Selective solvent annealing induced phase separation and dewetting in PMMA/SAN blend ultrathin films

Phase behaviors induced by solvent annealing in poly(methyl methacrylate) (PMMA) and poly(styrene‐ran‐acrylonitrile) (SAN) blend ultrathin films have been investigated by atomic force microscopy and grazing incidence small‐angle X‐ray scattering. Our results indicate that both the phase separation within the blend and the dewetting of the film induced by composition fluctuation take place upon the selective solvent annealing, producing complex structures containing upper droplets (of one phase) and mimic‐films (of the other rich‐phase). The use of acetic acid (the selective solvent for PMMA) generates PMMA mimic‐film and SAN droplets, while the introduction of DMF (exhibiting better solubility for SAN) vapor results in the formation of SAN mimic‐film and PMMA droplets. Essentially, the interaction at polymer/substrate interface, resultant wettability of selected component, solubility of PMMA and SAN in adopted solvent dominate not only the phase separation and the dewetting of the whole film but also the synergism of them. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1243–1251     

Dewetting behavior of Langmuir−Blodgett films of polystyrene‐b‐poly(methyl methacrylate) induced by solvent vapor annealing

In order to explore the degree of contact between hydrophilic blocks and the substrate, the dewetting behavior of Langmuir–Blodgett (LB) films of polystyrene‐block‐poly(methyl methacrylate) (PS‐b‐PMMA) induced by PMMA‐selective acetone vapor were investigated by atomic force microscopy (AFM) for the first time. With the annealing of acetone vapor, the LB films of PS‐b‐PMMA undergo the swelling and coalescing of aggregates, the formation of bicontinuous patterns, the formation of droplets, and the periodic increase and decrease of droplets. The emergence of the bicontinuous patterns indicates that the dewetting occurs via the spinodal dewetting mechanism. The periodic droplet evolution is a novel phenomenon observed for the first time and quite different from the single droplet evolution of spin‐coated films, which is probably due to the degree of contact between PMMA blocks and the substrate in the LB films being larger than that in the latter. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 825–830     

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.     

Multiscale dewetting of triblock copolymer thin film induced by solvent vapor

Multiscale dewetting of poly(styrene‐b‐ethylene/butylenes‐b‐styrene) (SEBS) triblock copolymer thin films induced by volatile solvent vapor treatment were observed in this study. Film rupture occurred at first and produced macroscopic holes. Near‐regular droplets (which represented a compromise between complete disorder and perfect order) could be formed at the last stage. The mechanism of solvent‐driven dewetting was discussed by comparing with that of thermal‐induced dewetting. Similar to thermal‐induced dewetting, the block copolymer thin films initially break up through the nucleation of holes that perforated the films. The rapid growing holes became unstable and formed nonequilibrium fingering patterns. The films exhibit autophobic or autodewetting phenomena. The velocity of the holes growth was nearly a constant (3.3 μm/min). The stages of the dewetting were quite similar to that found for homopolymer and block copolymer thin films dewetting on solid or liquid substrates under thermal treatment. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2874–2884, 2005     

Substrate surface energy dependent morphology and dewetting in an ABC triblock copolymer film

A gradient combinatorial approach was used to examine the effect of substrate surface energy on the morphology and stability of films of a poly(isoprene-b-styrene-b-ethylene oxide) triblock copolymer that exhibits an alternating gyroid morphology in the bulk. Atomic force microscopy data across our surface energy (water contact angle) library suggest a transformation to predominantly surface parallel lamellae with an antisymmetric ordering. For substrate water contact angles below 70 degrees the film exhibited autophobic dewetting from an adsorbed half-period triblock copolymer monolayer at longer annealing times. X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure analysis along gradient specimens indicated that the substrate surface energy governed the composition profile of the monolayer, and this variation in chemical expression was key to whether the film was stable or autophobically dewet. These observations demonstrate that enthalpic interactions, in addition to entropic considerations, can play a major role in autophobic dewetting of block copolymer films.     

