Visualization and quantitative evaluation of covalent bond scission in polymeric materials are critical in understanding their failure mechanisms and improving the toughness and reliability of the materials. Mechano-responsive polymers with the ability of molecular-level transduction of force into chromism and luminescence have evoked major interest and experienced significant progress. In the current review, we highlight the recent achievements in covalent mechanochromic and mechanoluminescent polymers, leading to a bridge between macroscopic mechanical properties and microscopic bond scission events. After a general introduction concerning polymer mechanochemistry, various examples that illustrate the strategies of design and incorporation of functional and weak covalent bonds in polymers were presented, the mechanisms underlying the optical phenomenon were introduced and their potential applications as stress sensors were discussed. This review concludes with a comment on the opportunities and challenges of the field.

Inspired by structures of antenna-reaction centers in photosynthesis, the complex micelle was prepared from zinc tetra-phenyl porphyrin (ZnTPP), fullerene derivative (PyC₆₀) and poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-b-PCL). The core-shell structure made the hydrophobic donor-acceptor system work in aqueous. In micellar core, coordination interaction occurred between ZnTPP and PyC₆₀ molecules which ensured the enhanced energy migration from the donor to the acceptor. The enhanced interaction between porphyrin and fullerene was confirmed by absorption, steady-state fluorescence and transient fluorescence. The generation of singlet oxygen and superoxide radical was detected by iodide method and reduction of nitro blue tetrazolium, respectively, which confirmed that electron transfer reaction in the complex micellar core occurred. Moreover, the complex micelle exhibited effective electron transfer performance in photodebromination of 2, 3-dibromo-3-phenylpropionic acid. The complex micellar structure endowed the donor-acceptor system with improved stability under irradiation. This strategy could be helpful for designing new electron transfer platform and artificial photosynthetic system.

Two copolymers of P1 and P2 comprising benzothiadiazole, 1, 4-bis(dodecyloxy)-benzene units were synthesized by Sonogashira coupling polymerization based on ethynyl-linked 1, 2, 5, 6-naphthalenediimide. Their thermal, optical, electrochemical as well as charge transport properties were studied. Bottom-gate top-contact organic field-effect transistors (OFETs) measurements of P1 and P2 thin films showed different charge transport behaviors. P1 displayed pure electron transport behaviors in OFETs with electron mobility up to 10^-3 cm²·V^-1·s^-1, while P2 exhibited hole transport features. The molecular structure analysis revealed that the structure of P1 has the acceptor-linker-acceptor' (A-L-A') characteristic, and P2 possesses the donor-linker-acceptor (D-L-A) structure feature. The results demonstrate that different molecular structures lead them to have distinct charge transport behaviors. In particular, the first pure electron transport copolymer in OFETs based on 1, 2, 5, 6-naphthalenediimide is achieved.

Choline phosphate (CP) as a novel zwitterion possesses specific and excellent properties compared with phosphorylcholine (PC), as well as its polymer, such as poly(2-(methacryloyloxy)ethyl choline phosphate) (PMCP), has been studied extensively due to its unique characteristics of rapid cellular internalization via the special quadrupole interactions with the cell membrane. Recently, we reported a novel PMCP-based drug delivery system to enhance the cellular internalization where the drug was conjugated to the polymer via reversible acylhydrazone bond. Herein, to make full use of this feature of PMCP, we synthesized the diblock copolymer poly(2-(methacryloyloxy)ethyl choline phosphate)-b-poly(2-(diisopropylamino)ethyl methacrylate) (PMCP-b-PDPA), which could self-assemble into polymersomes with hydrophilic PMCP corona and hydrophobic membrane wall in mild conditions when the pH value is ≥ 6.4. It has been found that these polymersomes can be successfully used to load anticancer drug Dox with the loading content of about 11.30 wt%. After the polymersome is rapidly internalized by the cell with the aid of PMCP, the loaded drug can be burst-released in endosomes since PDPA segment is protonated at low pH environment, which renders PDPA to transfer from hydrophobic to hydrophilic, and the subsequent polymersomes collapse thoroughly. Ultimately, the "proton sponge" effect of PDPA chain can further accelerate the Dox to escape from endosome to cytoplasm to exert cytostatic effects. Meanwhile, the cell viability assays showed that the Dox-loaded polymersomes exhibited significant inhibitory effect on tumor cells, indicating its great potential as a targeted intracellular delivery system with high efficiency.

