Synthesis and the effect of hydrophobic dodecyl end groups on pH-responsive micellization of poly(acrylic acid) and poly(ethylene glycol) triblock copolymer in aqueous solution

Abstract

A pH-responsive triblock copolymer of poly(acrylic acid)-b-poly(ethylene glycol)-b-poly(acrylic acid) containing hydrophobic dodecyl end groups (C12H25-PAA-b-PEG-b-PAA-C12H25) with narrow molecular weight distribution (M w/M n = 1.30) was synthesized via reversible addition-fragmentation chain transfer polymerization of acrylic acid (AA). Poly(ethylene glycol) (PEG) capped with S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate (DDATC) end groups was used as the macro chain transfer agent (PEG macro-CTA) and 2,2′-azobisisobutyronitrile (AIBN) as initiator for monomer acrylic acid. The effect of the hydrophobic dodecyl end groups on pH-sensitive self-association of C12H25-PAA-b-PEG-b-PAA-C12H25 in aqueous solution was investigated by fluorescence spectroscopy, dynamic light scattering and atomic force microscope. At pH ≥5.5, the solution behavior of C12H25-PAA-b-PEG-b-PAA-C12H25 is like polyelectrolyte in aqueous solution, and the effect of dodecyl end groups is negligible. At pH <5.0, the hydrophobic dodecyl end groups contribute dominantly to the pH-sensitive micellization and result in the formation of micelles with stronger hydrophobicity and larger size at low concentration (critical micelle concentration is 0.062 g/L). In the range of pH 2.5–3.5, the steady (R h ≈ 35.0 nm) and narrow size distributed micelles (polydispersity index, PDI < 0.2) can be obtained. The micelles formed by C12H25-PAA-b-PEG-b-PAA-C12H25 triblock copolymer in acidic solution are expected to have a core–shell–corona structure, where the hydrophobic dodecyl groups form the core, and weak hydrophobic PAA/PEG hydrogen-bonded complexes form the shell and the uncomplexed PAA, and PEG chain segments form the corona.