Weixiang Sun

Shape memory hydrogels containing over 76 wt% of water were synthesized in one-pot method and interpenetrating double network was formed by physically cross-linked gelatin network and chemically cross-linked polyacrylamide (PAM) network with graphene oxide (GO). The temporary shape was quickly fixed by cooling in ice water for 30 s after deformation at 80 °C for 10 s. Shape recovery started in 10 s under near infrared (NIR) irradiation and almost completed within 60 s depending on the curling angle. Small amount of GO in the hydrogels (≤1.5 mg/mL) played a key role of NIR energy absorption and transformation into thermal energy. The hydrogel without GO showed nonresponse to the NIR irradiation and cannot recover to its permanent shape by NIR irradiation. Temperature sweep was conducted in the cycle of 20 °C → 80 °C → 20 °C and the structure change in the hydrogels with temperature was investigated according to the storage modulus G' and tangent of the loss angle tan δ as a funct...

Rapid response and strong mechanical properties are desired for smart materials used in soft actuators. A bioinspired hybrid hydrogel actuator was designed and prepared by series combination of three trunks of tough polymer-clay hydrogels to accomplish the comprehensive actuation of…

ABSTRACT A facile method was explored to reinforce the poly(N-isopropylacrylamide) (PNIPAm)-hectorite nanocomposite hydrogel (NC gel) by heat treatment at temperature above the volume phase transition temperature (VPTT), which enhanced mechanical strength and reduced swelling capacity with denser microporous structure as compared with the non-treated NC gel. This reinforcement effect was increased by increasing treating temperature and/or treating time, which still remained even after swelling, indicating an irreversible structure change in the treated NC gels. Heat treatment became more effective when the NC gel contained higher clay content. The quartz crystal microbalance (QCM) data showed that the interaction between PNIPAm chains and clay platelets became stronger at temperatures higher than the VPTT, causing an increase in the crosslinking density of the treated NC gels. A strong hydrogel was obtained by heat treatment of the aqueous mixture of PNIPAm and hectorite clay for the first time. POLYM. COMPOS., 2014. © 2014 Society of Plastics Engineers

Dynamic polymer hydrogels with an environmental adaptive self-healing ability and dual responsive sol–gel transitions were prepared by combining acylhydrazone and disulfide bonds together in the same system. The hydrogel can automatically repair damage to it under both acidic (pH 3 and 6) and basic (pH 9) conditions through acylhydrazone exchange or disulfide exchange reactions. However, the hydrogel is not self-healable at pH 7 because both bonds are kinetically locked, whereas the hydrogel gains self-healing ...

ABSTRACT Stimulus-responsive hydrogels are utilized as smart materials in actuators for transforming external stimuli into actuation movements. Infrared (IR) irradiation is considered to be an ideal driving energy because it can penetrate into biomaterials without direct contact and can be remotely controlled. In the present work, a new IR-driving bilayer hydrogel actuator is prepared by stacking a graphene oxide (GO)-hectorite clay-poly(N-isopropylacrylamide) (PNIPAm) gel layer onto a hectorite clay-PNIPAm gel layer, synthesized through stepwise in situ polymerization. GO in the gel absorbs the IR irradiation and rapidly and efficiently transforms it into thermal energy, resulting in a much faster temperature increase in the GO-containing gel layer than that of the gel layer without GO, and the temperature of the former becomes higher than that of the latter. This bilayer structure with different temperatures changes the isotropic volume contraction into an anisotropic deformation, i.e., bending, which is always toward the GO-containing layer. Moreover, this bending occurs in the atmosphere, owing to the self-supporting capability of the tough gels. The repetition of the bending recovery is realized by turning the IR light on and off. According to these observations, the bilayer gel with GO provides a tough and IR-driving material for new soft actuators.

