Molecular structure and properties of click hydrogels with controlled dangling end defect

In this study, controlled amount of dangling ends is introduced to the two series of poly(ethylene glycol)‐based hydrogel networks with three and four crosslinking functionality by using click chemistry. The structure of the gels with regulated defect percentage is confirmed by comparing the results of low‐field NMR characterization and Monte Carlo simulation. The mechanical properties of these gels were characterized by tensile stress–strain behaviors of the gels, and the results are analyzed by Gent model and Mooney–Rivlin model. The shear modulus of the swollen gels is found to be dependent on the functionality of the network, and decreases with the defect percentage. Furthermore, the value of shear modulus well obeys the Phantom model for all the gels with varied percentage of the defects. The maximum extension ratio, obtained from the fitting of Gent model, is also found to be dependent on the functionality of the network, and does not change with the defect percentage, except at very high defect percentage. The value of the maximum extension ratio is between that predicted from Phantom model and the Affine model. This indicates that at the large deformation, the fluctuation of the crosslinking points is suppressed for some extend but still exists. Polymer volume fractions at various defect percentages obtained from prediction of Flory–Rehner model are found to be in well agreement with the swelling experiment. All these results indicate that click chemistry is a powerful method to regulate the network structure and mechanical properties of the gels. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1227–1236     

Energy diagrams and stability restrictions for electroelastic generators

The relation between electric and mechanical properties of dielectric elastomers allows using them as energy convertors under some conditions. The process is based on the relations between mechanical and electrical energy stored on the material. In this work, cycles of maximal mechanical energy to be converted into electrical energy are presented. The purpose of this article is to give a general formulation of electroelastic generator relations considering (1) Mooney‐Rivlin materials, (2) floating sample between electrodes and sample with compliant electrodes, and (3) explicit thermodynamic stability criteria. Estimations on the maximal energy to be converted are made. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2023–2028, 2010     

Elastic behavior of chitosan films

The thermoelastic behavior and equilibrium stress–strain properties of chitosan films lightly crosslinked with gluteraldehyde and swollen with water were studied. Precautions were taken to preclude changes in the swelling ratio of swollen sample films during the experiment. The results indicate that at relatively low extensions the elastic behavior of the biopolymer is entropic in origin. The equilibrium stress–strain isotherms of chitosan did not obey Mooney–Rivlin equation because of sharp increases in stress with extension ratio at high extensions. This is attributed mainly to interchain hydrogen-bonded interactions, but a possible contribution due to strain–induced crystallization cannot be ruled out. © 1997 John Wiley & Sons, Inc.     

On the relevance of the 8‐chain model and the full‐network model for the deformation and failure of networks formed through photopolymerization of multifunctional monomers

Crosslinked networks were synthesized by copolymerization of mono‐functional tert‐butyl acrylate (tBA) with diethyleneglycol dimethacrylate (DEGDMA) or polyethylene glycol dimethacrylates (PEGDMA). By varying the chain length and concentration of the difunctional PEGDMA, we obtained tBA‐PEGDMA copolymer networks while by varying the concentration of difunctional DEGDMA, we obtained tBA‐DEGDMA crosslinked networks. The various materials were submitted to large deformations through uniaxial tension tests. For moderate weight percent of crosslinking agent, up to 20%, the networks showed standard S‐shape stress–strain curves, characteristic of rubber‐like elasticity. Two macromolecular models, the 8‐chain model and the full‐network model, were applied to fit the uniaxial tensile response of the materials. Both models provide good representations of the overall uniaxial stress–strain response of each material. After fitting to stress–strain data, the network models were employed to predict the shear modulus and the elongation at break. Neither the 8‐chain nor the full network model were capable of predicting the failure strain or shear modulus, indicating these models are best used to describe stress–strain relations rather than predict mechanical properties for the network polymers considered here. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1226–1234, 2008     

Understanding mechanical characteristics of cellulose nanocrystals reinforced PHEMA nanocomposite hydrogel: in aqueous cyclic test

Cellulose nanocrystals (CNC) can be embedded within hydrogels to form tough and strong nanocomposite materials, which possess biomimetic properties from hydrogels including good biocompatibility, permeability and flexible mechanical characteristics. There are many potential applications for these strong nanocomposite hydrogels in medical devices, such as wound dressing or super absorbents. Whereas, the research on the mechanical properties of CNC reinforced nanocomposite remains at superficial level, and their nonlinear mechanical responses are rarely investigated in previous reports. Mechanical characteristics of CNC reinforced poly(2-hydroxyethyl methacrylate) (PHEMA) nanocomposite hydrogels, in terms of stress–strain correlations, fracture mechanism, and cyclic stretching responses, have been investigated in this work. Experimental results show that the modulus of the nanocomposite hydrogel tends to increase with increasing CNC content. Theoretical foundation for analysing the mechanical properties of hydrogels based on Mooney–Rivlin hyperelastic model, Voigt model and Reuss model has been developed and validated, which provides the prediction of the mechanical responses of CNC reinforced nanocomposite hydrogel to tension, especially the nonlinear responding behaviour.     

