In Situ Characterization of the Self-Assembly of a Polystyrene–Polydimethylsiloxane Block Copolymer during Solvent Vapor Annealing

Processive Pathways to Metastability in Block Copolymer Thin Films

Block copolymers (BCPs) self-assemble into intricate nanostructures that enhance a multitude of advanced applications in semiconductor processing, membrane science, nanopatterned coatings, nanocomposites, and battery research. Kinetics and thermodynamics of self-assembly are crucial considerations in controlling the nanostructure of BCP thin films. The equilibrium structure is governed by a molecular architecture and the chemistry of its repeat units. An enormous library of materials has been synthesized and they naturally produce a rich equilibrium phase diagram. Non-equilibrium phases could potentially broaden the structural diversity of BCPs and relax the synthetic burden of creating new molecules. Furthermore, the reliance on synthesis could be complicated by the scalability and the materials compatibility. Non-equilibrium phases in BCPs, however, are less explored, likely due to the challenges in stabilizing the metastable structures. Over the past few decades, a variety of processing techniques were introduced that influence the phase transformation of BCPs to achieve a wide range of morphologies. Nonetheless, there is a knowledge gap on how different processive pathways can induce and control the non-equilibrium phases in BCP thin films. In this review, we focus on different solvent-induced and thermally induced processive pathways, and their potential to control the non-equilibrium phases with regards to their unique aspects and advantages. Furthermore, we elucidate the limitations of these pathways and discuss the potential avenues for future investigations.

Solvent Vapor Annealing for Controlled Pore Expansion of Block Copolymer-Assembled Inorganic Mesoporous Films

Mesoporous inorganic thin films are promising materials architectures for a variety of high-value applications, ranging from optical coatings and purification membranes to sensing and energy storage devices. Having precise control over the structural parameters of the porous network is crucial for broadening their applicability. To this end, the use of block copolymers (BCP) as sacrificial structure-directing agents via micelle coassembly is a particularly attractive route, since the resultant pore size is directly related to scaling laws for the radius of gyration of the pore-forming macromolecule. However, tailoring the molecular weight of the BCP via bespoke synthesis is an elaborate process that requires precise control over highly sensitive reactions conditions. Alternative methods have emerged, based on supramolecular assembly or the addition of different swelling agents. Nevertheleses, to date, these present a negative impact on the structural order and pore size dispersity of the final inorganic mesoporous films. In this work, we propose a novel and effective method for control over pore size, porosity, and structural order, which relies on a synergistic combination of BCP selective swelling via solvent vapor annealing (SVA) and locking of the structure by condensation of the inorganic sol-gel precursors. The results obtained in this work for TiO₂ establish SVA as a new, straightforward, simple, and powerful route for the fabrication of mesoporous thin-film materials with controllable structural characteristics.

Selective sequential infiltration synthesis of ZnO in the liquid crystalline phase of silicon-containing rod-coil block copolymers

The combination of block copolymer (BCP) thin film self-assembly and selective infiltration synthesis of inorganic materials into one BCP block provides access to various organic-inorganic hybrids. Here, we apply sequential infiltration synthesis, a vapor-phase hybridization technique, to selectively introduce ZnO into the organic microdomains of silicon-containing rod-coil diblock copolymers and a triblock terpolymer, polydimethylsiloxane (PDMS)-b-poly{2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene} (PDMS-b-PMPCS) and PDMS-b-polystyrene-b-PMPCS (PDMS-b-PS-b-PMPCS), in which the PMPCS rod block is a liquid crystalline polymer. The in-plane cylindrical PDMS-b-PMPCS and core-shell cylindrical and hexagonally perforated lamellar PDMS-b-PS-b-PMPCS films were infiltrated with ZnO with high selectivity to the PMPCS. The etching contrast between PDMS, PS and the ZnO-infused PMPCS enables the fabrication of ZnO/SiO_x binary composites by plasma etching and reveals the core-shell morphology of the triblock terpolymer.

