Chemical Structure-Dependence of the Thermal Transition in Thermoresponsive Ethylene-Glycol-Containing Polymers as Characterized by EPR Spectroscopy

Variations in main and side chain lengths, along with the use of end groups of differing natures, are often identified as key parameters influencing phase transition temperatures (cloud point temperatures) in thermoresponsive polymers. Here, the phase transition behavior of thermoresponsive poly(oligo(ethylene glycol)acrylates) is systematically examined with respect to these three factors. Spin-probing electron paramagnetic resonance (EPR) spectroscopy is employed to investigate the impact of these parameters below and at the macroscopically observable thermal transition, as well as to characterize the nanoscale inhomogeneities associated with the transition. Optical transmission measurements are conducted to provide a comprehensive understanding of both, macroscopic and nanoscopic, events. Conducting EPR enables the early detection of the onset of the transition temperature range, well before optical detection. This combined approach enables us to establish the relative significance of these factors in determining the transition temperatures and the collapse processes. With this approach, this study finds that the (de)hydration behavior of the polymer chains is mainly determined by the length of ethylene glycol side chains but the individual single polymer end groups also have remarkable influence on the collapse behavior. The main chain length is identified as, relatively viewed, having the smallest effect on the thermal collapse on the molecular level.