Diblock Copolymer Thin Films Research Articles

Ionic liquid mediated one-step perpendicular assembly in block copolymer thin films for ultrafiltration membrane applications

The rapid rise of oil and gas, petrochemical, food processing, and pharmaceutical industries has added a complex mixture of contaminants like oil, particulates, metals, and other organic compounds to wastewater. Multipurpose membranes with precise pore structure can offer a solution to this problem and block copolymers (BCP) can be used to achieve such structures. Achieving vertical assembly of BCP domains provides a perfect template for developing uniform through film channels for separation and transport of material, besides being useful for the rectification of defects in photolithography patterns. Producing such morphologies may require extensive film/substrate processing and is not always feasible for scale-up. With this article, we demonstrate a facile solution casting method to induce vertical domain assembly in as-cast diblock copolymer thin films in the presence of an ionic liquid (IL) additive that preferentially segregates to one block and neutralizes interfacial interactions. We also show the tunability of domain sizes by controlling the additive concentration. These vertically aligned morphologies are important for the development of next-generation lithographic techniques and form excellent templates for ultrafiltration membranes with uniform pore sizes. This article demonstrates how IL additives can be used to obtain stable non-equilibrium morphologies in as-cast BCP films and the effect of BCP molecular mass, block volume fractions, and IL content on self-assembly.

Bayesian model calibration for block copolymer self-assembly: Likelihood-free inference and expected information gain computation via measure transport

We consider the Bayesian calibration of models describing the phenomenon of block copolymer (BCP) self-assembly, which is to infer model parameters and their uncertainty from noisy image data produced by microscopy or X-ray scattering characterization of BCP structures. To account for the long-range disorder in BCP structures, we introduce auxiliary variables in the model to represent this aleatory uncertainty. These variables, however, result in an integrated likelihood function for high-dimensional image data that is generally intractable to evaluate. We tackle this Bayesian inference problem using a likelihood-free inference (LFI) approach based on measure transport together with the construction of summary statistics for the image data. We show that expected information gains (EIG) from observed data about the model parameters can be computed with no significant additional cost. Lastly, we present a numerical case study using the Ohta–Kawasaki model for diblock copolymer thin film self-assembly and top-down microscopy characterization. We introduce several domain-specific energy and Fourier-based summary statistics for calibration and quantify their informativeness using EIG. The effect of various data corruptions, summary statistics, and experimental designs on the calibration results are studied using the proposed LFI method.

Bayesian model calibration for diblock copolymer thin film self-assembly using power spectrum of microscopy data and machine learning surrogate

Identifying parameters of computational models from experimental data, or model calibration, is fundamental for assessing and improving the predictability and reliability of computer simulations. In this work, we propose a method for Bayesian calibration of models that predict morphological patterns of diblock copolymer (Di-BCP) thin film self-assembly while accounting for various sources of uncertainties in pattern formation and data acquisition. This method extracts the azimuthally-averaged power spectrum (AAPS) of the top-down microscopy characterization of Di-BCP thin film patterns as summary statistics for Bayesian inference of model parameters via the pseudo-marginal method. We derive the analytical and approximate form of a conditional likelihood for the AAPS of image data. We demonstrate that AAPS-based image data reduction retains the mutual information, particularly on important length scales, between image data and model parameters while being relatively agnostic to the aleatoric uncertainties associated with the random long-range disorder of Di-BCP patterns. Additionally, we propose a phase-informed prior distribution for Bayesian model calibration. Furthermore, reducing image data to AAPS enables us to efficiently build surrogate models to accelerate the proposed Bayesian model calibration procedure. We present the formulation and training of two multi-layer perceptrons for approximating the parameter-to-spectrum map, which enables fast integrated likelihood evaluations. We validate the proposed Bayesian model calibration method through numerical examples, for which the neural network surrogate delivers a fivefold reduction of the number of model simulations performed for a single calibration task.

