Polymer Blend Thin Films Research Articles

Preferential Partition of Nanowires in Thin Films of Immiscible Polymer Blends

AbstractSummary: The preferential partition of silver nanowires in thin films of polystyrene/poly(vinyl pyrrolidone) (PS/PVP, with a 30/70 weight ratio) blends, that induces drastic blend morphology variation, is reported. The silver nanowires are fabricated with the anodic aluminum oxide templating method, and have a diameter of 300 nm and length of 10 µm. At a higher nanowire loading of 10 wt.‐%, the silver nanowires are entangled and selectively concentrate within the continuous PVP domain. If surface modified by thiols carrying hydrophobic tails, the silver nanowires become hydrophobic and prefer to stay within the discrete hydrophobic PS domains. At a lower nanowire loading of 5 wt.‐%, the nanowires are non‐entangled and concentrate at regions near the interfaces of the PS and PVP phases, which induces the formation of interconnected PS domains.Preferential partition of silver nanowires in thin films of PS/PVP (30/70) blends at a casting temperature of −10 °C.magnified imagePreferential partition of silver nanowires in thin films of PS/PVP (30/70) blends at a casting temperature of −10 °C.

Characterization of Thin Polymer Films on the Nanometer Scale with Confocal Raman AFM

AbstractThe combination of an atomic force microscope (AFM) with a Confocal Raman Microscope (CRM) has been used to study the composition of various thin films of polymer blends. The high spatial resolution of the AFM enables the morphological characterization of the polymer blends on the nanometer scale. Furthermore, when operating the AFM in Digital Pulsed Force Mode (DPFM), topographic information and local stiffness can be simultaneously recorded. This allows the material‐sensitive characterization of heterogeneous materials. Thin films where PMMA (at room temperature a glassy polymer) is blended with two different styrene‐butadiene rubbers are investigated. The presence of PMMA in both phase‐separated thin films allows the comparison of the mechanical properties of the two different rubber phases using DPFM‐AFM. When PMMA is blended with PET due to their similar mechanical properties (both are in the glassy state at room temperature) the assignment of the two phases to the corresponding polymers by AFM is rather difficult. Here, Raman spectroscopy provides additional information on the chemical composition of materials. In combination with a confocal microscope, the spatial distribution of the various phases can be determined with a resolution down to 200 nm. Therefore, the topographically different structures observed in AFM images can be associated to the chemical composition by using the Confocal Raman Microscope (CRM).

Spectroscopic Ellipsometry for Characterization of Thin Films of Polymer Blends

AbstractMorphology, composition, miscibility, interdiffusion, and interactions at interfaces are important quantities of polymer blends. Many of these parameters can be probed with spectroscopic ellipsometry. Ellipsometry in the visible spectral range is very suitable for determination of thicknesses and the high frequency refractive indices of thin organic films. However the spectral contrast is low for many polymers in comparison to infrared spectroscopic ellipsometry (IRSE) where specific contributions of the molecular vibrations are probed. In the presented study the infrared optical constants of a double layer (206.6 nm in total) of poly(n‐butyl methacrylate) (PnBMA) and poly(vinyl chloride) (PVC) and of the films of the single compounds have been determined with optical simulations using layer models. The multiple layer model served for simulation of the ellipsometric spectra taken after an annealing induced mixing process in a polymeric double layer. The ellipsometric spectra of a not completely mixed sample could be fitted in a three‐layer model, in which a mixed interphase in between the two layers of the polymers is formed due to interdiffusion.

Control of phase separation in blends of polyfluorene (co)polymers and the C 60-derivative PCBM

When creating thin films of polymer blends, interesting morphologies are formed because of phase separation. In particular for conjugated polymers, which are used as active material in optoelectronic devices, it is very important to understand the parameters that influence the phase separation process and to achieve control over the morphology. The overall goal of this blend morphology study is to contribute to the design of device structures with desired performance. Here we present results of morphology studies on thin films of polyfluorene-based blends with the fullerene derivative [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM). The polymers used are poly(9,9-dioctylfluorene) (F8) and four different copolymers of F8. The thin films are spin coated from chloroform solutions onto silicon substrates and their surface morphology is imaged by tapping mode atomic force microscopy (AFM). We observe that the size and the shape of the domains in the film depend on the structure of the polymer. The nature of the monomer that, together with F8, is building the repeating unit in the copolymers has a strong effect on the phase separation in the polymer: PCBM blend. Since phase separation is influenced by interactions between components of the blend and the solvent, these results indicate that the degree of chemical interaction between polymer, solvent and PCBM, is different for the different blends. For the systems that form larger domains there is a clear correlation between the domain size (area) and the polymer/PCBM blend ratio. We also observe that the spin speed affects the thickness of the films and that the domain size increases with increasing thickness, primarily due to longer drying times.

