Composition effect on dewetting of ultrathin films of miscible polymer blend

Two kinds of dewetting and their transition induced by composition fluctuation due to different composition in blend [poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN)] films on SiO_x substrate at 145 °C have been studied by in-situ atomic force microscopy (AFM). The results showed that morphology and pathway of dewetting depended crucially on the composition. Possible reason is the variation in intensity of composition fluctuation resulted from the change of components in polymer blend. Based on the discussion of this fluctuation due to the composition gradient, parameter of U_q0/E, which describes the initial amplitude of the surface undulation and original thickness of film respectively, has been employed to distinguish the morphologies of spontaneous dewetting including bicontinuous structures and holes. Prior to the investigation of dewetting, it is confirmed that this blend is miscible at 145 °C using grazing incidence ultra small-angle X-ray scattering (GIUSAX).     

Patterning thin polymer films by surface-directed dewetting and pattern transfer

The pattern evolution processes of thin polystyrene (PS) film on chemically patterned substrates during dewetting have been investigated experimentally. The substrates have patterns of self-assembly monolayers produced by microcontact printing with octadecyltrichlorosilane. Optical microscopy and atomic force microscopy images reveal that ordered micrometer scale pattern can be created by surface direct dewetting. Various pattern sizes and pattern complexities can be achieved by controlling the experimental parameters. The dewetting pattern has been transferred to form PDMS stamp for soft lithography.     

Composition fluctuations before dewetting in polystyrene/poly(vinyl methyl ether) blend thin films

We report structural development in blend thin films of deuterated polystyrene (dPS) and poly(vinyl methyl ether) (PVME) below 200 nm in two phase region during the incubation period before dewetting using neutron reflectivity (NR) and atomic force microscopy (AFM). As was predicted by the former optical microscope (OM) and small-angle light scattering (LS) measurements on blend thin films of protonated PS and PVME [Ogawa H, Kanaya T, Nishida K, Matsuba G. Polymer 2008;40:254–62.], the NR results clearly showed that the tri-layer structure consisting of the surface PVME layer, the middle blend layer and the bottom PVME layer was formed in the one phase region. After the temperature jump into the two phase region, it was found that the phase separation of the middle blend layer proceeded in the depth direction during the incubation period before dewetting, suggesting that the dewetting was induced by the composition fluctuations during the incubation period.     

Pattern formation by dewetting of polymer thin film

Strategies for the utilization of dewetting of polymer thin film to fabricate ordered patterns are reviewed. After a brief introduction to the polymer thin film dewetting theory, simulation results of pattern formation induced by physically and chemically patterned substrates, and physical confinement are then summarized. Experimental results including the mechanisms behind and the conditions for good quality of pattern formation based on the dewetting of polymer thin film induced by physical, chemical heterogeneous substrates, topographic structure on film surface, physical confinement and the movement of three-phase line are then discussed. A short introduction to the application of fabricated patterns is also discussed.     

Phase separation and dewetting in polystyrene/poly(vinyl methyl ether) blend thin films in a wide thickness range

Phase separation and dewetting processes of blend thin films of polystyrene (PS) and poly(vinyl methyl ether) (PVME) in two phase region have been studied in a wide film thickness range from 65 μm to 42 nm (∼2.5R_g, R_g being radius of gyration of a polymer) using optical microscope (OM), atomic force microscope (AFM) and small-angle light scattering (LS). It was found that both phase separation and dewetting processes depend on the film thickness and were classified into four thickness regions. In the first region above ∼15 μm the spinodal decomposition (SD) type phase separation occurs in a similar manner to bulk and no dewetting is observed. This region can be regarded as bulk. In the second region between ∼15 and ∼1 μm, the SD type phase separation proceeds in the early stage while the characteristic wavelength of the SD decreases with the film thickness. In the late stage dewetting is induced by the phase separation. In the third region between ∼1 μm and ∼200 nm the dewetting is observed even in the early stage. The dewetting morphology is very irregular and no definite characteristic wavelength is observed. It is expected that the irregular morphology is induced by mixing up the characteristic wavelengths of the phase separation and the dewetting. In the fourth region below ∼200 nm the dewetting occurs after a long incubation time with a characteristic wavelength, which decreases with the film thickness. It is considered that the layered structure is formed in the thin film during the incubation period and triggers the dewetting through the capillary fluctuation mechanism or the composition fluctuation one.     

Solvent vapor induced dewetting in diblock copolymer thin films

The dewetting pattern development of thin film of poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer has been studied after ‘annealing’ in the PMMA block selective solvent vapor. Initially, typical circular dewetted holes are observed. Further annealing, however, results in the formation of fractal-like holes. The heterogeneous stress induced by the residual solvent remaining in the film after spin-coating induces the anisotropy of the polymer mobility during the annealing process, which triggers the formation of the intriguing surface patterns.     

