Inhomogeneities in PNIPAM Aqueous Solutions: The Inside View by Spin Probe EPR Spectroscopy

Coil to globule transition in poly(N-isopropylacrylamide) aqueous solutions was studied using spin probe continuous-wave electronic paramagnetic resonance (CW EPR) spectroscopy with an amphiphilic TEMPO radical as a guest molecule. Using Cu(II) ions as the quencher for fast-moving radicals in the liquid phase allowed obtaining the individual spectra of TEMPO radicals in polymer globule and observing inhomogeneities in solutions before globule collapsing. EPR spectra simulations confirm the formation of molten globules at the first step with further collapsing and water molecules coming out of the globule, making it denser.

Automated summary

The coil to globule transition in poly(N-isopropylacrylamide) aqueous solutions was investigated using CW EPR spectroscopy with TEMPO radical as a guest molecule. By using Cu(II) ions as quencher, individual spectra of TEMPO radicals in polymer globule were obtained, showing inhomogeneities in solutions before globule collapsing. EPR spectra simulations confirmed the formation of molten globules initially, followed by further collapsing and densification of the globules as water molecules were released.