Session U4: Polymer Microstructures

On the Formation of an Ordered Array of Holes in a Polymer Film:What can Dew Formation Teach Us?

Systems driven far from equilibrium have a remarkable tendency to produce very ordered structures. Such ordered structures have been observed in many a situation where the material subjected to an external perturbing field responds to this perturbation by creating ordered periodic structures. We have used a system driven far from equilibrium to create structures that have subwavelength dimensions and can be made reproducibly. Ordered subwavelength structures are ubiquitous in nature. However, it is only recently that ordered macroporous materials with pore dimensions on the order of the wavelength of visible light have attracted much greater attention. This interest has been in large part due to their anticipated optical properties. We have demonstrated the use of a simple and robust method that uses evaporative cooling for the formation of ordered structures with dimensions that are controllable in a systematic way ranging from about 0.2$\mu $m to 20 $\mu $m. This method uses the formation, and subsequent crystallization of ``breath figures,'' to create the structures. When a cold solid or a liquid surface comes in contact with moist air, moisture condenses on the surface, forming water droplets that grow with time to form patterns on the surface. Such phenomena, referred to as ``breath figures,'' have been studied in detail, starting with the early works of Lord Rayleigh, Baker and Aitken, and more recently by Knobler and co- workers who demonstrated that it was possible to form a hexagonally ordered array of water droplets on a liquid surface as condensation proceeded. We have used ``breath figures'' to form three-dimensional, ordered macroporous arrays with controllable dimensions. We generated breath figures on dilute solutions of polystyrene and other conjugated polymers dissolved in volatile solvents. When solvent evaporation is complete, one is left with a two or a three dimensional array of holes. In this presentation we will discuss the mechanism of structure formation as well as point to some applications for these structured films.   

Block Copolymer Lithography

During the past half decade, extensive resources have been allocated to the development and implementation of new lithographic exposure tools for use by the microelectronics industry to pattern devices with critical dimensions of 50 nm and below. During this same timeframe relatively modest investments were made in the development of imaging materials. As feature dimensions shrink to below 50 nm, however, traditional materials such as chemically amplified photoresists may not be suitable to overcome significant new challenges with respect to problems such as line edge roughness and critical dimension control at the atomic and molecular level respectively. We explore and develop new materials and processes for advanced lithography in which self-assembling block copolymers are integrated into existing manufacturing processes for patterning high resolution structures that are useful for the fabrication of microelectronic devices. A principal concept is to combine the properties of advanced exposure tools (registration, pattern perfection) with the principles of molecular self-assembly (structures of molecular dimensions, thermodynamic control over pattern dimensions and line edge roughness) to meet strict criteria for device manufacturing at the nanoscale. Here we demonstrate that by depositing thin films of ternary blends of block copolymers and homopolymers on chemically nanopatterned substrates with tailored interfacial interactions, we can direct the assembly of perfectly ordered and registered domains with respect to the lithographically defined underlying surface pattern with considerable process latitude. We also demonstrate for the first time that it is possible to direct the assembly of block copolymer domains with non-linear patterns and arbitrary shapes.   

Microstructures of Polymer-Inorganic Hybrids

The study of polymer based self-assembly (bottom-up) approaches to multifunctional polymer-inorganic hybrid materials is an exciting emerging research area interfacing solid state and soft materials and offering enormous scientific and technological promise. By choice of the appropriate synthetic polymers as well as ceramic precursors unprecedented morphology control down to the nanoscale is obtained. Tailoring of the polymer--inorganic interface is of key importance. The structures generated on the nanoscale are a result of a fine balance of competing interactions, a typical feature of complex biological systems. The potential for new multifunctional materials lies in the versatility of the polymer chemistry as well as that of the inorganic chemistry that can be exploited in the materials synthesis. In the present contribution physical insights into the way how to direct microstructures of polymer-inorganic hybrid materials will be presented. In all cases cooperative self-assembly of organic and inorganic species is induced by amphiphilic macromolecules, either block copolymers or extended amphiphilic dendrons, which are blocked species with one block being highly branched. Resulting microstructures are discussed based on the phase behavior of the parent polymer systems that act as structure directing agents. Morphology diagrams of the resulting polymer-inorganic hybrid materials are presented illustrating differences in self-assembly of parent polymer and resulting hybrid systems. Finally, mechanistic structure formation studies are highlighted that elucidate necessary requirements for successful hybrid nanostructure control.   

Use of Polymer Micro-Structures for Drug \& Gene Delivery

The design of polymer microstructures, including polyelectrolyte-surfactant complex formation, plays an important role in the protection and controlled release of drugs {\&} DNA fragments. Two examples are presented: one for drug release and one for gene delivery. Non-viral gene therapy is a challenging problem that has not yet met much success even though numerous attempts have been made. The gene delivery illustration aims to present one specific approach on how DNA fragments can be delivered to a cell by using an electro-spun scaffold as a carrier, i.e., to consider how DNA fragments can be trapped into a scaffold for subsequent release and transfection. Our scheme is to capture the DNA fragments by taking advantage of the DNA coil-to-globule transition and to encapsulate the condensed DNA globule by using block copolymers. The supra-molecular capsule can then be incorporated into a nano-structured biodegradable polymer scaffold by means of electro-spinning. Subsequent DNA release to cells that adhere to the scaffolds was measured by using fluorescence microscopy.\newline\newline \textbf{\textit{Acknowledgements}}\newline \textbf{\textit{Financial Support:}}\newline \textbf{National Science Foundation, Polymers Program (DMR9984102 {\&} Creativity Extension Award), Center for Biotechnology at Stony Brook, ITG Grant, and NIH SBIR Grant to }\textbf{\textit{STAR.}}\newline \textbf{\textit{Main contributors}} include Professors Benjamin S. Hsiao and Michael Hadjiargyrou, Drs. Dufei Fang, Dehai Liang and Kwangsok Kim, Ms. K. Luu and Mr. J. Chiu.   

Functional Microstructures from Iron-Containing Block Copolymers

We have studied the properties of microstructures formed by diblock copolymers composed of an organic block such as polystyrene or polyisoprene, and an iron-containing block such as poly(vinyl ferrocene) or poly(ferrocenyldimethylsilane). We demonstrate that the thermodynamic state of these block copolymers can be controlled by altering the redox state of the ferrocene (Fc) moieties. Oxidizing only 8{\%} of the Fc block results in a 40 K drop in the order-disorder transition temperature. Fc is catalytically active in the oxidized state. Thus one can obtain catalysts from iron-containing block copolymers wherein both the support and the active sites are formed by self-assembly. An interesting property of ferrocene is the fact that its oxidation state can be altered reversibly by the application of small electric fields ($\sim $2V/cm). We are currently exploring the possibility of using electric fields to control the microstructure and function of our iron-containing block copolymers.