SIMULATION OF DIFFUSION-LIMITED STEP-GROWTH POLYMERIZATION IN 2D - EFFECT OF SHEAR-FLOW AND CHAIN RIGIDITY

Multimolecule Brownian dynamics simulation results for diffusion contr olled polymerization of bead-rod chain molecules in 2D solution are pr esented. Reaction between any two molecules undergoing Brownian diffus ion takes place if the reactive chain ends approach each other to with in a certain reaction radius, and if the chain end carrying segments a re collinear within certain specified limits. The second order reactio n rate constant is found to decrease with time as the molecular length s increase and the diffusivities decrease. Application of a shear flow is seen to result in alignment of the molecules along the flow direct ion, thereby enhancing the concentration of molecular pairs with paral lel orientation of reactive-end carrying chain segments, and hence the overall reaction rate. This effect is found to be more pronounced in the case of long rigid molecules as compared to flexible molecules bec ause of the slow rotation and high level of orientation under flow of the former. Even the molecular weight distribution (MWD) obtained duri ng polymerization may be affected. For example, longer molecules have lower diffusivities and hence lower reactivities, resulting in a narro wer MWD in the absence of flow, as compared to the results with the us ual assumption of molecular reactivity being independent of chain leng th. Furthermore, in the presence of an external flow, the longer molec ules orient to a higher degree and hence display a higher enhancement in reactivity. This results in a wider MWD of the polymer. The simulat ion results are in qualitative agreement with previous experimental da ta for solution polymerization of rod-like molecules.