Search

Smart hydrogels have attracted widespread attention and have displayed comprehensive applications in many fields. The development of a novel type of bilayer hydrogel with synergistic function of anisotropic deformation and photochromic property is extremely challenging. Many previous researches have focused on multiple response hydrogels. However, it hardly achieves the synergy of deformation and color-changing through a method with the advantages of simplicity, stability, practicality, and precise control. Herein, a near-infrared light (NIR) photothermal thermoresponsive graphene oxide/poly(N-isopropylacrylamide-β-cyclodextrin) (PNIPAM-β-CD/GO) hydrogel layer and a reversible photochromic ammonium molybdate tetrahydrate/poly(N-isopropylacrylamide-co-β-cyclodextrin) (PNIPAM-β-CD/Mo7) hydrogel layer were combined to obtain a bilayer hydrogel through the host–guest interactions between β-CD and the isopropyl group of NIPAM, exhibiting integrate anisotropic shape deformation and reversible photochromic behaviors. The designed actuators can undergo complex deformation owing to volume shrinkage caused by the photothermal effect under NIR of the PNIPAM-β-CD/GO layer, while the PNIPAM-β-CD/Mo7 hydrogel layer shows the characteristics of quickly color changing within 5 s when irradiated by a UV light source. The multiple responsiveness of hydrogels on temperature, NIR irradiation, and UV irradiation displays valuable advantages in multifunctional smart materials, such as intelligent soft robots.

Semicrystalline polymers can crystallize in the unique crystalline polymorph and show different phase behaviors under the high-pressure CO2 treatment. Understanding such unique crystallization and phase transition behavior is of fundamental importance for the CO2-assisited processing of semicrystalline polymers. Herein, we investigated the polymorphic crystalline structure, phase transition, and structure-property relationships of poly(L-lactic acid) (PLLA) treated by CO2 at different pressures (1–13 MPa) and crystallization temperatures (Tc’s, 10–110 °C). PLLA crystallized in the PLLA/CO2 complex crystals under 7–13 MPa CO2 at Tc ≤ 50 °C but the common a crystals under the high-pressure CO2 at Tc ≥ 70 °C. Solid-state nuclear magnetic resonance analysis indicated that the PLLA/CO2 complex crystals possessed weaker interactions between the PLLA chains than the common a crystals. The PLLA/CO2 complex crystals were metastable and transformed into the thermally stable a crystals via the solid-to-solid route during heating or annealing at the temperature above 50 °C. The complex crystals of PLLA produced at low Tc was more ductile than the a crystals due to the lower crystallinity and the plasticizing effect of CO2.

Semicrystalline polymers usually undergo multilevel microstructural evolutions under high-temperature annealing and stretching deformation; this is essential to tailor the physical properties of polymer products in industrial processing. Here, we choose poly( p-dioxanone) (PPDO), a typical biodegradable, biocompatible, and bioresorbable polymer, as a model semicrystalline polymer and investigated its polymorphic structural transition and crystalline lamellar evolution under high-temperature annealing and stretching. High-temperature annealing caused the α'-to-α phase transition of PPDO, accompanied by the improvement of crystallinity ( Xc) and thickening of crystalline lamellae. Tensile strength and Young's modulus of PPDO increased but the breaking strain decreased as the annealing temperature increased. Stretch-induced phase transition of PPDO depended strongly on the initial structure and stretching temperature ( Ts). The α-form PPDO transformed into its α' counterpart during stretching at low Ts. This phase transition was irreversible and did not retain the α form with the release of stress. However, no phase transition took place for the α-form PPDO stretched at high Ts (≥40 °C). Original lamellae of α-form PPDO changed into the fibrillar lamellae during stretching via the melt-recrystallization mechanism.

Controlled and homogeneous free-radical polymerization of acrylamide (AM) in aqueous phase was realized by using S,S′-bis(α,α′-dimethyl-α″-acetic acid)-trithiocarbonate as a reversible addition-fragmentation transfer (RAFT) agent. Linear increases in molecular weight with conversion and narrow molecular weight distribution were observed for polyacrylamide (PAM) throughout the polymerization. By this method, PAMs with controlled molecular weight (up to 1.0 × 106) and narrow molecular weight distribution (M w/M n < 1.2) were prepared. This study provides an effective method for synthesis of PAMs with narrow molecular weight distribution under environmentally friendly conditions.

