Polymerization, elastomeric networks

A number of variations of the above-mentioned full IPNs have also been stated in the literature. One of them involves having either Polymer I or II as linear (not crosslinked) polymer, in which case it is called semi-IPN. The other variation involves the formation of Polymer I and II simultaneously through two noninterfering polymerization processes (such as stepwise and chain polymerizations) in which case it is called simultaneous IPN (SIN). If a linear polymer is formed simultaneously with a crosslinked polymer, then we have a semisimultaneous IPN (semi-SINS). Still another type is taking a mixture of two linear polymers, and crosslinking both components simultaneously, in which case it is called interpenetrating elastomeric network (lEN). The common feature of... 

Polymerization of conjugated dienes like butadiene, isoprene, and chloroprenes involves activation of either or both of the double bonds to give 1,2 3,4 or 1,4 polymers (Figure 15.11). The residual unsaturation in the polymer chains provides convenient sites for the introduction of elastomeric network... 

Latex IPNs, by definition, have their origin in emulsion polymerization. Several types of latex IPNs exist. If one blends two kinds of latex particles, followed by film formation and cross-linking of both polymers, the material is called an interpenetrating elastomeric network, TEN. Usually, lENs form a three-dimensional mosaic structure (69,70). 

As it has been just aforementioned, for fast azobenzene-based artificial muscle-like actuators to be achieved, it is essential that the azo-chromophore used returns to its thermodynamically stable trans form in the dark as fast as possible. In this way, azobenzenes that undergo their thermal isomerisation through the rotational mechanism, of which push-pull azoderivatives are the most well-known example, will be valuable candidates for this aim since they are endowed with high thermal isomerisation rates at room temperature. In fact, the two only polysiloxane-based photoactive artificial muscle-like actuators exhibiting low relaxation times published so far are based on this strategy. Indeed, these systems contain the well-known push-pull azo-dyes 4-amino- (Camacho Lopez et al. 2004) and A-N, W-dimethylamino-4 -nitroazobenzene (Harvey and Terentjev 2007) as photo-active molecules, which are doped into host elastomeric networks but not covalently bonded to the polymeric structure decreasing thereby the stability of the final photo-actuator. 

The SANS experiment is applicable to polymeric networks containing some deuterium labeled chains. The chain geometry can be probed not only in the unperturbed network, but changes in chain shape and size can be measured as a function of strain or swelling. This enhances the applicability of SANS experiments for elastomeric systems. 

Distribution functions for the end-to-end separation of polymeric sulfur and selenium are obtained from Monte-Carlo simulations which take into account the chains geometric characteristics and conformational preferences. Comparisons with the corresponding information on PE demonstrate the remarkable equilibrium flexibility or compactness of these two molecules. Use of the S and Se distribution functions in the three-chain model for rubberlike elasticity in the affine limit gives elastomeric properties very close to those of non-Gaussian networks, even though their distribution functions appear to be significantly non-Gaussian. 

In any case, if this polymerized form of elemental sulfur is quenched (cooled rapidly), it becomes a solid. This solid is glassy at very low temperatures, but becomes highly elastomeric above its glass-transition temperature of approximately -30 °c.6 8 14 30 The situation is complicated by the presence of unpolymerized S8 molecules which would certainly act as plasticizers. So far, attempts to cross-link the elastomeric form into a network structure suitable for stress-strain measurements have not been successful. The polymer is unstable at room temperature, gradually crystallizing, and eventually reverting entirely to the S8 cyclics. 

Ti has also been incorporated into elastomeric Si-containing networks. These are prepared by Sn-catalyzed polymerization of tetraethoxysilane (TEOS) and oligomeric dimethyl silanols, containing 5 or 10 Si atoms. Part of the TEOS can be replaced by Ti(OiPr)4 or Ti tetrakis(2-ethylhexyloxide) ... 

In our laboratory, much attention has been devoted to the investigation of in situ sequential polyurethane/poly(methyl methacrylate) interpenetrating polymer networks (SEQ PUR/PAc IPNs) (2- ) in which the elastomeric polyurethane network is completely formed in the presence of the methacrylic monomers before the onset of the radical copolymerization which leads to the second network. To each polymerization process corresponds a typical kinetics, which however is not completely independent from each other ( -8). The results obtained with such SEQ IPNs show that the properties do in... 

A 10-year effort to design an artificial skin (1) has yielded a bilayer polymeric membrane comprising a top silicone elastomeric layer and a bottom layer consisting of a novel, highly porous cross-linked collagen-glycosaminoglycan (GAG) network. The polymeric bilayer is termed a Stage... 

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