ABSTRACT The present paper reports an interesting and versatile thermo-moldable property of poly(N,N-dimethylacrylamide)-clay nanocomposite hydrogels (NC gel). The NC gel can be thermo-pressed and molded to a desired shape at 80 °C, and the shape can be fixed by cooling to 20 °C. Mechanical test and small-angle X-ray scattering demonstrate that the molded NC gel still exhibits high strength with an elastic network structure. Viscoelastic measurements show that with increasing the temperature, the equilibrium shear modulus Ge decreases, and the creep and permanent deformation increases obviously, suggesting a decrease in the cross-linking density. Ge recovers when the gel is cooled down to room temperature, reflecting the recovery of the cross-linking density. This thermo-moldable property is attributed to reversible cross-linking in such NC gels by changing the temperature. This work provides a facile method to deform or pattern the tough NC gels after synthesis.

The binding interaction between poly(N-isopropylacrylamide) (PNIPAm) chains and hectorite clay platelets was directly detected by quartz crystal microbalance with dissipation (QCM-D) to explore the cross-linking mechanism in the strong nanocomposite hydrogel (NC gel), which is in situ polymerized with NIPAm in the clay suspension. PNIPAm chains were allowed to be adsorbed on the gold surface of the QCM electrode. A large frequency shift Δf in the QCM as introducing the clay indicated that a large amount of clay platelets were adsorbed on the deposited PNIPAm layer. The relationship between the dissipation shift ΔD and Δf revealed that the adsorption included two steps of fast initial buildup and following densification of the clay platelets. In dilute aqueous mixtures, the PNIPAm chain and clay formed aggregates as observed from the hydrodynamic diameter. A gelation state diagram was established for concentrated aqueous PNIPAm-clay mixtures. The Raman spectrum pointed out the confor...

Self-healing hydrogels were proposed to be used as biomaterials, because of the capability of spontaneously healing injury, but most of the reported self-healing hydrogels do not possess high mechanical strength and fast self-healing at the same time. Herein, we prepared graphene oxide (GO)-hectorite clay-poly(N,N-dimethylacrylamide) (PDMAA) hybrid hydrogels with enhanced mechanical properties and fast self-healing capability realized by near-infrared (NIR) irradiation. The physical cross-linking between clay sheets and PDMAA chains provided the hydrogel with mechanical strength to maintain its stability in shape and architecture. GO sheets in the hybrid hydrogels acted as not only a collaborative cross-linking agent but also as a NIR absorber to absorb the NIR irradiation energy and transform it to thermal energy rapidly and efficiently, resulting in a rapid temperature increase of the GO containing gels. The chain mutual diffusion and the reformation of physical cross-linking occu...

In the present work, hydrophilic monomer acrylamide (AM) was copolymerized with N-isopropylacrylamide (NIPAm) in an aqueous hectorite clay suspension to prepare PNIPAm-PAM-clay nanocomposite hydrogels (NC gels). With increasing AM content, the elongation at break of the copolymerized NC gels increased but the strength as well as the hysteresis during the loading-unloading cycle decreased, showing faster relaxation due to the more hydrophilic copolymer chains with the AM segments. The elongation at break of the copolymerized NC gels was independent of the notch length and notch type, while the fracture energy was greatly increased to 3000-5000 J m(-2) from 700 J m(-2) for the pure PNIPAm NC gels. The copolymer chains resulted in this notch insensitivity by easily dispersing the stress concentration at the notch tip through disorientation of the copolymer chains and clay platelets. The copolymerized NC gels also exhibited excellent self-healing capability; the cut surfaces were connected together by simply keeping in contact for a period of time (about 4 days at 20 °C). This self-healing was accelerated by increasing the treatment temperature (about 4 h at 80 °C).

The proper name of the product in this Macromolecules article is “Laponite ® ”, which is a synthetic hectorite. This term was originally published without the registered trademark symbol. ... Log in with your ACS ID. If you do not have an ACS ID, please register for one. ACS IDs are free, and you do not need to be a member to register. ... Because you do not have JavaScript enabled, the Quick Search functionality is not displayed here. Please enable JavaScript or use the Advanced Search Page.

