| IRG I. Mesoscopically helical order in chiral block copolymers |
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 Supermolecular, helical assemblies are a common structural motif exploited in far-ranging biological contexts, from the flagellar appendages of swimming microorganisms to the protein coats that sheath rod-like viruses. The screw-like structure of these biological assemblies is a consequence of the chiral subunits (proteins and amino acids) from which they are built, and this chirality imbues the structures with a right- or left-handed sense or twist. Grason and Russell, working collaboratively at the Materials Research Science and Engineering Center at UMass Amherst, demonstrate that a similar strategy may be employed in synthetic polymers, which are seen to self-assemble into helical structures. In general, block copolymers are assemble into array of mirror-symmetric, nano-structured arrays of ordered spherical, cylindrical and layered domains. This work uncovered a novel theoretical tool to both model and predict the self-assembled structures that emanate from constructing one of the constituent polymeric blocks from chiral segments. The theory shows that interactions between chiral segments “twist” the trajectories of the self-organized chain molecules. This twist is transmitted through the tension carried along the chains to the entire assembly, driving otherwise cylindrical assemblies to “buckle” into nanohelices organized in a hexagonal array. These results confirm the notion that mesoscopic, helical structures observed on tens of nanometer length scales in melts of the chiral block copolymer, poly-L-lactide-b-polystyrene, stems from the chiral nature of inter-molecular forces taking place at the nanometer lengthscale. Going forward, this opens a door to a new class of complex meso-chiral structures formed in chiral block copolymer materials yet to be discovered.
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| IRG II. A smooth cascade of structure refinement in thin sheets |
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 How does one make a rippled sheet terminate at a straight edge? If the sheet is sufficiently thin, such as a piece of paper or fabric, then the obvious solution of stretching it out flat will induce large stresses near the edge, possibly even tearing the sheet apart. One solution to this problem is suggested by the series of ever-smaller sharp folds of fabric generated near a curtain rod. Menon and Russell, working at the UMass Materials Research Science and Engineering Center, studied wrinkling patterns on an ultrathin floating raft of polystyrene, and discovered a new mechanism by which nature resolves such a conflict. In the middle of the film, a competition between gravity (which prefers shallow, frequent ripples) and the energy cost of bending the film (which favors longer, higher folds) determines the height and frequency of the folds. Near the edge, however, surface tension forces the film to lie flat. This refinement of structure occurred not by a hierarchy of pleats, but by a smooth cascade of ever-smaller wrinkles emerging as the edge is approached. This kind of hierarchical geometry is explained theoretically by the action of the surface tension of water that tends to “iron out” sharp features in the sheet. Similar types of smooth cascades may appear in other problems in materials science where a patterned surface hits an edge that is incompatible with the pattern.
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| Superseed. Ionic liquids as solution media for bioconjugation |
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Water soluble polymers, once reserved for commodity applications (i.e., shaving cream, emulsification processes, etc.) have emerged as valuable materials for medicine. Combining synthetic polymers with therapeutic proteins and cancer drugs improves the “therapeutic index” of the drugs, preventing their fast elimination from the body, and improving their availability for treating the disease. Emrick at the UMass Materials Research Science and Engineering Center found that ionic liquids provide an excellent environment for the attachment of proteins to synthetic polymers. Water soluble polymers like “polyMPC” shown in the figure are of interest for conjugation to proteins for medicine. However, the conjugation reaction itself is problematic when performed in water, due to loss of the reactive end-group to hydrolysis. Ionic liquids were found to provide an alternative medium for this conjugation, as the polar but aqueous-free characteristics of ionic liquids prevent competition of water for the reactive site at the polymer-chain-end, leading to clean, efficient conjugation. Such efficient and convenient bioconjugation strategies are critically important for scale-up and optimization of new polymer-based conjugates for medicine.meso-chiral structures formed in chiral block copolymer materials yet to be discovered. |
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| Superseed. Surfactant-mediated ion exchange and charge reversal at ionic liquid interfaces |
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 Bermudez at the UMass Materials Research Science and Engineering Center found that room-temperature ionic liquids (ILs) exhibit a unique set of properties due to their charged character, presenting opportunities for applications not possible using conventional organic solvents or water. This work showed combining positively or negatively charged molecules with ILs resulted in previously unknown interfacial behavior due to the electrostatic interactions between the charged molecule and the liquid. Specifically, sodium alkyl sulfates (negatively charged surfactants) and alkyl trimethylammonium bromides (positively charged surfactants) were found to segregate to the air-IL interface, and x-ray photoelectron spectroscopy revealed that the surfactant counter ions readily dissociate into the bulk, rather than staying close-by their oppositely charged partner. In addition, for an IL with an initial negative surface charge, the charge could be switched to positive by the addition of alkyl trimethylammonium bromides, giving access to a new method of manipulating surface charge of materials, which could have striking consequence as applied to coatings, ultra-thin films, and anti-fouling surfaces.
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Dip-Coating Crystallization on a Superhydrophobic Surface: A Million Mounted Crystals in a 1 cm2 Array
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Tom McCarthy and coworkers, working at the UMass Amherst MRSEC on Polymers, show that superhydrophobic surfaces emerging from water are, in fact, decorated with micron-size sessile water drops that rapidly evaporate. This simple technique proves useful for the preparation of very small liquid drops or puddles of controlled composition, and for preparation of arrays of crystalline substances of controlled size, termed dip-coating crystallization. Silicon wafers (silicon dioxide surfaces) were patterned to contain 3 mm (width) x 6 mm (length) x 40 mm (height) staggered rhombus posts in a square array (having a 20 mm distance between the posts). The surfaces exhibit superhydrophobicity – water drops “bead up” when in contact with the surface, with contact angles of qA/qR = 169°/156°). When a section of a wafer is submerged in and withdrawn from water, a completely dry surface emerges. If the same procedure is performed using aqueous sodium chloride as the liquid bath, individual crystals of the salt deposit on the top of each of the posts. “Dip-coating crystallization” using aqueous sodium chloride solution of 4.3M produces crystals with ~1 micron dimensions, while less concentrated salt solutions render crystals with ~500 nm dimensions.
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Highlights