Polymerization kinetics of rodlike molecules under quiescent conditions

The kinetics of solution polymerization to synthesize poly(p-phenylene terephthalamide), a rodlike polymer, was studied theoretically and experimentally. The reaction was conducted under quiescent conditions to eliminate effects of flow-induced orientation that are known to affect the kinetics of the process. Experiments included determining the degree of polymerization and polydispersity index for different times of reaction. Theoretical study comprised calculating the effective rate constant for the diffusion-controlled reaction using Smoluchowski's approach and computing the evolution of the molecular-weight distribution using the length-dependent effective rate constant obtained in the first part. Experiments showed significant slowing of the reaction and narrow molecular-weight distributions due to diffusion control. The degree of polymerization and the polydispersity index variation wit time is nearly the same of the tow initial monomer concentrations. The Smoluchowski theory predicts the effective rate constant to be k(eff) proportional to rho (-0.8)L(-3.5) for large L, with rho the local number concentration of rods and L the length of the reacting rodlike molecule. The model has tow parameters obtained by a least-squares fit to one set of date of degree of polymerization vs. time. Results of the population balance calculations using the computed effective rate constant matched with experimental data for degree of polymerization and polydispersity vs. time for both initial monomer concentrations. The model, augmented to account for slow, end-capping side reactions, predicted an optimal initial monomer concentration for obtaining the highest degree of polymerization for a fixed reaction time, as reported in previous experimental studies.