Design, synthesis and characterization of liquid crystalline polymers and networks containing transverse rods for potential auxetic materials
Negative Poisson's ratio (Auxetic) polymers are of much current interest. The phenomenon involves materials which expand laterally as they are stretched. To date there is no example of a molecularly designed material which exhibits this remarkable mechanical property. This dissertation reports on progress in design, synthesis and characterization of liquid crystalline polymers and networks for potential auxetic materials. The idea is to use main chain liquid crystalline polymers which contain terminally-attached and laterally-attached mesogenic groups. In the relaxed state, these laterally-attached mesogens (transverse rods) will align themselves along the direction of the terminally-attached mesogens. These transverse rods will become perpendicular to the mesophase director when upon stretching and therefore cause an auxetic response. Main chain liquid crystalline polymers containing different length and various percentages of transverse rods have been synthesized. All these polymers display a nematic liquid crystalline phase. The thermal behavior of these polymers was studied by DSC and polarized optical microscopy. This study shows that incorporation of the transverse rods into the main chain LC polymers does not affect the mesophase stability to a great extent. The longer transverse rods do not destabilize the mesophase when compared with their shorter transverse rod counterparts. This result suggests that the transverse rods are indeed oriented along the nematic director in the unstretched state. This study also suggests that there are two types of packing patterns existing in these main chain LC polymers: co-linear and zigzag packing patterns. It is possible that these two packing patterns are co-existing in these polymers but longer transverse rods and shorter flexible spacer favor the zigzag packing pattern. Polymer networks with different crosslinking density have been synthesized. A polymer network containing 16.7% percentage of the pentaphenyl transverse rods retains its liquid crystallinity. X-ray studies of these polymers show that there is an interchain distance increase for the polymers containing transverse rods when compared with their parent polymer (no transverse rod component) counterpart.