Postdoctoral research associate Li Yao (Hillmyer) has been exploring the self-assembly of oligomeric block copolymer coatings for use in lithographic and nanopatterning applications that push the current resolution limits. Block copolymers can be ideal platforms for providing nanoscale features, and have been used to prepare functional nanomaterials. Most block copolymers can only provide structures with the length scales between 20 and 30 nm. For emerging applications in microelectronics and magnetic storage, sub-10 nm or even sub-5 nm domain sizes are required to prepare ultrahigh density features. To achieve ultrasmall domain size, strongly segregated (i.e., large Flory-Huggins interaction parameters (χ)) block copolymer systems with low degrees of polymerization (N) are needed. Using a combination of anionic polymerization, end-capping, hydrogenation and controlled ring-opening polymerization, a set of poly(cyclohexylethylene)-b-poly(lactide) (PCHE-b-PLA) copolymers were prepared. Using dynamic mechanical testing, the order-disorder transition temperatures were estimated for these systems. Mean-field estimates for the degree of segregation at the order-disorder transition, were used to extract estimates of χ(T). These all organic (i.e., no exotic metal-containing blocks) block polymers are characterized by extremely large values of χ as compared to other related systems. Ordered thin films of PCHE-b-PLA with ultra-small vertical cylinders were then fabricated through spin-coating and solvent annealing. An AFM image showing sub-5 nm patterns from a PCHE-b-PLA material with a molar mass of 4.5 kg/mol post PLA etching is given in Figure, which was conducted at the Characterization Facility, University of Minnesota, on a Bruker Nanoscope V Multimode 8 with QNM. GISAXS measurement was conducted at Cornell High Energy Synchrotron Source G1 station with the wavelength of 0.10716 nm and the sample-detector distance of 1.06 m. This work has given some of the smallest patterns ever achieved in block polymer thin films.