KCl-Modulated Hydration and Subsequent Thermoresponsive Behavior of Poly(sulfobetaine)-Based Diblock Copolymer Thin Films

The KCl-modulated swelling of double thermoresponsive diblock copolymer (DBC) thin films in D2O atmosphere and their subsequent responsive behavior are investigated via spectral reflectance (SR), in situ time-of-flight neutron reflectometry (ToF-NR), and Fourier-transform infrared (FT-IR) spectroscopy. The copolymer consists of a short zwitterionic (poly(4-(N-(3-methacrylamidopropyl)-N,N-dimethylammonio) butane-1-sulfonate)) (PSBP) block and a long non-ionic poly(N-isopropylmethacrylamide) (PNIPMAM) block. DBC thin films are prepared via spin-coating from solutions containing 5 mM or devoid of KCl. The addition of KCl to the DBC thin films leads to a higher swelling ratio and D2O content during vapor treatment with D2O. Upon the subsequent heating of the hydrated DBC films, the films swell further and reach a maximum thickness before contracting. Two separate volume phase transition temperatures (VPTTs) are observed, namely where a further swelling plateau is reached (VPTTfs), and where contraction starts (VPTTc). Based on complementary SR studies of the effect of KCl on the swelling behavior of the respective PSBP and PNIPMAM homopolymer thin films, we conclude that the “further-swelling” period is mainly a consequence of the UCST-type phase transition of PSBP, whereas the “film contraction” period is due to the LCST-type phase transition of PNIPMAM in thin-film geometry. We observe that KCl reduces the VPTTc of the PNIPMAM blocks. Moreover, the salt migrates or aggregates inside the thin film upon heating, thereby forming a KCl enrichment layer in the intermediate section of the film. Furthermore, the observations by FT-IR prove that macroscopic and mesoscopic D2O absorption and desorption are correlated with the effect of KCl on hydration and de-hydration of the hydrophilic groups.

Interface Manipulations Using Cross-Linked Underlayers and Surface-Active Diblock Copolymers to Extend Morphological Diversity in High-χ Diblock Copolymer Thin Films

Top and bottom interfaces of high-χ cylinder-forming polystyrene-block-maltoheptaose (PS-b-MH) diblock copolymer (BCP) thin films are manipulated using cross-linked copolymer underlayers and a fluorinated phase-preferential surface-active polymer (SAP) additive to direct the self-assembly (both morphology and orientation) of BCP microdomains into sub-10 nm patterns. A series of four photo-cross-linkable statistical copolymers with various contents of styrene, a 4-vinylbenzyl azide cross-linker, and a carbohydrate-based acrylamide are processed into 15 nm-thick cross-linked passivation layers on silicon substrates. A partially fluorinated analogue of the PS-b-MH phase-preferential SAP additive is designed to tune the surface energy of the top interface. The self-assembly of PS-b-MH thin films on top of different cross-linked underlayers and including 0-20 wt % of SAP additive is investigated by atomic force microscopy and synchrotron grazing incidence small-angle X-ray scattering analysis. The precise manipulation of the interfaces of ca. 30 nm thick PS-b-MH films not only allows the control of the in-plane/out-of-plane orientation of hexagonally packed (HEX) cylinders but also promotes epitaxial order-order transitions from HEX cylinders to either face-centered orthorhombic or body-centered cubic spheres without modifying the volume fraction of both blocks. This general approach paves the way for the controlled self-assembly of other high-χ BCP systems.

Controlled Orientation of Silicon-Containing Diblock Copolymer Thin Films by Substrate Functionalization Under Vacuum

This work demonstrates a simple approach to control the orientation of the self-assembled nanostructured block copolymer thin films of polystyrene-block-polydimethylsiloxane (PS-b-PDMS) by functionalization of the oxide layer (SiO2) on the Si substrate followed by thermal annealing under low-pressure environmental conditions. The substrate can be functionalized through two-step grafting of hydroxy-terminated polystyrene brush (PS–OH brush) followed by hydroxy-terminated polydimethylsiloxane brush (PDMS–OH brush) onto the wafer substrate. By controlling the grafting ratio of PS–OH and PDMS–OH brushes, the affinities of the PS and PDMS blocks with the substrates can be fine-tuned to provide a neutral substrate in order to form perpendicular cylinders from the bottom after thermal annealing. Owing to the vacuum-driven orientation [i.e., thermal annealing under low-pressure environment conditions (∼10–4 Pa)], the orientation of the cylinders can be controlled at the air/polymer interface. Interestingly, by combining the vacuum-driven approach with substrate functionalization, perpendicular cylinders from the air/polymer interface and substrate/polymer interface can be generated, respectively. Consequently, well-aligned perpendicular cylinders with long-range ordering can be fabricated by the self-alignment process.

