Effects of tacticity and chiral center-to-dipole distance on mesogen-free liquid crystalline self-assembly of sulfonyl-containing comb-like polymers

• Published in: Polymer chemistry 2020-05, Vol.11 (17), p.318-331
• Main Authors: Bohannon, Caleb A, Kwok, Man-Hin, Li, Ruipeng, Zhu, Lei, Zhao, Bin
• Format: Article
• Language: English
• Subjects: Atacticity; Crystal structure; Crystallinity; Dipole interactions; Electronic devices; Epichlorohydrin; Ferroelectric materials; Ferroelectricity; Isotacticity; Liquid crystal polymers; Liquid crystals; NMR; Nuclear magnetic resonance; Phase transitions; Polyethers; Polymer chemistry; Polymers; Self-assembly; Substitution reactions; Tacticity
• ISSN: 1759-9954; 1759-9962
• DOI: 10.1039/d0py00199f

In this report, new approaches were implemented to achieve novel ferroelectric liquid crystalline self-assembly in mesogen-free comb-like isotactic polyoxypropylenes (iPOPs) bearing mono- or di-sulfonyl groups, which have potential to exhibit high spontaneous polarization for applications in advanced electronic devices. iPOPs with n -alkylsulfonylpentylthioether side chains (iPOP-SC 5 SO 2 C n , where C n is the alkyl tail with n being either 12 or 8) were prepared via a post-polymerization substitution reaction from isotactic poly[( R )-(−)-epichlorohydrin]. The effects of main-chain tacticity, chiral center-to-dipole distance, number of sulfonyl dipoles per side chain, and n -alkyl tail length on the liquid crystalline self-assembly behavior were investigated by comparing iPOPs with their atactic counterparts. First, when the sulfonyl dipole was placed far away from the chiral center in iPOP-SC 5 SO 2 C n compared with our previously reported isotactic polyethers with n -alkylsulfonyl side chains (iPOP-SO 2 C n ), the dipole-dipole interactions among the side chains decreased, which led to more liquid crystalline phases. Smectic E (SmE) and A (SmA) phases were observed after crystal melting; however, the main-chain chiral center was found to have an insignificant effect on the liquid crystalline assembly when compared to the atactic samples. Second, the longer C n tails increased the transition temperatures as a result of stronger van der Waals interactions. Third, after the thioether linkage in the side chain was oxidized into the sulfonyl group, the chiral center-dipole and dipole-dipole interactions in the resultant polymers ( i.e. , iPOP-SO 2 C 5 SO 2 C n ) enhanced. As a result, the liquid crystalline phase transitions ( i.e. , crystal → SmE → SmA → I) were pushed to higher temperatures. The knowledge gained in this study will help us further design and achieve the ferroelectric smectic C* phase in mesogen-free comb-like sulfonylated liquid crystalline polymers. Mesogen-free comb-like polyethers bearing strongly interacting mono- and di-sulfonylated side chains exhibit well-defined liquid crystalline self-assembly.