Dewetting and microphase separation in symmetric polystyrene‐block‐polyisoprene diblock copolymer ultrathin films

The kinetics of surface structure evolution in ultrathin films of low‐molecular‐weight polystyrene‐block‐polyisoprene (M_w: 7300 g mol^?1–7300 g mol^?1) diblock copolymer at temperatures below the bulk order‐to‐disorder transition temperature are presented. Films with two different thicknesses were studied as a function of annealing temperature using atomic force microscopy. These film thicknesses enabled the investigation of the competition between microphase separation and dewetting that resulted in two different morphologies: long‐range bicontinuous structures and random holes. Three distinctive stages of structure evolution were observed in bicontinuous structure, with the underlying mechanism compared with spinodal dewetting. Thicker films presented holes on their surfaces upon annealing at elevated temperatures, and kinetics of formation of the holes were discussed. We found that the molecular mobility determined the rates of dewetting, while the microphase separation hardly affected the dewetting process. © 2015 Society of Chemical Industry     

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.     

Conducting poly(styrene‐co‐divinylbenzene)/polypyrrole PolyHIPE composite foam prepared by chemical oxidative polymerization

Conducting poly(styrene‐co‐divinylbenzene)/polypyrrole (PPy) polyHIPE (polymerized high internal phase emulsion) composite foams were synthesized via chemical oxidative polymerization method. The effect of solvent and dopant type on the surface morphology and electrical conductivity of composite foams has been investigated. SEM micrographs showed that the morphology of PPy thin film on the internal surface of poly(styrene/divinylbenzene) (poly(St‐co‐DVB) polyHIPE support foam strongly depends on the solvent and dopant type used. Incorporation of dodecylbenzene solfunic acid‐sodium salt (DBSNa) as a dopant in chloroform solvent resulted in formation of a PPy thin film with higher molecular compact structure and electrical conductivity on the support foam as compared to other solvents and another dopant used. Fourier‐transform infrared spectroscopy was used to correlate the electrical conductivity of composite foams to their PPy structural parameters. As expected, the extended conjugation length of PPy in the presence of DBSNa dopant is the main reason for higher electrical conductivity of resultant composite foam. Electrical conductivity measurements revealed that the chemical aging of various conducting foams follows the first‐order kinetic model, which is a representative of a reaction‐controlled aging mechanism. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Surface tailoring of an ethylene propylene diene elastomeric terpolymer via plasma‐polymerized coating of tetramethyldisiloxane

In the research presented here, we explore the use of a low‐energy plasma to deposit thin silicone polymer films using tetramethyldisiloxane (TMDSO) (H(CH₃)₂? Si? O? Si? (CH₃)₂H) on the surface of an ethylene propylene diene elastomeric terpolymer (EPDM) in order to enhance the surface hydrophobicity, lower the surface energy and improve the degradation/wear characteristics. The processing conditions were varied over a wide range of treatment times and discharge powers to control the physical characteristics, thickness, morphology and chemical structure of the plasma polymer films. Scanning electron microscopy (SEM) shows that pore‐free homogeneous plasma polymer thin films of granular microstructure composed of small grains are formed and that the morphology of the granular structure depends on the plasma processing conditions, such as plasma power and time of deposition. The thicknesses of the coatings were determined using SEM, which confirmed that the thicknesses of the deposited plasma‐polymer films could be precisely controlled by the plasma parameters. The kinetics of plasma‐polymer film deposition were also evaluated. Contact angle measurements of different solvent droplets on the coatings were used to calculate the surface energies of the coatings. These coatings appeared to be hydrophobic and had low surface energies. X‐ray photoelectron spectroscopy (XPS) and photoacoustic Fourier‐transform infrared (PA‐FT‐IR) spectroscopy were used to investigate the detailed chemical structures of the deposited films. The optimum plasma processing conditions to achieve the desired thin plasma polymer coatings are discussed in the light of the chemistry that takes place at the interfaces. Copyright © 2004 Society of Chemical Industry     

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.     

