A new paradigm for the molecular basis of rubber elasticity

The molecular basis for rubber elasticity is arguably the oldest and one of the most important questions in the field of polymer physics. The theoretical investigation of rubber elasticity began in earnest almost a century ago with the development of analytic thermodynamic models, based on simple, highly-symmetric configurations of so-called Gaussian chains, i.e. polymer chains that obey Markov statistics. Numerous theories have been proposed over the past 90 years based on the ansatz that the elastic force for individual network chains arises from the entropy change associated with the distribution of end-to-end distances of a free polymer chain. There are serious conceptual objections to this assumption and others, such as the assumption that all network nodes undergo a simple volume-preserving linear motion and that all of the network chains have the same length. Recently, a new paradigm for elasticity in rubber networks has been proposed that is based on mechanisms that originate at the molecular level. Using conventional statistical mechanics analyses, Quantum Chemistry, and Molecular Dynamics simulations, the fundamental entropic and enthalpic chain extension forces for polyisoprene (natural rubber) have been determined, along with estimates for the basic force constants. Concurrently, the complex morphology of natural rubber networks (the joint probability density distributions that relate the chain end-to-end distance to its contour length) has also been captured in a numerical model (EPnet). When molecular chain forces are merged with the network structure in this model, it is possible to study the mechanical response to tensile and compressive strains of a representative volume element of a polymer network. As strain is imposed on a network, pathways of connected taut chains, that completely span the network along strain axis, emerge. Although these chains represent only a few percent of the total, they account for nearly all of the elastic stress at high strain. Here we provide a brief review of previous elasticity theories and their deficiencies, and present a new paradigm with an emphasis on experimental comparisons.     

Dynamic mechanical characterization of PMR polyimide/carbon fiber composites modified by fiber coating with silicones

Viscous and elastomeric silicones have been applied as interlayers to carbon fibers in order to develop a tougher, micro-crack resistant, thermally stable polyimide (PMR-15) composite. Carbon fiber is continuously coated with very high molecular weight polydimethylsiloxane (PDMS) and polyvinyl-methylsiloxane (PVMS). Dynamic mechanical properties of the composites have been determined and compared with uncoated carbon fiber reinforced PMR-15 polyimide composites. The presence of the interlayer is shown by the appearance of a new relaxation peak. The peak temperature is found to be a good indication of the degree of the cure of the silicone elastomer. Comparison of the storage moduli of uncoated and coated carbon fiber composites at the service temperature range of the composites indicates that the presence of the silicone interlayer affects the shear moduli of the composites. Apparent activation energy of the α transition of the matrix in the modified composites varies with the amount of interlayer and composition in concert with the impact strength.     

Carboxylated acrylo nitrile butadiene rubber latex/kaolin nanocomposites: preparation and properties

Carboxylated nitrile butadiene rubber (XNBR)–based nanocomposites with varying amounts of nanokaolin were produced by latex stage mixing. Sonication of the unmodified kaolin and the technique adopted for the preparation of the composite have helped to get a uniform dispersion of clay in XNBR matrix. Nanokaolin caused enhancement in the mechanical properties of the composites. Proper dispersion of the clay particles, partial exfoliation/intercalation of clay, and interaction of clay with the polar rubber latex made nanokaolin good reinforcing filler in XNBR latex. Swelling studies conducted in methyl ethyl ketone showed a decrease in the swelling index and solvent uptake confirming the hindrance exerted by clay and the possible clay–rubber interaction. Increase in complex modulus obtained from the strain sweep analysis is a further evidence for better rubber filler interaction. The composites were characterized by the scanning electron microscopy, X-ray diffraction analysis, and atomic force microscopy.     

The effects of polyethylene glycol and the coupling agent bis-(γ-triethoxysilylpropyl)-tetrasulfide (Si-69) on the interactions of silica-filled natural rubber

A Fourier Transform Infrared (FTIR) analysis using Fresnel Attenuated Total Reflectance (ATR) was performed on silica-filled cis-1,4-polyisoprene. Silica filler's detrimental effects on zinc-activated cure systems has been well documented. The silane coupling agent bis-(y-triethoxysilylpropyl)-tetrasulfide (Si-69) and polyethylene glycol (PEG) are industry standards used to offset the interactions caused by reaction between silica and the zinc-activated cure system. By adding PEG, it was found that the interaction peaks at 1040 and 1017 cm^-1 caused by the adsorption of natural rubber (NR) onto the surface of the silica were not formed. Also, by monitoring the zinc stearate peak at 1540 cm^-1, both Si-69 and PEG were found to reduce the soluble zinc ion reaction with the silica surface. Supporting evidence from the rheometer curves also shows that the additives reduce the cure retardation effects of the silica filler.     

