Orientation Of Microdomains Research Articles

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 (fPS = 68%, Mn = 53 kg mol−1), the other with PDMS block as majority (fPDMS = 67%, Mn = 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.

Complete and comprehensive orientation of cylindrical microdomains in a block copolymer sheet

Significant progress in cylindrical microdomain orientation is reported for a polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene triblock copolymer (SEBS-16) as analyzed by two-dimensional small-angle X-ray scattering. For this purpose, the morphological transition from sphere to cylinder was utilized. By applying compressional flow to the SEBS-16 specimen undergoing a morphological transition at 210 °C, superior cylinder orientation was achieved compared with the results obtained by applying the compressional flow to a cylindrical phase. The flow-treated specimens were further subjected to subsequent thermal annealing at 210 °C. It was found that the cylinder orientation proceeded by the subsequent thermal annealing even for flow-treated specimens with less oriented cylinders. Furthermore, the fraction of grains in which the plane of the hexagonal lattice of cylinders is oriented parallel to the specimen surface increased due to the subsequent thermal annealing, even for the specimen with the best cylinder orientation. Significant progress in cylindrical microdomain orientation is reported for a polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene triblock copolymer (SEBS-16) as analyzed by two-dimensional small-angle X-ray scattering. To achieve this goal, the morphological transition from sphere to cylinder was utilized. By applying compressional flow to the SEBS-16 specimen undergoing a morphological transition at 210 °C, superior cylinder orientation was achieved compared with the results obtained by applying compressional flow to the SEBS-16 specimen containing the cylindrical phase.

In situ GISAXS study of a Si-containing block copolymer under solvent vapor annealing: Effects of molecular weight and solvent vapor composition

The room-temperature solvent vapor annealing of polystyrene-b-polydimethylsiloxane (PS-b-PDMS) block copolymer films was studied in situ by grazing incidence small-angle X-ray scattering. Films of cylinder-forming PS-b-PDMS with molecular weight 16 kg/mol and 12.2 kg/mol, annealed under vapors of toluene:heptane with varying composition, exhibited swelling-ratio-dependent evolution of microdomain spacing and orientation. In a vapor made from a toluene:heptane 5:1 volumetric ratio liquid mixture, or from pure toluene, cylindrical microdomains reoriented from majority out-of-plane to in-plane during annealing, while drying led to shrinkage along the film normal and a large distortion of the hexagonal lattice of in-plane cylinders. Annealing under vapor from a toluene:heptane 1:5 volumetric ratio liquid produced a non-bulk lamellar structure in the 16 kg/mol PS-b-PDMS.

Isolating the Effect of Molecular Weight on Ion Transport of Non-Ionic Diblock Copolymer/Ionic Liquid Mixtures

The effects of molecular weight and microdomain orientation on the ionic conductivities of poly(styrene-b-methyl methacrylate) diblock copolymer/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (SM/IL) mixtures are assessed through complementary experimental and theoretical techniques. Small-angle X-ray scattering revealed that SM/IL mixtures have anisotropic lamellar morphologies, preferentially oriented parallel to the casting substrate. A method for quantifying the morphology factor, or microdomain orientation within the Sax–Ottino model, using 2-D SAXS data is presented and applied to SM/IL mixtures. Ionic conductivity increases by up to an order of magnitude with a 2-fold increase in molecular weight, even when accounting for the morphology type, composition, microdomain orientation, and PMMA/IL glass transition temperature. Self-consistent field theory calculations predict a nonuniform solvent distribution in PMMA/IL microdomains, suggesting that polymer mobility and ion transport are reduced near PS–PMMA microdomain interfaces. Thus, the increase in ionic conductivity with increasing block copolymer molecular weight is associated with having fewer PS–PMMA/IL interfaces per unit volume.

