Dielectric relaxation and the conformer equilibrium in the liquid and glassy states of beta-D-fructose

To investigate the ionic and molecular dynamics in the liquid and glassy st ates of beta -D-fructose, its dielectric relaxation spectra (12 Hz-500 kHz) and dynamic heat capacity (3.33 mHz) have been measured from 5 K above its melting point through the vitrification range, by allowing sufficient time for attainment of the conformer (or chemical) equilibria. Effects of the c hange in the conformer population on thermal cycling has been further studi ed. The dielectric behavior of liquid beta -D-fructose is characteristicall y different from that of other molecular liquids in three ways: (i) the con tribution to orientation polarization associated with the fast relaxation p rocess, which persists in the glassy state. is relatively high in the liqui d state of beta -D-fructose: (ii) this contribution decreases with temperat ure exceptionally rapidly on cooling; and (iii) the difference in the rates of the two process is exceptionally large. The dynamic heat capacity chang e through the vitrification region is similar to 160J/(mol K), and is sprea d over similar to 20 K range, and the enthalpy relaxation time is similar t o 50 s at 383 K. Transformation of beta -pyranose to other conformers and o ther conformer transformation equilibria change on thermal cycling with the result that the overall relaxation rate increases at T > 315 K and decreas es at T < 315 K. The relaxation spectrum becomes broader, the de conductivi ty increases and the rate of the Johari-Goldstein relaxation whose Arrheniu s energy is 42.1 kJ/mol increases. (C) 2001 Elsevier Science B.V. All right s reserved.