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.     

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.     

A paradigm shift in morphological architecture of PEO-b-PFOMA semi-fluorinated block copolymer thin films upon facile solvent annealing

A systematical study on the morphological transition of the micelle films of semi-fluorinated poly(ethylene oxide)-b-poly(1H,1H-dihydro perfluorooctyl methacrylate) (PEO-b-PFOMA) diblock copolymers was carried out upon perfluroalkanes (PF-5080) or α,α,α-trifluorotoluene (TFT) solvent annealing. Poorly ordered short cylindrical structures of the PEO_5k-b-PFOMA_21k micelle film underwent a phase inversion with PEO cores in the PFOMA continuous phase with a short period of PF-5080 solvent annealing. In contrast, the highly ordered morphology of PEO_10k-b-PFOMA_21k with PFOMA cores in the PEO continuous phase developed into cylindrical microdomains presumably via the fusion process. Prolonged annealing of the film transformed its morphology into inverted-spherical domains of PEO in the PFOMA continuous phase through long-range ordering by following the fission process. In order to find out a synthetic application of the morphology inversion strategy, an attempt was undertaken by adding a gold precursor to the PEO_10k-b-PFOMA_21k micelle solution, and as-cast thin films were prepared accordingly. Upon PF-5080 solvent annealing, the nanoparticles populated in self-assembled thin films resulted in inverted-spherical domains having gold nanoparticles populated in PEO cores surrounded by the PFOMA continuous phase. When the annealing solvent was changed to TFT, a highly ordered in-plane cylindrical morphology with respect to the substrate was achieved from the poorly ordered cylindrical microdomains of the PEO_5k-b-PFOMA_21k thin film, whereas an uneven cylindrical structure was produced from PEO_10k-b-PFOMA_21k.     

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 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.     

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     

Water-induced morphology evolution of block copolymer micellar thin films

Morphology evolution of diblock copolymer polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) micellar thin film in the presence of water was investigated. Surface holes with nanoscale cavities in hexagonal order could be induced by water treatment for certain periods. The nanoscale surface cavities could be transformed into isolated nanospheres in a dry environment or back to protruding densely packed spheres by toluene (a selective solvent for PS coronae) treatment. The morphology evolution of micellar thin film strongly depended on the slow evaporation of toluene solvent, the swelling of P4VP cores in the humid environment, and the subsequent movement of PS chains induced by air and toluene. The incompatibility between solvent and block, and that between the unlike blocks also played an important role in the morphology evolution.     

Nanoporous hard etch masks using silicon-containing block copolymer thin films

Nanoporous hard etch masks with various pore sizes were fabricated using a new type of silicon-containing block copolymers, polystyrene-block-poly(4-(tert-butyldimethylsilyl)oxystyrene) with different molecular weights. Since organic-inorganic block copolymers have a large difference in etch resistance between the organic and inorganic blocks, a hard etch mask of silicon oxide can be directly produced upon oxygen plasma treatment. Orientation and hexagonal arrays of cylindrical nanodomains were manipulated simply by adjusting the relative composition of selective and non-selective solvents in the annealing solvent. When the cylindrical nanostructures aligned perpendicular to the substrate surface were exposed to an oxygen plasma, hexagonally arranged nanopore arrays of silicon oxide with controlled pore sizes were fabricated. These nanoporous hard etch masks can be applied to the nanopatterning processes that require high aspect ratio structures.     

Structure and phase transition in thin films of block copolymer micelles complexed with inorganic precursors

Block copolymer micelle can be used as nano-reactor where separated domains serve as a compartment for the production of nanomaterials, ultimately creating nanocomposite materials. In this work, thin nanocomposite films generated from polystyrene-b-poly(acrylic acid) (PS-b-PAA) micellar solution in which small amount of inorganic precursor was added were investigated. The films were prepared by spin coating onto silicon substrate, and then solvent-annealed. As-spun films exhibit typical micellar structure with spherical shape along which inorganic nanoparticles are dispersed. Such morphology remains unchanged after solvent annealing for micellar films with small amount of inorganic precursor. However, further increase in the amount of inorganic precursors brings about the morphological changes, producing different organization of inorganic nanoparticles in composite films. This behavior was found to strongly depend on the types of precursors and solvents used for annealing. These results illustrate a simple yet useful route to generate the polymeric nanocomposites with diverse structure and composition.     

Enhanced light emission of nano-patterned GaN via block copolymer thin films

We demonstrate that the nanoscopic block copolymer patterns on GaN can enhance light extraction efficiency of GaN-based light emitting diodes. Nanoporous patterns were fabricated on a bare GaN substrate via self-assembly of poly(styrene-b-methyl methacrylate) block copolymers from which PMMA microdomains were selectively removed later on. A bare GaN surface was treated with a photo-crosslinkable thin layer of poly(styrene-r-methyl methacrylate) random copolymers to tune the cylindrical microdomain orientations. The nanoporous block copolymer thin film was controlled to be thicker than its typical repeat period in bulk by incorporating PMMA homopolymer into block copolymer. Consequently, the light extraction efficiency in photoluminescence spectra could be tuned with the thickness of nanopatterned thin film on GaN. This paper is dedicated to Professor Chul Soo Lee on the occasion of his retirement from Korea University.     

