Solvent Retention in Thin Spin-Coated Polystyrene and Poly(methyl methacrylate) Homopolymer Films Studied By Neutron Reflectometry

Responsive blends of block copolymers stabilize the hexagonally perforated lamellae morphology

Blends of block copolymers can form phases and exhibit features distinct from the constituent materials. We study thin film blends of cylinder-forming and lamellar-forming block copolymers across a range of substrate surface energies. Blend materials are responsive to interfacial energy, allowing selection of pure or coexisting phases based on surface chemistry. Blending stabilizes certain motifs that are typically metastable, and can be used to generate pure hexagonally perforated lamellar thin films across a range of film thicknesses and surface energies. This tolerant behavior is ascribed to the ability of blend materials to redistribute chains to stabilize otherwise high-energy defect structures. The blend responsiveness allows the morphology to be spatially defined through multi-tone chemical surface patterns.

Solvent-assisted self-assembly of block copolymer thin films

Solvent-assisted block copolymer self-assembly is a compelling method for processing and advancing practical applications of these materials due to the exceptional level of the control of BCP morphology and significant acceleration of ordering kinetics. Despite substantial experimental and theoretical efforts devoted to understanding of solvent-assisted BCP film ordering, the development of a universal BCP patterning protocol remains elusive; possibly due to a multitude of factors which dictate the self-assembly scenario. The aim of this review is to aggregate both seminal reports and the latest progress in solvent-assisted directed self-assembly and to provide the reader with theoretical background, including the outline of BCP ordering thermodynamics and kinetics phenomena. We also indicate significant BCP research areas and emerging high-tech applications where solvent-assisted processing might play a dominant role.

Thermally assisted nanotransfer printing with sub-20-nm resolution and 8-inch wafer scalability

Nanotransfer printing (nTP) has attracted considerable attention due to its good pattern resolution, process simplicity, and cost-effectiveness. However, the development of a large-area nTP process has been hampered by critical reliability issues related to the uniform replication and regular transfer printing of functional nanomaterials. Here, we present a very practical thermally assisted nanotransfer printing (T-nTP) process that can easily produce well-ordered nanostructures on an 8-inch wafer via the use of a heat-rolling press system that provides both uniform pressure and heat. We also demonstrate various complex pattern geometries, such as wave, square, nut, zigzag, and elliptical nanostructures, on diverse substrates via T-nTP. Furthermore, we demonstrate how to obtain a high-density crossbar metal-insulator-metal memristive array using a combined method of T-nTP and directed self-assembly. We expect that the state-of-the-art T-nTP process presented here combined with other emerging patterning techniques will be especially useful for the large-area nanofabrication of various devices.

Temperature-Dependent Spectroscopic Ellipsometry of Thin Polymer Films

Thin polymer films have found many important applications in organic electronics, such as active layers, protective layers, or antistatic layers. Among the various experimental methods suitable for studying the thermo-optical properties of thin polymer films, temperature-dependent spectroscopic ellipsometry plays a special role as a nondestructive and very sensitive optical technique. In this Review Article, issues related to the physical origin of the dependence of ellipsometric angles on temperature are surveyed. In addition, the Review Article discusses the use of temperature-dependent spectroscopic ellipsometry for studying phase transitions in thin polymer films. The benefits of studying thermal transitions using different cooling/heating speeds are also discussed. Furthermore, it is shown how the analysis and modeling of raw ellipsometric data can be used to determine the thermal properties of thin polymer films.

