Polymer hydrogels that are capable of spontaneously healing injury are being developed at a rapid pace because of their great potential in biomedical applications. Here, the self‐healing property of tough graphene nanocomposite hydrogels fabricated by using graphene peroxide as polyfunctional initiating and cross‐linking centers is reported. The hydrogels show excellent self‐healing ability at ambient temperature or even lower temperatures for a short time and very high recovery degrees (up to 88% tensile strength) can be achieved at a prolonged healing time. The healed gels exhibit very high tensile strengths (up to 0.35 MPa) and extremely high elongations (up to 4900%). The strong interactions between the polyacrylamide chains and the graphene oxide sheets are essential to the mechanical strengths of the healed gels.
Summary: Thermogravimetry and differential scanning calorimetry have been used to study the thermal and thermo‐oxidative degradation of polystyrene (PS) and a PS–clay nanocomposite. An advanced isoconversional method has been applied for kinetic analysis. Introduction of the clay phase increases the activation energy and affects the total heat of degradation, which suggests a change in the reaction mechanism. The obtained kinetic data permit a comparative assessment of the fire resistance of the studied materials.
The change in activation energy for the degradation of PS and the PS–clay nanocomposite with the extent of polymer conversion. 相似文献
Polymer hydrogels with characteristics distinct from those of solid materials are one of the most promising candidates for smart materials. Here, we report that a nanocomposite hydrogel (NC gel) consisting of a unique polymer/clay network structure, can exhibit complete self‐healing through autonomic reconstruction of crosslinks across a damaged interface. Mechanical damage in NC gels can be repaired without the use of a healing agent, and even sections of NC gels separated by cutting, from whichever the same or different kinds of NC gel, perfectly (re‐)combine by just contacting the cut surfaces together at mildly elevated temperatures. In NC gels, the autonomic fusion of cut surfaces as well as the self‐healing could be achieved not only immediately after being cut but also after a long waiting time.
Nanoparticles in a flexible polymer melt film often segregate to the substrate due to attractive depletion interactions between the nanoparticles and the substrate. Here, molecular dynamics simulations are performed to study the effect of chain stiffness on this segregation. The nanoparticles are modeled as spheres and the polymers as semi‐flexible bead‐spring chains. Both purely repulsive and attractive forces are considered, while assuming non‐selective interactions among all species. The nanoparticles are found to be well‐dispersed in the system having repulsive forces only and aggregate into clusters in the completely attractive system. For the repulsive system, adding chain stiffness substantially decreases the nanoparticles' segregation, and hence their concentration, at the substrate.
Summary: Nanocomposites were formulated by curing a sonicated mixture of epoxy resin, C18 clay, and acrylic rubber dispersants. At 5.5 phr (parts per hundred) organoclay loading and a rubber concentration of 15 phr, the tensile‐failure strain of the nanocomposite was found to be higher than that of epoxy nanocomposite, rubber‐dispersed epoxy, and pristine epoxy. A plausible mechanism for improvement of the failure strain of nanocomposites is proposed.
Stress strain curves of filled and unfilled epoxy specimen. 相似文献
Summary: A new strategy was developed to prepare disorderly exfoliated nanocomposites, in which a soft siloxane surfactant with a weight‐average molecular weight ( ) of 1 900 was adopted to modify the clay. The modified clay slurry was then mixed with silicone rubber by hand, and exfoliation was achieved. The proposed mechanism thereof was verified by TEM and XRD. The physical entanglement of the soft siloxane surfactant plays a vital role in the diffusion and intercalation of the matrix molecules during the compounding of the slurry‐polymer mixture. This simple method is applicable to other silicone‐based materials reinforced by clay.
TEM micrograph of silicone rubber/clay‐sil nanocomposite. 相似文献
This review article describes the preparation of polymer brushes by nitroxide‐mediated radical polymerization using either the ‘grafting to’ or the ‘grafting from’ approach. The use of TEMPO as a classical initiator is intensively described. More sophisticated nitroxides are also included in the discussion. Brush formation on flat surfaces such as wafers and also on particles is reported. Finally, some applications of polymer brushes are presented.
Summary: Migration of clay to the surface of nylon 6‐organically modified clay is investigated. The effect of annealing time and temperature on the migration of the clay is reported. Attenuated total reflectance FT‐IR spectrometry, X‐ray diffraction, and high‐resolution electronic microscopy are used in this study. The results obtained indicate that migration occurs predominantly in the samples with exfoliated structure. Migration increases in the temperature range of 250–275 °C. A further increase in temperature decreases the extent of migration. Migration increases with time at 250 °C. Annealing in the presence of oxygen decreases migration. It is suggested that the extent of migration depends on the concentration of the surfactant and the polymer in the exfoliated particles.
