Our goal is to develop combinatorial and high-throughput measurement approaches for rapid characterization of the structure and phase behavior of multicomponent fluid mixtures, as needed to advance the development of polymer-based formulations. Our approaches, which leverage microfluidic "lab on a chip" concepts, help industry discover, develop, and understand the complex fluids that are the basis of formulations, including paints, coatings, adhesives, pharmaceuticals, and personal care products.
Bringing new and optimized formulated products to market requires measurements that will allow rapid assessment of the structure and properties of multicomponent mixtures over large parameter spaces. To this end, we are developing microfluidic "lab on a chip" technologies that enable researchers to form, process, and analyze large numbers of fluid samples, using small amounts of material. As opposed to traditional microfluidic devices, which are typically designed for room temperature aqueous solutions, our devices are made for higher temperatures and organic fluids, including polymer mixtures. For example, NIST led the way by prototyping microfluidic devices in solvent-resistant, photo-patternable epoxies. We are also developing modular in situ measurements using Raman spectroscopy for chemical composition and extent of reaction, dynamic and static light scattering for measuring fluid and particulate nanostructures, and techniques for assessing interfacial tension and viscosity. In addition, integrated active mixing elements, temperature control, and UV-light sources enable on-device fluid processing, higher temperature reactions, and controlling interactions between several input components.
Polymer additives in formulations such as coatings, personal care goods, and pharmaceuticals can enhance key performance properties, but optimization of these components requires means to measure critical structure-property relationships over vast multivariate spaces.
This year, we succeeded in building a microfluidic platform that integrates the synthesis, processing and in-situ analysis of polymer surfactant materials. Our new instrument demonstrates how a "lab on a chip" approach can be used to screen families of specialty nanostructured additives in fluid formulations. A schematic of our device can be seen in the illustration above. To accommodate the temperatures and organic solvents needed to make and process many engineered polymers, we developed a novel class of microfluidic devices that are machined into aluminum plates, which are sealed with polyimide sheets (see below). The front end of this device is a continuous microreactor, capable of synthesizing complex polymer molecules from three or more input channels; these can consist of various monomers or macromomomers, solvent, or initiators and catalysts. A micro-stirrer ensures component mixing. Flow rates from each input determine the real-time fluid composition. By modulating the input channel flow rates, libraries can be created that systematically change, for example, co-monomer composition or initiator concentration. The total flow rate determines the residence time in the main winding reaction channel, which governs the ultimate conversion or polymer molecular weight. Embedded heating elements and thermocouples enable temperature control to 1 °C. Overall, the system requires less than 50 µL of material to produce a polymer solution sample. After the reaction channel, another input allows another fluid to be introduced. This could be, for example, a second solvent that changes the solution behavior of the polymer reaction products.
Building on our earlier work, this microfluidic system has a built-in dynamic light scattering (DLS) probe. This fiber- based detector enables rapid and nanometer precision measurement of the size and dispersion of polymer aggregates in solution. Accordingly, it is a highly powerful tool for determining whether the library of polymer molecules synthesized on the device form structures such as nanoscale micelles for drug delivery or monodisperse latex nanoparticles.
We demonstrated the power and operation of our integrated microfluidic device by having it synthesize a series of styrene-b- methacrylate block copolymer surfactants. The polymer library varied the composition and length of the methacrylate block. Subsequent addition of dodecane reduced the solvent quality for the styrene block, inducing the formation of nanoscopic micelles, but in some cases larger micrometer-sized aggregates (see below). The online DLS probe was able to map these solution behaviors over the library variables and determine optimal molecular parameters for micelle formation.
We reported this work via invited lectures at FY07 national meetings of the American Chemical Society and the Materials Research Society, and at two NIST Combinatorial Methods Center industry member workshops. In FY08, we will extend and employ this integrated DLS strategy to measure the behavior of specialty surfactants through processing routes that vary temperature and composition.