Enhanced Optoelectronic Properties of Polythiophene-g-Poly(dimethyl amino ethyl methacrylate)-b-Poly(diethylene glycol methyl ether methacrylate) Copolymers using “Grafting onto” Synthetic Strategy

The optoelectronic properties of polythiophene (PT) graft block copolymers are most important for fabricating optoelectronic devices, and recently, we reported a single-pot atom-transfer radical polymerization (ATRP) technique for preparation of PT graft block copolymers between thermoresponsive poly(diethylene glycol methyl ether methacrylate) (PDEGMEM) and pH-responsive poly(dimethyl amino ethyl methacrylate) (PDMAEMA) from the PT backbone via the "grafting from" strategy with an 11 mol % contamination. A "grafting onto" strategy has been opted to eliminate the contamination from the block copolymer where we synthesized poly(thiophene acetic acid) (P3TAA) followed by the coupling with PDEGMEM-b-PDMAEMA-Cl, PDMAEMA-b-PDEGMEM-Cl, and PDMAEMA-ran-PDEGMEM-Cl copolymers, produced separately by the ATRP technique. The polymers were characterized using ¹H NMR, SEC, etc. TEM study exhibits mostly vesicular morphology and optical properties measured using UV-vis and photoluminescence spectroscopy showing pH dependent behavior. dc conductivity values indicate semiconducting nature in the order P2 > P3 > P1. The abrupt hike of P2 (∼80 times) in conductivity at pH 3 from that of previously prepared P2 copolymers formed by the grafting from process is attributed to the absence of ∼11 mol % contamination. Conductivity decreases with increasing pH, due to coiling of the PT backbone in accordance with the blue shifts of λ_abs peaks. The current (I)-voltage (V) plots exhibit bimodal memory and organic mixed ionic and electronic conductivity. Higher current (3.3 mA for P2, pH 3) and electronic memory occur upon light irradiation than that of dark. Photoswitching property decreases with increase of pH, showing highest photocurrent gain of 8.05 for P2 at pH 3. Photocurrent gain follows the order P2 > P3 > P1 indicating P2 is the best to develop photoswitches in the P-series polymers. Fitting of growth and decay curves suggests that they are a two-stage process: photocurrent raises fast at the on state initially and then at a slower rate and similar at an off state. Impedance spectra suggest charge-transfer resistance and Warburg impedance values follow the order of P1 > P3 > P2, whereas capacitance value follows the opposite order.