An experimental study on saturation swelling of styrene–acryronitrile copolymer particles with styrene and acrylonitrile monomers

The saturation swelling behavior of styrene and acrylonitrile (SAN) copolymer particles with a styrene (St) and acrylonitrile (AN) comonomer mixture was investigated experimentally. The effects of the copolymer composition and the compositional inhomogeneity in SAN Copolymer particles on their swelling behavior were examined. The experimental results show that both the copolymer composition and the compositional inhomogeneity in SAN copolymer particles have little or no influence on the swellability of SAN copolymer particles with a St and AN comonomer mixture, as long as the weight fraction of AN monomer units in SAN copolymer particles is less than a certain value between 0.6 and 0.8. With increasing AN content in the copolymer particles beyond this value, however, the swellability of SAN copolymer particles gradually decreases. Semiempirical equations are proposed, which correlate the saturation concentration of each monomer in SAN copolymer particles as a function of the comonomer composition in the monomer droplets and the overall copolymer composition in SAN copolymer particles. © 1994 John Wiley & Sons, Inc.     

Topcoat‐Assisted Perpendicular and Straightly Parallel Coexisting Orientations of Block Copolymer Films

Highly ordered perpendicular orientation and straightly parallel orientation coexisting polystyrene‐block‐polydimethylsiloxane (PS‐b‐PDMS) cylindrical microdomains with 10 nm width can be realized by using polyvinyl acetate as a partially dewetted topcoat and solvent annealing with acetone vapor. During solvent annealing, the swelled topcoat begins to dewet and the dewetting rim sweeps the surface of the block copolymer films to align the cylindrical microdomains with the direction of dewetting propagation. However, the wetted region of the topcoat/PS‐b‐PDMS film forms with a perpendicular orientation due to reduced surface tension and sufficient concentration gradient in the solvent evaporation step. The orientational changes (perpendicular/straightly parallel orientation) in the dewetted/wetted area are also investigated according to the vapor pressure of solvent annealing. The degree of directionality of the swept PS‐b‐PDMS films according to the distance from the dewetting front, which is equivalent with time after sweeping, is examined. To control the direction of dewetting and complex structures within a specific area, an imprinting process is introduced to form topographical line–space patterns in the topcoat and perpendicular/parallel orientation of BCP patterns in the line–space patterns, respectively.

     

Responsive polymer/gold nanoparticle composite thin films fabricated by solvent-induced self-assembly and spin-coating

Self-assembled poly(4-vinylpyridine)-grafted gold (Au) nanoparticles (NPs) and polystyrene-b-poly(4-vinylpyridine) block copolymers were fabricated by the introduction of a selective solvent to a common solution. The assembled mixtures were spin-coated onto solid substrates to fabricate composite gold/polymer thin films composed of copolymer-hybridized Au NPs and independent copolymer micelles. The obtained composite Au thin films had variable localized surface plasmon resonance (LSPR) bands and microscopic morphologies upon vapor annealing with selective solvents because the adsorption and dissolving of solvent molecules into the films could rearrange the copolymer block. The hybrid nanostructured Au thin films may have potential in vapor sensing and organic assays.     

Dewetting Kinetics of Thin Polymer Films with Different Architectures: Effect of Polymer Adsorption

We have investigated the influence of the adsorption process on the dewetting behavior of the linear polystyrene film(LPS),the 3-arm star polystyrene film(3 SPS) and the ring polystyrene film(RPS) on the silanized Si substrate.Results show that the adsorption process greatly influences the dewetting behavior of the thin polymer films.On the silanized Si substrate,the 3 SPS chains exhibit stronger adsorption compared with the LPS chains and RPS chains; as a result,the wetting layer forms more easily.For LPS films,with the decrease of annealing temperature,the kinetics of polymer film changes from exponential behavior to slip dewetting.As a comparison,the stability of 3 SPS and RPS films switches from slip dewetting to unusual dewetting kinetic behavior.The adsorbed nanodroplets on the solid substrate play an important role in the dewetting kinetics by reducing the driving force of dewetting and increase the resistant force of dewetting.Additionally,Brownian dynamics(BD) simulation shows that the absolute values of adsorption energy(ε) gradually increase from linear polymer(-0.3896) to ring polymer(-0.4033) and to star polymer(-0.4264),which is consistent with the results of our adsorption experiments.     

