Alessandro Ghielmi

Long chain branching in emulsion polymerization

A kinetic model for evaluating the degree of polymerization of a branched polymer produced in emulsion is developed. Only chain branching occurring through chain transfer to polymer has been considered. The model accounts for chain compartmentalization and, when coupled to a model able to describe the evolution of the polymerization system, allows to evaluate the cumulative properties of the produced polymer both in the pre- and post-gel phases. The difficulties related to the description of a heterogeneous polymer chain population and of gel formation are overcome by using the “numerical fractionation” technique. A parametric analysis of both instantaneous and cumulative properties is reported and discussed, with special attention to the role of radical compartmentalization in determining the molecular weight properties of an emulsion produced polymer.

Role of active chain segregation in emulsion polymerization

Abstract The effects on the molecular weight development of chain segregation in emulsion polymerization are discussed both theoretically and experimentally. A large delay on gel formation induced by carrying out the polymerization reaction in emulsion instead of in bulk is predicted by the model. Two systems of practical interest (polyvinyl acetate and polybutyl acrylate) are then examined. In the first case, literature experimental data are compared with model predictions. The synergic effect of the different branching mechanisms on the molecular weight polydispersity is evidenced. In the second case, original experimental data obtained in a calorimetric reactor are presented. The predicted onset of gel formation at conversion values larger than those expected in bulk polymerization is experimentally verified.

Molecular weight distribution of crosslinked polymers produced in emulsion

A kinetic model for evaluating the chain length distribution of a branched polymer produced in emulsion was developed. Chain branching occurring through any intermolecular mechanism is considered, namely, crosslinking, chain transfer to polymer and propagation to terminal double bond. The model accounts for active chain compartmentalization and, when coupled to a model able to describe the evolution of the polymerization system, allows evaluation of the cumulative properties of the produced polymer both in the pregel and postgel phases. The numerical difficulties related to the description of a rather wide polymer chain population and of gel formation are overcome by using the ‘numerical fractionation’ technique. A parametric analysis of both instantaneous and cumulative properties is reported and discussed, with special attention to the role of radical compartmentalization in determining the molecular weight properties of a polymer produced in emulsion. Significant differences with the molecular weights computed using models developed for homogeneous (non compartmentalized) systems have been found. A comparison with the predictions of Florys statistical theory is also reported in terms of gel point and gel weight fraction.

Particle State Dependent Radical Desorption and Its Effect on the Kinetics of Emulsion Polymerization

Radical desorption from polymer particles is a kinetic event peculiar to the emulsion polymerization process. A careful modeling of this phenomenon is highly valuable in order to achieve accurate predictions of polymerization rate and average properties of molecular weight. In this work, radical desorption is described accounting for an aspect fully neglected in previous modeling literature. Specifically, particle state dependent desorption coefficients are used instead of a single average coefficient, and the corresponding rate expressions are developed and applied to the solution of the well-known Smith–Ewart equations. Parametric model simulations show that the higher level of detail introduced in the description of radical desorption improves the accuracy of the predicted values of the average number of radicals per particle, especially in the cases of high desorption rate and slow reactions in the aqueous phase.