Nanoscale inhomogeneities in thermoresponsive triblock copolymers

We present continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy data of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in aqueous solutions of poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronic or Poloxamer). TEMPO is notably smaller than the spin probes conventionally used in the context of polymer science and reveals the early emergence of small hydrophobic cavities when PPO strands of several molecules aggregate and collapse upon temperature increase. The occurrence of hydrophobic cavities appears independent of the overall molecular weight of the Pluronics, but clearly depends on the relative PPO/PEO contents. In all the cases studied, the volume fraction of hydrophobic cavities increases in a broad temperature range of ≥40 °C. The appearance of the hydrophobic regions does not seem to be directly correlated to micellization of the polymers. A decrease of the relative PPO amount in the polymers not only hinders collapse of the PPO strands, it can also be described as a site exchange of the spin probes between hydrophobic cavities and the surrounding medium. On the other hand, in cases of high PPO contents, spin probe exchange could not be observed. This suggests that one may potentially control the diffusion of small molecules between the micellar cores and the surrounding medium by adjusting the PEO/PPO ratio of the used Pluronics.