Wetting properties of thin liquid polyethylene propylene films

Enhanced Thermal Stability of Thermoplastic Polymer Nanostructures for Nanoimprint Lithography

Thermoplastic polymer micro- and nanostructures suffer pattern decay when heated to a temperature close to or above the polymer’s glass transition temperature. In this work, we report enhanced thermal stability of polycarbonate nanostructures at temperatures well above their glass transition temperatures. Based on this observation, we develop a unique technique for high-resolution polymer patterning by polymer reflows. This technique is characterized as the precise control of polymer reflows regardless of the annealing time, which avoids the time-domain nonlinear reflow of the polymer melt. We also implement thermal nanoimprinting in a step-and-repeat fashion, which dramatically increases the throughput of the thermal nanoimprint. The enhanced pattern stability against thermal reflow also allows for multiple imprinting at the same location to generate complex resist patterns from a simple mold structure. Since modern lithography often uses thin resist films (sub-100 nm) due to the restraint from the pattern aspect ratio, the unusual annealing behavior of thin polymer films is highly relevant in sub-100 nm lithographic processing.

Morphology of Poly(styrene- co-butadiene) Random Copolymer Thin Films and Nanostructures on a Graphite Surface

We studied the morphology of poly(styrene- co-butadiene) random copolymers on a graphite surface. Polymer solutions were spin coated onto graphite, at various concentrations and molecular weights. The polymer films and nanostructures were imaged using atomic force microscopy. Above the overlap concentration, thin films formed. However, total wetting did not occur, despite the polymers being well above their T_g. Instead, dewetting was observed, suggesting the films were in a state of metastable equilibrium. At lower concentrations, the polymers formed networks, nanoislands, and nanoribbons. Ordered nanopatterns were observed on the surface; the polymers orientated themselves due to π-π stacking interactions reflecting the crystalline structure of the graphite. At the lowest concentration, this ordering was very pronounced. At higher concentrations, it was less defined but still statistically significant. Higher degrees of ordering were observed with poly(styrene- co-butadiene) than polystyrene and polybutadiene homopolymers as the copolymer's aromatic rings are distributed along a flexible chain, which maximizes π-π stacking. At the two lowest concentrations, the size of the nanoislands and nanoribbons remained similar with varying molecular weight. However, at higher concentrations, the polymer network features were largest at the lowest molecular weight, indicating that in this case, a large proportion of shorter chains stay on top of the adsorbed ones. The contact angles of the polymer nanostructures remained mostly constant with size, which is due to the strong polymer/graphite adhesion dominating over line tension and entropic effects.

Crossover from semi-dilute to densely packed thin polymer films at the air-water interface and structure formation at thin film breakup

A series of poly(n-butyl acrylate) (PnBA, 5 to 32 kg mol-1) homopolymers and diblock copolymers with poly(ethylene glycol) (PEG, constant molecular weight of 0.3 kg mol-1) is synthesized for the purpose of the investigation of quasi-2D polymer films at the air-water interface. The presented compression isotherms show a transition from θ solvent behavior for PnBA homopolymers to good solvent conditions when the volume fraction of the PEG in the block copolymers is increased by decreasing the molecular weight of PnBA. A transition from a semi-dilute regime to a densely packed layer is observed in the pressure isotherms for all the polymers. In the densely packed films we found first evidence for thin film breakup of a thin polymer film directly at the air-water interface. Combination of results from Brewster-Angle-Microscopy and Surface X-ray scattering provide a consistent picture of the film breakup. Our results suggest a preferred length scale of 2.5 μm. This scenario is analogous to a spinodal mechanism driven by thermal fluctuations of the film height.

Dewetting Properties of Polystyrene Homopolymer Thin Films on Grafted Polystyrene Brush Surfaces

We have found that the critical molecular weight for auto dewetting of a homopolymer on a polymer brush of the same chemical composition is approximately NH = 0.5 NB, in good agreement with the mean field theory prediction. The measured velocity on the brush surface is at least an order of magnitude faster than that of the polystyrene (PS) on a homopolymer interface, with a greatly decreased dependence on the PS molecular weight. This suggests slippage as a possible mechanism for the dewetting dynamics.

