Surface-Constrained Foaming of Polymer Thin Films with Supercritical Carbon Dioxide

Abstract

Microcellular polymer foams afford a wide variety of attributes relative to their dense analogues, and efforts remain underway to establish viable routes to generate foams with substantially reduced pore cell size and increased pore cell density. Barrier constraints are applied in the present work to achieve diffusion-controlled isothermal foaming of thin polymer films in the presence of supercritical carbon dioxide (scCO 2). Poly(methyl methacrylate) (PMMA) films measuring ca. 95-100 µm in thickness are physically constrained between two impenetrable plates so that scCO2 exit diffusion is restricted to the film edges. Results obtained here demonstrate that the pore size can be systematically reduced to less than 100 nm in such systems by applying high saturation scCO 2 pressures, relatively low foaming temperatures (near the glass transition temperature of the scCO2-plasticized polymer), and a rapid pressure quench. Classical nucleation theory (CNT) modified to account for the compressible nature of scCO 2 is used to describe pore cell growth as a function of foaming temperature and scCO2 saturation pressure. Incorporation of a gradient model based on the Sanchez-Lacombe equation of state to account for PMMA-CO2 interfacial tension in conjunction with the CNT yields accurate predictions of foam cell densities as a function of relevant system parameters.

Assisted heterogeneous multinucleation and bubble growth in semicrystalline ethylene-vinyl acetate copolymer/expanded graphite nanocomposite foams: Control of morphology and viscoelastic properties

“…Similarly, there are some reports about clay reinforced nanocomposite foams based on semicrystalline polymers [5][6][7][8]. Moreover, microcellular foaming process is one of the techniques that have been studied for this purpose [9][10][11][12][13][14][15][16]. By rapid pressure quenching [17], melt blending using polymers with high glass tran-sition temperature (T g ) [4] and incorporating nanoclay into polymers [7], micron or submicron size cellular structures have been successfully obtained.…”

“…By rapid pressure quenching [17], melt blending using polymers with high glass tran-sition temperature (T g ) [4] and incorporating nanoclay into polymers [7], micron or submicron size cellular structures have been successfully obtained. Most polymers investigated in the previous researches have been amorphous, though there are several papers discussing microcellular foams of semicrystalline polymers [15,16,[18][19][20][21][22][23]. The influences of nanofiller characteristics such as aspect ratio and surface chemistry on the properties of solid polymer nanocomposites have been extensively studied [24][25][26][27].…”

“…The filler aspect ratio most likely changes the foam morphology that requires to be explored. For polymeric foaming processes using supercritical fluids, the most controllable parameters are processing variables such as sorption pressure and temperature [15]. The influence of the sorption pressure on the foam morphology has been studied at a constant sorption temperature.…”

“…The influence of the sorption pressure on the foam morphology has been studied at a constant sorption temperature. Besides, diffusivity of the supercritical fluid can be highly influential [15]. Furthermore, in the case of polymer nano/microcomposites, the filler characteristics like size, aspect ratio, geometric shape and filler/polymer interaction as well as the used fluid/gas are crucially important because the bubble size, shape and formation mechanism are greatly impacted by these factors [28,29].…”

Origins of the failure of classical nucleation theory for nanocellular polymer foams

“…This requires cell nucleation to take place rapidly when creating the foam. Classical nucleation theory (CNT) is often used to predict bubble nucleation rates [2][3][4][5][6] so that one can engineer polymer foams with smaller cell sizes -microcellular foams, for example. Further improvements are being sought by reducing cell sizes to nanometre dimensions to create nanocellular foams [2].…”