Monte Carlo simulations of phase separation in thin polymer blend films: scaling properties of morphological measures

Surface directed phase separation in thin polymer blend film has been studied with Monte Carlo simulations using a simple model based on reptation method. Time evolution of phase structure was characterized quantitatively by morphological measures (the Minkowski functionals) in addition to the inspection of concentration versus depth profiles. It was shown that the dynamical scaling hypothesis holds for the Minkowski functionals describing morphologies at the early stages of phase separation in thin films. Two time regimes with different scaling exponents (0.25, 0.33) were found for the growth of the characteristic length scale in the system corresponding to various transport mechanisms (diffusion along- and normally to the interface). Fast decrease in the morphological measures observed at the end of phase separation was attributed to the confinement of the thin film.

Phase Segregation of Thin Film Polymer Blends on Au Nanopatterned Si Substrates

We present a method for producing nano- to submicron scale, chemically heterogeneous surface patterns using an Ar ion mill. To observe the effects of the pattern on dewetting, thin films of PS and ...

Oxygen Plasma Processing of Silicon and Silica Substrates for Thin Films of Polymer Blends

The influence of plasma processing of silica and crystalline silicon substrates on the formation of polymeric layers of PS/PMMA blends by sorption from 50 vol. % concentration toluene solutions was analyzed. The morphology dependence of PS/ PMMA blend films on the type and condition of substrate processing was studied by X-ray photoelectron spectroscopy (XPS), ellipsometry and atomic force microscopy (AFM). It was shown that reduction of carbon and oxygen components from the surface contaminants as well as the existence of a nonstoichiometric SiOx layer on the Si surface contributed to the hydrophilicity of the substrate. These processes can be used to produce thin nanostructured polymer blend films.

Morphology of Immiscible Polymer Blend Thin Films Prepared by Spin-Coating

The morphologies of spin-coated thin films of the immiscible polymer blend of polystyrene and poly(tert-butyl acrylate) (PS/PtBA) are characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and scanning force microscopy (SFM) combined with selective dissolution. PS and PtBA are found to phase separate laterally in the thin films after spin-coating. The overall trend with blend composition is for PS islands to form when the PS:PtBA ratio is less than unity and for PtBA holes to form when the PS:PtBA ratio is larger than unity. The thickness of PS phases always exceeds that of the PtBA phases with both thicknesses increasing linearly with concentration of the spin-coating solution. The addition of P(S-b-tBA) diblock copolymer decreases the overall domain dimensions. After long annealing periods, PtBA is found to segregate preferentially to both the air−polymer interface and the polymer−substrate interface, effectively encapsulating PS droplet domains. A simple free energy model developed for the annealed specimens predicts a local free energy minimum as a function of PS droplet diameter, accounting for the observed stability of PS droplet phases and their very slow growth upon annealing. The surface tension of PtBA and the interfacial tension between PS and PtBA are shown to be critical parameters that affect the domain morphology in annealed PS/PtBA thin films.

Breakdown of dynamic scaling in thin film binary liquids undergoing phase separation.

The kinetics of phase separation in thin polymer blend films displaying discrete and bicontinuous domain morphologies are examined. For discrete domains, the correlation length xi grows as t(1/3), in agreement with a coalescence model. By plotting xi/d vs t/t(i) (initiation time), universal growth behavior is obtained for thickness values (d) from 1000 to 190 nm. In contrast, bicontinuous domains grow with a decreasing exponent, 0.62 to 0.28, as d decreases from 900 to 90 nm (i.e., no universal growth). This slowing down with reduced dimensionality suggests suppression of lateral hydrodynamic pumping.

