Govinda Ghimire - Electron Propagation within Redox-Active Microdomains in Thin Films of Ferrocene-Containing Diblock Copolymers

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  Publication Details (including relevant citation   information):

  DOI:   10.1021/acs.langmuir.5b02996 Langmuir 2015, 31, 1230712314


    This paper reports the electrochemical behavior of redox-active   microdomains in thin films of ferrocene-containing diblock   copolymers, polystyrene-block-poly(2-(acryloyloxy)ethyl   ferrocenecarboxylate) (PS-b-PAEFc).    PS-b-PAEFc with different PAEFc volume fractions   (PS154-b-PAEFc51,   PS154-b-PAEFc26,   PS154-b-PAEFc12;   fPAEFc = 0.47, 0.30, 0.17, respectively) was   synthesized by sequential atomic transfer radical   polymerization.  PS-b-PAEFc films of controlled   thicknesses (20-160 nm) were prepared on gold substrates via   spin-coating and characterized by ellipsometry.    Microdomains were observed via atomic force microscopy at   the surfaces of PS154-b-PAEFc51  and PS154-b-PAEFc26 films, but not   on PS154-b-PAEFc12 films.    Electrochemical behavior of films was assessed by cyclic   voltammetry and chronocoulometry in acetonitrile solution.     The   redox potential of ferrocene moieties was similar (ca.   +0.29 V vs.   Fc+/Fc)   regardless of fPAEFc and film   thickness.    For PS154-b-PAEFc51  and PS154-b-PAEFc26, thicker films   afforded larger faradaic peak currents and exhibited   diffusion-controlled voltammograms at faster sweep rates.    PS154-b-PAEFc26 produced   voltammograms less influenced by solvent-induced swelling than   PS154-b-PAEFc51, reflecting the   improved morphological stability of PAEFc microdomains by   redox-inert PS frameworks.  In contrast,   PS154-b-PAEFc12 films yielded   similar faradaic peak currents regardless of film thickness, and   exhibited voltammograms indicative of surface-confined species.    These observations suggest that   PS154-b-PAEFc51 and   PS154-b-PAEFc26 films contain   continuous PAEFc microdomains extending from the electrode to the   surface, in contrast to the   PS154-b-PAEFc12 films which   contain isolated PAEFc microdomains buried within the PS   matrix.  Electron propagation took place only through PAEFc   microdomains that could electrically communicate with the   underlying electrode.  Apparent diffusion coefficients   within PAEFc microdomains were similar (≈ 2 x 10-11  cm2/s) for   PS154-b-PAEFc51 and   PS154-b-PAEFc26.  The   relatively low efficiency in electron propagation was   attributable to ineffective electron self-exchange reaction   within the PAEFc microdomains and/or limited counter ion   migration through the acetonitrile-swollen microdomains.    These results provide guidance in design of redox active   metalloblock copolymers for various applications, which include   electrocatalysis, electrochemical mediation in enzyme sensors   and   redox-controlled molecular   deposition.  

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