Synthesis And Liquid Crystalline Behaviors Of Poly (_L-glutamate) Bearing Biphenyl Mesogens

Side chain liquid crystalline polymers(SCLCPs) are a type of polymer containing main-chains, mesogenic pendants, flexible spacer groups, and the tail groups attached to the mesogens. SCLCPs with rigid backbones can form hierarchical supermolecular ordering due to a biaxial orientation of rigid polymer main chain and mesogenic side chains. Polypeptides, especially poly(L-glutamates) with α-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. Therefore, this thesis is aiming to study the structure-phase-behavior relationship of two series of side chain liquid crystalline polypeptides with various main chain lengths and tail(alkyl or oligo-ethylene-glycol)lengths. The two contents of this thesis are as follows:Firstly, a series of liquid crystalline 4-alkyloxybiphenyl-poly(γ-propyl-L- gluta mate) conjugates(PPLGn1-g-BPCm1, n1 = 10, 54, 110, and 274; m1 = 4, 6, and 12)with end-on biphenyl mesogens and variable alkyl tails has been synthesized via a facile copper-mediated alkyne-azide [2+3] 1,3-dipolar cycloaddition. 1H NMR and FTIR analysis revealed a quantitative conjugation efficiency of each sample.Variable-temperature FTIR results indicated an irreversible α-helical to β-sheet transition of PPLG10-g-BPC6 in the solid state while PPLGn1-g-BPCm1 samples with longer main chain length(n ≥ 54) only adopted α-helical conformation in the temperature range between 25-200 °C. DSC, POM, and WAXD results collectively revealed that PPLGn1-g-BPCm1 samples underwent a reversible crystalline to liquid crystalline phase transition depending on the main-/side-chain lengths. PPL Gn1-g-BPCm1(n1 ≥ 54, m1 ≤ 6) samples formed highly stable smectic(SmC or SmE)phases with a calculated tilt angle of 73-90 o.Secondly, a series of polypeptides bearing biphenyl mesogenic side-chains and oligo-ethylene-glycol(OEG) tails(PPLGn2-g-BPOEGm2) has been synthesized via a1,3-dipolar cycloaddition with quantitative grafting density. 1H NMR and FTIR analysis comfirmed the molecular structures of the resulting polymers. DSC analysis revealed that the glass transition temperature(Tg) increased as the main chain length increased and decreased as the OEG length increased. POM results suggested that polypeptides with short OEG length(m2 = 2 and 3) showed no extinction while the ones with long OEG length(m2 = 7) exhibited clear point temperature at ~90 °C. TheWAXD results suggested that polypeptides with short OEG length(m2 = 2 and 3)showed smectic C phases.