Enhancing Molecular Chain Entanglement and π-π Stacking Toward the Improvement of Shape Memory Performance of Polyimide

Thermoplastic polyimides(PIs)with shape memory potential have received growing attention in recent years.In this work,highperformance thermoplastic PIs were fabricated by introducing PIs with chain rigidity(r-PI)into PI with chain flexibility(f-PI).The influences of molecular chain entanglement andπ-πinteractions on their thermomechanical and shape memory properties were investigated.The degree of molecular chain entanglement was quantitively characterized based on dynamic mechanical analysis(DMA).Theπ-πinteractions were investigated in detail by X-ray diffraction(XRD)and UV-Vis spectroscopy.It was found that the entanglement density increased andπ-πinteractions became stronger with the introduction of r-PI into f-PI,leading to the improvement of shape recovery.Moreover,a broad and increased glass transition temperature(T_(g))was achieved,endowing the PIs with multiple shape memory properties.The synergistic effects of increased entanglement density and enhancedπ-πinteractions were beneficial to regulating interchain interactions and thereby achieving high shape memory performance of the PIs.

Effects of Chain Entanglement on Liquid-Liquid Phase Separation Behavior of LCST-type Polymer Blends: Cloud Point and Decomposition Rate

By preparing homogenous blend samples with different degrees of chain entanglement, we report an anomalous contribution of chain entanglement to phase separation temperature and rate of poly（methyl methacrylate）/poly（styrene-co- maleic anhydride） （PMMA/SMA） blends presenting a typical lower critical solution temperature （LCST） behavior. The melt- mixed PMMA/SMA blends with a higher chain entanglement density present a lower cloud point （To） and shorter delay time, but lower phase separation rate at the given temperature than solution-cast ones, suggesting that for the polymer blends with different condensed state structure, thermodynamically more facilitation to phase separation （lower Tc） is not necessarily equivalent to faster kinetics （decomposition rate）. The experimental results indicate that the lower Tc of melt-mixed sample is ascribed to smaller concentration fluctuation wavelength （Am） induced by higher entanglement degree, while higher entanglement degree in melt-mixed sample leads to a confined segmental dynamics and consequently a slower kinetics （decomposition rate） dominated by macromolecular diffusion at a comparable quench depth. These results reveal that the chain packing in polymer blends can remarkably influence the liquid-liquid phase separation behavior, which is a significant difference from decomposition of small molecular mixtures.

Rheological Behaviour for Polymer Melts and Concentrated Solutions Part Ⅰ:A New Multiple Reptation Model to Predict the Nonlinear Visco-elasticity with Nagai Chain Constraints in Entangled Polymer Melts

An approach of stochastically statistical mechanics and a unified molecular theory of nonlinear viscoelasticity with constraints of Nagai chain entanglement for polymer melts have been proposed. A multimode model structure for a single polymer chain with n tail segments and N reversible entanglement sites on the test polymer chain is developed. Based on the above model structure and the mechanism of molecular flow by the dynamical reorganization of entanglement sites, the probability distribution function of the end-to-end vectr for a single polymer chain at entangled state and the viscoelastic free energy of deformation for polymer melts are calculated by using the method of the stochastically statistical mechanics. The four types of stress-strain relation and the memory function are derived from this thery. The above theoretical relations are verified by the experimentaf data for various polymer melts. These relations are found to be in good agreement with the experimental results

Dissolving of Ultra‑high Molecular Weight Polyethylene Assisted Through Supercritical Carbon Dioxide to Enhance the Mechanical Properties of Fibers

The molecular weight of ultra-high molecular weight polyethylene(UHMWPE)fbers is severely decreased compared with raw materials due to high temperature and strong shearing in the dissolving process.In this study,we reported a novel method to assist the dissolving of UHMWPE in parafn oil without severe degradation in order to improve the tensile strength of resultant fbers.UHMWPE fbers with relatively high molecular weight and more excellent disentanglement efect were prepared by gel-spinning with UHMWPE suspension treated with supercritical carbon dioxide(SC-CO_(2)).The dynamic thermomechanical,mechanical and crystalline properties of UHMWPE extracted fbers and drawn fbers were researched comprehensively.UHMWPE extracted fbers obtained after SC-CO_(2) treatment display a higher molecular weight.More importantly,it is clear that the disentanglement of UHMWPE gel fbers gained by processing SC-CO_(2) has been signifcantly promoted compared with that without SC-CO_(2) treatment from dynamic thermomechanical and rheological results,which could also be demonstrated from the cross-sectional morphology of UHMWPE extracted fbers.Furthermore,the tensile strength of UHMWPE fbers prepared through SC-CO_(2) treating is able to attain 30.11 cN/dtex,increased by 10.3%in comparison to UHMWPE fbers gained without assistance of SC-CO_(2).Beyond that,the thermal behavior and crystallization performance of UHMWPE extracted fbers and drawn fbers acquired by way of SC-CO_(2) treatment have also been enhanced.