Direct observation of nanoparticle embedding into the surface of a polymer melt

High-Temperature Melt Stamping of Polymers Using Polymer/Nanoporous Gold Composite Stamps

Parallel lithographic deposition of polymers onto counterpart substrates is a widely applied surface manufacturing operation. However, polymers may only be soluble in organic solvents or are insoluble at all. Solvent evaporation during stamping may trigger hardly controllable capillarity-driven flow processes or phase separation, and polymer solutions may spread on the counterpart substrates. Solvent-free stamping of melts prevents these drawbacks. Here, a stamp design for the deposition of melts is devised, which intrinsically circumvents ink depletion. The stamps' topographically patterned contact surfaces with protruding contact elements contacting the counterpart substrates consist of a nanoporous gold layer with a thickness of a few micrometers. The nanoporous gold layer is attached to a molten polymer layer, which is support for the nanoporous gold layer and ink reservoir at the same time. The nanoporous gold layer in turn stabilizes the topography of the stamps' contact surfaces. As examples, arrays of submicron microdots of polystyrene and poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE) are manufactured. The P(VDF-TrFE) microdots are partially crystalline, ferroelectric, and can be locally poled. It is envisioned that the methodology reported here can be automatized and may be extended to functional low-molecular-mass compounds, such as active pharmaceutical ingredients.

Surface and bulk relaxation of vapor-deposited polystyrene glasses

We have studied the liquid-like response of the surface of vapor-deposited glassy films of polystyrene to the introduction of gold nanoparticles on the surface. The build-up of polymer material was measured as a function of time and temperature for both as-deposited films, as well as films that have been rejuvenated to become normal glasses cooled from the equilibrium liquid. The temporal evolution of the surface profile is well described by the characteristic power law of capillary-driven surface flows. In all cases, the surface evolution of the as-deposited films and the rejuvenated films is enhanced compared to bulk and is not easily distinguishable from each other. The temperature dependence of the measured relaxation times determined from the surface evolution is found to be quantitatively comparable to similar studies for high molecular weight spincast polystyrene. Comparisons to numerical solutions of the glassy thin film equation provide quantitative estimates of the surface mobility. For temperatures sufficiently close to the glass-transition temperature, particle embedding is also measured and used as a probe of bulk dynamics, and, in particular, bulk viscosity.

On Demand Sequential Release of (Sub)Micron Particles Controlled by Size and Temperature

Polymeric devices capable of releasing submicron particles (subMP) on demand are highly desirable for controlled release systems, sensors, and smart surfaces. Here, a temperature-memory polymer sheet with a programmable smooth surface served as matrix to embed and release polystyrene subMP controlled by particle size and temperature. subMPs embedding at 80 °C can be released sequentially according to their size (diameter D of 200 nm, 500 nm, 1 µm) when heated. The differences in their embedding extent are determined by the various subMPs sizes and result in their distinct release temperatures. Microparticles of the same size (D ≈ 1 µm) incorporated in films at different programming temperatures T_p (50, 65, and 80 °C) lead to a sequential release based on the temperature-memory effect. The change of apparent height over the film surface is quantified using atomic force microscopy and the realization of sequential release is proven by confocal laser scanning microscopy. The demonstration and quantification of on demand subMP release are of technological impact for assembly, particle sorting, and release technologies in microtechnology, catalysis, and controlled release.

Size-Dependent Submerging of Nanoparticles in Polymer Melts: Effect of Line Tension

Adhesion of nanoparticles to polymer films plays a key role in various polymer technologies. Here we report experiments that reveal how silica nanoparticles adhere to a viscoelastic PMMA film above the glass transition temperature. The polymer was swollen with CO₂, closely matching the conditions of nanoparticle-nucleated polymer foaming. It is found that the degree by which the particles sink into the viscoelastic substrate is strongly size dependent and can even lead to complete engulfment for particles of diameter below 12 nm. These findings are explained quantitatively by a thermodynamic analysis, combining elasticity, capillary adhesion, and line tension. We argue that line tension, here proposed for the first time in elastic media, is responsible for the nanoparticle engulfment.