Phase separation induced ordered patterns in thin polymer blend films

Strategies for the utilization of phase separation to generate ordered pattern in polymer thin film are reviewed. First, the fundamental theory and factors influencing phase separation in polymer thin film are discussed. That is followed by a discussion of the formation of ordered patterns induced by phase separation in polymer thin films under the influence of a chemical heterogeneous substrate, convection or breath figures. The mechanisms and the conditions for well-ordered structures generated by phase separation are then discussed to show that multi-scaled/multi-component patterns, stimuli-responsive patterns may be developed by controlling the preparation conditions or exposing the sample to different environments more complex structures. Finally, applications of fabricated patterns in pattern generation and reproduction, antireflecting coating, catalysis, bio-chips and optoelectronics are also discussed.     

Combinatorial investigation of dewetting: polystyrene thin films on gradient hydrophilic surfaces

Film stability and dewetting is important to control for applications in coatings such as photoresists, paints, adhesives, lubricants, and biomaterials. We demonstrate the use of 2D combinatorial libraries to investigate thin film dewetting. Substrate libraries with gradients in contact angle (θ) were prepared by immersing Si-H passivated Si in a Piranha solution (H₂SO₄/H₂O₂/H₂O) at a controlled rate. Libraries of thin films of polystyrene on gradient etched silicon substrates containing orthogonal continuous variation of thickness were screened for dewetting behavior using automated optical microscopy. After comparing the high-throughput screening method to conventional studies of thickness effect on dewetting, a detailed morphological phase-map of the effects of contact angle on dewetting of polystyrene film was generated. Dewetting trends were visibly apparent. The number of polygons of dewetted polymer is sensitive to surface hydrophilicity as characterized by contact angle studies.     

Dewetting kinetics of thin polymer bilayers: Role of under layer

A combined experimental and computational study is presented to uncover the dewetting kinetics of the PS/PMMA system by changing the film thickness of the PMMA under layer. On the low M_w PMMA (M_w = 15 kg/mol) layer, the dewetting velocity of PS film firstly rapidly decreases (regime I), and then becomes almost invariant (regime II) with the increase of the film thickness of the liquid lower layer. Experiments suggest that the transition from regime I to regime II is correlative with the property of the solid substrates. The linear stability analysis of thin bilayers uncovers a bimodal behaviour of the instability under the experimental conditions and changeover of dominant mode of instability from one interface to the other is the major reason behind the switching of regimes. The nonlinear simulations closely mimic the experimental interfacial morphologies and suggest two different pathways of hole growth for regimes I and II under experimental condition. The simulations also indicate that the rapid reduction in the dewetting velocity is because of the increased excursion or penetration of the upper layer into the lower layer near the three phase contact line as the viscous resistance at the more viscous PMMA layer reduces with its increasing thickness. A qualitative match is thus found between the experimental and theoretical trends of the dewetting velocities. In addition, the experiments show that on a high M_w PMMA (M_w = 365 kg/mol) layer, the kinetics of hole growth of the PS layer is not affected by the PMMA layer thickness.     

Polymer blend latex films: Miscibility and polymer diffusion studied by energy transfer

Fluorescence non-radiative energy transfer experiments were used to study latex blend films composed of high molar mass poly(butyl acrylate-co-methyl methacrylate) (PBA-co-MMA) and much lower molar mass PBA-co-MMA latex of the same chemical composition (50:50 BA:MMA by weight). These blends take advantage of the strong chain length dependence of T_g so that the particles consisting of oligomeric polymer (“low-M”) have a much lower T_g than the corresponding high-M latex. This type of blend represents a useful strategy for obtaining latex coatings with a reduced VOC content. Here we report on experiments which follow the rate at which the low-M polymer mixes via diffusion with the high-M polymer in the latex films. The high-M latex are doubly labeled, containing both donor and acceptor dyes covalently bound to the PBA-co-MMA backbone. Diffusion of the unlabeled low-M polymer into this phase dilutes the dyes, increasing their separation and lowering the quantum efficiency for energy transfer.     

Phase separation in polymer blend thin films studied by differential AC chip calorimetry

AC chip calorimetry is used to study the phase separation behavior of 100 nm thin poly(vinyl methyl ether)/poly(styrene) (PVME/PS) blend films. Using the on-chip heaters, very short (10 ms-10 s) temperature jumps into the temperature window of phase separation are applied, simulating laser heating induced patterning. These temperature pulses produce a measurable shift in the glass transition temperature, evidencing phase separation. The effect of pulse length and height on phase separation can be studied. The thus phase separated PVME/PS thin films remix rapidly, in contrast with measurements in bulk. AC chip calorimetry seems to be a more sensitive technique than atomic force microscopy to detect the early stages of phase separation in polymer blend thin films.     

