Session J19: Focus Session: Hierarchically and Templated Ordered Systems I

Locally induced phase transitions in semi-crystalline block copolymers

This abstract is not available.   

Alignment of Hydrogen Bonded Liquid Crystalline Block Copolymers by Magnetic Fields

Hydrogen bonding between a poly(styrene-b-acrylic acid) backbone and an imidazole terminated biphenyl mesogen results in the formation of a side-group liquid crystalline block copolymer (LC BCP). We use a combination of FTIR, X-ray scattering and DSC to characterize the phase behavior of the PAA-LC system, which is largely dominated by the sub-stoichiometric saturation of the binding capacity of the chain. Alignment of a hierarchically ordered lamellar BCP was performed using a 5 T magnetic field at elevated temperature in the melt state and characterized by SAXS. The system exhibits a tilted smectic structure, which on alignment by the field displays scattering patterns akin to those observed in bookshelf or chevron-type structures. These results demonstrate that simple non-covalent interactions can be used to generate LC order and thus provide a convenient handle for subsequent alignment of BCP structures by magnetic fields.   

Using non-thermodynamic mechanisms to create tailored morphologies in polymer nanocomposites

A variety of non-thermodynamic approaches for both dispersion of nanoparticles (NPs) and the creation of organized, NP-rich phases in a polymer template has been investigated. To facilitate materials development, we have focused our preliminary efforts on commodity materials including colloidal silica and a poly(styrene-b-ethylenebutylene-b-styrene) copolymer (SEBS), processed using twin-screw extrusion and mineral oil as a solvent/diluents. Guidance and a theoretical framework is provided using a field-theoretic approach that combines the self-assembly behavior of the block copolymer matrix, the effects of solvent, and the effect of ligand-functionalized NP miscibility on morphology. The morphological behavior of these materials will be compared with the model predictions, as will the effectiveness of the non-thermodynamic mechanisms for creating controlled, tailored morphologies.   

Hierarchically Ordered Polymer/Block Copolymer/Nanoparticle Systems Enabled via Holographic Photopolymerization

Combining top-down and bottom-up methods in one process step creates novel hierarchical nanostructures with tailored properties. We demonstrate the ability to spatially pattern block copolymers or nanoparticles into periodic volume gratings using a one-step holographic patterning (HP) technique. The confinement of the second phase (e.g. block copolymer or nanoparticle) within the grating results in local preferential ordering of that phase, enabling single-step fabrication of complex hierarchical structures. As an example, this simple interfering photopolymer `writing' technique was utilized to generate a layer-in-layer structure of poly(ethylene oxide-b-epsilon-caprolactone) confined between crosslinked resin. The period of the volume grating was 200 nm, with the BCP occupying 100 nm of the 1-D structure. The BCP crystallized/phase separated to produce a lamellar structure with a 21 nm periodicity. This system exhibits interesting thermo-optical behavior during heating/cooling cycles. Alternatively, a layered polymer/nanoparticle composite was created through a one-step two-beam interference lithographic exposure of a dispersion of silica nanoparticles within a photopolymerizable mixture at a wavelength of 532 nm. The effects of exposure time and power, nanoparticle size, and periodicity on the nanocomposite structure were measured with transmission electron microscopy. The optical properties of the formed gratings were probed in real-time during formation. Collaborators on this work include M. Birnkrant and C. Li from Drexel University, A. Juhl and P. Braun from UIUC, and L. Natarajan, V. Tondiglia, and R. Vaia from AFRL.   

Super Helix Formation From a Partially Charged Diblock Copolymer

Polypeptoids, or N-substituted glycines, are a simplified biomimetic platform designed to combine attributes of biological polymers such as sequence specificity with the potential for hierarchical self assembly inherent to synthetic polymers. In this study, the aqueous self-assembly of a monodisperse partially charged amphiphilic diblock copolypeptoid is shown to result in giant super helices with diameters of approximately 615 nm and lengths ranging from 2-20 um. These structures are highly unusual in several ways including their large size, hierarchical structure, and homochirality. Sequence specific chemical modifications coupled with x-ray scattering have led to a model of self-assembly whereby lamellar stacks roll up to produce the large supramolecular structure. In order to understand the effect of charge density and location on the self-assembly process a series of polymers were synthesized pinpointing the charge to specific chain locations and varying the overall amount of available charge. While location of the negative charges is found to have a minimal effect on the self-assembly process, charge density is shown to have an important role. This role is complex, involving both electrostatic and hydrogen bond effects.   

