J. Phys. Chem. 1993,97, 10504-10508
10504
Electrochemical Characterization of a Thermoresponsive N-Isopropylacrylamide-Vinylferrocene Copolymer Film by the Use of Quartz Crystal Oscillators Noboru Oyama,' Tetsu Tatsuma, and Katsuhiko Takahashi Department of Applied Chemistry, Tokyo University of Agriculture and Technology, Naka-machi, Koganei, Tokyo 184, Japan Received: June 29, 1993"
N-Isopropylacrylamide was copolymerized with vinylferrocene (1 3: l ) , and an electrode was coated with a film of thus obtained copolymer. Electrochemical and other properties of the film were examined a t various temperatures by use of quartz crystal oscillators coupled with conventional electrochemical measurement systems. The copolymer film showed thermally induced volumetric phase transition; a t higher or lower temperatures than the transition temperature ( Tt),the film is shrunken or swollen, respectively, in aqueous media. The apparent diffusion coefficient for the homogeneous charge transport in the swollen film is a t least 5-fold larger than that in the shrunken film. The apparent formal potential of the film is also different between the swollen and shrunken states. Further, the phase transition temperature for the oxidized film was found to be higher than that for the reduced film. Thus, it was found that the copolymer film has two functions, namely, conversion of thermal information into electrochemical one and electrochemical control of phase transition or the transition temperature. A
Introduction
Poly(N4sopropylacrylamide) is a thermoresponsive polymer, the film of which shows volumetric phase transition at a specific phase transition temperature ( Tt).l At temperatures lower than T,, the film in an aqueous medium absorbs water and is swollen. On the other hand, at higher temperatures than T,, hydrophobic domains of the polymer aggregate and the film is shrunken. Such a thermoresponsive polymer is increasingly remarked as a functional material for drug release and so forth.* Further functionalization of the polymer is also envisaged by introducing other functional groups to the polymer. Gel consisting of a copolymer of N-isopropylacrylamide and a porphyrin derivative was found to exhibit phase transition induced by visible light.3 Introducing a redox active group to the polymer is expected to give a thermoresponsiveredox polymer. N-Isopropylacrylamide can be copolymerized with vinylferrocene to yield an electrochemically active copolymer (Figure 1A),4 which is hereafter referred to as poly(NIPAA/VF). An aqueous solution of this copolymer reportedly exhibits thermally induced phase separation, which is also caused by aggregation of hydrophobic domains, at higher temperatures than a specific temperature. In the present work, an electrode is coated with a film of poly(NIPAA/VF), which is expected to have the following functions: (A) conversion of thermal information into electrochemical information and (B) electrochemical control of phase transition or transition temperature. With these expected functions in mind, we characterized the poly(NIPAA/VF) in an NaC104 aqueous solution by means of cyclic voltammetry and by use of quartz crystal oscillators coupled with conventional electrochemicalmeasurement systems. Quartz crystal oscillators have served as microgravimetric sensors in both gaseous and liquid media.5 Mass loading onto the surface of a quartz crystal oscillator causes a decrease in the resonant frequency of the oscillator and the frequency decrease is proportional to the mass increase. To study not only mass change but also surface chemical and physical processes on a quartz crystal oscillator, it is preferable to examine the resonance properties of the oscillator on the basis of the electromechanical equivalent circuit model for a piezoelectric quartz crystal 0
To whom correspondence should be addressed. Abstract published in Aduance ACS Absrracrs, September 15, 1993.
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TTt Figure 1. (A) Molecular structure of NIPAA/VF copolymer. (B) Schematic illustration of the copolymer film in the swollen (left) and shrunken (right) states.
oscillator.6 In the recent papers, we studied the swelling and shrinking processes of a montmorillonite clay film7 and film property changes accompanying a redox reaction of a siloxane polymer having ferrocenyl group8 on the basis of piezoelectric admittance. Here this technique is applied to the analysis of film property changes as results of the phase transition induced by a redox reaction and a temperature change.