Pathway-Dependent Grain Coarsening of Block Copolymer Patterns under Controlled Solvent Evaporation

Solvent evaporation annealing (SEA) is a straightforward, single-step casting and annealing method of block copolymers (BCP) processing yielding large-grained morphologies in a very short time. Here, we present a quantitative analysis of BCP grain-coarsening in thin films under controlled evaporation of the solvent. Our study is aimed at understanding time and BCP concentration influence on the rate of the lateral growth of BCP grains. By systematically investigating the coarsening kinetics at various BCP concentrations, we observed a steeply decreasing exponential dependence of the kinetics power-law time exponent on polymer concentration. We used this dependence to formulate a mathematical model of BCP ordering under nonstationary conditions and a 2D, time- and concentration-dependent coarsening rate diagram, which can be used as an aid in engineering the BCP processing pathway in SEA and also in other directed self-assembly methods that utilize BCP-solvent interactions such as solvent vapor annealing.

Defining Swelling Kinetics in Block Copolymer Thin Films: The Critical Role of Temperature and Vapour Pressure Ramp

We studied the kinetics of swelling in high-χ lamellar-forming poly(styrene)-block- poly(lactic acid) (PS-b-PLA) block copolymer (BCP) by varying the heating rate and monitoring the solvent vapour pressure and the substrate temperature in situ during solvo-thermal vapour annealing (STVA) in an oven, and analysing the resulting morphology. Our results demonstrate that there is not only a solvent vapour pressure threshold (120 kPa), but also that the rate of reaching this pressure threshold has a significant effect on the microphase separation and the resulting morphologies. To study the heating rate effect, identical films were annealed in a tetrahydrofuran (THF) vapour environment under three different ramp regimes, low (rT<1 °C/min), medium (24 °C/min), for 60, 90 and 120 min, respectively, while the solvent vapour pressure and the substrate temperature were measured in real time. The translational order improved significantly with increasing the heating rate. The solvent mass uptake calculated for the different ramp regimes during annealing is linearly proportional to time, indicating that the swelling kinetics followed Case II diffusion. Two stages of the swelling behaviour were observed: (i) diffusion at the initial stages of swelling and (ii) stress relaxation, controlled at later stages. Films with a faster rate of increase in vapour pressure (rP>2 kPa/min) reached the pressure threshold value at an early stage of the swelling and attained a good phase separation. According to our results, highly ordered patterns are only obtained when the volume fraction of the solvent exceeds the polymer volume fraction, i.e., (φs≥φp), during the swelling process, and below this threshold value (φs=0.5), the films did not obtain a good structural order, even at longer annealing times.

The Influence of Annealing Atmosphere, Blending Ratio, and Molecular Weight on the Phase Behavior of Blend Materials

In the study of block copolymers, many parameters need to be adjusted to obtain good phase separation results. Based on block copolymer polystyrene-b-polycarbonate and homopolymer polystyrene, the effects of the annealing atmosphere, blending ratio, and molecular weight on phase separation were studied. The results show that annealing in air can inhibit the occurrence of phase separation. In addition, snowflake patterns are formed during phase separation. The blending ratio affects the quality of the pattern. The molecular weight affects the size of the pattern, and the size increases as the molecular weight increases. In this article, the influence of process conditions and materials on phase separation was discussed, which has laid a solid foundation for the development of block copolymer self-assembly in the future.

Large-Grained Cylindrical Block Copolymer Morphologies by One-Step Room-Temperature Casting

We report a facile method of ordering block copolymer (BCP) morphologies in which the conventional two-step casting and annealing steps are replaced by a single-step process where microphase separation and grain coarsening are seamlessly integrated within the casting protocol. This is achieved by slowing down solvent evaporation during casting by introducing a nonvolatile solvent into the BCP casting solution that effectively prolongs the duration of the grain-growth phase. We demonstrate the utility of this solvent evaporation annealing (SEA) method by producing well-ordered large-molecular-weight BCP thin films in a total processing time shorter than 3 min without resorting to any extra laboratory equipment other than a basic casting device, i.e., spin- or blade-coater. By analyzing the morphologies of the quenched samples, we identify a relatively narrow range of polymer concentration in the wet film, just above the order-disorder concentration, to be critical for obtaining large-grained morphologies. This finding is corroborated by the analysis of the grain-growth kinetics of horizontally oriented cylindrical domains where relatively large growth exponents (1/2) are observed, indicative of a more rapid defect-annihilation mechanism in the concentrated BCP solution than in thermally annealed BCP melts. Furthermore, the analysis of temperature-resolved kinetics data allows us to calculate the Arrhenius activation energy of the grain coarsening in this one-step BCP ordering process.

