Tunable pH and temperature response of weak polyelectrolyte brushes: Role of hydrogen bonding and monomer hydrophobicity

Academic Article

Abstract

  • We report on weak polyacid brushes with highly tunable pH and temperature response characteristics. This was achieved by synthesizing a series of homo- and copolymers which contain 2-alkylacrylic acids (aAAs) of increased hydrophobicity, i.e. acrylic acid (AA), methacrylic acid (MAA), or 2-ethylacrylic acid (EAA), using surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) polymerization. As revealed by contact angle measurements, in situ ellipsometry and AFM studies of brush swelling, the pH-response of PAA and PMAA brushes was similar, with brushes remaining highly swollen (swelling ratio 2.5-3.0) at low pH values. The PEAA brush, however, was unique as it showed low degrees of water uptake (<10%) at pH < 5 due to insolubility of this polyelectrolyte in acidic solutions, moderate swelling at high pH and relatively high (>70°) contact angles in the entire pH region from 2 to 8. Copolymer brushes of aAAs with N-isopropylacrylamide (NIPAM), denoted as P(AA-co-NIPAM), P(MAA-co-NIPAM) and P(EAA-co-NIPAM), demonstrated dual pH and temperature response, which was strongly dependent on the type of aAA co-monomer. P(AA-co-NIPAM) and P(MAA-co-NIPAM) brushes underwent large-amplitude pH-induced changes in brush swelling and water contact angle in the range of pH from 3 to 6, and were only weakly responsive to temperature in the transition region. In contrast, more hydrophobic P(EAA-co-NIPAM) brushes demonstrated both pH and temperature responses at physiologically relevant neutral/basic pH values even when the content of EAA units in the copolymer was as high as ∼50%. We discuss the role of inter- and intra-molecular hydrogen bonding and monomer hydrophobicity and ionization (quantified by FTIR) in determining pH ranges for brush response. These findings might enable control of molecular/cellular adhesion and flow at interfaces potentially useful in microfluidic and biomedical applications. © 2013 The Royal Society of Chemistry.
  • Published In

  • Soft Matter  Journal
  • Digital Object Identifier (doi)

    Author List

  • Lu Y; Zhuk A; Xu L; Liang X; Kharlampieva E; Sukhishvili SA
  • Start Page

  • 5464
  • End Page

  • 5472
  • Volume

  • 9
  • Issue

  • 22