Cross-linked elastomers, orientation

Cross-linked elastomers (the other main class of polymers studied by ESR) can be pre-strained above Tg to any desired extent (up to fracture) and their orientation stabilised by cooling below Tg before testing. At high pre-strains, strain-induced crystallization may occur providing a morphology essentially similar to that of synthetic fibres Cross-linked polymers may therefore be used to explore in a systematic manner, the role of strain and orientation in rmlecular fracture. 

The synthesis and phase behavior of the model polydiethylsiloxane networks have also been studied. The networks were made by hydrosilylation of well-defined vinyl and allyl telechelic siloxanes obtained by kinetically controlled polymerization of cyclic trisiloxane.314 The effects of molecular weight between the cross-linkings on segment orientation in polydiethylsiloxane elastomers were studied.315... 

Most elastomers are amorphous, but those with regular structures can crystallize when cooled to extremely low temperatures. Vulcanized soft rubber, which has a low cross-link density, when stretched crystallizes in a reversible process, and the oriented polymer has a high modulus (high stress for small strains, i.e., stiffness) and high tensile strength. 

In elastomer samples with macroscopic segmental orientation, the residual dipolar couplings are oriented as well, so that also the transverse relaxation decay depends on orientation. Therefore, the relaxation rate 1/T2 of a strained rubber band exhibits an orientation dependence, which is characteristic of the orientational distribution function of the residual dipolar interactions in the network. For perfect order the orientation dependence is determined by the square of the second Legendre polynomial [14]. Nearly perfect molecular order has been observed in porcine tendon by the orientation dependence of 1/T2 [77]. It can be concluded, that the NMR-MOUSE appears suitable to discriminate effects of macroscopic molecular order from effects of temperature and cross-link density by the orientation dependence of T2. 

Cross-linked liquid crystalline polymers with the optical axis being macroscopically and uniformly aligned are called liquid single crystalline elastomers (LSCE). Without an external field cross-linking of linear liquid crystalline polymers result in macroscopically non-ordered polydomain samples with an isotropic director orientation. The networks behave like crystal powder with respect to their optical properties. Applying a uniaxial strain to the polydomain network causes a reorientation process and the director of liquid crystalline elastomers becomes macroscopically aligned by the mechanical deformation. The samples become optically transparent (Figure 9.7). This process, however, does not lead to a permanent orientation of the director. 

An elastomer is a polymer that stretches and then reverts to its original shape. It is a randomly oriented amorphous polymer, but it must have some cross-linking so that the chains do not slip over one another. When elastomers are stretched, the random chains stretch out. The van der Waals forces are not strong enough to maintain them in that arrangement therefore, when the stretching force is removed, the chains go back to their random shapes. Rubber is an example of an elastomer. 

The elastomer that has preferable intralayer cross-linking (El, see Fig. 7) shows completely different behavior (see Fig. 12) (18,38). In this case, the switching time increases by less than a factor of 2, the polarization can still be determined, and measurement of the ferroelectric hysteresis shows no stabilization of the switching state present during cross-linking. Then, the coupling between the orientation of the mesogens and the network conformation is obviously very... 

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