ABSTRACT

Tailoring materials to spontaneously undergo changes in surface properties is a developing area of research. These stimuli-responsive films can undergo surface changes that include wettability, adhesion, interaction with cells or proteins and membrane permeability. There has been extensive study on stimuli-responsive films based on diblock copolymer brushes. These diblock brushes can undergo compositional changes at the surface in response to solvent, temperature, pH and ionic strength. These reversible changes in surface composition have considerable potential in the field of separation technology. Passive flow control of diblock copolymer brushes in microchannels will be studied. The literature contains several reports where the responsive behavior of grafted polyacrylamides has been used to control flow in microfluidic devices. The reversible rearrangement of diblock copolymer brushes should offer an alternative responsive system where, depending on the composition of the fluid stream, the surface can change its affinity for different solutes, alter flow rate or adjust flow between competing microchannels. The focus of this proposal is on the application of polymer brush synthesis and characterization to microchannels. Several simple testing platforms are described to assess the effectiveness of diblock brush rearrangement on passive flow control. Specifically, silane-based initiators for atom transfer radical polymerization will be used to prepare diblock copolymer brushes on the interior of glass capillaries or on the surface of flat glass substrates. The testing platforms all rely on pressure-driven flow using commercially available micropumps. Flow will be determined using liquid mass flow meters, which have nL/min flow sensitivity. Preliminary flow studies will be conducted with polyelectrolyte homopolymers. Three specific testing platforms are proposed. Initial screening of the polyelectrolyte homopolymers and diblock rearrangement will be conducted using a single glass capillary with one pump and one flow meter. In the second testing platform, a simple T-pattern will be created in PDMS using soft lithography. The PDMS mold will be combined with a glass substrate that has patterned brush chemistry so that the two channels possess different diblock brush compositions on the glass portion of the microchannel. Relative flow output will be monitored by liquid mass flow meters. The third testing platform is a simple split capillary system where competitive flow will again be monitored. Because most microfluidic analyses use aqueous streams, we will concentrate on hydrophobic/hydrophilic diblock brush systems where the hydrophilic block is a cationic or anionic polyelectrolyte. A wide variety of additional block compositions can be envisioned based on results from our prior support. Variables in the flow streams that will be examined include: pH, ionic strength, concentration of solutes, and polarity of the medium.

The intellectual merit of the proposed research is that this is first study to explore the application of diblock brush rearrangement as a flow control element in microchannels. To date, the use of surface-immobilized polymers in microchannels has focused on relatively primitive polymer systems. This study will employ state-of-the-art techniques in polymer synthesis to create stimuli-responsive coatings on the interior of microchannels. Successful control of flow in microchannels that is induced by compositional changes in the analyte stream will have a potentially large impact on the field of microfluidics and separation technology.

The broader impact of the proposed research is a multidisciplinary project that will train graduate students, including one woman, in organic, polymer and physical chemistry. Undergraduate students may also participate through an REU summer supplement, with mentoring by graduate students. Participants will present their results at ACS and Gordon Research conferences. Students will learn how to use dry box techniques, NMR, GPC, FTIR, TGA, ellipsometry and tensiometry during the course of their research. This work will contribute to general field of microfluidics by testing and assessing the hypothesis that diblock brush rearrangement can be used for passive flow control in microchannels.

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