Network formation, elastomer

For the explanation of reinforcement phenomenon, F. Bueche proposed a mechanism based on the splitting of the network chains between two adjacent particles that occur during network deformation. As is seen in Figure 3.412, due to the statistic character of the elastomer network formation, between two neighbour particles of ingredient, three types of bonds should appear [1200]. 

A liquid rubber must be more than just a low molecular weight counterpart of a conventional elastomer. Because of molecular weight, functionality, and network-formation requirements, a liquid rubber needs to be considered as a polymeric entity of its own. This point has been shown by an analysis of the network formed upon vulcanization of a liquid rubber (3). This analysis compares and contrasts two classes of liquid rubbers terminally functional and randomly functional prepoly-... 

Although the major interest in experimental and theoretical studies of network formation has been devoted to elastomer networks, the epoxy resins keep apparently first place among typical thermosets. Almost exclusively, the statistical theory based on the tree-like model has been used. The problem of curing was first attacked by Japanese authors (Yamabe and Fukui, Kakurai and Noguchi, Tanaka and Kakiuchi) who used the combinatorial approach of Flory and Stockmayer. Their work has been reviewed in Chapter IV of May s and Tanaka s monograph Their experimental studies included molecular weights and gel points. However, their conclusions were somewhat invalidated by the fact that the assumed reaction schemes were too simplified or even incorrect. It is to be stressed, however, that Yamabe and Fukui were the first who took into account the initiated mechanism of polymerization of epoxy groups (polyetherification). They used, however, the statistical treatment which is incorrect as was shown in Section 3.3. 

Epoxy networks may be expected to differ from typical elastomer networks as a consequence of their much higher crosslink density. However, the same microstructural features which influence the properties of elastomers also exist in epoxy networks. These include the number average molecular weight and distribution of network chains, the extent of chain branching, the concentration of trapped entanglements, and the soluble fraction (i.e., molecular species not attached to the network). These parameters are typically difficult to isolate and control in epoxy systems. Recently, however, the development of accurate network formation theories, and the use of unique systems, have resulted in the synthesis of epoxies with specifically controlled microstructures Structure-property studies on these materials are just starting to provide meaningful quantitative information, and some of these will be discussed in this chapter. 

A large number of macroscopic properties of elastomer networks are closely related to the density of network junctions and the extent of their fluctuations. Qualitatively, any increase of network density causes an increase in stress, whereas fluctuations of network junctions leads to a decreasing stress. It is generally believed that a formation of additional network junctions resulting fi-om the presence of filler particles in the elastomer matrix is one of the reasons for the improvement of mechanical properties of filled elastomers. However, the application of macroscopic techniques does not provide reliable results for the network structure in filled elastomers. Furthermore, a lack of information exists on the dynamic behavior of adsorption junctions. The present study fills the gap of knowledge in this area. 

It has been noted above that phase separation in thermoplastics is a common occurrence when two or more polymers are mixed and that miscibility is the uncommon event. This is exploited in toughening of thermosets by elastomers when phase separation occurs during the reaction that leads to three-dimensional network formation. If macroscopic phase separation is not desired then it is possible to achieve a different microscopic morphology and in some cases maintain some features of miscibility... 

The stress-strain behavior of thermosets (glassy polymers crosslinked beyond the gel point) is not as well-understood as that of elastomers. Much data were analyzed, in preparing the previous edition of this book, for properties such as the density, coefficient of thermal expansion, and elastic moduli of thermosets [20,21,153-162]. However, most trends which may exist in these data were obscured by the manner in which the effects of crosslinking and of compositional variation were superimposed during network formation in different studies, by... 

The hard segment aggregates act as physical crosslinks and-this thermoreversible three - dimensional network formation accounts for the properties of these thermoplastic elastomers. 

Diffusion coefficients are proportional to 1/M, the molecular weight of linear chains [7]. They are not well known and therefore neither is the interdigitated thickness. Hence, it is not possible to say whether the observed behavior has to be related to the interdiffusion depth, or to the number of crosslinks formed in the interfacial region or, most hkely, to both effects. These results given for the elastomer joints crosshnked by a sulfur-based vulcanizing system show that it is very difficult to separate interdiffusion and crosshnking mechanisms because the temperature influences both the chain mobihty and the kinetics of network formation. 

Non-linear viscoelastic properties were observed for fumed silica-poly(vinyl acetate) (PVAc) composites, with varying PVAc molar mass and including a PVAc copolymer with vinyl alcohol. Dynamic mechanical moduli were measured at low strains and found to decrease with strain depending on surface treatment of the silica. The loss modulus decreased significantly with filler surface treatment and more so with lower molar mass polymer. Copolymers with vinyl alcohol presumably increased interactions with silica and decreased non-linearity. Percolation network formation or agglomeration by silica were less important than silica-polymer interactions. Silica-polymer interactions were proposed to form trapped entanglements. The reinforcement and nonlinear viscoelastic characteristics of PVAc and its vinyl alcohol copolymer were similar to observations of the Payne effect in filled elastomers, characteristic of conformations and constraints of macromolecules. ... 

Latex lENs. Latex interpenetrating elastomer networks, latex lENs, are latex blends that have been cross-linked after film formation. They are named after the early works of Frisch and co-workers, who called these materials interpenetrating elastomer networks (Frisch et al. 1969b). Many latex blends, as used in coatings especially, are cross-linked as finally used in service. 

Crowe-Willoughby, J. A. Weiger, K. L. Ozcam, A. E. Genzer, J., Formation of Silicone Elastomer Network Eilms with Gradients in Modulus. Polymer 2010, 51, 763-773. 

Dusek, K. and Matejka L.,Advances in network formation and rheology during curing of epoxy resin systems including reactive liquid elastomers. Polym. Mat. Sci. Eng. 1987, 57, 765. 

Industrial interest in telechelics was stimulated hy the development of thermoplastic elastomers, which consist of ABA block and multiblock copolymers. Liquid telechelic polymers are the basis for reaction injection molding. Liquid telechelics that can be used for network formation offer processing advantages and may result in materials with improved properties (10). 

Stadler and coworkers have made important contributions to the field of supramolecular polymer chemistry through their studies of polybutadienes derivatized with hydrogenbonding phenylurazole derivatives (Figure 2). Lightscattering experiments, optical measurements, and thermal analysis were used to probe the formation of thermoplastic elastomers and elastomeric blends at lower temperatures, hydrogen bonding contributes to network formation and elastic behavior, whereas at higher temperatures these... 

The reinforcement induces important changes of the elastomer network, due to the formation of strong and weak bonds between the chain segments and reticulation centres located on the surface of active charge particles [1199]. The Mullins effect or the tension relaxation in networks is ascribed to the splitting of weak bonds. 

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