Inside the structure of a nanocomposite electrolyte membrane: how hybrid particles get along with the polymer matrix

Nanoscale Structure-Property Relations in Self-Regulated Polymer-Grafted Nanoparticle Composite Structures

Using a model system of poly(methyl methacrylate)-grafted silica nanoparticles (PMMA-NP) and poly(styrene-ran-acrylonitrile) (SAN), we generate unique polymer nanocomposite (PNC) morphologies by balancing the degree of surface enrichment, phase separation, and wetting within the films. Depending on the annealing temperature and time, thin films undergo different stages of phase evolution, resulting in homogeneously dispersed systems at low temperatures, enriched PMMA-NP layers at the PNC interfaces at intermediate temperatures, and three-dimensional bicontinuous structures of PMMA-NP pillars sandwiched between two PMMA-NP wetting layers at high temperatures. Using a combination of atomic force microscopy (AFM), AFM nanoindentation, contact angle goniometry, and optical microscopy, we show that these self-regulated structures lead to nanocomposites with increased elastic modulus, hardness, and thermal stability compared to analogous PMMA/SAN blends. These studies demonstrate the ability to reliably control the size and spatial correlations of both the surface-enriched and phase-separated nanocomposite microstructures, which have attractive technological applications where properties such as wettability, toughness, and wear resistance are important. In addition, these morphologies lend themselves to substantially broader applications, including: (1) structural color applications, (2) tuning optical adsorption, and (3) barrier coatings.

Effect of Nanoscale Confinement on Polymer-Infiltrated Scaffold Metal Composites

Most research on polymer composites has focused on adding discrete inorganic nanofillers to a polymer matrix to impart properties not found in polymers alone. However, properties such as ion conductivity and mechanical reinforcement would be greatly improved if the composite exhibited an interconnected network of inorganic and polymer phases. Here, we fabricate bicontinuous polymer-infiltrated scaffold metal (PrISM) composites by infiltrating polymer into nanoporous gold (NPG) films. Polystyrene (PS) and poly(2-vinylpyridine) (P2VP) films are infiltrated into the ∼43 nm diameter NPG pores via capillary forces during thermal annealing above the polymer glass transition temperature (T_g). The infiltration process is characterized in situ using spectroscopic ellipsometry. PS and P2VP, which have different affinities for the metal scaffold, exhibit slower segmental dynamics compared to their bulk counterparts when confined within the nanopores, as measured through T_g. The more attractive P2VP shows a 20 °C increase in T_g relative to its bulk, while PS only shows a 6 °C increase at a comparable molecular weight. The infiltrated polymer, in turn, stabilizes the gold nanopores against temporal coarsening. The broad tunability of these polymer/metal hybrids represents a unique template for designing functional network composite structures with applications ranging from flexible electronics to fuel cell membranes.

Unveiling the multiscale morphology of chemically stabilized proton exchange membranes for fuel cells by means of Fourier and real space studies

We recently presented the elaboration and functional properties of a new generation of hybrid membranes for PEMFC applications showing promising performances and durability. The strategy was to form, inside a commercial sPEEK membrane, via in situ sol-gel (SG) synthesis, a reactive SG phase able to reduce oxidative species generated during FC operation. In order to understand structure-properties interplay, we use a combination of direct space (AFM/3D FIB-SEM) and reciprocal space (SANS/WAXS) techniques to cover dimensional scales ranging from a hundred to few nanometers. AFM modulus images showed the SG phase distributed into spherical domains whose size increases with the SG uptake (ca. 100-200 nm range). Using contrast variation SANS, we observed that the sPEEK nanostructure is mostly unaffected by the insertion of the SG phase which presents a fractal-like multiscale structure. Additionally, the size of both the particles (aggregates/primary) is much too large to be sequestered in the ionic pathways of sPEEK. These findings indicate that the SG-NPs mainly grow within the amorphous interbundle domains. Noticeable rightward shift and widening of the ionomer peak are observed with the SG content, suggesting ion channel compression and greater heterogeneity of the ionic domain size. The SG phase develops in the interbundle regions with a limited impact on the water uptake but leading to a discontinuity of ionic conductivity. This Fourier and real spaces study clarifies the structure of the hybrid membranes and brings into the question the ideal distribution/localization of the SG phase to optimize the membrane's stabilization.