ABSTRACT Graphene oxide (GO) based hydrogels were proposed to be used as biomaterials and stimuli-response materials, but their poor mechanical properties restricted their applications. We enhanced GO-poly(N-isopropylacrylamide) (PNIPAm) hydrogels by hybrid with the hectorite clay through in situ polymerization for the first time. This clay was found to stabilize the GO in the aqueous suspension when a reducer was added in a redox initiating pair. These GO-clay-PNIPAm hybrid hydrogels exhibited a high mechanical strength and extensibility with the GO sheets as the cross-linker and with the hectorite clay as both the cross-linker and reinforcing agent. They were thermal-responsive with the volume phase transition at similar to 34 degrees C. Reduction of the GO with L-ascorbic acid under environmental friendly conditions resulted in a high conductivity to the graphene-clay-PNIPAm hydrogels. These graphene-clay-PNIPAm hydrogels still had desirable mechanical properties. This finding has provided an easy method to prepare strong and stimuli-response graphene-polymer hydrogels to meet the demand for the newly developed soft matter. (c) 2013 Elsevier Ltd. All rights reserved.

The aging of salt-free hectorite suspensions with different concentrations (cL = 2.9, 3.2 and 3.5 wt%) stored for 2 days or 4 days was studied by rheology at different temperatures.  The evolution of storage and loss moduli G’ and G” during aging followed aging time-temperature superposition.  The temperature dependence of the shift factor aT, which reflected the aging kinetics, was interpreted by the reaction-limited colloidal aggregation (RLCA) mechanism with counterion condensation in calculating the double-layer interaction of the charged clay particles.  Temperature dependence of the plateau modulus and yield stress of the suspension aged for 800 s was modeled with the soft glassy rheology (SGR) theory.  The estimated noise temperature x indicated that the sample aged at higher temperature corresponded to a deeper quench in the nonergodic state.  Under larger amplitude of oscillatory shear, the suspension exhibited a strain rate-frequency superposition (SRFS).  The shearing eliminated the effects of aging and heating.

The effects of the concentration of polyethylene glycol (PEG) and NaCl on the gelation kinetics of aqueous Laponite suspensions were investigated. The PEG concentration cp was increased from 0.063 wt.% to 1.0 wt.%, and the NaCl concentration cs was increased from 3.0 mM to 5.0 mM. The gelation process was monitored with the small amplitude oscillatory shear (SAOS) rheology. At each NaCl concentration, the time curves of storage and loss moduli G′ and G″ at different PEG concentrations were shifted horizontally to construct a master curve. Master curves for different NaCl concentrations were shifted further into a main master curve. The results were understood in terms of an increase in steric repulsion with increasing cp. A model based on the theory of interaction-limited coagulation was proposed to describe the dependence of the shift factor on the PEG concentration, taking into account of the interactions between particles. The quantitative description of the observed results has been achieved with this model.

The effect of poly(ethylene glycol) (PEG) on the nonlinear viscoelasticity of Laponite suspensions containing NaCl was investigated with large amplitude oscillatory shear rheology. The molecular weight (Mw) of PEG was 4k, 10k, and 35k, and the concentration of PEG was varied from 0.063 wt% to 2.4 wt%. The dynamic strain sweep showed that the nonlinearity appeared at \gamma_0 > 30% with a stress overshoot at \gamma_0 = 50% ~ 70%. The intensity ratio I_{3/1} from Fourier-transform increased with gamma_0 when entering the nonlinear regime and leveled off at \gamma_0 >= 100% with higher slope and constant value of the PEG of higher Mw or lower concentration. I_{3/1} revealed the structure difference in the suspensions induced by adsorbing PEG in the nonlinear regime. The minimum- and large-strain rate viscosities \eta_M and \eta_L appeared at lower \gamma_0 with higher maximum following the same dependency as I_{3/1} on PEG Mw and concentration. The overall nonlinearity parameters N_E and N_V were proposed in this paper and demonstrated to reflect the difference in the laponite suspensions with PEG more clearly and more effectively.