Mesophases stress-induced

PROPERTIES OF SPECIAL INTEREST Low glass transition temjjerature, mesophase behavior including reversible stress-induced mesophase formation accompanied by necking-denecking phenomena in cyclic deformation of elastomers at room temperature and above. 

In spite of the clear analogy between stress-induced mesophase formation and stress-induced crystallization under flow, there is a significant difference between the two processes. Thermodynamically, for a phase transition accompanied by a change in length AL, the relationship between the force/ and the temperature T is given by [71,72]... 

Godovsky, Y. K. Valetskaya, L. A., Mesophase Elastomers Stress-Induced Mesophase Formation in Poly(diethylsloxane) Networks and Their Thermome-chanical Behavior. Polym. Bull. 1991, 27, 221-226. 

A molecular chain of PDES contains no mesogenic elements still two columnar mesophases Xi and X2 exist above the melting point of the crystalline state. In cross-linked PDES elastomers stretching can induce an amorphous-mesophase transition. With increasing stress the transition shifts to higher temperature. Kinetically this transition follows a 2D growth model. 

Elasticity is a macroscopic property of matter defined as the ratio of an applied static stress (force per unit area) to the strain or deformation produced in the material the dynamic response of a material to stress is determined by its viscosity. In this section we give a simplified formulation of the theory of torsional elasticity and how it applies to liquid crystals. The elastic properties of liquid crystals are perhaps their most characteristic feature, since the response to torsional stress is directly related to the orientational anisotropy of the material. An important aspect of elastic properties is that they depend on intermolecular interactions, and for liquid crystals the elastic constants depend on the two fundamental structural features of these mesophases anisotropy and orientational order. The dependence of torsional elastic constants on intermolecular interactions is explained, and some models which enable elastic constants to be related to molecular properties are described. The important area of field-induced elastic deformations is introduced, since these are the basis for most electro-optic liquid crystal display devices. 

The property these materials exhibit of greatest relevance here is their stress-strain isotherms. Some obtained for PDES (270) as a function of degree of mesomorphic structure are shown schematically in Figure 5. The curves exhibit deld points similar to those shown by partially crystalline polymers, and the overall shapes of the curve differ greatly with the increase in the amounts of mesophase present, either present initially or induced by the deformation. As is generally the case, formation of a second phase leads to irreversibility in the stress-strain isotherms. 

---