Spatial and Temporal Dependence of Diffusion in Polystyrene Thin Films on Silicon and Carbon Surfaces

The strong inhibition of chain diffusion in thin polystyrene (PS) films has been observed near an attractive silicon surface, and diffusion remains inhibited out to distances of several radii of gyration from the surface. The present study seeks to determine the time dependence of the diffusion coefficient, and to examine the effect of a carbon surface on this diffusion. The sputter-deposited carbon surface may serve as a model for carbon-black particles employed in nanocomposites, which have recently been observed to reduce diffusion throughout a nanocomposite layer. The experiments employed a thin (∼15 nm) deuterated polystyrene (dPS) marker layer sandwiched between two normal PS layers. Deuterium profiles were monitored in the annealed samples by secondary ion mass spectrometry. Strong segregation was observed at the silicon surface, but was inhibited at the carbon surface, allowing the diffusion behaviour to be studied in the latter case over longer annealing times. A finite-element computer program was developed to fit the observed diffusion profiles. The variation of the diffusion coefficient with depth is shown to be consistent with previous results, and diffusion is comparable at both the carbon and silicon surfaces. The diffusion coefficient decreases roughly in proportion to t−1/2, and is discussed in the context of reptation theory.

In situ observation of the melting behaviour of PEO single crystals on a PVPh substrate by AFM

PVPh sublayer thickness dependent melting of PEO single crystals.

Instability, self-organization and pattern formation in thin soft films

The free surface of a thin soft polymer film is often found to become unstable and self-organizes into various meso-scale structures. In this article we classify the instability of a thin polymer film into three broad categories, which are: category 1: instability of an ultra-thin (<100 nm) viscous film engendered by amplification of thermally excited surface capillary waves due to interfacial dispersive van der Waals forces; category 2: instability arising from the attractive inter-surface interactions between the free surface of a soft film exhibiting room temperature elasticity and another rigid surface in its contact proximity; and category 3: instability caused by an externally applied field such as an electric field or a thermal gradient, observed in both viscous and elastic films. We review the salient features of each instability class and highlight how characteristic length scales, feature morphologies, evolution pathways, etc. depend on initial properties such as film thickness, visco-elasticity (rheology), residual stress, and film preparation conditions. We emphasize various possible strategies for aligning and ordering of the otherwise isotropic structures by combining the essential concepts of bottom-up and top-down approaches. A perspective, including a possible future direction of research, novelty and limitations of the methods, particularly in comparison to the existing patterning techniques, is also presented for each setting.

Probing interfacial properties using a poly(ethylene oxide) single crystal

Poly(ethylene oxide) (PEO) single crystals were grown from dilute solution using a self-seeding method. The PEO single crystals with uniform dimensions, homogeneous chemical and physical properties were used as a simplified ultrathin film system to probe the interfacial properties of different substrates. In situ studying the annealing and melting behavior of PEO single crystals on the PAA, amorphous PEO and PVA substrates were carried out using an atomic force microscope (AFM) equipped with a hot stage. The interaction force between the PEO modified probe and various substrates was measured at different temperatures, and the universal dependence of the interaction force between the probe and polymer substrate on the temperature was demonstrated. The wetting and dewetting behavior of PEO melt on the PAA and amorphous PEO and PVA substrates were observed and the spreading coefficient (S) was proposed to prejudge the spreading behavior of a polymer ultra-thin film on a solid substrate according to the interaction force. Different melting points were found and the initial melting of the PEO single crystals occurred at 51, 54 and 61 °C on the PAA, PEO and PVA substrates, respectively. How the interfacial energy affects the melting point of single crystals was demonstrated, and the theoretical prediction agrees well with the experimental results.

Dewetting of Polymer Bilayers: Morphology and Kinetics

The kinetics of de-wetting a polycarbonate (PC) film from a poly(styrene-coacrylonitrile) (SAN) copolymer film was monitored using optical microscopy. Whereas the SAN layer was stable upon annealing at 190°C, the PC layer dewetted the SAN and formed holes whose diameter increased linearly with time. Auger electron spectroscopy measurements confirmed that PC was fully removed from the interior of the hole. Upon varying the AN content, the dewetting velocity was found to be a minimum near 0.27 weight percent AN. This result is consistent with the interfacial thermodynamics between PC and SAN. Atomic force microscopy was used to provide a unique image of the hole profile.