Self‐organized organic nanostructures: structure formation in thin polymer blend films

ŁCorrespondence to: S. Walheim, Forschungszentrum Karlsruhe, Institut fur Nanotechnologie, 76021 Karlsruhe, Germany. E-mail: stefan.walheim@int.fzk.de parallel to the film surface, depending on the surface energy of the substrate. By using chemically heterogeneous prestructured substrates, made by micro-contact printing, any pattern can be used to induce a compositional variation in the polymer film (see Fig. 1). Thus, by controlling the phase morphology, one can create surfaces with new properties so that those systems are applicable for lithography masks, optical components or biological sensors. In addition, a laterally isotropic phase separation can lead to new physical properties. For instance, if we remove one of the two polymers having a morphology with a lateral

Phase-sensitive acoustic microscopy of polymer thin films

The three-dimensional images obtained by scanning acoustic microscopy with vector contrast (PSAM), contain significant qualitative and quantitative information that is not easily obtainable by other methods. We employ this technique to examine homopolymer and polymer blend thin films. The complex V( z) functions derived from the images, and the results obtained by image processing and meticulous analysis are employed to render the morphology, composition and micro-mechanical properties of the polymer films. In addition, ways by which the information inherent in the phase images can be extracted are examined. This is highly desirable, as the phase images contain very useful additional information.

Hierarchic Structure Formation in Binary and Ternary Polymer Blends

The phase morphology of multi-component polymer blends is governed by the interfacial interactions of its components. We discuss here the domain morphology in thin films of model binary and ternary polymer blends containing polystyrene, poly(methyl metacrylate), and poly(2-vinylpyridine) (PS, PMMA, PVP). When sandwiched between two non-polar surfaces, characteristic lateral phase morphologies are observed after the film formation by spin-coating. We discuss here two techniques, by which hierarchical lateral structures in polymer films can be made. The first method makes use of two simultaneously occurring interfacial instabilities. The second technique employs the effect of a variation of the enthalpic interaction parameters in a ternary polymer mixture on its lateral polymer phase morphology.

Surface-directed phase separation in nanometer polymer films: self-stratification and pattern replication

AbstractPhase separation occurs in thin films of polymer blends when molecular mobility is promoted by a temperature above the glass transition but inside the twophase region (temperature quench), or a common solvent added to the polymers (solvent quench). This phenomenon can be altered by a homogeneous surface or pre-patterned substrate, resulting, e.g., in self-stratification or pattern replication, respectively. Such self-organisation processes ordering polymer phases were observed for model polymer blends (deuterated/hydrogenated polystyrene, dPS/hPS, and deuterated/partially brominated PS, dPS/PBrS, both with hPSpolyisoprene diblocks added; dPS/poly(vinylpyridine) and PBrS/PVP) with highresolution ion beam techniques (Nuclear Reaction Analysis, profiling and mapping mode of dynamic Secondary Ion Mass Spectrometry) and Atomic Force Microscopy. The self-stratification process is strongly affected by both the range as well as the strength of the surface/polymer interactions. This is illustrated for the temperature-quenched blends with surface-active copolymer additives tuning the interactions exerted by both external surfaces. The pattern transfer from the substrate to the films is demonstrated for the solvent-quenched blends. Patterndirected composition variations (SIMS maps) coincide with free surface undulations (AFM images). The most effective pattern replication is achieved for the length scale of phase domain morphology comparable with the pattern periodicity and for carefully adjusted polymer/substrate interactions.

Short- and Long-Range Interactions in Thin Films of Polymer Blends in Microchannels

We present the surface structures formed under the void microchannels of an elastomeric mold when thin films of polystyrene/polybutadiene (PS/PB) blends are annealed with the mold placed on the film. Exactly opposite types of structure result under identical annealing conditions, depending only on the height difference in the as-cast thin film between the PS and PB phases. Recessed patterns form under the microchannel by long-range interactions when this height is more than 60 nm; the usual protruding patterns result in the void microchannels due to the short-range interactions of capillarity when the height is less than 40 nm. The height of fully developed meniscus of the mobile PB in solid PS is believed to be the factor that determines the type of pattern formation.

