Molecular mobility in amorphous maltose and maltitol from phosphorescence of erythrosin B.
We have used phosphorescence from erythrosin B (tetraiodofluorescein) dispersed in amorphous thin films of maltose and maltitol at mole ratios of 0.8:10(4) dye:sugar to monitor the molecular mobility of these matrixes over the temperature range from -25 to over 110 degrees C. Analysis of the emission peak frequency and bandwidth (full width at half-maximum) and time-resolved intensity decay parameters provided information about thermally activated modes of matrix mobility that enhanced the rate of dipolar relaxation around the triplet state and the rate of intersystem crossing to the ground state (k(TS0)). Detectable dipolar relaxation began in the glassy state about 50 degrees C below T(g) in both maltose and maltitol; the relaxation rate, however, while 3-4 orders of magnitude slower than literature values for the beta relaxation determined from dielectric relaxation, had an activation energy only 2-fold smaller. Dipolar relaxation was further enhanced in the melt above T(g); the dipolar relaxation rates in the melt scaled nearly exactly with rates for the alpha relaxation determined from dielectric relaxation. