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.
Monodomains of smectic C liquid crystalline elastomers were prepared by controlled stretching of a polydomain thin film. Specimens were prepared from this film for stress–strain study. A Poisson's ratio of zero was found at small strains for uniaxial stretching perpendicular to the director. For uniaxial strain parallel to the director, Poisson's ratios of approximately 0.4–0.5 were found (by extrapolation) for very small strains. A modulus anisotropy of 8 was found for stretching parallel versus perpendicular to the director.magnified image
Stress–strain results are reported for two series of smectic C main chain liquid crystalline elastomers (MCLCEs). The structural variable in one series is crosslink density; in the other, transverse rod content. With increasing crosslinker content, the LCEs exhibit increased modulus, a shorter polydomain‐to‐monodomain (P–M) plateau, and a lower elongation at break. Increasing transverse rod content leads to lower clearing temperatures and lower modulus. For each series, stress–strain measurement was made simultaneously with Poisson's ratio determination. For the variable crosslinker series, typical behavior showed the Poisson's ratio increased at small strain, rose to a maximum about 50% strain, then decreased with greater strain. For the transverse rod series, typical behavior also showed the Poisson's ratio increased at small strain, rose to a maximum about 50% strain, then decreased with greater strain. In both series, the maximum in the Poisson's ratio corresponded to the onset of the P–M transition. Possible deformation mechanisms are presented.
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