while high temperature promotes the transient R 00 phase transforming into γ 0 phase. The different final states at temperatures below and above glass transition temperature is due to the competition and coupling between external work and thermal activation. External work from tensile deformation is responsible for the appearance of the transient R 00 phase and the final mesomorphic state at temperature below glass transition temperature, while thermal activation drives the transition from the transient R 00 phase to γ 0 phase. All these phase transitions occur in the plastic deformation region, which shows a close correlation between the mechanical response and the structural evolution during tensile deformation.

Unlike thermotropic liquid-crystalline C(3)-symmetric molecules with flexible chains, the herein-designed fully rigid three-armed molecules (C(3)-symmetric and unsymmetric) create a fancy architecture for the formation of lyotropic liquid crystals in water. First, hollow columns with triple-stranded helices, analogous to helical rosette nanotubes, are spontaneously constructed by self-organization of the rigid three-armed molecules. Then, the helical nanotubes arrange into hexagonal liquid-crystalline phases, which show macroscopic chirality as a result of supramolecular chiral symmetry breaking. Interestingly, the helical nanotubes constructed by the fully rigid molecules are robust and stable over a wide concentration range in water. They are hardly affected by ionic defects at the molecular periphery, that is, further decoration of functional groups on the molecular arms can presumably be realized without changing the helical conformation. In addition, the formed columnar phases can be aligned macroscopically by simple shear and show anisotropic ionic conductivity, which suggests promising applications for low-dimensional ion-conductive materials.

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.

The supreme mechanical performance of natural rubber (NR) is commonly attributed to strain-induced crystallization (SIC). The SIC of NR during uniaxial stretch has been extensively investigated, whereas that under multiaxial deformation has been rarely reported, which is close to real service conditions (i.e., tire). In this work, the crystallization behavior of NR under biaxial stretch was studied with in situ synchrotron radiation wide-angle X-ray diffraction in combination with a custom-built biaxial stretch machine. It is observed that biaxial stretch frustrates the SIC of NR: within λ x /λ y < 1.6, where λ x and λ y are stretch ratios of two mutually perpendicular axes, no crystallization emerges even under large drawing ratio until sample fracture at ambient temperature. This finding challenges the common wisdom of the self-reinforcement mechanism of SIC in NR under multiaxial deformation in real service conditions. A theoretical SIC model is proposed, which can decouple the contributions of conformational entropy reduction ΔS f and amorphous chain orientation f to final Gibbs free energy change (ΔG) during multiaxial deformation. This model quantitatively renders a reproduction of the crystallinity during the biaxial stretch, which is well consistent with experimental results and can be further generalized for flow-induced crystallization of semicrystalline polymers.