In the present work, we demonstrated the recyclability and precisely targeted reparability of amino functionalized multiwall carbon nanotubes–epoxy resin based on dynamic covalent Diels–Alder (DA) network (NH2–MWCNTs/DA-epoxy) by exploring the photothermal conversion of CNTs to trigger the reactions of dynamic chemical bonds. The covalent cross-linked networks of NH2–MWCNTs/DA-epoxy resin change their topology to linear polymer by thermally activated reverse Diels–Alder (r-DA) reactions at high temperatures, which endues the resin with almost 100% recyclability. The self-healing property of the epoxy resin was confirmed by the complete elimination of cracks after the reconstruction of DA network induced by heating or near-infrared (NIR) irradiation. For heat-triggered self-healing process, heat energy may also act on those uninjured parts of the resin and cause the dissociation of the whole DA network. Therefore, redundant r-DA and DA reactions, which have no contribution to self-healing, are also triggered during thermal treatment, resulting in not only a waste of energy but also the deformation of the sample under external force. Meanwhile, for the NIR-triggered self-healing process, the samples can maintain well their original shape without observable deformation after irradiation. The NIR-triggered healing process, which uses MWCNTs as the photothermal convertor, have very good regional controllability by simply tuning the MWCNTs content, the distance from NIR laser source to sample, and the laser power. The injured samples can be locally repaired with high precision and efficiency without an obvious influence on those uninjured parts.Keywords: Diels−Alder reaction; epoxy resin; photothermal conversion; recycle; self-healing;

•Sparsely partial-wrapped Py-PPDO-b-PEG@MWCNTs hybrid nano-aggregates with tunable morphology were prepared.•Dispersion and percolation efficiency of MWCNTs were obviously improved by the sparsely partial wrapping strategy.•The PCL/Py-PPDO-b-PEG@MWCNTs composite films showed significantly improved electrical conductivity and mechanical properties.Carbon nanotubes (CNTs) is a great potential reinforcing additives or conductive fillers for polymer composites. It is well known that interfacial properties between CNTs and polymer matrix have a critical effect on the properties of composites. Here, we demonstrate a novel strategy for preparing high efficient percolation networks of CNTs in polymer composite by partially wrapping of MWCNTs with crystallization induced self-assembly of pyrene end-capped poly(p-dioxanone)-block-poly(ethylene glycol) (Py-PPDO-b-PEG). The wrapped surface can prevent the aggregation of CNTs, while the bare surface may still induce enough interconnection of CNTs, resulting in high efficient percolation network. The wrapping density, which could be easily engineered by control the crystallization temperature of PPDO block, is essential to the formation of percolation networks. The composite film prepared from Py-PPDO-b-PEG@MWCNTs and PCL exhibited much improved conductivity especially at very low nanotube concentration compared to those from neat MWCNTs without any wrapping. Especially using Py-PPDO-b-PEG@MWCNTs prepared at 40 °C as the precursors, the composite film exhibits both best electrical conductivity (3–11 orders of magnitude higher than that of PCL/MWCNTs composites films at same MWCNTs contents) and mechanical properties, which could be attributed to the optimized wrapping density of this sample. Dense or excessively sparse wrapping may impede the interconnection and dispersity of the MWCNTs, respectively, and therefore resulted in decreased conductivities.

Poor thermal stability and thermoplastic processability are the main obstacles for wide application of polyvinyl alcohol, which is a typically water-soluble polymer with excellent mechanical properties, biocompatibility, barrier properties and biodegradability. In this work, a novel method was demonstrated for preparing PVA based material with improved thermal stability, low additive loading, and without impairing the inherent good properties of PVA. Poly (1-vinyl-3-ethyl-imidazolium bromide) (PEtVIm-Br) was synthesized and used as both a thermal stabilizer for PVA and a compatibilizer for PVA/graphene nanocomposites. A 49% improvement in tensile strength and 65% improvement in the elongation at break were achieved by incorporating 2.5 wt% of PEtVIm-Br and 0.5 wt% of graphene into PVA matrix. The thermal stability of the PVA/PEtVIm-Br-graphene composite has been investigated by thermogravimetric analysis (TGA), TGA-DSC simultaneous thermal analysis, and TGA coupled with FTIR. With the addition of PEtVIm-Br-graphene, the elimination reaction of PVA at the initial stage of pyrolysis was suppressed remarkably, while the activation energy for pyrolysis of the nanocomposite increased obviously, resulted in a much improved thermal stability, comparing to pure PVA. The volatile products of pure PVA and PVA composites were detected by TGA/FTIR and Py-GC/MS. The improvement in thermal stability of PVA/PEtVIm-Br and PVA/PEtVIm-Br-graphene composite comparing to pure PVA may attribute to the ability of capturing free radicals of PEtVIm-Br and the barrier effect of well-dispersed graphene.

