Aggregation Behavior of Mono-endcapped Hydrophobically Modified Poly(sodium acrylate)s in Aqueous Solution

Area of Science:

• Polymer Science
• Physical Chemistry
• Materials Science

Background:

• Hydrophobically modified polymers self-assemble into aggregates in aqueous solutions.
• Understanding polymer aggregation is crucial for designing advanced materials and drug delivery systems.

Purpose of the Study:

• To investigate the aggregation behavior of mono-endcapped hydrophobically modified poly(sodium acrylate)s.
• To determine the critical aggregation concentration (CAC) under varying conditions of pH and salt concentration.
• To elucidate the influence of polymer molecular weight on aggregation.

Main Methods:

• Synthesis of polymers with varying molecular weights (800–31,700) via radical polymerization.
• Titration microcalorimetry to measure thermodynamic parameters of aggregation.
• Steady-state fluorescence spectroscopy to determine CAC.

Main Results:

• At pH 5, CAC increases with molecular weight, ranging from 0.2–2.4 mM.
• At pH 9, CAC increases significantly to ~80 mM due to electrostatic repulsions.
• An anomalous decrease in CAC with increasing molecular weight was observed for polymers with M(n)<1400 at pH 9, attributed to counterion gradients.
• 1 M sodium citrate suppressed this anomaly, lowering CAC to levels similar to pH 5 conditions.

Conclusions:

• Polymer molecular weight and solution pH critically influence the aggregation of hydrophobically modified poly(sodium acrylate)s.
• Electrostatic repulsions at higher pH significantly increase the CAC.
• Counterion binding plays a key role in modulating aggregation behavior, especially for lower molecular weight polymers.