Molecular confinement of solid and gaseous phases of self-standing bulk nanoporous polymers inducing enhanced and unexpected physical properties
Graphical abstract
Introduction
Nanoporous polymers are of great scientific and technological importance [1]. Their tunable chemistry, high surface area, and improved mechanical properties offer promising new applications in catalysis, filtration [2], [3], hydrogen storage [4], [5], gas separation and adsorption [6], [7], energy conversion [8], and microelectronics [9], [10]; while their superior thermal properties are suited for applications as high performance thermal insulators in building trade, aeronautics, or even space launch [11], [12]. Several strategies have been developed for the production of these novel materials, often taking advantage of polymer blends or block copolymers self-assembly followed by swelling or removal of a dispersed nanometric phase [13], [14], [15], [16], [17], [18], [19]. In recent years, the use of physical blowing agents (e.g. CO2) to induce the nucleation of pores inside homogeneous polymers with high chemical affinity for the blowing agent has gained popularity as this provides a pathway to exploit the unique properties of nanostructured porous polymers in advanced materials for large scale applications [19], [20], [21], [22], [23], [24], [25], [26].
The nanometric architecture, defined by the presence of nanopores with sizes below 100 nm, provides polymeric materials with a high surface area, in which the confinement of a gaseous phase leads to a drastic reduction of thermal conductivity (i.e. the Knudsen effect already demonstrated in aerogels and porous ceramics) [27], as recently shown experimentally in bulk nanoporous materials (thick nanocellular foams) phase–separated poly (methyl methacrylate) (PMMA)-based systems [28]. Some indirect evidence of changes in the polymer phase behavior within these nanoporous materials were found, along with an enhancement of several mechanical properties compared to microcellular foams [29]. Such improvements were explained by a possible confinement of the polymer chains into pore walls with nanometric dimensions below 60 nm, by analogy to thin films [30]. Yet no experimental evidence exists of a relation between a hypothetical polymer chains confinement in the nanometric polymer matrix and macroscopic physical properties of bulk nanoporous polymers (i.e. self-standing samples with significant thickness of about or over 1 mm, or thousands of times thicker than the size of the pores within), in particular when materials have a closed porous structure.
Section snippets
Experimental section
Materials: Neat poly (methyl methacrylate) homopolymer (PMMA, Tg = 112° C, ρ = 1.18 g/cm3, MwPMMA ≈ 83000 g/mol, MnPMMA ≈ 43000 g/mol, IpPMMA ≈ 1.9) and a triblock copolymer poly (methyl methacrylate)-co-poly (butyl acrylate)-co-poly (methyl methacrylate) (MAM, 36 wt% poly (butyl acrylate), ρ = 1.08 g/cm3, MwMAM ≈ 180000 g/mol, MnMAM ≈ 85000 g/mol, IpMAM ≈ 2.1) were gently provided by Altuglas-Arkema Company (France) in the form of pellets.
Fabrication of PMMA-based nanoporous polymers: 90/10
Results and discussion
Here, we obtain an experimental evidence of the confinement and scale-related effects on the physical properties of bulk nanoporous polymers. To provide such evidences we have produced a series of bulk porous materials with various degrees of confinement, which is achieved by inducing micro and nano porosity into homogeneous and nanostructured polymer matrices based on commercially available PMMA using CO2 as physical blowing agent. Moreover, the use of both fabrication strategies, i.e.
Conclusions
We demonstrate the presence of a confinement effect both in solid and gaseous phases of bulk nanoporous PMMA-based materials, produced by a CO2 sorption and selective swelling or foaming process. In the solid phase, this effect implies conformational changes and immobilization of the PMMA chains, which have been detected by Raman and Dielectric spectroscopy. As a consequence, the glass transition temperature of these materials presents an increase of up to 11° C in nanoporous samples with pore
Acknowledgements
Financial support from The Dow Chemical Company USA, FPI grant BES-2013-062852 (B. Notario) from the Spanish Ministry of Education, MINECO (MAT 2012-34901), and the Junta of Castile and Leon (VA035U13) is gratefully acknowledged. J. Pinto and B. Notario contributed equally to this work.
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