Role of hydrogen bonding and hydrophobic interaction in the volume collapse of a poly(ethylenimine) gel

A cationic polyelectrolyte gel was prepared through the cross-linking of linear poly(ethylenimine) (LPEI) with ethylene glycol diglycidyl ether. The cross-linking degree was ca. 12% by means of elemental analysis, A pH change cycle (3 to 12 and vice versa) brought about a discontinuous transition near pH 5.9 (swelling) and pH 10.7 (deswelling); thus, a large hysteresis appeared in the swelling curves. In contrast, the gel underwent a continuous swelling-deswelling change without a hysteresis during the cyclical change of NaCl concentration from 0 to 0.1 M and vice versa, A careful analysis of the changes in pH of an aqueous gel dispersion during titration with NaOH followed by a back titration with HCl demonstrated that the origin of the hysteresis in the swelling curves was due to a hysteresis in the deprotonation-protonation process, The formation through hydrogen bonding of a stable structure between the LPEI chain segments in the network and water molecules as the solvent is considered to be the reason for the hysteresis in the deprotonation-protonation process. However, this does not account for the gel undergoing discontinuous transitions in the protonation and deprotonation processes. We therefore assumed that the transition takes place when the repulsive force due to the -NH2+- and -NH+ < groups overcomes the hydrogen bonding as the attractive force or vice versa. To support this assumption, the effects of urea and anionic surfactants were investigated. Urea facilitated the swelling of the gel at higher pH levels during the protonation process, which suggests that urea inhibits the formation of the hydrogen bond under consideration, On the other band, anionic surfactants such as sodium dodecyl sulfate (SDS) effectively collapsed the gel due not only to the neutralization of the cationic charges but also to hydrophobic interaction; the diameter of the gels was about half that of the gel collapsed at pH > 10.7. However, urea exhibited no influence on the SDS-induced gel collapse, thus indicating that urea inhibits the formation of hydrogen bonds but does not affect the hydrophobic interaction.