Self-Assembly of Architecturally Complex Block Copolymers

The term self-assembly denotes the formation of complex structures from simpler building blocks, resembling the manner in which Nature generates functional systems. In this pursuit, block copolymers present a great opportunity to study the interactions, dynamics and self-assembly of soft matter. Block copolymers have the ability to self-assemble into thermodynamically stable microphase segregated domains of precise shape and size, which are controlled by the chemistry of the constituent blocks, their size and connectivity, temperature and solvent conditions. Specifically, in this body of work two different types of branched copolymers with polystyrene (PS) and polyisoprene (PI) constituents are studied. The complex architectural arrangements studied include miktoarm block copolymers with a PS-PI-(PI)2 configuration and symmetric (PS)n(PI)n (AnBn) heteroarm star copolymers with an equal number of PS and PI arms emanating from a common junction point. In the case of the miktoarm PS-PI-(PI)2 block copolymers, the increase in complexity beyond a linear architecture clearly modifies the dynamics and structure of the macromolecules. I found by static and dynamic light scattering that these copolymers self-assemble into spherical micelles with cores composed of the unsolvated linear PS blocks and coronas formed by the well-solvated branched PI blocks. After thorough characterization of the micellization behavior, I followed the adsorption kinetics of these macromolecular ensembles using in situ phase modulated ellipsometry. In order to properly characterize their adsorption process from the initial fast adsorption to pseudo-equilibrium, a modeling framework that incorporates the effects of mass transport and dynamic relaxation/reorganization events occurring at the solid/fluid interface is adopted. The results demonstrate commonality between adsorption of micelle-forming surfactant-like copolymers and biomimetic vesicles formed by small-molecule surfactants, both of which are systems whose adsorption behaviors are dominated by rearrangements on the surface. The solution behavior of a set of AnBn heteroarm star copolymers was also examined. The systematic doubling of the number of arms in the symmetric (PS)n(PI)n (AnBn) hetero-arm star copolymers results in a doubling of the total molecular weight while keeping the composition fixed. Dynamic light scattering experiments in n-hexane showed a loss of flexibility in the stars as the number of arms increase, an effect related to the increased intramolecular interactions as branching increases. Bimodal hydrodynamic size distributions for stars with 2 and 4 arms are observed, but only monomodal size distributions are observed for stars with 8 and 16 arms over the concentration range studied. Thus as the number of arms increases, the system becomes unable to self-assemble, mainly due to shielding posed by the longer PI blocks. By studying these architecturally complex block copolymers, fundamental knowledge of how the structure and dynamics of polymeric materials can be modified by macromolecular design, solvent conditions and confinement is generated. This knowledge enhances our understanding of architecturally complex macromolecules and the role of chain architecture on behavior