The aqueous two-phase copolymerization (ATPP) of acrylamide and cationic monomers was carried out in poly(ethylene glycol) (PEG) aqueous solution. The effects of inorganic salts on the ATPP process were systematically investigated and obvious differences were observed between univalent and multivalent anions. For the ATPP system with added univalent anions such as Cl−, the critical conversion for the phase separation decreased as the particle size increased, but the polymerization kinetics and final molecular weight of the copolymers were similar with that of the salt-free system. However, after adding multivalent anions such as SO42− into the ATPP system, the critical conversion for the phase separation decreased sharply, the polymerization rate slowed down, the final molecular weight of the copolymers increased, and the particle size greatly increased. Moreover, the system underwent a unique particle formation process. A mechanism for the role of anions in the ATPP process was proposed. The salting-out effects of the univalent anions screen the electrostatic repulsion between cationic species and lead to the formation of gravel-like particles. But for multivalent anions, when the concentration is low, their salting-out effect is involved in the particle formation process and results in the formation of gravel-like particles as well. When the concentration becomes high, their bridge effect is dominant, making large and spherical particles appear.

Nitroxide-mediated polymerization of methyl methacrylate (MMA) was carried out using 4,4′-dimethoxydiphenyl nitroxide-based alkoxyamine as a mediator. This alkoxyamine can be easily prepared and is able to control the nitroxide-mediated polymerization of MMA up to a conversion of 65%. A linear increase of the number-average molecular weight with conversion was observed and polymers with narrow molecular weight distributions (PDI = 1.2–1.4) were obtained. The living character was also confirmed by chain extension from the prepared poly(methyl methacrylate) macroinitiator with MMA or styrene. ESR (electron spin resonance) results indicate that the living chain fraction is more than 85%. Additionally, the MMA polymerization cannot be well controlled by the diphenylnitroxide-based alkoxyamine because of the unwanted cross combination reaction between two nitroxides; the prepared polymer shows a bimodal and broad molecular weight distribution.

An organic acid, camphorsulfonic acid (CSA) and three strong polar compounds, diethyl malonate (DEM), acetylacetone (AAT) and malononitrile (MN) were used to accelerate the polymerization of methyl methacrylate (MMA) mediated by 4,4′-dimethoxydiphenyl nitroxide (DMDPN). Polymerization with CSA proceeded quite fast but was not “living”. When DEM, AAT and MN were used as accelerators, MN supplied the best results. A conversion of 45% was achieved in 3 h with an optimal [MN]/[alkoxyamine] molar ratio of 3 : 1 and the dispersity index was even lower than that without any additive. Electron spin resonance (ESR) study showed the NO–C bond could be weakened by MN, resulting in a slightly increase in kd. Both of the weakening of the NO–C bond and the dipole–dipole interaction between MN and DMDPN attribute to the rate acceleration, while the dipole–dipole interaction plays the pivotal role.

Although stereocomplex (sc) crystallization is highly effective for improving the thermal resistance of poly(lactic acid) (PLA), it is much less predominant than homocrystallization in high-molecular-weight (HMW) poly(l-lactic acid)/ poly(d-lactic acid) (PLLA/PDLA) racemic blends. In this contribution, the sc crystallization of HMW PLLA/PDLA racemic blends was facilitated by using comblike PLAs with cellulose acetate as the backbone. Competing crystallization kinetics, polymorphic crystalline structure, and structural transition of comblike PLLA/PDLA blends with a wide range of MWs were investigated and compared with the corresponding linear/comblike and linear blends. The HMW comblike blend is preferentially crystallized in sc polymorphs and exhibits a faster crystallization rate than does the corresponding linear blend. The sc crystallites are predominantly formed in nonisothermal cold crystallization and isothermal crystallization at temperatures above 120 °C for the comblike blends. Except for the facilitated sc formation in primary crystallization, synchrotron radiation WAXD analysis indicates that the presence of a comblike component also facilitates the transition or recrystallization from homocrystallite (hc) to sc crystallite upon heating. Preferential sc formation of comblike blends is probably attributable to the favorable interdigitation between enantiomeric branches and the increased mobility of polymer segments. After crystallization under the same temperature, the comblike blends, which mainly contain sc crystallites, show smaller long periods and thinner crystalline lamellae than do the corresponding PLLA with homocrystalline structures.