Swelling, dewetting and breakup in thin polymer films for cultural heritage

The removal of ultrathin amorphous polymer films in contact with nonsolvent/solvent binary mixtures is addressed by means of neutron reflectometry and atomic force microscopy. The high resolution of neutron scattering makes it possible to resolve the distribution profiles of heavy water and benzyl alcohol inside Laropal®A81, often employed as a protective varnish layer for Culture Heritage in restoration of easel paintings. The swelling kinetics and distribution profiles were recorded as a function of time and increasing benzyl alcohol concentration in water. The varnish film swells by penetration of the good solvent. At higher concentrations water-filled cavities appear inside the varnish and grow with time. Contrary to homogeneous dissolution dewetting is observed at late stages of exposure to the liquid which leads to the Breakup of the film. The high resolution measurements are compared to bulk behaviour characterized by the ternary phase diagram and the Flory-Huggins interaction parameters are calculated and used to predict the swelling and solvent partition in the films. Distinct differences of the thin film to bulk behaviour are found. The expectations made previously for the behaviour of solvent/non-solvent mixtures on the removal of thin layers in the restoration of easel paintings should be revised in view of surface interactions.

Nanoscale viscosity of confined polyethylene oxide

Complex fluids near interfaces or confined within nanoscale volumes can exhibit substantial shifts in physical properties compared to bulk, including glass transition temperature, phase separation, and crystallization. Because studies of these effects typically use thin film samples with one dimension of confinement, it is generally unclear how more extreme spatial confinement may influence these properties. In this work, we used x-ray photon correlation spectroscopy and gold nanoprobes to characterize polyethylene oxide confined by nanostructured gratings (<100nm width) and measured the viscosity in this nanoconfinement regime to be ∼500 times the bulk viscosity. This enhanced viscosity occurs even when the scale of confinement is several times the polymer's radius of gyration, consistent with previous reports of polymer viscosity near flat interfaces.

Temperature-Controlled Solvent Vapor Annealing of Thin Block Copolymer Films

Solvent vapor annealing is as an effective and versatile alternative to thermal annealing to equilibrate and control the assembly of polymer chains in thin films. Here, we present scientific and practical aspects of the solvent vapor annealing method, including the discussion of such factors as non-equilibrium conformational states and chain dynamics in thin films in the presence of solvent. Homopolymer and block copolymer films have been used in model studies to evaluate the robustness and the reproducibility of the solvent vapor processing, as well as to assess polymer-solvent interactions under confinement. Advantages of utilizing a well-controlled solvent vapor environment, including practically interesting regimes of weakly saturated vapor leading to poorly swollen states, are discussed. Special focus is given to dual temperature control over the set-up instrumentation and to the potential of solvo-thermal annealing. The evaluated insights into annealing dynamics derived from the studies on block copolymer films can be applied to improve the processing of thin films of crystalline and conjugated polymers as well as polymer composite in confined geometries.

Synergic Swelling of Interactive Network Support and Block Copolymer Films during Solvent Vapor Annealing

We report the effect of "interactive" polymer network (PN) supports on the solvent-vapor processing of thin polymer films. Densely cross-linked surface-attached network exhibits under experimental time scale a glassy swelling behavior with the conformational states and solvent-uptake clearly sensitive to the degree of solvent vapor saturation in the atmosphere. Pretreatment of the thermally cured PN films by complete immersion or by swelling in saturated chloroform vapors facilitates relaxation of the residual stresses and induces irreversible changes to the network structure as revealed by the swelling/deswelling tests. The presence of a polymer film on top of the PN support results in a mutual influence of the layers on the respective swelling kinetics, steady-state solvent uptake, and chain dynamics. Using UV-vis ellipsometry, we revealed a significantly faster swelling and higher solvent uptake of glassy PN layer below a polymer film as compared to a single PN layer on silicon substrate. Remarkably, the swelling of the network support continues to increase even when the overall swelling of the bilayer is in a steady-state regime. Block copolymer films on PN supports exhibit a faster ordering dynamics and exceptional stability toward dewetting as compared to similar films on silicon wafers. The mechanical stress produced by continuously swelling PN is suggested to account for the enhanced segmental dynamics even at low solvent concentration in the block copolymer film. Apart from novel insights into dynamics of solvent uptake by heterogeneous polymer films, these results might be useful in developing novel approaches toward fast-processing/annealing of functional polymer films and fibers.