Formation and characterization of perfluorocyclobutyl polymer thin films

Perfluorocyclobutyl (PFCB) polymers are a new class of materials that show promise as selective layer materials in the development of composite membranes for gas separations, such as carbon dioxide/methane (α_{pure gas} = 38.6) and oxygen/nitrogen (α_{pure gas} = 4.8) separations. In many of the flat sheet applications, a thin film of the selective layer that is free of major defects must be coated onto a support membrane. A focus of this study was to elucidate the impacts of solvents, polymer concentration, and dip‐coating withdrawal speed on PFCB thin film thickness and uniformity. An extension was proposed to the Landau–Levich model to estimate the polymer film thickness. The results show that the extended model fits the thickness‐withdrawal speed data well above about 55 mm/min, but, at lower withdrawal speeds, the data deviated from the model. This deviation could be explained by the phenomenon of polymer surface excess. Static surface excesses of polymer solutions were estimated by applying the Gibbs adsorption equation using measured surface tension data. Prepared films were characterized by ellipsometry. Refractive index was found to increase with decreasing film thickness below about 50 nm, indicating densification of ultrathin films prepared from PFCB solutions below the overlap concentration. Atomic force microscopy was used to characterize surface morphologies. Films prepared from tetrahydrofuran and chloroform yielded uniform nanolayers. However, films prepared using acetone as solvent yielded a partial dewetting pattern, which could be explained by a surface depletion layer of pure solvent between the bulk PFCB/acetone solution and the substrate. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Inhibition of thin polystyrene film dewetting via phase separation

By addition of a small amount of poly(methyl methacrylate) (PMMA) into polystyrene (PS), we present a novel approach to inhibit the dewetting process of thin PS film through phase separation of the off-critical polymer mixture (PS/PMMA). Owing to the preferential segregation of PMMA to the solid SiO_x substrate, a nanometer thick layer, rich in PMMA phase, is formed. It is this diffusive PMMA-rich phase layer near the substrate that alters the dewetting behavior of the PS film. The degree of inhibition of dewetting depends on the concentration and molecular weight of PMMA component. PMMA with low (15.9k) and intermediate (102.7k) molecular weight stabilizes the films more effectively than that with a higher molecular weight (387k).     

Morphology change of asymmetric diblock copolymer micellar films during solvent annealing

The morphology change of an asymmetric polystyrene-block-poly(2-vinyl pyridine) (PS-b-PVP) diblock copolymer micellar film was investigated during solvent vapor annealing in chloroform. Initially, smaller islands in nanometer-length scale form at the film surface. Further annealing results in the growth of the islands composed of the PS-b-PVP cylinders above the bottom brush layer. For comparison, a film of the block copolymer prepared from THF solution (without micellar structure) was also studied. The surface morphology of the film from THF evolves via spinodal dewetting mechanism during solvent vapor annealing. At a long time solvent vapor annealing, the two kinds of the films display the same surface morphologies, which are determined by the interplay between the surface field and autodewetting.     

Polyamide/polyacrylonitrile thin film composites as forward osmosis membranes

Thin film composites (TFCs) as forward osmosis (FO) membranes for seawater desalination application were prepared. For this purpose, polyacrylonitrile (PAN) as a moderately hydrophilic polymer was used to fabricate support membranes via nonsolvent‐induced phase inversion. A selective thin polyamide (PA) film was then formed on the top of PAN membranes via interfacial polymerization reaction of m‐phenylenediamine and trimesoyl chloride (TMC). The effects of PAN solution concentration, solvent mixture, and coagulation bath temperature on the morphology, water permeability, and FO performance of the membranes and composites were studied. Support membranes based on low PAN concentrations (7 wt %), NMP as solvent and low coagulation bath temperature (0 °C) demonstrated lower thickness, thinner skin layer, more porosity, and higher water permeability. Meanwhile, decreasing the PAN solution concentration lead to higher water permeance and flux and lower reverse salt flux, structural parameter, and tortuosity for the final TFCs. Composites made in N,N‐dimethylformamide presented lower permeance and flux for water and salt and higher salt rejection, structural parameter, and tortuosity. FO assay of the composites showed lower water permeance values in saline medium comparing to pure water. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44130.     

