Synthesis and characterization of heterocyclic polymers: polybenzoxazoles and cyanate ester networks

Abstract

High molecular weight soluble aromatic polybenzoxazoles (PBO) were synthesized from various bis( ortho amino phenols) and aromatic diacid chlorides via solution step polymerization in Nmethyl- 2-pyrrolidone (NMP). It was demonstrated that reaction temperatures of about 0 Cor lower were highly desired in order to obtain high molecular weight polyhydroxy amides. It was postulated that this lower temperature prevented pyridine catalyzed side reactions with the phenol group, which would otherwise either limit molecular weight or produce branched or gelled structures. The polyhydroxy amides were successfully solution cyclized at elevated temperatures (170 C) and complete cyclization was demonstrated with reactions times of 6 hours or less. The reSUlting fluorinated PBO materials were soluble in amide solvents at concentrations of at least 15% at 25 C. The PBO based on terephthaloyl chloride and the 6F bis(o-aminophenols) (2,2- bis(3 -amino-4-hydroxypheny I )-1,1,1-3,3,3-hexafluoropropane was systematically investigated. It was utilized as a thermally stable component of multiphase microphase separated block and graft copolymers. The second component investigated was based upon the thermally labile structure derived from polypropylene oxide (PPO). Block copolymers were prepared from either hydroxyl or aminophenyl terminated polypropylene oxide. The materials were characterized by solution, thermal and microscopic techniques. In particular, the potential of these materials to develop nanofoam-like structure was demonstrated. The resulting materials could be foamed by selectively degrading the labile block or graft at elevated temperatures in air. The foamed matrix was characterized via thermal and microscopic techniques and density values were compared to those of the homopolymer. It was concluded that the block copolymer produced nanofoam-like structures with pore sizes in the desired submicron range, but the graft copolymers appeared to collapse during the foaming process and were not further characterized. The resulting systems are of interest as an approach to lowering the dielectric constant of insulators, appropriate for microelectronic applications.

The second portion of the thesis focused on cyanate ester or polycyanurate networks generated from dicyanate monomers. The influence of electron withdrawing and donating groups on the behavior of the network forming curing process was investigated. It was demonstrated that electron withdrawing characteristics, such as carbonyl, sulfone, or phenylphosphine oxide increased the reactivity of the cyanate group above the melting point of the monomer. Unfortunately, these electron withdrawing groups promoted a small processing 3window between the melting point and the exothermic cure temperature. However, by introducing an aryl ether bond, para or meta, to the cyanate group, wide highly desirable processing windows were demonstrated. In particular, the phenylphosphine oxide diphenyl ether combination produced good processabilty, cured glass transition temperatures of 280 C and high char yield in air, suggesting that the materials may show enhanced fire resistance. Adhesive bond strength of several materials were also investigated and it is suggested that future efforts in this area might be desirable.