Block-Random Copolymers Richard A. Register, Princeton University, DMR Block copolymers spontaneously form an internal nanoscale structure, which makes them useful for everything from melt- processible rubbers to templates for high- density memory devices. Ideally, such polymers can be heated to mix the blocks and erase the nanoscale structure, which then forms on cooling through an “ordering temperature” T O. Both the size of the nanostructures and the value of T O increase with the length of the block copolymer chain, so it has previously been difficult to prepare polymers which form large-period nanoscale structures that can still be erased by heating. [1] B.S. Beckingham and R.A. Register, “Synthesis and Phase Behavior of Block-Random Copolymers of Styrene and Hydrogenated Isoprene”, Macromolecules, 44, (2011). Left: Schematic illustration of a block-random copolymer, where the top block is a homopolymer of red beads, and the bottom block is a random copolymer of red and blue beads. Middle: This block-random copolymer self-assembles into a lamellar structure, consisting of alternating layers of the homopolymer block (red) and the random block (purple, from an intimate mixture of red and blue beads). Right: Small-angle x-ray scattering (SAXS) confirms the lamellar structure and indicates that the period is 27 nm. All the sharp SAXS peaks disappear on heating to 105°C, which is T O for this polymer. By substituting a random copolymer, whose composition can be tuned continuously, for one of the blocks, it becomes possible to decouple the nanoscale period from T O. We recently validated 1 this approach by synthesizing such “block-random” copolymers from the common industrial monomers styrene and isoprene; polymers with molecular weights greater than 30 kg/mol still had T O comparable to or less than the temperature of boiling water.

NOVA’s “Making Stuff”: Outreach to Middle-School Students Richard A. Register, Princeton University, DMR Throughout Fall 2010, the Princeton Center for Complex Materials (PCCM) worked with middle- school teachers from nearby Trenton to build knowledge of materials science, in preparation for the NOVA television program “Making Stuff”, which aired in January-February 2011 on PBS. On March 1, the Register Group performed a pair of 40-minute auditorium shows on the Princeton University campus for these teachers’ students, highlighting the unique and useful properties of polymers as a complement to the coverage in “Making Stuff”. Augmenting the presentation and hands-on activities was a background PowerPoint slideshow, illustrating concepts of polymer chain entanglements, crosslinking, and reversible crosslinking. Demos included swelling and deswelling of a superabsorbent polymer gel, the elasticity of rubber and viscoelasticity of Silly Putty, and the preparation of "slime" or "gak" (fully or partially hydrolyzed polyvinylacetate, reversibly crosslinked with borate), distributed as free samples to all takers. Top: Rick Register (PI, in red shirt) near the end of the auditorium show, demonstrating how the physical crosslinking in “slime” (created by adding borax) can be reversed by adding vinegar, quickly transforming the material from a free-flowing clear liquid to a gel and back to a liquid. Bottom: Graduate student Sheng Li (right) helps an attendee make her own pink “gak”, while a second attendee snaps a photo.