Synthesis and Properties of Side-Chain Liquid Crystalline Polypeptides Bearing Various Alkyl Spacers and Oligo-Ethylene-Glycol Tails

A series of polypeptides bearing various alkyl (i.e., propyl, hexyl, or octyl) spacers, biphenyl mesogens, and oligo-ethylene-glycol (OEG) tails (i.e., PPLG 26 -BPOEG m , PHLG 34 -BPOEG m , and POLG 32 -BPOEG m [ m = 3 or 7]) has been synthesized via 1,3-dipolar cycloaddition reactions with quantitative grafting densities. Fourier transform infrared results reveal that the polypeptides adopted an α -helical conformation in the solid-state. Differential scanning calorimetry (DSC) analysis reveals that the glass transition temperatures ( T g s) of the polypeptides with OEG 3 tails decrease with the increasing length of the alkyl spacers. For the samples containing alkyl spacers of the same length, T g significantly decreases with the increasing lengths of the OEG tails. Polarized optical microscope (POM) results reveal that all polypeptides exhibit strong birefringence at room temperature. Polypeptides with long alkyl spacers (i.e., hexyl or octyl) or OEG 7 tails show isotropic transitions, while PPLG 26 -BPOEG 3 show no clear point temperature before thermal decomposition. DSC, POM, and wide-angle X-ray scattering results collectively reveal that polypeptides with long alkyl spacers (i.e., hexyl or octyl) and OEG 3 tails undergo a reversible smectic E phase to isotropic phase transition, while polypeptides with OEG 7 tails undergo a reversible liquid crystalline phase to isotropic phase transition. The transition temperatures increase with the increasing length of alkyl spacers.