Dewetting Dynamics of Nanofilled Polymer Thin Films

Effect of Solvent on Convectively Driven Silica Particle Assembly: Decoupling Surface Tension, Viscosity, and Evaporation Rate

The process of convectively self-assembling particles in films suffers from low reproducibility due to its high dependency on particle concentration, as well as a variety of interactions and physical parameters. Inhomogeneities in flow rates and instabilities at the air-liquid interface are mostly responsible for reproducibility issues. These problems are aggravated by adding multiple components to the dispersion, such as binary solvent mixtures or surfactant/polymer additives, both common approaches to control stick-slip behavior. When an additive is used, not only does it change the surface tension, but also the viscosity and the evaporation rate. Worse yet, gradients in these three properties can form, which then lead to Marangoni currents. Here, we use a series of alcohols to study the role of viscosity independently of other solvent properties, to show its impact on stick-slip behavior and interband distances. We show that mixtures of glycerol and alcohol or poly(acrylic acid) and alcohol lead to more complex patterning. Marangoni currents are not always observed in co-solvent systems, being dependent on the rate of solvent evaporation. To produce homogeneous particle assemblies and control stick-slip behavior, gradients must be avoided, and the surface tension and viscosity need both be carefully controlled.

The influence of polymer architectures on the dewetting behavior of thin polymer films: from linear chains to ring chains

The dewetting behavior of ring polystyrene (RPS) film and linear polystyrene (LPS) film on silanized Si substrates with different grafting densities and PDMS substrate was investigated. Results showed that polymer architectures greatly influenced the dewetting behavior of the thin polymer film. On the silanized Si substrate with 69% grafting density, RPS chains exhibited stronger adsorption compared with LPS chains, and as a result the wetting layer formed more easily. For LPS films, with a decreased annealing temperature, the stability of the polymer film changed from non-slip dewetting via apparent slip dewetting to apparently stable. However, for RPS films, the polymer film stability switched from apparent slip dewetting to apparently stable. On the silanized Si substrate with 94% grafting density, the chain adsorption became weaker and the dewetting processes were faster than that on the substrate with 69% grafting density at the same experimental temperature for both the LPS and RPS films. Moreover, on the PDMS substrate, LPS films always showed non-slip dewetting, while the dewetting kinetics of RPS films switched from non-slip dewetting to slip dewetting behaviour. Forming the wetting layer strongly influenced the stability and dewetting behavior of the thin polymer films.

Fabrication of nanocomposites by covalent bonding between noble metal nanoparticles and polymer matrix

Noble metal nanoparticles capped with novel aminothioalkil ligands are used to fabricate polymer nanocomposites. The nanoparticles are permanently attached to the polymer matrix through covalent bonding.

Molecular insight into nanoscale water films dewetting on modified silica surfaces

In this work, molecular dynamics simulations are adopted to investigate the microscopic dewetting mechanism of nanoscale water films on methylated silica surfaces.

Thermal dewetting behavior of polystyrene composite thin films with organic-modified inorganic nanoparticles

The thermal dewetting of polystyrene composite thin films with oleic acid-modified CeO2 nanoparticles prepared by the supercritical hydrothermal synthesis method was investigated, varying the nanoparticle concentration (0-30 wt %), film thickness (approximately 50 and 100 nm), and surface energy of silanized silicon substrates on which the composite films were coated. The dewetting behavior of the composite thin films during thermal annealing was observed by an optical microscope. The presence of nanoparticles in the films affected the morphology of dewetting holes, and moreover suppressed the dewetting itself when the concentration was relatively high. It was revealed that there was a critical value of the surface energy of the substrate at which the dewetting occurred. In addition, the spatial distributions of nanoparticles in the composite thin films before thermal annealing were investigated using AFM and TEM. As a result, we found that most of nanoparticles segregated to the surface of the film, and that such distributions of nanoparticles contribute to the stabilization of the films, by calculating the interfacial potential of the films with nanoparticles.

