Crosslinking mechanisms polymer chain conformation

An elastomer may be defined as a crosslinked polymer network whose temperature is above its glass transition temperature. The molecular mechanism responsible for rubber elasticity is based on changes in chain conformation brought about by the overall strain (see Figure 1.13). Clearly, the number of possible chain conformations must be fewer in case (c) than in case (a), resulting in a reduction of entropy (Flory, 1953, Chapter 11). Statistically all possible chain conformations are equally likely, assuming negligible... 

In conclusion, simple symmetry considerations allow for a successful orientation of poly domain elastomers using mechanical fields. In principle knowledge is only needed of the local chain conformation of the LC polymer on which the elastomer is based, and the consistent mechanical deformation must be applied. Nevertheless, the chemical constitution of the whole polymer network has to be considered. Often, the orientational behavior is strongly influenced by the crosslinking topology. As a mle of thumb, prolate chain conformations are increasingly preferred when the crosslinker concentration is increased and when the crosslinker molecules are more rod-shaped [90, 91]. 

The orientation of polydomain polymers by mechanical or viscous flow fields can be achieved easiest if a macroscopic chain anisotropy that coincides with the local symmetry of the LC phase structure is induced and fixed by chemical crosslinking. Eor nematic or Sa main chain polymers which locally show a prolate (see Sect. 3) chain anisotropy, a uniaxial deformation leads to a globally prolate chain conformation. If the chain conformation of the LC polymer is locally oblate, a globally oblate chain conformation can be induced by either uniaxial compression or -equivalently - biaxial stretching of the sample (Lig. 9). 

Thermoelastic measurements on such samples reveal a spontaneous elongation along n at the transition to the smectic phase, indicating a prolate polymer backbone conformation in the smectic elastomer [137]. On another hand, SANS results for end-on side-chain polymers in the smectic phase indicate an oblate chain conformation, with the backbone preferentially confined in the plane of the layers (Sect. 2.2). Thus, the chain distribution and macroscopic shape of the smectic elastomer change their sign if crosslinking is made under uniaxial mechanical stress in the isotropic and/or nematic phase. This result is remarkable and indicates that the oblate chain conformation of a smectic end-on polymer can be easily turned into prolate by a low uniaxial extension during solvent evaporation. 

Polypeptide homopolymers (typically PBLG) with rigid a-helix conformation can form LC structures at a high concentration and temperature. When the solution is cooled, a transparent, mechanically self-supporting gel is always observed [42, 87-91]. The gel formation was found to be concentration and temperature dependent and completely reversible. It is well known that the physical or chemical crosslinks are necessary for polymer gels. Flexible polymers can easily form crosslinking domains with crystalline or semicrystalline structures. However, for rigid polypeptide chains, it is less clear how the rodlike polypeptides participate extensively in intermolecular crosslinks. 

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