Ellipsometry-based combination of isothermal sorption-desorption measurement and temperature programmed desorption technique: A probe for interaction of thin polymer films with solvent vapor

An environmental chamber equipped with an in situ spectroscopic ellipsometer, programmatic vapor pressure control, and variable temperature substrate holder has been designed for studying polymer coating behavior during an exposure to a solvent vapor and also for probing the residual solvent in the film afterwards. Both sorption-desorption cycle at a constant temperature and temperature programmed desorption (TPD) of the residual solvent manifest themselves as a change of the film thickness. Monitoring of ellipsometric angles of the coating allows us to determine the thickness as a function of the vapor pressure or sample temperature. The solvent vapor pressure is precisely regulated by a computer-controlled pneumatics. TPD spectra are recorded during heating of the film in an oil-free vacuum. The vapor pressure control system is described in detail. The system has been tested on 6-170 nm thick polystyrene, poly(methyl methacrylate), and poly(2-vinyl pyridine) films deposited on silicon substrates. Liquid toluene, water, ethanol, isopropanol, cyclohexane, 1,2-dichloroethane, and chlorobenzene were used to create a vapor atmosphere. Typical sorption-desorption and TPD curves are shown. The instrument achieves sub-monolayer sensitivity for adsorption studies on flat surfaces. Polymer-solvent vapor systems with strong interaction demonstrate characteristic absorption-desorption hysteresis spanning from vacuum to the glass transition pressure. Features on the TPD curves can be classified as either glass transition related film contraction or low temperature broad contraction peak. Typical absorption-desorption and TPD dependencies recorded for the 6 nm thick polystyrene film demonstrate the possibility to apply the presented technique for probing size effects in extremely thin coatings.

Experimental investigation of the glass transition of polystyrene thin films in a broad frequency range

In this study, we investigate the α process of a polystyrene thin film using inelastic neutron scattering (INS), dielectric relaxation spectroscopy (DRS), and thermal expansion spectroscopy (TES). The DRS and TES measurements exhibited a decrease in glass transition temperature (T_{g}) with film thickness. On the other hand, an increase in T_{g} was observed in INS studies. In order to interpret this contradiction, we investigated the temperature dependence of the peak frequency (f_{m}) of the α process probed by DRS and TES. The experiments revealed an increase in the peak frequency (f_{m}) with decreasing film thickness in the frequency region. This observation is consistent with the observed decrease in T_{g} with thickness. Interestingly, the increase in T_{g} with film thickness was confirmed by fitting the temperature dependence measurements of the peak frequency with the Vogel-Fulcher-Tammann equation, within the frequency region probed by INS. The discrepancy between INS and DRS or TES descriptions of the α process is likely to be attributed to a decrease in the apparent activation energy with film thickness and reduced mobility, due to the impenetrable wall effect.

Rapid ordering of block copolymer thin films

Block-copolymers self-assemble into diverse morphologies, where nanoscale order can be finely tuned via block architecture and processing conditions. However, the ultimate usage of these materials in real-world applications may be hampered by the extremely long thermal annealing times-hours or days-required to achieve good order. Here, we provide an overview of the fundamentals of block-copolymer self-assembly kinetics, and review the techniques that have been demonstrated to influence, and enhance, these ordering kinetics. We discuss the inherent tradeoffs between oven annealing, solvent annealing, microwave annealing, zone annealing, and other directed self-assembly methods; including an assessment of spatial and temporal characteristics. We also review both real-space and reciprocal-space analysis techniques for quantifying order in these systems.

