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.     

Restructuring in block copolymer thin films: In situ GISAXS investigations during solvent vapor annealing

Block copolymer (BCP) thin films have been proposed for a number of nanotechnology applications, such as nanolithography and as nanotemplates, nanoporous membranes and sensors. Solvent vapor annealing (SVA) has emerged as a powerful technique for manipulating and controlling the structure of BCP thin films, e.g., by healing defects, by altering the orientation of the microdomains and by changing the morphology. Due to high time resolution and compatibility with SVA environments, grazing-incidence small-angle X-ray scattering (GISAXS) is an indispensable technique for studying the SVA process, providing information of the BCP thin film structure both laterally and along the film normal. Especially, state-of-the-art combined GISAXS/SVA setups at synchrotron sources have facilitated in situ and real-time studies of the SVA process with a time resolution of a few seconds, giving important insight into the pathways and mechanisms of SVA induced restructuring. We give a short introduction to the GISAXS method and review recent theoretical studies, experimental techniques such as sample preparation and in situ chambers together with SVA protocols, and we review and discuss experimental results. We conclude by giving an outlook on emerging developments of the in situ real-time GISAXS scattering technique in combination with new approaches to control BCP thin film structures using SVA.     

Study of the time evolution of the surface morphology of thin asymmetric diblock copolymer films under solvent vapor

The time development of the surface morphology of asymmetric polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) thin films ‘annealing’ in methanol vapor, a selective solvent for minority P4VP block, was investigated by atomic force microscopy(AFM). For PS-b-P4VP with cylindrical structure in bulk, as annealing time progressed, the surface morphology underwent structural transitions from featureless topography to hybrid morphology of cylindrical and spherical pits, to cylinders, to nanoscale depressions, back to cylinders again. The different film thickness made the number of the transitions observed, at any given annealing time, different. The thicker the film is the more transitions at a given annealing time can be observed. If the film was not thick enough, depressions appeared. For PS-b-P4VP with spherical structure in bulk, it displayed nanoscale depressions with the annealing time increasing. A possible mechanism of the transition of morphologies during solvent annealing was proposed.     

Solvent annealing assisted self-assembly of hydrogen-bonded interpolymer complexes of AB block copolymer/C homopolymer in thin film

A simple process of solvent annealing has been shown to produce ordered self-assembly structures of poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP)/poly(4,4′-oxydiphenylenepyromellitamic acid) (POAA) block copolymer/homopolymer blends in thin film, where POAA chains selectively interact with P4VP blocks by strong interpolymer hydrogen-bonding. By simply exposing the thin film to benzene/NMP (0.97/0.03, in volume) vapor mixture, ordered microphase-separated structures with PS spherical microdomains distributed within P4VP/POAA complexes matrix were obtained. The formation of the microphase-separated structures could be attributed to the substantial mobility of PS blocks and P4VP/POAA complexes and enhanced repulsion between them under the benzene/NMP mixture vapor. When the volume ratio of benzene to NMP increased to 0.98/0.02, the increasing benzene in the mixture vapor induced the adhesive collision of spherical microphase-separated structures to form long “pearl necklaces”. With increasing volume ratio of benzene to NMP to 0.99/0.01, an ordered “pearl necklace” array oriented parallel to the film surface formed. The self-assembly structures were studied by FTIR spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Finally, possible mechanism of self-assembly and formation of microphase morphology was proposed.     

Mechanisms of ordering in block copolymer sub-monolayer films upon selective solvent annealing

The solvent annealing induced two-dimensional ordering in poly(styrene–ethylene/butylenes–styrene) (SEBS) triblock copolymer sub-monolayer films with a thickness of 16 nm equaling half of the bulk domain thickness l₀ was investigated by time-resolved ex-situ atomic force microscopy (AFM). Cyclohexane, a selective solvent for majority poly(ethylene/butylenes) block was used. The detailed pathway information on the ordering of highly regular hexagonal spheres was obtained by repeatedly taking images of the same marked area on the sample surface after ex-situ annealing treatments. Two different ordering mechanisms were observed under two different solvent annealing conditions: 1) under a well-sealed environment with a slow cyclohexane evaporation rate, poorly-ordered short cylinders first break into spheres, then the evolution of spherical phase takes place to gradually improve the orderliness of spheres, and finally well-ordered hexagonal spheres are formed; 2) under a poorly-sealed environment with a fast cyclohexane evaporation rate, “holes” and “islands” are instantaneously formed on the sample surface first, and then the surface gradually get even annihilating hole and island areas. Within the hole areas, the transition from short cylinders to hexagonal spheres takes place via a not well-defined microphase separation structure, while in the island areas, the transitions between cylinders and spheres take place to improve the orderliness of cylinders, and finally the enough-ordered cylinders transform into hex-spheres.     