Extraction of interaction parameters from specular neutron reflectivity in thin films of diblock copolymers: an "inverse problem".

Diblock copolymers have been shown to undergo microphase separation due to an interplay of repulsive interactions between dissimilar monomers, which leads to the stretching of chains and entropic loss due to the stretching. In thin films, additional effects due to confinement and monomer-surface interactions make microphase separation much more complicated than in that in bulks (i.e., without substrates). Previously, physics-based models have been used to interpret and extract various interaction parameters from the specular neutron reflectivities of annealed thin films containing diblock copolymers (J. P. Mahalik, J. W. Dugger, S. W. Sides, B. G. Sumpter, V. Lauter and R. Kumar, Interpreting neutron reflectivity profiles of diblock copolymer nanocomposite thin films using hybrid particle-field simulations, Macromolecules, 2018, 51(8), 3116; J. P. Mahalik, W. Li, A. T. Savici, S. Hahn, H. Lauter, H. Ambaye, B. G. Sumpter, V. Lauter and R. Kumar, Dispersity-driven stabilization of coexisting morphologies in asymmetric diblock copolymer thin films, Macromolecules, 2021, 54(1), 450). However, extracting Flory-Huggins χ parameters characterizing monomer-monomer, monomer-substrate, and monomer-air interactions has been labor-intensive and prone to errors, requiring the use of alternative methods for practical purposes. In this work, we have developed such an alternative method by employing a multi-layer perceptron, an autoencoder, and a variational autoencoder. These neural networks are used to extract interaction parameters not only from neutron scattering length density profiles constructed using self-consistent field theory-based simulations, but also from a noisy ad hoc model constructed previously. In particular, the variational autoencoder is shown to be the most promising tool when it comes to the reconstruction and extraction of parameters from an ad hoc neutron scattering length density profile of a thin film containing a symmetric di-block copolymer (poly(deuterated styrene-b-n-butyl methacrylate)). This work paves the way for automated analysis of specular neutron reflectivities from thin films of copolymers using machine learning tools.

Failure and Mechanical Properties of Glassy Diblock Copolymer Thin Films

Block copolymer self-assembly is affected by nanoscale confinement, which has long been known to affect interchain entanglements and dynamics of polymers. While most previous work on confined polymer glasses has focused on the properties of homopolymers, the mechanical response of glassy block copolymer thin films is still relatively unexplored. By uniaxially deforming glassy lamellar diblock copolymer films with different morphologies via molecular dynamic simulations, we demonstrate that the toughness of the films with fingerprint morphologies is higher compared to homopolymers and oriented lamellar films due to the increase in the randomness of domain orientations and entanglements. We show that the thickness impact on the mechanical properties of the block copolymers is not as big as that of the homopolymer systems. In the strain localization analysis of the block copolymer films, there are the plastic rearrangements initially clustered at the boundary between the two phases of the lamellae until close to failure when the plasticity transitions to the center of a domain. In the block copolymer systems, crazes in the thinnest films exhibit distinct behaviors compared to thicker films. Our studies of the film mechanics provide molecular insights into how segmental mobility and entanglements interplay with position and morphology to control the mechanics of thin polymer films.

Kinetics of Water Transfer Between the LCST and UCST Thermoresponsive Blocks in Diblock Copolymer Thin Films Monitored by In Situ Neutron Reflectivity

