Synthesis And Characterization Of Main Chain And Side Chain Grafted Liquid Crystal-Poly-L-Lactide

Poly(lactic acid)(PLA) exists as a polymeric helix, and belongs to the family of aliphatic polyesters. It is a thermoplastic, high-strength, high-modulus polymer, which attracts much attention for high-optical power in electro-optical modulation.PLA segment includes spiral structure when it is prepared without racemization can be high-optically active. This makes PLA useful role in electro-optical modulation as optical elements and optical switching, et al. However, highly optical pure PLA could easily crystallize leading to the difficulty in preparing material with one or two dimensional orientational ordering. Liquid crystalline PLLA polymer shows the characteristics of both thermodynamic stability of polymer and orientational ordering of liquid crystal. A new orientational ordering PLA material with relative low degree of crystalline is expected to be obtained by combining liquid crystal groups into the PLA segment chemical strucuture.This paper presents the synthesis of two series of side chain grafted liquid crystal-polylactide and main chain grafted liquid crystal-polylactic acid copolymers. Polymerizable Acrylate of Poly-L-LA with mesogenic group containing alkoxyl end group (m=4,8) AOLAn060PPOm (n= 10,20,30,40) were prepared and polymerized to get P-AOLAnO6OPPOm. The main chain grafted liquid crystal-polylactic acid copolymers HOLAnOmOPPO3mOLAnOH-co-GC (m=3,6) were prepared first by co-polymerization between HOLAnOmOPPOmOLAnOH (m=3,6; n=10,20,30,40) and glutaryl chloride (GC).The oligo-LLA segments with controlled molecular weight and narrow molecular weight distribution were obtained by ring-opening polymerization of lactide using mOPPO6OH and HOmOPPOmOH as coinitiators. The 1H-NMR and GPC (Gel permeation chromatography) results verified the structure of O-LLA segments and also the degree of polymerization is calculated to be corresponding to the feed ratios of the lactide and the coinitiators HO-R chemicals. The molecular weight and molecular weight distribution of side chain and main chain grafted polymers were also measured by GPC. The results indicated that the polymerization degrees of side chain grafted polymers were about 9. but with a low molecular weight distribution around 1.2. The molecular weight of main chain grafted polymers were about 20 kDa-30 kDa, and molecular weight distribution,1.8, after the condensation polymerization were performed.All the copolymers showed thermotropic smectic liquid crystalline behaviors by the measurement of DSC (Differential scanning calorimetry), POM (Polarized optical microscope) and XRD (X-ray diffraction). Also, the helical chain of polymers locates on theα-form crystal, as evaluated by XRD.CD (Circular dichroism) spectroscopy was used to measure the circular dichroism of O-LLA segments in polymers. In the polymer solution, CD spectra shows a positive single circular dichroism absorptions band around 210nm. It verified the presence of Spiral Structure of O-LLA segments.