Critical salt effects in the swelling behavior of a weak polybasic brush

Area of Science:

• Polymer Science
• Surface Chemistry
• Materials Science

Background:

• Polymer brushes are crucial in surface modification and nanotechnology.
• Understanding their response to environmental stimuli is key for advanced applications.
• Poly(2-(diethylamino)ethyl methacrylate) (PDEA) is a pH-responsive polymer.

Purpose of the Study:

• To investigate the swelling behavior of PDEA brushes in response to pH and ionic strength.
• To elucidate the mechanisms behind the observed transitions and hysteresis.
• To compare the kinetics of pH-induced and salt-induced conformational changes.

Main Methods:

• Synthesis of PDEA brushes on silica using ARGET ATRP with two initiator approaches.
• In situ ellipsometry for equilibrium measurements of brush thickness.
• Kinetic studies with high temporal resolution to analyze transition rates.

Main Results:

• pH-induced swelling and collapse transitions exhibit hysteresis.
• Swelling (protonation, solvent ingress) is faster than collapse (charge neutralization, solvent expulsion).
• A critical conformational change occurs at low electrolyte concentrations (0.5-1 mM) due to brush hydrophobicity.
• Salt-induced swelling and collapse kinetics are nearly identical, unlike pH-induced transitions.

Conclusions:

• Hysteresis in pH response is attributed to a dense outer skin formed during collapse.
• The narrow range of salt-induced conformational change suggests hydrophobicity dominates over osmotic effects.
• The differing kinetics of pH and salt responses indicate distinct mechanisms triggered by environmental stimuli.