AbstractThe kinetics of water transfer between the lower critical solution temperature (LCST) and upper critical solution temperature (UCST) thermoresponsive blocks in about 10 nm thin films of a diblock copolymer is monitored by in situ neutron reflectivity. The UCST‐exhibiting block in the copolymer consists of the zwitterionic poly(4‐((3‐methacrylamidopropyl)dimethylammonio)butane‐1‐sulfonate), abbreviated as PSBP. The LCST‐exhibiting block consists of the nonionic poly(N‐isopropylacrylamide), abbreviated as PNIPAM. The as‐prepared PSBP80‐b‐PNIPAM400 films feature a three‐layer structure, i.e., PNIPAM, mixed PNIPAM and PSBP, and PSBP. Both blocks have similar transition temperatures (TTs), namely around 32 °C for PNIPAM, and around 35 °C for PSBP, and with a two‐step heating protocol (20 °C to 40 °C and 40 °C to 80 °C), both TTs are passed. The response to such a thermal stimulus turns out to be complex. Besides a three‐step process (shrinkage, rearrangement, and reswelling), a continuous transfer of D2O from the PNIPAM to the PSBP block is observed. Due to the existence of both, LCST and UCST blocks in the PSBP80‐b‐PNIPAM400 film, the water transfer from the contracting PNIPAM, and mixed layers to the expanding PSBP layer occurs. Thus, the hydration kinetics and thermal response differ markedly from a thermoresponsive polymer film with a single LCST transition.

Area Quasi-minimizing Partitions with a Graphical Constraint: Relaxation and Two-Dimensional Partial Regularity

We consider a variational model for periodic partitions of the upper half-space into three regions, where two of them have prescribed volume and are subject to the geometric constraint that their union is the subgraph of a function, whose graph is a free surface. The energy of a configuration is given by the weighted sum of the areas of the interfaces between the different regions and a general volume-order term. We establish existence of minimizing configurations via relaxation of the energy involved, in any dimension. Moreover, we prove partial regularity results for volume-constrained minimizers in two space dimensions. Thin films of diblock copolymers are a possible application and motivation for considering this problem.

Vacuum-Driven Orientation of Nanostructured Diblock Copolymer Thin Films.

This work aims to demonstrate a facile method for the controlled orientation of nanostructures of block copolymer (BCP) thin films. A simple diblock copolymer system, polystyrene-block-polydimethylsiloxane (PS-b-PDMS), is chosen to demonstrate vacuum-driven orientation for solving the notorious low-surface-energy problem of silicon-based BCP nanopatterning. By taking advantage of the pressure dependence of the surface tension of polymeric materials, a neutral air surface for the PS-b-PDMS thin film can be formed under a high vacuum degree (∼10-4 Pa), allowing the formation of the film-spanning perpendicular cylinders and lamellae upon thermal annealing. In contrast to perpendicular lamellae, a long-range lateral order for forming perpendicular cylinders can be efficiently achieved through the self-alignment mechanism for induced ordering from the top and bottom of the free-standing thin film.

Poly(sulfobetaine)-Based Diblock Copolymer Thin Films in Water/Acetone Atmosphere: Modulation of Water Hydration and Co-nonsolvency-Triggered Film Contraction.

The water swelling and subsequent solvent exchange including co-nonsolvency behavior of thin films of a doubly thermo-responsive diblock copolymer (DBC) are studied via spectral reflectance, time-of-flight neutron reflectometry, and Fourier transform infrared spectroscopy. The DBC consists of a thermo-responsive zwitterionic (poly(4-((3-methacrylamidopropyl) dimethylammonio) butane-1-sulfonate)) (PSBP) block, featuring an upper critical solution temperature transition in aqueous media but being insoluble in acetone, and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block, featuring a lower critical solution temperature transition in water, while being soluble in acetone. Homogeneous DBC films of 50-100 nm thickness are first swollen in saturated water vapor (H2O or D2O), before they are subjected to a contraction process by exposure to mixed saturated water/acetone vapor (H2O or D2O/acetone-d6 = 9:1 v/v). The affinity of the DBC film toward H2O is stronger than for D2O, as inferred from the higher film thickness in the swollen state and the higher absorbed water content, thus revealing a pronounced isotope sensitivity. During the co-solvent-induced switching by mixed water/acetone vapor, a two-step film contraction is observed, which is attributed to the delayed expulsion of water molecules and uptake of acetone molecules. The swelling kinetics are compared for both mixed vapors (H2O/acetone-d6 and D2O/acetone-d6) and with those of the related homopolymer films. Moreover, the concomitant variations of the local environment around the hydrophilic groups located in the PSBP and PNIPMAM blocks are followed. The first contraction step turns out to be dominated by the behavior of the PSBP block, whereas the second one is dominated by the PNIPMAM block. The unusual swelling and contraction behavior of the latter block is attributed to its co-nonsolvency behavior. Furthermore, we observe cooperative hydration effects in the DBC films, that is, both polymer blocks influence each other's solvation behavior.

