Rigid chains

Spiro polymers are also sometimes classified as ladder polymers, and molecules in which the ladder structure is interrupted by periodic single bonds are called semiladders. Consisting entirely of fused ring structures, ladder polymers possess very rigid chains with excellent thermal stability. 

Nonthermoprocessible Condensation Polyimides. These are obtained from condensation of aromatic dianhydrides with aromatic diamines. They are linear noncross-linked resins but their rigid chain stmcture and strongly hydrogen-bonded character leads to systems which do not melt or soften before decomposition. 

FIG. 11 Adsorbed amount as a function of bulk concentration for a non-interacting (empty symbols) and adsorbing (full symbols) wall. Diamonds and triangles correspond to a system with semi-rigid chains, circles and squares for flexible chains [28]. 

FIG. 13 Average center-of-mass position of flexible chains of length / with respect to the nearest solid surfaces for different /. Diamonds denote a system of semi-rigid chains in which the opposite effect is observed [28]. 

On the other hand, as the width of a closed three-dimensional gap V is decreased, L(T>)) gradually decreases for absolutely flexible chains whereas for semi-rigid chains it goes through a minimum at D = 2 monomer diameters and then grows again for D = 1 (Fig. 15). 

A large number of SAHs described in the literature combine synthetic and natural macromolecules in the network structure. The natural components are usually starch, cellulose, and their derivatives. It is assumed that introduction of rigid chains can improve mechanical properties (strength, elasticity) of SAH in the swollen state. Radical graft polymerization is one of the ways to obtain such SAH. 

Papkov, S. P. Liquid Crystalline Order in Solutions of Rigid-Chain Polymers. Vol. 59, pp. 75— 102. 

Tsvetkov, V. and Andreeva, L. Flow and Electric Birefringence in Rigid-Chain Polymer Solutions. Vol. 39, pp. 95-207. 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

These two different approaches for attaining an oriented state in flexible-chain and rigid-chain polymers indicate that the fundamental property of macromolecules - their flexibility - is of great importance to the orientation processes. However, the mechanism of the transition into the oriented state and the properties of highly oriented systems exhibit many features characteristic of both rigid- and flexible-chain polymers. 

Hence, Flory s theory offers an objective criterion for chain flexibility and makes possible to divide all the variety of macromolecules into flexible-chain (f > 0.63) and rigid-chain (f < 0.63) ones. In the absence of kinetic hindrance, all rigid-chain polymers must form a thermodynamically stable organized nematic phase at some polymer concentration in solution which increases with f. At f > 0.63, the macromolecules cannot spontaneously adopt a state of parallel order under any conditions. 

For rigid-chain crystallizable polymers, spontaneous transition into the nematic phase is accompanied by crystallization intermolecular interactions should lead to the formation of a three-dimensional ordered crystalline phase. 

Fig. 1. Schematic representation of the structure of a crystallizable rigid-chain polymer point defects are located at end joints...

Hence, the main aim of the technological process in obtaining fibres from flexible-chain polymers is to extend flexible-chain molecules and to fix their oriented state by subsequent crystallization. The filaments obtained by this method exhibit a fibrillar structure and high tenacity, because the structure of the filament is similar to that of fibres prepared from rigid-chain polymers (for a detailed thermodynamic treatment of orientation processes in polymer solutions and the thermokinetic analysis of jet-fibre transition in longitudinal solution flow see monograph3. ... 

Fig. 21 a-c. Schematic representation supramolecular structure of a crystalline rigid-chain polymer (a), an idealized ECC of a flexible-chain polymer (b) and an orientationally crystallized sample with a spatial ECC framework (c)... 

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... 

Stable microcapsules require the combination of polymer solutions containing both flexible and rigid chains. 

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