Rigid Rod Polymers Having Flexible Side Chains. 3. Structural Investigations on a Novel Layered Mesophase Formed by Thermotropic Poly(1,4-phenylene-2,5-dialkoxy terephthalate)s

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Abstract
A series of thermotropic poly(1,4-phenylene-2,5-di-n-alkoxy terephthalate)s with the length of the n-alkoxy side chains being varied between n = 2 and n = 18 carbon atoms has been examined by optical microscopy, x-ray diffraction and DSC. All members of this series have a rigid backbone with attached flexible side chains and form mesomorphic melts. In polymers with side chains shorter than 7 C-atoms the mesophases are found to be nematic; longer side chains lead to the observation of a novel, layered mesophase (modification A). The layer spacing in this mesophase as revealed by x-ray analysis is shown to increase linearly with increasing length of the side chains. The observed increment of 1.25Å per CH2 unit shows the side chains to protrude normal to the main chain axis. A comparison of the molecular size of the polyester molecule with the layer spacing suggests that the side chains interdigitate in the mesophase. A similar layered structure with a smaller increment (0.82Å/CH2 unit) is found in the solid state. In contrast to modification A this structure (modification B) is highly crystalline. Packing considerations again suggest interdigitated side chains whereas the smaller spacing of the layers points to a staggered arrangement of the main chains.