Session B4: Macromolecular Assemblies: Structure and Dynamics

\textbf{Dynamics of Chain Exchange in Block Copolymer Micelles}

Block copolymer micelles are rarely at equilibrium. The primary reason is the large number of repeat units in the insoluble block, $N_{core}$, which makes the thermodynamic penalty for extracting a single chain (``unimer exchange'') substantial. As a consequence, the critical micelle concentration (CMC) is rarely accessed experimentally; however, in the proximity of a critical micelle temperature (CMT), equilibration is possible. We have been using time-resolved small angle neutron scattering (TR-SANS) to obtain a detailed picture of the mechanisms and time scales for chain exchange, at or near equilibrium. Our model system is poly(styrene)-\textit{block}-poly(ethylene-\textit{alt}-propylene)) (PS-PEP), in the PEP-selective solvent squalane (C$_{30}$H$_{62})$. Equivalent micelles with either normal (hPS) or perdeuterated (dPS) cores are initially mixed in a blend of isotopically substituted squalane, designed to contrast-match a 50:50 hPS:dPS core. Samples are then annealed at a target temperature, and chain exchange is revealed quantitatively by the temporal decay in scattered intensity. The rate of exchange as function of concentration, temperature, $N_{core}$, $N_{corona,}$ and chain architecture (diblock versus triblock) will be discussed.   

Assemblies of Cellulose Nanocrystals.

The entropically driven coassembly of nanorods (cellulose nanocrystals, CNCs) and different types of nanoparticles (NPs), including dye-labeled latex NPs, carbon dots and plasmonic NPs was experimentally studied in aqueous suspensions and in solid films. In mixed CNC-NP suspensions, phase separation into an isotropic NP-rich and a chiral nematic CNC-rich phase took place; the latter contained a significant amount of NPs. Drying the mixed suspension resulted in CNC-NP films with planar disordered layers of NPs, which alternated with chiral nematic CNC-rich regions. In addition, NPs were embedded in the chiral nematic domains. The stratified morphology of the films, together with a random distribution of NPs in the anisotropic phase, led to the films having close-to-uniform fluorescence, birefringence, and circular dichroism properties.   

Mega-supramolecules for safer, cleaner fuel

Guided by the statistical mechanics of ring-chain equilibrium, we designed and synthesized polymers that self-assemble into ``mega-supramolecules'' ($\ge $5,000 kg/mol) at low concentration ($\le $0.3{\%}wt) in hydrocarbon liquids. Experimental results accord with model predictions that end-functional polymers, which distribute among cyclic and linear supramolecules, can form a significant population of mega-supramolecules at low total polymer concentration---if, and only if$,$ the backbones are long (\textgreater 400 kg/mol) and end-association strength is optimal (16-18\textit{kT}). Hydrocarbon liquid fuels are the world's dominant power source (34{\%} of global energy consumption).~~Transportation relies heavily on such liquids, presenting the risk of explosive post-impact fires. The collapse of~the~World Trade Center~on September 11, 2001~inspired us to revisit polymers for mist control to mitigate post-impact fuel explosions. Rheological and both light and neutron scattering measurements of long end-functional polymers having polycyclooctadiene backbones and acid or amine end groups verify formation of mega-supramolecules. Post-impact flame propagations experiments show that mega-supramolecules control misting. Turbulent flow measurements show that mega-supramolecules reduce drag like ultra-long covalent polymers. With individual building blocks short enough to avoid hydrodynamic chain scission (400\textless $M_{w}$ [kg/mol] $\le $1,000) and reversible linkages that protect covalent bonds, they respond reversibly to flow through pumps and filters without degradation. Mega-supramolecules had no adverse effect on power output, fuel efficiency or emissions in diesel engines. In fact, they gave a 12{\%} reduction in diesel soot. Thus, long end-associative polymers may open the way to fuel additives that reduce pollution and improve transportation safety and security.   

Charge Effects on the Self-Assembly of Protein Block Copolymer Nanostructures

Self-assembly of globular protein-polymer block copolymers into nanostructured phases provides a simple method for structural control in biomaterials. Electrostatics play a major role in the self-assembly of these structures from aqueous solutions. While the specific distribution of charge on the protein plays a relatively minor role in self-assembly, large changes in the total charge have a large impact on the concentration at which the proteins self-assemble. While for near-neutral proteins salt screening promotes disassembly and suggests that electrostatic interactions are attractive, proteins with a highly asymmetric charge have repulsive interactions that suppress self-assembly. Using a zwitterionic block in the bioconjugate was also explored as a means to promote self-assembly; however, zwitterionic fusions self-assemble over a narrower range of composition than fusions of any of the nonionic polymers explored. This suggests that dipolar attractions in charge-asymmetric protein-polymer materials play a significant role in the driving force for self-assembly. However, the sensitivity of zwitterionic materials to salt conditions in the buffer also provides a powerful handle for tuning polymer solubility, enabling salt to be used as a method to induce self-assembly.   

Giant Surfactants based on Precisely Functionalized POSS Nano-atoms: Tuning from Crystals to Frank-Kasper Phases and Quasicrystals

In creating new functional materials for advanced technologies, precisely control over functionality and their hierarchical ordered structures are vital for obtaining the desired properties. Giant polyhedra are a class of materials which are designed and constructed via deliberately placing precisely functionalized polyhedral oligomeric silsesquioxane (POSS) and fullerene (C$_{\mathrm{60}})$ molecular nano-particles (MNPs) (so-called ``nano-atoms'') at the vertices of a polyhedron. Giant surfactants are consisted of polymer tail-tethered ``nano-atoms'' which are deliberately and precisely functionalized POSS or C$_{\mathrm{60}}$ molecular nano-particles (MNPs). The ``nano-atom'' heads and polymer tails thus have drastic chemical differences to impart amphiphilicity. These giant surfactants capture the essential structural features of their small-molecule counterparts in many ways but possess much larger sizes, and therefore, they are recognized as size-amplified versions of small molecule surfactants. Two of the most illustrating examples are a series of novel giant tetrahedra and a series of giant giant surfactants as building blocks to construct into hierarchical ordered super-lattice structures ranging from crystals, Frank-Kasper phases and quasicrystals in the condensed bulk states, reveals evidently the interconnections between soft matters and hard matters in sharing their common structures and fundamental knowledge.