A novel fluorinated triblock copolymer incorporating 2-ethylhexyl methacrylate (EHMA), tert-butyl methacrylate (tBMA) and 1H, 1H, 2H, 2H-perfluorodecyl acrylate (FA) (PEHMA-b-PtBMA-b-PFA) was first synthesized using three successive reversible addition fragmentation chain transfer (RAFT) polymerization and the subsequent hydrolyzing at acidic condition. The as-fabricated triblock copolymer exhibited an interesting morphology evolution from the multi-compartment rod-like structure to spherical structure along with the addition of a selective solution. At the same time, a visible phase separation domain could be seen in the core area due to the existence of fluorocarbon segments. Furthermore, the self-assembly behavior of the triphilic copolymer at different pH was also verified by transmission electron microscopy, as well as the dynamic light scattering. These stimuli-responsive multi-compartment nanostructures may have potential applications in drug delivery.

The conformation of silk fibroin (SF) frozen with polyethylene glycol (PEG) at a molecular weight from 2 kDa to 20 kDa and a mass ratio of PEG:SF from 1:5 to 10:1 was studied by spectral and microscopic methods. It is found that the conformation transition of SF from random coil to β-sheet could be induced by the stress resulting from PEG crystallization at -20℃, and greatly depended on the cooling rate, PEG:SF mass ratio and PEG molecular weight. These findings provide a new method for the preparation of desired SF nanofibers.

To enhance the mechanical properties of three-dimensional graphene aerogels with aramid fibers, graphene/organic fiber aerogels are prepared by chemical reduction of graphene oxide in the presence of organic fibers of poly(p-phenylene terephthalamide) (PPTA) and followed by freeze-drying. Thermal annealing of the composite aerogels at 1300℃ is adopted not only to restore the conductivity of the reduced graphene oxide component but also to convert the insulating PPTA organic fibers to conductive carbon fibers by the carbonization. The resultant graphene/carbon fiber aerogels (GCFAs) exhibit high electrical conductivities and enhanced compressive properties, which are highly efficient in improving both mechanical and electrical performances of epoxy composites. Compared to those of neat epoxy, the compressive modulus, compressive strength and energy absorption of the electrically conductive GCFA/epoxy composite are significantly increased by 60%, 59% and 131%, respectively.

In the present work, poly(propylene glycol) (PPG) was block copolymerized to form polylactide-poly(propylene glycol)-polylactide (PL-PPG-PL) triblock copolymers for preparing flexible stereocomplex PL (scPL) blend films. The scPL blend films were prepared by solution blending of poly(L-lactide)-PPG-poly(L-lactide) (PLL-PPG-PLL) and poly(D-lactide)-PPG-poly(D-lactide) (PDL-PPG-PDL) triblock copolymers before film casting. The influences of PL end-block lengths (2×10⁴ and 4×10⁴ g/mol) and blend ratios (75/25, 50/50 and 25/75 W/W) on the stereocomplexation and mechanical properties of the blend films were evaluated. From DSC and WAXD results, the 50/50 blend films had complete stereocomplexation. Phase separation between the scPL and PPG phases was not observed from their SEM images. The tensile stress and elongation at break increased with the sterecomplex crystallinities and PL end-block lengths. The PPG middle-blocks enhanced elongation at break of the scPL films. The results showed that the PL-PPG-PL triblock structures did not affect stereocomplexation of the PLL/PDL block blending. In conclusion, the phase compatibility and flexibility of the scPL films were improved by PPG block copolymerization.

