| Nuclear magnetic resonance spectroscopy was used to study the lyotropic liquid-crystalline system cis-poly(p-phenylene benzobisoxazole) (PBO) in polyphosphoric acid (PPA). Polymer concentrations of 5%, 8%, 11%, and 14% by weight, all exhibiting some degree of liquid-crystallinity, were analyzed with {dollar}sp1{dollar}H, {dollar}sp{lcub}31{rcub}{dollar}P and {dollar}sp{lcub}13{rcub}{dollar}C NMR. Moisture uptake experiments showed minimal impact on the solutions after one hour of atmospheric exposure. Chain scission was observed, however, in the extremely hygroscopic polyphosphoric acid upon immersion in a water bath. After one hour of submersion, the expected coagulation of the polymer was not evident through either spectral changes or spin-lattice relaxation time increases.; Variable temperature {dollar}sp{lcub}31{rcub}{dollar}P and {dollar}sp{lcub}13{rcub}{dollar}C spin-lattice relaxation time measurements of the PBO/PPA solutions provided concentration-independent activation energies for thermal motion ranging from 3.4 to 28 kcal/mol. Aside from a flow onset near 40{dollar}spcirc{dollar}C, no other obvious thermal transitions were evident. Unusual spectral area increases and peak shifts were observed through variable temperature {dollar}sp{lcub}31{rcub}{dollar}P NMR experiments. These concentration-dependent effects appear to be a result of competition between residual-water dynamics, PPA hydrolysis, and polymer/solvent complexation. |
