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-connectivitydriven 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.

Cancellation of Auxetic Properties in F.C.C. Hard Sphere Crystals by Hybrid Layer-Channel Nanoinclusions Filled by Hard Spheres of Another Diameter

“…Novel structures [26][27][28][29][30][31][32][33][34], nanostructures [35,36], and models [37][38][39][40][41][42][43][44], real materials (e.g., polymers [45,46], composites [47], or foams [48,49]), and metamaterials [50,51] with auxetic properties have been designed and reported. These advances in real metamaterials would not have been possible without more basic research [52][53][54], analysis [55][56][57][58][59], and optimization [60,61] of auxetic models on various levels. This effort is motivated by the extraordinary, counterintuitive elastic properties and potential applications of auxetics [62][63][64][65][66][67][68].…”

Removing Auxetic Properties in f.c.c. Hard Sphere Crystals by Orthogonal Nanochannels with Hard Spheres of Another Diameter

“…Today, novel structures [ 36 , 37 ], nanostructures [ 38 ], and metamaterials [ 39 , 40 , 41 ] with auxetic properties are being developed. This would not have been possible without extensive theoretical studies in the form of basic research [ 42 , 43 , 44 ] or analysis and optimizations [ 45 , 46 ] of novel auxetic model structures and metamaterials. The study of materials with inclusions at the structural level is one of the possible directions of such optimizations [ 47 , 48 , 49 , 50 , 51 ].…”

Partially Auxetic Properties of Face‐Centered Cubic Hard‐Sphere Crystals with Nanochannels of Different Sizes, Parallel to [001]‐Direction and Filled by Other Hard Spheres

“…This attracts more and more researchers to study known auxetic systems and look for new ones. [9][10][11] The scope of research on auxetics today covers all scales from nanostructures to macrostructures, as well as theoretical [12,13] and experimental [14,15] research. Theoretical models are an important element in a deeper understanding of the mechanisms of auxeticity [16][17][18][19][20] and the search for new ideas for creating auxetics.…”