Rigid-chain polymers linear

In order to give a general idea about the appearance of liquid crystalline state and about the properties of the systems containing these polymers, some examples of linear rigid-chain polymers follow ... 

Cayton RH, Chisholm MH, Huffman JC, Lobkowsky EB (1991) l,8-Naphthyridinyl-2,7-dioxy bridged quadruply bonded molybdenum-molybdenum system, an insight into the nature of a linear rigid chain polymer [M-M... M-M]. Angew Chem Int Edit 30 862-864... 

At present, it is known that the structures of the ECC type (Figs 3 and 21) can be obtained in principle for all linear crystallizable polymers. However, in practice, ECC does not occur although, as follows from the preceding considerations, the formation of linear single crystals of macroscopic size (100% ECC) is not forbidden for any fundamental thermodynamic or thermokinetic reasons60,65). It should be noted that the attained tenacities of rigid- and flexible-chain polymer fibers are almost identical. The reasons for a relatively low tenacity of fibers from rigid-chain polymers and for the adequacy of the model in Fig. 21 a have been analyzed in detail in Ref. 65. 

The molar mass dependence of the intrinsic viscosity of rigid chain polymers cannot be described by a simple scaling relation in the form of Equation (36) with molar mass independent of K and a. over a broad molar mass range. Starting from the worm-like chain model, Bohdanecky proposed [29] the linearizing equation... 

The conformation of macromolecules in linear polymers can be characterized by the vector h which connects the chain ends (end-to-end vector). For rigid-chain polymers the conformation of macromolecules is usually described by the persistence length a (worm-Uke chain model) which is related to /i as ... 

Fibres from rigid-chain aromatic polyamides (para-aramids), which are viewed as single-phase structures, are obviously more amenable to a detailed analysis than fibres from linear flexible-chain polymers with segregated crystalline and noncrystalline regions. The absence of chain folding in p ra-aramids also simplifies the situation. Compilation of early results showed that the chain orientation with respect to the fibre axis has a strong effect on fibre modulus this was later confirmed by more detailed studieswhich resulted in the formulation of a relationship between the fibre modulus E and the orientation, expressed as ... 

Even from the generalized (and often oversimplified) picture presented here, it should be clear that understanding of the relationship between the fibre-formation conditions, structure and properties is still incomplete. As far as fibres from linear flexible-chain polymers are concerned, it is the concept of the degree of chain extension that needs to be developed and quantified, particularly in relation to the limiting role of the transient network. For fibres from rigid-chain polymers it is particularly the formation of defects during the fibre manufacture and their effect on fibre properties that need to be understood. 

It is possible to classify polymers by their structure as linear, branched, cross-linked, and network polymers. In some polymers, called homopolymers, merely one monomer (a) is used for the formation of the chains, while in others two or more diverse monomers (a,p,y,...) can be combined to get different structures forming copolymers of linear, branched, cross-linked, and network polymeric molecular structures. Besides, on the basis of their properties, polymers are categorized as thermoplastics, elastomers, and thermosets. Thermoplastics are the majority of the polymers in use. They are linear or branched polymers characterized by the fact that they soften or melt, reversibly, when heated. Elastomers are cross-linked polymers that are highly elastic, that is, they can be lengthened or compressed to a considerable extent reversibly. Finally, thermosets are network polymers that are normally rigid and when heated do not soften or melt reversibly. 

---