Toughening of poly(L‐lactide)/multiwalled carbon nanotubes nanocomposite with ethylene‐co‐vinyl acetate

Poly(L ‐lactide)/multiwalled carbon nanotubes (PLLA/MWCNTs) nanocomposite recently attracts much attention because of its excellent comprehensive properties including improved thermostability, tensile strength, and conductivity. However, the nanocomposite exhibits similar brittleness compared with unmodified PLLA. In this work, a polar elastomer, that is, ethylene‐co‐vinyl acetate (EVA), was introduced into PLLA/MWCNTs nanocomposite. The selective distribution of MWCNTs and the effects of EVA on crystalline structure of PLLA were investigated using scanning electron microscope, transmission electron microscope, differential scanning calorimetry, and wide angle X‐ray diffraction. The results show that the presence of EVA induces the change of the distribution of MWCNTs in the nanocomposites, and consequently, the cold crystallization of PLLA is prevented. With the increase of EVA content, both the ductility and the impact resistance of PLLA/FMWCNTs are improved greatly, indicating the toughening effect of EVA on PLLA/MWCNTs nanocomposite. The decreased tensile strength and modulus can be compensated through annealing treatment. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Synthesis and characterization of semicrystalline triblockcopolymers of isotactic polystyrene and polydimethylsiloxane

iPS‐b‐PDMS‐b‐iPS triblock copolymers were prepared by hydrosilylation of vinyl‐terminated isotactic polystyrenes (iPS) with α,ω‐bis(dimethylsilane)‐terminated poly(dimethylsiloxane)s (PDMS). As a function of the molecular weights of the two components, the triblock copolymer composition was varied between 9.0 and 98 wt % iPS. The resulting triblock copolymers remained soluble during block copolymer synthesis due to slow iPS crystallization in solution. At iPS content exceeding 31 wt %, the iPS crystallization was achieved by postpolymerization annealing and melt processing. The triblock copolymers melted above 200 °C with melting temperatures very similar to those of the corresponding iPS homopolymers. Nanostructure and microstructure formation of both amorphous and semicrystalline triblock copolymers were examined by means of light microscopy, atomic force microscopy, and TEM measurements. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

A critical local energy release rate criterion for fatigue fracture of elastomers

Using Digital Image Correlation on high‐resolution images, the full strain field near the tip of a crack propagating under cyclic loading in an elastomer was characterized. We show unambiguously, and for the first time, the existence of a strongly localized and highly oriented process zone close to the crack tip and propose a simple physical model introducing a local energy release rate g_local = W_unloadingH₀, where W_unloading is the unloading strain energy density in uniaxial tension at the maximum strain measured at the crack tip, and H₀ is the undeformed size of the highly stretched zone in the loading direction. Remarkably, the crack growth rate under cyclic loading is found to fall on a master curve as a function of g_local for three elastomers with different filler contents and crosslinking densities, while the same crack growth rate as a function of the applied macroscopic energy release rate G, differs by two orders of magnitude for the same three elastomers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1518–1524, 2011     

Localised Actuation in Composites Containing Carbon Nanotubes and Liquid Crystalline Elastomers

A liquid crystalline elastomer–carbon nanotube (LCE‐CNT) composite displays a reversible shape change property in response to light. The development of some systems such as tactile devices requires localised actuation of this material. A method is reported that combines mechanical stretching and thermal crosslinking of an LCE‐CNT for creating sufficiently well‐aligned liquid crystal units to produce localised actuation. The method demonstrates that it is feasible to optically drive a LCE‐CNT film within a localised area, since only the walls of the stretched parts of the film contain aligned LC domains.     

Liquid‐Crystalline Soft Actuators with Switchable Thermal Reprogrammability

Thermal reprogrammability is essential for new‐generation large dry soft actuators, but the realization sacrifices the favored actuation performance. The contradiction between thermal reprogrammability and stability hampers efforts to design high‐performance soft actuators to be robust and thermally adaptable. Now, a strategy has been developed that relies on repeatedly switching on/off thermal reprogrammability in liquid‐crystalline elastomer (LCE) actuators to resolve this problem. By post‐synthesis swelling, a latent siloxane exchange reaction can be induced in the common siloxane LCEs (switching on), enabling reprogramming into on‐demand 3D‐shaped actuators; by switching off the dynamic network by heating, actuation stability is guaranteed even at high temperature (180 °C). Using partially black‐ink‐patterned LCEs, selectively switching off reprogrammability allows integration of completely different actuation modes in one monolithic actuator for more delicate and elaborate tasks.     

