Session D19: Focus Session: Thin Films Copolymers II

Packing Frustration and its Effects on Phase Transitions in Block Copolymer Films

Packing frustration in ordered asymmetric block copolymers arises from the non-uniform stretching of chains required to fill the Wigner-Seitz unit cells of the ordered structure. While its effects are clearly important for block copolymers in the bulk, these effects may be even more important for thin block copolymer films. This is because the Wigner-Seitz unit cells of surface block copolymer domains differ significantly for those of the bulk, giving rise to a more non-uniform chain stretching in the surface layers. Packing frustration plays an important role in the phase stability of multilayer films of spherical block copolymers that show transitions from hexagonal close packed (HCP) to face-centered orthorhombic (FCO) to nearly BCC as the number n of layers increases. Additions of short corona block homopolymers reduce the larger packing frustration of the HCP surface and bulk layers by segregating to the regions of the Wigner-Seitz cell where stretching would be most extreme. These additions increase the n at which a transition from HCP to FCO occurs. The order-disorder temperature of spherical and cylindrical block copolymers also decreases on going from a bilayer to a monolayer. A monolayer of cylinders has a square Wigner-Seitz cell whereas in the bulk, the cell is hexagonal; a monolayer of spheres has a Wigner-Seitz cell that is a hexagonal prism whereas the cells of the bulk BCC, FCO and HCP are significantly more uniform. Finally packing frustration is probably also partly responsible the very different cylinder to sphere order-order transition temperatures observed for diblock and triblock copolymers in thin films relative to bulk samples. Results from scanning force microscopy, transmission electron microscopy and grazing incidence transmission electron microscopy will be used to illustrate these effects experimentally and, where possible, these will be compared to predictions of self-consistent field theory simulations.   

Direct Observation of the BCC (100) Plane in Thin Films of Sphere-forming Diblock Copolymers

In sphere-forming diblock copolymers, periodic arrays of spheres are arranged in a body-centred cubic (BCC) lattice structure in bulk. However, in thin films different surface morphologies were observed as a function of the film thickness, and the transition from the hexagonal array to the BCC (110) arrangement of spheres on film surfaces was located with respect to the increase of the film thickness. Here we report the first direct observation of the BCC (100) plane in thin films of poly (styrene-$b$-methyl methacrylate) diblock copolymers on homogeneous substrates. By balancing the surface energies of both blocks, the lower energy BCC (100) plane corresponding to a square arrangement of half spheres, formed on film surfaces when the film thickness was commensurate with the spacing, L$_{100}$, between (100) planes or greater than 2 L$_{100}$. A hexagonal arrangement of spheres was only observed when the thickness was less than 2 L$_{100}$ and incommensurate with 1 L$_{100}$. Monte Carlo (MC) simulation confirmed our experimental observation and was used to investigate the transition of the arrangement of spheres as a function of the film thickness.   

Development of X-ray and neutron scattering approaches for the characterization of block copolymer thin films

One set of challenges to the implementation of block copolymer (BCP) lithography as a nanomanufacturing technology comprises the limitations of traditional approaches to characterize the nanometer-scale features of the block copolymer template. Common techniques, such as atomic force microscopy and scanning electron microscopy, provide local measurements of relatively small areas limited to the sample surface. We report on the development of neutron and X-ray based scattering techniques to provide global, quantitative characterization of three-dimensional microdomain structure and orientation, and demonstrate the application of these techniques to the investigation of thermal gradient directed self-assembly of BCP thin films. Additionally, preliminary data on the development of grazing-incidence small-angle neutron scattering (GI-SANS) for the characterization of BCP structure and orientation at buried interfaces will be discussed.   

Neutron Reflectivity Studies of Thin Films of Symmetric Block Copolymer and PS-Modified C$_{60}$

C$_{60}$ has a miscibility limit of approximately 1wt{\%} in polystyrene as indicated by wide angle x-ray scattering and molecular dynamics simulations. In order to use block copolymers as a template for creating ordered arrays of fullerenes, it necessary to increase the concentration of C$_{60}$ while maintaining dispersion. C$_{60}$ grafted with six polystyrene arms (C$_{60}$PS$_{6})$ was shown to be miscible with PS up to 80wt{\%} as C$_{60}$ aggregation was not observed. Because C$_{60}$PS$_{6}$ offers improved miscibility with PS, we have investigated C$_{60}$PS$_{6}$ with a symmetric poly(deuterated styrene-$b$-methyl methacrylate) block copolymer (dPS-PMMA) using neutron reflectivity (NR). NR was used to examine the effects of nanoparticle concentration and size, annealing conditions, and thin film thickness on the location and formation of a C$_{60}$PS$_{6 }$layer and block copolymer phase behavior. Reflectivity results indicate the formation of dPS-PMMA lamellae oriented parallel to the silicon substrate after thermal annealing for films prepared from 4wt{\%} solution at all concentrations of C$_{60}$PS$_{6}$ investigated. Similar results were seen for films prepared at different thicknesses and annealed at constant temperature.   

