Highlights

Creating well-organized conducting nanostructures in a flexible polymer matrix provides platforms for numerous applications in optics, sensors, and wave-guiding structures. Working in the Materials Research Science and Engineering Center on Polymers at the University of Massachusetts Amherst, Thayumanavan and Russell achieved self-assembled hybrid structures from diblock copolymers and gold nanoparticles, where the nanoparticles are exquisitely arranged in circular patterns as directed by the polymer template. Key to this process was the synthesis of block copolymers having a photocleavable junction point; after light-initiated polymer fragmentation, the template is left with sulfur-rich functional groups having an affinity for metallic surfaces, such as gold and silver nanoparticles. The accompanying image shows the results of this process, with the dark circular areas indicating the presence of gold nanoparticles arranged along the edges of the circular polymer pattern.

Understanding the commonplace mechanical instability known as “wrinkling” has the potential to impact numerous materials and technological applications, such as optical surfaces, adhesives and flexible electronic devices. Understanding how wrinkles change with strain is important for many of these applications. While homogeneous strain will result in a homogeneously wrinkled surface, a systematic understanding of pre-existing strain inhomogeneity, which is commonly found in practical materials, has remained elusive.  Crosby, leading a collaborative effort in the Materials Research Science and Engineering Center on Polymers at UMass, developed novel fabrication methods to find that a surface with inhomogeneously distributed wrinkles will develop a surprising pattern of wrinkles on the surface, in which the wrinkles have exhibit homogeneous geometric dimensions at very small applied strains (~6%).

Polymers having “conjugated structures”, allowing them to conduct electricity, hold great potential for flexible and ink-jet printable electronic devices and inexpensive plastic solar cells. Work by Hayward and Emrick in the Materials Research Science and Engineering Center (MRSEC) on Polymers at the University of Massachusetts Amherst has shown how to coax such polymers to twist into conducting wires thousands of times smaller than the twisted cables used in common electronic devices. These structures are based on polythiophenes, which tend to crystallize into ribbons only a few nanometers in thickness. Such crystalline nanostructures are important to determining the electronic properties of materials based on this polymer, and the ability to control their formation holds promise for improving the performance of devices. Remarkably, the inclusion of functional groups that bind salt ions leads to twisting of these nanowires into helices that join together into double helices, reminiscent of DNA, and larger bundles containing multiple strands. These materials provide opportunities to 1) study the still poorly understood driving forces for helical assembly, and 2) tune the electronic properties of conjugated polymers.

The crumpling or crushing of paper, aluminum foil, or even a car fender is an everyday occurrence that is surprisingly rich in new physical and materials principles.  Working in the Materials Research Science and Engineering Center (MRSEC) on Polymers at the University of Massachusetts Amherst, Menon and Russell used X-ray microtomography experiments on foils crushed into a ball to understand their detailed 3D structure. These measurements reveal that the internal 3-dimensional geometry of a crumpled ball is in many respects isotropic and homogeneous. Crumpling recapitulates classic nonequilibrium problems such as turbulence, where a system driven by long-wavelength and low-symmetry forces shows only rather subtle fingerprints of the forcing mechanism.  However, the researchers found local nematic ordering of the sheet into parallel stacks, a surprising results that will help physicists and materials scientists better understand these complex structures. The extent of this stacking or layering increased with the volume fraction, or degree of compression.  Stacking of the material into thicker “walls” may be either an alternative or an assistive mechanism to the formation of ridges, and impart structural rigidity to the material.

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.

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.