Polymerization galvanostatic

On the basis of experimental findings Heinze et al. propose the formation of a particularly stable, previously unknown tertiary structure between the charged chain segments and the solvated counterions in the polymer during galvanostatic or potentiostatic polymerization. During the discharging scan this structure is irreversibly altered. The absence of typical capacitive currents for the oxidized polymer film leads them to surmise that the postulated double layer effects are considerably smaller than previously assumed and that the broad current plateau is caused at least in part by faradaic redox processes. 

The disadvantages described above in terms of the irreversibility of the polyion response stimulated further research efforts in the area of polyion-selective sensors. Recently, a new detection technique was proposed utilizing electrochemically controlled, reversible ion extraction into polymeric membranes in an alternating galvanostatic/potentiostatic mode [51]. The solvent polymeric membrane of this novel class of sensors contained a highly lipophilic electrolyte and, therefore, did not possess ion exchange properties in contrast to potentiometric polyion electrodes. Indeed, the process of ion extraction was here induced electrochemically by applying a constant current pulse. 

Let us consider, for instance, the response mechanism of a polycation-selective galvanostatically controlled sensor. The polymeric membrane is in contact with a NaCl solution. The membrane of the sensor is formulated with a lipophilic salt, for instance, tetradodecylammonium dinonylnaphthalenesulfonate (TDDA-DNNS), which has a relatively high affinity to protamine. Even though protamine is presented in the sample, spontaneous extraction does not take place due to the high lipophilicity of TDDA-DNNS, thus the initial concentration of protamine or sodium cations in the membrane is close to zero. 

A. Shvarev and E. Bakker, Pulsed galvanostatic control of ionophore-based polymeric ion sensors. Anal. Chem. 75, 4541-4550 (2003). 

A molecularly imprinted polypyrrole film coating a quartz resonator of a QCM transducer was used for determination of sodium dodecyl sulphate (SDS) [147], Preparation of this film involved galvanostatic polymerization of pyrrole, in the presence of SDS, on the platinum-film-sputtered electrode of a quartz resonator. Typically, a 1-mA current was passed for 1 min through the solution, which was 0.1 mM in pyrrole, 1 mM in SDS and 0.1 M in the TRIS buffer (pH = 9.0). A carbon rod and the Pt-film electrode was used as the cathode and anode, respectively. The SDS template was then removed by rinsing the MlP-film coated Pt electrode with water. The chemosensor response was measured in a differential flow mode, at a flow rate of 1.2 mL min-1, with the TRIS buffer (pH = 9.0) as the reference solution. This response was affected by electropolymerization parameters, such as solution pH, electropolymerization time and monomer concentration. Apparently, electropolymerization of pyrrole at pH = 9.0 resulted in an MIP film featuring high sensitivity of 283.78 Hz per log(conc.) and a very wide linear concentration range of 10 pM to 0.1 mM SDS. 

In situ polymerization, and electrochemical polymerization in particular [22], is an elegant procedure to form an ultra thin MIP film directly on the transducer surface. Electrochemical polymerization involves redox monomers that can be polymerized under galvanostatic, potentiostatic or potentiodynamic conditions that allow control of the properties of the MIP film being prepared. That is, the polymer thickness and its porosity can easily be adjusted with the amount of charge transferred as well as by selection of solvent and counter ions of suitable sizes, respectively. Except for template removal, this polymerization does not require any further film treatment and, in fact, the film can be applied directly. Formation of an ultrathin film of MIP is one of the attractive ways of chemosensor fabrication that avoids introduction of an excessive diffusion barrier for the analyte, thus improving chemosensor performance. This type of MIP is used to fabricate not only electrochemical [114] but also optical [59] and PZ [28] chemosensors. 

Poly(bithiophene) films from these two ionic liquids are morphologically similar (Figure 7.14), even though the redox behavior (Figure 7.9) is markedly different, suggesting that the dominant differences in the films produced are on an atomic or sub-micron rather than macroscopic level. The morphology ofthe poly (bithiophene) films appears to be similar to that described by Roncali et al. [74] who reported a thin film on the surface of the electrode, covered by a thick brittle powdery deposit, from the galvanostatic polymerization of bithiophene in acetonitrile. The nodular structures are smaller in the poly (bithiophene) films than in the poly (thiophene), which is consistent with the formation of shorter chain polymers [73], but this does not... 

Galvanostatic, potentiostatic, or potentiodynamic techniques can be used to electro-polymerize suitable monomeric species and form the corresponding film on the electrode. The potentiodynamic experiment in particular provides useful information on the growth rate of conducting polymers. The increase in current with each cycle of a multistep CV is a direct measure of the increase in the surface of the redoxactive polymer and, hence, a measure of relative growth rates (Fig. 3). 

Figure 11.7 Atomic force microscopy images measured in situ from fresh PPy film formed in a 0.25 M TBACIO4/PC solution (the conditions of galvanostatic polymerization are indicated in the caption to Figure 11.5a). The images were taken during two consecutive potentiodynamic cycles, at (a) 2V and (b) 4 V, as indicated. Area scanned 1 X 1 (Im.

Most laboratory setups employ a three-electrode potentiostated system to ensure effective potential control and to maximize the reproducibility of the polymerization process. The positioning of the auxiliary electrode is critical in that it determines the electrical field generated, which can influence the quality and evenness of the polymer deposited. The electrode system shown in Figure 2.2 includes a reference electrode. A two-electrode cell can also be used, usually with galvanostatic (constant current) electropolymerization methods, but care must be taken to avoid overoxidation of the PPy through poor control of the potential. 

To achieve high conductivities, the polythiophene paradox must be overcome. The polymerization process and conductivity of the resultant material are influenced by the concentration of monomer used during polymerization116 because, if this is too low, the overoxidation reaction predominates, at least when galvanostatic polymerization is used. Synthesis at reduced temperatures will help avoid overoxidation and can be used to increase the conductivity of the resultant material.117... 

In the last few decades, conductive polymers have found exciting new relevance in the non-rechargeable (primary) and rechargeable (secondary) batteries for electrical storage. The durability studies of polymeric electrodes may be made galvanostatically or potentios-tatically to evaluate their life in battery applications [49-51]. 

Fig. 5 Voltage changes during the galvanostatic electropolymerization of aniline to prepare PANI inverse opals. The inset sketches represent the stages of the formed PANI replica inside the PS template at the indicated points, with the SEM images showing the corresponding PANI inverse opals after stopping the polymerization at these points and subsequent PS template dissolution [76]...

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