Effect of a conjugated/elastic block sequence on the morphology and electronic properties of polythiophene based stretchable block copolymers†
We report the synthesis, morphology, and electronic properties of intrinsically stretchable AB-type, ABA-type, and BAB-type block copolymers (BCPs) of poly(3-hexylthiophene) (P3HT: A block) and elastic poly(octylene oxide) (POO: B block). By incorporating the soft coils, all BCP thin films exhibited clear self-assembled fibrillary nanostructures and largely improved stretchable mechanical properties within the films. The results from the absorption spectra and AFM images indicated that the self-assembled structures of the P3HT blocks in the block copolymers were related to the backbone sequences. Also, the phased-separated morphology of the AB-, ABA-, and BAB-block copolymers from GI-SAXS showed the average nanofibril d-spacing ranging from 17.9, 21.6, and 13.9 nm, respectively, which led to the same order on the formed nanofibril density. Among all the BCPs, only the ABA-type BCP thin film remains perfectly smooth and no wrinkle or crack is observed even up to the strain of 100%. These results indicate the significance of the conjugated/elastic block sequence for the morphology and physical properties of the prepared block copolymers. The resistive memory devices fabricated in the configuration of PDMS (substrates)/CNTs (bottom electrode)/BCP thin films (active layer)/Al (top electrode) exhibited non-volatile flash characteristics. In particles, the as-fabricated resistive memory device with the ABA-type BCP could endure up to 80% strain and an excellent ON/OFF current ratio was also achieved after 500 continuous stretching/releasing cycles under 50% strain. This work highlights the importance of molecular architectures on the conjugated/elastic ratios of BCPs for stretchable devices.