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Polymerizations in Supercritical Carbon Dioxide Chemical Reviews, 1999, Vol. 99, No. 2 545 concluded that dipole-quadrupole interactions be- tween CO2 and the acrylate unit contributed to the polymer solubility. Increasing the free volume of the polymer also increased solubility at temperatures above 100 °C, where the polar interactions are decreased. The solubility of fluorine-containing polymers has been investigated. McHugh noted that while poly- ethylene is insoluble in CO2, even up to 270 °C and 2750 bar, perfluorinated ethylene-propylene copoly- mer has greatly improved solubility (200 °C, 1000 bar).18 Other work revealed that the interactions between CO2 and poly(trifluoropropyl methyl silox- ane) are anomalously high, and the authors at- tributed this finding to specific interactions between CO2 and the polar fluorine containing group.16,19 More recently high-pressure 19F NMR studies have indicated specific interactions between CO2 and fluorocarbons.20 These solubility requirements dictate the types of polymerization techniques employed in polymer syn- thesis. For example, amorphous fluoropolymers have been synthesized homogeneously in supercritical CO2. In contrast, many polymerization reactions in CO2 are conducted under heterogeneous processes, either as precipitation, dispersion, or emulsion polymerizations, due to the inherent insolubility of most polymers in CO2. Thus, important research in this field involves the synthesis of surfactants, which are a critical component of dispersion and emulsion polymerizations, for use as stabilizers in CO2. These stabilizers have been successfully employed in the heterogeneous polymerizations of CO2-insoluble poly- mers. The nature of the surfactant controls polymer properties and morphology, and these factors need to be considered in surfactant design. The fractionation of polymers is another important area affected by polymer solubility in supercritical CO2. Careful and controlled lowering of the density of a polymer solution allows precipitation of the highest molecular weight polymer fraction.1 For example, CO2-soluble synthetic oils such as per- fluoroalkyl polyethers, chlorotrifluoroethylene oligo- mers, and poly(dimethylsiloxane) (PDMS) were fractionated with supercritical CO2.10 In the case of PDMS, the starting sample with a number average molecular weight (Mn) of 4 × 105 g/mol was separated into six fractions with Mn ranging from 4 × 102 g/mol to 1 × 106 g/mol. Another important feature of supercritical CO2 in polymerization is plasticization, which results in the lowering of the polymer’s glass transition tempera- ture (Tg). Polymers have been shown to become highly plasticized by CO2. This plasticization allows important effects such as the removal of residual monomer from the polymer, incorporation of addi- tives, and formation of foams. Table 1 summarizes some of the studies on the effect of CO2 plasticization on polymers. The effects of high-pressure CO2 on the glass transition temperature and mechanical proper- ties of polystyrene (PS) have been measured by Young’s modulus and creep compliance.21 This work demonstrated the severe plasticization of PS as a Table 1. Summary of Literature Studies on the Swelling and Plasticization of Homopolymers, Copolymers, and Polymer Blends by CO2 polymeric material technique Homopolymers ref(s) DSCa 170 DSC 22,32 swelling/sorption 23 DSC 171 DSC 30,32 sorption 16,19,38 DSC 32 DSC 33 DSC 22,32,33 sorptive dilation 29 creep compliance 27 33 swelling/sorption 172 DSC 22,32 creep compliance 24,26,27 swelling/sorption 23,172,173 dielectric 25 measurements DSC 33 DSC 33 creep compliance 21,24,26 DSC 22 swelling/sorption 23,172 DSC 32 infrared spectroscopy 31 DSC 32 sorptive dilation 28 sorptive dilation 28 DSC 22,32 DSC 33 swelling/sorption 172 Copolymers creep compliance 26,172 DSC 32 DSC 32 Polymer Blends DSC 22 DSC 174 sorption 175 a DSC ) differential scanning calorimetry. function of CO2 pressure. In work by Choiu, dif- ferential scanning calorimetry (DSC) was used to study the plasticization of polymers by CO2 at pres- sures up to 25 bar.22 Even at such modest pressures as 25 bar, reductions in the glass transition temper- atures of up to 50 °C were observed. Other studies included measurements of polymer swelling and creep compliance as a function of time to determine Tgs of polycarbonate (PC), PS, and PMMA in CO2.23,24 Beckman examined the plasticization of PMMA by CO2 as a function of pressure using dielectric mea- surements.25 Johnston performed an in-depth study of the solubility of CO2 in the polymer and its effect on the Tg vs CO2 partial pressure behavior for PMMA, PS, and a random copolymer of methyl methacrylate and styrene.26 Johnston has also used in situ measurements of creep compliance to deter- mine the Tg depression of PMMA and poly(ethyl methacrylate) in CO2.27 As evidenced by the above poly(aryl ether ether ketone) (PEEK) polycarbonate (PC) substituted polycarbonates poly(2,6-dimethylphenylene oxide) (PPO) poly(dimethylsiloxane) (PDMS) poly(ether imide) polyethylene (PE) poly(ethylene terephthalate) (PET) poly(ethyl methacrylate) (PEMA) poly(iminoadipoylimino- hexamethylene) (nylon-6,6) polyisoprene poly(methyl methacrylate) (PMMA) poly(oxymethylene) poly(propylene) (PP) polystyrene (PS) polysulfone polyurethane poly(vinyl benzoate) poly(vinyl butyral) poly(vinyl chloride) (PVC) poly(vinylidene fluoride) (PVF2) poly(vinylpyridine) poly(methyl methacrylate-co-styrene) acrylonitrile butadiene styrene (ABS) high impact polystyrene PMMA/PVF2 PS/PC PS/poly(vinylmethyl ether)PDF Image | Polymerizations in Supercritical Carbon Dioxide
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