Photoinduced cooperative reorientation in polymer blend films with hydrogen-bonded mesogenic side groups

Thermally enhanced photoinduced cooperative reorientation in hydrogen (H)-bonded polymer blend films was investigated. The films consisted of two kinds polymethacrylates with hexamethylene spacer groups terminated with 4-oxycinnamic acid (P6CA), 4-oxybenzoic acid (P6BA) or 4-(4′-oxyphenyl)benzoic acid (P6PBA) in the side chains. The films were subjected to linearly polarized ultraviolet (LPUV) light and subsequent annealing. Sufficient cooperative in-plane molecular reorientation in P6CA–P6BA blend films was achieved for the first time. In P6CA–P6PBA blend films, however, reorientation was not observed. The molecular weight, as well as the thermal properties of the homopolymers and the degree of photoreaction all played an important role in the cooperative reorientation behavior of the blend films. Finally, uniform alignment control of low-molecular-weight liquid crystals (LCs) on the molecularly reoriented polymer blend films perpendicular to the polarization E of LPUV light was obtained.     

Particle nucleation for dewetting initiation of thin polymer films

A quantitative comparison between spontaneous dewetting and particle nucleation for thin (thickness = 17nm) polystyrene (PS) films on nonwettable silicon (Si) surfaces is presented both experimentally and theoretically. Performing experiments in a class 100 clean room, we found that ～ 23% of the observed dry patches formed because of dust particles, while the majority of the holes formed via the well known spontaneous dewetting process. The result was verified qualitatively by diffusion theory, which, however, predicted a diminished role for the airborne particles, leading to the conclusion that pre‐existing particles on the Si surfaces and/or the polymer solutions contribute substantially to the dewetting process. The driving force of particle motion into the polymer film is examined by placing aluminum oxide (Al₂O₃) particles on PS films. Finally, the effect of particle geometry is studied by placing gold (Au) disks on the free surface of PS films. An optically continuous PS film is found to be present around the periphery of the disk particles, even after the completion of the dewetting process in the rest of the sample. An attempt to explain dewetting inhibition at the vicinity of the micro‐disks, on the basis of molecular interactions developed in the system Au/PS/Si, is finally presented. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 138–145, 2005     

Capillarity-induced patterning of thin polymer films and related regular dewetting structures

When a thin polymer film is spin-coated on a solid substrate and heated above its glass transition temperature (T_g) while in conformal contact with a patterned polydimethylsiloxane (PDMS) mold, capillarity forces the polymer melt to fill up the void space of the mold, thereby forming a negative replica of the polymer after mold removal. In this study, particular emphasis is given to the nodal dewetting phenomenon in a thin, laterally confined polymer film that sticks to a substrate. When heated above the glass transition temperature, the film dewets from the substrate through the generation of nodal waves, with the PDMS wall acting as a node. Various superposed waves were observed depending on the intrinsic period of dewetting and the confining width of the cavity.     

Methacrylate-based polymer films useful in lithographic applications exhibit different glass transition temperature-confinement effects at high and low molecular weight

The effects of confinement on polymer films are important in applications related to photoresists. To optimize resolution, methacrylate polymers used in photoresists are often low molecular weight (MW). We use ellipsometry and fluorescence to study how the glass transition temperature (T_g) is affected by confinement in silica-supported films of low and high MW poly(1-ethylcyclopentyl methacrylate) (PECPMA) and poly(methyl methacrylate) (PMMA). With decreasing nanoscale thickness, T_g is nearly invariant for high MW (M_n = 22.5, 188 and 297 kg/mol) PECPMA but decreases for low MW PECPMA, with T_g – T_g,bulk = −7 to 8 °C in a 27-nm-thick film (M_n = 4.1 kg/mol) via ellipsometry and −15 °C in a 17-nm-thick film (M_n = 4.9 kg/mol) via fluorescence. Fluorescence studies using a 20-nm-thick dye-labeled layer in multilayer, bulk PECPMA films reveal a much greater perturbation to T_g in the free-surface layer for low MW PECPMA, which propagates tens of nanometers into the film. The effect of MW in single-layer monodisperse PMMA films is even more striking; T_g increases with confinement for high MW but decreases for low MW, with T_g – T_g,bulk = 9 °C in a 12-nm-thick film (nominal MW = 509 kg/mol) and −16 °C in a 17-nm-thick film (nominal MW = 3.3 kg/mol). The strong influence of MW on confinement effects in PECPMA and PMMA is in contrast to the previously reported invariance of the effect with MW in supported polystyrene films, reconfirmed here.     

