Research in the Locklin Lab
Our research focuses on growing functional polymers from surfaces using
different surface initiated polymerization techniques.
Surface Initiated Polymerization
Much of our research is focused around growing functional polymers from surfaces in a “grafting from” method using different surface initiated polymerization techniques. Surface-bound initiators are tethered to a substrate (such as glass, metal, or plastic) and the polymer is grown directly from the initiator, resulting in the covalent attachment of polymer chains to the surface. In a densely packed environment, the polymer chains adopt an upright conformation, forming what are called polymer brushes. Irreversibly immobilized polymer chains have excellent long-term stability, even adverse environments, which make them attractive for a wide variety of applications. Currently, we are using ring-opening polymerization (ROP), Kumada catalyst-transfer polycondensation (KCTP), Stille catalyst-transfer polycondensation (SCTP), atom-transfer radical polymerization (ATRP) and conventional free radical polymerization to develop functional coatings for the following applications: stimuli responsive surfaces, photo-induced mechanical motion, sensors for biological arrays, antimicrobial coatings and enzymatic biofuel cells.
Reactive polymer brushes add complexity to surfaces through the use of postpolymerization modification. Using a photo-initiated free radical polymerization, polymers are grafted from a surface, resulting in a greater volume of functional groups as the brush extends into the third dimension. Utilizing a grafting from method, as opposed to a grafting to method (which results in films of low grafting density), gives rise to unique interfacial properties, such as wettability, adhesion and self-assembly. Postpolymerization modification can be used to add delicate functionalities to the polymer that would normally not survive the polymerization. For instance biomolecules and nanostructures easily degrade in most polymerization conditions. Also, complex structures such as surface-attached bottlebrush polymers can easily be generated with these reactive platforms. Our main goal is to generate surfaces that contain spatially-resolved functionalities through the use of various patterning techniques. We can use the patterned substrates for different applications, including biological arrays, microfluidic devices and cellular studies.
Orski, S.V.; Poloukhtine, A.A.; Arumugam, S.; Mao, L.; Popik, V.V.; Locklin, J. "High Density Orthogonal Surface Immobilization via Photoactivated Copper-Free Click Chemistry," J. Am. Chem. Soc. 2010, 132, 11024-11026. DOI: 10.1021/ja105066t
Arnold, R.M.; Locklin, J. "Self-Sorting Click Reactions That Generate Spatially Controlled Chemical Functionality on Surfaces," Langmuir 2013, 29, 5920-5926. DOI: 10.1021/la4012857
Conjugated Polymer Brushes
Conjugated polymers are used to make organic electronic devices. We have adapted the standard Kumada catalyst-transfer polycondensation (KCTP) and Stille catalyst-transfer polycondensation (SCTP) to the formation of surface-bound electroactive conjugated polymer brushes. This has been called the “skyscraper approach to nanoelectronics.” The direct binding with the electrode surface (typically ITO) eliminates grain boundaries to help shuttle electrons or holes from the device to generate current. Electrode surfaces are functionalized with monolayers containing an aryl bromide moiety that undergo oxidative addition with a pre-catalyst to form Ni(0) or Pd(0) species. This organometallic constitutes the initiating complex and goes on to catalyze the polymerization. Polymerization precedes in a repeating cycle of oxidative addition, transmetallation and reductive elimination, using AB-type monomers containing a halogen and a transmetallating group (a Grignard or stannane).
We are also probing a new polymerization method called direct (hetero)arylation polymerization (DHAP). Traditional methods of synthesis involve metal-functionalized transmetallating agents, which often generate toxic byproducts. DHAP brings about a new mechanistic pathway to avoid such toxic byproducts making it a greener synthesis for conjugated polymers. A transition metal catalyst is inserted into a C–H bond of an aryl unit with the help of acetate ligands. We are currently probing this technique to synthesize donor-acceptor polymers for organic photovoltaic devices.
