Weak polyelectrolyte brushes: re-entrant swelling and self-organization

We have studied the combined influence of pH and ionic strength on the properties of brushes of a weak polyion, poly(acrylic acid), in conditions of grafting density close to the mushroom-brush crossover. By combining atomic force microscopy AFM and quartz crystal microbalance, we show that at low i...

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Bibliographic Details
Published in:Soft matter 2020-08, Vol.16 (33), p.7727-7738
Main Authors: Senechal, Vincent, Saadaoui, Hassan, Vargas-Alfredo, Nelson, Rodriguez-Hernandez, Juan, Drummond, Carlos
Format: Article
Language:English
Subjects:
Acrylic acid
Addition polymerization
Atomic force microscopy
Brushes
Chemical Sciences
Condensed Matter
Crossovers
Grafting
Ionic strength
Ionization
Ions
Microbalances
Microscopy
Mushrooms
pH effects
Physics
Polyacrylic acid
Polyelectrolytes
Polymers
Quartz crystal microbalance
Quartz crystals
Self-assembly
Silica
Silicon dioxide
Soft Condensed Matter
Swelling
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Summary:We have studied the combined influence of pH and ionic strength on the properties of brushes of a weak polyion, poly(acrylic acid), in conditions of grafting density close to the mushroom-brush crossover. By combining atomic force microscopy AFM and quartz crystal microbalance, we show that at low ionic strengths the conformational change of grafted polyions is non-monotonic with increasing pH due to the counterintuitive variation of the ionization degree. Thus, reentrant swelling of the polymer chains is observed with increasing pH. This effect is more important at low polymer grafting densities, when it is accompanied by in-plane heterogeneous distribution at intermediate pH values. In addition, we observed self-assembly on the polymer brush (formation of holes and islands) at pH values below p K a , due to the short-range attractive interaction between uncharged grafted chains. The sensitivity of the ionization of grafted chains to the physicochemical environment was also studied by measuring the interaction force between a silica tip and polymer brushes by atomic force microscopy. The dependence of the ionization of polyions on the presence of the tip points toward important charge regulation effects, in particular at pH values corresponding to partial ionization of the polyion. We have studied the influence of pH and ionic strength on brushes of poly(acrylic acid) at grafting densities close to the mushroom-brush crossover. Heterogeneous morphology and non-monotonous variation of charge density with pH were observed.
ISSN:1744-683X
1744-6848
DOI:10.1039/d0sm00810a