线形低密度聚乙烯／废胶粉热塑弹性体动态硫化性能研究

利用动态硫化法制备了线形低密度聚乙烯(LLDPE)/废胶粉(GTR)热塑弹性体。重点研究了两种交联剂:硫和过氧化二异丙苯(DCP)对共混物性能的影响。加入一定量的苯乙烯-丁二烯-苯乙烯(SBS)共聚物作为增容剂。结果表明,经过DCP动态硫化后的共混物的力学性能比简单共混的共混物有明显的提高,而加入硫磺体系对共混物力学性能影响不大甚至有所下降。通过红外光谱、热分析(DSC)和扫描电镜(SEM)对共混物的热行为和表面形态研究表明,加入DCP交联剂使LLDPE、SBS和胶粉之间发生了交联反应,从而增加了胶粉颗粒与LLDPE间的界面相容性,使其热塑性弹性体的力学性能得以提高。     

Damping of Magnetorheological Elastomers

The damping property of magnetorheological (MR) elastomers is characterized by a modified dynamic mechanical-magnetic coupled analyzer. The influences of the external magnetic flux density, damping of the matrix, content of iron particles, dynamic strain, and driving frequency on the damping properties of MR elastomers were investigated experimentally. The experimental results indicate that the damping properties of MR elastomers greatly depend on the interfacial slipping between the inner particles and the matrix. Different from general composite materials, the interfacial slipping in MR elastomers is affected bythe external applied magnetic field.     

Rapid,metal‐free room temperature vulcanization produces silicone elastomers

Silicone room temperature vulcanization elastomers are usually formed through either a platinum‐catalyzed hydrosilylation or tin‐catalyzed moisture cure. In this article, we show that it is possible to create robust, transparent silicone elastomers without the need for metal catalysts. Hydrogen‐terminated silicone polymers are crosslinked by tri‐ or tetraalkoxysilane crosslinkers in a condensation process catalyzed by the presence of trispentafluorophenylborane catalyst to give elastomers and alkane by‐products. This procedure allows for very fast cure times (< 30 s to a tack free state): the process is more conveniently controlled with the addition of a small amount of solvent. Physical and mechanical properties are readily modified by control of the chain length of the starting polymer, the functionality and nature of the alkoxy group on the crosslinker. Organofunctional groups, useful for further polymer modification, can optionally be incorporated by judicious choice of readily available starting materials. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Strain energy function of swollen polybutadiene elastomers studied by general biaxial strain testing

The stress–strain behavior of polybutadiene elastomers with various degrees of swelling is investigated for general biaxial strain. Although a number of previous observations have suggested that the classical neoHookean (NH) model describes the uniaxial data of an elastomer in a highly swollen state, the NH model evidently fails to reproduce the biaxial data. This result indicates that the successful fit of the NH model for the uniaxial data of highly swollen elastomers, which has long been recognized, is superficial. We show that the biaxial data in all of the various swollen states including the neat (unswollen) state are satisfactorily described by an Ogden‐type strain energy function with a single set of parameters. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 721–728, 2010     

A new spectral memory of filled rubbers

We recently discovered that shearing particle‐reinforced rubbers in oscillation at a frequency f_a at a small strain γ_a (e.g., ～1% strain) for time t_a can often produce a spectrum hole or drop in the strain‐dependent dissipation spectra of the materials. The location of the hole (or localized perturbation in the loss modulus or loss tangent) depends on the aging strain amplitude γ_a. The depth of this hole is influenced by both the oscillatory aging frequency f_a and the aging duration t_a, and follows a simple power relationship of the product of f_a and t_a. The exponent for the power relationship is a function of filler concentration. These attributes of the spectral hole in filled rubbers are not sensitive to the frequency used to postanalyze the hole. This new memory effect occurs at very small strains and involves material stiffening during the strain aging, and both of those features are quite different from the Mullins effect in filled elastomers. We interpret this newly discovered memory character of filled rubbers from a much broader concept of structure pinning in a condensed frustrated system and consider that the agglomeration of filler particles in rubber matrix shares common physics with granular materials and glass‐forming materials. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 859–869, 2010