Herein we developed a novel strategy for preparing biodegradable polylactide (PLA) based materials with improved crystallinity, mechanical properties and rheological behaviour by introducing a long-chain branched block copolymer (LB-PCLA) of PLA and poly-ε-caprolactone (PCL). The LB-PCLA copolymer was synthesized by single hydroxyl-terminated PLA (PLA-OH) and three hydroxyl-terminated PCL (PCL-3OH) precursors. The crystallinity and crystal morphology of PLA/LB-PCLA blends were investigated by a differential scanning calorimetry (DSC) instrument and polarized optical microscopy (POM). The morphology and domain size of PLA/LB-PCLA blends were investigated by transmission electron microscopy (TEM). The irregular dispersed droplet shape of the LB-PCLA copolymer suggested that the interfacial interaction between the PLA and PCL phases was obviously compatible because of the copolymerization and the branched structure of the LB-PCLA. This phase morphology is responsible for the enhancement in crystallinity, crystallization rate, and toughness of the PLA/LB-PCLA blends compared to neat PLA and PLA/PCL blends. The elongation at break for the PLA/LB-PCLA blend with 15 wt% of the LB-PCLA copolymer was about 210%, an increase of 30 times compared with that of neat PLA. The rheological behaviour also shows that the LB-PCLA copolymer and PLA/LB-PCLA-15 have more pronounced shear thinning behaviour and longer relaxation time than neat PLA and PLA/PCL blends with 15 wt% of the PCL, which can be attributed to the long-chain branched structure of the LB-PCLA copolymer.

A novel and facile method was developed for morphological controlling of self-assemblies prepared by crystallization induced self-assembly of crystalline-coil copolymer depending on the combination effect of crystallization and micellization. The morphological evolution of the self-assemblies of alternating poly(p-dioxanone)-block-poly(ethylene glycol) (PPDO–PEG) multiblock copolymer prepared by different solvent mixing methods in aqueous solution were investigated. “Chrysanthemum”-like and “star anise”-like self-assemblies were obtained at different rates of solvent mixing. The results suggested gradually change in solvent quality (slowly dropping water into DMF solution) leaded to a hierarchical micellization-crystallization process of core-forming PPDO blocks, and flake-like particles were formed at the initial stage of crystallization. Meanwhile, crystallization induced micellization process occurred when solvent quality changed drastically. Shuttle-like particles, which have much smaller size than those of flake-like particles, were formed at the initial stage of crystallization when quickly injecting water into DMF solution of the copolymer. Therefore, owing to the different changing rate of solvent quality, which may result in different combination effect of crystallization and micellization during self-assembly of the copolymer, PPDO–PEG self-assemblies with different hierarchical morphology in nano scale could be obtained.

Nanofibers from poly(lactic acid) (PLA) homopolymer and poly(p-dioxanone)-b-poly(ethylene glycol) multi-block copolymer (PPDO-b-PEG) with different phase separation morphologies depending on the crystallization induced self-assembly of PPDO-b-PEG are prepared by single spinneret electrospinning. Mixing solvents of chloroform–dimethyl formamide (CHCl3–DMF) with different compositions are used for controlling the crystallization of the PPDO block and therefore the phase separation in the obtained nanofibers. The crystallization of the PPDO block has a very important influence on the morphology of the PPDO-b-PEG nanoparticles in the spinning solution. In spinning solutions with low DMF content, nanoparticles with irregular shapes and non-compact inner structures are formed because the degree of crystallization of the PPDO block is relatively low, and a discontinuous sea-island phase separation is formed in the obtained electrospun nanofibers. Meanwhile, in spinning solutions with high DMF content, the copolymer can form flake-like nanoparticles with a relatively high degree of crystallization. The flake-like shape favors compact aggregation of the PPDO phase during formation of the nanofibers, and a continuous core–shell phase separation of the nanofibers is obtained.

