EPR Spectroscopic Characterization of Local Nanoscopic Heterogeneities during the Thermal Collapse of Thermoresponsive Dendronized Polymers

Abstract

The collapse transition of thermoresponsive dendronized polymers was characterized on a molecular scale by CW EPR spectroscopy. Aggregation of the polymer is triggered by dynamic structural inhomogeneities of a few nanometers, and the dehydration of the polymer chains proceeds, despite the sharp phase transition, over a temperature interval of at least 30 °C (see picture).

EPR Spectroscopy Provides a Molecular View on Thermoresponsive Dendronized Polymers Below the Critical Temperature

“…Two representative CW EPR spectra of TEMPO in an aqueous solution of 10 wt.-% PG1(ET) well above and below the critical temperature (T C ) of 328C [26] are displayed in Figure 1a. The low temperature spectrum shows common features of a nitroxide in fast rotational motion.…”

“…[16,18,19,26] TEMPO (98%, Aldrich) was used as received. Distilled water was further purified by a MilliQ System (Millipore) to achieve a resistivity of 18.2 MV cm.…”

“…As described previously, the polymer solutions were ramp heated (>30 K min À1 ) to a maximum temperature of 65 8C, which was then decreased stepwise for the actual measurements. [26] Data Analysis and Interpretation CW EPR spectral simulations were performed with a Matlab program utilizing the fast-motion routine of the Easyspin software package for EPR. [27] While at and above T C , two spectral contributions in an aqueous hydrophilic environment (denoted E1) and in a more hydrophobic environment (denoted E2) could be discerned, the EPR spectra below T C were fitted to simulations assuming a single spin probe species (denoted E) with an isotropic hyperfine coupling constant a E and an isotropic g-value parameter g E .…”

“…[23] In a recent study, we could show that the dehydration of the dendronized polymers proceeds via the formation of structural heterogeneities on the nanometer length scale, which trigger the aggregation of different polymer chains at the critical temperature. [26] While macroscopic turbidity measurements suggested a very sharp phase transition of the dendronized polymers (within 1 K), [16,17] the dehydration of the formed mesoglobule is more accurately described as a non-equilibrium process that is governed by two distinct de-swelling processes proceeding over a temperature interval of at least 30 K.…”

“…Although the aggregation temperature is mainly determined and the aggregation in general triggered by the hydrophobicity of the peripheral dendritic units, the dehydration efficiency above T C was found to be governed by the hydrophobicity of the dendritic cores. [26] In this paper, we evaluate if a similar relationship can be observed for single dendronized polymers below T C .…”