Optically Patternable Metamaterial Below Diffraction Limit

We report an optically patternable metamaterial (OPM) for ultraviolet nanolithography below the diffraction limit. The OPM features monolayered silver nanoislands embedded within a photosensitive polymer by using spin-coating of an ultrathin polymer, oblique angle deposition, and solid-state embedment of silver nanoislands. This unique configuration simultaneously exhibits both negative effective permittivity and high image contrast in the ultraviolet range, which enables the surface plasmon excitation for the clear photolithographic definition of minimum feature size of 70 nm (≲ λ/5) beyond the near-field zone. This new metamaterial provides a new class of photoresist for ultraviolet nanolithography below the diffraction limit.

Facilitated embedding of silver nanowires into conformally-coated iCVD polymer films deposited on cloth for robust wearable electronics

We propose that a silver nanowire (AgNW)-embedded conducting film can be monolithically applied onto an arbitrary cloth with strong adhesion and environmental stability. We employ a vapor-phase method, initiated chemical vapor deposition (iCVD), for conformal coating of a scaffold polymer film on the cloth. AgNWs are applied on the surface of iCVD polymer films, and the embedding of AgNWs is completed within only 20 s on heating the polymer-coated cloth to 70 °C. Crosslinking the copolymer at 120 °C renders the AgNW-embedded conducting films on the cloth not only thermally and chemically stable, but also mechanically robust. Moreover, when a hydrophobic encapsulating polymer layer is added on the AgNW-embedded film via iCVD, it substantially improves the stability of the cloth against thermal oxidation under hot and humid conditions, showing applicability of the technology to wearable electronics. With these robust conducting films, we demonstrate the fabrication of a waterproof cloth-based heater and circuit for a seven-segment display, thus, confirming the wide applicability of the technology developed in this study.

Dynamics of nanocubes embedding into polymer films investigated via spatially resolved plasmon modes

Integration of nanoparticles into thin films is essential for the development of functional materials, studies of fundamental interfacial processes, and exploitation of inherent properties from the particles themselves. In this work, we systematically investigate the process of incorporation of silver nanocubes into thin polystyrene films at temperatures just above the polymer glass transition. The process of nanocrystal incorporation can be precisely monitored via far-field spectroscopy to observe the response of spatially resolved hybrid plasmon modes. Each plasmon resonance has a distinct dynamic range and maximum sensitivity forming a complementary set of nanorulers that operates over a distance comparable to the edge length of the cubes. The approach explored in this work is a general robust method for the development of long-range polychromatic nanorulers.

Optical and surface enhanced Raman scattering properties of Au nanoparticles embedded in and located on a carbonaceous matrix

Au nanoparticles (NPs) on the surface and embedded in a matrix have been the subject of studies dealing with a variety of spectroscopic and sensing applications.

Noble metal nanoparticles embedding into polymeric materials: From fundamentals to applications

This review covers some key concepts related to embedding of the noble metal nanoparticles in polymer surfaces. The metal nanoparticles embedded into the polymer matrix can provide high-performance novel materials that find applications in modern nanotechnology. In particular, the origin of various processes that drive the embedding phenomenon, growth of the nanostructure at the surface, factors affecting the embedding including role of surface, interface energies and thermodynamic driving forces with emphasis on the fundamental and technological applications, under different conditions (annealing and ion beams) have been discussed. In addition to the conventional thermal process for embedding which includes the measure of fundamental polymer surface properties with relevant probing techniques, this review discusses the recent advances carried out in the understanding of embedding phenomenon starting from thin metal films to growth of the nanoparticles and embedded nanostructures using novel ion beam techniques.

