Main-chain liquid crystalline polymers bearing periodically grafted folding elements†
A series of main-chain liquid crystalline polymers (MCLCPs) carrying a biphenyl mesogen and a flexible alkylene spacer in the backbone was prepared; a unique feature of these polymers is that they have a non-mesogenic pendant segment, namely, an alkyl, PEG or fluoroalkyl segment, which is periodically located along the backbone. Due to the presence of these periodic substituents, the chain folds in a zigzag fashion, permitting the collocation of the mesogenic biphenyl units within one layer and the pendant segments in alternate layers of a lamellar morphology generated by these polymers. Most of the polymers were found to exhibit a stable smectic mesophase upon melting that appears to retain the folded chain conformation, which becomes disordered only after the isotropization transition. From the variation of the interlamellar spacing, estimated from SAXS studies, as a function of the pendant alkyl segment length, it was evident that the pendant segments adopted an extended all-trans conformation and these were fully interdigitated. Furthermore, by comparing the d-spacing of a sample that had longer alkylene backbone segments (C10 instead of C6), we were able to show that the linear variation remains valid for an increase in both the length of the pendant alkyl chain as well as that of the backbone alkylene segment. This observation serves as further evidence for the zigzag folded chain conformation adopted by this class of periodically substituted MCLCPs. Furthermore, the study also reveals the role of aromatic mesogens in enhancing the propensity to adopt this conformation and attain lamellar morphologies, wherein the dimensions are regulated by only the grafting density and the grafted segment length and not by the molecular weight of the polymer.