Orientation and relaxation behaviors of lamellar microdomains of poly(methyl methacrylate)-b-poly(n-butyl acrylate) thin films as revealed by grazing-incidence small-angle X-ray scattering

The orientation behavior and domain spacing relaxation of the phase-separated structure of poly(methyl methacrylate-b-n-butyl acrylate) (PMMA-PnBA) thin films were investigated by grazing-incidence small-angle X-ray scattering (GISAXS). Moreover, low-energy GISAXS (tender X-ray) was performed for the in-depth profiling of the microphase-separated structure. PMMA-PnBA forms a lamellar structure. Parallel orientation of the lamellae was induced by thermal annealing. XPS measurements indicated that surface segregation of the PnBA component occurred rapidly, within 1 min after annealing at 160 °C. GISAXS measurements revealed that the apparent degree of orientation and the domain spacing D|| of the parallel-oriented lamellae increased with annealing time, and the final D|| was close to the domain spacing of the bulk material. The domain spacing relaxation took longer than the orientation of the lamellae of the block copolymers. Furthermore, it was found that D|| in the vicinity of the surface was greater than inside the film, which indicates that the domain spacing near the surface relaxed faster than inside due to the higher mobility of the polymer chain. The orientation behavior and domain spacing relaxation of the phase-separated structure of poly(methyl methacrylate-b-n-butyl acrylate) thin films were investigated by grazing-incidence small-angle X-ray scattering (GISAXS). Moreover, low-energy GISAXS was performed for the in-depth profiling of the lamellar structure. The domain spacing (D||) relaxation took longer than the orientation of the lamellae. Furthermore, the D|| in the vicinity of the surface was greater than inside the film, which indicates that the D|| near the surface relaxed faster than inside due to the higher mobility of the polymer chain.

Microdomain orientation control of PS-b-PMMA films enabled by wettability relay of graphene

A substrate-independent method to control the orientation of PS-b-PMMA film is presented by utilizing monolayer graphene coated PS-r-PMMA copolymer layer.

Extending Dynamic Range of Block Copolymer Ordering with Rotational Cold Zone Annealing (RCZA) and Ionic Liquids

Scalable and low-cost methods to align and orient block copolymer (BCP) films and membranes are critical for their adaptation for nonlithographic applications. Cold zone annealing (CZA) can align BCP microdomains and is scalable via roll-to-roll (R2R) manufacturing. However, the efficacy of orientation by CZA is strongly dependent on the thermal zone velocity (Vcza). Optimization of this rate can be time-consuming and tedious. To address this shortcoming, we report rotational or radial CZA (RCZA) that provides a combinatorial approach to efficiently determine how linear Vcza rate impacts microdomain orientation. RCZA rapidly identifies the optimal CZA velocities for perpendicular orientation of cylinders in polystyrene-block-poly(methyl methacrylate) films that previously required tens of measurements [Macromolecules 2012, 45, 7107], demonstrated here with much finer velocity resolution using three overlapping radial regimes. Notably, the efficacy of CZA for perpendicular alignment rapidly decays for Vcza > 10 μm/s. To overcome this limitation, the addition of 2 wt % 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide sufficiently alters the surface tension and segmental relaxations via reduced viscosity to increase the processing window for perpendicular cylinders, approximately 75% at Vcza ≈ 330 μm/s, approaching R2R speeds. Further increasing ionic liquid content to 5 wt % leads to mostly parallel orientation due to surface wetting. Ionic liquids can dramatically increase BCP processing speeds for applications, such as membranes, and RCZA can efficiently map out the optimal processing parameters.

Perpendicular Block Copolymer Microdomains in High Aspect Ratio Templates.

Perpendicular orientation of lamellar microdomains in a high interaction parameter block copolymer was obtained within high aspect ratio gratings functionalized with a preferential sidewall brush. The experiments used polystyrene-block-polydimethylsiloxane (PS-b-PDMS) with molecular weight 43 kg/mol within trenches made using interference lithography. The perpendicular alignment was obtained for both thermal and solvent annealing, using three different solvent vapors, for a range of film thicknesses and trench widths. A platinum (Pt) layer at the base of the trenches avoided the formation of a wetting layer, giving perpendicular orientation at the substrate surface. The results are interpreted using self-consistent field theory simulation and a Ginzburg-Landau analytic model to map the energies of lamellae of different orientations as a function of the grating aspect ratio and the surface energies of the sidewalls and top and bottom surfaces. The model results agree with the experiment and provide a set of guidelines for obtaining perpendicular microdomains within topographic features.

