A main-chain nematic polymer melt/solution exhibits macroscopic orientational order of main polymer chains, i.e., a preferred (nematic) direction. It has long been known that in such polymeric liquid crystals spatial density/concentration variations and distortions of the nematic direction are coupled, obeying a vectorial continuity constraint whose rigidity increases with chain length. Its vectorial nature precludes the application to flexible chains, where backfolds (hairpins) are present and apolar nematic symmetry is manifest, which has been its puzzling feature from the beginning. We now establish a description of the splay–density coupling in the case of arbitrary backfolding, devising a continuity constraint for the “recovered” polar order of the chain tangents and introducing hairpins as its new type of sources. Performing detailed Monte Carlo simulations of nematic monodomain melts of “soft” worm-like chains with variable length and flexibility, we show via their structure factors that the weakening of the coupling due to the backfolding can be consistently quantified on the macroscopic level.

中文翻译：

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半挠性主链向列聚合物与发夹的张径-密度耦合

主链向列聚合物熔体/溶液表现出主聚合物链的宏观取向顺序，即，首选（向列）方向。早就知道，在这种聚合物液晶中，遵循矢量刚度随链长而增加的矢量连续性约束，在空间密度/浓度向列方向上的变化和畸变是耦合的。它的矢量性质阻止了它在存在反向折叠（发夹）并且明显表现出非极性向列对称性的柔性链中的应用，这从一开始就是令人困惑的特征。现在，我们对任意反向折叠情况下的张径-密度耦合进行描述，为链切线的“恢复”极性顺序设计一个连续性约束，并引入发夹作为其新型来源。对可变长度和柔性的“软”蠕虫状链向列单畴熔体进行详细的蒙特卡洛模拟，通过其结构因素，可以在宏观水平上一致地量化由于反向折叠而导致的耦合弱化。