Effect of Film Thickness and Domain Spacing on Defect Densities in Directed Self-Assembly of Cylindrical Morphology Block Copolymers

Directed assembly of block copolymer thin films is recognized as a high-throughput, low-cost complement to optical lithography with the ability to overcome the 32 nm natural resolution limit of conventional lithographic techniques. For bulk block copolymer systems, desired feature sizes ranging from 5 to 100 nm can be obtained by controlling the molecular weight and composition of a block copolymer, as long as the bulk order–disorder temperature (ODT) is such that the copolymer is well-segregated at the processing conditions. However, our studies on graphoepitaxially aligned cylindrical morphology block copolymer monolayer and bilayer films demonstrate that, as domain sizes are reduced, the block copolymer becomes increasingly susceptible to an unacceptably high density of thermally generated defects, resulting in a significant reduction of the ODT. Thus, in thin films, the minimum feature spacing accessible is limited by thermal defect generation and not by the bulk ODT. Our experimental studies on monolayer films of cylindrical morphology polystyrene-<i>b</i>-poly(2-vinyl pyridine) with microdomain spacings approaching 20 nm reveal that defect densities and the ODT are surprisingly sensitive to variations as small as 2 nm in the microdomain spacing. Additionally, the monolayer and bilayer ODT differ by nearly 100 °C when the monolayer domain spacing is 20 nm, while the difference is only 20 °C when the monolayer domain spacing is 22 nm. We explain this behavior using a quantitative estimation of the energetic cost of defect production in terms of the domain spacing, χ<i>N</i>, and block copolymer composition. These studies reveal unexpected consequences on the equilibrium defect densities of thin film block copolymers which must be accounted for when designing a block-copolymer-based directed-assembly process

Bicontinuous Block Copolymer Morphologies Produced by Interfacially Active, Thermally Stable Nanoparticles

Polymeric bicontinuous morphologies were created by thermal annealing mixtures of poly(styrene-<i>b</i>-2-vinylpyridine) (PS-<i>b</i>-P2VP) block copolymers and stabilized Au-core/Pt-shell (Au–Pt) nanoparticles. These Au–Pt nanoparticles have a cross-linked polymeric shell to promote thermal stability and are designed to adsorb strongly to the interface of the PS-<i>b</i>-P2VP block copolymer due to the favorable interaction between P2VP block and the exterior of the cross-linked shell of the nanoparticle. The interfacial activity of these Au–Pt nanoparticles under thermal annealing conditions leads to decrease in domain size of the lamellar diblock copolymer. As nanoparticle volume fraction ϕ_p was increased, a transition from a lamellar to a bicontinuous morphology was observed. Significantly, the effect of these shell-cross-linked Au–Pt nanoparticles under thermal annealing conditions was similar to those of traditional polymer grafted Au nanoparticles under solvent annealing conditions reported previously. These results suggest a general strategy for producing bicontinuous block copolymer structures by thermal processing through judicious selection of polymeric ligands, nanoparticle core, and block copolymer

Morphology Evolution of PS-<i>b</i>-P2VP Diblock Copolymers via Supramolecular Assembly of Hydroxylated Gold Nanoparticles

We report on the strong segregation of core–shell Au nanoparticles, with a shell layer consisting of a random copolymer brush of styrene and vinylphenol (PS-<i>r</i>-PVPh-SH), in poly­(styrene-<i>b</i>-2-vinylpyridine) (PS-<i>b</i>-P2VP) diblock copolymer. Because of the formation of multiple hydrogen bonds between the hydroxyl groups within the shell of the nanoparticles and the pyridine group in PS-<i>b</i>-P2VP, the Au nanoparticles were strongly localized into P2VP domains with a very high volume fraction of nanoparticles (ϕ_p ∼ 0.53). The spatial distribution of Au nanoparticles, observed by transmission electron microscopy (TEM), is compared with results of previous experiments where homopolymers were blended with block copolymers. If the diameter <i>d</i> of the nanoparticles is much less than the width <i>D</i> of the P2VP lamellar domains, these nanoparticles are more uniformly distributed across the P2VP domain than if <i>d</i> is comparable to <i>D</i>, in which case the nanoparticles are pushed toward the center of the P2VP domains. This behavior is similar to that observed when homopolymers are blended with block copolymers. Novel morphological transitions from spherical to cylindrical P2VP morphologies and from lamellae to cylindrical PS morphologies were observed during coassembly of these functional nanoparticles with block copolymers

