β Scribed by Tsvetkov, V. N.
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The flow birefringence and the photoelastic effect in films have been used to study the molecular optical anisotropy of stereoisomers of polystyrene (PS), poly(pβmethyl styrene) (PMS), poly(methyl methacrylate) (PMMA), and polyβ(nβbutyl methacrylate) (PBMA). Considerable differences in the optical anisotropy are observed between the atactic and isotactic structures of the molecular chain. These differences change in value and sign for various polymers and in some cases cause a change of the sign of the birefringence. The observed effect is due to the difference between the character of the rotation of the side radicals of the macromolecule in the isotactic and atactic isomers. The molecular optical anisotropy is shown to be especially sensitive to side group rotation in those cases where the positive anisotropy of the principal molecular chain is to a considerable degree compensated by a negative anisotropy of similar absolute value in the side groups. In these cases the study of the molecular optical anisotropy may serve as a sensitive method for studying the stereoregularity. The strain birefringence observed in films of crystallizable polymers is a sum of the two effects: the orientation of chains in the amorphous phase and the orientation of the stereoblocks forming crystallites. These two processes can be differentiated owing to differences in their time dependences, and experimental study of these may serve as a sensitive method for revealing traces of a crystalline phase in a polymer.
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