Improved mechanical stability of acetoxypropyl cellulose upon blending with ultranarrow PbS nanowires in Langmuir monolayer matrix

Cellulose and cellulose derivatives have long been used as membrane fabrication. Langmuir monolayer behavior, which naturally mimics membranes, of acetoxypropyl cellulose (APC) and lead sulfide (PbS) nanowire mixtures at different volume ratios is reported. Surface pressure (π)-area (A) isotherms of APC and PbS nanowires mixtures at different volume ratios show a gradual decrease in the monolayer area with increasing volume fraction of PbS nanowires. Change of surface potential with monolayer area at different volume ratios also reveals a gradual increase in the surface potential indicating incorporation of PbS nanowires within APC matrix. The compressibility and elastic constants measurements reveal an enhancement of the elasticity upon incorporation of PbS nanowires up to certain volume fractions. An enhancement in stability of the blend is observed upon PbS nanowire incorporation to the APC matrix. Rheological measurements also support the robustness of the mixture of APC and PbS nanowires in 3D bulk phase. Such robust ultrathin films of cellulose based-nanowire blend obtained by means of the Langmuir technique may lead to novel routes for designing cellulosic-based thin films and membranes.

Self-organized pathways to nanopatterns exploiting the instabilities of ultrathin confined bilayers

Self-organized interfacial instabilities of an ultrathin bilayer confined between a pair of rigid surfaces is explored. The bilayers are classified based on the macroscopic dewetting behaviors of the liquid films sandwiched between a pair of confining surfaces having surface energy higher or lower than the liquid films. Linear and nonlinear analyses employing the governing equations originating from the continuum description together with molecular dynamics (MD) simulations unveil the salient spatiotemporal features of the dewetting process. The study uncovers that, under the destabilizing influence of the intermolecular interactions, the interface of a confined bilayer can deform into interesting embedded and encapsulated patterns with nanoscale periodicity. The continuum and MD simulations collectively show the detailed route to dewetting starting from the formation of holes in the early stage, their growth to achieve equilibrium contact angle at the intermediate phase, and then to evolve into the equilibrium morphologies at the later stage. Examples are shown where the length and the time scales of the simulated nanostructures from both the continuum and MD approaches are found to agree with the same obtained from the linear stability analysis. We also highlight the deviations that are observed in the continuum and MD approaches. The study confirms that the reduced stabilizing interfacial tension at the liquid-liquid interface together with enhanced intermolecular interaction because of the thinness of the layers can be an alternative strategy for pattern miniaturization exploiting the instabilities of a thin confined bilayer. Further, the study shows that use of topographical patterns on the bounding surfaces can impose periodic order to the holes, droplets, columns, and channels, which can find important applications in the diverse areas of nanotechnology.

Suppression of the coffee ring effect by hydrosoluble polymer additives

A simple and novel method has been demonstrated for avoiding coffee ring structure based on hydrosoluble polymer additives during droplet evaporation. The polymer additives lead to the motion of the contact line (CL) resulted from the viscosity and Marangoni effect. The viscosity provides a large resistance to the radially outward flow. It results in a small amount of spheres deposited at droplet edge, which do not facilitate the pinning of the CL. The Marangoni effect resulted from the variation of polymer concentration at droplet edge during droplet evaporation contributes to the motion of the CL. Thus, uniform and ordered macroscale SiO(2) microspheres deposition is achieved. What's more, the coffee ring effect can be eliminated by different hydrosoluble polymer. This method will be applicable to a wide of aqueous system and will be of great significance for extensive applications of droplet deposition in biochemical assays and material deposition.

Effect of film thickness on morphological evolution in dewetting and crystallization of polystyrene/poly(ε-caprolactone) blend films

