Session R34: Thin Films of Block Copolymers and Hybrid Materials: Hierarchical Structures

Thin Films of Supramolecular Nanocomposites

Supramolecular nanocomposites, composed of polymers, small molecules and nanoparticles, offer numerous opportunities to achieve nanoparticle assemblies with high spatial precision and to incorporate different built-in functionalities by simply varying building blocks. However, as multi-component systems, building blocks are mixed together without forming covalent bonds. There are different energetic contributions governing their phase behavior in bulk and in thin films. Energetically, these contributions are comparable, typically in the range of a few kcal/mol. This makes it feasible to access a rich library of nanostructured composites and enables one to manipulate dynamic nanoparticle assemblies. However, the rather flat energy landscape also presents challenges to precisely control the assemblies in a predictable manner. Here, we present our recent studies on the phase behavior of supramolecular nanocomposites in thin films. We qualitatively describe the effect of the particle-polymer interaction, the polymer chain conformation, the surface tension of each component and the supramolecular morphology on the nanoparticle assemblies in thin films. These basic studies led to well-defined 3-D nanoparticle assemblies of single type nanoparticle and nanoparticle mixtures in thin films. Furthermore, I will discuss our interesting explorations on the dynamics of nanoparticle assemblies in thin films of supramolecular assemblies.   

Hierarchical Structuring in Block Copolymer Nanocomposites through Two Phase Separation Processes Operating on Different Time Scales

The ability to assemble nanoparticles (NPs) hierarchically, with control over their positioning and spacing, is considered an important step toward applications where collective properties are sensitive to the morphology of the NP aggregates. Using block copolymers as matrices for organizing metal and semiconductor NPs through microphase separation leads to hierarchical NP assemblies, but control over NP location within the hosting domains is usually limited to one-dimensional distribution. The presentation will demonstrate by both experimental evidence and mesoscopic simulations that functionalizing the nanoparticles with polymeric ligands that are incompatible with both blocks, but to considerably different extents, leads to hexagonally-packed NP assemblies in every other domain of the copolymer film. Such choice of polymeric components leads to a situation where the microphase separation of the block copolymer precedes the macrophase separation of the NPs from the copolymer. Thus, when the latter finally sets in, it occurs within the confines of the domains hosting the NPs. In the hexagonally-packed arrays formed by this process, the interparticle distances are controlled by the thickness of the nanoparticle coating.   

Hierarchical pattern formation through photo-induced disorder in block copolymer/additive composite films

Segregation strength in hybrid materials can be increased through selective hydrogen bonding between organic or nanoparticle additives and one block of weakly segregated block copolymers to generate well ordered hybrid materials. Here, we report the use of enantiopure tartaric acid as the additive to dramatically improve ordering in poly(ethylene oxide-block-tert-butyl acrylate) (PEO-b-PtBA) copolymers. Phase behavior and morphologies within both bulk and thin films were studied by TEM, AFM and X-ray scattering. Suppression of PEO crystallization by the interaction between tartaric acid and the PEO block enables the formation of well ordered smooth thin films. With the addition of a photo acid generator, photo-induced disorder in PEO-b-PtBA/tartaric acid composite system can be achieved upon UV exposure to deprotect PtBA block to yield poly(acrylic acid) (PAA), which is phase-miscible with PEO. Due to the strong interaction of tartaric acid with both blocks, the system undergoes a disordering transition within seconds during a post-exposure baking. With the assistance of trace-amounts of base quencher, high resolution, hierarchical patterns of sub-micron regions of ordered and disordered domains were achieved in thin films through area-selective UV exposure using a photo-mask.   

Hierarchical multiscale patterned flexible PDMS elastomeric film and its ice-retarding properties

Hierarchical structures in nature inspired development of artificial micro-nano structures in recent years, because these structures exhibit unique properties like tunable adhesion and wetting. We demonstrate a simple yet versatile method to fabricate micro-nano surface based on combination of PDMS nano-imprinting and UVO lithography. Nanoscale patterned PDMS is fabricated by imprinting digital recording media discs (CD/DVD) pattern. The micro pattern was then built by selective densification of patterned PDMS by exposing to UVO through a bigger mask like TEM grid or wire mesh. The nano imprinted pattern remains unaffected during the UVO treatment. We observed that tunable hierarchical structures with height up to 900 nm can be created by simply controlling UVO exposure time. This method provides potential applications in various fields such as superhydrophocity, icephobicity, microfludics and solar cell. We demonstrate that these hierarchical surface exhibits improved icephobicity comparing to flat hydrophobic surface. Icephobocity experiments were carried out in a controlled humidity and temperature chamber. Patterned PDMS film coatings were cooled to -10 $^{\mathrm{o}}$C at a relative humidity of 65{\%}. Temporal formation of ice was observed under optical microscopy.   

