Synthesis and Characterization of the Polyhydroxyamide/Polymethoxyamide Family of Polymers

The poly(hydroxy-amide) [PHA] family of polymers possess not only high temperature stability, but also form a more thermally stable polymer (polybenzoxazole) [PBO] during thermal decomposition. The synthesis occurs in solution at low temperature, preventing premature ring cyclization to PBO. The molecular structure was confirmed from 1H-, 13C-NMR, and elemental analysis. One drawback for the highly aromatic PHA is the lack of a softening temperature before the onset of ring closure to PBO. Thus, poly(methoxy-amide) [PMeOA] and random copolymers of PHA and PMeOA have been synthesized and characterized. From microcalorimetry, the peak heat release capacity values for the copolymers is typically higher than that of the pure PHA (while still much lower than most other polymers); however, differential scanning calorimetry (DSC) analysis has shown glass transition temperatures to as low as 151°C for the copolymers. Thermo-gravimetric analysis (TGA) displays a three-step degradation associated with two ring closures to PBO from PHA and PMeOA (with the release of small low-fuel molecules) and the thermal degradation of PBO. The magnitude of the weight loss associated with each step is dependant on the percent of PHA, PMeOA, and aliphatic content in the copolymer. TGA and DSC analysis of PMeOA films under isothermal conditions show degradation and demethylation (cyclization) occurring simultaneously above 300°C and FT-IR displays benzoxazole ring forming under the same condition. Fibers have been spun from the molten state of PMeOA using a Micro-compounder. Fibers spun at 320°C, with average diameter of 180 μm, show an ultimate tensile strength, elongation at break, and Young's modulus of 60 MPa, 2.2% and 3.6 GPa, respectively. Films were also made by spin-coating or solvent casting to observe the effect of cyclization on tensile properties. These films had an ultimate tensile strength and Young's modulus of 120 MPa and 3 GPa, respectively.