Elastomeric state

Elastomeric state Elastomer modifiers Elastomer process Elastomers... 

Both low molecular weight materials [145] and polymers [146,147] can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides. It can also occur with flexible chains, however, and it is these flexible chains in the elastomeric state that are the focus of the present discussion. One reason such liquid-crystalline elastomers are of particular interest is the fact that (i) they can be extensively deformed (as described for elastomers throughout this chapter), (ii) the deformation produces alignment of the chains, and (iii) alignment of the chains is central to the formation of liquid-crystalline phases. Because of fascinating properties related to their novel structures, liquid-crystalline elastomers have been the subject of numerous studies, as described in several detailed reviews [148-150]. The purpose here will be to mention some typical elastomers exhibiting liquid crystallinity, to describe some of their properties, and to provide interpretations of some of these properties in molecular terms. 

Thermal Activation Energies. The relaxation that occurs as the polymer passes from the glassy state to the elastomeric state can be described using an Arrhenius type equation. 

The Burger model represents the behavior above Tg in the elastomeric state, while the single dash-pot simulates melt flow at higher temperatures. Around the transition between the glassy and elastomeric states, the modulus of elasticity drops 10 to 10 fold, and the material becomes flexible and... 

The use of a polymer is, of course, determined by the application. Polymers can be used in the glassy state (examples of this are PMMA or Plexiglas, polystyrene, and epoxy), or in the rubbery or elastomeric state, where they are soft (examples include polyisoprene or polybutadiene). The limit of usability for both groups is determined by the glass transition temperature. 

Their composition induces the formation of a morphology in which polystyrene spheres are distributed with a cubic symmetry in an elastomeric matrix of polybutadiene. At the service temperature the spherical polystyrene nodules are in the glassy state, whereas the polybutadiene chains connecting them are in the elastomeric state. Figure 5.36 shows the rigid nodules of PS in their role of physical cross-links, which are responsible for the reversibility of the deformations undergone by the sample. 

Introduction. In the case of polymers, liquid crystallinity ( mesomor-phism ) frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides (263). It can also occur with flexible chains, and it is these flexible chains in the elastomeric state that are relevant here. Some reasons why such liquid-crystalline elastomers are of particular interest is the fact that (i) they can be extensively deformed, (ii) the deformation produces alignment of the chains. 

Polymer networks are conveniently characterized in the elastomeric state, which is exhibited at temperatures above the glass-to-rubber transition temperature T. In this state, the large ensemble of configurations accessible to flexible chain molecules by Brownian motion is very amenable to statistical mechanical analysis. Polymers with relatively high values of such as polystyrene or elastin are generally studied in the swollen state to lower their values of to below the temperature of investigation. It is also advantageous to study network behavior in the swollen state since this facilitates the approach to elastic equilibrium, which is required for application of rubber elasticity theories based on statistical thermodynamics. ... 

Rajan G S, Vu Y T, Mark J E and Myers Ch L (2004) Thermal and mechanical properties of pol3T)ropylene in the thermoplastic elastomeric state, Eur Polym J 40 63-71. 

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