Dewetting behavior of a block copolymer/homopolymer thin film on an immiscible homopolymer substrate

Numerous previous studies have established that the addition of a microphase-ordered AB diblock copolymer to a thin homopolymer A (hA) film can slow, if not altogether prevent, film rupture and subsequent film dewetting on a hard substrate such as silica. However, only a few reports have examined comparable phenomena when the hA/AB blend resides on a soft B-selective surface, such as homopolymer B (hB). In this work, the dewetting kinetics of thin films composed of polystyrene (PS) and a symmetric poly(styrene-b-methyl methacrylate) (SM) diblock copolymer on a poly(methyl methacrylate) substrate is investigated by hot-stage light microscopy. Without the SM copolymer, the dewetting rate of the PS layer is constant under isothermal conditions and exhibits Arrhenius behavior with an apparent activation energy of approximately 180 kJ/mol. Addition of the copolymer promotes a crossover from early- to late-stage dewetting kinetics, as evidenced by measurably different dewetting rates. Transmission electron microscopy reveals the morphological characteristics of dewetted PS/SM films as functions of film thickness and SM concentration.     

Morphology Evolution of Block Copolymer Blend Thin Films Induced by Solvent Vapor Annealing

ABSTRACT

Self-assembly of binary block copolymer blends in thin film induced by solvent vapor annealing has been systematically studied. The diblock copolymers polystyrene-b-poly(2-vinylpyridine) with different molecular weights and volume fractions were blended with different molar ratios to cast thin films on silica substrate by spin coating. The films were annealed separately in the vapor of ethanol or toluene over time to induce morphology transformations from spheres, gyroids, and bicontinuous nanostructures, depending on the blending ratio, solvent selectivity, and annealing time, as investigated by atomic force microscopy and X-ray photoelectron spectroscopy. The formation and transformation mechanism of the self-assembly structure are discussed in the context of solvent-copolymer interactions. This study provides new insights into the simple manipulation of self-assembled nanostructures of block copolymer thin films.     

Achieving structural control with thin polystyrene-b-polydimethylsiloxane block copolymer films: The complex relationship of interface chemistry,annealing methodology and process conditions

The structure of thin microphase-separated polystyrene-block-polydimethylsiloxane (PS–PDMS) films has been studied using state-of-the-art top-down and cross-sectional electron microscopy. This is the first time that the profile of PS–PDMS films has been measured in situ and these measurements allowed us to image the shape of the PDMS domains within the film as well as examine the wetting behavior of the block copolymer film on a variety of substrates. It was found that for each polymer, substrate chemistry and annealing method combination examined, there was a small range of film thicknesses whereby the films exhibited the optimal characteristics of high levels of ordering without dewetting or multilayering. Specifically, the optimum thickness for films treated by thermal annealing was greater than that for the equivalent solvent annealed film; a change that was correlated with morphology variations related to solvent swelling of the solvent annealed films. The surface chemistry also induced changes in the optimum film thickness. Selective surfaces were shown to control whether a PDMS wetting layer was formed or not, leading to either thicker or thinner wetting optimum film thicknesses; while undulating morphologies were observed for less selective surfaces. Concomitant changes in the periodicity were then hypothesized to occur as a result of confinement effects and the selectivity of the surface.     

Wetting behavior of lightly sulfonated polystyrene ionomers on silica surfaces

The wetting/dewetting behavior of thin films of lightly sulfonated low molecular weight polystyrene (SPS) ionomers spin-coated onto silica surfaces were studied using atomic force microscopy (AFM), contact angle measurements, and electron microscopy. The effects of the sulfonation level, the choice of the cation, the solvent used to spin-coat the films, and the molecular weight of the ionomer were investigated. Small angle X-ray scattering was used to determine the bulk microstructure of the films. The addition of the sulfonate groups suppressed the dewetting behavior of the PS above its glass transition temperature, e.g. no dewetting occurred even after 240 h of annealing at 120 degrees C. Increasing the sulfonation level led to more homogeneous and smoother surfaces. The choice of the cation used affected the wetting properties, but not in a predictable manner. When tetrahydrofuran (THF) or a THF/methanol mixed solvent was used for spin-casting, a submicron-textured surface morphology was produced, which may be a consequence of spinodal decomposition of the film surface during casting. Upon annealing for long times, the particles coalesced into a coherent, nonwetted film.     