Near-IR dye sensitization of polymer blend solar cells

We have fabricated ternary blend solar cells based on poly(3-hexylthiophene) (P3HT) as a donor material, poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (N2200) as an acceptor material, and a silicon naphthalocyanine derivative (SiNc) as a near-IR sensitizer. In order to discuss how molecular structures impact the dye sensitization in P3HT/N2200 solar cells, we studied two SiNc molecules with different axial groups: SiNc10 with decyldimethylsilyl oxide and SiNc6 with trihexylsilyl oxide. As a result, P3HT/N2200/SiNc10 ternary solar cells exhibited a power conversion efficiency of 1.4%, which is improved by 73% compared to P3HT/N2200 binary solar cells and also by 17% compared to P3HT/N2200/SiNc6 ternary solar cells. We discuss the origin of the improvement in the device performance in terms of dye location in ternary blend films.     

Analysis of polymer blend morphologies from transmission electron micrographs

Thermomechanical properties of polymer blends seem to depend on their morphology on microscales and in particular on the size of the dispersed phase particles and/or their distances (ligament thickness). Precise characterization of morphologies by few simple geometrical parameters is often a quite delicate task, in particular because of the strong polydispersity of these systems. We present here a simple method based on image analysis of transmission electron micrographs (TEM) to estimate both distributions in particle size and ligament thickness. We first reconstruct three-dimensional distributions in particle size from two-dimensional measurements and show in particular that corrections from section thickness become significant when thickness is comparable to particle size. Knowing the distribution in particle size, we extend the model initially proposed by Wu to estimate the distribution in ligament thickness. This method provides a more detailed relation between the distribution in particle size and the distribution in ligament thickness. Advantages and limitations of the method are illustrated by practical examples on polyamide-12 systems filled with various particle dispersions.     

Effect of loading-rate on fracture micromechanism of methylmethacrylate-butadiene-styrene polymer blend

Methylmethacrylate-butadiene-styrene (MBS) polymer blends having two different types of rubber particle distribution, monomodal and bimodal, were prepared, and their fracture properties and fracture mechanisms were investigated under quasi-static and impact loading. A fracture property, maximum J-integral J_max, was evaluated at both loading-rates, and it was shown that J_max values of the bimodal MBSs are much greater than that of the monomodal with small particles, and slightly better than that of the monomodal with large particles. Thick damage zones were observed in the crack-tip regions in the bimodal and monomodal with large particles, indicating larger energy dissipation during fracture initiation than in the monomodal with small particles in which damage zone is much thinner. TEM micrographs exhibit that extensive plastic deformation under quasi-static rate and multiple craze formation under impact loading rate are the primary toughening mechanisms in the bimodal MBS blends. By assessing both fracture properties and transparency, the bimodal blend with blend ratio: 2.5/7.5 (=140 nm/2.35 μm; total rubber particle content is 10 wt%) was proved to show the best performance as MBS polymer blend with satisfiable transparency and high fracture resistance.     

Film instability induced evolution of hierarchical structures in annealed ultrathin films of an asymmetric block copolymer on polar substrates

With an atomic-force microscope and a grazing-incidence small-angle X-ray scattering we studied ex situ the evolution of hierarchical structures in isothermally annealed ultrathin films of asymmetric polystyrene-block-poly(methyl methacrylate) P(S-b-MMA) that dewetted on polar substrates via a mechanism involving nucleation and growth. Film instability causes the surface to acquire an undulating thickness through incommensurability, producing not only the relief structures on a micrometer scale but also mesophase-separated domains on a nanometer scale. The dewetted morphologies strongly influence the ordering behavior of the nanoscale domains. The noncylindrical nanostructures become stable at the curved edges of the relief microstructures in the destabilized P(S-b-MMA) films, for which a preferential wetting of the PS block with the free surface is prohibited. Additionally, the shape of relief structures as result of film instability correlates with the formation of mesophase-separated nanodomains. At early stages of film instability, the formation of parallel-oriented PMMA cylindrical nanodomains increases the deformation energy and it further persists to force the shape of relief structures between irregular holes to have a facet-wedge shape. However, those relief structures are expected to be not at equilibrium. At high temperatures, the relief structures between irregular holes progressively developed to form hemispherical-cap drops accompanied by a transformation of cylindrical into noncylindrical nanodomains at curved surfaces.