Theory of Hierarchical Morphologies in Binary Blends of AB/CD Diblock Copolymers

The self-assembled structures formed in binary blends of $AB$/$CD$ diblock copolymers are studied using the real space Self-Consistent Field Theory (SCFT), focusing on the cases with attractive $A/C$ and repulsive $B/D$ interactions. The attractive $A/C$ interaction prevents macroscopic phase separation, whereas the repulsive $B/D$ interaction leads to the formation of complex nanoscopic structures. The combination of these features makes the $AB$/$CD$ blend an ideal model system for the study of hierarchical self-assembly. Our results demonstrate that the $B/D$ separation leads to the emergence of hierarchal alternate lamellar, cylinders and checker board morphologies from the classical lamellar structure. Similar behavior in the cylindrical phase, where an increase in the $BD$ interaction leads to a phase transition from the classical hexagonally packed cylinders to alternating cylinders, has also been predicted. The theoretical predictions are consistent with available experiments and, more importantly, provide an interesting route for the engineering of hierarchically ordered structures using block copolymer blends.   

Solution Construction of Multigeometry Nanoparticles and Multicompartment Superstructures from Block Copolymer Mixtures

Novel soft objects with both compositional and geometric complexity at nanoscale have been constructed through solution supramolecular assembly from block copolymer mixtures due to their non-ergodic character. The mixture is composed of two block copolymers with distinctive hydrophobic blocks but the same poly(acrylic acid) hydrophilic block. First, multigeometry nanoparticles, due to segregation of unlike block copolymer molecules into multiple subdomains trapped within the same micelle-like structures, have been assembled in tetrahydrofuran/water solution. Through carefully designed molecular architecture, mixing ratio and pathway kinetics, both size and shape of subdomains can be controlled to produce a novel class of multigeometry nanoparticles, including sphere-sphere, sphere-cylinder, cylinder-cylinder, cylinder-disk, and sphere-disk hybrid nanoparticles. Second, hierarchical multicompartment superstructures including particle chains, rings and other nano to micro cluster formations, have been built up from pre-formed multigeometry nanoparticles by taking advantage of their surface anisotropy and the controlled particle-particle association. The interparticle association can be achieved via either covalent or non-covalent bindings due to different post-polymerization chemical modifications with hydroxyethyl acrylate or crown ether functionalities, respectively.   

Mixed Solvent Strategy for the Dispersion of PCBM in Block Copolymer Thin Films

In this work a model system of self assembling cylinder forming polystyrene-b-poly(ethylene oxide) (PS-$b$-PEO) block-copolymer (BCP) and photosensitive phenyl-C61-butyric acid methyl ester (PCBM) nanoparticles were utilized to study extent of nanoparticle dispersion into BCP thin films. We studied effects of different solvents and mixture of solvents for casting variable amount of PCBM loaded PS-$b$-PEO films on the final morphology of the films. Atomic force microscope (AFM) as well as transmission electron microscope (TEM) was employed to study the dispersion of PCBM into PS-$b$-PEO matrix. We were able to disperse more than fifty percent PCBM (wt./wt.) in the film, which is higher than the percolation threshold of nanoparticles, without forming PCBM clusters. Grazing incidence small angle scattering (GISAXS) results show that the mixed solvent strategy resulted in change of domain sizes of thin films due to change of effective interaction parameters. It was found by AFM scratch test that the film thickness is highly dependent on the casting solvent mixtures and nanoparticle concentration.   

Dynamics of block copolymer / nanoparticle composites

We present results of a large scale coarse grained computer simulation for block copolymer nanoparticle composites, Hybrid Cell Dynamics Simulation. Dynamics of the nanoparticles is found to strongly influence block copolymer nanostructure dynamics and vice versa. Different ratios of nanoparticle diameter and block copolymer domain spacing were investigated. The effect of the external fields such as electric or magnetic fields on the dynamics of the particles was incorporated into the computer model and found to influence block copolymer matrix structure. For example, the nanoparticles can controllably induce phase transitions between different block copolymer morphologies. The simulation results gave insights on underlying physical mechanisms in recent experiments on such systems.   