Mimicking the rice leaf--from ordered binary structures to anisotropic wettability

In this paper, we report a method to fabricate a series of surfaces with large-area ordered binary arrays by controllable dewetting. The binary structure arrays consist of an ordered-stripe array and droplet-row array. In order to expand the system, polystyrene (PS) and poly(methyl methacrylate) (PMMA) are introduced in this experiment for investigation in detail. Through adjustment of the polymer solution concentration and the modified underlying pattern on substrate, the surface topographies can be controlled simply. Accordingly, three types of topographies with ordered binary arrays have been obtained by thermal annealing. These unique surfaces mimic the natural rice leaf structurally, which also displays anisotropic wettability for water droplet as natural surfaces. This method points out a new way for the manufacture of functional surfaces.

Structural signal of a dynamic glass transition

Pentaphenyl trimethyl trisiloxane is an isotropic liquid at room temperature with a dynamic glass transition at 224 K. Using x-ray reflectivity, we see surface density oscillations (layers) develop below 285 K, similar to those seen in other metallic and dielectric liquids and in computer simulations. The layering threshold is approximately 0.23 times the liquid-gas critical temperature. Upon cooling further, there is a sharp increase at 224 K in the persistence of the surface layers into the bulk material, i.e., an apparently discontinuous change in static structure at the glass transition.

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

Direct embedding of metal nanoparticles (NPs) into the surface of a polymer melt is observed by TEM and a new embedding mechanism proposed. Upon annealing above the glass transition temperature of polystyrene (PS), NPs (20 nm gold) are rapidly covered by a thin PS wetting layer, h* approximately 1.3-1.8 nm (i.e., about two or three monomers). Because it creates capillary pressure on a NP, this "universal" wetting layer is proposed to be responsible for NP embedding. The value of h* is independent of the molecular weight of PS and constant during the embedding process. The value of h* is found to be similar to the equilibrium wetting layer thickness of a polymer melt spreading on a metal substrate. Using a model that includes the spreading coefficient, long-range van der Waals interactions, and a chain-stretching penalty, h* is shown to be independent of the molecular weight of the polymer. Using this model and the measured value of h*, the interfacial energy between Au NP and PS is estimated to be 8.7 J/m2.

Dewetting of thin polystyrene films under confinement

The dewetting behavior of thin polystyrene (PS) film has been investigated by placing an upper plate with a ca. 140 nm gap from the underlying substrate with the spin-coated thin polymer films. Three different kinds of dewetting behaviors of thin PS film have been observed after annealing according to the relative position of the PS film to the upper plate. Since the upper plate is smaller than the underlying substrate, a part of the polymer film is not covered by the plate. In this region (I), thin PS film dewetting occurs in a conventional manner, as previously reported. While in the region covered by the upper plate (III), the PS film exhibits unusual dewetted patterns. Meanwhile, in the area right under the edge of the plate (II) (i.e., the area between region I and region III), highly ordered arrays of PS droplets are formed. Formation mechanisms of different dewetted patterns are discussed in detail. This study may offer an effective way to improve the understanding of various dewetting behaviors and facilitate the ongoing exploration of utilizing dewetting as a patterning technique.

Capillary wave fluctuations and intrinsic widths of coupled fluid-fluid interfaces: an x-ray scattering study of a wetting film on bulk liquid