The overall crystallization kinetics and spherulite morphologies of miscible poly(ethylene oxide) (PEO)/1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM] [PF₆]) mixtures were studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and rheological measurements. The finer crystal structures were further detected by wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). Crystallization of PEO is largely suppressed by[BMIM] [PF₆] addition especially at higher ionic liquid (IL) concentrations above 20 wt%. Both the overall crystallization rate and the spherulite growth decrease with the increase of IL content and crystallization temperature; however, the crystallization mechanism keeps unchanged as evidenced by the similar Avrami exponent n and WAXD results. The addition of[BMIM] [PF₆] could induce more nuclei to some extent, but the induction time of crystallization is evidently prolonged, and a linear to non-linear transition of the spherulite growth (R ∝ t to R ∝ t^1/2) can be observed. At higher IL concentration, the spherulite texture changes apparently from particular serrated to branch surface due to the diffusion-controlled growth and the dilution effect, which also as a main factor contributes to the increasing trend of the long period of crystals.

We device a relaxed lattice model (RLM) to study the mechanism of glass transition, which unifies the cage-effects from particle-particle interaction and entropy. By analyzing entropy in RLM with considering the influence of interactions on equilibrium, we demonstrate that glass transition is a second-order phase transition. For a perfect one-dimensional linked particle system like linear polymer under normal pressure, the free volume at glass transition is rigorously deduced out to be 2.6%, which provides a theoretical basis for the iso-free volume of 2.5% given by Willian, Landel and Ferry (WLF) equation. Extending to system with dead particles linked with higher dimensions like branched or cross-linked chains under positive or negative pressure, free volume at glass transition is varied, based on which we construct a phase diagram of glass transition in the space of free volume-dead particle-pressure. This demonstrates that free volume is not the single parameter determining glass transition, while either dead particles like cross-linked points or external force fields like pressure can vary free volume at the glass transition.

Biphenyl-contained monomer of 1, 4-bis[2-(3, 4-epoxy cyclohexyl ethyl) dimethylsilyl] biphenyl (BP-SiH-EP) was prepared via hydrosilylation reaction of 1, 4-bis(dimethylsilyl) biphenyl (BP-SiH) and 1, 2-epoxy-4-vinylcyclohexane in the presence of Karstedt's catalyst. ¹H-NMR, ¹³C-NMR and FTIR were used to characterize the structure of the obtained monomer. BP-SiH-EP was then cured by methyl hexahydrophthalic anhydride (MeHHPA) with 1-cyanoethyl-2-ethyl-4-methylimidazole as an accelerator. The polymerization behavior was studied by DSC. The results of DMA measurement demonstrate that the cured BP-SiH-EP/MeHHPA can maintain high storage modulus (>1 GPa) in a wide range of temperature up to 176℃. According to the damping factor curve of DMA, cured BP-SiH-EP/MeHHPA exhibits a high glass transition temperature (T_g) of 192℃, which is 20℃ higher than that of cured 1, 4-bis[2-(3, 4-epoxy cyclohexyl ethyl) dimethylsilyl] benzene (DEDSB)/MeHHPA. TGA results show that cured BP-SiH-EP/MeHHPA has good thermal stability (T_5%=339℃) due to the high heat-resistance of rigid biphenyl group. Moreover, the crosslinking density of cured BP-SiH-EP/MeHHPA should be lower than that of cured DEDSB/MeHHPA estimated from their chemical structures, which conflicts with the calculated results based on the rubber elasticity equation. The inconsistence indicates that the calculated crosslinking densities are not comparable, possibly owing to their differences in the rigidity of polymer chains and intermolecular interaction.

The performance of reinforced rubber compounds depends on the filler composition while the reinforcement and dissipation mechanisms still remain unclear. Herein linear and nonlinear dynamic rheological responses of carbon black/silica hybrid filler filling nature rubber compounds are investigated. The rheological contributions of dynamically retarded bulk phase and filler network are revealed to be crucial at high and low frequencies, respectively, and the bulk phase is shown to be of vital importance for the occurrence of nonlinear Payne effect at mediate frequencies. A framework for simultaneously solving reinforcement and dissipation varying with filler composition and content is suggested, providing a new perspective in understanding the filling effect for manufacturing high-performance rubber materials.