Poly(N-isopropylacrylamide-co-acrylamide-co-maleic acid) (P(NIPAM-AM-MA)) hydrogel has been synthesized by free radical polymerization. The incorporation of functional monomer in the hydrogel was confirmed by Fourier transform infrared spectrometer (FTIR). Swelling measurements and differential scanning calorimeter (DSC) were employed to investigate the volume phase transition of P(NIPAM-AM-MA) hydrogel. P(NIPAM-AM-MA) shows higher swelling ratio and LCST than poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAM-AM)). The adsorption behavior of copper(II) (Cu2+) ions on P(NIPAM-AM-MA) hydrogel is temperature and pH-dependent. The adsorption isotherm is well fitted by the Freundlich model and the adsorption kinetics can be described by the pseudo-second order equation. In 20 mL of CuSO4 solution containing 4 mg Cu2+, the adsorption capacity reaches 24.4 mg g−1 dry hydrogel at 30 °C and pH = 5. After the volume phase transition, the Cu2+-loaded P(NIPAM-AM-MA) hydrogel can release almost 90% of absorbed water containing few heavy metal ion. Synchrotron radiation small-angle X-ray scattering (SAXS) was used to study the effect of temperature and Cu2+ ions on the microstructure of P(NIPAM-AM-MA) hydrogel. The occurrence of volume phase transition increases the size of cross-linked domains and mass fractal dimension, while the presence of Cu2+ ions has an opposite effect. The adsorbed hydrogel can be easily regenerated by hydrochloric acid and reused in the following adsorption process. This pH and temperature sensitive hydrogel may be used for water purification and enrichment of heavy metal ions.

A facile and nontoxic strategy for the preparation of amphiphilic quasi-block copolymers and nanoparticles has been developed utilizing miniemulsion polymerization. By volume phase transition behavior of poly(N-isopropyl acrylamide) (PNIPAM) and thermally induced interfacial absorption, amphiphilic copolymer was generated, which can self-assemble into core–shell nanoparticles in situ. Stearyl methacrylate (SMA) was used as hydrophobic monomer and to form crystalline core, because it has pendent long alkyl side chain and can form crystalline domain to synthesize comb-like polymers. A volatile organic cosolvent was used to enhance the formation process and particles size, and the formation mechanism was investigated. Impacts of cosolvent concentration, monomer feed ratio and polymerization temperature on the morphology and thermal characteristics of particles were studied by DLS, TEM and DSC. 1H NMR, 13C NMR and GPC were carried out to analyze the chemical structure and composition of copolymers. The triads sequence and average block length are analyzed to determine the sequence distribution of copolymers. Volume phase transition temperature (VPTT) of PNIPAM copolymer was first increased to nearly 40 °C with hydrophobic monomer rather than hydrophilic monomer. In addition, hydrophilic property of copolymer and the drug release behavior were studied to understand further applications. These amphiphilic quasi-block copolymers show great potential in the controlled delivery system for drugs.

The increasing use of rare earth elements (REEs) in a wide range of high-tech applications, leading to global ever-increasing demands, has stimulated the development of new technologies for REEs separation and recovery. Herein, an efficient porous graphene oxide/poly (N-isopropyl acrylamide-maleic acid) [GO/P(NIPAM-MA)] cryogel was synthesized by cryopolymerization and was employed as the REEs recovery adsorbent. The cryogel exhibits stable, microporous interpenetrating structure and thermosensitive characteristics. The adsorption follows the pseudo-second-kinetic model indicating that the adsorption process was controlled by the adsorption sites with the equilibrium adsorption capacity of La3+ was 33.1 mg/g. The Langmuir isotherm model fits the data better, revealing the homogenous adsorption for REEs on the surface of the adsorbent. Besides, the adsorbent displayed preferential adsorption of La3+ compared to Cu2+, Co2+, Ni2+, Nd3+, Yb3+ in a binary solution suggested the high level of La3+ separation efficiency that exceeded 95%. In general, the eco-friendly, sustainability and higher efficiency characteristics of GO/P(NIPAM-MA) cryogel provide a promising route for recycling rare earth elements from wastewater.

An interpenetrating network (IPN) hydrogel with a highly enhanced elongation-at-break has been prepared using poly(ethylene glycol) (PEG)-swollen poly(2-acrylamide-2-methylpropane sulfonic acid) (PAMPS) as the first network and polyacrylamide (PAM) as the second network. The new IPN hydrogel of PAMPS–PEG/PAM has remarkably high elongation-at-break (∼2100%) in tensile deformation, which is 4 times larger than common PAMPS/PAM IPN hydrogel and PAMPS/PAM–PEG IPN hydrogels synthesized by incorporating PEG into the second network. The microstructures of single network (SN) and IPN hydrogels were investigated by synchrotron radiation small-angle X-ray scattering (SAXS). SAXS results were analyzed by Guinier, Ornstein–Zernike (OZ), and generalized Ornstein–Zernike (GOZ) models. It was found that the incorporated PEG increases the size of cross-linked domains and decreases the fractal dimension of domains. The toughening mechanism of PEG on IPN hydrogel was discussed. It is proposed that the largely enhanced toughness of PAMPS–PEG/PAM IPN hydrogel is due to the increased size of physical cross-linked domains, hydrogen bonding between the first and second network, and the increased pull-out resistance of PAM chains under deformation.

