The Synthesis And Properties Of Soluble Hyperbranched Polybenzoxazoles And Their Copolymers

PBO is a kind of high-performance aromatic heterocyclic polymer with outstanding physicalproperties, such as excellent mechanical properties and extremely high thermal stabilities. As akind of ragid conjugated polymer, PBO also possesses good optical properties and showspromising applications in organic electroluminescent materials and photoelectronic devices.However, the ragid backbone of PBO makes it only soluble in a few strong acids, which makes theprocessing of PBO difficult and hampers its practical applications. At the same time,hyperbranched polymers have attracted great interests in recent years owing to their specificproperties in comparison to linear polymers, such as modifications of terminal groups,good solubility and lower viscosity. As a result of the decreased interchain entanglement,nanocavities created by the branching points, and large amount of peripheral functional groups,hyperbranched polymers show smaller viscosity and better solubility than their linear counterpartswith similar molecular weight. The optical, thermodynamic, and electrical properties ofhyperbranched polymer were also different from the the linear polymers because of the branchedstructure. Moreover, the outer functional groups in hyperbranched polymers can also allow thecopolymerization with other polymers and introduce the advantageous properties ofhyperbranched polymers to the matrix.With the above consideration, this dissertation focuses on the synthesis of hyperbranchedPBOs and investigation of the influence of structural variation on their solubility and physicalproperties. Meanwhile, we have also attempted to investigate the effects of covalently bondedhyperbranched PBOs on the properties of other polymers, such as epoxy resins and linear PBO,which were fabricated by copolymerization of hyperbranched PBOs and corresponding polymerprecursors. In chapter2, we have synthesized hyperbranched Polybenzoxazoles (HBPBOs) by a one-potpolycondensation of2,2-Bis (3-amino-4-hydroxyphenyl) hexafluoropropane and1,3,5-benzenetricarboxylic acid in Polyphosphoric acid. The HBPBOs exhibited good solubility inorganic solvents due to the branched structure and the flexible hexafluoropropane groups in mainchains. The structure and terminal functional groups could be tailored by adjusting the molar ratioof two monomers. FT-IR, NMR and XRD measurements confirmed the structure of HBPBOs,while TGA, UV-vis and photoluminescence spectra, combined with the comparison with linearPBOs demonstrated the intriguing optical properties and good thermal stabilities of HBPBOs.The good solubility of HBPBOs and peripheral functional groups also permitted their usageas molecular reinforcement for polymer composites. Therefore, in chapter3, we have fabricatedcomposites of HBPBOs and epoxy resins. HBPBOs were first blended with epoxy monomers, andthen copolymerized with epoxy groups via ring-open polymerization during curing. It is found thatthe incorporation of HBPBOs has improved the optical and thermal properties, but has nosignificant influence on the mechanical properties of epoxy resins. Moreover, the changes insurface wettabilities of HBPBOs/epoxy composites varied with incorporation of HBPBOs bearingdifferent peripheral functional groups.Finally, to investigate the influence of hyperbranched polymers on properties of linearpolymer, we have prepared copolymers of HBPBO and linear PBO by a two-step method.Different amount of HBPBOs with terminal o-aminophenol groups were first reacted withprecursors of linear PBO, the precursors of copolymers were then cyclized by heat treatmentyielding hyperbranched-linear PBO copolymer films. FT-IR, XRD, DSC, TGA and tensile testwere used to probe the structure and properties of copolymer films. It is found that the thermalstabilities of copolymers remained unchanged upon the incorporation of HBPBOs, while the glasstransition temperatures and modulus of copolymers increased with increasing the amount ofHBPBOs due to the crosslinking effect of HBPBOs to linear PBO chains. Due to the nanocavitiesin HBPBOs, the copolymers also showed lowered dielectric constant as compared to linear PBOs,which made our copolymers promise cadidates for microelectric applications.