1. Thin Film Block Copolymers for Lithography
Block copolymers demonstrate potential for use in next-generation lithography due to their ability to self-assemble into well-ordered periodic arrays on the 5-100 nanometer length scale. Successful lithographic application of block copolymers relies on three critical conditions being met: high Flory-Huggins interaction parameters (χ) which enable formation of less than 10 nm features, reactive ion etch selectivity between blocks for facile pattern transfer during template formation, and thin-film self-assembly control. The work on this project involves the synthesis and self-assembly of block copolymers exhibiting large χ and low degree of polymerization (N) enabling formation of 5 nm features. Bulk and thin film self-assembly of a variety of morphologies is facilitated by a combination of spin-coating, solvent and thermal annealing, and top coat deposition techniques. As observed by SAXS and AFM, these materials exhibit some of the smallest block copolymer features in the bulk and in thin films reported to date.
2. Graphene Elastomers, Foams and Aerogels
Graphenes are 2D sheets of carbonaceous material that possess extraordinary mechanical properties, thermal and electrical conductivities, and high surface area. However, most of the methods that have been discovered to mass produce graphenes often require costly and tedious purification, along with associated high energy consumption, to achieve the most attractive forms of the material. One of the solutions to reduce the cost of synthesizing graphene while preserving its excellent properties is to use a precursor such as graphene oxide (GO). GO is commonly prepared using the modified Hummers’s method, which combines chemical functionalization with physical exfoliation through strong stirring or sonication of high purity graphite. Through this process, the resulting GO sheets can be functionalized with numerous reactive groups including carboxylic acids, hydroxyls, and epoxides, that can be exploited in other reactions. This research aims to synthesize functional polymer/GO composites for use in various applications, such as temperature/pressure sensors, repositionable adhesives, oil absorbers, gas barrier/separation membranes, etc. This wide range of applications is possible due to the unique chemical and physical properties of graphene derivatives, such as high electrical conductivity and mechanical integrity, in combination with the processability and versatility of polymers. To make these objectives feasible, strategies to effectively disperse GO, reduce GO, and utilize a range of specific interactions between polymers and graphene derivatives will be exploited.
3. Nature Inspired Green Approach to Fiber Manufacturing using Thiol-ene Photopolymerization
Can we make fibers without solvent or heat? This is the challenging question we are addressing in this research. There are a number of processes to make thin fibers including electrospinning, melt blowing, and recently developed ForcespinningTM. However, use of solvents or heat to lower viscosity for processing is common to all existing polymer fiber manufacturing methods and a greener approach to making fibers remains a challenge. However, in nature spiders and silkworms have developed benign ways of making silk fibers with high strength and toughness without involving harsh solvents or heat energy. A spider’s approach of chemically linking small functional units into long chain molecules and solid fibrillar structures while simultaneously extruding the fibers is fundamentally different from current synthetic fiber manufacturing methods, where extrusion of pre-formed long chain polymers is facilitated with organic solvent or heat. Drawing inspiration from nature, we have developed a process where we use light to trigger a thiol-ene chemical interaction that rapidly transforms small reactive molecules in liquid form into solid threadlike structures as they are forced out of a capillary at high speeds. Besides being a greener process, not involving use of solvents or heat, the fibers are also mechanically robust and have excellent chemical and thermal stability. Further, the fiber surfaces have residual functional groups that allow a variety of subsequent chemical modifications. Work is underway towards that direction and also to adapt this approach into other fiber manufacturing methods.
4. Catechol Based Multifunctional Materials
Catechol-containing biopolymers constitute a fascinating class of polymers well known for their intriguing chemical structure and physiological functions including photoprotection, radical scavenging and metal-ion chelation. Although they have a suite of properties not common to many known organic materials, efforts to exploit those properties in technologically relevant materials have been few compared to other biopolymers such as cellulose, chitin or collagen. In recent research, we found that natural catechol and catechol-like synthetic biopolymers have potential as antioxidant additives for certain commercial polymers. For example, an efficient, environmentally friendly, and water applied flame retardant surface nanocoating based on polydopamine (PDA) was developed for foamed materials such as polyurethane (PU). The PDA nanocoating, deposited by simple dip-coating in an aqueous dopamine solution, consists of a planar sublayer and a secondary granular layer structure that evolve together, eventually turning into a dense, uniform, and conformal layer on all foam surfaces. In contrast to flexible PU foams that are known to be highly flammable without flame retardant additives, micro combustion calorimetry (MCC) and thermogravimetric analysis (TGA) confirm that the neat PDA is relatively stable with a strong tendency to form carbonaceous, porous char that is highly advantageous for flame retardancy. Another example is exploiting PDA in order to enable the formation of block copolymer (BCP) nanopatterns on a variety of soft material surfaces. This chemically functionalized, biomimetic layer served as a reactive platform for subsequently grafting a surface neutral layer to perpendicularly orient lamellae-forming block copolymer. Moreover, scanning electron microscopy observations confirmed that a block copolymer nanopattern on a poly(ethylene terephthalate) substrate was not affected by bending with a radius of ~0.5 cm. This procedure enables nondestructive, plasma-free surface modification of chemically inert, low-surface energy soft materials, thus overcoming many current chemical and physical limitations that may impede high throughput, roll-to-roll nanomanufacturing.
5. Patterning by Photochemically Directing the Marangoni Effect
Recently, structured surface topography has been shown to play a crucial role in many emerging technologies. For example, topographically structured films can be used as anti-fouling coatings, adhesion promoters between two surfaces, and energy efficiency enhancements of light emitting diodes and photovoltaics. To meet the great demand for these various applications, we are developing a new polymer patterning methodology based on directing convection in thin films. We exploit the Marangoni effect which causes fluid flow in response to surface energy gradients. We apply photochemical patterning techniques to prescribe surface energy patterns in polymer thin films. Upon heating the solid polymer ﬁlm to a liquid state, the polymer ﬂows from the low surface energy regions (unexposed) to high surface energy regions (exposed) according to the Marangoni effect. This ﬂow creates smooth, three-dimensional topography reflective of the exposure pattern. We believe this methodology will be potentially useful as a high-throughput and contact-free patterning technique that is compatible with existing lithography tools, but does not require an etching or wet development step.