Investigation of Stimuli Responsive Block Copolymer Thin Film Morphology Using Gradient Libraries

We report the rapid characterization of block copolymer thin film morphology using gradient libraries. This approach allows the simultaneous exploration of a large parameter space thus reducing the need for multiple experiments and extensive data work-up to elucidate the critical parameters controlling nanoscale structure and domain size. We examined a stimuli-responsive block copolymer that undergoes a chemical deprotection and corresponding morphology change above a thermal threshold. Temperature and film thickness were varied using orthogonal continuous gradients on a single substrate to monitor auto-catalytic propagation fronts, nanoscale morphology, and chemical structure. Additionally, a systematic library of block copolymers was investigated to determine the effect of molecular weight and volume fraction on film structure. The film morphologies were investigated using optical microscopy and atomic force microscopy, while chemical structure changes were analyzed using polarization modulation IR reflection absorption spectroscopy. By using these characterization techniques we are able to map morphological and chemical structural changes in a rapid fashion, as well as demonstrate that surface functionality affects the kinetics of the chemical deprotection reaction.   

Photo-induced Ordering of Block Copolymers Systems

The microphase separation of block copolymers and the resulting morphologies are governed by the segregation strength and the relative volume fractions of the different blocks. The segregation strength of disordered block copolymers can be enhanced by incorporation of additives that can interact selectively with one of the blocks. In this work, we discuss how disordered Pluronic block copolymer surfactants, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) when mixed with weakly interacting, protected additives undergo photo-induced ordering upon exposure to UV light and baking. Exposure to UV light and heating causes deprotection reaction resulting in formation of strongly interacting groups on the additives. This strong interaction with one of the blocks of the block copolymer causes segregation of the PEO and PPO blocks and thus leads to formation of ordered morphologies. Using such a photo-induced ordering strategy, arbitrary patterns of ordered nanostructures can be made in an otherwise disordered block copolymer film.   

Photocontrol over the Disorder-to-Order Transition (DOT) in Thin Film of Polystyrene-\textit{block}-Poly(methyl methacrylate) Block Copolymers Containing Photodimerizable anthracene functionality

Reversible photocontrol over the disorder-to-order transition (DOT) of block copolymers can be used to fabricate defect-free, long-range ordered nanomaterials over macroscopic distances by ``photo-combing'' the microdomains. Here, we randomly copolymerized anthracene functionalities in a ``dilute'' way with 2-hydroxyethyl methacrylate as the middle block, forming deuterated polystyrene-\textit{block}-poly(2-(methacryloyloxy)ethyl anthracene-9-carboxylate-\textit{random}-2-hydroxyethyl methacrylate)-\textit{block}-poly(methyl methacrylate) ($d_{8}$-PS-$b$-P(9AnEMA-$r$-HEMA)-$b$-PMMA) triblock copolymers. Upon UV irradiation of a thin film of the phase-mixed triblock copolymer, photodimerization of anthracene links the junction of $d_{8}$-PS and PMMA blocks and produces an artificial interface to force a phase-separation, i.e. a UV-induced DOT, as evidenced by small angle neutron scattering (SANS). Reversible photocontrol over the DOT can be achieved by taking advantage of photodimerization and photodissociation of anthracene.   

Microphase Separation in Thin Films of Block Copolymer Supramolecular Assemblies: Composition Dependent Morphological Transitions and Molecular Architecture Effect

Block copolymer based supramolecular assemblies (SMAs) recently have attracted lot of attention because of their potential application as nanotemplates. These SMAs are prepared by attaching small molecules selectively to one of the blocks of the copolymer through physical interactions. In the present study, the phase behavior of SMAs formed by polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) with 2-(4'-hydroxybenzeneazo)benzoic acid (HABA) was investigated with respect to the molar ratio (X) between HABA and 4VP monomer unit in bulk as well as in thin films. It will be shown that these SMAs show some interesting composition dependent and solvent induced pathway dependent phase transitions. Moreover, the orientation of cylindrical or lamellar microdomains of P4VP(HABA) depends on the selectivity of the solvent as well as on the degree of swelling of the thin film. Furthermore, it will be shown that the molecular architecture of the block copolymer influences the orientation and ordering of microdomains in the SMA. Hence, whereas, the cylindrical and lamellar microdomains of SMA composed of a P4VP-b-PS-b-P4VP triblock copolymer were perpendicular to the substrate, those composed from a PS-b-P4VP diblock of similar composition had in-plane orientation of the microdomains.   

