Effect Of Ionic Liquids On Foaming Performance Of Poly (Methylmethacrylate) In Supercritical CO₂

Microcellular polymer foams, which are porous materials with the cellsize about10m, are prepared by supersaturation method with supercriticalfluid (CO₂) as the blowing agent. They have small and uniform cell sizes, highcell densities, excellent physical-mechanical properties. However, it is difficultto obtain the materials with high cell size and expansion ratio simultaneouslydue to the low CO₂solubilities of polymers. In this work, we used the ionicliquid (IL) as a third component to change the properties of polymer, thusimprove its foaming behaviors.ILs have attracted growing interest as a new type of green solvents,which are completely composed of anions and cations. Lots of studies haveshown that CO₂is significantly soluble in ILs, but the ILs solubility in CO₂isvery small. In polymer science, ILs are mainly used as polymerization media,polymer solvents and functional additives.In this study, we used a rapid pressure-quenching process with ILs as thefoaming additives, CO₂as the blowing agent to investigate the foaming performance of poly(methylmethacrylate)(PMMA) by changing temperature,pressure and the content of ILs, for the PMMA/1-dodecyl-3-methylimidazoli-um hexafluorophosphate ionic liquid ([C₁2MIM]PF₆) blends. The introductionof compatibility [C₁2MIM]PF₆decreased the glass transition temperature (Tg)of the blends, the foaming conditions are divided into three regions accordingto the main morphological difference of cells. At relatively low temperaturesand pressures, neat PMMA could not be foamed, while the addition of[C₁2MIM]PF₆facilitated the formation of cells. By increasing temperature orpressure, neat PMMA foams were obtained with wide cell size distribution,while the blend foams were formed with narrowed cell size distribution andincreased cell size. At high temperatures and pressures, neat PMMA wasfoamed with narrow cell size distribution and high cell density, while themajor effect of [C₁2MIM]PF₆was to increase the cell size. The resultsindicated that the plasticization effect of [C₁2MIM]PF₆and the decreasedenergy barrier for the initial cell nucleation accounted for the differences onthe cell morphology between neat PMMA and the blend foams.For the IL of different kinds of anions and cations, hydrophobic[C_nMIM]PF₆is compatible with PMMA, and hydrophilic [C_nMIM]Br isincompatible with PMMA. The Tgof compatible blends is lower than the incompatible, and the IL with long alkyl chain has better plasticization, biggercell size, low expansion ratio and cell density.