Si-Chong Chen

Multi-stimuli sensitive supramolecular hydrogel formed by host–guest interaction between PNIPAM-Azo and cyclodextrin dimers

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.

Synthesis and micellization of amphiphilic multi-branched poly(p-dioxanone)-block-poly(ethylene glycol)

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.

Dynamic Origin and Thermally Induced Evolution of New Self‐Assembled Aggregates from an Amphiphilic Comb‐Like Graft Copolymer: A Multiscale and Multimorphological Procedure

In the past several decades, the self-assembly of block copolymers in selective solvents has attracted extensive interest due to the formation of various aggregates including spherical and cylindrical micelles, vesicles, tubes, helices, toroids, and other complex forms. Research has been focused on aggregates with spherical, flowerlike, tubular or sheet-like superstructures, obtained through the hierarchical self-assembly of Janus micelles or Janus nanoparticles, owning to their formation mechanisms and potential applications in biomedical materials and new nanodevices. The self-assembled aggregates can also be constructed from amphiphilic comb-like graft copolymers in water or organic solvents. Despite that many interesting aggregates including petal-like micelles, spindle-like micelles, wormlike micelles, chiral helices, and so on, are obtained by the change of structural and environmental parameters, in most cases conventional spherical micelles and vesicles are observed. In comparison with block copolymers, self-assembled aggregates of amphiphilic comb-like graft copolymers reveal the morphological characteristics of obviously low diversity and complexity. Considering the shape plays a crucial role in determining physical and chemical properties of aggregates, the controlled fabrication and switch of morphologies have been paid a great attention, which has been implemented by regulating the conditions, such as solvent, pH, redox, and so on. Typically, crystallization is an important factor utilized to control the self-assembly of semicrystalline block copolymers consisting of the crystallizable block and amorphous block, which has been actively researched recently. Inspired by this specific class of copolymer, we have designed an amphiphilic comb-like graft copolymer consisting of poly(p-dioxanone) (PPDO) as a crystallizable hydrophobic side chain and poly(vinyl alcohol) (PVA) as a hydrophilic main chain, and found that this copolymer presented an unique self-assembled behavior. In fact, recently we reported a “star anise”-like nanoaggregate from a PPDO-based branched alternating multi-block copolymer. Herein, by directly dispersing this PPDO-based amphiphilic comb-like graft copolymer into water, more complex and regular aggregates with a well-defined snowflake-like superstructure was obtained, and their origin involving a dynamically disorder–order change from nanoto submicroscale was visualized. Moreover, the aggregates showed thermally induced multimorphological evolution from a snowflake-like to cluster-like structure. As an original and significant work, this superstructure enriches the self-assembled morphology of amphiphiles, especially comb-like macromolecules. Furthermore, the direct water phase self-assembled strategy of PPDO-based amphiphilic copolymers and temperature-adjusted morphological change could provide a new idea to construct complex multimorphological and multiscale objects. The precursors of the copolymers were synthesized through ring-opening polymerization and the acylation, respectively. The copolymers were prepared in DMSO by a coupling reaction between carboxyl groups presenting on PPDO chain-end and hydroxyl groups of PVA (Scheme 1). The information of the molecular structure of copolymers is listed in the Supporting Information, Table S1. The self-assembly was achieved by adding PVA500-g5.6%PPDO15 to water at room temperature, heating the system to homogenous phase (to erase the thermal history), and aging it at 25 8C. Tests of dynamic light scattering (DLS) indicated the presence of monodispersed and stable particles with 791 nm of intensity-averaged hydrodynamic diameter ( ) and a polydispersity index (PDI) of 0.053 (Figure 1a, their and PDI after 7 days was shown in the Supporting Information, Figure S1). Transmission electron microscopy (TEM) images revealed well-defined snowflakelike aggregates having about 260 nm of average thickness, 530 nm of average width, and 620 nm of average length, respectively (marked by short arrows in Figure 1band 1c). For monodisperse spherical aggregates, they have same diffusion coefficient in every direction, and their size from CONTIN analysis is independent of scattering angles. Conversely, for the anisotropic or polydisperse aggregates, the results of DLS are dependent of scattering angles. By multi-angle DLS experiment, we found that the diameter and apparent [a] G. Wu, Dr. S.-C. Chen, Dr. X.-L. Wang, Dr. K.-K. Yang, Prof. Y.-Z. Wang National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan) State Key Laboratory of Polymer Materials Engineering College of Chemistry, Sichuan University 29 Wangjiang Road, Chengdu 610064 (P.R. China) Fax: (+86)28-85410259 E-mail : chensichong@scu.edu.cn yzwang@scu.edu.cn Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201103961.

Nanofibers with Very Fine Core-Shell Morphology from Anisotropic Micelle of Amphiphilic Crystalline-Coil Block Copolymer

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.

Biodegradable polylactide based materials with improved crystallinity, mechanical properties and rheological behaviour by introducing a long-chain branched copolymer

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-e-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.

Temperature dependent morphological evolution and the formation mechanism of anisotropic nano-aggregates from a crystalline-coil block copolymer of poly(p-dioxanone) and poly(ethylene glycol)

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.

Photothermal Conversion Triggered Precisely Targeted Healing of Epoxy Resin Based on Thermoreversible Diels–Alder Network and Amino-Functionalized Carbon Nanotubes

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.

Phase separation in electrospun nanofibers controlled by crystallization induced self-assembly

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.

PREPARATION AND CHARACTERIZATION OF BIODEGRADABLE POLY(p‐DIOXANONE)/HYDROXYAPATITE COMPOSITES

Poly(p‐dioxanone) (PPDO)/hydroxyapatite (HA) composites with high viscosity‐average molecular weights (say, 343,000 g/mol) were prepared through the in situ ring‐opening polymerization of p‐dioxanone (PDO) with HA nanopaticles (n‐HA) and organically modified HA (EG‐HA) in the presence of triethylaluminum as the catalyst. The structures of n‐HA and EG‐HA synthesized were characterized by Fourier transform infrared spectroscopy and powder X‐ray diffractometer. Scanning electron microscopy and transmission electron microscopy were used to observe the dispersibility and morphology of nanopaticles in the PPDO matrix and the ethanol solution, respectively. The thermal properties were examined with differential scanning calorimeter and thermogravimetric analyzer. In conclusion, the PPDO/EG‐HA composites have better thermal stability and higher overall crystallization rate than both the PPDO/HA composites and the neat PPDO. A nucleating effect of n‐HA was observed as the crystallization temperature of PPDO increased.

Fennel-like nanoaggregates based on polysaccharide derivatives and their application in drug delivery.

Biodegradable polymeric nanoparticles, which hold hierarchical morphologies, are of importance for controlled drug delivery. In this work, nanoparticles with fennel-like morphology were prepared from graft copolymers of hydroxyethylcellulose and poly(p-dioxane). The effect of microstructure parameters on the morphological transition of the nano-aggregates was studied and the nanoparticles were investigated as a carrier of hydrophobic drug. The morphology and drug release property of the nanoparticles were found to be related with the degree of substitution and molecular weight of graft chain of the copolymer.