Kinetic Monitoring of Block Copolymer Self-Assembly Using In Situ Spectroscopic Ellipsometry

Understanding the kinetic pathways of self-assembly in block copolymers (BCPs) has been a long-standing challenge, mostly due to limitations of in situ monitoring techniques. Here, we demonstrate an approach that uses optical birefringence, determined by spectroscopic ellipsometry (SE), as a measure of domain formation in cylinder- and lamellae-forming BCP films. The rapid experimental acquisition time in SE (ca. 1 sec) enables monitoring of the assembly/disassembly kinetics of BCP films during solvent-vapor annealing (SVA). We demonstrate that upon SVA, BCP films form ordered domains that are stable in the swollen state, but disorder upon rapid drying. Surprisingly, the disassembly during drying strongly depends on the duration of solvent exposure in the swollen state, explaining previous observations of loss of order in SVA processes. SE thus allows for decoupling of BCP self-assembly and disordering that occurs during solvent annealing and solvent evaporation, which is difficult to probe using other, slower techniques.

Solvent effects leading to a variety of different 2D structures in the self-assembly of a crystalline-coil block copolymer with an amphiphilic corona-forming block

We describe a polyferrocenyldimethylsilane (PFS) block copolymer (BCP), PFS27-b-P(TDMA65-ran-OEGMA69) (the subscripts refer to the mean degrees of polymerization), in which the corona-forming block is a random brush copolymer of hydrophobic tetradecyl methacrylate (TDMA) and hydrophilic oligo(ethylene glycol) methyl ether methacrylate (OEGMA). Thus, the corona is amphiphilic. This BCP generates a remarkable series of different structures when subjected to crystallization-driven self-assembly (CDSA) in solvents of different polarity. Long ribbon-like micelles formed in isopropanol, and their lengths could be controlled using both self-seeding and seeded growth protocols. In hexanol, the BCP formed more complex structures. These objects consisted of oval platelets connected to long fiber-like micelles that were uniform in width but polydisperse in length. In octane, relatively uniform rectangular platelets formed. Finally, a distinct morphology formed in a mixture of octane/hexanol, namely uniform oval structures, whose height corresponded to the fully extended PFS block. Both long and short axes of these ovals increased with the initial annealing temperature and with the BCP concentration. The self-seeding protocol also afforded uniform two-dimensional structures. Seeded growth experiments, in which a solution of the BCP in THF was added to a colloidal solution of the oval micelles led to a linear increase in area while maintaining the aspect ratio of the ovals. These experiments demonstrate the powerful effect of the amphiphilic corona chains on the CDSA of a core crystalline BCP in solvents of different hydrophilicity.

Real-time and in situ observation of structural evolution of giant block copolymer thin film under solvent vapor annealing by atomic force microscopy

An instrumentation technique for real-time, in situ and real space observation of microphase separation was proposed for ultra-high molecular weight block copolymer thin films (1010 kg mol⁻¹, domain spacing of 180 nm) under high solvent vapor swelling conditions. This is made possible by a combination of a homebuilt chamber which is capable of supplying sufficient amount of vapor, and force–distance curve measurements which gives real-time swollen film thickness and allow active feedback for controlling the degree of swelling. We succeeded in monitoring the domain coarsening of perpendicular lamellar structures in polystyrene-block-poly(methyl methacrylate) thin films for eight hours via tapping mode imaging. During the annealing process, the thickness reached a maximum of 8.5 times that of the original film. The series of temporal real space topographic images obtained via this method allowed us to study, for the first time, the growth exponent of the correlation length under solvent vapor annealing.

Real-time and in situ observation technique was proposed for ultra high molecular weight block copolymer thin films under solvent vapor annealing.

