(345b) Modeling of Precipitation and Dispersion Copolymerization of Fluorinated Monomers in Supercritical Carbon Dioxide
Several recent works have shown that technologically relevant fluorinated semicrystalline copolymers can be produced by free radical polymerization of the two vinyl monomers vinylidene fluoride (VDF) and hexafluoropropylene (HFP) in supercritical carbon dioxide (scCO2). Being the polymer insoluble in the medium, the process is heterogeneous and both precipitation and dispersion regimes are possible.
The copolymer molecular weight distribution (MWD) is heavily affected by the interphase area, Ap, of the polymer phase: bimodal or monomodal MWDs are obtained under precipitation (lower Ap) or dispersion conditions (higher Ap), respectively. This finding is incompatible with the assumption of reaction taking place in the continuous phase only (as proposed by Ahmed et al.) and strongly supports a mechanistic picture where the reaction is going on in both the phases, continuous and dispersed. Accordingly, in this work a heterogeneous copolymerization model consistent with the latter 2-loci schematization is developed as an evolution of a previous homopolymerization model. It is accounting for radical transport between the phases, diffusion limitations affecting propagation and termination, as well as chain length dependent rate constant for the termination reactions occurring in the polymer-rich, dispersed phase. As expected, the resulting model involves a very large number of kinetic parameters, which evaluation is indeed asking for a major effort. This is especially true because we are dealing with multiple monomers and heterogeneous systems. Therefore, a specific parameter evaluation strategy has been used in order to estimate most of them a priori while minimizing their evaluation by direct fitting to the experimental data. With this respect, preliminary information on the system behaviour at very low conversion (where the contribution of the reaction in the continuous phase is dominant) as well as on the system thermodynamics were found especially helpful. In particular, taking advantage of the recently proposed lattice free volume theory, coupled with a semiempirical scaling law for the chain length dependence of the diffusion coefficients, monomer and radical diffusion coefficients in both phases are estimated from knowledge of the system thermodynamics (namely, easily accessible pressure-volume-temperature data). This way, transport coefficients as well as diffusion limited reaction rate constants are evaluated. The model predictions compare quite favourably with the experimental results of conversion and molecular weight distribution. Therefore, the proposed modeling strategy is expected to represent a convenient and robust approach to the simulation of complex, multiphase and reacting systems such those under examination here.
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