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Our PRF-sponsored research aims to examine the structure and interactions of self-organized, complex block copolymers, both when self-assembled into thin films at the solid-fluid interface, but also in solution, where they form micelles, vesicles, or nanoparticles. Copolymers consisting of two or more chemically dissimilar blocks share distinctive traits with small-molecule surfactants, perhaps most notably the ability to spontaneously organize into ensembles comprised of many single molecules. The size and shape of those supramolecular ensembles are determined by the composition, connectivity, and sequence of the constituent chains and the set of interactions those materials have with their solution environment. Polymer micelles or vesicles are of practical importance for applications such as drug delivery or enhanced oil recovery, and those supramolecular ensembles also are of broad interest as building blocks in nanotechnology. Nevertheless, our understanding of the links between polymer architecture, self-assembled structure, and dynamics and interactions is quite limited.

Figure 1.  Architecturally-complex copolymers studied, including mikto-arm (mixed arm) star-like copolymers (left) consisting of a branched polyisoprene block (blue) and a polystyrene end block (red); and highly branched star block copolymers (right) having polystyrene core blocks and poly(2-vinylpyridine) coronas (green).

To address this we are investigating the self-assembly of branched and multi-component block copolymers, focusing on how chain topology and interactions influences structure and surface properties, especially surface relaxations. We have used light scattering to determine the structure of self-organized, branched block copolymers in preferential solvents and a variety of surface sensitive techniques to examine kinetics of preferential adsorption and thin film morphology. Our investigations to date have involved three-arm “mikto-arm” (mixed arm) star-like copolymers consisting of polystyrene and polyisoprene and highly branched stars having arms consisting of poly(2-vinylpyridine-block-polystyrene) diblock copolymers, both of which are represented schematically in Figure 1. The former set of copolymers has been studied in n-hexane, which is preferential for polyisoprene and assembly and surface structure of the latter set has been examined in toluene, which is preferential for polystyrene.

The main experimental method used in the adsorption studies is phase modulated ellipsometry, which provides high sensitivity, a low signal-to-noise ratio, and time resolution as short as 10 ms. Moreover, a feedback loop provides excellent long-time stability, enabling the kinetics of adsorption to be followed over very long periods. Using these copolymer systems and complementary insights obtained through modeling, we have demonstrated that the kinetics of assembly and surface relaxations are significantly impacted by architecture and solution concentration. In cases where the assembly is conducted from a micellar solution, as in the PS/PI mikto-arm star copolymer system, the kinetics of preferential adsorption depend strongly on surface relaxation/ reorganization events because the diffusion-controlled regime ends rather quickly (within a few 10's of seconds). As shown in Figure 2, the adsorption process is also significantly impacted by the dynamic equilibrium that exists between single chains and supramolecular ensembles.

Figure 2.  Results from phase modulated ellipsometry and dynamic light scattering showing how kinetics of preferential adsorption (left) and hydrodynamic size distribution of micellar aggregates (right), respectively, are affected by concentration of micelles in solution.  These studies of the assembly and dynamics of self assembled miktoarm star-like copolymers reveal exchange of chains between free solution and the micellar structures that in turn impact relaxations and reorganization of the supramolecular ensembles at the solid-fluid interface.

In the case of the highly-branched polystyrene/polyvinylpyridine (PS/PVP) star-block copolymers, we found that the adsorption process depends sensitively on star composition: the adsorption of symmetric stars appears to follow a random sequential adsorption (RSA) model, and for all styrene:vinylpyridine ratios examined, the evolution of layer structure is strongly influenced by surface relaxation events. In fact, model fitting shows that at lower concentrations the adsorption of the PS/PVP stars is dominated by surface reorganization events, rather than diffusion. We have also used neutron reflectometry to examine the nanoscale structure of ultrathin films formed by adsorption of the PS/PVP star-block copolymers; as shown in Figure 3, we find that due to internal crowding, not all of the PVP arms are able to reach the surface.

Figure 3.  Scattering length density (SLD) profile for a highly asymmetric PS/PVP star assembled on a silicon surface.  The fit indicates that not all of the PVP is able to “find” the surface.

Final steps in the project involve examining nanoscale structure of the star block copolymers in various solution environments using liquids reflectometry and to examine the dynamic behavior and solution structure of these copolymers using light scattering methods.  We also plan on examining structural response of surface tethered, multicomponent systems as solvent environment is changed. These systems are being made both by preferential adsorption of multicomponent block copolymers at the solid-fluid interface and also by surface grafting, which involves reacting polymers onto solid substrates. The results emerging from our studies provide a basis for understanding how macromolecular architecture and composition can be used to tune self-assembled structure and properties.  A clear and final challenge is to link the solution structure and dynamics to specific surface relaxation processes occurring upon adsorption.

These research activities have fostered the development of a graduate student and a postdoctoral researcher. Through this work they have developed expertise in dynamic light scattering and in neutron reflectivity through participation in scattering experiments at national user facilities (such as the NIST Center for Neutron Research and the Spallation Neutron Source at Oak Ridge National Laboratory). As a result of those research activities as well as through training and mentoring, our research group has increased its capabilities by expanding the suite of techniques that can be brought to bear on understanding the formation, organization, and properties of polymer-modified interfaces.