J. Phys. Chem. B 2005, 109, 14335-14343
14335
Evaluating the Influence of Deposition Conditions on Solvation of Reactive Conducting Polymers with Neutron Reflectivity Andrew Glidle, Charlotte S. Hadyoon, Nikolaj Gadegaard, and Jon M. Cooper Bioelectronics Research Group, Department of Electronics, Glasgow UniVersity, Glasgow, G12 8LT, United Kingdom
A. Robert Hillman* and Robert W. Wilson Department of Chemistry, UniVersity of Leicester, Leicester, LE1 7RH, United Kingdom
Karl S. Ryder Department of Chemistry, UniVersity of Leicester, Leicester, LE1 7RH, United Kingdom, and Department of Chemistry, Loughborough UniVersity, Loughborough, LE11 3TU, United Kingdom
John R. P. Webster ISIS Facility, Rutherford Appleton Laboratory, Didcot, Oxfordshire, OX11 0QX, United Kingdom
Robert Cubitt Institut Laue-LangeVin, 38042 Grenoble Cedex 9, France ReceiVed: March 23, 2005; In Final Form: May 12, 2005
We describe in situ neutron reflectivity (NR) and RAIRS studies of the chemical modification of films of a polypyrrole-based conducting polymer derived from the pentafluorophenyl ester of poly(pyrrole-N-propanoic acid) (PFP) electrodeposited on electrode surfaces. We explore the role of the solvent in controlling the rate of reaction with solution-based nucleophiles (amines, which react with the ester to form amides). By varying the identity of the solvent (water vs acetonitrile) and the neutron contrast (deuteration), we find that both the identity of the solvent and its population within the film are paramount in determining chemical reactivity and electroactivity. IR signatures allow monitoring of the reaction of solution-based amine-tagged species such as amino-terminated poly(propylene glycol), ferrocene ethylamine, and lysine with film-based ester functionalities: the carbonyl bands show ester/amide interconversion and some hydrolysis to acid. Timedependent spectral analysis shows marked variations in reaction rate with (i) (co-)polymer composition (replacement of some fluorinated ester-functionalized pyrrole with unfunctionalized pyrrole), (ii) the solvent to which the polymer film is exposed, and (iii) the rate of polymer deposition. NR data provide solvent profiles as a function of distance perpendicular to the interface, the variations of which provide an explanation for film reactivity patterns. Homopolymer films are relatively hydrophobic, thus hindering reaction with species present in water solutions. Incorporating pyrrole groups raises the solvent populationsdramatically for waters thereby facilitating entry and reaction of aqueous-based lysine. Changing film deposition rate yields films with different absolute levels of solvent and reactivity patterns that are dependent on the size of the reactant molecules: more rapid deposition of polymer gives films with a more open structure leading to a higher solvent content and thence increased reactivity. These results, supported by XPS and AFM data, allow assembly of composition-structure-reactivity correlations, in which the controlling feature is film solvation.
Introduction In this paper we address an issue that is central to construction of sophisticated interfacial structures at electrode surfaces, namely the extent to which one can modify a film (here a polymer, but the methodology is quite general) after its deposition. The issue is important because the “cartoons” frequently used to represent surface modification procedures generally describe synthetic aspirations, rather than directly determined structural accomplishments. Since postdeposition modification of a filmshere, an electroactive conducting polymersgenerally involves diffusion into the film of a species * Address correspondence to this author. E-mail:
[email protected].
from the bathing solution, we explore the notion that control over film solvation must be related to the ease with which it can be chemically functionalized. The strategy adopted is to determine (i) the nature and extent of reaction (the spatially averaged composition), using infrared spectroscopy, and (ii) the spatial locations of entering reactant and solvent and of departing product (spatial structure), using neutron reflectivity. The time dependences of these responses, together with electrochemical assay of conducting polymer spine and modifying redox functionalities, provide information on reactivity. The end result is that we are able to explore in situ film compositionstructure-reactivity correlations as a function of copolymer and
10.1021/jp0515030 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/07/2005
14336 J. Phys. Chem. B, Vol. 109, No. 30, 2005 solvent identity. The crucial outcome is that these correlations are all controlled by the population within the film of solvent, an electroinactive, transparent, and generally spectroscopically silent species whose role is frequently overlooked. As has been widely reviewed,1,2 electrodeposited conducting organic polymers have found many applications since their discovery in the mid-1980s. Among these are systems drawn from fields as diverse as analytical3 and materials4 science, electrochromic displays,5 light emission6 and photovoltaic devices,7 battery technology,8 energy storage,9 catalysis,10 chemical11 and biochemical12 sensors, antibiofouling surfaces,13 MEMS,14 and electronic muscles.15 In attempting to realize such applications, it is generally recognized that the ability to characterize and manipulate film solvent content is crucial: this is because the solvent facilitates the dynamics of both the polymer itself (chain motion) and mobile species (ion, solvent, and reactant exchange and transport). Although some techniques, notably the EQCM,16 have proved to be very useful in observing changes in film solvation, quantitative statements regarding absolute solvent populations are rare. Here we report the use of neutron reflectivity to accomplish not only this latter goal but also to reveal the spatial distribution of solvent within a conducting polymer film. A frequently claimed advantage of electrochemical control to effect polymer deposition is the ability to influence material properties such as charge-transfer rates, deposition yields, polymer morphology, structure, and composition.1,2 Electrochemistry offers this opportunity through control of parameters such as deposition rate (via either potentiostatic or galvanostatic control), monomer concentration, electrolyte character and the inclusion of additives/brighteners, or the use of solutions containing multiple monomer species (so as to produce copolymer films).17 While the effects of varying these parameters on material performance can be measured with electrochemical, spectroscopic, or scanned probe microscopies, direct measurements of the underlying influence on these variations of the internal polymer microstructure are harder to perform. One of the few techniques that can “see” inside polymer films with a spatial resolution on the nanometer scale is Neutron Reflectivity (NR).18 In terms of fundamental characteristics, this technique has similarities with X-ray and optical reflectivity methods, but it operates on a different length scale and has the advantage of being isotopically sensitive. Thus, when used to observe polymer films exposed to a solution, the contrast of the solvent molecules within the polymer film can be changed simply by substituting hydrogenous for deuterated solvents. A particular polymer system we have found extremely versatile as the basis for the formation of chemical and biochemical sensors, antibiofouling surfaces, and templates for nanoscale architectures has been the pentafluorophenyl ester of pyrrole-N-propanoic acid (PFP).19 After deposition, the activated ester group in this polymer can readily be made to undergo a substitution reaction with solution-based molecules containing a nucleophilic moiety (such as a primary or secondary amine group). An advantage of this postdeposition modification strategy in creating polymer interfaces with different functionalities is that it is not necessary to devise synthetic routes to differently functionalized monomers for each particular application. Such synthesis can be both laborious and (in unfortunate circumstances) lead to functionalized monomers that prove difficult to electropolymerize. The postdeposition modification reactions of poly(PFP) proceed readily when the substituting nucleophilic molecules are dissolved in polar organic solvents such as acetonitrile,
Glidle et al. dimethyl sulfoxide, or dimethylformamide.20 In principle, this appears very attractive, but we have found that application in aqueous media is problematical. Specifically, if a purely aqueous solvent is used, it has been found that for all but the smallest nucleophiles (MW