Precise control of magnetic and dielectric nanoparticle placement within block copolymer templates for the fabrication of 3D magneto-dielectric metamaterials

Magneto-dielectric metamaterials fabricated using high permeability (high-$\mu$) nanoparticles (NPs) with precise control over position and orientation could yield superior electromagnetic properties with low loss. In addition, proper tuning of the effective dielectric constant of the host composite could yield more efficient devices with a wider bandwidth. Lin et al. recently reported the use of strong interactions between NPs and one segment of weakly segregated block copolymer (BCP) systems to drive the assembly of well-ordered morphologies while confining the NPs specifically in the desired spherical, cylindrical or lamellar domains. Here we used this approach to assemble high-$\mu$ NPs into well ordered systems. Specifically, FePt nanoparticles functionalized with H-bonding donating ligands were shown to induce strong segregation in weakly segregated BCP systems. In addition, different NP/polymer segment interactions, such as $\pi$-$\pi$ interactions, were introduced to incorporate dielectric NPs in order to tune the effective permittivity of the material. Small-angle X-ray scattering was used to track the morphological evolution of the composite. Transmission electron microscopy was used to investigate the location of the NPs in their respective polymer domains.   

Morphological studies on supramolecular hybrids comprising a block copolymer and semiconductor nanoparticles

Well-ordered periodic nanostructures have been attaining much attention due to their high potential for nano-applications. Nanophase-separated structures of block copolymer/inorganic nanoparticle hybrids are one of good candidates for such applications. Here we report a systematic study on preparation and morphological observation of hybrids composed of a block copolymer and hydroxy-capped cadmium selenide nanoparticles (h-CdSe) via hydrogen bonding. Three polystyrene-$b$-poly(4-vinylpyridine) (PS--P4VP) block copolymers with the same PS chain length but with different P4VP chain length were synthesized for hybrid preparation. Each PS--P4VP was mixed with h-CdSe by varying a weight ratio of PS--P4VP:h-CdSe. A hybrid composed of h-CdSe and PS--P4VP bearing long P4VP blocks represents a single nanophase-separated structure, where domain spacing expansion and morphology transition induced by addition of h-CdSe were observed. On the other hand, macrophase separation accompanied by overflow of h-CdSe from nanophase-separated domains was observed in hybrids which contain PS--P4VP bearing short P4VP blocks. These results are attributed to hydrogen-bonding formation and the stoichiometric balance of functional groups.   

Nanoparticle distribution in complex block-copolymer morphologies

We present our work on the distribution of nanoparticles (NPs) having various shapes (sphere, rod or disk) in different types of directed-self-assembled block-copolymer (BCP) morphologies using hybrid particle-field simulations. The BCP patterns are first obtained by modeling a nanoscale template consisting of ordered posts that are attracted to one of the blocks of BCPs. Once a desired pattern is obtained, we run simulations using the pattern as the initial condition while also including nanoparticles with different shapes, sizes and positions. By calculating the mean-field free energy of the entire system, we study the role that chain stretching and nanoparticle shape and size play in the equilibrium location of the NPs in the BCP matrix. Our results can have important implications in directing the self-assembly of multi-component hierarchical materials.   

Directed Nanorod Assembly Using Block Copolymer-Based Supramolecules

Nanorods display many unique electrical, mechanical, and optical properties unavailable in traditional bulk materials, and are attractive building blocks toward functional materials. The collective properties of anisotropic building blocks often depend strongly on their spatial arrangements, interparticle ordering, and macroscopic alignment. We have systematically investigated the phase behavior of nanocomposites composed of nanorods and block copolymer (BCP)-based supramolecules forming spherical, cylindrical and lamellar morphologies. Initial exploration showed that the nanorods can be readily dispersed in polymeric matrix and the overall morphology of nanorod-containing supramolecular nanocomposite depends on the nanorod-polymer interactions, inter-rod interactions and entropy associated with polymer chain deformation. The energetic contributions from the components of the system can be tailored to disperse nanorods with control over inter-rod ordering and the alignment of nanorods within BCP microdomains.[1] By varying the supramolecular morphology and composition, arrays, sheets, and interconnected networks of nanorods are demonstrated that may prove useful for fabrication of optically and electrically active nanodevices. 1. Thorkelsson, K. et al. Nano letters 2012, 12, 498   

Morphological Control of Charged Block Copolymer Micelle Complexes in Dilute Aqueous Media

Amphiphilic block copolymers dispersed in a selective solvent can be self-assembled into various aggregates such as spherical and cylindrical micelles and bilayer vesicles. Block copolymers typically possess hundred repeat units, leading to kinetically stable or trapped assemblies due to the lack of molecular chain exchange between aggregates in solution; thus, aggregated morphologies are highly path dependent. Here, we demonstrate the amphiphilic block copolymer micelle (BCM) complexes with pH-tunable electrostatic interactions between two differently charged corona blocks in aqueous media. The combination of preformed micellization of each BCM with the dissociation control of the corona blocks provides a distinct assembly pathway. This is to say that the sequential mixing of charged BCMs reveals the effects of both corona complexation (through inter-component interactions) and the manipulation of interfacial curvature between core and corona within a micelle (through the intra-molecular block conformations), resulting in unique complex morphologies such as crystal-like hexagonal prisms, hierarchical spheres, and twisted peapods.