In this Article, the morphological evolution in the blend thin film of polystyrene (PS)/poly(ε-caprolactone) (PCL) was investigated via mainly AFM. It was found that an enriched two-layer structure with PS at the upper layer and PCL at the bottom layer was formed during spinning coating. By changing the solution concentration, different kinds of crystal morphologies, such as finger-like, dendritic, and spherulitic-like, could be obtained at the bottom PCL layer. These different initial states led to the morphological evolution processes to be quite different from each other, so the phase separation, dewetting, and crystalline morphology of PS/PCL blend films as a function of time were studied. It was interesting to find that the morphological evolution of PS at the upper layer was largely dependent on the film thickness. For the ultrathin (15 nm) blend film, a liquid-solid/liquid-liquid dewetting-wetting process was observed, forming ribbons that rupture into discrete circular PS islands on voronoi finger-like PCL crystal. For the thick (30 nm) blend film, the liquid-liquid dewetting of the upper PS layer from the underlying adsorbed PCL layer was found, forming interconnected rim structures that rupture into discrete circular PS islands embedded in the single lamellar PCL dendritic crystal due to Rayleigh instability. For the thicker (60 nm) blend film, a two-step liquid-liquid dewetting process with regular holes decorated with dendritic PCL crystal at early annealing stage and small holes decorated with spherulite-like PCL crystal among the early dewetting holes at later annealing stage was observed. The mechanism of this unusual morphological evolution process was discussed on the basis of the entropy effect and annealing-induced phase separation.

Stability and dewetting of metal nanoparticle filled thin polymer films: control of instability length scale and dynamics

We investigate the influence of gold nanoparticle addition on the stability, dewetting, and pattern formation in ultrathin polymer-nanoparticle (NP) composite films by examining the length and time scales of instability, morphology, and dynamics of dewetting. For these 10-50 nm thick (h) polystyrene (PS) thin films containing uncapped gold nanoparticles (diameter approximately 3-4 nm), transitions from complete dewetting to arrested dewetting to absolute stability were observed depending on the concentration of the particles. Experiments show the existence of three distinct stability regimes: regime 1, complete dewetting leading to droplet formation for nanoparticle concentration of 2% (w/w) or below; regime 2, partial dewetting leading to formation of arrested holes for NP concentrations in the range of 3-6%; and regime 3, complete inhibition of dewetting for NP concentrations of 7% and above. Major results are (a) length scale of instability, where lambdaH approximately hn remains unchanged with NP concentration in regime 1 (n approximately 2) but increases in regime 2 with a change in the scaling relation (n approximately 3-3.5); (b) dynamics of instability and dewetting becomes progressively sluggish with an increase in the NP concentration; (c) there are distinct regimes of dewetting velocity at low NP concentrations; (d) force modulation AFM, as well as micro-Raman analysis, shows phase separation and aggregation of the gold nanoparticles within each dewetted polymer droplet leading to the formation of a metal core-polymer shell morphology. The polymer shell could be removed by washing in a selective solvent, thus exposing an array of bare gold nanoparticle aggregates.

Direct observation of interfacial C60 cluster formation in polystyrene-C60 nanocomposite films

Large interfacial C(60) clusters were directly imaged at the supporting film-substrate interface in physically detached polystyrene-C(60) nanocomposite films by atomic force microscopy, confirming the stabilizing mechanism previously hypothesized for thin polymer films. Additionally, we found that the C(60) additive influences basic thermodynamic film properties such as the interfacial energy and the film thermal expansion coefficient.

Control of dispersion state of silsesquioxane nanofillers for stabilization of polystyrene thin films

The influence of the dispersion states of the nanofillers on the dewetting behavior of the polymer thin film was investigated. Polyhedral oligomeric silsesquioxanes (POSS) with various substituents were added into polystyrene (PS) thin films as the nanofillers. The dewetting rate of the films drastically changed with the surface substituents of POSS additives. Neutron reflectivity measurements indicated that the difference of the dewetting rate was associated with the dispersion state of POSS additives in the films. POSS with phenethyl groups (PhPOSS), which homogeneously dispersed into the films, resulted in the decrease of the glass transition temperature of PS and the enhancement of the dewetting of the films. POSS with a fluoroalkyl group (CpPOSS-R f) segregated to the film surface and showed the retardation of the dewetting by the decrease of the surface energy of the film. POSS with hydroxyl groups (CpPOSS-2OH) segregated to the film surface and film-substrate interface and led to the elimination of the dewetting, suggesting the importance of the interfacial segregation for the inhibition of dewetting. These results revealed the strong relationship between the dispersion state of the nanofillers and the dewetting of the nanofilled films.

