Controlled Foaming of Polymer Films through Restricted Surface Diffusion and the Addition of Nanosilica Particles or CO₂-philic Surfactants

Abstract

Synergistic use of surface barriers and nanoscale additives is investigated as alternate means by which to promote bubble nucleation in, and thus improve the porosity of, poly(methyl methacrylate) (PMMA) thin films (i) constrained between impenetrable plates, (ii) modified with either nanosilica particles, commercial short-chain fluorosurfactants, or designer CO2-philic block/graft copolymers, and (iii) exposed to high-pressure CO2. Resultant foamed films exhibit a vast array of micro/mesocellular morphologies in the presence of supercritical, as well as liquid, CO2 and demonstrate that copolymer micelles afford better control over bubble nucleation (with pore cell densities, N, approaching 1012 cells/cm3) relative to hard nonporous nanoparticles, which alone increase N by more than 2 orders of magnitude at low CO2 pressures. Incorporation of these nanoscale additives in a surface-constrained polymer matrix serves to enhance foaming through concurrent restriction of CO2 diffusion, heterogeneous nucleation of CO2 bubbles, and/or reduced interfacial tension between PMMA and CO2.

Low density polyethylene/silica nanocomposite foams. Relationship between chemical composition, particle dispersion, cellular structure and physical properties

“…This means that the nucleating effectiveness of the particles decreases as the dispersion level decreases. All these findings are in concordance with results presented in previous studies [21,35,36]. In these works it was reported that for high silica contents, high aggregates and low dispersion levels were found, which resulted in a low nucleating effect of nanofillers.…”

PMMA-sepiolite nanocomposites as new promising materials for the production of nanocellular polymers

“…Larger cell densities yield to smaller cell sizes. This result is commonly observed in systems filled with nanoparticles [17,22,23,27]. Besides, the volume fraction occupied by the nanometric cell increases as sepiolite content increases.…”

“…Spherical nanosilicas [22][23][24] and layered montmorillonite-type nanoclays [18,25,26] have been proved to be suitable nucleating agents in different polymeric matrices. For instance, in the work of Siripurapu et al [27] ultraporous PMMA films were produced using silica particles. A 12 wt% of these particles led to a reduction in the cell size from 10 to almost 1 μm.…”

Nanoporous polymeric materials: A new class of materials with enhanced properties

“…Heterogeneous materials (i.e., systems in which heterogeneous nucleation occurs) offer another two main approaches to increase the nucleation rate: the addition of particles (mainly nanoparticles) to the polymer matrix [20,[49][50][51][52] or the use of block copolymers [47,[53][54][55][56]. Urbanczyk and coworkers [52] added nanoclays (montmorillonite (MMT)) to poly(styrene-co-acrylonitrile) (SAN) to obtain porous materials with pore sizes between the nanometric and micrometric scales at saturation pressures up to 30 MPa.…”