Synthesis, Morphology, and Properties of Poly(3-hexylthiophene)-block-Poly(vinylphenyl oxadiazole) Donor-Acceptor Rod-Coil Block Copolymers and Their Memory Device Applications

Novel donor-acceptor rod-coil diblock copolymers of regioregular poly(3-hexylthiophene) (P3HT)-block-poly(2-phenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole) (POXD) are successfully synthesized by the combination of a modified Grignard metathesis reaction (GRIM) and atom transfer radical polymerization (ATRP). The effects of the block ratios of the P3HT donor and POXD pendant acceptor blocks on the morphology, field effect transistor mobility, and memory device characteristics are explored. The TEM, SAXS, WAXS, and AFM results suggest that the coil block fraction significantly affects the chain packing of the P3HT block and depresses its crystallinity. The optical absorption spectra indicate that the intramolecular charge transfer between the main chain P3HT donor and the side chain POXD acceptor is relatively weak and the level of order of P3HT chains is reduced by the incorporation of the POXD acceptor. The field effect transistor (FET) hole mobility of the system exhibits a similar trend on the optical properties, which are also decreased with the reduced ordered P3HT crystallinity. The low-lying highest occupied molecular orbital (HOMO) energy level (-6.08 eV) of POXD is employed as charge trap for the electrical switching memory devices. P3HT-b-POXD exhibits a nonvolatile bistable memory or insulator behavior depending on the P3HT/POXD block ratio and the resulting morphology. The ITO/P3HT(44)-b-POXD18/Al memory device shows a non-volatile switching characteristic with negative differential resistance (NDR) effect due to the charge trapped POXD block. These experimental results provide the new strategies for the design of donor-acceptor rod-coil block copolymers for controlling morphology and physical properties as well as advanced memory device applications.