Elsevier

Polymer

Volume 113, 24 March 2017, Pages 27-33
Polymer

Molecular confinement of solid and gaseous phases of self-standing bulk nanoporous polymers inducing enhanced and unexpected physical properties

https://doi.org/10.1016/j.polymer.2017.02.046Get rights and content

Highlights

  • Molecular confinement of polymer chains on self-standing nanoporous PMMA is evidenced.

  • Scale-reduction related phenomena appear on both polymer and gaseous phases.

  • Influence on the macroscopic physical properties of the confinement is demonstrated.

Abstract

In this work it is provided the first evidence of the polymer chains confinement within self-standing pore walls of nanoporous materials based on poly (methyl methacrylate) (PMMA). This was made possible by producing a series of porous samples with a wide range of pore sizes between 90 nm and 3 μm using processes combining CO2 sorption, selective block copolymer swelling or homogeneous physical foaming. Mobility restrictions of the PMMA chains in the porous samples with pore size below 200 nm was consistently demonstrated with several experimental techniques, including differential scanning calorimetry, Raman spectroscopy, and broadband dielectric spectroscopy.

In addition, several scale-reduction phenomena related to the constitutive elements of the porous materials, both in the polymeric and gaseous phases, and to the porous architecture are identified. The significance of these phenomena on macroscopic electrical conductivity and permittivity of the nanoporous materials is demonstrated, and the presented observations support previous explanations of improved mechanical properties and thermal insulation of this type of nano-materials.

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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