Birefringence and stress relaxation in polyisobutylene

Relaxation of birefringence and stress in simple extension has been studied for polyisobutylene at -26.0°C and 25.0°C for extension ratios ranging from about 1.2 to 2.0. The dependence of both the stress and birefringence on the extension ratio, for a given time, is well described by equations of the Mooney–Rivlin form. The Mooney–Rivlin treatment of the birefringence experiments was found to reinforce but not add to information available from the stress-relaxation experiments alone. At 25.0°C, the stress-optical coefficient is found to be stress dependent. Possible explanations of this behavior are discussed. An experiment is also reported in which the birefringence is observed under zero stress immediately after cutting a strained sample. The zero stress birefringence is believed to be due to regions of stress-induced crystallization taking a finite time to melt after the stress is removed.     

Recoverable strain storage capacity of shape memory polyethylene

Shape memory polymers (SMPs) are an important class of smart materials. So far the focus of such polymers was to find suited triggers for various application fields. Thus, the potential of most of these macromolecular networks regarding their maximally storable strain capability was not explored. In this study, the polyethylenes HDPE, LDPE, and ethylene‐1‐octene (EOC) were systematically investigated with respect to their strain storage potential. To achieve maximum strains, the polymers were chemically cross‐linked in such a way that they are at the borderline between thermoplastics and elastomers. All investigated polymers showed higher strain storage than literature reported systems and exhibited excellent shape memory parameters. The highest stored strain was found for networks of EOC with fully recoverable 1400%. Interestingly, this value could not be enlarged by using EOCs with higher molecular weight, which is probably due to increasing content of entanglements as confirmed by Mooney‐Rivlin. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1033–1040     

国内外PVB材性研究

主要介绍了国内外研究PVB(聚乙烯醇缩丁醛)材性的现状。国内外研究表明,PVB是应变率及温度敏感材料。应变率增加,弹性模量变大;温度升高,弹性模量和剪切模量均下降。同时,国内外进行了少量的实验,研究PVB的本构模型。总结发现,PVB的本构模型可描述为线弹性、弹塑性、线性粘弹性和非线性粘弹性四种,但本质上PVB是非线性粘弹性材料,不同的环境条件与计算要求可选择不同的本构模型。目前,国内外学者比较认可的是用超弹性考虑其非线性,用Maxwell模型考虑其粘弹性。     

Tensile strength of Iβ crystalline cellulose predicted by molecular dynamics simulation

The mechanical properties of Iβ crystalline cellulose are studied using molecular dynamics simulation. A model Iβ crystal is deformed in the three orthogonal directions at three different strain rates. The stress–strain behaviors for each case are analyzed and then used to calculate mechanical properties. The results show that the elastic modulus, Poisson’s ratio, yield stress and strain, and ultimate stress and strain are highly anisotropic. In addition, while the properties that describe the elastic behavior of the material are independent of strain rate, the yield and ultimate properties increase with increasing strain rate. The deformation and failure modes associated with these properties and the relationships between the material’s response to tension and the evolution of the crystal structure are analyzed.     

Enhanced nonisothermal crystallization of a series of poly(butylene succinate‐co‐terephthalate) biopolyesters by poly(vinyl butyral)

Enhanced nonisothermal crystallization of a series of poly(butylene succinate‐co‐terephthalate) (PBST) by poly(vinyl butyral) (PVB) as a macromolecular nucleating agent has been examined systematically with various techniques and theoretical modeling. The role of PVB depends strongly on the butylene terephthalate content, PVB content, and cooling rate. The (0.3–0.7 wt %) PVB reduces the spherulitic size, but considerably increases the peak temperature of crystallization, for example, by 28 °C for the PBST with 50 mol % terephthalic acid. The effects of PVB are believed to stem from its unique molecular structure. Both the hydroxyl and butyral groups of PVB may synergistically participate in nucleating PBSTs for crystallization because of favorable secondary interaction and affinity of butyral groups with butylene succinate units of PBSTs. Only the Tobin model suffices to describe the nonisothermal crystallization kinetics, while the modified Avrami model is suitable for limited crystallinity. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 658–672     

Characterization and gel point of randomly linked high cis-polybutadiene networks

Cis—Polybutadiene networks (98% cis), crosslinked in the bulk state by gamma irradiation were characterized in terms of their mechanical properties. Data from stress-strain analyses, swelling degrees, stress relaxation and random—linking theory are in accordance and the estimated gel points practically coincide. Trapped entanglements contributed up to 30% to the network modulus, from which also the plateau modulus was determined. The tensile data were obtained from Mooney—Rivlin representations. It was concluded that the assumption of additive contributions from chemical and entanglement crosslink densities in the Langley theory works well in the range of doses investigated here. A comparison with a tube—like approach of these networks is made.     