Influences of chain heterogeneity on instability of polymeric thin films: Dewetting of polystyrenes, polychloromethylstyrenes and its copolymers

This study compares the stability of various polymeric thin films supported on SiO(x)/Si substrate. Dewetting behaviors of polystyrenes (PS), polychloromethylstyrenes, and random poly(styrene-co-chloromethylstyrene)s are investigated by utilizing atomic force microscopy. A systematic addition of the chloromethylstyrene (ClMS) unit into PS chain causes the increase of segment polarity, affecting interfacial and interchain interactions in thin films. It is found that stability of the polymeric films depends on two major parameters, ratio of the ClMS unit and film thickness. For approximately 5 nm thick film, the addition of only 5 mol% ClMS unit causes a drastic increase of its stability, attributed to the enhanced interfacial interactions between ClMS group and SiO(x) layer. Further increasing the ClMS mole ratio to 20, 45, and 100% is accompanied by a systematic increase of the film stability. Thicker films (thicknesses approximately 22 and approximately 45 nm) of the copolymer with 5 mol% ClMS unit exhibit rather different behavior. They are found to be less stable compared to the PS films. However, the films of copolymers with ClMS unit of 20, 45, and 100% are still much more stable than the PS films. These dewetting behaviors of the copolymers are correlated to the interfacial interactions, interchain interactions and segmental segregation in thin films.

Controlled arrangement of nanoparticle arrays in block-copolymer domains

This Review describes recent results on the precise spatial distribution control of metal and semiconductor nanoparticles into domains of microphase-separated block copolymers. Specific focus is directed towards selective incorporation into a specific microphase of a block copolymer. Details on theoretical aspects concerning nanoparticle incorporation as well as practical examples are given. Furthermore, examples on applications and technological aspects of the resulting nanoparticle/polymer nanocomposites are provided.

Influence of molecular architecture on the dewetting of thin polystyrene films

The control of dewetting for thin polymer films is a technical challenge and of significant academic interest. We have used polystyrene nanoparticles to inhibit dewetting of high molecular weight, linear polystyrene, demonstrating that molecular architecture has a unique effect on surface properties. Neutron reflectivity measurements were used to demonstrate that the nanoparticles were uniformly distributed in the thin (ca. 40 nm) film prior to high temperature annealing, yet after annealing, they were found to separate to the solid substrate, a silanized silicon wafer. Dewetting was eliminated when the nanoparticles separated to form a monolayer or above while below this surface coverage the dewetting dynamics was severely retarded. Blending linear polystyrene of similar molecular weight to the polystyrene nanoparticle with the high molecular weight polystyrene did not eliminate dewetting.

Dewetting of thin polystyrene films absorbed on epoxy coated substrates

Various characteristics of dewetting of thin polystyrene (PS) films absorbed on highly cross-linked epoxy-coated and silicon oxide covered substrates are studied as a function of PS film thickness (20<h<1300 A) by optical microscopy, atomic force microscopy, and x-ray and neutron reflectivity. For a silicon oxide covered substrate, the nucleation of holes and growth (NG) mechanism is observed for h>h(c1) whereas the spinodal dewetting (SD) occurs through the growth of surface undulations for h<h(c1), where h(c1) is approximately 4R(g). For an epoxy-coated substrate, the NG mechanism is observed for h>h(c2) while the SD mechanism is observed for h<h(c2), where h(c2) is approximately 6R(g). We demonstrate that the highly cross-linked epoxy-coated silicon substrate leads to retardation of the PS film dewetting in comparison to the silicon oxide covered silicon substrate. Moreover, we confirm that the epoxy-coated substrate leads to a significant decrease in the fraction of dewetted area at the apparent equilibrium stage of dewetting due to the anchoring effect of PS molecules caused from the cross-linked networks of the epoxy layer. In contrast the retardation effect of the epoxy-coated substrate on the rate of dewetting is more remarkable for relatively thinner PS films (h< approximately 800 A) than thicker films ( approximately 800<h<1300 A) since the short-range intermolecular interactions are dominant for relatively thin PS films. Thus the highly cross-linked epoxy-coated substrate has a large influence on the kinetics, morphology, and mechanism of dewetting of thin PS films.