Spinodal clustering induced dewetting and non-monotonic stabilization of polymer blend films at high nanofiller concentrations

We examine the effects of high fullerene nanoparticle (f-NP) concentrations, ?_f-NP ∼ (10–20) mass% on polystyrene (PS)/polybutadiene (PB) blend thin film stability. Dewetting of the polymer blend around spinodally clustered f-NPs in this high concentration limit leads to a spinodal like dewetting morphology. This is in contrast to our previously observed results on the stabilization effects of f-NPs on PS/PB blend thin films in the intermediate f-NP concentration range of 7–10 mass%, wherein, after saturating the polymer–blend interface, the NPs stabilize the film by segregating to the blend–substrate interface. We determine three regimes of polymer blend film stability as a function of filler concentration: a) ?_f-NP < 7 mass% where preferential segregation of the f-NPs to the polymer–polymer interface leads to macroscopic dewetting, b) ?_f-NP ∼ (7–10) mass% where PS/PB blend films exhibit complete film stability, and c) ?_f-NP ∼ (11–20) mass%, where spinodal clustering of the f-NPs gives rise to polymer–NP phase exclusion and subsequent dewetting.     

Morphology and glass transition of thin polylactic acid films

The aim of this study is to highlight the effect of the morphology of polylactic acid (PLA) thin films on the glass transition in this confined geometry. For that purpose, poly(L ‐lactic acid) and poly(D ‐lactic acid) polymer films were prepared on different surfaces such as mica and silicon surfaces by using two different solvents. The films exhibit different morphologies starting from individual macromolecular chains at very low concentration to homogeneous films at higher concentrations. Globular to elongated rodlike structures of the PLA macromolecules were observed according to the polarity of the surface and the solvent. The T_g of PLA enantiomeric film, determined by adhesion force using atomic force microscopy (AFM) as a function of temperature, was confirmed by ellipsometric measurements. Surprisingly, this value is independent of the morphology of thin enantiomeric film. A balance between the attractive surface effects and a decrease of the crystallinity of PLA on the mobility in the amorphous region will be discussed. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Numerical simulation of the self‐assembly of a polymer–polymer–solvent ternary system on a heterogeneously functionalized substrate

The spinodal decomposition of a polymer–polymer–solvent ternary blend spin coated on heterogeneously functionalized substrate is studied in a three‐dimensional numerical model. The Cahn‐Hilliard equation was used to describe the free energy profile of the domain. The mechanism of the morphology evolution was studied quantitatively. The well‐established linear relationship of the characteristic length, R(t), with t^1/3 can be observed in the simulation results. The functionalized substrate greatly affected the morphology evolution of blends with different solvent concentrations. The results indicated that a critical time can be observed, at which the evolution rate changes abruptly, also after which the compatibility of the surface morphology to the functionalized substrate pattern increases at a much lower pace in the diluted solution. In the condensed solution, the compatibility actually decreases beyond the critical time. The influence of solvent evaporation is investigated and a sharper interface was observed in the case with solvent evaporation and film thickness reduction. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Atomic force microscopy characterization of ultrathin polystyrene films formed by admicellar polymerization on silica disks

