Electron spin resonance (e.s.r.) spectra of the vanadyl probe VO(H20)~ + in chemically cross-linked polyacrylamide (PAA) gels with pore diameters of 7-58 A were used to analyse the changes in gel structure as a function of temperature and solvent composition. The PAA networks were swollen by water and acetone/water mixtures containing 0.1-0.6 volume fraction of acetone. Spectroscopic evidence has been obtained that indicates the absence of bulk water in all the networks studied. Depending upon experimental conditions, bimodal distributions of pore sizes have been detected for all networks. The appearance of this distribution is triggered by lowering the gel temperature in networks swollen by water; alternatively, the bimodal distribution appears for a homogeneous gel at a constant temperature, by adding acetone to the swelling fluid. Unlike the ionic gels, the transition from a homogeneous PAA gel to a gel having a bimodal distribution of pore sizes is gradual as a function of temperature in water swollen gels, and of the acetone content in gels swollen by the solvent mixtures. The microviscosity in the pores appears to depend on the free volume in the pores; this is evident from the Fujita-Dolittle plots in PAA swollen by water, at three different temperatures.
Посилання на статтю:
Study of polymer networks using paramagnetic probes / Gary C. Rex* and Shulamith Schlick0 // Polymer. – 1992. – Vol 33. – P. 2036-2042.