Influence of Polycation Composition on Electrochemical Film Formation

The effect of polyelectrolyte composition on the electrodeposition onto platinum is investigated using a counterion switching approach. Film formation of preformed polyelectrolytes is triggered by oxidation of hexacyanoferrates(II) (ferrocyanide), leading to polyelectrolyte complexes, which are physically crosslinked by hexacyanoferrate(III) (ferricyanide) ions due to preferential ferricyanide/polycation interactions. In this study, the electrodeposition of three different linear polyelectrolytes, namely quaternized poly[2-(dimethylamino)ethyl methacrylate] (i.e., poly{[2-(methacryloyloxy)ethyl]trimethylammonium chloride}; PMOTAC), quaternized poly[2-(dimethylamino)ethyl acrylate] (i.e., poly{[2-(acryloyloxy)ethyl]trimethylammonium chloride}; POTAC), quaternized poly[N-(3-dimethylaminopropyl)methacrylamide] (i.e., poly{[3-(methacrylamido)propyl]trimethylammonium chloride}; PMAPTAC) and different statistical copolymers of these polyelectrolytes with N-(3-aminopropyl)methacrylamide (APMA), are studied. Hydrodynamic voltammetry utilizing a rotating ring disk electrode (RRDE) shows the highest deposition efficiency DE for PMOTAC over PMAPTAC and over POTAC. Increasing incorporation of APMA weakens the preferred interaction of the quaternized units with the hexacyanoferrate(III) ions. At a sufficient APMA content, electrodeposition can thus be prevented. Additional electrochemical quartz crystal microbalance measurements reveal the formation of rigid polyelectrolyte films being highly crosslinked by the hexacyanoferrate(III) ions. Results indicate a different degree of water incorporation into these polyelectrolyte films. Hence, by adjusting the polycation composition, film properties can be tuned, while different chemistries can be incorporated into these electrodeposited thin hydrogel films.

. 1 H-NMR spectra in D2O recorded for P(MOTAC-co-APMA-15%) before (blue) and after (red) quaternization, peaks shifting upon quaternization are assigned as indicated in the sketch at the top right. Figure S2. 1 H-NMR spectra in D2O recorded for P(OTAC-co-APMA-15%) before (blue) and after (red) quaternization, peaks shifting upon quaternization are assigned as indicated in the sketch at the top right (* solvent impurities). . 1 H-NMR spectra in D2O recorded for P(MAPTAC-co-APMA-15%) before (blue) and after (red) quaternization, peaks shifting upon quaternization are assigned as indicated in the sketch at the top right.

Characterization of Synthesized Polymers
The synthesized polymers were characterized by 1 H-NMR to determine the amount of APMA polymerized into the corresponding copolymer systems. Therefore, the signal of methyl groups of the quaternized units is set to 9 for the MOTAC and OTAC copolymer systems. The APMA content is determined via the integral corresponding to the methylene groups adjacent to amide and amine function of APMA (equations and schemes below, and Figures S4, S5). In the case of the MAPTAC polymer systems the signal of the methyl groups (quaternary amine function) and methylene group adjacent to the quaternary amine function overlap with the methylene group adjacent to the APMAs amine function. Thus, the signal of the methyl and methylene groups of MAPTAC and the methylene group of APMA is set to 11 + x. x corresponds to the integral value found for the APMA methylene group adjacent to the amide function ( Figure S6). The APMA contents determined by this procedure are shown in Table S1. For P(MOTAC-co-APMA-x%), the APMA content is comparable to the initial monomer ratio used in the synthesis (5 %, 10 %, 15 % and 30 % of APMA). A larger amount is incorporated for the P(MAPTAC-co-APMA-x%) and P(OTAC-co-APMA-x%) copolymer systems.

Collection efficiencies & deposition efficiencies
The so-called collection efficiency N is calculated to evaluate how much of the disk-generated ferricyanide is collected at the ring [1]. It is defined as the ratio of the limiting disk and ring current. For the HCF couple the collection efficiency is around 0.5. It is decreased upon film and complex formation. As the visible collection efficiencies N are the lowest for copolymers containing MOTAC units, this indicates that this polymer is the most effective with respect to film formation. Moreover, the increasing APMA-content results in increased N values for all polymers. This again shows that the film formation is reduced when decreasing the possibilities of bridging points. Collection efficiencies were determined from the average current (averaged for backward and forward scan) at 0.46 V.
The so-called deposition efficiencies were estimated corresponding to the given equation: The charge required for the full reduction of the deposited ferricyanides is obtained by the difference of integration of the disk current (backward scan) and integration of the sigmoidal curve of ferrocyanide oxidation (forward scan). Thus, the cathodic disk peak current is determined. As some deposited ferricyanide is also detected at the ring, the corresponding difference in the integrals of backward and forward scan corrected by dividing by the N value is added to this expression. The calculated charge is compared to the total charge used for the oxidation of ferrocyanides. This is derived as twice the integral over the disk forward scan as the backward scan cannot be used due to the presence of the cathodic peak. Thus, the DE provides the information how much of the produced ferricyanide is used for film formation onto the disk electrode. Worthy to note, the baseline of forward and backward scan was compared and their difference subtracted from the cathodic disk peak area to eliminate contributions from capacitive background disk currents.

Electrochemical Quartz Crystal Microbalance
Only minor changes in resistance during the measurements indicate a rigid film that allows for application of the Sauerbrey-Equation. This equation correlates changes in the eigenfrequency of a specifically cut quartz crystal to mass changes per area [2,3]. The so-called sensitivity factor Cf then only relies on fundamental properties of the Quartz crystal rendering any calibration to be redundant. The used AT-cut Quartz crystal exhibit a sensitivity factor of 56.6 Hz µ g -1 cm².