High‐resolution electronic microscopy image of an exfoliated nylon‐6/clay sample in which migration has occurred. 相似文献
Sixteen parallel polymerization reactions of 2‐ethyl‐2‐oxazoline have been performed at different temperatures in an automated synthesizer that allowed individual heating of each reactor. During the reactions samples were taken automatically, which were characterized by means of both online GPC and offline GC, in order to optimize the reaction temperature and to determine the activation energy of the polymerization.
Photolabile polymer brushes with tailored length containing a photoremovable protecting group (NVOC) are prepared via the SI‐ATRP method. Upon light irradiation, the NVOC group is removed to generate controlled densities of free amine groups (PAMA) randomly distributed along the brush. The presence of the ionizable groups induces a photo‐triggered swelling response. The swelling degree can be tuned by the irradiation dose. A dual (light and pH), tunable response is demonstrated.
A directed diffusion approach is used to create atomistic models of crosslinked epoxy. In polymerization‐based approaches for preparing epoxy model structures, conversions higher than 95% are difficult to achieve due to very slow diffusion of unreacted monomers and crosslinkers in the partially formed network. This problem is overcome by creating very long bonds in the polymerization stage, and then relaxing these to equilibrium values by using a directed‐diffusion‐based relaxation strategy. The method minimizes the use of custom code by relying on the in‐built functionality in LAMMPS package (S. Plimpton, J. Comput. Phys. 1995 , 117, 1). The approach allows for near‐complete conversion (≈99%) and the thermal and volumetric properties of the structures so prepared show good agreement with experimental data.
These studies provide evidence for the ability of a commercially available, defined, hyaluronan‐gelatin hydrogel, HyStem‐C?, to maintain both mouse embryonic stem cells (mESCs) and human induced pluripotent stem cells (hiPSCs) in culture while retaining their growth and pluripotent characteristics. Growth curve and doubling time analysis show that mESCs and hiPSCs grow at similar rates on HyStem‐C? hydrogels and mouse embryonic fibroblasts and Matrigel?, respectively. Immunocytochemistry, flow cytometry, gene expression and karyotyping reveal that both human and murine pluripotent cells retain a high level of pluripotency on the hydrogels after multiple passages. The addition of fibronectin to HyStem‐C? enabled the attachment of hiPSCs in a xeno‐free, fully defined medium.
A simple method to fabricate polymer nanocomposites with single‐walled carbon nanotubes is reported, in which the nanotubes were reacted with poly(L ‐lysine) by using high‐speed vibration milling. The nanocomposites obtained were characterized by Fourier transform infrared (FT‐IR), UV–Vis spectroscopy, and thermogravimetric methods. The morphology as well as the dispersion of the carbon nanotubes were determined by scanning and transmission electron microscopy.
The storage moduli, shear moduli and surface morphologies of poly(vinyl alcohol) (PVA) and alumina hybrid hydrogels were investigated. The storage moduli of hybrid hydrogels with higher alumina contents were found to be 1.5 times higher than those of PVA gels. This increase in modulus might be attributed to the cohesion of alumina to the PVA network.
SEM photograph of Al7 PVA/alumina hybrid hydrogel. The photograph was taken with a magnification of × 220. 相似文献
The aim of the study is the investigation of the percolation phenomena in some model copolymer systems. Diblock, triblock, random copolymers, and a blend of homopolymers are studied. For this purpose, we developed an idealized model of polymeric systems. The positions of polymer segments are restricted to vertices of a simple cubic lattice. The chains are at good solvent conditions – the excluded volume is the only interaction between the segments of the chain. The properties of the model chains are determined by means of Monte Carlo simulations with a sampling algorithm based on chain's local changes of conformation. The differences and similarities in the percolation behavior are shown and discussed. The percolation threshold is found to be very weakly dependent on the chain length, however, it appears that the main factor that influenced the percolation threshold is the screening effect of other parts of chains.
Films of an α‐cyclodextrin/poly(ε‐caprolactone) inclusion complex have been successfully prepared and show high transparency and heat resistance in comparison to the pure polymer film. The physical properties, such as transparency, mechanical properties, and thermal stability, of the α‐CD‐PCL‐IC films are found to depend on the α‐cyclodextrin‐to‐polymer stoichiometry.
The present features review article discusses the crystallisation of the polymer matrix when containing silicate layers. The accent is put on nylons (polyamides) and poly(ethylene oxide) as typical hydrophilic polymers and, poly(propylene) from the hydrophobic group. The effects of the clay, either intercalated or exfoliated, on the crystallisation behaviour of the matrix are highlighted. In addition, the crucial aspects of the semicrystalline morphology of the matrix in the presence of the clay platelets are also debated. The overall crystallisation rate is reported to slow down for most of the crystallisable polymer matrices on account of a retarding growth effect exerted by the clay platelets. As far as the location of the exfoliated clay platelets in the polymer matrix is concerned, they are assumed to be rejected from the crystalline phase in the interspherulitic space.