Controlling Self-Assembly in Gyroid Terpolymer Films By Solvent Vapor Annealing

The efficacy with which solvent vapor annealing (SVA) can control block copolymer self-assembly has so far been demonstrated primarily for the simplest class of copolymer, the linear diblock copolymer. Adding a third distinct block-thereby creating a triblock terpolymer-not only provides convenient access to complex continuous network morphologies, particularly the gyroid phases, but also opens up a route toward the fabrication of novel nanoscale devices such as optical metamaterials. Such applications, however, require the generation of well-ordered 3D continuous networks, which in turn requires a detailed understanding of the SVA process in terpolymer network morphologies. Here, in situ grazing-incidence small-angle X-ray scattering (GISAXS) is employed to study the self-assembly of a gyroid-forming triblock terpolymer during SVA, revealing the effects of several key SVA parameters on the morphology, lateral order, and, in particular, its preservation in the dried film. The robustness of the terpolymer gyroid morphology is a key requirement for successful SVA, allowing the exploration of annealing parameters which may enable the generation of films with long-range order, e.g., for optical metamaterial applications.

Templated Self-Assembly of a PS- Branch-PDMS Bottlebrush Copolymer

The self-assembly of block copolymers (BCPs) with novel architectures offers tremendous opportunities in nanoscale patterning and fabrication. Here, the thin film morphology, annealing kinetics, and topographical templating of an unconventional Janus-type "PS- branch-PDMS" bottlebrush copolymer (BBCP) are described. In the Janus-type BBCP, each segment of the bottlebrush backbone connects two immiscible side chain blocks. Thin films of a Janus-type BBCP with M_n = 609 kg/mol exhibited 22 nm period cylindrical microdomains with long-range order under solvent vapor annealing, and the effects of as-cast film thickness, solvent vapor pressure, and composition of the binary mixture of solvent vapors are described. The dynamic self-assembly process was characterized using in situ grazing-incidence X-ray scattering. Templated self-assembly of the BBCP within lithographically patterned substrates was demonstrated, showing distinct pattern orientation and dimensions that differ from conventional BCPs. Self-consistent field theory is used to elucidate details of the templated self-assembly behavior within confinement.

Double-Layer Morphologies from a Silicon-Containing ABA Triblock Copolymer

A combined experimental and self-consistent-field theoretical (SCFT) investigation of the phase behavior of poly(stryrene- b-dimethylsiloxane- b-styrene) (PS- b-PDMS- b-PS, or SDS32) thin films during solvent vapor annealing is presented. The morphology of the triblock copolymer is described as a function of the as-cast film thickness and the ratio of two different solvent vapors, toluene and heptane. SDS32 formed terraced bilayer morphologies even when the film thickness was much lower than the commensurate thickness. The morphology transitioned between bilayer cylinders, bilayer perforated lamellae, and bilayer lamellae, including mixed structures such as a perforated lamella on top of a layer of in-plane cylinders, as the heptane fraction during solvent annealing increased. SCFT modeling showed the same morphological trends as a function of the block volume fraction. In comparison with diblock PS- b-PDMS with the same molecular weight, the SDS32 offers a simple route to produce a diversity of well-ordered bilayer structures with smaller feature sizes, including the formation of bilayer perforated lamellae over a large process window.

High-Precision Solvent Vapor Annealing for Block Copolymer Thin Films

Despite its efficacy in producing well-ordered, periodic nanostructures, the intricate role multiple parameters play in solvent vapor annealing has not been fully established. In solvent vapor annealing a thin polymer film is exposed to a vapor of solvent(s) thus forming a swollen and mobile layer to direct the self-assembly process at the nanoscale. Recent developments in both theory and experiments have directly identified critical parameters that govern this process, but controlling them in any systematic way has proven non-trivial. These identified parameters include vapor pressure, solvent concentration in the film, and the solvent evaporation rate. To explore their role, a purpose-built solvent vapor annealing chamber was designed and constructed. The all-metal chamber is designed to be inert to solvent exposure. Computer-controlled, pneumatically actuated valves allow for precision timing in the introduction and withdrawal of solvent vapor from the film. The mass flow controller-regulated inlet, chamber pressure gauges, in situ spectral reflectance-based thickness monitoring, and low flow micrometer relief valve give real-time monitoring and control during the annealing and evaporation phases with unprecedented precision and accuracy. The reliable and repeatable alignment of polylactide cylinders formed from polystyrene-b-polylactide, where cylinders stand perpendicular to the substrate and span the thickness of the film, provides one illustrative example.

