Double Twist in Helical Polymer “Soft” Crystals

Abstract

In natural and synthetic materials having non-racemic chiral centers, chirality and structural ordering each play a distinct role in the formation of ordered states. Configurational chirality can be extended to morphological chirality when the phase structures possess low liquid crystalline order. In the crystalline states the crystallization process suppresses the chiral helical morphology due to strong ordering interactions. In this Letter, we report the first observation of helical single lamellar crystals of synthetic non-racemic chiral polymers. Experimental evidence shows that the molecular chains twist along both the long and short axes of the helical lamellar crystals, which is the first time a double-twist molecular orientation in a helical crystal has been observed.PACS numbers: 61.41. + e Biological materials and their fascinating functions in the human genome have been extensively explored and continue to stimulate new research directions in materials science. Synthetic polymers are similar to proteins and deoxyribonucleic acids (DNA) with respect to their long chain nature, but synthesizing polymers which possess properties similar to biomaterials require, at the very least, an introduction of chirality. The chirality effect on the material properties and structures of small molecular liquid crystals indicates that a series of new phases exist through the introduction of chiral centers which have interesting electro-optical behaviors [1][2][3][4][5]. A helical morphology with a pitch length of several micrometers is typical of chiral liquid crystalline (LC) phases, but exists only in low ordered LC phases. In highly ordered smectic crystal phases, the helical morphology is suppressed by the crystallization process, leading to the traditional parallel close packing scheme in three-dimensional space [1]. We expect that by directionally connecting small LC molecules with covalent bonds to form main-chain nonracemic chiral LC polymers will lead to an enhancement of the conformational chirality strength. The chirality strength should be strong enough to compete with the parallel close packing scheme during crystallization and stabilize the helical morphology in a crystalline state.The material studied is a main-chain chiral polyester synthesized from ͑R͒-͑2͒-4 0 -͕v-͓2-͑ p-hydroxy-o-nitrophenyloxy͒-1-propyloxy͔-1-nonyloxy͖-4-biphenyl carboxylic acid. The polymer has a spacer of nine methylene units, and is abbreviated as PET͑R ‫ء‬ ͒-9, This polymer was specifically synthesized by an A-B type of condensation to ensure strict head-to-tail connections between adjacent repeating units [6,7]. The polymer possesses right-handed chiral centers ‫͒ء͑‬ along the mainchain backbone. The specific rotation of the monomer is 228.5 ± . The molecular weight of PET͑R ‫ء‬ ͒-9 is approximately 16 000 g͞mol with a polydispersity of 2 after fractionation, as measured by gel permeation chromatography based on polystyrene standards.Polymer thin films (approximate thickness of 50-100 nm) were prepared by casting a 0.05 (wt) % te...

Enantioselective control of lattice and shape chirality in inorganic nanostructures using chiral biomolecules

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