Electrical and mechanical behavior of filled rubber. III. Dynamic loading and the rate of recovery

Following the earlier articles in this series, the changes in the electrical resistivity and mechanical behavior as a result of static and dynamic deformation have been studied. Cyclic shear and tensile loading were used to follow the changes in stress and resistivity with strain, including the recovery with time from the effects of a large strain as monitored by the small‐strain behavior. The recovery of resistivity from a prestrain was not complete even after 7 days at room temperature or at 50 °C, but swelling with a solvent and subsequent drying produced rapid recovery. It appears from the detailed results that there are two strain regions. Below about 10% the resistance and the modulus are strongly dependent on the filler–filler structure, which can break down and reform fairly readily, but the changes at higher strains are probably influenced by changes in the elastomer matrix and also by slippage at the filler–rubber interface. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1649–1661, 2005     

Analysis of the Interface Crack for Rubber-like Materials

The interface crack between two dissimilar rubber materials under mixed load is asymptotically analyzed in this paper. It is shown that the crack tip field is composed of one expanding sector and two shrinking sectors. For the case considered, the interface is located in the expanding sector. Analytical solutions are obtained for both the expanding sector and the shrinking sectors. The structures of the crack tip field obtained in this paper is compared with those in previous works for different constitutive equations.     

Radiation preparation of nano-powdered styrene-butadiene rubber (SBR) and its toughening effect for polystyrene and high-impact polystyrene

Nano-powdered styrene-butadiene rubber (NPSBR) was synthesized based on the styrene-butadiene rubber (SBR) latex via gamma radiation crosslinking followed by spray drying. Two functional monomers, 2-ethyl hexyl acrylate (2-EHA) and trimethylolpropane triacrylate (TMPTA) were used as crosslinking agents. It was found that both 2-EHA and TMPTA can improve the radiation crosslinking of SBR latex. Transmission electron microscope (TEM) and scanning electron microscope (SEM) revealed that the NPSBR has a particle size similar to that of SBR latex with a diameter of 100 nm due to the high degree of crosslinking of SBR. Mechanical testing results showed that NPSBR could toughen polystyrene (PS) and high-impact polystyrene (HIPS) effectively. In addition, NPSBR is more suitable to toughen HIPS than PS at low rubber content.     

Thermal analysis of polymeric materials looked at from the environmental and recycling point of view

This paper presents DSC investigations on the curing kinetics of an epoxy-polyester (EP/PE) powder coating system with two different accelerators and on the crystallization behaviour of a semicrystalline thermoplastic containing regranulate. In addition thermal degradation and entropy relaxation effects of an amorphous, thermoplastic due to different injection moulding parameters are discussed. Thermogravimetry results are presented for quantitative analysis of rubber, and for elucidating problems which may arise during the injection moulding of thermoplastic regranulate which is obtained from painted rejects. The measurements were carried out using NETZSCH DSC 200 (heat flux DSC) and thermobalances TG 439 and TG 209.     

Deformation and molecular orientation in films of metallocene polypropylene,ethylene–butylene rubber,and their 80/20 (w/w) blend revealed by the simultaneous kinetic measurement of microscopic infrared dichroism,macroscopic stress,and mesoscale strain

Simultaneous kinetic measurement of microscopic infrared dichroism, macroscopic stress, and mesoscale strain was used to study the deformation mechanisms of metallocene polypropylene (MPP), ethylene–butylene rubber (EBR), and their blend (MPP/EBR = 80/20 w/w). As with pure MPP, the molecular orientation in the blend is dominated by the necking of the isotactic polypropylene matrix. During the necking passage through the mesoscale sampling area, the molecular orientation of the polypropylene matrix in the blend is smaller than that in the pure polypropylene film at the same level of mesoscale strain. However, the orientation of the EBR dispersed phase in the blend is larger than that in the pure EBR film. This may result from the partial miscibility of the two ingredients in the amorphous phases and their resultant strong interfacial interaction. The large stress supported by the MPP matrix extends to the island of the EBR domain and leads to its large deformation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1520–1531, 2005     

Hysteresis loss as a measure of metal–rubber adhesion

As in the case of reinforcing filler-induced increase in hysteresis in rubbers, placement of aluminum (A1) foil to the surface of a rubber blend of epichlorohydrin rubber and carboxylated nitrile base induces increased hysteresis of the rubber due to adhesion between Al and the rubber blend. Changes in hysteresis loss due to Al foil can be correlated with the peel strength of Al-rubber-Al joints. © 1995 John Wiley & Sons, Inc.