Atomic force microscopy was used to study the characteristics of polymer films formed via admicellar polymerization (the polymerization of monomers solubilized in adsorbed surfactant aggregates). The investigated system included cetyltrimethylammonium bromide (C₁₆TAB) as a cationic surfactant, styrene, 2,2′‐azobisisobutyrilnitrile as an initiator, and polished silica disk substrates. Our goal was to examine changes in the properties and morphology of the formed polymer films due to changes in the surfactant and monomer feed levels. Normal tapping and phase‐contrast modes in air were used to image the nanoscopic and microscopic morphologies of the polystyrene‐modified silica. The root‐mean‐square roughness of the surface before and after modification was statistically analyzed and compared. The images were captured with loading‐force set‐point ratios of 0.2–0.9, and this allowed us to examine the stability of the polystyrene films. In the first series, for which the feed ratio of C₁₆TAB to styrene was kept constant and the total feed concentration was varied, a uniform layer of a polystyrene film was observed along with some nanometer‐size aggregates at high feed concentrations of both C₁₆TAB and styrene. These droplets eventually agglomerated with the film beneath and formed larger macrodroplets in a ring arrangement. At lower concentrations, droplets and holes were observed that eventually agglomerated to form a bicontinuous thin film. In the second experimental series, the concentration of C₁₆TAB was kept constant, and the feed ratio of C₁₆TAB to styrene was varied. A smooth thin film was observed at high concentrations of styrene. This film could be deformed and/or removed to expose the silica surface beneath. At lower styrene loadings, the polystyrene film became unstable and formed dropletlike aggregates, possibly because of either the uneven adsolubilization of the styrene monomer within the admicelle or the dewetting effect during washing and drying. The structure of the polystyrene film formed on a smooth silica disk was very dependent on the amount of the surfactant fed to the system; this contrasted with the results on precipitated silica. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 36–46, 2003     

Influence of Substrate Properties on the Dewetting Dynamics of Viscoelastic Polymer Films

We studied the dewetting process of thin polystyrene (PS) films on silicon substrates, coated with a thin, irreversibly adsorbed polydimethylsiloxane (PDMS) layer, by optical microscopy and atomic force microscopy. Besides demonstrating the exceptional potential of dewetting for a sensitive characterization of rheological properties of PS thin films, characterized by a stress-relaxation time, τ₁, we focused on the influence of the frictional behaviour (energy dissipation mechanism) at the interface between the PDMS-coated silicon wafer and the PS film on the dewetting process. Our results show that the initial stages of dewetting depend sensitively on the thickness and the way the PDMS layer was adsorbed. The maximum width of the dewetting rim at τ₁ increased with increasing PDMS layer thickness, which can be interpreted as an increase of the effective, velocity-dependent slippage length. Interestingly, τ₁ was found to be almost independent of the substrate properties. Our results demonstrate that dewetting is a really powerful approach for rheological and frictional studies of thin polymer films.     

Structural,optical, and electrical characterization of the poly[9,9‐dioctylfluorenyl‐2,7‐diyl]‐co‐1,4‐benzo‐(2,1,3)‐thiadiazole thin film fabricated by electrostatic spray technique

The article presents the efficient fabrication of the Poly[9,9‐dioctylfluorenyl‐2,7‐diyl]‐co‐1,4‐benzo‐(2,1,3)‐thiadiazole (F8BT) thin film using the electrostatic spray technique. Electrostatic atomization of the in‐house developed F8BT polymer ink was achieved at considerable low voltage. The structural and optical characterizations of the fabricated F8BT thin film were thoroughly investigated. Furthermore, the organic diode structure with electrostatic spray deposited F8BT thin film was fabricated and its performance was analyzed by performing current voltage measurement. The current–voltage characteristic curve of the organic diode showed nonlinear diode like behavior, thereby confirming the proper interference established between organic diode adjacent layers. The space charged limited current mechanism has been found to be dominant in the fabricated organic device with carrier mobility value of 5.65 e^?4 cm²V^?1s^?1. POLYM. ENG. SCI., 54:675–681, 2014. © 2013 Society of Plastics Engineers     

Thermodynamics and kinetics control of support layer synthesis for enhanced thin film composite membrane performance