An x-ray specular reflectivity (XR) and off-specular diffuse scattering (XDS) study of the coupled thermal capillary fluctuations and the intrinsic profiles of two interacting fluid-fluid interfaces is presented. The measurements are carried out on complete wetting films of perfluoromethylcyclohexane (PFMC) on the surface of bulk liquid eicosane (C20), as a function of film thickness 30<D<160 A. In order to facilitate the analysis and interpretation of the data with minimal complexity, approximate methods for calculating scattering intensities are developed to take into account the subtleties of thermal diffuse scattering from layered liquid surfaces. With these methods, the calculations of XR/XDS intensities are reduced to a single numerical integration of simple functions in real space. In addition, an analytic expression is derived for small-angle XR that contains Debye-Waller-like factors with effective capillary roughness and takes into account the partial correlations of the two interfaces. The expression for the XR is quantitatively accurate so long as the reflection angle is small enough that the scattering from interfaces is distinguishable from bulk scattering. The results of the XR and XDS data analysis indicate that the capillary fluctuations at the two interfaces of the wetting films are partially correlated and their coupling is consistent with the van der Waals interactions. The relatively large intrinsic width (4 approximately 6A) of the liquid-liquid interface observed for thicker films (D greater than or similar to 50 A) is comparable to the value expected for the bulk liquid-liquid interface (D-->infinity), determined by either the radius of gyration (5.3 A) or the bulk correlation length (4.8 A) of the alkane C20. The intrinsic liquid-vapor interfacial width is sharper (approximately 2 A) and remains essentially constant over the entire probed range of D .

Design of a new invertible polymer coating on a solid surface and its effect on dispersion colloidal stability

This article presents a new approach to building up self-adjustable invertible polymer coatings at solid surfaces. The approach is based on a two-step process. In the first step, the surface of dispersed TiO2 or silicon wafers was functionalized with the aid of a reactive copolymer, viz., poly(styrene-alt-maleic anhydride) (PSM), to which, in the second step, the chains of amphiphilic oligoester have been tethered. The latter contains both hydrophilic poly(ethylene glycol) and hydrophobic aliphatic dibasic acid moieties being alternately distributed along the oligomer chains. It is shown that the titania modified in this way can form stable suspensions in both polar (water) and nonpolar (toluene) media. Moreover, multiple drying/redispersion cycles demonstrate the ability of the modified titania particles, after their removal from one type of dispersion and consequent drying, to be redispersed in dispersing media strongly differing by polarity from that of the previous medium. An environmentally induced switching of the surface properties has been observed via the measurement of the wetting contact angles and scanning force microscopy (SFM) of silicon wafers covered by PSM with tethered oligoester chains. These experiments give strong support for the predicted capability of such polymer coatings to switch their environmental appearance (i.e., to behave as a self-adjustable invertible interface because of the ability of the tethered amphiphilic oligoester chains to change their conformations in response to environmental changes in such a manner so as to adapt and enhance their compatibility with the surrounding media).

Observation of surface layering in a nonmetallic liquid

Oscillatory density profiles (layers) have previously been observed at the free surfaces of liquid metals but not in other isotropic liquids. We have used x-ray reflectivity to study a molecular liquid, tetrakis(2-ethylhexoxy)silane. When cooled to T/Tc approximately 0.25 (well above the freezing point for this liquid), density oscillations appear at the surface. Lateral order within the layers is liquidlike. Our results confirm theoretical predictions that a surface-layered state will appear even in dielectric liquids at sufficiently low temperatures, if not preempted by freezing.

Influence of physical and chemical heterogeneity shape on thin film rupture

It is known that the breakup times for thin liquid films on solid surfaces can be substantially smaller if the surface is heterogeneous, either chemically or physically. In this paper we explore issues related to the effect of the shape of the physical and chemical heterogeneities on the breakup time and the thinning behavior. We consider two shapes, sinusoidal and exponential, for both physical and chemical gradients and compare the breakup times for these two different forms of gradients. Furthermore, the wavelength of the sinusoidal gradients and the length scale of the exponential gradients are varied and the effects of these on the breakup times and the film evolution are determined. For the sinusoidal gradients, we also obtain analytical results for shape evolution that are valid at short times and for small amplitude perturbation of the physical/chemical heterogeneities. The fastest growing modes are determined for spinodal breakup and also for both shapes (sinusoidal and exponential) of physical and chemical heterogeneities. The breakup times for the fastest growing modes from the linear and the nonlinear studies are compared for spinodal breakup and these results are also compared with those for both chemical and physical heterogeneities, of both sinusoidal and exponential shapes. Results show that the presence of heterogeneities, in general, accelerates the breakup of the film. In the linear regime, the growth rates are the same for the chemical and physical heterogeneities and spinodal breakup, and the effect of the heterogeneities is manifested as increased amplitude of initial perturbation. The effect of the chemical and physical heterogeneities dominate the film dynamics at early times, becoming less important at later times. The growth rates and equivalently the breakup times for the films on heterogeneous surfaces depend on the length scale over which physical/chemical gradients occur, and as the length scale approaches zero, which implies that the gradients become very steep, the effect of the heterogeneities on the breakup times becomes small.