A novel plasticizer (lacti-glyceride) synthesized by esterification of glycerol with l-lactic acid was developed for thermal processing of PVA/PLA blend. Meanwhile, the stannous octoate (Sn(Oct)2) was used as the catalyst for the transesterification reaction of PVA, PLA, and the plasticizer. The results showed that the PVA/PLA blends plasticized with lacti-glyceride have improved thermal processability and mechanical properties compared to those plasticized with glycerol, because the lacti-glyceride may act not only as a plasticizer but also as a compatibilizer for PVA and PLA. SEM images indicated that PVA/PLA blends catalyzed with Sn(Oct)2 had better compatibility than those without catalyst. Water contact angle measurements showed that the PVA/PLA blends plasticized with lacti-glyceride have much more hydrophobic surfaces than those blends plasticized with glycerol. The blends will find wider applications than PVA.

We reported here a novel three stimuli sensitive hydrogel that was constructed by the formation of host–guest complexes between poly(N-isopropylacrylamide) (PNIPAM) containing azobenzene groups and cyclodextrin dimers connected by disulfide bonds. The obtained hydrogel gives a smart response to the stimuli of temperature, light, and reduction, manifested in the form of a sol–gel phase transition.

Well-defined, reversibly light-responsive amphiphilic diblock copolymer grafted with spiropyran, was prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization. The copolymer self-assembles into polymeric micelles in water and exhibits reversible dissolution and re-aggregation characteristics upon ultraviolet (UV) and visible (Vis)-light irradiation. The fluorescence response of spiropyran immobilized onto the copolymer was light switchable. When nitrobenzoxadiazolyl derivative (NBD) dyes are encapsulated into the core of the micelles, a reversible, light-responsive, dual-color fluorescence resonance energy transfer (FRET) system is constructed and processed, which is well regulated by alternatively UV/vis irradiation. We anticipate these photoswitchable and FRET lighting up nanoparticles will be useful in drug delivery and cell imaging or tracking synchronously.Well-defined amphiphilic diblock copolymer decorated with spiropyran exhibits good reversible photoswitchable and FRET light-responsive properties, which would have a potential application in drug delivery and cell imaging and tracking synchronously.

Poly(p-dioxanone)-block-polyethylene glycol diblock copolymers functionalized with pyrene moieties (Py-PPDO-b-PEG) at the chain ends of PPDO blocks were synthesized for preparing anisotropic micelles with improved stability. The micellization and crystallization of the copolymers were investigated by nano differential scanning calorimetry (Nano DSC), transmission electron microscopy (TEM), UV–vis spectrophotometery, fluorophotometer, and dynamic light scattering (DLS), respectively. The results indicated that the aggregation of pyrene induced by intermolecular interaction lead to micellization of Py-PPDO-b-PEG at much lower concentrations than those of PPDO-b-PEG copolymers without pyrene moieties. The aggregation of pyrene moieties may also serve as nucleation agent and therefore enhance the crystallization rate of PPDO blocks. Fluorescence measurements by using Nile Red as the fluorescent agent indicated that the micelles of Py-PPDO-b-PEG have high stability and load capacity for hydrophobic molecules.Crystallization induced micellization of poly(p-dioxanone)-block-polyethylene glycol diblock copolymer functionalized with pyrene moiety were studied. The aggregation of pyrene moieties promotes both micellization of the copolymer and crystallization of the PPDO blocks by serving as heterogeneous nucleation agent, resulted in much higher micelle stability than copolymer without pyrene moieties.

