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

Diffusiophoresis in complex and confined fluids

Moving fluids at the micro- and nano-scales requires a different approach compared to the traditional methods based on pressure gradients. The increase of surface/volume ratio as the length of the systems decreases the efficiency of the latter. The use of thermodynamic forces such as electric fields, chemical potential and temperature gradients is crucial for effective transport when the local perturbation of the fluid at the interfaces becomes relevant. The specific case of chemical potential gradients in fast-moving components of a solution driving the movement of colloids, polymers and other moieties is known as diffusiophoresis. In this thesis, we study diffusiophoresis using a combination of theory and computer simulations. We use non-equilibrium thermodynamics, starting from the entropy production to build our theoretical framework aiming to discuss some subtleties present in previous works. As the first case of study, we perform non-equilibrium molecular dynamics to analyse the movement of a very large colloid using the Derjaguin-Anderson approximation, reducing the problem to a diffusio-osmotic flow. In the simulations, we drive the system out of equilibrium by applying microscopic representations of the chemical potential gradient that are compatible with periodic boundary conditions. We then report the first applications of such a method to a colloidal system and compare it with simulations where explicit concentration gradients are imposed. Our approach is more convenient as we decouple diffusiophoresis from strong advective effects. We find a non-monotonic relation between the diffusiophoretic mobility of the colloid and the strength of the interaction with the solution. Finally, we report a numerical study of polymer diffusiophoresis, finding that the existing theory for solid particles is not accurate for polymer coils. Moreover, we observe that the hydrodynamic flow through the polymer is less screened than for pressure-driven flows

New constraints on the postglacial shallow-water carbonate accumulation in the Great Barrier Reef

More accurate global volumetric estimations of shallow-water reef deposits are needed to better inform climate and carbon cycle models. Using recently acquired datasets and International Ocean Discovery Program (IODP) Expedition 325 cores, we calculated shallow-water CaCO3 volumetrics and mass for the Great Barrier Reef region and extrapolated these results globally. In our estimates, we include deposits that have been neglected in global carbonate budgets: Holocene Halimeda bioherms located on the shelf, and postglacial pre-Holocene (now) drowned coral reefs located on the shelf edge. Our results show that in the Great Barrier Reef alone, these drowned reef deposits represent ca. 135 Gt CaCO3, comparatively representing 16-20% of the younger Holocene reef deposits. Globally, under plausible assumptions, we estimate the presence of ca. 8100 Gt CaCO3 of Holocene reef deposits, ca. 1500 Gt CaCO3 of drowned reef deposits and ca. 590 Gt CaCO3 of Halimeda shelf bioherms. Significantly, we found that in our scenarios the periods of pronounced reefal mass accumulation broadly encompass the occurrence of the Younger Dryas and periods of CO2 surge (14.9-14.4 ka, 13.0-11.5 ka) observed in Antarctic ice cores. Our estimations are consistent with reef accretion episodes inferred from previous global carbon cycle models and with the chronology from reef cores from the shelf edge of the Great Barrier Reef