Ordering of block copolymer/nanoparticle composite thin films

The ordering behavior of polymer nanocomposites composed of gold nanoparticles confined in the polystyrene (PS) domains of PS based block copolymers was investigated. The results reveal that the self‐assembly of nanoparticles in the PS domains improved the ordering of microdomains. This is attributed to the presence of nanoparticles that reduced the degree of segregation of the system, causing slow phase separation. This facilitates the packing of the cylindrical microdomains, leading to a well‐ordered structure of the composite. When particles were incorporated into the major domains of cylindrically ordered block copolymer, the connectivity of the domains allowed particles to move to the top of the film to gain additional entropy of the system. In contrast, when particles were organized in parallel cylinders in the block copolymer, they were confined in the cylinders which prevented them from diffusing in the depth direction. The aggregation of nanoparticles was amplified when the composite was annealed in air. We believe that the results from this study will enable more understanding of the effects of nanoparticles on the ordering of block copolymer/nanoparticle composite thin films and will provide a tool in the fabrication of composite thin films. Copyright © 2012 Society of Chemical Industry     

Perpendicular oriented cylinders via directional coalescence of spheres embedded in block copolymer films induced by solvent annealing

Polystyrene-b-poly(methyl acrylate) (PS-b-PMA) block copolymer with PS volume fraction of 25.2 vol% was synthesized by atom transfer radical polymerization. Non-pretreated silicon wafers were used as the substrates to prepare perpendicular oriented PS cylinders in PMA matrix via solvent annealing which could induce the transformation of spheres to vertically oriented and hexagonally packed cylinders. The spherical microdomains were formed after the evaporation of solvents from the solutions of the block copolymer in selective solvents mixed from methanol, acetone and dichloromethane. The thickness of films could be as thick as 1000 nm, which were much thicker than usual cases and the cylinders came from the directional coalescence of the spheres, thus any pre-treatments of the substrates were not required for perpendicular orientation. The structures were characterized by small angle X-ray scattering (SAXS), transmission electron microscope (TEM), atom force microscopy (AFM) and grazing incidence small angle X-ray scattering (GISAXS).     

Morphology of semicrystalline oxyethylene/oxybutylene block copolymer thin films on mica

The morphology of as-cast and annealed thin films of four symmetric semicrystalline block copolymers on mica was investigated by tapping mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). It is found that the morphology of the thin films is dependent on chain length of oxyethylene/oxybutylene block copolymers. The as-cast thin films of the shorter E_mB_n block copolymers on mica exhibit a multi-layered lamellar structure parallel to the surface, in which the stems of the E crystals in the first half polymer layer contacting mica are parallel to the mica surface and perpendicular to the mica surface in the upper polymer layers. In contrast, the as-cast thin film of longer E₂₂₄B₁₁₄ exhibits a structure with mixed orientations of lamellar microdomains on a half polymer layer parallel to the surface. After annealing, the multi-layered structure on mica is transformed into a half-layered, densely branched structure, which is formed following a diffusion-limited aggregation mechanism, opposed to the featureless half-layered structure on silicon. Upon annealing, the upper polymer layers gradually retreat and the remaining area becomes thicker, but in contrast the first half polymer layer contacting mica becomes thinner due to wetting and the parallel orientation of the E crystal stems. The densely branched structure and the different chain orientations of the E crystal stems in the first half polymer layer contacting mica are attributed to the strong interaction between the E block and mica, as revealed by our previous work. The width of branches was employed to analyze the kinetics of secondary crystallization. It is also found that the width of the branches and the velocity of crystal front decrease as the chain length increases.     

Solvent annealing thin films of poly(isoprene-b-lactide)

The use of solvent annealing to control the microdomain orientation and long-range ordering in poly(isoprene-b-d,l-lactide) thin films was investigated using scanning force microscopy and grazing incidence small angle X-ray scattering (GISAXS). Benzene and chloroform were used as annealing solvents. Both were found to improve the long-range order in the films. Additionally, at high concentrations of chloroform in the film, a perpendicular orientation of cylinders was observed where the solvent was able to mediate interfacial interactions sufficiently to prevent a preferential segregation of one of the blocks to the surface. In situ GISAXS measurements made during solvent swelling and evaporation allowed an examination of the film morphology over a wide range of solvent concentrations providing a efficient route to optimize conditions for morphology control by solvent annealing.     

