Modeling molecular weight distribution, vinyl content and branching in the reactive extrusion of high density polyethylene

摘要

We study reactive modification of high density polyethylene by organic peroxides.We improve a previous model capable of describing average molecular weights and vinyl content to incorporate the calculation of long-chain branching and complete molecular weight distributions. We calculate average molecular weights and long-chain branching indexes for molecules with a given number of vinyls, by applying a double moment technique over those variables. No moment technique was applied to the vinyl content so that model has the capability of calculating those average quantities for every vinyl content. To calculate molecular weight distribution we apply probability generating function definitions with respect to the molecular size, to the mass balances of radical and polymer species of a given number of vinyls. We use two distinct definitions of probability generating functions, each one directly applicable either to the number or weight distributions. These probability generating functions are numerically inverted to obtain the corresponding calculated molecular weight distribution. Results give a deeper insight into the evolution of vinyl groups and branching points formation. MWD and vinyl content predictions are compared with qualitative experimental data showing the model capabilities.