Phase Separated Morphology of Ferroelectric–Semiconductor Polymer Blends Probed by Synchrotron X‑ray Methods

Control of the domain size and morphology of ferroelectric-semiconductor polymer blend thin films is essential for producing working organic ferroelectric resistive switches that can be used for low-cost, flexible memory applications. However, improvements in characterization techniques that can selectively probe these polymers are still needed. The unique core-level absorption profiles of these polymers make synchrotron based soft X-ray techniques ideal to achieve contrast and chemical sensitivity between polymers and characterize thin film morphology. Transmission soft X-ray microscopy and scattering reveal that a phase separated structure exists through the bulk for a blend of a semicrystalline semiconducting polythiophene with a functionalized side chain and a well-studied ferroelectric polymer. Surface sensitive soft X-ray spectroscopy and wide-angle X-ray scattering suggest a potential enhancement of polythiophene at the film surface, and that the surface layer is more amorphous in character. This work demonstrates the utility of soft X-rays to characterize ferroelectric-semiconductor polymer blends both in the bulk and at the film surface. Understanding differences in composition and morphology between the bulk and thin film interfaces is critical to further improve organic-based memory technology

Surface Organization of a Perfluorocarbon-Functionalized Polystyrene Homopolymer

We use the perfluorocarbon-functionalized polymer, poly­(4-((1<i>H</i>,1<i>H</i>,2<i>H</i>,2<i>H</i>-perfluorodecyl)­oxycarbonyl)­styrene) [PPFOCS], as a model system with both surface molecular segregation and molecular orientation to test the capabilities of a near-edge X-ray absorption fine structure (NEXAFS) spectroscopy analysis scheme for polymer surfaces. Both NEXAFS spectroscopy and angle-resolved X-ray photoelectron spectroscopy (XPS) show segregation of the −(CF₂)₇CF₃ chain to the air/polymer interface with the styrenic portion underneath. Postedge analysis of the NEXAFS spectra indicates a low carbon atom density surface layer, of thickness 1.0–1.4 nm, due to the overlayer of perfluorocarbon chains. An analysis of the NEXAFS C 1s → π*_CC and C 1s → σ*_C–F transitions accounting for the different depth distributions of the phenyl rings and fluorocarbon helices reveals strong orientational ordering with the orientational order parameter <i>S</i>_CC for the phenyl ring equal to −0.27 and for the C–F bonds in the fluorocarbon helix <i>S</i>_C–F equal to −0.13. The <i>S</i>_CC and <i>S</i>_C–F determined for the polymer with the ester-linked side chain are considerably higher than those reported previously (−0.039 and 0, respectively) for a polymer [poly­(4-(1<i>H</i>,1<i>H</i>,2<i>H</i>,2<i>H</i>-perfluorodecyl)­oxymethylstyrene)] with an identical side chain that was ether linked to the styrene phenyl ring. We tentatively attribute the high orientation in the PPFOCS to the partial conjugation between the ester group and the phenyl ring providing a relatively stiff linkage between the perfluorocarbon helix and the phenyl ring

Temperature Dependence of the Diffusion Coefficient of PCBM in Poly(3-hexylthiophene)

Interest in new functional small molecule and polymer blends, such as polymer–fullerene bulk heterojunction (BHJ) organic solar cells motivates the development of new methods to measure the diffusion coefficient of molecular species (e.g., PCBM) in polymers. The aim of this study is to systematically improve our understanding of the relevant material and processing parameters needed to control the microstructure of BHJ organic solar cells in order to develop a more complete understanding of how to improve its power conversion efficiency. Here, we fabricate a terraced monolayer–bilayer sample of P3HT and P3HT/PCBM and use this structure to quantify both the volume fraction of miscible PCBM in P3HT and the diffusion coefficient of disordered PCBM in disordered P3HT. Our findings reveal that the diffusion coefficient for disordered PCBM in P3HT is strongly dependent on the annealing temperature (i.e., increasing by 3 orders of magnitude when doubling the annealing temperature) and weakly dependent on the PCBM concentration. The temperature-dependent diffusion coefficients were fit with an Arrhenius relationship to determine an activation energy for the diffusion of disordered PCBM through P3HT. Ultimately, this report demonstrates that the self-assembly of the P3HT:PCBM BHJ solar cell during annealing and cooling is not limited by the diffusion of deuterated PCBM in P3HT with the nanostructure of PCBM being controlled by the relative volume fractions of ordered and disordered P3HT