Surface segregation of conformationally asymmetric polymer blends

We have generalized the Edwards' method of collective description of dense polymer systems in terms of effective potentials to polymer blends in the presence of a surface. With this method we have studied conformationally asymmetric athermic polymer blends in the presence of a hard wall to the first order in effective potentials. For polymers with the same gyration radius Rg but different statistical segment lengths lA and lB the excess concentration of stiffer polymers at the surface is derived as delta rho A(z=0) approximately (lB-2 - lA-2)ln(R2g/l2c), where lc is a local length below of which the incompressibility of the polymer blend is violated. For polymer blends differing only in degrees of polymerization the shorter polymer enriches the wall.

Deviations from liquidlike behavior in molten polymer films at interfaces

We have performed x-ray specular and diffuse scattering on liquid polymer films and analyzed the spectra as a function of film thickness and molecular weight. The results show that films whose molecular weight is close to the entanglement length behave as simple liquids except that the shortest wavelength is determined by the radius of gyration (R(g)) rather than the monomer-monomer distance. When the molecular weight is higher than the entanglement length, the strong deviations from liquidlike behavior are observed. We find that the long wavelength cutoff vector, q(l,c), scales with film thickness, d as d(-1.1+/-0.1) rather than the usual d(-2) expected for simple liquids. If we assume that these deviations are due to surface pinning of the polymer chains, then our results are consistent with the formalism developed by Fredrickson et al. to explain the capillary wave spectrum that can propagate in a polymer brush.

Glass transition in ultrathin polymer films: a thermal expansion study

The glass transition process gets affected in ultrathin films having thickness comparable to the size of the molecules. We observe systematic broadening of the glass transition temperature (T(g)) as the thickness of an ultrathin polymer film reduces below the radius of gyration but the change in the average T(g) was found to be very small. The existence of reversible negative and positive thermal expansion below and above T(g) increased the sensitivity of our thickness measurements performed using energy-dispersive x-ray reflectivity. A simple model of the T(g) variation as a function of depth expected from sliding motion could explain the results.

Pattern formation and dewetting in thin films of liquids showing complete macroscale wetting: from "pancakes"to "swiss cheese"

Based on the complete 3D numerical solutions of the nonlinear thin film equation, we address the problems of surface instability, dynamics, morphological diversity and evolution in unstable thin films of the liquids that display complete macroscale wetting. The twin constraints of complete macroscale wettability and nanoscale instability produce a variety of microscopic morphological phases approximating sharp crystal surfaces with flat tops resembling a mesa or a micro "pancake" or a slice of Swiss cheese. While the maximum thickness of flat regions is found to be independent of the initial film thickness, the precise lateral morphology of microdomains formed depends on the film thickness. As the film thickness is increased, the initial pathway of evolution changes from the formation of small spherical droplets, to long mesas (parapets) and islands, to circular holes, all of which eventually resolve by ripening into a collection of round pancakes at equilibrium. However, beyond a certain transition thickness, a novel metastable honeycombed morphology, resembling a membrane or a slice of Swiss cheese, is uncovered, which is produced by an abrupt "freezing" of the evolution during hole growth. In contrast, the spinodal dewetting in thin films of partially wettable systems always engenders spherical droplets at equilibrium. The equilibrium dewetted area from simulations, as well as from simple mass balance, is shown to decline linearly with the initial film thickness.

Spinodal-like dewetting of thermodynamically-stable thin polymer films

Energetic considerations indicate that long-range Van der Waals forces stabilize thin polystyrene (PS) films against height fluctuations on silicon substrates. Nevertheless, we report here on the amplification of capillary waves of specific wavelengths for 15 nm thick PS films on silicon, ultimately leading to dewetting in a "spinodal-like" process. However, the temporal dependence of the wavelength of the growing instability does not agree with the "classical" spinodal dewetting mechanism. Therefore, this phenomenon is ascribed to the existence of "structural" forces resulting either from the restructuring of the films or from density variations within the films during annealing, in accordance with recent theoretical treatments. The process is shown not to be limited to polystyrene films, which indicates the generality of our findings.