Effects of local order parameter dependent transport coefficient in diblock copolymers under applied electric fields.

We present an approach for constructing thermodynamically consistent time-dependent models relevant to thin films of diblock copolymers in applied electric fields. The approach is based on the principles of linear irreversible thermodynamics, and, in this work, it is applied to study the effects of electric fields on thin films of incompressible diblock copolymers. Enforcement of local incompressibility constraint at all times leads to a local order parameter dependent transport coefficient in the model for the diblock copolymers. The dependence of the transport coefficient on the local order parameter is used to relate it with the diffusion constant of Rouse chains and leads to sensitivity of the model to initial conditions. In addition, transient behavior is found to be affected when compared with an ad hoc model assuming a constant transport coefficient. Numerical results such as electric field induced alignment of lamellae domains due to the field are found to be in qualitative agreement with experiments. This approach opens up a systematic way of developing kinetic models for simulating effects of electrolytes added to thin films containing diblock copolymers in the presence of applied electric fields.

Combining Particle-Based Simulations and Machine Learning to Understand Defect Kinetics in Thin Films of Symmetric Diblock Copolymers

The self-assembly of soft matter provides a practical and scalable route toward the production of nanostructured materials, with minimal need for direct intervention at nanoscopic length scales. Symmetric diblock copolymers, which can self-assemble into a lamellar phase, are a prototype for this class of materials. In this work, we introduce a machine learning model that is trained by intermediate time-scale simulations of a soft, coarse-grained model. The aim of the model is to simulate defect kinetics in the lamellar morphology, as the material relaxes toward equilibrium. To do so, we exploit the physical characteristics of overdamped dynamics and formulate the problem of time evolution as a Markov chain. The trained artificial neural network (ANN) predicts a time-independent transition probability from one time step to the next. As a result, we arrive at a method that can be repeatedly applied to generate long-time trajectories. Predicting defect kinetics in this manner provides hitherto unavailable insights into the late-time dynamics of block copolymer relaxation. The neural network is purposely designed to be independent of input size, which enables training on small systems, and enabling predictions over large scales. As a demonstration of these capabilities, in this work, we leverage the ANN to obtain information about the statistics of defect motion and lifetimes over a long-range ordering process.

Micro and Nanopatterned Ion Exchange Membranes for Applications in Electrochemical Devices

Topographical patterns on ion exchange membranes’ (IEMs’) surfaces have been show to improve the performance and energy efficiency in electrochemical energy conversion and water treatment processes. Substituting flat IEMs with patterned/profiled IEMs in electrodialysis reduces spacer channel mass transfer limitations by increasing the interfacial area between the liquid solution and membrane materials. Additionally, the extended surface of the profiled cation-exchange and anion-exchange membranes augment the spacer channel conductivity without obfuscating bulk liquid flow. Topographically patterned ion exchange membranes are fabricated using several methodologies that include photo-/soft-lithography, hot pressing, solution casting and 3D printing. The minimum lateral feature size that can be obtained by these methods is practically on the order of a few microns and potentially could be as low as few hundred nanometers. Block copolymer (BCP) lithography is a nanopatterning platform that generates lateral feature sizes of 5 to 50 nm. Thin films of diblock copolymers can self-assemble on a neutral layer resulting in lamella, cylinder, or sphere morphologies when the film is thermal or solvent vapor annealed. The morphology of the film primarily depends on volume fraction (fA, fB) of each block and interaction parameter between the blocks (χ). Poly(styrene-block-methylmethacrylate) (PS-b-PMMA) is one of the most studied block copolymer for BCP lithography because it form perpendicular aligned nanostructures via thermal annealing without topcoat layers to control the free surface interfacial energy. The etch contrast between the styrene and MMA blocks also makes it easier to transfer the pattern onto substrates – especially if sequential infiltration synthesis (SIS) is used.In this work, for the first time, a method to prepare nanopatterned IEMs using BCP lithography is reported. The SIS process post PS-b-PMMA self-assembly converts MMA into alumina creating organic-inorganic hard mask. The styrene block is removed via oxygen plasma in a reactive ion-etcher leaving behind a nanoporous alumina template on a silicon wafer. Sulfonated poly(arylene ether ether ketone) (SPEEK) is dropcasted onto the nanoporous alumina and the solvent is evaporated. Once releasing the SPEEK from the substrate, a nanopatterned cation-exchange membrane is attained. The electrochemical properties of the nanopatterned membrane such as conductivity, permselectivity and transference number are measured and compared to a flat, non-patterned membrane. The efficiency of these nanopatterned membranes for simple electrochemical devices, like a hydrogen pump, will also be presented.This work is supported from NSF Award # 1703307. Figure 1