Polymer Side Chain Modification Alters Phase Separation in Ferroelectric-Semiconductor Polymer Blends for Organic Memory

Side chain modification of a semiconducting polythiophene changes the resulting phase separation length scales when blended with a ferroelectric polymer for use in organic ferroelectric resistive switches. The domain size of the semiconducting portion of blends of poly­[3-(ethyl- 5-pentanoate)­thiophene-2,5-diyl] (P3EPT) and poly­(vinylidene fluoride-<i>co</i>-trifluoroethylene) (PVDF-TrFE) in thin film blends are smaller than previously reported and easily controllable in size through simple tuning of the weight fraction of the semiconducting polymer. Furthermore, P3EPT has a relatively high degree of crystallinity and bimodal crystallite orientations, as probed by wide-angle X-ray scattering. Resistive switches fabricated from blends of P3EPT and PVDF-TrFE show memristive switching behavior over a wide range of polythiophene content and good ON/OFF ratios

Crystalline Polymorphs of [6,6]-Phenyl‑C₆₁-butyric Acid <i>n</i>‑Butyl Ester (PCBNB)

The thermotropic behavior of [6,6]-phenyl-C₆₁-butyric acid <i>n</i>-butyl ester (PCBNB) in powder and thin film form was investigated using X-ray diffraction and transmission electron microscopy. Upon heating PCBNB powder above its glass-transition temperature, an amorphous-to-crystalline transition (i.e., cold crystallization) and a subsequent melting of these crystals were observed. A thin film of PCBNB was observed to order on a simple hexagonal lattice (HEX) with the <i>c</i> axis preferentially oriented normal to film at an annealing temperature of 180 °C but became disordered above 200 °C, consistent with the powder results. However, when annealed at 160 °C, the PCBNB thin film ordered on a superlattice of the HEX as indicated both by electron diffraction and high-angle annular dark field scanning TEM (HAADF-STEM) images. The formation of the HEX superlattice polymorph was independent of both solvent and substrate and could be formed both on heating from the amorphous as cast state and by cooling from the HEX structure formed at a higher temperature. HAADF-STEM shows that the superlattice corresponds to a regular deficiency of PCBNB molecules on every fifth (1 1̅ 0 0) plane of the HEX structure

Producing Small Domain Features Using Miktoarm Block Copolymers with Large Interaction Parameters

We demonstrate that small domain features (∼13 nm) can be obtained in a series of polystyrene (PS) and poly­(lactic acid) (PLA) block copolymers, PS–(PLA)₂ and (PS)₂–(PLA)₂, that combine miktoarm molecular architectures with large interaction parameters. To supplement the experimental work, we used self-consistent field theory in tandem with the random phase approximation to explore and contrast the phase behavior of AB_<i>n</i> and A_<i>n</i>B_<i>n</i> types of miktoarm block copolymers. Specifically, AB₂ and A₂B₂ were found to be effective molecular architectures for inducing strong shifts in phase boundaries with copolymer composition and to simultaneously tune domain feature sizes. The performance of these systems is markedly different from the corresponding linear diblock copolymers and indicates the potential of macromolecular architecture control for future applications in lithography

Nanostructured Supramolecular Block Copolymers Based on Polydimethylsiloxane and Polylactide

Hierarchical self-assembly has been demonstrated with diblock copolymers comprising poly­(dimethylsiloxane) (PDMS) and poly­(lactide) (PLA) with supramolecular, 4-fold hydrogen-bonding junctions. PDMS with a single ureidoguanosine unit at the end was synthesized by a postpolymerization strategy. PLA with a single 1,7-diamidonaphthyridine was synthesized by ring-opening polymerization from the appropriate functional initiator. Selective association of the end groups to form distinct, noncovalent connections between the respective homopolymers in blends was established by ¹H NMR spectroscopy. The orthogonal self-assembly of the resulting pseudoblock copolymer, driven by immiscibility between the polymer constituents was demonstrated. Bulk polymer blends were prepared that have approximately symmetric composition and a 1:1 end-group stoichiometry. Small angle X-ray scattering combined with differential scanning calorimetry and transmission electron microscopy provide unambiguous evidence for the adoption of a lamellar morphology having long-range order, nanoscopic domain dimensions (20 nm pitch), and a sharp domain interface defined by the supramolecular building blocks