Rheological, Rheooptical and Physical Chemical Studies on Nematic Polymers and their Solutions

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9417374 Berry Rheological and rheooptical studies are being carried out on nematic polymers and their solutions. The linear and nonlinear viscoelastic properties of this technologically important class of materials are complex owing to the inherent anisotropy of the properties of an ordered, nematic phase. Four nematogenic polymers are to be studied to provide a systematic variation in the molecular characteristics giving rise to nematic order. Three of the systems are solutions of chains with large persistence lengths characterizing their conformation, but with differing degrees of rigidity to the molecular conformation. Two have high rigidity of structure, but with persistence lengths differing by a factor of two: poly (1,4-phenylene- 2,6-benzobisthiazole), PBZT, and poly (1,4-phenylene-2,6- benzobisoxazole), PBZO, with the former having the higher persistence length. The third, poly(1,4-phenylene- terephthalate, has a persistence length similar to PBZO, but is believed to have a less rigid structure. The fourth sample is a poly(1-olefin sulfone) known to form an ordered fluid over a range of temperatures. The proposed studies include static and dynamic light scattering on aligned monodomains to determine the Frank curvature elasticities for distortion of the director field, and associated viscosities, as functions of chain length, concentration and other variables. Third harmonic generation on the same samples will be used to assess surface layers of special alignment, including a biaxial nematic alignment, that may be important to rheological behavior. The emphasis will be on the use of creep and recovery and rheooptical measurements on nematic preparations, with no prior treatment to induce overall alignment, augmented by studies on monodomain samples to determine whether the preparations align in a slow shear flow. %%% The rheology of isotropic solutions of nematogenic polymers comprising rodlike sequences catenated by monomers imparting a no n rodlike linkage will also be studied. The study is directed to elucidation of aspects of the molecular dynamics of semiflexible chains. It is postulated that chains may exhibit a marked suppression in molecular mobility, perhaps leading to a structural glass similar to one proposed theoretically, but never found, for nematogenic rodlike polymers. This could occur owing to the severe restriction to rotational motions expected for each of the rodlike subunits, leading to an overall suppression of molecular diffusion, and solid-like behavior under small stresses. A yield behavior to fluid-like behavior is anticipated at larger stresses, owing to the relatively weak intermolecular interactions leading to the structural glass.