Studies of Block Copolymer Thin Films and Mixtures with an Ionic Liquid
Block copolymers are capable of self-assembling into structures on the 10-100 nm length scale. Structures of this size are attractive for applications such as nanopatterning and electrochemical membrane materials. However, block copolymer self-assembly in these examples is complicated by the presence of surfaces in the case of thin films and the presence of an additive, such as an ionic liquid, in the case of electrochemical membrane materials. Improved understanding of the structure and thermodynamics of such systems is necessary for the development of structure-property relationships in applications for block copolymers, such as nanopatterning and electrochemical devices.
To address the challenge of block copolymer thin film characterization over large areas, resonant soft X-ray scattering (RSoXS) has been applied to characterize order formation in copolymer thin films. Using theory and experiment, the dramatic chemical sensitivity of RSoXS to subtle differences in the bonding energies of different blocks of a copolymer is demonstrated. The unambiguous identification of structure and domain size in block copolymer thin films using RSoXS enables a quantitative comparison of the bulk block copolymer structure and domain size, leading to improved understanding of the impact of surfaces on block copolymer self-assembly.
The self-assembly of block copolymer/ionic liquid mixtures has been characterized as a function of block copolymer composition and molecular weight, mixture composition, and temperature using small-angle X-ray scattering (SAXS), optical transmission characterization, wide-angle X-ray scattering (WAXS), and differential scanning calorimetry (DSC). The resulting phase behavior is reminiscent to that of block copolymer mixtures with a selective molecular solvent and lamellar, cylindrical, ordered spherical micelles, and disordered phases are observed. Analysis of order-disorder transitions and molecular weight scaling analysis qualitatively indicates that the segregation strength between block copolymer phases increases with ionic liquid loading. DSC characterization of the thermal properties of the block copolymer/ionic liquid mixtures reveals two composition dependent regimes. At high block copolymer concentrations, a "salt-like" regime corresponding to an increase in the block copolymer glass transition temperature is observed, while at intermediate block copolymer concentrations, a "solvent-like" regime corresponding to a decrease in the block copolymer glass transition temperature is observed.
The distribution of ionic liquid within microphase-separated domains of a block copolymer has been characterized using contrast matched small-angle neutron scattering (SANS) and DSC. The ionic liquid is shown to partition selectively into domains formed by one block of a block copolymer in agreement with studies of the phase behavior of ionic liquid/block copolymer mixtures. Unexpected differences in ionic liquid partitioning are observed in mixtures containing a deuterated versus hydrogenated ionic liquid.