Pulsed dynamic NMR of fibers from thermotropic liquid-crystal polymers

Semiflexible liquid-crystal polyesters were melt spun into highly oriented fibers with tensile moduli up to E = 22 GPa and studied by pulsed dynamic NMR techniques. Analysis of the various 2H NMR experiments provided detailed information about molecular order and dynamics of these systems. As-spun and annealed fibers are characterized by a high degree of orientational and conformational order of the polymer chains. In addition, practically all director axes are aligned in draw direction. High modulus and strength result from these highly oriented chain configurations. Molecular motion in annealed fibers is heterogeneous, as two components are detected. One component exhibits similar fast dynamics as observed in the anisotropic melt (liquid-crystalline phase), while the other shows a drastic motional decrease (crystalline phase). Decomposition of various NMR relaxation curves into two components yields a crystallinity of (55 * 5) %, subject to the annealing conditions. However, the degree of order is independent of the thermal history, in agreement with only subtle changes of the mechanical properties upon annealing.