PI-b-PMMA diblock copolymers: nanostructure development in thin films and nanostructuring of thermosetting epoxy systems

Poly(isoprene-block-methyl methacrylate) (PI-b-PMMA) block copolymers with different block ratios have been used to generate nanostructures both in thin films and by nanostructuring a thermosetting epoxy system. Obtained morphologies have been analyzed in terms of atomic force microscopy. The nanostructuring of thin films was carried out by thermal and solvent vapor annealing, in which the copolymer films were exposed to acetone vapors, selective solvent for methyl methacrylate (PMMA) block. By solvent vapor annealing thin films of both copolymers self-assembled into a hexagonally packed cylindrical morphology. Thermal annealing was carried out above the glass transition temperature of both blocks, obtaining worm-like and lamellar morphologies, depending on the block ratio. One of the copolymers has also been used for nanostructuring an epoxy thermosetting system. Morphologies consisting of spherical-shaped PI domains dispersed in a continuous epoxy matrix in which PMMA remained miscible were obtained, independently of the copolymer amount.     

An in situ GISAXS study of selective solvent vapor annealing in thin block copolymer films: Symmetry breaking of in-plane sphere order upon deswelling

Thin films of asymmetric poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) block copolymers are studied by means of in situ grazing-incidence small-angle X-ray scattering (GISAXS) during solvent vapor annealing in tetrahydrofuran, a solvent selective for the PS majority block of the copolymer. Upon swelling, PS-b-P4VP block copolymers form hexagonal arrays of spherical P4VP microdomains in a PS matrix in films 7–9 layers thick. Deswelling the films induces a transition from hexagonal to face-centered orthorhombic (fco) symmetry, which is stable only at ∼7 layers of spherical microdomains. Dry films show co-existing hexagonal and orthorhombic symmetries when the solvent is removed slowly, whereas instantaneous solvent removal suppresses the fco structure, resulting in films with only hexagonal structure. The in-plane order of microdomains is significantly deteriorated in dry films independent of the solvent removal rate.Spherical block copolymer microdomains are known to undergo a transition from hexagonal to orthorhombic packing in isothermally annealed thin films when the number of sphere layers is increased from 4 to 5. In this paper, in situ GISAXS experiments reveal that a similar transition occurs during solvent vapor annealing in a selective solvent. Interestingly, the transition from hexagonal to orthorhombic packing of spheres occurs as solvent is removed from a thin block copolymer film. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 331–338     

Coupling of microphase separation and dewetting in weakly segregated diblock co-polymer ultrathin films

We have studied the coupling behavior of microphase separation and autophobic dewetting in weakly segregated poly(ε-caprolactone)-block-poly(L-lactide) (PCL-b-PLLA) diblock co-polymer ultrathin films on carbon-coated mica substrates. At temperatures higher than the melting point of the PLLA block, the co-polymer forms a lamellar structure in bulk with a long period of L ～ 20 nm, as determined using small-angle X-ray scattering. The relaxation procedure of ultrathin films with an initial film thickness of h = 10 nm during annealing has been followed by atomic force microscopy (AFM). In the experimental temperature range (100-140 °C), the co-polymer dewets to an ultrathin film of itself at about 5 nm because of the strong attraction of both blocks with the substrate. Moreover, the dewetting velocity increases with decreasing annealing temperatures. This novel dewetting kinetics can be explained by a competition effect of the composition fluctuation driven by the microphase separation with the dominated dewetting process during the early stage of the annealing process. While dewetting dominates the relaxation procedure and leads to the rupture of the ultrathin films, the composition fluctuation induced by the microphase separation attempts to stabilize them because of the matching of h to the long period (h ～ 1/2L). The temperature dependence of these two processes leads to this novel relaxation kinetics of co-polymer thin films.     

