Thermo- and UV-responsive amphiphilic nanogels via reversible [4+4] photocycloaddition of PEG/PCL-based polyurethane dispersions

Abstract

Amphiphilic nanogels are emerging as promising materials for several applications due to their high loading capacity of hydrophobic substances. In this context, a thermoresponsive anthracene end-capped hyperbranched polyurethane (HPU-AM) based on poly(ethylene glycol), polycaprolactone-triol, and isophorone diisocyanate, was used as a platform for nanogel preparation. Both polymer synthesis and functionalization with anthracene were one-pot and controlled. HPU-AM self-assembled in aqueous solutions to form micelles, while heating above the solution cloud point (Tcp) led to phase separation and globule formation, as verified by 1H nuclear magnetic resonance and dynamic light scattering experiments. Nanogel dispersions were prepared by UV-irradiation of the globule dispersions. The UV-light (λ = 365 nm) triggers the [4+4] cycloaddition of the anthracene chain-ends, yielding chemically cross-linked nanogels. Their hydrodynamic radius was tuned from 28 to 50 nm by varying the quench temperature (T > Tcp) and HPU-AM concentration. Evidence of nanogel softness, spherical morphology, narrow size distributions, and phase-segregated inner structure was provided by microscopy, light scattering, and Nile-red loading experiments. The hydrodynamic volume of the nanogels was reversibly reduced up to 85% by heating from 15 to 45 °C. Finally, irradiation of the nanogel dispersions at λ = 280 nm cleaved the anthracene dimer, partially reverting the crosslinking, and promoting nanogel disaggregation into micelles of hydrodynamic radius of 7 nm.