Self-assembly of well-defined fatty acid based amphiphilic thermoresponsive random copolymers

Abstract

A series of amphiphilic random copolymers, consisting of poly(ethylene glycol) methyl ether methacrylate (PEGMA) and 2-(methacryloyloxy)ethyl stearate (SAMA), were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. The compositions of SAMA and PEGMA in the copolymers were determined by proton nuclear magnetic resonance (¹H NMR) spectroscopy, and these were further used to calculate reactivity ratios of the SAMA and PEGMA monomers using the extended Kelen–Tüdös method at high monomer conversions. Crystallinity was observed in the copolymers with high (30% or higher) fatty acid content, studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) measurements and polarized optical microscopic (POM) study. The copolymers showed reversible phase transition in response to temperature cycles in aqueous medium with lower critical solution temperatures (LCST) between 42 and 65 °C, as determined by UV-vis spectrophotometer. The LCST values decreased with the increase in hydrophobic SAMA content in the copolymer. Self-assembly behavior of the copolymers to micellar structures in aqueous solution was investigated by measurement of critical micelle concentration using fluorescence spectroscopy, dynamic light scattering (DLS) and scanning electron microscopy (SEM). The in vitro cytotoxicity study using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction (MTT) assay established no cytotoxic response in the presence of fatty acid based copolymers.