Implications for Auxetic Response in Liquid Crystalline Polymers: X‐Ray Scattering and Space‐Filling Molecular Modeling

Abstract

Five transverse rod (TR)‐containing main‐chain liquid crystalline polymers (LCPs) are examined in both the powder and fiber forms by wide angle X‐ray diffraction. Data from the diffraction experiments are consistent with the site‐connectivity‐driven TR‐reorientation mechanism for intrinsic auxetic character in these macromolecules. Shifts in peak maxima, intensity distributions in the interchain interaction region, and calculated volume increase accompanying fiber formation are all in line with expectations from considerations of this mechanism. The narrow window for experimental conditions necessary for fiber preparation and subsequent X‐ray observation of this phenomenon is detailed. A close examination of space‐filling Corey–Pauling–Koltun (CPK) molecular models reveals the details of local packing of rods and connected polymer chains in the quiescent nematic polymer melt prior to fiber drawing. It is found that two rod‐reorientation possibilities exist each of which leads to increased interchain separation as required for auxetic response. Suggestions for future experiments and materials design for new LCPs are described.