Combining Graphoepitaxy and Electric Fields toward Uniaxial Alignment of Solvent-Annealed Polystyrene–b–Poly(dimethylsiloxane) Block Copolymers

We report a combined directing effect of the simultaneously applied graphoepitaxy and electric field on the self-assembly of cylinder forming polystyrene-b-poly(dimethylsiloxane) block copolymer in thin films. A correlation length of up to 20 μm of uniaxial ordered striped patterns is an order of magnitude greater than that produced by either graphoepitaxy or electric field alignment alone and is achieved at reduced annealing times. The angle between the electric field direction and the topographic guides as well as the dimensions of the trenches affected both the quality of the ordering and the direction of the orientation of cylindrical domains: parallel or perpendicular to the topographic features. We quantified the interplay between the electric field and the geometry of the topographic structures by constructing the phase diagram of microdomain orientation. This combined approach allows the fabrication of highly ordered block copolymer structures using macroscopically prepatterned photolithographic s...

Fluorescence Recovery after Photobleaching and Single-Molecule Tracking Measurements of Anisotropic Diffusion within Identical Regions of a Cylinder-Forming Diblock Copolymer Film.

This work demonstrates ensemble and single-molecule diffusion measurements within identical regions of a cylinder-forming polystyrene-poly(ethylene oxide) diblock copolymer (PS-b-PEO) film using fluorescence recovery after photobleaching (FRAP) and single-molecule tracking (SMT). A PS-b-PEO film (∼4 μm thick) with aligned cylindrical PEO microdomains containing 10 μM sulforhodamine B (SRB) was prepared by directional solvent-vapor penetration (SVP) of 1,4-dioxane. The ensemble diffusion behavior of SRB in the microdomains was assessed in FRAP studies of circular photobleached regions (∼7 μm in diameter). The SRB concentration was subsequently reduced by additional photobleaching, and the diffusion of individual SRB molecules was explored using SMT in the identical area (∼16 × 16 μm(2)). The FRAP data showed anisotropic fluorescence recovery, yielding the average microdomain orientation. The extent of fluorescence recovery observed (∼90%) demonstrated long-range microdomain connectivity, while the recovery time dependence provided an ensemble measurement of the SRB diffusion coefficient within the cylindrical microdomains. The SMT data exhibited one-dimensional diffusion of individual SRB molecules along the SVP direction across the entire film thickness, as consistent with the FRAP results. Importantly, the average of the single-molecule diffusion coefficients was close to the value obtained from FRAP in the identical area. In some cases, SMT offered smaller diffusion coefficients than FRAP, possibly due to contributions from SRB molecules confined within short PEO microdomains. The implementation of FRAP and SMT measurements in identical areas provides complementary information on molecular diffusion with minimal influence of sample heterogeneity, permitting direct comparison of ensemble and single-molecule diffusion behavior.

In Situ SAXS Studies of Structural Relaxation of an Ordered Block Copolymer Melt Following Cessation of Uniaxial Extensional Flow

A hexagonally ordered styrene–ethylene-co-butylene–styrene triblock copolymer melt is studied during and immediately following uniaxial extensional flow using small-angle X-ray scattering. Strips of polymer melt are stretched between counter-rotating drums inside an oven designed for in situ synchrotron studies. Previous study of this sample demonstrated deformation and reorientation of PS microdomains and identified a critical Hencky strain ∼0.2 at which the structure is dramatically disrupted. To further probe microstructural dynamics, we study relaxation behavior of structure and stress in samples stretched in the melt to various Hencky strains. Upon cessation of stretching below the critical strain, structural relaxation is strongly retarded; stretching to higher strain leads to more rapid relaxation. At all strains, microdomain orientation relaxation following flow is slower than relaxation of flow-induced deformation and mechanical stress. The extension rate dependence of the sample’s response is also explored; higher rates result in faster relaxation of both stress and structure.

Single-molecule tracking studies of flow-induced microdomain alignment in cylinder-forming polystyrene-poly(ethylene oxide) diblock copolymer films.

