Poly optical birefringence

As is known, the polypeptide a-helix molecules are rod-shaped if the helical internal structures are smeared out. Therefore, we may expect a phase separation in their solutions also. Indeed, Robinson (27) found in 1956 a phase separation in several solutions of the a-helix, poly-y-benzyl-L-glutamate, in which the second phase, separated out as small droplets, showed an optical birefringence. ITie critical concentration is of course a function of the molecular length. 

Fig. 26. Qualitative compatison of substrate materials for optical disks (187) An = birefringence IS = impact strength BM = bending modulus HDT = heat distortion temperature Met = metallizability WA = water absorption Proc = processibility. The materials are bisphenol A—polycarbonate (BPA-PC), copolymer (20 80) of BPA-PC and trimethylcyclohexane—polycarbonate (TMC-PC), poly(methyl methacrylate) (PMMA), uv-curable cross-linked polymer (uv-DM), cycHc polyolefins (CPO), and, for comparison, glass.

As discussed in section 7.1.6.4, semidilute solutions of rodlike polymers can be expected to follow the stress-optical rule as long as the concentration is sufficiently below the onset of the isotropic to nematic transition. Certainly, once such a system becomes nematic and anisotropic, the stress-optical rule cannot be expected to apply. This problem was studied in detail using an instrument capable of combined stress and birefringence measurements by Mead and Larson [109] on solutions of poly(y benzyl L-glutamate) in m-cresol. A pretransitional increase in the stress-optical coefficient was observed as the concentration approached the transition to a nematic state, in agreement of calculations based on the Doi model of polymer liquid crystals [63]. In addition to a dependence on concentration, the stress-optical coefficient was also seen to be dependent on shear rate, and on time for transient shear flows. 

The last polymer mentioned in Table 10.9 is poly(p-phenylene terephthalamide). This polymer is spun from its solution in pure sulphuric acid (100%), a dope that exhibits mesomorphic (=liquid crystalline) behaviour it is optically anisotropic and is nematic in character. A number of other polymers, containing rigid elements in the chain have melts of polymer liquid crystals, with a high birefringence and a non-linear optical behaviour in electric fields. 

Recently, the orientation behavior of poly-(IV-5) was compared to both poly-(IV-4) and a statistical copolymer poly-IV-5-COEN (COEN=cyclooctene) [51]. The experiments shotved that, compared to poly-(IV-5), samples of poly-(IV-5-COEN) were easier to orientate, a finding that was attributed to the more flexible chain. The dependence of the magnetic field and the temperature on the degree of orientation, measured by the birefringence and the optical transparency, is shown in Fig. 6. 

Systematic application of hydrodynamic and light-scattering methods has been made in conjunction with optical rotatory studies upon poly-e-Y-car-bobenzoxy-L-lysine (Applequist and Doty, 1961 Daniel and Katchalski, 1960, 1961) and poly-L-lysine (Applequist and Doty, 1961). Poly-e-Y-car-bobenzoxy-L-lysine in dimethyl formamide displays light scattering, viscosity, and flow birefringence behavior that is consistent with a predomi-... 

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