Conventional networks

In order to emphasize the additional phenomena resulting from liquid crystallinity, we adopt the following postulates of a classical polymer network (Treloar, 1975)  

the probability of R, the end-to-end vector of a strand of a polymer network, is given by 

The three orthogonal components of the mean square end-to-end distance are equal. 

According to the affine deformation postulate for the bulk deformation A, a symmetrical rank two tensor, the end-to-end distance vector deforms from Ro to 

Define the extension ratio along the extension direction, Z axis, by A = z, then 

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Ladder polymers are double-strand linear polymers. Their permanenee properties are superior even to those of conventional network polymers. The latter are randomly cross-linked, and their molecular weight ean be redueed by random scission events. When a chemical bond is broken in a ladder polymer, however, the second strand maintains the overall integrity of the molecule and the fragments of the broken bond are held in such close proximity that the likelihood of their recombination is enhanced. 

Small molecular mass liquid crystals do not respond to extension and shear stress. Liquid crystalline polymers may exhibit a high elastic state at some temperature due to the entanglements. However, the liquid crystalline network itself is an elastomer, showing rubber elasticity. In the presence of external stress, liquid crystalline networks deform remarkably and then relax back after the release of stress. The elasticity of liquid crystalline networks is more complicated than the conventional network, such as the stress induced phase transition, the discontinuous stress-strain relationship and the non-linear stress optical effect, etc. 

In order to understand the pecuHarities in the deformation of hypercrosslinked polystyrenes, let us first consider the deformation of conventional network styrene copolymers (Fig. 7.38). The copolymer incorporating 3% DVB exhibits two physical states, glassy and rubbery, with a narrow transition zone... 

As shown in Fig. 7.41, the deformation of all hypercrossHnked beads noticeably increases at a temperature above 100°C. However, this phenomenon is not related to trivial transition of polystyrene segments from glassy to rubber-Hke state. The very nature of deformation of hypercrosslinked materials differs fundamentally from the deformation of conventional networks under rubber-Hke elasticity. 

These arbitrarily chosen p values are consistent with the well-known effect of swelling in diminishing the ratio Cp/C. in conventional networks. Although the numerical data are uncertain, the qualitative nature of the anisotropy seems clear. Young s modulus for small extensions of the dry network from the state of ease should correspondingly be smaller in the direction of original stretch than perpendicular to it, but the calculation of this aspect of the anisotropy as well as its experimental measurement is more difficult. 

One remaining possibility that is less costly from an energy point of view but needs to be carefully controlled is to incorporate additives called flow improvers. These materials favor the dispersion of the paraffin crystals and in doing so prevent them from forming the large networks which cause the filter plugging. The conventional flow improvers essentially change the CFPP and pour point, but not the cloud point. They are usually copolymers, produced, for example, from ethylene and vinyl acetate monomers ... 

Block copolymers with stmctures such as A—B or B—A—B ate not thermoplastic elastomers, because for a continuous network to exist both ends of the elastomer segment must be immobilized in the hard domains. Instead, they are much weaker materials resembling conventional unvulcanized synthetic mbbers (4). 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

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