CHEMISTRY FOR CONTROLLING THE ORIENTATION OF BLOCK COPOLYMERS (BCPS) IN THIN-FILMS
The ability to control the orientation of the self-assembled BCP domains in thin-films is important in areas such as microelectronics, magnetic data storage and membranes for filtration. The shape and size of the microdomain can be controlled by varying the molecular weights and relative volume fractions of each block in the BCP. The orientation of the microdomains in thin-films is mainly controlled by the interaction of each block with the substrate and the free surface through wetting energetic and polymer confinement effects resulting from the film thickness. We have focused on (1) developing a new simple chemistry for surface modification to control the BCP domain orientation and (2) understanding the assembly rules for BCP thin-films on these modified substrate. To achieve vertical orientation of BCP domains in thin-films we have synthesized a new type of surface-modifying copolymer with a distribution of a third polar monomer which is capable of anchoring via side groups to an oxide surface or photo-crosslinking with itself to create a non-preferential surface. We have developed an in-depth understanding of the chemistry of the surface modifying copolymer, including the effects of its composition on the assembly of lamella and cylinder forming BCPs in thin- films.
The challenge and excitement in this area of surface engineering for biological studies is the application of traditional organic/polymer chemistry and engineering tools to accommodate the complexity presented by a biological system. Synthetic surfaces which present specific ligands or a combination of ligands can aid in identifying the nature of cellular interactions which lead to specific cell response. We have been investigating surface grown polymer brushes and crosslinked thin-films as templates for immobilization of biomolecules. The goal is to gain quantitative insight into the binding process and its subsequent effect on cell interactions.
MORPHOLOGICAL EFFECTS ON ELECTRO-OPTIC (EO) PROPERTIES
EO activity is the voltage-dependence of the refractive index, which is a critical property for applications such as modulation, and optical signal filtering. In studies of the polymer-chromophore interactions, we have found that it has an important and relatively unexplored role in determining the macroscopic order parameter. Our findings indicate that poled order in confined domains depends on the polymer host architecture, domain size, nature of interaction between the chromophore and polymer host, and concentration of the chromophores within the domains. Specifically we examined two different block copolymer (BCP) architectures, namely linear-diblock copolymer and linear-dendritic copolymers as hosts for encapsulating chromophores. A linear-dendritic morphology of the BCP is especially effective as it efficiently disperses the chromophores into small domains (< 10 nm), and keeps them apart within the domains due to the dendritic architecture. The insight into morphological effects on EO activity that we have developed complements the ongoing efforts in chromophore synthesis as values exceeding 100 pm/V at 1550 nm can be potentially achieved with synthetically less demanding chromophores.
DEVELOPMENT OF BLOCK COPOLYMER MATERIALS FOR NANOFABRICATION
The thermodynamically driven process of spontaneous self-assembly of block copolymers (BCP’s) has inspired a range of functional materials where domain confinement is of importance. We have developed new synthetic routes to well–defined organic-inorganic hierarchical structures. These hybrid materials with multiple length scales of ordering are of great interest for nanopatterning where in a single step a soft mask can be converted into a hard mask. One such example of an inorganic precursor is polyhedral oligomeric silsesquioxane (POSS), which is known to form nanometer size crystalline or glassy aggregates and is converted to silica on exposure to oxygen plasma. We have developed anionic polymerization route in synthesizing POSS-containing block copolymers, PS-b-PMAPOSS and PMMA-b-PMAPOSS, which results in the formation of well-defined self-assembled hierarchical nanostructures. Solvent annealing resulted in vertically oriented lamellae and cylinders which is converted to a hard mask in a single step by highly selective oxygen plasma etching. The size of the resulting cylinders is less than 10 nm, which is one of the smallest dimensions reported using BCP lithography. This work is an active collaboration with Prof. Hayakawa at Tokyo Institute of Technology.
SELF-ASSEMBLED ELECTROACTIVE UNITS AS FUNCTIONAL INTERFACES
We develop new chemical strategies and organic molecules for functionalization of inorganic surfaces in which molecular-scale phenomena such as surface states and charge transfer process can be read out by using field-effect transistors and ultrafast infrared spectroscopy. Inorganic oxides including SiO2 and ZnO are of particular interest. Through the UW-MRSEC collaboration we have been able to apply the self-assembly of electroactive molecules to electronic interfaces to tap into the charge transfer characteristics, static dipole moment and induced polarization of chromophores. We have pursued these concepts with self-assembled monolayer’s of C60 and dipolar chromophores.