Polylactide-b-poly(ethylene-co-butylene)-b-polylactide (PLA–PEB–PLA) triblock copolymers containing PLA segments with different stereo-regularities such as poly(L-lactide) (PLLA), poly(D-lactide) (PDLA), and poly(D,L-lactide) (PDLLA) were prepared via the ring-opening polymerization of various lactides using α,ω-dihydroxy PEB as the macromolecular initiator. Molecular weight and chemical composition of copolymers were adjusted by changing the monomer-to-initiator ratio. Morphological, thermal, mechanical, and shape memory behaviors of PLA–PEB–PLA were explored. As confirmed by small angle X-ray scattering (SAXS) and transmission electrical microscopy (TEM), PLA–PEB–PLA adopted ordered microphase-separated morphology, depending on the copolymer composition and crystallizability of PLA segments. Spherical, hexagonally packed cylindrical, and lamellar structures were observed in PLA–PEB–PLA upon increasing the volume fraction of PLA. However, the morphological order was diminished in PLLA–PEB–PLLA/PDLA–PEB–PDLA enantiomeric blends, due to the preferential stereocomplexation of PLLA and PDLA segments before microphase separation. PLA–PEB–PLA showed the properties of thermoplastic elastomers. Their Young's modulus and tensile strength increased while the strain at break decreased upon increasing the fraction of PLA hard segments or with the crystallization or stereocomplexation of PLA domains. Interestingly, PLA–PEB–PLA elastomers showed shape memory behavior, which could be controlled by the crystallizability of PLA hard segments.

The effects of poly(vinylidene fluoride) (PVDF) on the crystallization kinetics, competing formations of homocrystallites (HCs) and stereocomplexes (SCs), polymorphic crystalline structure, and HC-to-SC crystalline reorganization of the poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemic mixture were investigated. Even though the PLLA/PDLA/PVDF blends are immiscible, blending with PVDF enhances the crystallization rate and SC formation of PLLA/PDLA components at different temperatures that are higher or lower than the melting temperature of the PVDF component; it also facilitates the HC-to-SC melt reorganization upon heating. The crystallization rate and degree of SC crystallinity (Xc,SC) of PLLA/PDLA components in nonisothermal crystallization increase after immiscible blending with PVDF. At different isothermal crystallization temperatures, the crystallization half-time of PLLA/PDLA components decreases; its spherulitic growth rate and Xc,SC increase as the mass fraction of PVDF increases from 0 to 0.5 in the presence of either a solidified or a molten PVDF phase. The HCs formed in primary crystallization of PLLA/PDLA components melt and recrystallize into SCs upon heating; the HC-to-SC melt reorganization is promoted after blending with PVDF. We proposed that the PVDF-promoted crystallization, SC formation, and HC-to-SC melt reorganization of PLLA/PDLA components in PLLA/PDLA/PVDF blends stem from the enhanced diffusion ability of PLLA and PDLA chains.

Linear, 3-arm, and 6-arm star-shaped stereo diblock copolymers of l- and d-lactic acid (PLLA-b-PDLA) with high molecular weights (MWs) were synthesized via two-step ring-opening polymerization (ROP) with 1-dodechanol, glycerol, and d-sorbitol as the initiators, respectively. The chemical structure, nonisothermal and isothermal crystallization kinetics, crystalline structure, lamellar morphology, and mechanical thermal properties of PLLA-b-PDLAs with different macromolecular topologies were investigated. Compared to the high-molecular-weight (MW) poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemic blends, PLLA-b-PDLAs exhibit faster crystallization upon cooling and isothermal melt crystallization; they crystallize exclusively in stereocomplex (sc) crystallites under all of the conditions investigated. This is attributable to the enhanced interactions between enantiomeric blocks linked covalently. Macromolecular topology influences the crystallization kinetics and crystalline structure of PLLA-b-PDLAs significantly. The crystallization temperature upon cooling, melting temperature, degree of crystallinity, spherulitic growth rate, crystallite size, long period, and crystalline layer thickness of PLLA-b-PDLA decrease with increasing branching number because of the retarding effect of branching on the crystallization rate and crystallizability. Because of the formation of high-melting-point sc crystallites, both the linear and star-shaped PLLA-b-PDLAs exhibit better thermal resistance and higher storage moduli at high temperature than does homocrystalline PLLA.