结晶和去润湿竞争条件下分子结构对超薄膜表面形貌的影响

将星形支化结构的聚己内酯, 包括六臂星形聚己内酯(HPCL)和树枝状星形聚己内酯(DPCL), 以及线形聚己内酯(LPCL)室温旋涂于云母片上,通过原子力显微镜(AFM)观察分子结构对星形支化聚己内酯超薄膜的润湿-去润湿性质的影响. 在旋涂过程中, 薄膜的形成受去润湿和结晶竞争的控制. 差示扫描量热(DSC)测试结果表明, 当相对分子质量相同时, 结晶性的顺序是: DPCL最弱, HPCL次弱, LPCL最强. 依据分子结构和相对分子质量的影响, 即去润湿和结晶竞争的结果, LPCL、HPCL和DPCL的超薄膜表现出不同的表面形态, 包括尺寸不同的完整的球晶、开口的球晶、树枝状片、分散的颗粒.     

Template-directed adsorption of block copolymers on alkanethiol-patterned gold surfaces

Functionalized alkanethiols have been self-assembled on gold to modify the wetting properties of the surface and promote or hinder the adsorption of block copolymers containing both hydrophobic and hydrophilic blocks. X-ray photoelectron spectroscopy (XPS) studies of spin-coated polyethylene-block-poly(ethylene oxide) (PE-b-PEO) copolymers on 16-mercaptohexadecanoic acid (MHDA)-, octadecanethiol (ODT)-, and 1H,1H,2H,2H-perfluorodecanethiol (PFDT)-covered surfaces have been performed. In the case of an 80 wt % PEO block copolymer, spin-coating on a gold surface precovered with MHDA results in a polymer film thick enough to completely attenuate Au 4f photoelectrons; spin-coating on the more hydrophobic ODT and PFDT monolayers leads to significantly thinner polymer films and incomplete attenuation of the gold photoelectrons. The opposite results are observed when a 20 wt % PEO block copolymer is used. Angle-resolved XPS studies of the 80 wt % PEO block copolymer spin-coated onto an MHDA-covered surface indicate that the PE blocks of the polymer segregate to the near-surface region, oriented away from the hydrophilic carboxylic acid tails of the monolayers; the surface concentration of PE is further enhanced by annealing at 90 degrees C. Microcontact printing and dip-pen nanolithography have been used to pattern gold surfaces with MHDA, and the surfaces have been backfilled with ODT or PFDT, such that the unpatterned regions of the surface are covered with hydrophobic monolayers. In the case of backfilling with PFDT, spin-coating the 80 wt % PEO copolymer onto these patterned surfaces and subsequent annealing results in the block copolymer preferentially adsorbing on the MHDA-covered regions and forming well-defined patterns that mimic the MHDA pattern, as determined by scanning electron microscopy and atomic force microscopy. Significantly worse patterning, characterized by micron-sized polymer droplets, results when the surface is backfilled with ODT instead of PFDT. Using PFDT and MHDA, polymer features having widths as small as 500 nm have been formed. These studies demonstrate a novel method to pattern block copolymers with nanoscale resolution.     

Morphologies in solvent-annealed thin films of symmetric diblock copolymer

We have systematically studied the thin film morphologies of symmetric poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer after annealing to solvents with varying selectivity. Upon neutral solvent vapor annealing, terraced morphology is observed without any lateral structures on the surfaces. When using PS-selective solvent annealing, the film exhibits macroscopically flat with a disordered micellar structure. While PMMA-selective solvent annealing leads to the dewetting of the film with fractal-like holes, with highly ordered nanoscale depressions in the region of undewetted films. In addition, when decreasing the swelling degree of the film in the case of PMMA-selective solvent annealing, hills and valleys are observed with the coexistence of highly ordered nanoscale spheres and stripes on the surface, in contrast to the case of higher swelling degree. The differences are explained qualitatively on the basis of polymer-solvent interaction parameters of the different components.     

Ordered droplet formation by thin polymer film dewetting on a stripe-patterned substrate

The dewetting process of thin polystyrene (PS) film on flat and stripe-patterned substrates is presented. Different dewetting processes were observed when the thin PS films annealed at above the glass transition temperature on these different kinds of substrates. The final dewetting on the flat substrate led to formation of polygonal liquid droplets, while on the stripe-patterned substrate, the droplets were observed to align at the centers of the stripes. A possible explanation for the dewetting process on the stripe-patterned substrate is proposed.