Charge Dependence of Local Hydration Dynamics in Poly(Acrylic Acid) Solutions

Area of Science:

• Polymer Chemistry
• Physical Chemistry
• Materials Science

Background:

• Understanding polymer hydration is crucial for applications like superabsorbent polymers.
• The role of polymer protonation state in local water dynamics is not fully understood.

Purpose of the Study:

• To investigate how the protonation state of poly(acrylic acid) (pAA) affects its local solvation environment.
• To correlate local water dynamics with polymer chain conformation and hydration behavior.

Main Methods:

• Overhauser dynamic nuclear polarization (ODNP) relaxometry for localized hydration measurements.
• Molecular dynamics (MD) simulations to complement experimental findings.
• Varying polymer concentration and solution pH to probe different regimes.

Main Results:

• Three distinct regimes of local water dynamics were identified, ranging from bulk-like to subdued diffusivity.
• Deprotonated carboxylic acid groups promote a hydrophilic environment, extended chains, and bulk-like water incorporation.
• Protonated carboxylic acid groups lead to collapsed polymer conformations and slower hydration dynamics.
• Hydration behavior of pAA-stat-PEGMEA copolymers is influenced by monomer ratios.

Conclusions:

• Local water dynamics are directly linked to polymer chain conformation and the protonation state of carboxylic acid groups.
• This study provides molecular insights into the superabsorbent properties of poly(acrylic acid).
• The findings highlight the importance of carboxylic acid groups in mediating polymer hydration.