Stephen M. Martin

Preferred domain orientation in block copolymer fibers after solvent annealing

Block copolymer fibers represent an emerging class of nanomaterials with attractive properties and functionalities. Previous work has used coaxial electrospinning to prepare block copolymer fibers in thermally stable sheaths, which allow for thermal annealing of the block copolymers to form ordered nanostructures. However, the resulting nanostructures are mostly concentric due to isotropic boundary conditions at the sheaths. Herein, we describe a strategy of utilizing solvent annealing to achieve ordered block copolymer nanostructures in fibers. Cylinder- and lamella-forming poly(methyl methacrylate-block-styrene) (PMMA-b-PS) block copolymers are synthesized via reversible addition–fragmentation chain transfer polymerization and electrospun into fibers. After solvent annealing, the block copolymers can form long-range ordered nanostructures and exhibit a preferred domain orientation that is perpendicular to the fiber axis. Complementary to thermal annealing of coaxially electrospun block copolymer fibers, the method of solvent annealing provides an alternative approach for controlling the block copolymer domain orientation in fibers.

Effect of molecular packing density and intermolecular interactions on solute transport in supported liquid crystalline membranes

The transport of organic solutes in 4-(trans-4′-4′-n-pentylcyclohexy)-benzonitrile (PCH5) and 4-cyano-4′-octylbiphenyl (8CB) supported liquid crystal (LC) membranes was reported in order to broaden the understanding of mechanisms for solute transport in ordered mesophases. PCH5 was chosen as it exhibits a lower packing density than 8CB but retains a similar molecular structure. Higher sorption and diffusion coefficients were measured in PCH5 compared to 8CB, due primarily to the lower packing density in the PCH5 nematic phase. Solutes capable of forming a single hydrogen bonding with LC molecules exhibited faster diffusion than those with no hydrogen-bonding ability or those capable of multiple hydrogen-bonding interactions. Measurements of relative solubility of pyridine and salicylic acid in 8CB, PCH5 and kerosene confirmed that intermolecular hydrogen bonding plays a stronger role than π−π interactions in solute absorption. Finally, in the absence of differences in hydrogen bonding, higher dipole–dipole interactions resulted in lower diffusivities in LCs.