Research

The ψ group is focused on using polymers to address some of the key challenges in the areas of renewable energy and environmental science. Particular areas of interest include Superoleophobic surfaces, Superhydrophobic Surfaces, Ice-Repellent Surfaces, Membranes, Polymer Nanocomposites, Thermoelectrics, Solar Cells, and Liquid-liquid Separations.


Some of the research projects that we are currently pursuing include:


Membranes for oil-water separation.


There is an acute need for the development of new energy-efficient solutions to separate oil-water mixtures as both the production of oil and oil-transport engender a severe environmental risk in sensitive ecosystems. In many ways 2010 was a banner year highlighting this risk, as evidenced by the deepwater horizon oil-spill disaster off the coast of Louisiana, the Chinese tanker that ruptured on the Great Barrier Reef in the Indian Ocean and the Deep Horizon Gulf Rig that exploded and sank.


Mixtures of oil and water are classified based on the size of oil droplet (doil) – free oil if doil > 150 microns, dispersed oil if 20 microns < doil < 150 microns and emulsified oil if doil < 20 microns. We have recently developed a novel solution for the separation of free oil, dispersed oil, and oil-water emulsions based on the design of hygro-responsive (from the Greek word ‘hygra’ meaning liquid) surfaces. These surfaces, counter-intuitively, are wet by water, but are still able to repel low surface tension oils like rapeseed oil or hexadecane . This makes these porous surfaces ideal for gravity-based separation of oil and water as they allow the higher density liquid (water) to flow through while preventing the flow of the lower density liquid (oil). We have also developed strategies that allow us to use these membranes for the continuous separation of surfactant stabilized oil-in-water and water-in-oil emulsions. For more information see Kota et al., “Hygro-responsive membranes for effective oil-water separation”, Nature Communications, 2012,  3:1025, DOI: 10.1038/2027.


In addition, we have also recently developed a novel methodology that uses an electrical potential to separate out almost all types of oil-water mixtures. See Kwon et al., “On-demand separation of oil-water mixtures”, Advanced Materials, 2012, 24 (27), 3666-3671.












Developing mechanically robust, transparent, superoleophobic surfaces.


The wetting of surfaces has generated immense interest in material scientists for decades because of its prominent role in a wide range of daily phenomena and commercial applications. The simplest measure of wetting on a smooth surface is the equilibrium contact angle (θ), given by the Young’s equation. Surfaces that display contact angles greater than 90° with water are considered hydrophobic, while surfaces that display contact angles greater than 150° and low contact angle hysteresis (i.e. the difference between the advancing θadv and the receding contact angle θrec) are generally considered to be superhydrophobic.


The most widely-known example of a natural superhydrophobic surface is the surface of the lotus leaf (Nelumbo nucifera). Numerous studies have suggested that it is the combination of surface chemistry and roughness on multiple scales on the lotus leaf’s surface that allows for the trapping of air underneath a water droplet, thereby imbuing the leaf with its characteristic superhydrophobicity. However, a liquid with a markedly lower surface tension like hexadecane (γlv = 27.5 mN/m) rapidly wets the lotus surface leading to a contact angle of ~ 0°, clearly demonstrating the leaf’s oleophilicity. Indeed, in spite of the plethora of superhydrophobic surfaces now available, there are no naturally occurring superoleophobic surfaces, i.e. surfaces that display contact angles greater than 150° with organic liquids such as alkanes having appreciably lower surface tensions than water. 


In recent work, we developed the first-ever superoleophobic surfaces by considering the effects of re-entrant surface texture on surface wettability. Further, to aid the systematic engineering of non-wetting surfaces, we developed four design parameters that allow us to provide an a priori estimation of both the apparent contact angles, as well as the robustness of the composite interface, supported with a given contacting liquid. We are currently investigating various methodologies to develop mechanically robust and transparent superoleophobic surfaces. Such surfaces are expected to have a wide range of commercial applications, including the development of surfaces with enhanced solvent-resistance, stain-resistant textiles, ‘non-stick’ coatings, controlling protein and cell adhesion on surfaces, engineering surfaces with enhanced resistance to organic solvents, reduction of biofouling and the development of finger-print resistant surfaces for flat-panel displays, cell-phones and sunglasses.









Directed self-assembly of nanoparticles in specific block copolymer domains


The micro-phase separation of diblock copolymers (DBCP) produces different phase morphologies, from spheres to lamellae to interconnected networks, depending on the relative length of each polymer block. Further, DBCP’s spontaneously self-assemble into regular domains whose size can be tuned from a few to several tens of nanometers, depending on the molecular weight of each block. These domains in turn can be used to sequester nanoscale inclusions, allowing for precise control over the size, particle density and spatial location of various inorganic nanoparticles. Such control over the spatial distribution of nanoparticles (NP) provides the opportunity to tailor the electronic, optical or magnetic properties of the obtained nanocomposite.


Various research groups have used different techniques to direct the assembly of inorganic NP’s to individual DBCP domains, including in-situ and ex-situ synthesis of nanoparticles, chemical binding and electrophoretic deposition. However, by far the most typical experimental method employed for directing the nanoparticle assembly involves the surface modification of nanoparticles, to improve their compatibility with one of the DBCP phases. Such surface modification is also anticipated to be essential in preventing nanoparticle aggregation.


In previous work, we discussed various strategies for the dispersion of spherical nanoparticles in homopolymer melts and found a simple heuristic that dictated nanoparticle miscibility. An experimentally produced phase diagram suggested that as long as the nanoparticle radius (a) was smaller than the polymer radius of gyration (Rg), the nanoparticles could be well dispersed within the polymer matrix. This miscibility condition was found to remain valid despite chemical dissimilarity, as demonstrated by the miscibility of polyethylene (PE) nanoparticles in linear polystyrene (PS) or PS nanoparticles in poly (methyl methacrylate) (PMMA).


We are currently investigating if it may be similarly possible to direct the assembly of spherical nanoparticles in individual DBCP domains depending on the relative magnitude of the nanoparticle radius and the radius of gyration of each DBCP block. One of the key applications of this work will be in the development of high efficiency, hybrid block copolymer – inorganic nanoparticle solar cells.



 
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  Polymers, Surfaces and Interfaces (PSI - ψ) group

   Research

Also see Tuteja et al., Science, 2007, 318, 1618-1622.

Also see Tuteja et al., Science, 2007, 318, 1618-1622.

Also see Tuteja et al., PNAS, 2008, 105, 47, 18200-18205

A polymer radius of gyration – nanoparticle radius phase diagram, with the solid markers representing systems where phase separation was detected and the open markers denote miscible systems.

Also see Mackay et al., Science, 2006, 311, 1740-1743

   Outreachhttp://www-personal.umich.edu/~atuteja/Outreach.html

Also see Kota et al., Nature Communications, 2012, 3:1025, DOI: 10.1038/2027, and Kwon et al.Advanced Materials, 2012, 24 (27), 3666-3671