The morphological evolution and phase transition of a branched crystalline-coil multi-block copolymer, poly(p-dioxanone)-block-poly(ethylene glycol) (PPDOstar-b-PEG), in aqueous solution under heating and cooling were investigated. The changes in size and morphology of the nano-aggregates were monitored by dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). A semitransparent and uniform dispersion of nano-aggregates with star anise-like morphology was obtained from PPDOstar-b-PEG at room temperature. The dispersion gradually turned transparent during heating to 80 °C because of the melting of the crystallized PPDO blocks. The crystals with low regularity melted first leading to dissociation of the star anise nano-aggregates to flake-like particles. The copolymer formed sphere-like micelles when the temperature was high enough for melting all PPDO crystals. During the cooling run, a hysteresis of phase transition was observed because of the supercooling of crystallization. The morphological evolution of the copolymer micelle suggested that the formation of the star anise-like nano-aggregates was a hierarchical assembly process. A “crystallization induced hierarchical assembly” mechanism was therefore proposed to explain the formation of the star anise-like nano-aggregates. Metastable flake-like nano-particles formed at the initial stage of crystallization of PPDO blocks. The hydrophobic core of the flake was composed of several crystal lamellae or plates piled up in a layer-by-layer fashion. With further crystallization of PPDO blocks, the flakes tended to aggregate because of the variation of the hydrophilic–hydrophobic balance. The active edge of crystalline lamellae in the hydrophobic core of one flake may induce two different growth modes: epitaxial growth with amorphous spherical micelles and interparticle interpenetration crystallization in the amorphous region of other flakes. The branched structure of the nano-particles was therefore formed driven by interparticle interpenetration crystallization and epitaxial crystallization simultaneously.

A novel and facile strategy, combining anisotropic micellization of amphiphilic crystalline-coil copolymer in water and reassembly during single spinneret electrospinning, was developed for preparing nanofibers with very fine core–shell structure. Polyvinyl alcohol (PVA) and polyethylene glycol-block-poly(p-dioxanone) (PEG-b-PPDO) were used as the shell and the crystallizable core layer, respectively. The core–shell structure could be controllably produced by altering concentration of PEG-b-PPDO, and the chain length of the PPDO block. The morphology of the nanofibers was investigated by Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM). X-ray rocking curve measurements were performed to investigate the degree of ordered alignment of the PPDO crystalline lamellae in the nanofiber. The results suggested that the morphology of nanoparticles in spinning solution plays very important role in determining the phase separation of nanofibers. The amphiphilic PEG-b-PPDO copolymer self-assembled into star anise nanoaggregates in water solution induced by the crystallization of PPDO blocks. When incorporated with PVA, the interaction between PVA and PEG-b-PPDO caused a morphological transition of the nanoaggregates from star anise to small flake. For flake-like particles, their flat surface is in favor of compact stacking of PPDO crystalline lamellae and interfusion of amorphous PPDO in the core of nanofibers, leading to a relatively ordered alignment of PPDO crystalline lamellae and well-defined core–shell phase separation. However, for star anise-like nanoaggregates, their multibranched morphology may inevitably prohibit the compact interfusion of PPDO phase, resulting in a random microphase separation.Keywords: anisotropic micelle; core−shell nanofiber; crystalline-coil copolymer; single spinneret electrospinning

A novel multi-branched crystalline-coil block copolymer composed of hydrophilic polyethylene glycol (PEG) block and multi-branched crystallisable poly(p-dioxanone) (PPDO) block was prepared. Firstly, multi-branched PPDO was prepared via polycondensation of AB2-type HOOC-PPDO-2OH precursor, which was synthesized by using 2,2-bis(hydroxymethyl)propionic acid as initiator for ring opening polymerization of p-dioxanone; then the multi-branched PPDO-b-PEG copolymer was obtained by coupling the end hydroxyl group of multi-branched PPDO with carboxylated mPEG using dicyclohexylcarbodiimide as dehydrator. The molecular structures of polymers formed in each step were characterized by NMR and GPC. The results confirmed the successful preparation of the target product, and the molecular characteristics of the multi-branched PPDO, such as chain length of the blocks and branch density, could be facilely controlled. In addition, the micelle of the copolymer in aqueous solution was investigated by fluorescent probe, TEM, DLS, DSC and NMR. The results indicated that the copolymer in aqueous solution can form “star anise”-like micelles and the micellization behavior was determined by the composition and molecular architecture of the copolymer.

Nano aggregates in aqueous medium with a novel “star anise”-like morphology were prepared from a branched alternating multi-block copolymer composed of 3-arm star-like hydrophobic poly(p-dioxanone) block and linear hydrophilic poly(ethylene glycol) block. The influence of block length on the morphology of the nano aggregate was investigated.

