Investigation of the nanocellular foaming of polystyrene in supercritical CO2 by adding a CO2-philic perfluorinated block copolymer

Amorphous Polymers' Foaming and Blends with Organic Foaming-Aid Structured Additives in Supercritical CO2, a Way to Fabricate Porous Polymers from Macro to Nano Porosities in Batch or Continuous Processes

Organic polymers can be made porous via continuous or discontinuous expansion processes in scCO2. The resulting foams properties are controlled by the interplay of three groups of parameters: (i) Chemical, (ii) physico-chemical, and (iii) technological/process that are explained in this paper. The advantages and drawbacks of continuous (extrusion, injection foaming) or discontinuous (batch foaming) foaming processes in scCO2, will be discussed in this article; especially for micro or nano cellular polymers. Indeed, a challenge is to reduce both specific mass (e.g., ρ < 100 kg·m-3) and cell size (e.g., average pore diameter ϕaveragepores < 100 nm). Then a particular system where small "objects" (coreshells CS, block copolymer MAM) are perfectly dispersed at a micrometric to nanometric scale in poly(methyl methacrylate) (PMMA) will be presented. Such "additives", considered as foaming aids, are aimed at "regulating" the foaming and lowering the pore size and/or density of PMMA based foams. Differences between these additives will be shown. Finally, in a PMMA/20 wt% MAM blend, via a quasi one-step batch foaming, a "porous to nonporous" transition is observed in thick samples. A lower limit of pore size (around 50 nm) seems to arise.

Nanocellular Foaming Behaviors of Chain-Extended Poly(lactic acid) Induced by Isothermal Crystallization

Recently, the fabrication of semicrystalline polymer foams with a nanocellular structure by supercritical fluids has been becoming a newly developing research hotspot, owing to their peculiar properties and prospective applications. In this work, a facile and effective isothermal crystallization-induced method was proposed to prepare nanocellular semicrystalline poly(lactic acid) (PLA) foams using CO₂ as a physical blowing agent. Styrene-acrylonitrile-glycidyl methacrylate (SAG) as a chain extender (CE) was introduced into PLA through a melt-mixing method to improve the crystallization behavior and melt viscoelasticity of PLA. The chain extension reaction between PLA and SAG occurred successfully as well as the branching and micro cross-linking structures were generated in chain-extended PLA (CPLA) samples, which were confirmed by Fourier transform infrared spectra, gel fraction, and intrinsic viscosity measurements. Owing to the nucleation effect of branching points and the restricted movement of PLA molecular chains by the formation of branching and/or microcross-linking structures, a large number of small spherocrystals were generated in CPLA samples, which was beneficial to produce nanocells. Nanocellular CPLA foams were prepared successfully, when the foaming temperature was 125 °C. As the SAG content increased, the cell size of various PLA foams decreased from 364 ± 198 to 249 ± 100 nm and their volume expansion ratio increased from 1.15 ± 0.05 to 2.22 ± 0.01 times, gradually. When the foaming temperature increased from 125 to 127 °C, an interesting transition from nanocells to microcells could be observed in CPLA foam with the CE content of 2 wt %. Finally, the formation mechanism of nanocells in various PLA foams was proposed and clarified using a schematic diagram.

Advanced Polymers for Reduced Energy Consumption in Architecture

In an effort to slow the progress of climate change, the current scientific community has focused on the reduction of greenhouse gases in order to limit the global average temperature inflation to less than 2 °C. The improvement of thermally controlled construction materials can potentially result in lower energy homes/reduced emissions, and lowering the thermal conductivity of insulation materials improves home energy efficiency. Nanoporous insulation foams impart a drastic decrease in thermal conductivity but many polymer properties must be assessed to produce these materials. Passive phase-change materials also represent another key energy-saving device to control heat flux within a living space. Research into unique polymeric systems provides a novel means of encapsulation or creating polymeric cross-linked matrices to prevent leakage and improve mechanical robustness. These two areas of polymer research in architecture represent key advancements for construction materials aimed toward energy savings and energy-related emissions control.

Silica-Assisted Nucleation of Polymer Foam Cells with Nanoscopic Dimensions: Impact of Particle Size, Line Tension, and Surface Functionality

Core-shell nanoparticles consisting of silica as core and surface-grafted poly(dimethylsiloxane) (PDMS) as shell with different diameters were prepared and used as heterogeneous nucleation agents to obtain CO₂-blown poly(methyl methacrylate) (PMMA) nanocomposite foams. PDMS was selected as the shell material as it possesses a low surface energy and high CO₂-philicity. The successful synthesis of core-shell nanoparticles was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy. The cell size and cell density of the PMMA micro- and nanocellular materials were determined by scanning electron microscopy. The cell nucleation efficiency using core-shell nanoparticles was significantly enhanced when compared to that of unmodified silica. The highest nucleation efficiency observed had a value of ∼0.5 for nanoparticles with a core diameter of 80 nm. The particle size dependence of cell nucleation efficiency is discussed taking into account line tension effects. Complete engulfment by the polymer matrix of particles with a core diameter below 40 nm at the cell wall interface was observed corresponding to line tension values of approximately 0.42 nN. This line tension significantly increases the energy barrier of heterogeneous nucleation and thus reduces the nucleation efficiency. The increase of the CO₂ saturation pressure to 300 bar prior to batch foaming resulted in an increased line tension length. We observed a decrease of the heterogeneous nucleation efficiency for foaming after saturation with CO₂ at 300 bar, which we attribute to homogenous nucleation becoming more favorable at the expense of heterogeneous nucleation in this case. Overall, it is shown that the contribution of line tension to the free energy barrier of heterogeneous foam cell nucleation must be considered to understand foaming of viscoelastic materials. This finding emphasizes the need for new strategies including the use of designer nucleating particles to enhance the foam cell nucleation efficiency.

From micro/nano structured isotactic polypropylene to a multifunctional low-density nanoporous medium

A homogeneous low-density nano-porous medium of isotactic polypropylene (iPP) with a low thermal conductivity was fabricated using supercritical carbon dioxide (scCO₂).

Block Copolymer-Assisted Microcellular Supercritical CO2 Foaming of Polymers and Blends

The behaviour in supercritical CO2 of block copolymers containing styrenic, butadiene, and methacrylic or perfluroalkyl blocks is studied in view of a specific swelling and foaming by a gas dissolution process. These block copolymers are considered as neat materials or as additives in blends e.g in polystyrene (PS) or polymethylmethacrylate (PMMA) matrices.

In both cases (neat or blend) the copolymers may exhibit a structuration at a micro or nano level. The phase separated (nano) structures depend on the block type and the concentration of copolymers in the polymer matrix, so that micelles, vesicles, lamellas, or warm-like structures are generated.

Furthermore when one block is chosen as a highly CO2-philic moiety, the nanostructures are able to act as CO2 reservoirs. The result is the possibility to control microcellular foaming, or sometimes nanocellular foaming, of commodity amorphous polymers such as PMMA and PS. Besides, at room temperature, the blocks can be either glassy or rubbery in order to freeze the growth and coalescence of cells during foaming.

Different cellular polymers were elaborated by varying either the copolymer type or the foaming conditions (saturation pressure, temperature, depressurization rate). Cell sizes are accessible in a range from 0.2 to 200 μm, and densities from 0.40 to 1 g/cm3.

It is also shown that nanostructuring polymers are also efficient to produce polymer foams with oriented / structured voids.

This new approach could be used to produce nanocellular or ultra microcellular polymer foams in a simple process, using blending and extrusion.