Stability of ultrathin nanocomposite polymer films controlled by the embedding of gold nanoparticles

Thin and ultrathin polymer films combined with nanoparticles (NPs) are of significant interest as they are used in a host of industrial applications. In this paper we describe the stability of such films (hpoly ≤ 30 nm) to dewetting, specifically, how the development of a spinodal instability in a composite NP-polymer layer is controlled by the embedding of Au NPs. At working temperatures (T = 170 °C) above the polymer glass transition temperature (Tg ≈ 100 °C) the absence of Au NPs leads to film rupture by nucleation dewetting, while their presence over a large surface area enhances the development of a spinodal instability without destroying the film continuity. When the NPs embed, the surface undulations are suppressed. The dynamics change from an unstable to a stable state, and the thin composite NP-polymer layer returns to a flat configuration, while the wavelength of the pattern remains constant. Moreover, we demonstrate from a thermodynamic perspective that NPs will remain on the surface or embed in the polymer film depending on their free energy, which is determined by the NP interactions with the underlying polymer, the native SiOx layer, and the Si substrate.

Fabrication of gold nanoparticle-polymer composite particles with raspberry, core-shell and amorphous morphologies at room temperature via electrostatic interactions and diffusion

Composite particles with varying morphologies composed of gold nanoparticles (Au NPs) and polymers were fabricated based on a combination of electrostatic interactions between the polymer particles and Au NPs and diffusion processes. The positively charged polymer particles were prepared from amino-terminated polystyrene (PS-NH2) and amino-terminated 1,2-polybutadiene (PB-NH2). Adsorption of citrate-stabilized Au NPs resulted in three different distribution states of Au NPs in the polymer particles, depending on the glass transition temperature (Tg) and molecular weight of the polymer. The adsorption of Au NPs onto PS-NH2 particles produced raspberry-like composite particle morphologies, while the NPs instead diffused into the PB-NH2 particles, since the Tg of PB-NH2 is below room temperature. The diffusion of Au NPs could be controlled by varying the molecular weight of the PB-NH2 and the diameter of the NPs, and both core-shell and amorphous distributions were successfully achieved.

Effect of Au nanoparticle spatial distribution on the stability of thin polymer films

The stability of thin poly(methyl-methacrylate) (PMMA) films of low molecular weight on a solid substrate is controlled by the areal coverage of gold nanoparticles (NPs) present at the air-polymer interface. As the polymer becomes liquid the Au NPs are free to diffuse, coalesce, and aggregate while the polymer film can change its morphology through viscous flow. These processes lead at the same time to the formation of a fractal network of Au NPs and to the development of spinodal instabilities of the free surface of the polymer films. For thinner films a single wavelength is observed, while for thicker films two wavelengths compete. With continued heating the aggregation process results in a decrease in coverage, the networks evolve into disordered particle assemblies, while the polymer films flatten again. The disordering occurs first on the smallest scales and coincides (in thicker films) with the disappearance of the smaller wavelength. The subsequent disordering on larger scales causes the films to flatten.

Tunable wetting of nanoparticle-decorated polymer films

In this paper, amine-modified silica nanoparticles (NPs) with diameters (d) from 15 to 230 nm are covalently linked to poly(styrene-random-acrylic acid) (P(S-ran-AA)) films, and wettability is studied as a function of diameter and NP surface coverage. During attachment, films swell and exhibit long and short scale roughness, consisting of a ridged, honeycomb structure, approximately 1 mum wide and 45-50 nm deep, which encircles nanoscale features 10-15 nm high and approximately 50 nm apart. A maximum NP coverage of approximately 70% was achieved for d less than or nearly equal to the nanoscale roughness induced by surface swelling. For d several times greater than this nanoscale roughness, the maximum coverage was limited by interparticle repulsion and reached only approximately 30%. For NPs with diameters of 15-106 nm, the water contact angle increased from 75 degrees to 120 degrees as NP coverage increased from 0 to 70%. At low and high NP coverage, the Wenzel and Cassie models, respectively, accurately describe the data. However, at intermediate NP coverage, neither model is satisfactory. An increase in surface roughness alone cannot account for this discrepancy. Atomic force microscopy (AFM) studies show that the NPs partially embed into the swollen P(S-ran-AA) surface, suggesting that the amine-coated NPs are wet by the copolymer, exposing low surface energy styrene. These studies demonstrate that control over surface properties of coatings, such as wetting, can be achieved by selecting NP sizes that complement film roughness.