Dr. Nedoma’s Research

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Polymer Nanomaterials

Commercial polymers accounted for $110 bn in U.S. revenues last year¹, driven by the low cost of materials and ease of processing. The physical properties of polymers can be precisely tailored to an application by controlling the chemical synthesis, composition, and processing conditions. Multifunctionality is engineered into a product by compositing polymers with external materials or blending different species of polymer. Properties like electronic conductivity, hardness, light absorption, birefringence, and impact resistance, to name but a few, can be incorporated into a material by selecting the appropriate combination of polymers and additives. Nanostructured composites exhibit the bulk mechanical properties of a single material, desirable characteristics of each component, and a large interfacial area between components. The international market for nanomaterials is nascent, expected to reach $5.5 bn by 2016 , and is focused on cutting edge applications in healthcare, electronics, and energy generation and storage. Polymer-based nanomaterials are one of the most promising candidates for growth in these developing sectors.

Mission Statement

This research aims to control the structure and properties of polymer nanomaterials by harnessing fundamental thermodynamic and kinetic driving forces. Focusing particularly on self-assembling materials, like block copolymers and semi-crystalline polymers, we use simple systems to build models that are broadly applicable. Moving beyond pure polymers and bench-scale science, we aim to translate polymer physics to scalable technologies. Optimizing the safety and efficiency of nanomaterials manufacturing is integral to developing sustainable new industries.

Plastic Solar Cells

Polymer-based solar cells are an under-commercialized technology that promises to fit into the niches (literally!) that are not suitable for silicon solar panels. Flexible, inexpensive, and easy-to-process, plastic solar cells have inspired a range of products ideas for on-the-go charging of mobile devices: messenger bags, umbrellas, clothing, tents, etc. The greatest obstacles to commercial plastic solar cells are low power conversion efficiencies (PCE’s)³ and short lifetimes (< 10,000 hours)⁴. My research tackles both these problems by exploring fundamental ways to control the structure and properties of plastic solar cells. Rather than compete with silicon solar panels, plastic technologies will advance by focusing on applications that exploit the unique properties that only polymers can provide.

Polymer-based organic photovoltaics are most commonly made from blends of a conjugated polymer and fullerene, the electron donor and acceptor, respectively. Charge separation requires a photo-induced exciton to diffuse to the interface between the donor and acceptor, where the electron separates from the hole and can perform electrical work. Bulk heterojunctions are highly dispersed mixtures of the electron donor and acceptor, yielding a large interface for charge separation. The greatest efficiencies are expected when the characteristic length of the bulk heterojunction is ~10 nm because that is the diffusion length of an exciton. Creating nanonstructured bulk heterojunctions is only half of the challenge, preventing them from coarsening is a separate problem. The phase separated donor and acceptor structures tend to coarsen in order to decrease the free energy of the interface, resulting in decreased device performance. Block copolymers address both challenges by self-assembling into controlled nanostructures that are thermodynamically equilibrated, and therefore stable. Early attempts to use block copolymers in solar cells, nicely summarized in a review by Paul Topham⁵, have uncovered a complex interplay of thermodynamic and kinetic forces. Continuing research and creative processing are needed to realize the potential of block copolymer-based solar cells.

Polymer blends and miscibility

The overwhelming majority of homopolymer pairs are not miscible, owing to the large molecular weight of each molecule. Yet most consumer plastics are composites of one or more types of plastic in order to harness the useful properties of different polymer species. The food packaging industry, in particular, has developed complex work-arounds to produce plastic composites. Potato chip bags contain multiple layers of polypropylene, aluminum, and polyethylene, separated with compatibilizing agents to make the layers adhere. The resulting product exhibits desirable barrier properties to block the diffusion of water and oxygen, as well as mechanical strength and compliance. It cannot be easily recycled, however, because polypropylene and polyethylene must be segregated (hence the resin code numbering system), otherwise they would separate like oil and water in the recycling smelter.

Many applications require a material with homogenous bulk properties, but the attributes of several different polymers. Micro- and nanoscale compositing can achieve apparent homogeneity on the bulk scale whilst retaining the unique properties of the constituent materials. High impact polystyrene, for example, is a block copolymer comprising polystyrene for hardness and polybutadiene for impact absorption. Immiscible as homopolymers, the polystyrene and polybutadiene are reactively blended, forming covalent bonds between the species that prevent large scale phase separation. A fundamental understanding of the thermodynamics and dynamics of polymer blends is essential for designing nanophase separated materials. My work has examined the rare instance of a miscible polymer pair in order to design more effective compatibilizing agents (block copolymers) for immiscible polymer blends.

References

[1] IBISWorld, Plastic & Resin Manufacturing in the US: Market Research Report, July 2015, Website: http://www.ibisworld.com/industry/default.aspx?indid=473.

[2] Freedonia, World Nanomaterials to 2016 – Demand and Sales Forecasts, Market Share, Market Size, Market Leaders, 2012, website: http://www.freedoniagroup.com/World–Nanomaterials.html.

[3] National Renewable Energy Laboratory, Best Research-Cell Efficiencies, 6 June 2015, website: http://www.nrel.gov/ncpv/images/efficiency_chart.jpg.

[4] Ritesh Tipnis, Jan Bernkopf, Shijun Jia, John Krieg, Sergey Li, Mark Storch, and Darin Laird. Large-area organic photovoltaic module–Fabrication and performance. Solar Energy Materials and Solar Cells 93(4), 442-446, 2009.

[5] Paul Topham, Andrew J. Parnell, and Roger C. Hiorns. Block copolymer strategies for solar cell technologies. Journal of Polymer Science Part B: Polymer Physics 49(16), 1131-1156, 2011.