Spontaneous Evolution of Nanostructure in Composite Films Consisting of Mixtures of Two Different Block Copolymer Micelles

Diblock copolymers consisting of two immiscible polymer blocks covalently bonded together form various self-assembled nanostructures such as spheres, cylinders, and lamellae in bulk phase. In a selective solvent, however, they assemble into micelles with soluble corona brushes and immiscible cores. Both polystyrene-poly(4-vinylpyridine) (PS-b-P4VP) and polystyrene-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymers form micelles with PS coronas and P4VP or P2VP cores in a PS selective solvent (toluene). By varying the mixture ratio between PS-b-P4VP and PS-b-P2VP, composite films based on the micellar mixtures of PS-b-P4VP and PS-b-P2VP were obtained by spin-coating, followed by the solvent annealing with tetrahydrofuran (THF) vapor. Since THF is a solvent for both PS and P2VP blocks and, at the same time, a non-solvent for the P4VP block, PS-P2VP micelles transformed to lamellar multilayers while PS-P4VP micelles remained intact during the THF annealing. The spontaneous evolution of nanostructure in composite films consisting of lamellae layers with BCP micelles were investigated in detail by cross-sectional TEM and AFM.   

Tunable Nanoparticulate Scaffolds from Self assembly in Surfactant Mesophase

We examine the organization of silica nanoparticles in non-ionic surfactant, C$_{12}$E$_{9}$/water, H$_{1}$ phase. We show, using SAXS, TEM , Freeze Fracture and Microscopy, that particles with sizes smaller than the mesophase characteristic size, $a$, template the mesophase; particles with size $\sim \quad a$, partition into a dispersed phase and into strand-like aggregates. Particles of size $> \quad a$ phase separate to form particulate strands organized into a scaffold. The particulate network forms by expulsion of particles from growing H$_{1}$ phase domains; particles are concentrated in the isotropic phase and jam at domain boundaries. Changing the cooling rate into the H$_{1}$ phase changes the nucleation of domains -- hence providing a route to tune the network mesh size. When the surfactant is heated to the micellar phase, the particles redisperse readily. SANS confirms that C$_{12}$E$_{9}$ forms a bilayer on the silica particles, preventing their irreversible aggregation. PEI coated particles (with size $>a)$ also phase separate to form networks. The network of such coated particles can be crosslinked to obtain a free standing nanoparticulate scaffold.   

Mechanical properties of holographically-defined porous polymers and carbonized polymers

A new class of porous materials, so called a holographically-defined porous material (HDPM), has been available by the interference of multiple laser beams. Since the solid volume fraction ($f_{V})$ can be systematically adjusted without altering periodicity, we studied the mechanical behavior of polymer-HDPM as a function of $f_{V}$ using nanoindentation. The observed elastic modulus of the polymer-HDPM has a significantly lower value than what expected from theoretical models at a high $f_{V}$. We confirmed that polymer-HDPM has two different plastic deformation modes from the transition in modulus and in energy absorption associated with the deformation. We also studied carbon-HDPM and compared these materials with the polymer-HDPM in energy absorption. Although carbon is a highly elastic material in bulk, carbon-HDPM absorbs energy by inelastic deformation including highly localized cracks. The energy absorption per residual indentation volume is proportional to a cube of indentation depth for all carbon-HDPM. Interestingly, when a carbon-HDPM has a partially disconnected network, its behavior was clearly different with the connected and rather similar to polymer-HDPM.   

Controlling Hierarchically Self-Assembly in Supramolecular Tailed-Dendron Systems

We study the self-assembly of a dendritic macromolecular system formed by a second-generation dendron and a polymer chain emanating from its focal point. We use supramolecular ionic interactions to attach to the periphery of the dendrons sulphated alkyl tails. The resulting ``triblock copolymers'' have a molecular architecture similar to a four-arm pitchfork with varying arms and holder lengths. The bulk morphologies observed by SAXS and TEM show thermodynamically stable, hierarchical ``inverted'' hexagonal or lamellar structures. The structural models for the molecular packing emerging from experimental findings are benchmarked to available self-consistent field theories (SCFT) and experiments and theoretical predictions are found in perfect agreement. The present results show that supramolecular systems based on tailed dendrons and surfactants can be used to scale up of the structural organization from the liquid crystalline length scale to the block copolymer length scale, while preserving the inverted unconventional morphologies offering new possibilities in the design of nanostructured materials.   

Evolution of Ordered Block Copolymer Serpentines into a Macroscopic, Hierarchically Ordered Web

Cylinder-forming diblock copolymer solutions are confined in a restricted geometry comprised of a spherical lens placed upon a flat substrate. At the micrometer scale, the synergy of the controlled evaporative self-assembly of a polymer solution and controlled fingering instabilities mediated by the interaction between the polymer and substrate yields intriguing concentric serpentine microstructures of diblock copolymer over large areas. Selective solvent vapor annealing then transforms these microstructures into a macroscopic web-like pattern composed of regularly arranged microporous mesh arrays, at the same time forming domains of closely packed, nanoscopic hexagonal cylinders of diblock copolymer that are vertically oriented to the surface of the web at the nanoscale. This approach thus utilizes two consecutive self-assembly processes to precisely organize unique nanomaterials into spatially ordered structures that can serve as functional materials for potential applications in optical, electronic, and photonic devices, templates for complex structures and pattern transfer, among other areas.