Experimental Section Materials. N-Isopropylacrylamidewas purchased from Tokyo Kasei (Japan) and recrystallized from a mixed medium of benzene and n-hexane (ca. 1:l). Vinylferrocene from Aldrich and azobisisobutyronitrile from Kanto Chemical (Japan) as a polymerization initiator were used as obtained. AT-cut quartz crystal oscillators (5 MHz) with Au electrodes were used for piezoelectric measurements unless otherwisenoted. Geometrical area of electrochemically active and piezoelectrically active electrodes were 0.5 and 0.283 cm2,respectively. Au-coated glass plate (0.5cm2) was used for conventional and electrochemical measurements. Preparation of Polymer Film-Coated Electrodes. The copolymer was synthesized in benzene containing 1.9 M N-isopro-
0022-365419312097-10504%04.00/0 0 1993 American Chemical Society
A Thermoresponsive Copolymer Film pylacrylamide, 0.1 M vinylferrocene, and 0.07 M azobisisobutyronitrile. This solution was continuously stirred for 6 h at 60 OC under nitrogen atmosphere, then mixed with a large amount of acetone, and finally mixed with n-hexane to precipitate the copolymer. Thus obtained copolymer was thoroughly rinsed with n-hexane, dried, and then stored as powder. An aqueous solution of the copolymer ( < l o OC,2 wt %) was cast on an electrode (12 p L cm-2) and dried up. Thickness of the film thus obtained was about 5 pm in a dried state. Measurement Procedures. Measurements were performed in a temperature-controlled aqueous solution of NaC104 (0.1 M unless otherwise noted). Temperature of the solution was monitored by use of a thermocouple. Electrode potential was controlled with potentiostat/function generator PS-07 (Toho Technical Research, Japan). Potential-step measurements were performed using electrochemical analyzer CS- 1090 (Toa Dempa, Japan). A saturated sodium chloride caromel electrode (SSCE) and a platinum wire were used as reference and counter electrodes, respectively. Admittance of a quartz crystal oscillator was measured by use of 4192A LF impedance analyzer (HewlettPackerd), and all parameters set and data obtained were transferred to a personal computer PC-9801 (NEC, Japan) via GP-IB interface for subsequent data processing and analysis. A quartz chemical analyzer QCA917 (Seiko EG & G, Japan) was used for in situ monitoring of the resonant frequency and resonant resistance.
The Journal of Physical Chemistry, Vol. 97,No.40, 1993 10505
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Results and Discussion Thermally Induced Phase Transition. Ratio of acrylamide unit and ferrocene unit for the synthesized copolymer was determined by elemental analysis as 13:l. Thermally induced phase separation of an aqueous solution of the copolymer was observed at higher temperatures than ca. 21 OC by means of a differential scanning calorimeter, while the separation was observed at around 24 OC in transmittance measurements of the solution (transmittance is lower in the phase separated state). The copolymer film prepared by casting an aqueous solution of the copolymer ( is not advantageous for electron hopping. Thus, larger D,, at E the lower temperature is probably caused by higher mobility of D 'irl the polymer chain and/or a counter ion due to the swollen structure of the film. It was thus found that the apparent diffusion coefficients for the charge transport can be controlled by temperature. It means that conversion of thermal information into electrochemical one -2 -1 0 was attained. Further design of the polymer or selection of an appropriate electrolyte could bring about complete inhibition of log ([NaCIO,] / M) the redox process at high temperatures and achieve thermally Figure 5. Dependencies of the apparent formal potential (E!,) of the induced switching behavior of the film. poly(NIPAA/VF) film-coatedelectrode on the NaCIOd concentratin at Temperature Dependency of Apparent Formal Potential. Figure 10 ( O ) , 15 (m), 20 (A), 25 (A), 30 (O), and 35 (0)OC. 4 depicts the temperature dependencyof apparent formal potential of the ferrocene/ferricinium couple (Fc/Fc+), which is derived concentration of the supporting salt (NaC104 here) were examined from Figure 2. Apparent formal potential is defined as the average a t a range of temperature (Figure 5). Contribution of Ej is about of the anodic and cathodic peak potentials of a cyclic voltam-5 mV/decade.ll As can be seen in the figure, plots of E: mogram. Here we note again that apparent formal potentials at against a common logarithm of the salt concentration gave roughg 15 and 35 OC