Correlation of morphological parameters and mechanical performance of polyamide‐612/poly (ethylene–octene) elastomer blends

Blends of polyamide‐612 (PA‐612) and maleic anhydride grafted poly (ethylene–octene) elastomer (POE‐g‐MA) as an impact modifier have been prepared in the composition range of 0–35 wt. % of POE‐g‐MA and subsequently investigated for their structural, thermal, mechanical, dynamic mechanical properties and morphological attributes. X‐ray diffraction studies revealed a decrease in crystallinity whereas the thermal properties such as onset to degradation temperature and crystallization temperature remained broadly unaffected. Nearly three‐fold increase in the impact strength is registered accompanied by substantial increase in tensile failure strain, though tensile modulus (E) and tensile yield strength (σ _y) decreased with increase in impact modifier content. Dynamic mechanical analysis exhibited a singularity response in the loss factor in the temperature range of ~10°C–50°C. Micromechanical aspects were analyzed using conventional theoretical models for low strain mechanical response (E) such as rule of mixtures and foam model and for high strain mechanical response (σ _y) such as Nikolais–Narkis model and porosity model. Impact toughness and strain‐at‐break of the investigated composition were successfully correlated to the domain size (D_n) of the dispersed phase and their inter‐particle distances (τ). Scanning electron microscopy showed the coalescence of domains of the dispersed phase at higher POE content and thus reiterates the crucial role of inter‐particle distance in controlling the toughening mechanism of POE blended PA‐612. Copyright © 2013 John Wiley & Sons, Ltd.     

Rubber networks containing unattached macromolecules. III. Nonlinear stress relaxation in the system ethylene–propylene terpolymer with ethylene–propylene copolymer and behavior of extracted networks

Further stress relaxation experiments, mostly at 50°C, are reported on mixtures of crosslinkable ethylene–propylene terpolymer with saturated ethylene–propylene copolymer (molecular weights 3.6 and 45 × 10⁴) containing up to 50% by weight of copolymer, crosslinked by sulfur to leave the saturated copolymer unattached and free to reptate in the copolymer network. Stress relaxation was measured in small simple elongations (stretch ratio about 1.15) on samples which had been extracted to remove a large part of the unattached copolymer and dried. The relative increase in modulus at long times (10⁴ sec) increased with the proportion extracted; at short times (1 sec), extraction of the lower molecular weight copolymer increased the modulus to about the same extent but extraction of the higher molecular weight copolymer affected it very little. The relaxation modulus of the copolymer extracted from sample 50H (50% copolymer of high molecular weight), obtained by difference, agreed with that for the total copolymer except for a small difference probably attributable to molecular weight selectivity in the extraction. Stress relaxation was measured on sample 50H at six higher elongations up to a stretch ratio of 3. The dependence of stress on time and strain was consistent with an analysis based on the following assumptions: (a) linear additivity of the network and unattached copolymer contributions, (b) strain–time factorization of the stress contributions from the individual components, (c) a strain dependence for the unattached component corresponding to the presence of a Mooney–Rivlin C₂ term only, (d) a strain dependence for the network component which does not follow the Mooney–Rivlin equation but is dominated by a simple neo-Hookean term.     

Tensile behavior of Nafion and sulfonated poly(arylene ether sulfone) copolymer membranes and its morphological correlations

The tensile stress–strain behavior of Nafion 117 and sulfonated poly(arylene ether sulfone) copolymer (BPSH35) membranes were explored with respect to the effects of the strain rate, counterion type, molecular weight, and presence of inorganic fillers. The yielding properties of the two films were most affected by the change in the strain rate. The stress–strain curves of Nafion films in acid and salt forms exhibited larger deviations at strains above the yield strain. As the molecular weight of the BPSH35 samples increased, the elongation at break improved significantly. Enhanced mechanical properties were observed for the composite membrane of BPSH35 and zirconium phenylphosphonate (2% w/w) in comparison with its matrix BPSH35 film. The stress‐relaxation behavior of Nafion and BPSH35 membranes was measured at different strain levels and different strain rates. Master curves were constructed in terms of plots of the stress‐relaxation modulus and time on a double‐logarithm scale. A three‐dimensional bundle‐cluster model was proposed to interpret these observations, combining the concepts of elongated polymer aggregates, proton‐conduction channels, and states of water. The rationale focused on the polymer bundle rotation/interphase chain readjustment before yielding and polymer aggregate disentanglements and reorientation after yielding. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1453–1465, 2006     

