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
Mechanically Stable Thermally Cross-linked Poly(acrylic acid)/Reduced Graphene Oxide Aerogels: Porous carbonaceous materials, such as graphene-based aerogels (GAs), are very attractive because they possess a unique combination of properties including high surface area, excellent thermal and electrical conductivity, and low mass density (a few mgs per cubic centimeter) compared to dense analogs. Such aerogels have a wide range of potential applications in energy storage devices, polymer composites, sensors, etc. However, one major drawback for GAs is that they are often very fragile, due to the weak Van der Waals interactions between adjacent two-dimensional graphene sheets, resulting in a delicate structure. We recently demonstrate a method to enhance the mechanical properties of a specific type of GA composed of graphene oxide (GO) using a small amount of inexpensive, commercially available and thermally cross-linkable poly(acrylic acid) (PAA). Additionally, GO can be reduced to form reduced graphene oxide (rGO) in a single step using hydroiodic acid (HI) vapor. The thermally crosslinked graphene oxide aerogels (termed XPAA/rGO) exhibit dramatically improved mechanical properties, while all the other attractive features of GAs (high surface area, high porosity, high electrical conductivity, low density) are maintained.
Highly Elastic Poly(dimethylsiloxane)/ Graphene Oxide Composite Elastomers: Polymer-based membranes are attractive for industrial gas separation processes due to their low cost, small process footprint and ability to be mass produced. Addition of filler particles is a simple way to tailor the performance of these membranes. In our study, we have successfully synthesized a PDMS/GO elastomer with enhanced mechanical properties and gas selectivity. The GO platelets can act as a filler to improve the gas barrier performance of the otherwise highly permeable PDMS membrane. With only 8 wt% (3.55 vol%) of GO incorporated in the PDMS matrix, the permeability of several gases can be reduced to 99.9% when compared to the neat PDMS, which was attributed to the tortuous pathways formed by the gas-impermeable GO sheets. Notably, our experimental results demonstrated that the selectivity of CO2/N2 meets the minimum desirable selectivity for post-combustion carbon capture processes.
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. 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. Drawing inspiration from nature, we have developed a reactive fiber spinning processes based on melt blowing and electrospinning platform to produce cross-linked nonwoven fiber mats. In these processes, 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. Our aim is to better understand the process in terms of fiber diameter and morphology with particular focus on the photopolymerization reaction kinetics and material properties. Efforts are also ongoing to develop UV curable monomers that can generate nonwovens with different targeted end properties. As a part of this research, collaborative effort between the Ellison and Reineke groups have led to the production of fibers that are both renewable and degradable to address the problem of microfibers pollution.
4. Bio-derived and Biodegradable Polymers
Salicylic acid based biodegradable polymers: Polyesters constitute around 10% of the global plastic market with aromatic polyesters, such as poly(ethylene terephthalate) (PET), being the most prevalent because of their attractive properties. As for most commercial plastics, polyesters are primarily derived from fossil resources and are not readily degradable, which raises a number of sustainability concerns. Degradable polymers with commercially competitive properties are considered attractive alternatives because they could partly address plastic waste concerns. However, some polyesters labeled as degradable are not readily degradable in natural environments but rather require more aggressive conditions. Therefore, designing polymers with competitive properties from sustainable feedstocks that rapidly degrade under mild conditions is an attractive strategy for addressing the current plastic waste problem. The works on this project involve the design and synthesis of readily degradable polymers with competitive properties by utilizing the key chemical structure, salicylic acid moieties. While an aromatic group provides good thermal and physical properties, aryloxide group induce the rapid fragmentation. Polymer properties (e.g., glass transition, thermal degradation, tensile strength, oxygen barrier properties), miscibility with other polymers (e.g., PET), degradability under aqueous (e.g., seawater) and composting conditions are evaluated for potential future applications.
Toughening PLA: As members of the Center for Sustainable Polymers, we study sustainable materials with the goal of improving their mechanical properties to replace a wide range of non-sustainable (petroleum-based) polymers, limiting the negative environmental effects of plastics. For example, one of our research interests is toughening poly(lactide) (PLA). PLA is commercially available (synthesized from corn) and is industrially compostable, making it a sustainable alternative to petroleum-based polymers which buildup in landfills and the environment. However, PLA is intrinsically brittle due to its rapid physical aging. Our research involves designing block copolymer (BCP) additives to enhance its toughness and ductility. We study the mechanical properties and toughening mechanism by performing tensile tests, in-situ tensile tests during small angle x-ray scattering, and transmission electron microscopy.
5. Recycling and Reuse
Plastic pollution is one of the most pressing global environmental issues we face today, in part due to the continued rise in production and use of disposable plastic products. Polyolefins (e.g. polyethylene (PE), isotactic polypropylene (iPP)) and polyesters (e.g. poly(ethylene terephthalate) (PET)) are two of the most prevalent types of polymers in the world accounting for ∼80% of total non-fiber plastic production. Recycling, despite being intrinsically environmentally friendly and sometimes economically viable, remains at a surprisingly low level (<9% in the U.S.) with most plastic waste ending up in landfills. One reason for this low rate of recycling stems from the challenge of recycling mixed waste streams and multicomponent plastics. In mixed waste streams, physical presorting of components prior to recycling requires significant effort, which translates to added cost. For multicomponent plastics (e.g., multilayer films such as food wrappers), the individual plastic components cannot be efficiently physically separated, and they are immiscible with poor interfacial adhesion when melt reprocessed. Thus, direct recycling of mixed plastics by melt reprocessing results in products that lack desired end-use properties. In our research group, we have explored multiblock copolymers (MBCPs) as compatibilizers in both PE/iPP and PE/PET system. We evaluated their utility as adhesive tie layers in multilayer films and compatibilizer additives for melt-reprocessed blends. Another reason for low rate of recycling, especially with polyolefins such as iPP, is that the strength and toughness is sacrificed in the process of blending different grades of the same polymer, rendering the subsequent product less commercially attractive. To this end, our group has delved into crafting additives from diblock copolymers which offer competitive mechanical properties while requiring lower concentrations than are necessary with traditional ethylene-propylene rubber (EPR) toughening agents.
6. 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.