were almost independent of scan rate at all scan straight lines, and the slope changes a t around the phase transition rates used in the experiments (1-300 mV/s). This thermally temperature. Although activities of ferrocene and water in the induced change in the potential may be explained as follows. The film may depend on the salt concentration, the observed redox process of the poly(NIPAA/VF) film can be formulated dependencies of Eipp are caused probably by difference in as follows: transference numbers of the cation and anion (see eq 5); the change in the slope of those plots at around the transition FC, r+C+, t+A-, t P - , + mH,O, F? Fc+f+ A-, + temperature may imply a change in the transferase numbers of t+C+, e- mH,O, (4) ions, which is induced by the phase transition. At higher temperatures, namely in the shrunken state, the where A- and C+ are anion and cation, respectively and t- and slope is about -43 mV/decade. In the case where ion activities t+ are transference numbers of anion and cation, respectively, at in the film do not change or they are much smaller than the the film/solution interface. The subscripts f and s represent the activities in the solution, the minus slope means that ion transport film and solution phases. The number of water transported with during redox reactions at the film/solution interface is rather ions, m,is not necessarily an integer and sign of it is not necessarily selective toward anion. In this case, the transference number of plus. Thus, the apparent formal potential of Fc/Fc+ couple in C104- ion is estimated as about 0.8. On the other hand, in the the film, E:,,, is formulated as follows: case where ion activities in the film are much larger than those in the solution, the minus slope means that ion transport is rather selective toward cation. As will be described below, microgravi( t + - t-) In u(A-,) m In a(H,Of)) Ej (5) metric measurements using quartz crystal oscillators support anion where Efand Ej are formal potential of the ferrocene in the film selective behavior of the film. At lower temperatures, namely in and junction potenial, respectively, and a represents activity, on the swollen state, the slope is about -5 mV/decade and therefore the assumption that contribution of H+and OH- transport is the film is selective to neither the cation nor the anion. That is, negligible. Since the junction potential is almost independent of a Na+ ion (or C104- ion) is more difficult to be transported a t temperature,*O the observed temperature dependence of EL,, is the shrunken filmlsolution interface than a C104- ion (or Na+ ascribed to changes in transference numbers of ion and activities ion), while both ions are transported easily at the swollen film/ of ferrocene, ions, and water in the film. solution interface. To investigate the temperature dependence of the apparent As to poly(viny1ferrocene) film,6aJ1J2 which is more hydroformal potential in further detail, dependencies of Ef, on the phobic than the present copolymer, slope of the plots was -40 to
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A Thermoresponsive Copolymer Film
The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10507 loo00
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Figure 6. Frquency change vs. potential curveg for poly(NIPAA/VF) film-coated quartz crystal oscillator in a 0.1 M NaClOd aqueous solution at 10 (A), 20 (B), 25 (C), and 35 (D) OC. The scan rate is 10 mV 5-l.
-45 mV/decade at both 8 and 30 "C. This behavior may be analogous to that of the poly(NIPAA/VF) film in the shrunken state. Dependencies of tbe Film Properties on Potential and Temperature. By the use of the NIPAA/VF copolymer film-coated quartz crystal oscillator, frequency change was monitored in the course of cyclic voltammetry (Figure 6). Geometrical area of the piezoelectrically active electrode was 0.283 cm*. Similar results could be obtained at steady state, as will be described below regarding Figure 7. For the shrunken film (at 35 "C), the resonant frequency in the oxidized state was lower than that in the reduced state. If it is assumed that the film is sufficiently rigid, this behavior indicates that mass of the film increases with oxidation and decreases with reduction; namely, this corresponds to anion-exchange behavior of the film. For the swollen film (at 10 "C), the resonant frequency in the oxidized state was higher than that in the reduced state. Such a behavior is not expected from Figure 5 showing comparable transference number for each ion at temperatures
