Experimental and theoretical investigations will be performed to better understand how amphiphilic diblock copolymers can be induced to form regular nanoscopic structures on the liquid-air interface, and thus in 2-dimensions. Flexible diblock copolymers of poly (styrene) -b- poly (ethylene oxide) have been shown to spread from a quickly evaporating solvent to form dots, spaghetti and continents depending on the solution concentration and % hydrophilic block. The length scales of these macromolecules are naturally on the order of tens to hundreds of nanometers. The phenomenon observed of pattern formation is not a thermodynamically stable result, but rather, a competition between the solution droplet spreading (and thus separating the polymers) and an increase in solution concentration due to evaporation (and thus decreasing polymer separation) that ultimately causes aggregation of the hydrophobic block. In collaboration with Dr. Anette Hosoi of MIT, a theoretical model was derived that predicts this kinetically trapped behavior without the use of fitting parameters; only the real physical parameters of the system are used. These studies will now be extended to the experimental observation and theoretical description of diblock copolymers with different physical properties: geometries other than linear coil-coil chains and different rigidities as defined by persistence length. Binary mixtures, of diblocks and of diblocks with homopolymers, will be used to better control the relative compositions of the hydrophilic and hydrophobic contents. In this manner, the general applicability of the model will be tested. Furthermore, the possible techniques and magnitudes of forces required to shear align the spaghetti structures will be probed both experimentally and theoretically. The goals are both to create structures on the nanoscale and to orient them in order to provide surfaces with both topographical and chemical heterogeneity.
NON-TECHNICAL SUMMARY:
Most structures in biological systems spontaneously organize on the smallest length scales by a process known as self-assembly. This research utilizes many of the same forces as biological systems, that is, the organization and separation of hydrophilic (water-loving) and hydrophobic (oil-loving) sections of a single large molecule, in this case a diblock copolymer. These polymers have been observed to form two-dimensional nanoscopic structures in a preparation technique that involves spreading the polymers on a water surface and changing only the concentration of the solution or the relative sizes of the hydrophilic and hydrophobic parts of the polymer. This work couples theoretical and experimental work to better understand and control the creation of these tiny structures in self-assembly. The research actively involves undergraduates as the key research participants with active mentoring and training of postdoctoral researchers. The impact of an active and engaging research program on undergraduates is important in encouraging and sustaining future scientists.
