Modeling Heterogeneous copolymerization of fluorinated monomers in scCO2

Abstract

In recent years the use of supercritical carbon dioxide (scCO2) in polymer processing and reaction has emerged as a “green” alternative to replace both environmentally hazardous organic solvents and water, whose utilization involves the production of large amount of waste streams. Moreover, scCO2 is non-toxic, inexpensive, and often exhibits intense plasticizing effect on polymers. In this work, the copolymerization of fluorinated monomers in scCO2 is explored. Batch free-radical copolymerization reactions of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) have been carried out at temperature of 50 C and pressure of 400 bar using DEPDC as initiator and different amounts of stabilizer (FLK-7004A). Conversion, composition, molecular weight and particle size have been measured by gravimetry, 19F-NMR, gel permeation chromatography and electron microscopy, thus providing a comprehensive characterization of the product. In the case of VDF precipitation homopolymerization in scCO2, bimodal MWDs have been reported. Two different hypotheses regarding the phase behavior of the system have been proposed to explain such eterogeneity, the first invokes chain-transfer to polymer in an homogeneous reaction system, the second mass transport limitations of the radicals between supercritical and polymer phase in an heterogeneous reaction system. The experimental data here presented support the second scenario; namely, monomodal high-MWDs are obtained carrying out the reaction under stable dispersion conditions, thus favoring the radical transport from the continuous to the dispersed phase by increasing the interfacial area. Accordingly, a mathematical model accounting for two reaction loci (continuous and dispersed phases) as well as the interphase transport of radicals, has been developed and validated by comparison with the experimental results. Even though some parameter fitting was required, the final model prediction ability is satisfactory, especially in terms of copolymer composition and molecular weight distribution. Such agreement is believed to represent an effective confirmation of the presumed two loci reaction mechanism.