Ionization and pH Stability of Multilayers Formed by Self-Assembly of Weak Polyelectrolytes
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The formation of polyelectrolyte multilayers using the layer-by-layer sequential self-assembly of a weak polybase and polyacid onto a surface of Si crystal and their stability under acidic conditions were studied. The system was 12%, 20%, 25%, 37%, 90%, or 98% quaternized poly-4-vinylpyridine (Q12, Q20, Q25, Q37, Q90, or Q98) as polybases (generally abbreviated as QPVP) and poly(methacrylic acid) (PMAA) as a polyacid in buffered D2O. The adsorption and ionization of pyridine rings and carboxylic groups were quantified using Fourier transform infrared spectroscopy in attenuated total reflection mode. At pH = 5, film growth was primarily controlled by electrostatic interactions. When QPVP was included into the top layer, ionization of carboxylic groups was increased compared to its value in solution. The amount of the induced charge increased linearly with the total charge density of the polybase (calculated as a sum of ethylated pyridinium and protonated pyridinium groups), suggesting the formation of both types of electrostatic pairs (COO--NR+ and COO--NH+). The degree of ionization of both pyridine groups and carboxylic groups within the film responded to the charge of the polymer included in the top layer and oscillated as a function of layer number. The average ionization of the carboxylic groups within the multilayer was suppressed when the multilayer was topped with PMAA, and it was enhanced when the top layer was the polybase. The effect was rather long-ranged and persisted through over three polymer layers with a decay length of about 100 . When the pH of the buffer solution was lowered below pH 2, multilayers dissolved due to protonation of carboxylic groups and disruption of polymer/polymer ionic contacts. The multilayer stability under acidic conditions was dependent on the alkylation degree of QPVP. With the exception of the largest alkylation degrees, the stability of the film increased with the alkylation degree of QPVP. The multilayer destruction correlated with the protonation of the small (<5%) residual fraction of carboxylic groups that was included into polymer/polymer ionic pairs and with the number of ionic polymer/polymer contacts per QPVP chain, suggesting that the number of polymer/polymer contacts during self-assembly was a critical parameter in multilayer stability. The results show the effect of the QPVP charge density on the ionization of self-assembled PMAA and on the pH stability of polyelectrolyte multilayers.