Spectral collapse temperature

The spectral parameters for component A again coincide with those of TEMPO in pure water ( a 48.3 MHz), i.e., this spin probe is located in a strongly hydrated, hydrophilic environment. The observed decrease of ai o by 3.7 MHz for species B ( hnal at 65 °C) is indicative of much more hydrophobic and less hydrated surroundings for these spin probes (comparable to chloroform or tert-hutyl alcohol [83]). At temperatures below the collapse temperature Tq, only the hydrophilic spectral component A is observed since all dendritic units are water-swollen. Above the critical temperature of 33 °C an increasing fraction of hydrophobic species B is observed with increasing temperature. The dehydration of the dendritic units thus leads to a local phase separation with the formation of hydrophobic cavities. Unlike in PNIPAAM hydrogels (Sect. 3.1), here hydrophobic regions are not observed below the macroscopic collapse temperature. 

T[i from 13c NMR spectral collapse temperatures, was first noted in ref. 16, based on published NMR data by Axelson and Mandelkem.1 We were puzzled at the timel as to why an NMR technique in the megaherz range would yield static values of T/j, but accepted it as an empirical finding. A recent paper by Axelson, Mandelkem, and coauthors has clarified this question for us even though these authors discounted our association of T// with Xg as being a fortuitous circumstance. 1 We intend to treat the new experimental results in considerable detail elsewhere at a later date. However, a brief commentary with one illustration can clarify this situation for present purposes as regards Fig. 6. 

The same features are observed in the osmylation of arene donors. Thus, osmium tetra-oxide spontaneously forms complexes with arenes, and the systematic spectral shift in the CT bands parallels the decrease in the arene IP [59]. The same osmylated adducts are obtained thermally on leaving mixtures to stand in the dark or upon irradiation of the CT bands at low temperature. Time-resolved spectroscopy establishes that irradiation of the CT band of the anthracene/osmium tetraoxide complex leads directly to the radical-ion pair ANT+, 0s04, which then collapses to the osmium adduct (with a rate constant fc 109 s 1) in competition with back ET [59]. 

Fig. 5 (a) Temperature-dependent CW EPR spectra of TEMPO in an aqueous solution with PNIPAAM-based hydrogels and (b) comparison of the fraction of spin probes in hydrophihc enviromnent a. as found from combining spectral simulations of both species (according to Scombined = a Sa + (1 Wa)>Sb) and a macroscopic (APM-based) observation of hydrogel collapse. (c) Model of network collapse as seen by EPR spin probes individual pockets continuously collapse before the macroscopic collapse happens... 

Xe NMR spectra in some heterogeneous polymer systems consist of two resonances which collapse to a single resonance as a function of temperature. Two different resonances arise from spatially separated, distinct sorption environments and spectral collapse occurs when xenon atoms diffuse from one environment to the other at a sufficiently fast rate. ... 

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