Elastomer synthesis molecular weight distribution

The synthesis of well defined block copolymers exhibiting controlled molecular weight, low compositional heterogeneity and narrow molecular weight distribution is a major success of anionic polymerization techniques 6,7,14-111,112,113). Blocks of unlike chemical nature have a general tendency to undergo microphase separation, thereby producing mesomorphic phases. Block copolymers therefore exhibit unique properties, that prompted numerous studies and applications (e.g. thermoplastic elastomers). 

Furthermore, LCEs have been prepared by block copolymerization and hydrogen bonds (Cui et al., 2004 Li et al., 2004). Li et al. (2004) proposed a musclelike material with a lamellar structure based on a nematic triblock copolymer (Components 8a-c, Fig. 3.10). The material consists of a repeated series of nematic (N) polymer blocks and conventional rubber (R) blocks. The synthesis of block copolymers with well-defined structures and narrow molecular-weight distributions is a crucial step in the production of artificial muscle based on triblock elastomers. Talroze and coworkers studied the structure and the alignment behavior of LC networks stabilized by hydrogen bonds under mechanical stress (Shandryuk et al., 2003). They synthesized poly[4-(6-acryloyloxyhexyloxy)benzoic acid], which... 

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. 

As mentioned in Section 3, typical aramid-6-polyether elastomers are synthesized by the polycondensation reaction of polyether diol with the aramid compound I in the presence of transesterification catalysts. Under these conditions, the synthesis of aramid-6-polyester elastomers gave only low molecular weight elastomers with a broad segment length distribution due to transesterification reactions of the polyester segments. This result inidicated that in the obtaining of aramid-6-polyester elastomers, transesterification catalysts should be avoided. Later, a method for the obtaing of this type of elastomers was developed, which consisted in the copolymerization of an activated acyl lactam-terminated aramid compound II with polyester diols in the molten state, in the absence of transesterification catalysts [40,42]. Compound II was obtained by the reaction of N-(p-aminobenzoyl) caprolactam with terephthaloyl chloride, as shown in Scheme 8 [61]. 

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