Optical effect Order-disorder transition

In Chapter 8 we will present a detailed discussion of the isotropic phase molecular orientation effects by an applied optical field from a short intense laser pulse. It is shown that both the response time and the induced order Q depend on the temperature vicinity (T-in a critical way they both vary as (r- which becomes very large near T. This near-r critical slowing down behavior of the order parameter Q of the isotropic phase is similar to the slowing down behavior of the order parameter S of the nematic phase discussed in the previous section. Besides the nematic isotropic phase transition, which is the most prominent order disorder transition exhibited by Uquid ciystals, there are other equally interesting phase transition processes among the various mesophases, such as smectic-A smectic-C, which will be discussed in Chapter 4. 

From now on we will consider first of all that any appreciable perturbation of the COTTON effects consecutive to a modification in the external conditions is the result of a chemical reaction before envisaging the influence of conformations. Up til now, we have not needed to take into consideration order-disorder transitions to interpret the behaviour of the optically active polymers that we have synthesized. 

Conformations of the polymers were studied by CD and optical rotation measurements. Poly-L-lysine is known to exist in disordered, helical and -conformation, depending on the temperature, pH of the system and the solvent used. The side chain of the polymer has a significant effect on the backbone conformation. At neutral pH, poly-L-lysine exists in a random coil structure while at pH above 10, the e-amino group becomes a neutral form and the polymer undergoes transition to a helical structure. In order to elucidate the effect of base substituents on the conformation of poly-L-lysine, CD spectra of the copolymer were measured. 

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