Morphology and strain rate effects on heat generation during the plastic deformation of polyethylene/carbon black nanocomposites

The phenomenon of internal heat generation during the plastic deformation of polyethylene/carbon black nanocomposites at high strain rates was investigated using a high resolution thermal camera. Material morphology, strain rate and carbon black (CB) content were found to be critical factors that affected heat generation during tensile testing, and consequently changed the mechanical behaviour. Two processing methods (M1 and M2) were used to prepare the materials, with CB contents of 0.5, 1 and 3 wt.%. The results showed a significant increase in internal heat generation after yielding, with temperatures exceeding 70 °C for materials processed using M1 and 55 °C for materials processed using M2. The temperature increase was dependent on the processing method, the CB content and the strain rate. The increase in temperature due to plastic heat generation affected the properties of the material, reducing the plastic hardening and reducing the tensile strength at high strain rates. This is of significance when considering the use of these materials in applications involving high strain rates, such as impact protection.     

Temperature and variable strain rate sensitivity in recycled HDPE

The recycling of post-consumer plastics and their utilization as raw materials to develop value-added products has become an important goal worldwide. The present work is concerned with the thermo-mechanical analysis of recycled high-density polyethylene (HDPE) under uniaxial tensile loading. The main focus is to propose a one-dimensional phenomenological model able to describe the influence of temperature and strain rate on the mechanical behavior. Tensile tests were performed over a wide range of temperatures (from 25°C to 100°C). Each experiment was performed under controlled strain rate varying from 7.25 × 10⁻⁵ s⁻¹ to 7.25 × 10⁻³ s⁻¹ in steps. It is shown that only one tensile test performed at three different temperatures is necessary to fully identify experimentally all material parameters that arise in the theory. Thus, with this experimental procedure, the number of tests used to evaluate the mechanical properties of recycled HDPE is significantly reduced. The experiments are compared with the model predictions and show good agreement.     

Elastomeric Properties of Polysiloxane Networks. Bimodal Elastomers that are Spatially Inhomogeneous and Others that are Very Broadly Multimodal

End‐linking poly(dimethylsiloxane) was used to prepare bimodal elastomers networks so as to have inhomogeneous nanostructures, and also to prepare others having very broadly multimodal chain‐length distributions. Macroscopic phase separation, probably high crosslink density clusters, was observed to occur in some of the bimodal networks. The mechanical properties in simple extension and in equilibrium swelling were measured. The bimodal elastomers that were not obviously inhomogeneous showed very good mechanical properties, but the macroscopically phase‐separated networks, and the broadly multimodal network were weak. Analysis of the Mooney‐Rivlin profiles suggests that the reinforcing mechanism could have a structural component in addition to that from the limited extensibilities of the short chains. The mechanical properties and the extents of swelling support the cluster conjecture, in accord with previous morphological studies on spatially‐inhomogeneous polysiloxane elastomers.     

Synthesis and characterization of photocrosslinked poly(ε‐caprolactone)s showing shape‐memory properties

Poly(ε‐caprolactone) (PCL) with a pendent coumarin group was prepared by solution polycondensation from 7‐(3,5‐dicarboxyphenyl) carbonylmethoxycoumarin dichloride and α, ω‐dihydroxy terminated poly(ε‐caprolactone) with molecular weights of 1250, 3000, and 10,000 g/mol. These photosensitive polymers underwent a rapid reversible photocrosslinking upon exposure to irradiation with alternating wavelengths (>280/254 nm) without a photoinitiator. The thermal and mechanical properties of the photocrosslinked films were examined by means of differential scanning calorimetry and stress–strain measurements. The crosslinked films exhibited elastic properties above the melting temperature of the PCL segment along with significant decrease in the ultimate tensile strength and Young's modulus. Shape‐memory properties such as strain fixity ratio (R_f) and strain recovery ratio (R_r) were determined by means of a cyclic thermomechanical tensile experiments under varying maximum strains (ε_m = 100, 300, and 500%). The crosslinked ICM/PCL‐3000 and ‐10,000 films exhibited the excellent shape‐memory properties in which both R_f and R_r values were 88–100% for tensile strain of 100–500%; after the deformation, the films recovered their permanent shapes instantaneously. In vitro degradation was performed in a phosphate buffer saline (pH 7.2) at 37 °C with or without the presence of Pseudomonas cepacia lipase. The presence of the pendent coumarin group and the crosslinking of the polymers pronouncedly decreased the degradation rate. The crosslinked biodegradable PCL showing a good shape‐memory property is promising as a new material for biomedical applications. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2422–2433, 2009