The desalination performance of a thin film composite (TFC) membrane hinges highly on the surface characteristics of support membrane. In this study, good wettability, regular pore size, and moderate roughness were identified as the critical surface properties of support membrane in forming a defect‐free, uniform, and structurally stable polyamide film. These features were tailored by adjusting the thermodynamics and kinetics properties of the polymer solution via polysulfone (PSf)/polyvinylpyrrolidone (PVP) ratio as well as using N,N‐dimethylacetamide (DMAc) and N,N‐dimethylformamide as co‐solvent. It was found that the membrane formation was controlled kinetically by altering the PSf/PVP ratio but thermodynamically by using co‐solvent. The TFC membrane with better desalination performance was achieved with the support membrane prepared at PSf/PVP ratio of 0.941 without any co‐solvent. The resulting TFC membrane attained the highest permeability ratio of 0.691 bar^?1 for water/NaCl filtration in this study. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45802.     

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.     

Morphology of poly(styrene‐block‐dimethylsiloxane) copolymer films

The morphologies of poly(styrene‐block‐di‐methylsiloxane) (PS‐b‐PDMS) copolymer thin films were analyzed via atomic force microscopy and transition electron microscopy (TEM). The asymmetric copolymer thin films spin‐cast from toluene onto mica presented meshlike structures, which were different from the spherical structures from TEM measurements. The annealing temperature affected the surface morphology of the PS‐b‐PDMS copolymer thin films; the polydimethylsiloxane (PDMS) phases at the surface were increased when the annealing temperature was higher than the PDMS glass‐transition temperature. The morphologies of the PS‐b‐PDMS copolymer thin films were different from solvent to solvent; for thin films spin‐cast from toluene, the polystyrene (PS) phase appeared as pits in the PDMS matrix, whereas the thin films spin‐cast from cyclohexane solutions exhibited an islandlike structure and small, separated PS phases as protrusions over the macroscopically flat surface. The microphase structure of the PS‐b‐PDMS copolymer thin films was also strongly influenced by the different substrates; for an asymmetric block copolymer thin film, the PDMS and PS phases on a silicon substrate presented a lamellar structure parallel to the surface at intervals. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1010–1018, 2007     

Evolution of Rim Instabilities in the Dewetting of Slipping Thin Polymer Films

Using optical microscopy, we investigated the amplification of instabilities of the moving rim which formed during dewetting of slipping polymer films. At the onset, the wavelength of the rim instability grew in time and proportional to the width of the rim. At later stages, these instabilities led to finger and subsequent droplet formation. Droplet size was found to be proportional to the width of the rim at break-off of droplets, which, in turn, was proportional to the initial film thickness. Our experiments suggest that the decrease of the dewetting velocity with increasing width of the rim is the key mechanism responsible for this instability. Droplet formation provided a possibility for self adjustment of the dewetting front resulting in a constant mean self-regulated dewetting velocity. This mean velocity was significantly higher than the velocity for the corresponding stable rim.     

Synthesis and characterization of homogeneously sulfonated poly(ether ether ketone) membranes: Effect of casting solvent

Poly(ether ether ketone) (PEEK) was homogeneously sulfonated to have various degrees of sulfonation from 48 to 83%. The sulfonated PEEK (sPEEK) membranes were prepared by a solvent casting method using a few solvents such as N,N‐dimethyl formamide, N,N‐dimethyl acetamide, and 1‐methyl‐2‐pyrrolidinone. The effect of casting solvent on the membrane morphology and properties was investigated. The sulfonation degree and ion exchange capacity were determined by a back titration method, and the morphology of membrane by SEM. It has been demonstrated that the surface morphology and properties of sPEEK membranes, such as water uptake, methanol permeability, ion conductivity, and mechanical strength, were considerably affected by the type of solvent, where the DMAC‐sPEEK system showed the best performance in the polymer electrolyte membrane application for DMFC. This solvent effect on the membrane morphology and properties was caused by interaction strength (hydrogen bonding) between polymer and solvent. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008