Rigid chain polymers anisotropic solutions

Orientational order appears in the solutions of rigid-chain polymers because a random mutual arrangement of their macromolecules is possible only up to a certain concentration of the solution. To retain a minimal volume (minimal free energy) above a certain critical concentration, asymmetric macromolecules must acquire an ordered mutual arrangement, which corresponds to a transition to the state typical for liquid crystals. In this case the solution becomes anisotropic. The degree of this anisotropy is still less than strict three-dimensional ordering typical of crystalline systems, but at the same time it differs from that of the isotropic state typical of amorphous systems. 

The polydispersion of the macromolecules, which results in their partial fractionation by lengths between the isotropic and anisotropic phases, plays an important role in nematic ordering in solutions of rigid-chain polymers [50-53, 115-117]. 

Fig. 2.4. Dependence of the critical concentration of the transition of a solution of a rigid-chain polymer into the anisotropic state (v ) on axial ratio x solid line theoretical curve O experimental points for PBA-DMAA + Ua [21] forPPTA-H SO [22] A forPBA-DMAA + Lia[23].

Fig. 9.23. Temperature dependences of the anisotropic viscosity coefficients of p-azoxyanisole (a) and solutions of rigid-chain polymers in different regions of the phase diagram (b).

Many cellulose derivatives form lyotropic liquid crystals in suitable solvents and several thermotropic cellulose derivatives have been reported (1-3) Cellulosic liquid crystalline systems reported prior to early 1982 have been tabulated (1). Since then, some new substituted cellulosic derivatives which form thermotropic cholesteric phases have been prepared (4), and much effort has been devoted to investigating the previously-reported systems. Anisotropic solutions of cellulose acetate and triacetate in tri-fluoroacetic acid have attracted the attention of several groups. Chiroptical properties (5,6), refractive index (7), phase boundaries (8), nuclear magnetic resonance spectra (9,10) and differential scanning calorimetry (11,12) have been reported for this system. However, trifluoroacetic acid causes degradation of cellulosic polymers this calls into question some of the physical measurements on these mesophases, because time is required for the mesophase solutions to achieve their equilibrium order. Mixtures of trifluoroacetic acid with chlorinated solvents have been employed to minimize this problem (13), and anisotropic solutions of cellulose acetate and triacetate in other solvents have been examined (14,15). The mesophase formed by (hydroxypropyl)cellulose (HPC) in water (16) is stable and easy to handle, and has thus attracted further attention (10,11,17-19), as has the thermotropic mesophase of HPC (20). Detailed studies of mesophase formation and chain rigidity for HPC in dimethyl acetamide (21) and for the benzoic acid ester of HPC in acetone and benzene (22) have been published. Anisotropic solutions of methylol cellulose in dimethyl sulfoxide (23) and of cellulose in dimethyl acetamide/ LiCl (24) were reported. Cellulose tricarbanilate in methyl ethyl ketone forms a liquid crystalline solution (25) with optical properties which are quite distinct from those of previously reported cholesteric cellulosic mesophases (26). 

Rigid and semirigid polymer chains form anisotropic structures in the melt or in solution, which result in high orientation in the solid state... 

Lyotropic LCPs are polymers whose solutions exhibit liquid crystallinity, that is, anisotropic domains in a fluid system, over a characteristic range of concentrations. In more concentrated solutions the system may be multiphasic and contain crystalline particles, amorphous gel particles and anisotropic solution coexisting with one another. Upon dilution, the anisotropic liquid crystalline solution turns biphasic, where anisotropic and isotropic solutions of the same polymer in the same solvent coexist. Upon further dilution, the solution becomes fully isotropic. Polymers that exhibit lyotropic mesomorp-hicity are either stiff-backbone polymers with strong interchain interaction in the absence of solvent or polymers whose backbones are so extended and rigid that, upon breakup of their crystalline order by the addition of some solvent, the stiff polymer chains retain substantial measure of parallel alignment to remain in mobile anisotropic domains. 

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