Kinetics of Surface and Interfacial Fluctuations in Phase Separating Polymer Blend Films

Surface and interfacial fluctuations in phase separating thin film polymer blends are investigated using atomic force microscopy (AFM) and forward recoil spectrometry. The root-mean-square roughness at the surface (Rq,s) and the interface beneath the wetting layer (Rq,i) are quantified using AFM. Rq,s is found to scale with the initial film thickness (l0) during the early and intermediate stages, consistent with the morphology evolution mechanism reported earlier; however, the entire temporal evolution of Rq,s does not scale with l0. A fast Fourier transform of the AFM images uncovers low (ql) and high (qh) wavenumber fluctuations at both the surface and interface. The former fluctuation is associated with capillary fluctuations, whereas the latter reflects spinodal decomposition within the film. As the minority phase in the bulk of films undergoes 2D coarsening, the high wavenumber surface and interface fluctuations follow power-law dynamics, qh,s ∼ tαs and qh,i ∼ tαi, respectively. Both −αs and −αi increase with film thickness and reach an asymptotic value of about 1/3, suggesting that decreasing film thickness inhibits the growth of short wavelength fluctuations. Furthermore, −αs is systematically less than −αi, suggesting that the surface fluctuations are hindered relative to interfacial ones and that the surface and internal fluctuations are not conformal.

Thickness and Composition Dependence of the Glass Transition Temperature in Thin Homogeneous Polymer Blend Films

This research has been supported by the Korea Science and Engineering Foundation through the Hyperstructured Organic Materials Research Center, by the Center for Advanced Functional Polymers, and by the Korea Research Foundation (1998- 017-E00211).

Dynamic Density Functional Study on the Structure of Thin Polymer Blend Films with a Free Surface

Using the dynamic density functional method, we studied the structural evolution of a thin film of polymer blends placed between the solid substrate and free surface. We observed various types of structural instabilities such as the spinodal wave and the roughening of the free surface due to droplet formation. We proposed a simple theoretical argument based on the Neumann triangle condition among the interfacial tensions to construct a phase diagram for the instability of the free surface and confirmed this by a series of dynamic density functional simulations.

Confinement induced stabilization in polymer blend thin films

Using atomic force microscopy, film stability is investigated for poly(methyl methacrylate) (PMMA)/poly(styrene-ran-acrylonitrile) (SAN) thin film blends ranging from 10nm to a few microns thick and deposited on an oxide covered silicon substrate. In addition to characterizing the surface morphology, the PMMA rich phase which wets the surface and oxide, is selectively etched to reveal the underlying or interfacial phase morphology. For 50/50 blends at 158°C, films are found to be stable for thickness values between 20 and 100nm. Dewetting and phase separation mechanisms are invoked to understand why films are unstable below and above this range, respectively. By annealing 25/75, 50/50 and 75/25 thin film blends between 158 and 200°C, a stability diagram is constructed and shows that blends with low PMMA volume fractions (i.e. wetting component) are more stable than blends rich in PMMA. In some cases, phase separation is observed at temperatures as low as 158°C, which is about 12°C below the bulk lower critical solution temperature (LCST). Film stability is analyzed by predicting the PMMA concentration necessary to produce a surface wetting layer at its equilibrium thickness.

Three-Dimensional Information on the Phase Domain Structure of Thin Films of Polymer Blends Revealed by Secondary Ion Mass Spectrometry

We describe the technique of dynamic secondary ion mass spectrometry developed to determine three-dimensional phase domain structures of films of polymer blends. The polymers are composed of light elements or are labeled with deuterium or heavy elements. The applicability of this method to various polymer blends forming thin and ultrathin films with flat and undulated air/film interface is discussed.

Late stages of ordering of thin polymer films on chemically heterogeneous substrates: energetics and metastability

By a direct numerical integration of the dynamical evolution equations, we study phase separation of thin polymer blend films in two-dimensional models. The focus of our work is on the late-time morphology of the films. We first investigate morphological transitions in a thin film on a flat, homogeneous substrate. In particular, we probe the transition between a completely wetting and a partially wetting surface morphology. This transition is controlled by a competition between surface interactions and the interfacial energy in a thin film. This competition becomes even more pronounced when a chemically patterned substrate is used. For films cast on a patterned substrate, we consider several different substrate pattern widths and surface interaction strengths in our simulations. We also probe effects of thermal fluctuations on the late-time film morphology by carrying out simulations to many different final quench temperatures. Along with quench simulations, we further study film morphology by slow cooling the film from an initial high temperature. Comparing the morphology of films for a quench case to a slow-cooling case, we identify possible barriers to reaching equilibrium in experimental situations under a quench.