Using Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry to Depth Profile Nanoparticles in Polymer Nanocomposites.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a versatile surface-sensitive technique for characterizing both hard and soft matter. Its chemical and molecular specificity, high spatial resolution, and superior sensitivity make it an ideal method for depth profiling polymeric systems, including those comprised of both inorganic and organic constituents (i.e., polymer nanocomposites, PNCs). To best utilize ToF-SIMS for characterizing PNCs, experimental conditions must be optimized to minimize challenges such as the matrix effect and charge accumulation. Toward that end, we have successfully used ToF-SIMS with a Xe+ focused ion beam to depth profile silica nanoparticles grafted with poly(methyl methacrylate) (PMMA-NP) in a poly(styrene-ran-acrylonitrile) matrix film by selecting conditions that address charge compensation and the primary incident beam angles. By tracking the sputtered Si+ species and fitting the resultant concentration profile, the diffusion coefficient of PMMA-NP was determined to be D = 2.4 × 10-14 cm2/s. This value of D lies between that measured using Rutherford backscattering spectrometry (6.4 × 10-14 cm2/s) and the value predicted by the Stokes-Einstein model (2.5 × 10-15 cm2/s). With carefully tuned experimental parameters, ToF-SIMS holds great potential for quantitatively characterizing the nanoparticles at the surfaces and interfaces within PNC materials as well as soft matter in general.

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.

Grafted Nanoparticle Surface Wetting during Phase Separation in Polymer Nanocomposite Films.

Wetting of polymer-grafted nanoparticles (NPs) in a polymer nanocomposite (PNC) film is driven by a difference in surface energy between components as well as bulk thermodynamics, namely, the value of the interaction parameter, χ. The interplay between these contributions is investigated in a PNC containing 25 wt % polymethyl methacrylate (PMMA)-grafted silica NPs (PMMA-NPs) in poly(styrene-ran-acrylonitrile) (SAN) upon annealing above the lower critical solution temperature (LCST, 160 °C). Atomic force microscopy (AFM) studies show that the areal density of particles increases rapidly and then approaches 80% of that expected for random close-packed hard spheres. A slightly greater areal density is observed at 190 °C compared to 170 °C. The PMMA-NPs are also shown to prevent dewetting of PNC films under conditions where the analogous polymer blend is unstable. Transmission electron microscopy (TEM) imaging shows that PMMA-NPs symmetrically wet both interfaces and form columns that span the free surface and substrate interface. Using grazing-incidence Rutherford backscattering spectrometry (GI-RBS), the PMMA-NP surface excess (Z*) initially increases rapidly with time and then approaches a constant value at longer times. Consistent with the areal density, Z* is slightly greater at deeper quench depths, which is attributed to the more unfavorable interactions between the PMMA brush and SAN segments. The Z* values at early times are used to determine the PMMA-NP diffusion coefficients, which are significantly larger than theoretical predictions. These studies provide insights into the interplay between wetting and phase separation in PNCs and can be utilized in nanotechnology applications where surface-dependent properties, such as wettability, durability, and friction, are important.

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 (Tg). 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 Tg. The more attractive P2VP shows a 20 °C increase in Tg 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.

Kinetic Monitoring of Block Copolymer Self-Assembly Using In Situ Spectroscopic Ellipsometry.

Understanding the kinetic pathways of self-assembly in block copolymers (BCPs) has been a long-standing challenge, mostly due to limitations of in situ monitoring techniques. Here, we demonstrate an approach that uses optical birefringence, determined by spectroscopic ellipsometry (SE), as a measure of domain formation in cylinder- and lamellae-forming BCP films. The rapid experimental acquisition time in SE (ca. 1 sec) enables monitoring of the assembly/disassembly kinetics of BCP films during solvent-vapor annealing (SVA). We demonstrate that upon SVA, BCP films form ordered domains that are stable in the swollen state, but disorder upon rapid drying. Surprisingly, the disassembly during drying strongly depends on the duration of solvent exposure in the swollen state, explaining previous observations of loss of order in SVA processes. SE thus allows for decoupling of BCP self-assembly and disordering that occurs during solvent annealing and solvent evaporation, which is difficult to probe using other, slower techniques.