Reversible morphology control of three-dimensional block copolymer nanostructures by the solvent-annealing-induced wetting in anodic aluminum oxide templates

In this work, the authors study the fabrication of three-dimensional block copolymer nanostructures in which the morphologies can be reversibly controlled. Polystyrene-block-polydimethylsiloxane (PS-b-PDMS), a promising candidate for nanolithography, is introduced into cylindrical nanopores of anodic aluminum oxide (AAO) templates using a solvent annealing–induced nanowetting in templates (SAINT) method. Not only the morphologies of the infiltrated PS-b-PDMS nanostructures can be tuned by the annealing solvents, but also the solvent-vapor-controlled morphologies can be altered reversibly by annealing the samples repeatedly between different solvent vapors.     

Effect of nanoparticle surface functionality on microdomain orientation in block copolymer thin films under electric field

The electric field induced microdomain orientations has been an interesting research topic. In this article, the effect of nanoparticle surface functionality on microdomain alignments in block copolymer/nanoparticle hybrid thin films was investigated with transmission electron microscopy experiments. The presence of gold nanoparticles influenced the microdomain orientation behaviors of block copolymer/nanoparticle thin films. The possibility for complete alignment normal to the substrate was illustrated by controlling electric field strength, concentration, and surface ligands of nanoparticles. This work provides basic and essential data to understand the properties and behaviors of emerging block copolymer/nanoparticle hybrid thin films.     

AFM study of crystallization and melting of a poly(ethylene oxide) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} in ultrathin films

Crystallization and melting of a poly(ethylene oxide) (PEO) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) in ultrathin films have been studied using atomic force microscopy (AFM) coupled with a hot stage. The PEO and PMPCS block possess the number-average molecular weights (M_n) of 5300 and 2100 g/mol, respectively. The ultrathin films on the mica and glow-discharged carbon surfaces were obtained by static dilute solution casting at room temperature. Isothermal melt crystallization from ultrathin films always leads to flat-on lamellae. Selective area electron diffraction (SAED) experiments have demonstrated that the PEO blocks crystallize with a monoclinic structure identical to that of homo-PEO and the chain direction is perpendicular to the substrate. At T_c<44 °C, the monolayer crystals are dendrites. At T_c>48 °C, square-shaped crystals are formed with the (100) and (020) planes as the crystal edges. At 44 °C≤T_c≤48 °C, an intermediate monolayer morphology is observed. The monolayer thickness increases monotonically with increasing T_c. At the same T_c, the monolayer lamellae with the top and bottom amorphous layers contacting with the atmosphere and the substrate possess a significantly larger overall thickness than the long period of the crystals in bulk. For the spiral terraces induced by screw dislocation, the thickness of each terrace is close to that of the monolayer formed at the same T_c, and their melting is mainly determined by the terrace thickness.     

Macrolattice structure of a block copolymer in a selective solvent

Macrolattice structure in the ordered phase of a poly(styrene-b-butadiene-b-styrene) (SBS, with the bulk morphology of spherical polystyrene microdomains in the polybutadiene matrix) dissolved in a selective solvent (dodecyl methacrylate, C₁₂MA, or a 75/25 w/w mixture of C₁₂MA and butylene diacrylate, BDA) which mixes preferentially with the polybutadiene matrix was examined by means of transmission electron microscopy. The use of C₁₂MA/BDA mixture as the selective solvent provided the opportunity of freezing the macrolattice structure upon UV-initiated polymerization of the acrylic monomers when the SBS content is above ca. 60 wt%. Results indicated clearly a body-centered cubic structure, in contrast to the simple cubic packing previously proposed.     

Lamella reorientation in thin films of a symmetric poly(l-lactic acid)-block-polystyrene upon crystallization at different temperatures

The thin films of a symmetric crystalline-coil diblock copolymer of poly(l-lactic acid) and polystyrene (PLLA-b-PS) formed lamellae parallel to the substrate surface in melt. When annealed at temperatures well above the glass transition temperature of PLLA block (T_g^PLLA), the PLLA chains started to crystallize, leading to reorientation of lamellae. Such reorientation behavior exhibited dependence on the correlation between the crystallization temperature (T_c), the glass transition temperature of PS (T_g^PS), the peak melting point of PLLA crystals (T_m^PLLA), and the end melting point of PLLA crystals (T_m,end^PLLA). When annealed at (T_c=) 80 °C (T_c < T_g^PS < T_ODT, order-disorder transition temperature), 123 °C (T_g^PS < T_c < T_m^PLLA < T_ODT), 165 °C (T_g^PS < T_m^PLLA < T_c < T_m,end^PLLA < T_ODT), the parallel lamellae became perpendicular to the substrate surface, exclusively starting at the edge of surface relief patterns. Meanwhile, the corresponding lamellar spacing was significantly enhanced. The PLLA crystallization between PS layers was hypothesized to account for the lamella reorientation during annealing. The crystallization, chain conformation, and possible chain folding mechanisms were discussed, based on detailed analysis of the lamellar structure before and after crystallization.     

