Synthesis and Characterization of Poly(acrylic acid) based Microgels for Formulation Applications

Abstract

pH-responsive microgels are cross-linked polymer particles which can swell/deswell in response to pH and salt concentration of their external surroundings. This project focuses on the pH-responsive microgels particularly based on poly(acrylic acid) (PAA) hydrophobically modified with poly(ethers) i.e. poly(propylene glycol) (PPG) and poly(tetrahydrofuran) (PTHF). Interestingly, these poly(ethers) are not widely used to incorporate in PAA based hydrogels yet. We expect that these microgels might be further used as carriers for both hydrophilic and hydrophobic active ingredients with controlled-release response triggered by pH and salt concentration.
With chemical cross-linking, we prepared well-defined PAA micro-spherical hydrogels cross-linked with either PPG- or PTHF-divinyl acrylate end groups via surfactant free emulsion polymerization and acid-hydrolysis. The chemical structure of the microgels was confirmed by FT-IR and NMR. At high pH, electrostatic repulsion between ionized carboxyl groups of PAA contributed to the swelling of the microgels. Moreover, the shape factor (ρ = RG/RH) obtained by DLS and SLS informed that the microgels contain a dense core with a loose shell.
Additionally, physical association between PAA and poly(ethers) in solutions was investigated using various techniques. Whilst 1H-NMR does not appear to be too helpful to detect polymer association, DOSY-NMR already provided indications for the association at low Mw but T2 solvent relaxation approach required higher Mw samples for a clear effect to be visible.
Furthermore, we performed preliminary experiments using a dialysis method and UV-Vis spectroscopy to investigate the release of active ingredients (AIs; benzyl alcohol and paracetamol) from Carbopol® 690, commercial PAA hydrogels, as a function of pH. The results show that the release of AIs was significantly sustained comparing with the control samples. However, the effect of pH (or the swelling of Carbopol®) on the release of AIs is rather unclear.

Date of Award	6 Nov 2018
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Jeroen S Van Duijneveldt (Supervisor) & Julian Eastoe (Supervisor)