Marshall, N; Sontag, S.K.; Locklin, J. "Surface-initiated polymerization of conjugated polymers," Chem. Comm. 2011, 47, 5681-5689. DOI: 10.1039/C1CC10483G
Yang, L.; Sontag, S.K.; LaJoie, T.W.; Li, W.; Huddleston, N.E.; Locklin, J.; You, W. "Surface-Initiated Poly(3-methylthiophene) as a Hole-Transport Layer for Polymer Solar Cells with High Performance," ACS Appl. Mater. Interfaces. 2012, 4, 5069-5073. DOI: 10.1021/am301401n
Huddleston, N.E.; Sontag, S.K.; Bilbrey, J.A.; Sheppard, G.R.; Locklin, J. "Palladium-Mediated Surface-Initiated Kumada Catalyst Polycondensation: A Facile Route Towards Oriented Conjugated Polymers," Macromol. Rapid Comm. 2012, 33, 2115-2120. DOI: 10.1002/marc.201200472
Antimicrobials are part of an ever-expanding toolbox to fight infection. The inclusion of these materials in household items and medical supplies presents both chemical and engineering challenges. Parameters such as miscibility, stability and even appearance can be crucial in these applications. More importantly, these materials must not contribute to the evolution of drug-resistant bacteria, a problem that plagues antibiotics. We are fabricating antimicrobial polymers that function indiscriminately of application, vector, or bacterial strain, and can be incorporated into bulk materials as well as on a surface. In our previous work, photochemical grafting of a pendant benzophenone was used to immobilize antimicrobial polymers to any surface containing a C–H bond. We have shown that the material has substantial antimicrobial capacity against both Gram-positive and Gram-negative bacteria (>98% microbial death).
Dhende, V.; Samanta, S.; Jones, D.M.; Hardin, I.R.; Locklin, J. "One-Step Synthesis of Permanent, Nonleaching, Ultrathin Antimicrobial Coatings for Textiles and Plastics," Applied Materials & Interfaces, 2011, 3, 2830-2837. DOI:10.1021/am200324f
Polymers fashioned with stimuli-responsive chemistry can be used to fabricate dynamic devices that respond to external stimuli such as changes in pH, temperature and light. We have constructed light-active stimuli-responsive bulk hydrogel materials through conventional radical polymerization and ATRP. Polyacrylamide copolymer hydrogels equipped with benzene-1,2-diol (catechol) moieties are sensitive to changes in pH, and, when complexed to aqueous ferric ions, these complexes are reversible. Small changes in the network architecture may influence a change in the macroscopic properties of these materials. Our goal is to exploit this reversible organometallic chemistry to develop new light-active degradable adhesives among other stimuli-responsive materials.
White, E.M.; Yatvin, J.; Grubbs III, J.B.; Bilbrey, J.A., Locklin, J. "Advances in smart materials: Stimuli-responsive hydrogel thin films" J. Polym. Sci. Part B Polym. Phys., 2013, 51, 1084-1099. DOI: 10.1002/polb.23312
Block copolymers (BCPs) are a unique class of polymers that undergo microscale phase transitions with thermal and/or solvent annealing, leading to many different morphologies. In the synthesis of BCPs, one monomer is polymerized followed by chain extension with a different monomer to form AB diblock copolymers. We are currently fabricating phase separated morphologies that utilize an easily-abstracted sacrificial component, which can be used to fabricate nanosensors, patterned surfaces and highly ordered templates.
Surface-Initiated Ring Open Polymerization
One common method to form polymer films is surface-initiated ring opening polymerization (SI-ROP). SI-ROP of bio-sourced monomers have resulted in thin films of 10 nm or less, but optimizing the catalyst will lead to controlled polymerization and generate thicker polymer brush films. We are looking into degradable brushes which can be used to protect certain surfaces or functionalities and be easily removed afterwards.
In situ sensing is a common goal of many research areas where removal and examination of an analyte is difficult or unfeasible, such as in load-bearing materials. By taking advantage of recent developments in mechanophore technology, we are attempting to synthesize polymeric materials that can be tailored to produce an obvious macroscopic signal in response to mechanical stress or damage. Such chemical responders can be included in low weight percent to minimize detrimental effects on the properties of the bulk polymer. This approach to smart materials allows the integration of smart sensing into common polymer materials.
Surface Plasmon Resonance Spectroscopy
Surface plasmon resonance (SPR) spectroscopy is a powerful method to investigate binding events on interface between organic and metal films. In a typical SPR configuration, light travels through prism, reaches the organic-metal interface and reflects back. At a specific incident angle, called the SPR angle, light resonantly couples with free oscillating electrons of metal and excites surface plasmons. This causes a loss of reflectance energy, which is monitored by photodiode. The SPR angle depends on both the thickness and refractive index of the organic layer absorbed on metal film. Based on this concept, we have modified metal films by covalent attachment of organic molecules to study the kinetics and mechanism of various reactions. Moreover, we combine SPR with cyclic voltammetry (CV) using a three-electrode system to elucidate the dynamics of small molecule absorption on gold surface as well as mechanistic steps in polymerizations.