Synthesis and morphology of new asymmetric star polymers of poly[4-(9,9-dihexylfloren-2-yl)styrene]-block-poly(2-vinylpyridine) and their non-volatile memory device applications

Abstract

We report the synthesis, morphology, and properties of new 3-arm AB22 and 5-arm AB44 asymmetric amphiphilic star polymers of poly[4-(9,9-dihexylfloren-2-yl)styrene]-block-poly(2-vinylpyridine) (P(St-Fl)-b-P2VP), with narrow molecular weight distributions (Mw/Mn < 1.11). The asymmetric star polymers were synthesized by living anionic polymerization using 1,1-diphenylethylene (DPE)-functionalized P(St-Fl) with two or four benzyl bromide moieties, followed by linking reaction with a living anionic P2VP. Depending on the arm numbers of P2VP, lamellar and hexagonally perforated lamellae morphologies were observed for the AB22 and AB44P(St-Fl)-b-P2VP from the results of SAXS, WAXS, and TEM. The [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) incorporated in the polymer matrix increased the order degree of the liquid-crystal-like morphology in the P(St-Fl) block. The prepared AB22 and AB44P(St-Fl)-b-P2VP could be blended with PCBM for electrical write-once-read-many times (WORM) memory devices application. The threshold voltage (−2.8 to −3.9 V) and ON/OFF ratio (103 to 106) of the WORM characteristic were obtained by tuning the PCBM content. The ON or OFF state of the WORM memory devices could remain over 104 s without any degradation. The optical absorption and photoluminescence results indicated the formation of charge transfer complexation between PCBM and the 2-vinylpyridine or pendent fluorene chromophores. The switching behavior was further explained by the charge injection dominated thermionic emission in the OFF state and field-induced charge transfer in the ON state. These experimental results provide a new strategy to prepare a conjugated moiety containing asymmetric star polymers and their potential electrical memory device applications.