Mesogen-Initiated Linear Polyglycerol Isomers: The Ordering Effect of a Single Cholesterol Unit on “Sticky” Isotropic Chains

Abstract

Synthesis, thermal properties, and the liquid crystalline (LC) order of polymers consisting of a single mesogenic cholesterol unit and flexible, linear polyglycerol (PG) or poly(glyceryl glycerol) (PGG) chains have been investigated. Incorporation of the single mesogen has been achieved by using cholesterol directly as an initiator for the oxyanionic ring-opening polymerization (ROP) of ethoxyethyl glycidyl ether (EEGE) or isopropylidene glyceryl glycidyl ether (IGG). The controlled polymerization allowed the synthesis of a series of peculiar rod–coil type polyethers with molecular weights of 600–2300 g/mol, representing a degree of polymerization (DPn) of 4–30 for both PG and PGG with the polydispersity Mw/Mn in the range of 1.07–1.25. The resulting linear PGs exhibit extremely stable thermotropic LC order in a broad temperature range up to 260 °C, forming mainly layered smectic A (SmA) phases with varying layer thicknesses, depending on the degree of polymerization of the respective polymer structure. LC phases were observed up to a chain length of 26 glycerol units, while PGGs showed no LC order. This is explained both by the steric hindrance of the branched monomer units and the higher hydrophilicity of the polymer backbone. Permethylation of the cholesterol-PG samples resulted in strongly reduced LC order or in the entire loss of the self-assembly in LC phases, which is a consequence of the disappearance of hydrogen-bonding between the functional coil segments. Detailed characterization of the phase behavior of the polymers has been achieved by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and small-angle X-ray scattering (SAXS), confirming the smectic layer structure of the materials.