Salt-Dependent Phase Transition Behavior of Doubly Thermoresponsive Poly(sulfobetaine)-Based Diblock Copolymer Thin Films.

The water vapor-induced swelling, as well as subsequent phase-transition kinetics, of thin films of a diblock copolymer (DBC) loaded with different amounts of the salt NaBr, is investigated in situ. In dilute aqueous solution, the DBC features an orthogonally thermoresponsive behavior. It consists of a zwitterionic poly(sulfobetaine) block, namely, poly(4-(N-(3'-methacrylamidopropyl)-N,N-dimethylammonio) butane-1-sulfonate) (PSBP), showing an upper critical solution temperature, and a nonionic block, namely, poly(N-isopropylmethacrylamide) (PNIPMAM), exhibiting a lower critical solution temperature. The swelling kinetics in D2O vapor at 15 °C and the phase transition kinetics upon heating the swollen film to 60 °C and cooling back to 15 °C are followed with simultaneous time-of-flight neutron reflectometry and spectral reflectance measurements. These are complemented by Fourier transform infrared spectroscopy. The collapse temperature of PNIPMAM and the swelling temperature of PSBP are found at lower temperatures than in aqueous solution, which is attributed to the high polymer concentration in the thin-film geometry. Upon inclusion of sub-stoichiometric amounts (relative to the monomer units) of NaBr in the films, the water incorporation is significantly increased. This increase is mainly attributed to a salting-in effect on the zwitterionic PSBP block. Whereas the addition of NaBr notably shifts the swelling temperature of PSBP to lower temperatures, the collapse temperature of PNIPMAM remains unaffected by the presence of salt in the films.

Orthogonal Nanoprobes Enabling Two-Color Optical Super-Resolution Microscopy Imaging of the Two Domains of Diblock Copolymer Thin Film Nanocomposites

Orthogonal Nanoprobes Enabling Two-Color Optical Super-Resolution Microscopy Imaging of the Two Domains of Diblock Copolymer Thin Film Nanocomposites

Electromechanical response of lamellar forming ionic diblock copolymer thin films

Electromechanical responses of lamellar forming neutral-charged diblock copolymers (BCPs) in thin films are investigated using coarse-grained molecular dynamics simulations, where the electric permittivity of polymer monomer is described by explicitly introducing freely rotating dipoles. Effects of an applied electric field on the ion transport, morphological change and stress build-up are studied in detail. In particular, we focus on the responses to strong electric fields in terms of the ion dissociation and stress distribution along the film thickness. The different stages of BCP evolution under the electric field are characterized. This work paves a way towards establishing a modeling scheme that describes the response of ionic polymers in the presence of applied electric field.

Surface properties of thin films of poly(2,3,4,5,6-pentafluorostyrene) and diblock copolymers based on 2,3,4,5,6-pentafluorostyrene and 2-hydroxyethyl methacrylate

Surface properties of thin films of poly(2,3,4,5,6-pentafluorostyrene) and diblock copolymers based on 2,3,4,5,6-pentafluorostyrene and 2-hydroxyethyl methacrylate

Dispersity-Driven Stabilization of Coexisting Morphologies in Asymmetric Diblock Copolymer Thin Films

Despite decades of research, characterization of the effects of polymer chain dispersity on the structural properties of block copolymer thin films remains challenging. We present an integrated exp...