Creation of Ordered Patterns by Dewetting of Thin Films on Homogeneous and Heterogeneous Substrates

Spontaneous formation of locally ordered patterns during dewetting of thin films on homogeneous and heterogeneous substrates is investigated based on the 3-D nonlinear equation of motion. Physicochemical heterogeneities engender the rapid formation of the primary holes that serve as "seeds" for the formation of locally ordered structures. The secondary multiring structure surrounding the primary hole evolves by one of the following two different pathways depending on the film thickness vis-à-vis the location of the minimum in the spinodal curve: (A) Thick films evolve by the formation of secondary satellite holes that originate from a ring-like depression behind the rim of the primary hole. The process of ordering is repeated until the true spinodal holes appear on the remaining substrate. (B) Ordering in a relatively thin film occurs by the formation of droplets caused by the disintegration of the elevated rim that surrounds the primary hole. The radial distance between the successive ordered layers is close to the spinodal length scale, lambda(m). Droplets within the same layer are separated by a distance slightly greater than lambda(m), whereas holes within the same layer are separated by a distance slightly less than lambda(m). The number density of holes or droplets in the ordered pattern is of the same order as the predictions of the spinodal theory. The number of ordered layers and the size of the locally ordered domain depend significantly on the relative magnitudes of the time scales for the following events: (1) formation of the primary hole, (2) growth of holes (inverse of hole-growth velocity), (3) formation of a secondary feature (hole or droplet) adjacent to the primary hole, (4) true spinodal rupture far from the primary hole. The morphology of an ordered structure can therefore be tailored by modulation of the film thickness and the short- and long-range intermolecular interactions (substrate surface properties), since these affect the time scales 1 to 4 in different ways.

Dewetting patterns and molecular forces: a reconciliation

We studied the dewetting of thin liquid polymer films from solid surfaces. Our experimental results lead to a consistent picture demonstrating the interplay between short- and long-range interfacial forces. Observations comprise nucleation and spinodal dewetting, as well as thermal nucleation of holes. The effective interface potential of the system, as reconstructed from the morphology of the dewetting patterns, agrees quantitatively with what is computed from the optical properties of the system. This shows that the assumption of additivity of dispersion potentials in multilayer systems yields good results.

Interface localization-delocalization transition in a symmetric polymer blend: a finite-size scaling Monte Carlo study

Using extensive Monte Carlo simulations, we study the phase diagram of a symmetric binary (AB) polymer blend confined into a thin film as a function of the film thickness D. The monomer-wall interactions are short ranged and antisymmetric, i.e., the left wall attracts the A component of the mixture with the same strength as the right wall does the B component, and this gives rise to a first order wetting transition in a semi-infinite geometry. The phase diagram and the crossover between different critical behaviors is explored. For large film thicknesses we find a first order interface localization-delocalization transition, and the phase diagram comprises two critical points, which are the finite film width analogies of the prewetting critical point. Using finite-size scaling techniques we locate these critical points, and present evidence of a two-dimensional Ising critical behavior. When we reduce the film width the two critical points approach the symmetry axis straight phi=1/2 of the phase diagram, and for D approximately 2R(g) we encounter a tricritical point. For an even smaller film thickness the interface localization-delocalization transition is second order, and we find a single critical point at straight phi=1/2. Measuring the probability distribution of the interface position, we determine the effective interaction between the wall and the interface. This effective interface potential depends on the lateral system size even away from the critical points. Its system size dependence stems from the large but finite correlation length of capillary waves. This finding gives direct evidence of a renormalization of the interface potential by capillary waves in the framework of a microscopic model.

Enhanced instability in thin liquid films by improved compatibility

We investigated experimentally the morphological evolution of thin polydimethylsiloxane films sandwiched between a silicon wafer and different bounding liquids with interfacial tensions varying by 2 orders of magnitude. It is shown that increasing the compatibility between film and bounding liquid by adding a few surfactant molecules results in a faster instability of shorter characteristic wavelength. Inversely, based on the characteristic parameters describing the instability we determined extremely small interfacial tensions with a remarkable accuracy.