Reordering transitions during annealing of block copolymer cylinder phases

While equilibrium block-copolymer morphologies are dictated by energy-minimization effects, the semi-ordered states observed experimentally often depend on the details of ordering pathways and kinetics. Here, we explore reordering transitions in thin films of block-copolymer cylinder-forming polystyrene-block-poly(methyl methacrylate). We observe several transient states as films order towards horizontally-aligned cylinders. In particular, there is an early-stage reorganization from randomly-packed cylinders into hexagonally-packed vertically-aligned cylinders; followed by a reorientation transition from vertical to horizontal cylinder states. These transitions are thermally activated. The growth of horizontal grains within an otherwise vertical morphology proceeds anisotropically, resulting in anisotropic grains in the final horizontal state. The size, shape, and anisotropy of grains are influenced by ordering history; for instance, faster heating rates reduce grain anisotropy. These results help elucidate aspects of pathway-dependent ordering in block-copolymer thin films.

Latent Alignment in Pathway-Dependent Ordering of Block Copolymer Thin Films

Block copolymers spontaneously form well-defined nanoscale morphologies during thermal annealing. Yet, the structures one obtains can be influenced by nonequilibrium effects, including processing history or pathway-dependent assembly. Here, we explore various pathways for ordering of block copolymer thin films, using oven-annealing, as well as newly disclosed methods for rapid photothermal annealing and photothermal shearing. We report the discovery of an efficient pathway for ordering self-assembled films: ultrarapid shearing of as-cast films induces "latent alignment" in the disordered morphology. Subsequent thermal processing can then develop this directly into a uniaxially aligned morphology with low defect density. This deeper understanding of pathway-dependence may have broad implications in self-assembly.

Arbitrary lattice symmetries via block copolymer nanomeshes

Self-assembly of block copolymers is a powerful motif for spontaneously forming well-defined nanostructures over macroscopic areas. Yet, the inherent energy minimization criteria of self-assembly give rise to a limited library of structures; diblock copolymers naturally form spheres on a cubic lattice, hexagonally packed cylinders and alternating lamellae. Here, we demonstrate multicomponent nanomeshes with any desired lattice symmetry. We exploit photothermal annealing to rapidly order and align block copolymer phases over macroscopic areas, combined with conversion of the self-assembled organic phase into inorganic replicas. Repeated photothermal processing independently aligns successive layers, providing full control of the size, symmetry and composition of the nanoscale unit cell. We construct a variety of symmetries, most of which are not natively formed by block copolymers, including squares, rhombuses, rectangles and triangles. In fact, we demonstrate all possible two-dimensional Bravais lattices. Finally, we elucidate the influence of nanostructure on the electrical and optical properties of nanomeshes.

Nanoscale patterning methods based on self-assembly promise to revolutionize the fabrication of high-tech devices, but suffer from a limited number of possible lattice symmetries. Here, the authors use a laser zone annealing technique to pattern block copolymers into any 2D mesh motif they desire.

Millisecond Ordering of Block Copolymer Films via Photothermal Gradients

For the promise of self-assembly to be realized, processing techniques must be developed that simultaneously enable control of the nanoscale morphology, rapid assembly, and, ideally, the ability to pattern the nanostructure. Here, we demonstrate how photothermal gradients can be used to control the ordering of block copolymer thin films. Highly localized laser heating leads to intense thermal gradients, which induce a thermophoretic force on morphological defects. This increases the ordering kinetics by at least 3 orders of magnitude compared to conventional oven annealing. By simultaneously exploiting the thermal gradients to induce shear fields, we demonstrate uniaxial alignment of a block copolymer film in less than a second. Finally, we provide examples of how control of the incident light field can be used to generate prescribed configurations of block copolymer nanoscale patterns.

Accurate and facile determination of the index of refraction of organic thin films near the carbon 1s absorption edge

A practical and accurate method to obtain the index of refraction, especially the decrement δ, across the carbon 1s absorption edge is demonstrated. The combination of absorption spectra scaled to the Henke atomic scattering factor database, the use of the doubly subtractive Kramers-Kronig relations, and high precision specular reflectivity measurements from thin films allow the notoriously difficult-to-measure δ to be determined with high accuracy. No independent knowledge of the film thickness or density is required. High confidence interpolation between relatively sparse measurements of δ across an absorption edge is achieved. Accurate optical constants determined by this method are expected to greatly improve the simulation and interpretation of resonant soft x-ray scattering and reflectivity data. The method is demonstrated using poly(methyl methacrylate) and should be extendable to all organic materials.

