Toward molecular auxetics: Main chain liquid crystalline polymers consisting of laterally attached para-quaterphenyls
Polymers containing para-quaterphenyl rods laterally attached to the polymer main chain have been synthesized. The molecular design choices which led to use of the quaterphenyl rod to achieve auxetic response will be discussed. Preparative chemistry of the monomers will be described along with the details of the polymerization reaction. Both polyethers and polyesters were prepared. The length of the flexible alkyl main chains which link the rods plays an important role in determining whether the resulting polymers will exhibit liquid crystallinity. Polymers with longer alkyl links exhibit nematic liquid crystallinity and the rods appear to have their long axes oriented roughly parallel and along the main chain direction. X-ray scattering experiments were performed on the polymers both in the quiescent (unstretched) and the stretched states. Results from these experiments suggest that, under tensile strain, site connectivity driven rod-reorientation occurs in these materials giving rise to an increase in the interchain distance for these polymers. This increase in the interchain separation is consistent with our concept of a molecular level auxetic mechanism for this type of designed polymer. (c) 2005 WILEY-VCH Verlag GmbH P Co. KGaA, Weinheim.