Random coil history

As mentioned already, distinction of single and multiple helical chains is not easy by the fiber diffraction method. This problem is very easily solved by the C NMR method, if these polymorphic structures can be identified with the aid of sample history and other experimental techniques and mutual conformational conversions among them can be manipulated by a series of physical treatments under a controlled manner, as illustrated in Fig. 24.3 [16-19]. The single chain form can be obtained from a sample of either the single helix by dehydration or the triple helix by lyophilization from DMSO solution. Even a multiple-stranded helix can be completely dispersed as a result of the conformational transition to a random coil form in DMSO... 

The orientation and morphology of thermotropic LCP parts are strongly dependent on the process conditions near the melt temperature because of the inherent molecular anisotropy and very long relaxation times. Random coil thermoplastic polymers are much less dependent on processing history near the melt temperature, and orientation requires a secondary step such... 

APPENDIX 5.1 HISTORY OF THE RANDOM COIL MODEL FOR POLYMER CHAINS ... 

For polyacrylamide there are two rheological effects which can be explained in terms of its random coil structure. Firstly, it was discussed above that polyacrylamide is much more sensitive than xanthan to solution salinity and hardness. This is explained by the fact that the salinity causes the molecular chain to collapse, which results in a much smaller molecule and hence in a lower viscosity solution. The second effect which can be explained in terms of the polyacrylamide random coil structure is the viscoelastic behaviour of this polymer. This is shown both in the dynamic oscillatory measurements and in the flow through the stepped capillaries (Chauveteau, 1981). When simple models of random chains are constructed, such as the Rouse model (Rouse, 1953 Bird et al, 1987), the internal structure of these bead and spring models gives rise to a spectrum of relaxation times, Analysis of this situation shows that these relaxation times define response times for the molecule, as indicated in the simple Maxwell model for a viscoelastic fluid discussed above. Thus, because of the internal structure of a flexible coil molecule, one would expect to observe some viscoelastic behaviour. This phenomenon is discussed in much more detail by Bird et al (1987b), in which a range of possible molecular models are discussed and the significance of these to the constitutive relationship between stress and deformation rate and deformation history is elaborated. 

The history of the creation of the first smart polymeric system dates back to the works of Kuhn and Katchalsky (88, 89). They demonstrated that collagen fibers changed dimension reversibly on transition from cyclic helices to random coils when immersed cyclically between salt solution and water. This was referred to as a mechanochemical system capable of transforming chemical energy to mechanical work. 

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