Optimizing self-consistent field theory block copolymer models with X-ray metrology

A block copolymer self-consistent field theory (SCFT) model is used for direct analysis of experimental X-ray scattering data obtained from thin films of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) made from directed self-assembly. In a departure from traditional approaches, which reconstruct the real space structure using simple geometric shapes, we build on recent work that has relied on physics-based models to determine shape profiles and extract thermodynamic processing information from the scattering data. More specifically, an SCFT model, coupled to a covariance matrix adaptation evolutionary strategy (CMAES), is used to find the set of simulation parameters for the model that best reproduces the scattering data. The SCFT model is detailed enough to capture the essential physics of the copolymer self-assembly, but sufficiently simple to rapidly produce structure profiles needed for interpreting the scattering data. The ability of the model to produce a matching scattering profile is assessed, and several improvements are proposed in order to more accurately recreate the experimental observations. The predicted parameters are compared to those extracted from model fits via additional experimental methods and with predicted parameters from direct particle-based simulations of the same model, which incorporate the effects of fluctuations. The Flory-Huggins interaction parameter for PS-b-PMMA is found to be in agreement with reported ranges for this material. These results serve to strengthen the case for relying on physics-based models for direct analysis of scattering and light signal based experiments.

Pathways to Mesoporous Resin/Carbon Thin Films with Alternating Gyroid Morphology

Three-dimensional (3D) mesoporous thin films with sub-100 nm periodic lattices are of increasing interest as templates for a number of nanotechnology applications, yet are hard to achieve with conventional top-down fabrication methods. Block copolymer self-assembly derived mesoscale structures provide a toolbox for such 3D template formation. In this work, single (alternating) gyroidal and double gyroidal mesoporous thin-film structures are achieved via solvent vapor annealing assisted co-assembly of poly(isoprene-block-styrene-block-ethylene oxide) (PI-b-PS-b-PEO, ISO) and resorcinol/phenol formaldehyde resols. In particular, the alternating gyroid thin-film morphology is highly desirable for potential template backfilling processes as a result of the large pore volume fraction. In situ grazing-incidence small-angle X-ray scattering during solvent annealing is employed as a tool to elucidate and navigate the pathway complexity of the structure formation processes. The resulting network structures are resistant to high temperatures provided an inert atmosphere. The thin films have tunable hydrophilicity from pyrolysis at different temperatures, while pore sizes can be tailored by varying ISO molar mass. A transfer technique between substrates is demonstrated for alternating gyroidal mesoporous thin films, circumventing the need to re-optimize film formation protocols for different substrates. Increased conductivity after pyrolysis at high temperatures demonstrates that these gyroidal mesoporous resin/carbon thin films have potential as functional 3D templates for a number of nanomaterials applications.

Pathway-engineering for highly-aligned block copolymer arrays

While the ultimate driving force in self-assembly is energy minimization and the corresponding evolution towards equilibrium, kinetic effects can also play a very strong role. These kinetic effects, such as trapping in metastable states, slow coarsening kinetics, and pathway-dependent assembly, are often viewed as complications to be overcome. Here, we instead exploit these effects to engineer a desired final nano-structure in a block copolymer thin film, by selecting a particular ordering pathway through the self-assembly energy landscape. In particular, we combine photothermal shearing with high-temperature annealing to yield hexagonal arrays of block copolymer cylinders that are aligned in a single prescribed direction over macroscopic sample dimensions. Photothermal shearing is first used to generate a highly-aligned horizontal cylinder state, with subsequent thermal processing used to reorient the morphology to the vertical cylinder state in a templated manner. Finally, we demonstrate the successful transfer of engineered morphologies into inorganic replicas.

