Controlling the collapse/swelling transition in charged hydrogels

Area of Science:

• Polymer Science
• Materials Science
• Physical Chemistry

Background:

• Charged hydrogels exhibit pH-dependent swelling due to ionic interactions.
• Understanding swelling behavior is crucial for applications in drug delivery and soft robotics.

Purpose of the Study:

• To systematically investigate parameters controlling the swelling transition of charged hydrogels.
• To characterize the influence of crosslink density, salt concentration, and pendant polyethylene glycol (PEG) chains on hydrogel swelling.

Main Methods:

• Combined theoretical analysis and experimental investigations.
• Characterized transition pH and transition width.
• Varied hydrogel crosslink density (subchain molecular weight) and solution salt concentration.
• Incorporated pendant polyethylene glycol (PEG) side chains.

Main Results:

• Swelling varied non-monotonically with salt concentration (initially increasing, then decreasing).
• Swelling remained sensitive to pH even at high salt concentrations.
• Transition pH depended solely on gel charge chemistry for long subchains, resulting in sharp transitions.
• Decreasing subchain length and/or salt concentration shifted transition pH and broadened the transition.
• Pendant PEG chains significantly increased swelling, unaffected transition pH, and narrowed transition width.

Conclusions:

• Hydrogel swelling is a complex interplay of electrostatic interactions, polymer network structure, and solution conditions.
• PEGylation offers a method to enhance hydrogel swelling and control transition sharpness.
• Findings provide insights for designing hydrogels with tailored swelling properties for specific applications.