Biomedical subjects

Mark C Lonergan

Publications and source records attributed to Mark C Lonergan.

4 recordsLinked to original sources

PubMed2005

Charge transport in a mixed ionically/electronically conducting, cationic, polyacetylene ionomer between ion-blocking electrodes.

The electrical behavior of the cationic, polyacetylene-based, conjugated ionomer, poly[(2-cyclooctatetraenylethyl)trimethylammonium trifluoromethanesulfonate], sandwiched between gold electrodes is reported. The steady-state current of this mixed ionically/electronically conducting system is assigned to be unipolar diffusive hole transport for voltages below approximately 1.4 V, giving way to bipolar migratory transport above approximately 1.4 V. In the low-voltage regime, a non-Faradaically controlled doping model is proposed where p-doping at the anode is balanced by the charging of an ionic double layer at the cathode. In the high-voltage regime, n- and p-type regions extend from the electrodes as the voltage becomes sufficient to drive disproportionation and the electric field required by the redistribution of ions begins to substantially influence carrier transport. The assignment of a transport mechanism is primarily based on analyzing the decay of the steady-state system under short-circuit and open-circuit conditions. First, it is shown that the power describing the power-law decay of the short-circuit current is characteristic of the steady-state carrier profile. Second, it is argued that a component of the time-dependent, open-circuit voltage decaying more rapidly than the time scale for ion motion is indicative of a substantial migratory component to steady-state transport, as observed in the high-voltage regime. The hole and electron mobilities are estimated to be on the order of 10(-7)-10(-6) cm(2) V(-1) s(-1).

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PubMed2004

A conjugated polymer pn junction.

Dopant counterion diffusion has made the conjugated polymer pn homojunction a challenging target for decades. We report the electrochemical fabrication of a polyacetylene pn homojunction based on internally compensated forms where the dopant counterions are covalently bound to the polymer backbone. After drying under vacuum, the pn junction exhibits diode behavior with the ratio of the forward to reverse current at 2 V being 7. Despite such modest diode behavior, the fabricated pn junction is significant because it demonstrates the utility of internal compensation in the fabrication of metastable interfaces between dissimilarly doped conjugated polymers.

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PubMed2004

Unidirectional current in a polyacetylene hetero-ionic junction.

Unidirectional electronic current is reported for a device based on the interface between an anionically functionalized and a cationically functionalized polyacetylene. The unidirectional current in this mixed ionically/electronically conducting system is electronic but is regulated by asymmetry in the ionic processes.

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PubMed2002

Electrochemical characterization of polyacetylene ionomers and polyelectrolyte-mediated electrochemistry toward interfaces between dissimilarly doped conjugated polymers.

The electrochemical characterization of thin films of the ionically functionalized polyacetylene analogues poly(tetramethylammonium 2-cyclooctatetraenylethanesulfonate) (P(A)) and poly[(2-cyclooctatetraenylethyl)trimethylammonium trifluoromethanesulfonate] (P(C)) is reported along with an electrochemical approach to the fabrication of interfaces between dissimilarly doped conjugated polymers. Such interfaces are of interest because of the central role analogous interfaces based on silicon play in conventional microelectronics. The cationically functionalized P(C) can be both oxidatively (p-type) and reductively (n-type) doped to a conductive state, whereas the anionically functionalized P(A) can only be p-type doped. The voltammetry of P(C) displays relatively sharp waves with minimal history or relaxation effects. In contrast, the voltammetry of P(A) exhibits broader doping waves and a dependence on electrochemical history. The apparent formal potentials reported in 0.075 M Me4NBF4/CH3CN were -1.04 V versus SCE for the n-doping of P(C) and 0.40 and 0.30 V versus SCE for the p-doping of P(C) and P(A), respectively. These values depend on electrolyte concentration consistent with a Donnan potential due to the selective partitioning of ions between the electrolyte and polymer. Electrochemical quartz crystal microbalance data demonstrate that the p-type doping of P(A) and the n-type doping of P(C) proceed with the loss of ions from the polymer film and the formation of the internally compensated state. Voltammetry in tetrabutylammonium poly(styrenesulfonate)/CH3CN supporting electrolyte is also reported. It is demonstrated how a polyanion supporting electrolyte in concert with a conjugated ionomer can be used to control redox chemistry by governing the sign of ions available for charge compensation. In particular, we demonstrate the self-limiting oxidation of P(A) to inhibit deleterious overoxidation and prepare the precisely internally compensated state; the selective oxidation of P(A) over P(C), despite their similar apparent formal potentials; and the inhibition of the reoxidation of the n-doped form of P(C). The use of such polyelectrolyte-mediated electrochemistry in the fabrication of interfaces between dissimilarly doped conjugated polymers is discussed.

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