Flow-based approaches are promising routes to preparation of aligned block copolymer microdomains within confined spaces. An in-depth characterization of such nanoscale morphologies within macroscopically nonuniform materials under ambient conditions is, however, often challenging. In this study, single-molecule tracking (SMT) methods were employed to probe the flow-induced alignment of cylindrical microdomains (ca. 22 nm in diameter) in polystyrene-poly(ethylene oxide) diblock copolymer (PS-b-PEO) films. Films of micrometer-scale thicknesses were prepared by overlaying a benzene solution droplet on a glass coverslip with a rectangular glass plate, followed by solvent evaporation under a nitrogen atmosphere. The microdomain alignment was quantitatively assessed from SMT data exhibiting the diffusional motions of individual sulforhodamine B fluorescent probes that preferentially partitioned into cylindrical PEO microdomains. Better overall microdomain orientation along the flow direction was observed near the substrate interface in films prepared at a higher flow rate, suggesting that the microdomain alignment was primarily induced by shear flow. The SMT data also revealed the presence of micrometer-scale grains consisting of highly ordered microdomains with coherent orientation. The results of this study provide insights into shear-based preparation of aligned cylindrical microdomains in block copolymer films from solutions within confined spaces.

A Top Coat with Solvent Annealing Enables Perpendicular Orientation of Sub‐10 nm Microdomains in Si‐Containing Block Copolymer Thin Films

Achieving sub‐10 nm high‐aspect‐ratio patterns from diblock copolymer self‐assembly requires both a high interaction parameter (χ, which is determined by the incompatibility between the two blocks) and a perpendicular orientation of microdomains. However, these two conditions are extremely difficult to achieve simultaneously because the blocks in a high‐χ copolymer typically have very different surface energies, favoring in‐plane microdomain orientations. A fully perpendicular orientation of a high‐χ block copolymer, poly(styrene‐block‐dimethylsiloxane) (PS‐b‐PDMS) is realized here using partially hydrolyzed polyvinyl alcohol (PVA) top coats with a solvent annealing process, despite the large surface energy differences between PS and PDMS. The PVA top coat on the block copolymer films under a solvent vapor atmosphere significantly reduces the interfacial energy difference between two blocks at the top surface and provides sufficient solvent concentration gradient in the through‐thickness direction and appropriate solvent evaporation rates within the film to promote a perpendicular microdomain orientation. The effects of interfacial energy differences and the swellability of PVA top coats controlled by the degree of hydrolysis on the orientation of micro­domains are examined. The thickness of the BCP film and top coats also affects the orientation of the BCP film.

Electric-Field-Induced Control and Switching of Block Copolymer Domain Orientations in Nanoconfined Channel Architectures

Successfully controlling and switching cylinder-forming diblock copolymer domain orientations located in nanoconfining channel architectures has been achieved by the application of electric fields. Two degenerate structures result when the block copolymer thin films are confined in intersecting crossed channels. Here, cylinder domains align in one of two energetically degenerate structures having orientations of 45° or 135° with respect to the channels in the crossed areas. Electric fields are then introduced to further control the orientation of microdomains in both as-cast and phase-separated films. The cylinder domains then become oriented parallel to the electric field, and a single structure is obtained in the crossed areas. Finally, by introducing quadrupolar electrodes, we demonstrate the ability to switch block copolymer domain orientations between the two degenerate structures. These results suggest many follow-on applications in fundamental polymer physical chemistry as well as new approaches for static and dynamic nanostructure design and control.

Alignment and Structural Evolution of Cylinder-Forming Diblock Copolymer Thin Films in Patterned Tapered-Width Nanochannels

Control over the orientation of cylindrical block copolymer microdomains and the position of dislocations using tapered nanochannels is herein demonstrated. Dislocations are introduced periodically as the channels become progressively wider. Our high-temperature atomic force microscopy (AFM) enables us to probe real-time and real-space dislocation movement directly. Upon annealing, dislocations are observed to move to their equilibrium positions, where the average free energy of the polymer chains reaches a minimum. Smaller confinement widths impose a powerful guiding field for templating domain alignment, as the deviations of the dislocations at the narrower end of the channel from their equilibrium positions are minimal. Finally, the fluctuations of dislocations across the wedge are studied under different temperatures, and the perpendicular diffusion coefficients and activation energy are determined. These experiments demonstrate the ability to control the placement of periodically located dislocations...

