Study On The Effect Of Alkyl Spacers On The Phase Behavior Of Side-chain Liquid Crystalline Polypeptides

Side chain liquid crystalline polymers(SCLCPs) are a type of liquid crystalline polymers with mesogens in the side chains. SCLPs with rigid backbones displayed unique hierarchical supermolecular ordering from a biaxial orientation of the rigid main chain and mesogenic side chain. Polypeptides, especially poly(L-glutamate)s with ?-helical conformations and consequently rigid backbones are able to form a variety of liquid crystalline phases and highly ordered supermolecular structures after incorporation of mesogenic pendants. This thesis is aiming to study the structure-phase-behavior relationship of a series of side chain liquid crystalline polypeptides with various alkyl spacers, bipenyl mesogens, and oligo-ethylene-glycol(OEG) tails.1. A series of polypeptides bearing various alkyl(i.e., propyl, hexyl, or octyl)spacers, biphenyl mesogens, and OEG tails(i.e., PPLG26-g-BPOEGm, PHLG34-g-BPOEGm, and POLG32-g-BPOEGm, m = 3 or) has been synthesized via 1,3-dipolar cycloaddition reactions with quantitative grafting densities. 1H NMR and FTIR analysis comfirmed the molecular structures of the resulting polymers. FTIR analysis also revealed that the resulting polypeptides adopted an ?-helical conformation in the solid-state.2. Differential scanning calorimetry(DSC), polarized optical microscopy(POM),and wide angle X-ray diffraction(WAXD) were used to study the phase behaviors of the resulting side-chain liquid crystalline polypeptides. DSC analysis revealed that the glass transition temperatures(Tg) of the polypeptides with OEG3 tails decreased with increasing the length of alkyl spacers. For the samples with the same alkyl spacers, Tg was significantly decreased with increasing the lengths of OEG tails. POM results revealed that all polypeptides exhibited strong birefringence at room temperature.Polypeptides with long alkyl(i.e., hexyl or octyl) or OEG7 tails showed isotropic transitions while PPLG26-g-BPOEG3 showed no clear point temperature before decomposition temperature. DSC, POM, and WAXS results collectively revealed that at the temperatures above respective Tg, PPLG26-g-BPOEG3 exhibited a phase transition of smectic E ? smectic C while PHLG34-g-BPOEG3 and POLG32-g-BPOEG3 with longer alkyl spacers possessed a phase transition of smectic E ?isotropic. Polypeptides with OEG7 tail showed similar phase transitions of liquid crystal ? isotropic. The clear point temperature increased from 73.5 to 110 °C as the alkyl spacers changed from propyl to octyl groups.