Molecular Organization in Nematic Polymers. 2. Evolution of the Mesophase

We have probed the orientational dynamics of a nematic polymer melt in a magnetic field by broad-line proton NMR and its thermomechanical behavior upon solidification by torsion braid analysis. The objective of this work has been to understand the nature of the evolution of polymeric mesophases aging in the nematic phase. The experimental polymer was an aperiodic copolyester containing both aromatic and aliphatic structural units and having molecular weight in the range (0.4–1.6) × 10⁴. Using a model for magnetic orientation we obtained values of a characteristic orientation time, τ, as a function of molecular weight and aging time in the nematic state. We found that τ, which is proportional to rotational viscosity, is proportional to molecular weight to a power that has a striking dependence on the age of the fluid. The exponent decreases approximately from 6 to 1 depending on isothermal age of the fluid in the range of several minutes to hours. Thermomechanical analysis of solid samples obtained from the nematic fluids revealed with aging a reduced tendency of the solid to order. This tendency is linked to the mesophase evolution to a more stable state which has longer range orientational order and aligns very rapidly in external magnetic fields relative to the unaged nematic state. Studies of orientation dynamics aimed at verifying physical theories must therefore define precisely the mesophase age and its texture.