Substrate-Independent Lamellar Orientation in High-Molecular-Weight Polystyrene-b-poly(methyl methacrylate) Films: Neutral Solvent Vapor and Thermal Annealing Effect

Lamellar microdomain orientation in polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) films was controlled by a solvent vapor annealing process, where the high-molecular-weight block copolymer (BCP) was used to self-assemble in a large period of 105 nm. A neutral solvent annealing with tetrahydrofuran vapor screened the difference in the surface energy between the two blocks and the interfacial interactions of the substrate with each block, leading to the substrate-independent perpendicular orientation of lamellar microdomains. Together with thermal annealing of the solvent-annealed BCP film, we demonstrate that highly ordered line arrays of perpendicularly oriented lamellae were well guided in topographic line and disk photoresist patterns composed of the PS-attractive cross-linked copolymer, where the interlamellar d-spacing compliant to the patterns was dependent on the confinement types.

High density array of CdS nanoporous films on ITO from self-assembly of block copolymer

Among many methods to prepare nanoporous structure in inorganic films, block copolymers have been widely employed. In this work, we have systematically studied the orientation of the cylindrical PS microdomains in polystyrene-b-polymethyl methacrylate (PS-b-PMMA) block copolymer thin films on a conducting ITO substrate and obtained perfect vertically orientated PS cylinders by modifying the substrate. Furthermore, the well-ordered morphology of the block copolymer thin film has been used successfully as a template to prepare nanoporous semicon CdS thin film with hexagonal crystalline phase. The facile and effective approach would be universal and open new avenues for fabrication of some nanomaterials.

The effect of nanoparticle on microdomain alignment in block copolymer thin films under an electric field

Controlling microdomain structure of block copolymers (BCPs) under electric field has been one of the most challenging research tasks. In this study, we examined the effect of nanoparticles on the microdomain orientations in BCP/nanoparticle thin films under electric field using cross-sectional transmission electron microscopy experiments. Gold and cadmium selenide nanoparticles with a tailored surface property were incorporated to control microdomain orientations in BCP/nanoparticle thin films by varying dielectric constant of one constituting block. It was revealed that the microdomain orientation of BCP/nanoparticle thin films under electric field was suppressed by the introduction of gold nanoparticles. Thus, it can be inferred that gold nanoparticles can show a shielding effect under external electric field. The effect of complementary parameters such as NPs concentration, exposure time, and field strength were also demonstrated. In addition, it was also found that the suppression effect lessened with cadmium selenide nanoparticles having a dipole from the noncentrosymmetric structure. This work can provide fundamental data for understanding of microdomain alignment behavior of BCP/nanoparticle system under electric field.

Effect of nanoparticle surface functionality on microdomain orientation in block copolymer thin films under electric field

The electric field induced microdomain orientations has been an interesting research topic. In this article, the effect of nanoparticle surface functionality on microdomain alignments in block copolymer/nanoparticle hybrid thin films was investigated with transmission electron microscopy experiments. The presence of gold nanoparticles influenced the microdomain orientation behaviors of block copolymer/nanoparticle thin films. The possibility for complete alignment normal to the substrate was illustrated by controlling electric field strength, concentration, and surface ligands of nanoparticles. This work provides basic and essential data to understand the properties and behaviors of emerging block copolymer/nanoparticle hybrid thin films.

Pathways of cylindrical orientations in PS-b-P4VP diblock copolymer thin films upon solvent vapor annealing.

The orientation changes of perpendicular cylindrical microdomains in polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) thin films upon annealing in different solvent vapors were investigated by in situ grazing incidence small-angle X-ray scattering (GISAXS) and ex situ scanning force microscopy (SFM). The swelling of P4VP perpendicular cylinders (C⊥) in chloroform, a non-selective solvent vapor, leads to the reorientation to in-plane cylinders through a disordered state in a particular kinetic pathway in the phase diagram upon drying. On the other hand, the swelling of the P4VP perpendicular cylinders in a selective solvent vapor (i.e., 1,4-dioxane) induces a morphological transition from cylindrical to ellipsoidal as a transient structure to spherical microdomains; subsequent solvent evaporation resulted in shrinkage of the matrix in the vertical direction, merging the ellipsoidal domains into the perpendicularly aligned cylinders. In this paper, we have discussed the mechanism based on the selectivity of the solvent to the constituting blocks that is mainly responsible for the orientation changes.