Long-chain-branched poly(p-dioxanone)s (LCB-PPDOs) with different branch densities were prepared via the chain-extending reaction of hydroxyl group terminated linear bi-functional PPDO (2a-PPDO) and star-like tri-functional PPDO (3a-PPDO) prepolymers, which were synthesized by the ring-opening polymerization of p-dioxanone (PDO) using 1,4-butanediol (BD) and trimethylolpropane (TMP) as multi-functional initiators, respectively. The undesirable gelation was successfully depressed by adjusting the chain length and feed ratio of prepolymers. The average molecular weight between branch points (Mb) and the average number of branch per 100,000 g/mol (Bn) of LCB-PPDOs were calculated from the 1H NMR spectra. The average number of branch ranged from 0 to 6.72 branch points per 100,000 g/mol, and the number-average molecular weights between branch points ranged from 6900 to 20,500 g/mol. The results of differential scanning calorimetry (DSC) showed that the crystallization behavior of LCB-PPDOs was changed evidently with the branch density. Small-amplitude dynamic oscillatory rheometer was used to investigate the rheological properties of the melts of LCB-PPDO including zero-shear viscosity, storage modulus, relaxation times and loss angle, which largely depended on the branch density and length of LCB-PPDOs. Therefore, the rheological behaviors of PPDO can be well-controlled via synthesizing LCB-PPDOs with the desired architectures.Long-chain-branched poly(p-dioxanone) was prepared by using hexamethylene diisocyanate as chain-extender of bi- and tri-functional PPDO prepolymers. The undesired gelation was depressed by adjusting the molecular weight and content of the prepolymer.

Triblock copolymer of poly(p-dioxanone) and polyethylene glycol end-capped with pyrene moieties ((Py-PPDO)2-b-PEG) was synthesized and used as modifier for multi-wall carbon nanotubes (MWCNTs). Nano-aggregates ((Py-PPDO)2-b-PEG@MWCNTs) with shish-kebab like partially wrapped morphology and very good stability were obtained by incorporating the copolymer with MWCNTs. The bare MWCNT sections of (Py-PPDO)2-b-PEG@MWCNTs were able to induce π–π interactions with graphene (GE) and resulted in a novel GE/(Py-PPDO)2-b-PEG@MWCNTs hybrid. The dispersity of GE in solution or polymer matrix was therefore greatly improved. The PCL nanocomposite films using GE/(Py-PPDO)2-b-PEG@MWCNTs as hybrid nanofiller exhibited obviously improved mechanical properties especially at very low hybrid nanofiller content. The influence of the nanofiller content and feed ratio of GE/MWCNTs on the mechanical properties of composites films was evaluated. When the feed ratio of GE to MWCNTs is 2:8 and the total loading of nanofiller is only 0.01 wt%, the tensile strength of the composite film increased by 163% and the elongation at break increased by 17% compared to those of neat PCL. These results can be attributed to fine dispersion of the nanofillers in PCL matrix and the hybrid interactions between GE and MWCNTs. Therefore, this work provides a novel method for preparing polymer nanocomposites with high mechanical performance and low nanofiller loading.The PCL nanocomposite films prepared by using GE/(Py-PPDO)2-b-PEG@MWCNTs as hybrid nanofiller exhibited a greatly improved mechanical property at very low hybrid nanofiller content.

Nano aggregates in aqueous medium with a novel “star anise”-like morphology were prepared from a branched alternating multi-block copolymer composed of 3-arm star-like hydrophobic poly(p-dioxanone) block and linear hydrophilic poly(ethylene glycol) block. The influence of block length on the morphology of the nano aggregate was investigated.

Nanofibers from poly(lactic acid) (PLA) homopolymer and poly(p-dioxanone)-b-poly(ethylene glycol) multi-block copolymer (PPDO-b-PEG) with different phase separation morphologies depending on the crystallization induced self-assembly of PPDO-b-PEG are prepared by single spinneret electrospinning. Mixing solvents of chloroform–dimethyl formamide (CHCl3–DMF) with different compositions are used for controlling the crystallization of the PPDO block and therefore the phase separation in the obtained nanofibers. The crystallization of the PPDO block has a very important influence on the morphology of the PPDO-b-PEG nanoparticles in the spinning solution. In spinning solutions with low DMF content, nanoparticles with irregular shapes and non-compact inner structures are formed because the degree of crystallization of the PPDO block is relatively low, and a discontinuous sea-island phase separation is formed in the obtained electrospun nanofibers. Meanwhile, in spinning solutions with high DMF content, the copolymer can form flake-like nanoparticles with a relatively high degree of crystallization. The flake-like shape favors compact aggregation of the PPDO phase during formation of the nanofibers, and a continuous core–shell phase separation of the nanofibers is obtained.