Enhancement of thermoelectric performance of N-type Bi2Te3 based thin films via in situ annealing during magnetron sputtering

In this work, n-type Bi₂Te₃ based thin films were prepared in 300 °C via DC magnetron sputtering, and influences of sputtering power and annealing time on thermoelectric properties of films were investigated. The raise of sputtering power brings about the improvement of deposited rate and enhancement of grain size. Taking the consideration that the large-sized grains are to phonon scattering, we determine the medial power of 30 W as the basic technical parameters for the purpose of further optimizing performance through an in situ annealing process. Subsequently, thin-film treated by in situ annealing process acts out an obvious reduction in electrical conductivity attributed to the decrease in carrier concentration. Especially, the film annealed for 40 min shows an enhancement in the Seebeck coefficient and leads to a maximum power factor 0.82 m W m⁻¹ K⁻² at 543 K.     

Effect of thermal annealing in atmosphere on photoluminescence of BTEO–PPV films

The photoluminescence (PL) spectra of poly[2,5‐bis‐(tri‐ethoxy)‐1,4‐phenylene vinylene] (BTEO–PPV) films are blue‐shifted with increasing thermal annealing temperature. It is known from the UV–vis absorption spectra that thermal annealing decreases the conjugation length of the polymer. For BTEO–PPV films, unlike with MEH–PPV films, the symmetric triethoxy side groups further block aggregation of the polymer chains. The absorption Fourier transfer infrared spectra showed that thermal annealing in atmosphere destroyed the chain structure of BTEO–PPV film by thermal oxidation to form aldehyde groups, which resulted in low PL efficiency of the annealed films. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

A novel method for sulfonation of microporous polystyrene divinyl benzene copolymer using gaseous SO2 in the waste air streams

In this study a novel sulfonation method for microporous polystyrene divinyl benzene copolymer (PSDBP) was introduced. In our sulfonation system gaseous SO₂ is used as the sulfonation agent and planned to be obtained from waste gas streams. The proposed method, therefore, combines SO₂ control and clean sulfonation technology in a single compact design.Molded polymeric monoliths of the PSDBP containing imprisoned H₂O₂ solution inside the pores (PSDBPH₂O₂) were produced in disk shapes. Dry gas mixture containing 3000 ppm SO₂ is fed into PSDBPH₂O₂ disk reactor with a flow rate of 0.8 L/min and effluent gas composition in terms of SO₂ was measured. Breakthrough curves for varied initial H₂O₂ amount were used to calculate SO₂ adsorption capacity and sulfonation degree of the PSDBPH₂O₂ disks.Successful sulfonation of PSDBPH₂O₂ was verified by the changes in its morphological structure and formed sulfone bonds determined by SEM and IR analyses, respectively. Maximum adsorption capacity for PSDBPH₂O₂ for the initial H₂O₂ volume percentage of 13% was determined as 57 mg SO₂/g polymer. It should be noted that SO₂ adsorption was observed only in H₂O₂ imprisoned polymer disks. Sulfonation degree of PSDBPH₂O₂ which attained maximum SO₂ amount is calculated as 10%.     

Controlled dispersion of a porphyrin/fullerene donor‐acceptor complex in semiconducting polymer thin films: Intermolecular interactions of polymers with porphyrin and fullerene

Thin films composed of semiconducting polymers [poly(2‐vinyl naphthalene), poly(4‐diphenyl aminostyrene), poly(1‐vinyl pyrene), and poly(3‐hexyl thiophene‐2,5‐diyl)], zinc(II)?5,10,15,20‐tetra‐(2‐naphthyl)porphyrin, and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester blends were prepared to investigate the controlled dispersion of porphyrin molecules in semiconducting polymer thin films. Tailoring the intermolecular interactions between the polymer/fullerene, polymer/porphyrin, and porphyrin/fullerene systems was found to be an effective method of controlling the dispersion. When the polymer/porphyrin interactions were enhanced, intermixed porphyrin/fullerene donor–acceptor complex domains were formed, whereas under conditions where the polymer/porphyrin interactions were weakened, the complex assembled at the borders between the polymer and fullerene phases. This concept could potentially be applied to various combinations of porphyrin/fullerene systems in semiconducting polymer thin films to develop polymer solar cells with excellent performance. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41629.