嵌段共聚物薄膜自组装热退火技术进展

大分子自组装能使嵌段共聚物薄膜中微相结构进行自发地有序排列,形成高度图案化的微观形貌,继而赋予薄膜特殊的物理与化学功能。热退火诱导技术是制备嵌段共聚物自组装薄膜的一种主要手段,具有操作简单和实验条件易于控制的优点。综述了多种新型热退火诱导技术以及制备高取向、低缺陷密度自组装薄膜的方法,并对多种新型技术的特点进行了总结。     

嵌段共聚物薄膜自组装热退火技术进展

大分子自组装能使嵌段共聚物薄膜中微相结构进行自发地有序排列,形成高度图案化的微观形貌,继而赋予薄膜特殊的物理与化学功能。热退火诱导技术是制备嵌段共聚物自组装薄膜的一种主要手段,具有操作简单和实验条件易于控制的优点。综述了多种新型热退火诱导技术以及制备高取向、低缺陷密度自组装薄膜的方法,并对多种新型技术的特点进行了总结。     

Tailoring block copolymer nanoporous thin films with acetic acid as a small guest molecule

Block copolymers offer the fabrication of mesoporous thin films with distinct nanoscale structural features. In this contribution, we present the use of acetic acid (CH₃COOH) as a low‐molecular‐weight guest molecule to tune the supramolecular assembly of poly[styrene‐block‐(4‐vinylpyridine)] (PS‐b‐P4VP), offering a versatile and straightforward method to obtain tailored nanostructured films with controlled topography and pore size. Spin‐coating toluene solutions of PS‐b‐P4VP, with a variable amount of CH₃COOH, leads to micellar thin films, where the micelles contain the carboxylic acid as a guest molecule. The size can be conveniently modified in these films (from 48 to 75 nm) by varying the amount of organic acid in the starting solutions. Subsequent surface reconstruction of micellar films using ethanol leads to ring‐shaped copolymer nanoporous films with modulated diameter. Controlling the micelle reconstruction process, cylindrical porous films are also obtained. Interestingly, changing the type of aliphatic carboxylic acid leads to a modification of the observed film morphology from micelles to out‐of‐plane P4VP cylinders (or lamellae) in a PS matrix. © 2019 Society of Chemical Industry     

Perpendicular lamellae induced at the interface of neutral self-assembled monolayers in thin diblock copolymer films

We obtained perpendicular lamellar orientations in thin films of symmetric polystyrene-block-poly(methyl methacrylate), PS-b-PMMA, on self-assembled monolayers (SAMs) of 3-(p-methoxyphenyl)propyltrichlorosilane (MPTS) prepared on silicon wafers. In contrast to completely parallel lamellae on silicon wafers having a native oxide layer, perpendicular lamellae at the MPTS interface with parallel lamellae at the air interface were directly observed by transmission electron microscopy (TEM) in cross-sectional view. The perpendicular lamellae at the MPTS interface were attributed to the non-preferential (neutral) MPTS-covered substrate to both PS and PMMA blocks. The neutrality of the SAMs of MPTS was confirmed by the similar interfacial tension values of the SAMs of MPTS with PS and PMMA, estimated by contact angle measurements.     

The nanostructure and dewetting of block copolymer thin films annealed in different neutral solvents

The morphology of polystyrene‐b‐poly(2‐vinyl pyridine) thin films annealed under various neutral solvents was investigated. The morphological transition depends on the vapor pressure of the solvent, the quantity of the solvent in the film, and annealing time. We introduced the volume fraction of solvent in a film (Q) to correlate these factors to the morphology. At low Q, the amount of solvent that penetrates into the film is limited and it cannot induce enough chain mobility. Thus, thin film shows short stripes or a worm‐like structure. At high Q, the great diffusion of solvent into the film facilitates polymer mobility, leading to an ordered structure. Our results also suggested that the dewetting mechanism of thin film depends on Q. At low Q, dewetting develops via the nucleation and growth. At high Q, the condensation of solvent on the surface removes some polymer and dewetting is dominated by spinodal mechanism. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

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.     

Silicon and carbon substrates induced arrangement changes in poly(styrene-b-isoprene-b-styrene) block copolymer thin films

Poly(styrene-b-isoprene-b-styrene) (SIS) block copolymer ordering in thin films was studied using two selective substrates as carbon and silicon. Atomic force microscopy (AFM) and contact angle measurements were employed to examine the affinities between domains and surrounding interfaces. The surface morphology was examined by AFM using different amplitude ratios. Results showed polyisoprene (PI) domain layer formation in the outermost film layer. On the other hand, the layer close to substrate adopted different arrangements on silicon and carbon substrates. Topographical and phase images revealed that in both substrates with the thickest films, the interactions between substrate and block domains were not enough to induce surface ordering being the morphology independent of employed substrate. However, decreasing film thickness, SIS thin films displayed a variety of arrangements such as perforated lamellae and cylindrical morphologies. Depending on substrate, these morphologies were achieved in different film thicknesses. Finally, the thinnest film did not adjust to characteristic domain spacing commensurability and terraces formation was observed. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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.     

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.