Competitive crystallization kinetics, polymorphic crystalline structure, and transition of poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemic blends with a wide range of molecular weights (MWs) were symmetrically investigated. Stereocomplex (sc) crystallites are exclusively formed in the low-MW racemic blends. However, stereocomplexation is remarkably depressed, and homocrystallization becomes prevailing with increasing MWs of PLLA and PDLA. Suppressed stereocomplexation in high-MW (HMW) racemic blends is proposed to be due to the low chain diffusion ability and restricted intermolecular crystal nucleation/growth. Equilibrium melting point of sc crystallites first increases and then decreases as MW increases. Crystallinity and relative fraction of sc crystallites in racemic blends enhance with crystallization temperature (Tc), and the sc crystallites are merely formed at Tc170 °C because of their higher thermodynamic stability. In situ wide-angle X-ray diffraction (WAXD) analysis reveals that the stereocomplexation and homocrystallization are successive rather than completely simultaneous, and the stereocomplexation is preceding homocrystallization in isothermal crystallization of HMW racemic blends. Both initial crystalline structure of homocrystallites (hc) and MW influence the heating-induced hc-to-sc transition of HMW racemic blend drastically; the hc-to-sc transition becomes easier with decreasing Tc and MW. After crystallization at the same temperature, sc crystallites show smaller long period than their hc counterparts.

A novel in situ formed gel system with potential biodegradability and biocompatibility is developed by mixing the diblock and triblock poly(lactic acid)/poly(ethylene glycol) (PLA/PEG) copolymers with opposite configurations of PLA blocks. In situ gelation of such system is extremely fast, which happens within 10 s after mixing. In situ gelation, gel-to-sol transition, crystalline structure, microstructures, and mechanical properties of PLA-PEG/PLA-PEG-PLA enantiomerically mixed gels are significantly influenced by the mixing ratio, degree of polymerization for PEG block in triblock (DPPEG,tri) and diblock copolymers (DPPEG,di). It is found that in situ gelation of PLA-PEG/PLA-PEG-PLA enantiomeric mixture just happen at relatively smaller PLA-PEG/PLA-PEG-PLA mass ratio and larger DPPEG,tri. Hydrodynamic diameters of PLA-PEG and PLA-PEG-PLA copolymers in dilute solution increase remarkably upon mixing, indicating the formation of bridging networks. Stereocomplexed crystallites are formed for the PLA hydrophobic domains in PLA-PEG/PLA-PEG-PLA enantiomeric mixtures. As indicated by synchrotron-radiation SAXS analysis, the enantiomeric mixture changes from a compactly to loosely aggregated structure and the intermicellar distance enhances with increasing DPPEG,tri, DPPEG,di, or PLA-PEG-PLA fraction. Gelation mechanism of PLA-PEG/PLA-PEG-PLA enantiomeric mixture is proposed, in which part of PLA-PEG-PLA chains act as the connecting bridges between star and flower-like micelles and the stereocomplexed crystallites in micelle cores act as physically cross-linked points.

N -phenylmaleimide (PMI) is used to copolymerize with vinyl chloride (VC) for improving the heat resistance of PVC. The variations of instantaneous and integral copolymer compositions with conversion for VC/PMI copolymerization system were investigated quantitatively by use of the recurrence method. The copolymer composition variation could be controlled and optimized by means of selecting a third monomer and its content. For VC/PMI suspension copolymerization, acrylonitrile (AN) and methyl methacrylate (MMA) are preferably selected to improve copolymer composition homogeneity and heat resistance. There were different effects of control parameters on the selectable monomer feed fraction ranges. The influences of the third monomer on the ranges of monomer feed fraction with integral terpolymer composition error were studied in detail. Subsequently, on the basis of prediction, control and optimization for variation of copolymer composition with conversion and the suspension terpolymerization technology feature, the optimum regions of monomer feed fraction for VC/PMI/AN and VC/PMI/MMA terpolymerizations are obtained.

Nanogel (hydrophilic polymer nanoparticles) core-silica shell nanoparticles were successfully fabricated via hydrolysis and condensation reaction of tetraethoxysilane (TEOS). Transition-metal tetrafluoroborate-containing nanogels were used as templates for fabrication in inverse systems (cyclohexane as continuous phase). Magnetic, hollow silica particles were subsequently formed by removing the polymer core and converting iron salts to iron oxides via heat treatment. We propose that the formation of the core-shell morphology is induced by the promoted precipitation of silica species at the surface of nanogels due to the interaction between silica species and transition-metal tetrafluoroborate. The influence of the synthesis parameters (type and amount of salts, pH of the nanogels, and amount of TEOS) on the particle morphology was systematically investigated. The pore properties and specific surface area of the hollow silica particles could be modified by the varying the amount of salt.