Interfacial Energy-Controlled Top Coats for Gyroid/Cylinder Phase Transitions of Polystyrene-block-polydimethylsiloxane Block Copolymer Thin Films

Block copolymers (BCPs) with a high Flory-Huggins interaction parameter (χ) can form well-defined sub-10 nm periodic structures and can be used as a template for fabrication of various functional nanostructures. However, the large difference of surface energy between the blocks commonly found in high-χ BCPs makes it challenging to stabilize a useful gyroid morphology in thin film form. Here, we used an interfacial-energy-tailored top-coat on a blended film of a polystyrene-block-polydimethylsiloxane (PS-b-PDMS) BCP and a low-molecular-weight PDMS homopolymer with a hydrophilic end functional group. The top coat consisted of a random mixture of 40% hydrolyzed poly(vinyl acetate)-random-poly(vinly alcohol) (PVA-r-PVAc, PVA40) and PVAc homopolymer. At the optimized top-coat composition, gyroid nanostructures with sub-10 nm strut width were achieved down to ∼125 nm film thickness, which is only 3 times the lattice parameter of the gyroid structure. This is in marked contrast with a mixed morphology of gyroid and cylinders obtained for other compositions of the top coat. Self-consistent field theoretic simulations were used to understand the effect of the interfacial energy between the top coat and BCP/homopolymer blends on the phase transition behavior of the BCP/homopolymer films.

Morphology, directed self-assembly and pattern transfer from a high molecular weight polystyrene-block-poly(dimethylsiloxane) block copolymer film

The self-assembly of block copolymers with large feature sizes is inherently challenging as the large kinetic barrier arising from chain entanglement of high molecular weight (MW) polymers limits the extent over which long-range ordered microdomains can be achieved. Here, we illustrate the evolution of thin film morphology from a diblock copolymer of polystyrene-block-poly(dimethylsiloxane) exhibiting total number average MW of 123 kg mol^-1, and demonstrate the formation of layers of well-ordered cylindrical microdomains under appropriate conditions of binary solvent mix ratio, commensurate film thickness, and solvent vapor annealing time. Directed self-assembly of the block copolymer within lithographically patterned trenches occurs with alignment of cylinders parallel to the sidewalls. Fabrication of ordered cobalt nanowire arrays by pattern transfer was also implemented, and their magnetic properties and domain wall behavior were characterized.

UV-solvent annealing of PDMS-majority and PS-majority PS-b-PDMS block copolymer films

The response of polystyrene-block-poly(dimethylsiloxane) (PS-b-PDMS) thin films to UV exposure during solvent vapor annealing is described, in order to improve their applicability in nanolithography and nanofabrication. Two BCPs were examined, one with the PS block as majority (f _PS = 68%, M _n = 53 kg mol^-1), the other with PDMS block as majority (f _PDMS = 67%, M _n = 44 kg mol^-1). A 5 min UV irradiation was applied during solvent vapor annealing which led to both partial crosslinking of the polymer and a small increase in the temperature of the annealing chamber. This approach was effective for improving the correlation length of the self-assembled microdomain arrays and in limiting subsequent flow of the PDMS in the PDMS-majority BCP to preserve the post-anneal morphology. Ordering and orientation of microdomains were controlled by directed self-assembly of the BCPs in trench substrates. Highly-ordered perpendicular nanochannel arrays were obtained in the PDMS-majority BCP.

Influence of film structure on the dewetting kinetics of thin polymer films in the solvent annealing process

On a non-wetting solid substrate, the solvent annealing process of a thin polymer film includes the swelling process and the dewetting process. Owing to difficulties in the in situ analysis of the two processes simultaneously, a quantitative study on the solvent annealing process of thin polymer films on the non-wetting solid substrate is extremely rare. In this paper, we design an experimental method by combining spectroscopic ellipsometry with optical microscopy to achieve the simultaneous in situ study. Using this method, we investigate the influence of the structure of swollen film on its dewetting kinetics during the solvent annealing process. The results show that for a thin PS film with low Mw (Mw = 4.1 kg mol(-1)), acetone molecules can form an ultrathin enriched layer between the PS film and the solid substrate during the swelling process. The presence of the acetone enriched layer accounts for the exponential kinetic behavior in the case of a thin PS film with low Mw. However, the acetone enriched layer is not observed in the case of a thin PS film with high Mw (Mw = 400 kg mol(-1)) and the slippage effect of polymer chains is valid during the dewetting process.