Interfacial properties of polystyrene thin films as revealed by neutron reflectivity

We have studied the glass transition temperature (T(g)) and molecular mobility of polystyrene (PS) thin films near the interface between the polymer thin film and substrate with bilayer thin films consisting of surface hydrogenated PS (h-PS) and bottom deuterated PS (d-PS) using neutron reflectivity. With decreasing the thickness of the bottom d-PS layer, T(g) near the interface between the polymer thin film and substrate increased compared to bulk T(g) and a drastic increase of T(g) was observed for the bottom d-PS layer <155 Å thick. The orientation of polymer chains at the interface is supposed to be related to the increase of T(g) near the interface between the polymer and substrate. The polymer chain mobility decreased with thickness even for the bottom d-PS layer with no discernible change of T(g). It is considered that the numerous contacts between polymer chains and substrate are related to the decrease of mobility near the interface between the polymer thin film and substrate.

Tailoring the near-surface composition profiles of pressure-sensitive adhesive films and the resulting mechanical properties

We present a possibility of tailoring the near-surface composition profiles of pressure sensitive adhesive (PSA) films by an exposure to atmospheres of different relative humidities (RHs). The statistical copolymer P(EHA-stat-20MMA) with a majority of ethylhexylacrylate (EHA) and a minority of methylmethacrylate (MMA), being cast from a toluene based solution, is chosen as a model system. The near-surface composition profile is probed with X-ray reflectivity. All probed samples show an enrichment of PMMA at the sample surface; however, the near-surface PMMA content strongly increases with increasing RH. The influence of the RH on the composition profile is present down to a depth of 50 nm. Therefore the surface tensions being derived from contact angle measurements do not show any measurable humidity dependence. In contrast, in a mechanical tack test with a smooth punch surface, a strong influence is probed. This observation can be explained by considering the integrated PMMA content over an appropriate near-surface region and the resulting impact on the cavitation process.

Distributions of glass-transition temperature and thermal expansivity in multilayered polystyrene thin films studied by neutron reflectivity

We performed neutron reflectivity measurements on multilayered polymer thin films consisting of alternatively stacked deuterated polystyrene (d-PS) and hydrogenated polystyrene (h-PS) layers ∼200 Å thick as a function of temperature covering the glass-transition temperature T(g), and we found a wide distribution of T(g) as well as a distribution of the thermal expansivity α within the thin films, implying the dynamic heterogeneity of the thin films along the depth direction. The reported anomalous film thickness dependences of T(g) and α were reasonably understood in terms of the distributions, showing that the surface mobile layer and the bottom hard interfacial layer are, respectively, responsible for the depressions of T(g) and α with decreasing film thickness. The molecular mobility in each layer is also discussed in relation to the distribution of T(g), based on the results on mutual diffusion at the layer interface.

Single molecule fluorescence microscopy investigations on heterogeneity of translational diffusion in thin polymer films

Translational diffusion of single perylene diimide molecules in 25 nm thin polymer films was investigated by single molecule widefield fluorescence microscopy. Spatial heterogeneities in single molecule motion were detected and analyzed by a new, quantitative method which draws a comparison of log-Gaussian fits of experimentally determined diffusion coefficient-distributions and diffusion coefficient-distributions from Monte Carlo random walk simulations. Heterogeneities could be observed close to the glass transition temperature, but disappear at ca. 1.1 × T(g). At higher temperatures, heterogeneities do not exist or they average out on the time and length scales of observation. The observed heterogeneities also explain why the dependency of diffusion coefficients on temperature does not follow Vogel-Fulcher-Tammann behavior.