Surface Interaction Influence on Transition Behavior of Block Copolymer Films

The phase transitions for the films of block copolymers (BCPs) on the modified surface, like the order-to-disorder transition (ODT), was investigated by in-situ grazing incidence small angle x-ray scattering (GISAXS). The selective interactions at the surface by PS-brush substrate that favors the preferential interactions with the PS component of the block copolymer enhance the parallel orientation of the lamellar microdomains to the film surface. The thickness dependences of transition temperatures for BCP films on the preferential surface will be discussed in terms of the temperature dependence of $\chi$ between two block components. Interestingly, with decreasing film thickness, typical behavior of the BCP films on the preferential interactions indicates an increase of transition temperature, which is in quite contrast to a decrease of transition temperature for the films on the balanced interfacial interactions.   

Ultrathin-films of diblock copolymers under shear

The behavior of diblock copolymers ultra thin-films under confined-shear conditions has been studied using coarse-grained Langevin dynamics simulations. The conformational properties of the diblock copolymers within the thin-film and the overall film's structural behavior are examined as functions of the composition (volume fraction) $f$, the segregation strength, $\Phi$, and the strength of the shear-field, $\dot{\gamma}$. Below the order-disorder transition (ODT) the film generates a rich variety of structures composed by a monolayer of compressed micelles. Once a shear field is applied and above a critical shear rate, the system re-self-assembles into cylindrical micelles with orientation parallel to the shear flow. This study draws much of what is known from simulation and experiment on the formation of cylinders under shear and provides deeper insights into shear-induced sphere-to-cylinder order-order transition and realignment of cylinders. In addition to formation of cylinders formation parallel to the sheared direction, we have identified the conditions under which the formation of cylinders under the influence of shear flow have orientation {\it perpendicular} rather than parallel to the shear flow. This order-order steady-state orientation transition is temperature dependent.   

Chi effect on Transition Behaviors of BCP Films on PS-brushe Substrate

The order-to-disorder transitions (ODTs) for the films of symmetric block copolymers on a modified surface, in the weak segregation regime, were investigated by in-situ grazing incidence small angle x-ray scattering (GISAXS). The selective interactions at the modified substrate that favors the preferential interactions with the one component of the block copolymer enhance the parallel orientation of the lamellar microdomains to the film surface. We discuss the thickness dependences of transition temperatures for the films of symmetric polystyrene-b-poly(methyl methacryltate) (PS-b-PMMA) and polystyrene-b-polyisoprene (PS-b-PI) on preferential surfaces in terms of the temperature dependence of Chi between two block components.   

Complex Transitions of Block Copolymer in Melt and Film Geometry

The complex transitions such as the order-to-order transition (OOT) and the order-to-disorder transition (ODT) for an asymmetric polystyrene-block-polyisoprene (PS-b-PS), where PS block is a major component and is preferentially interacting with a PS grafted substrate, were investigated by in-situ small-angle x-ray scattering (SAXS) and grazing incidence x-ray scattering (GISAXS) in film geometry. Block copolymer in the bulk possesses an OOT pathway of a lamellar morphology (LAM), perforated layered structure (PL), (Fddd+GYR)-GYR-DIS, On the other hand, the PS-b-PI film that preferentially interacts with a PS grafted substrate shows LAM-HML-GYR-DIS, where HML indicates a hexagonally modulated layer structure with epitaxial orientation.   

Effect of chain architecture on the surface energy of block copolymer melts

Careful experimental studies of ABA triblock copolymer melts by Khanna et al. [Macromolecules, {\bf 39}, 9346, 2006] have detected an entropic advantage of placing the A-rich domain next to a surface as opposed to the B-rich domain. This preference is also corroborated by a self-consistent field theory (SCFT) calculation. Their proposed explanation is that B blocks avoid the surface, because otherwise they would lose the entropy associated with bridging and looping. However, a more thorough investigation of the SCFT reveals that the preference is due to an entropic advantage of having the end segments of the A-rich domain next to a surface. Furthermore, we use a simple lattice-model argument to intuitively explain that the entropy due to bridging and looping becomes irrelevant when the contour length of the B block is much smaller than the width of the B-rich domain, which is the normal situation in triblock copolymer melts. Our new chain-end explanation could be tested experimentally by examining other architectures such as A$_2$B starblock copolymers.