This Perspective highlights how entanglement effects on rubber elasticity can be unveiled by a combination of different macroscopic and microscopic methods, taking advantage of new developments in proton low-field NMR spectroscopy as applied to bulk and swollen rubbers. Specifically, the application of a powerful yet routinely applicable double-quantum method, combined with a back-extrapolation procedure over results measured at different degrees of swelling, allows one to characterize the recently introduced “phantom reference network” state, which only reflects contributions of actual cross-links and topologically trapped entanglements. We further present an assessment of the qualitative yet popular Mooney–Rivlin analysis of mechanical data, where the influence of entanglement contributions on the fitted, purely empirical parameters C 1 and C 2 is reconsidered in the context of different tube models of rubber elasticity. We also review the impact of entanglements on results of equilibrium swelling experiments and address the validity of the common Flory–Rehner approach, where we stress its qualitative nature and the need to use NMR observables for a correct estimation of the relevant volume fractions. We discuss semiquantitative estimations of the cross-link density from these macroscopic experiments with its microscopic determination by NMR on the example of lowly cross-linked synthetic and natural poly­(isoprene) rubber prepared by a novel UV-based curing protocol of dried latex based upon thiol–ene chemistry, which in contrast to previously studied thermally peroxide-cured natural rubber contain only small amounts of short-chain defects. We find that the entanglement effects in these samples can best be described by the Heinrich–Straube tube model with positive scaling exponent ν > 0.3 as well as by the slip-link model of Ball et al./Edwards–Vilgis with a slip parameter η > 0.1. A comparison with literature results demonstrates that these findings are not universal in that the apparent entanglement contribution depends significantly on the sample (in)­homogeneity, i.e., of the NMR-determined fraction of inelastic defects and spatial cross-linking inhomogeneities. This means that conclusions on the validity or invalidity of specific tube theories cannot be drawn without careful consideration of the network microstructure.

According to classic single-chain theories of rubber elasticity such as the affine or phantom models, the length N and the state of stretching R/R 0 of network chains are directly reflected in the magnitude of segmental orientation correlations, as quantified by a dynamic order parameter S f R 2 /N (this relation holds for chains in the bulk and q solvent). S can be determined by suitable NMR techniques, and is a viable molecular probe of the network structure, as it is directly proportional to the elasticity modulus. Furthermore, previous studies (see Saalwächter et al., Soft Matter, 2013, XX, XXX, in this issue for a review) have convincingly demonstrated the validity of the phantom model for the prediction of S in equilibrium-swollen networks. We here investigate changes in the degree of local chain stretching reflected in S as a function of the degree of (partial) swelling Q ¼ V/V 0 in different networks prepared in bulk and in states of increasing dilution. Previous work has already revealed a non-monotonic dependence of S on Q, indicating strongly subaffine local deformation in the early stages of swelling. The width of the distribution in S is also accessible, and increases significantly during the early stage of swelling, indicating the well documented presence of swelling heterogeneities. We find that beyond this early stage, the network chains deform affinely. A back-extrapolation of the affine deformation range to zero swelling allows for conclusions on the actual crosslink density of the networks corrected for the effect of topological or packing constraints, which we refer to as the "phantom reference state" of a bulk network. Deviations of the network elasticity from this state constitute non-classical contributions, possible origins of which are discussed.

No abstract

Abstract:The investigation of mica and mica/epoxy-composites has always been of high importance and has gained increased attention in recent years due to their significant role as insulation material in the electrical industry. Electrical insulation represents a key factor regarding the reliability and lifespan of high voltage rotating machines. As the demand for generating power plants is increasing, rotating machines are of intrinsic importance to the electrical energy supply. Therefore, impeccable functioning is of immense importance for both the producers of high voltage machines as well as the energy suppliers. Thus, cost reduction caused by shorter maintenance times and higher operational lifespan has become the focus of attention. Besides the electrical properties, composites should offer compatible chemical and mechanical, as well as thermal characteristics for their usage in insulating systems. Furthermore, knowledge of several occurring stresses leading to the final breakdown of the whole insulation is required. This review aims to give an overview of the properties of pure components, the composite, and the possible occurring failure mechanisms which can lead to a full understanding of insulation materials for high voltage rotating machines and therefore establish a basis for prospective optimizations.

Photopolymerization offers substantial advantages in terms of time, temperature, energy consumption, and spatial control of the initiation. The application however is strongly limited due to the constrained penetration of light into thick films. Strategies to overcome the problem of limited curing depth, as well as to improve the curing of shadow areas, involve dual curing, frontal polymerization, and upconversion of particles. Whereas excellent results have been accomplished applying photofrontal polymerization on a theoretical level, few studies report on practical applications achieving high curing depth within short time. This study aims to investigate the potential of photofrontal polymerization, performed only with photoinitiator and light, for the fast and easy production of several-centimeter-thick (meth)acrylic layers. Monomer/ initiator systems were evaluated with respect to their optical density as well as photobleaching behavior. Moreover, depth-dependent polymerization was studied in specimens of varying monomer ratio and photoinitiator concentration. When an ideal photoinitiator concentration was selected, curing up to 52 mm in depth was accomplished within minutes.