Rotational constraints polymer conformation

Tadoroko et al. provide a new method for the calculation of the elasticity of an isolated helical chain and the distribution of strain energy to the internal co-ordinates under conditions that the rotational angle per monomer is constant. For the calculation of three-dimensional elastic constants, the space group symmetry of the unit cell may be used to reduce the memory size required for polymers such as poly(vinyl alcohol) which have unsymmetrical repeat units. The constraints imposed upon tie molecules in semi-crystalline polyethylene are considered to restrict them to three conformations, the deforma-... 

One of the assumptions made in the isotropic model, that is the chromophores can rotate freely, may be responsible for this discrepancy. When the NLO chromophores are linked to the polymer backbone either directly or through short spacers, the motions of both die main chain and the side groups are coupled. This results in conformational constraints on chromophore ordering. Herminghaus et al. and Robin et al. developed simple models which take into account the effects of such conformational constraints and yield dss/dai ranging from 3 to 6. 

While the flow and creep of a polymer are necessarily associated with translational motion, the internal rotational shape change is taking place at the same time. So the total movement is a combination of the two. As before, when polymers approach the ideal flexible model with copious internal rotation, then their behaviour ceases to be sensitive to local conformational constraints and can... 

If the long polymer chains are interconnected by widely separated cross-links, the portions of chains between the cross-links can still assume the three states mentioned. In the case of melt state, a loosely cross-linked melt is called a rubber. The extent to which the original molecules can assume new conformations is limited because of the topological constraints of the cross-links. If the number of cross-links is increased, the portions of chains between cross-links become shorter. Finally, these sections of chains may be so short that rotations around single bonds and segmental diffusion are no longer possible and the system resembles a permanent glass even at temperatures at which an uncross-linked polymer would be a melt. 

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