Synthesis And Characterization Of AB₂ Copolymer Containing Azobenzene Liquid Crystalline Blocks

This dissertation highlighted the developments of star polymers, amphiphilic polymers and azobenzene-containing liquid crystalline polymers. A series of linear block copolymers composed of poly(ethylene oxide) (PEO) and side-chain liquid crystalline azobenzene-containing polymethacrylate (PEO-b-PMMAZO) and AB2 polymers (PEO-(PMMAZO)2) were designed and synthesized. Furthermore, their thermotropic phase behaviors, liquid crystalline behaviors and self assembly behaviors were studied.1. Monofunctional macroinitiator PEO-Br was prepared by esterification of PEO andα-bromoisobutyryl bromide. Linear copolymer PEO-b-PMMAZO was synthesized via atom transfer radical polymerization (ATRP) using PEO-Br as initiator and 6-(4′-methoxy-4-oxy-azobenzene)hexyl methacrylate (MMAZO) as monomer in chlorobenzene (total weight content of monomer and initiator was 30% ) and characterized by nuclear magnetic resonance (NMR), Fourier transfer infrared ray (FT-IR) and gel permeation chromatography (GPC). Thermotropic phase behaviors and liquid crystalline behaviors of PEO-b-PMMAZO were studied by differential scanning calorimetry (DSC) and polarized optical microscope (POM) and self assembly behavior by atomic force microscope (AFM). It was found that the phase transition temperatures of TS-N and TN-I of PMMAZO increased with increasing molecular weight of PEO-b-PMMAZO while the melting temperature of PEO (Tm) decreased. Size of PEO-b-PMMAZO micelles formed in selective solvent increased with Mn increasing.2. Bifunctional macroinitiator PEO-Br2 was synthesized with two steps, the synthesis of 2, 2-bis(methyl-α-bromoisopropyl) propionic acid (BPPA) by esterification of 2, 2-bis(hydroxymethyl) propionic acid andα-bromoisobutyryl bromide and the synthesis of PEO-Br₂ by esterification of PEO and BBPA using dicyclohexyl carbodiimide (DCC) as the efficient condensation agent. AB₂ polymer (PEO-(PMMAZO)₂) was synthesized via ATRP using PEO-Br₂ as bifunctional macroinitiator and MMAZO as monomer in chlorobenzene (total weight content of monomer and initiator was 30% ) and characterized by NMR, FT-IR and GPC. Kinetic study showed that it was a first order reaction referred to the monomer MMAZO. Molecular weight increased along with conversion and polydispersities were narrow (<1.3), indicating the good control of the polymerization. Thermotropic phase behaviors and liquid crystalline behaviors of PEO-(PMMAZO)₂ were studied by DSC and POM and self assembly behaviors by AFM. It was found that TS-N and TN-I of PMMAZO increased with increasing molecular weight of PEO-(PMMAZO)₂ while Tm of PEO decreased. Compared with PEO-b-PMMAZO, Tm of PEO in PEO-(PMMAZO)₂ decreased more rapidly while TS-N and TN-I of PMMAZO didn't change too much, indicating PEO chain in PEO-(PMMAZO)₂ was confined more severely. Size of PEO-(PMMAZO)₂ micelles formed in selective solvent also increased with Mn increasing. Micelles changed to mallets when polymer content was increased to certain degree. Compared with PEO-b-PMMAZO micelles, size of PEO-(PMMAZO)₂ micelles was larger, due to the unique AB2 structure and the existence of side group.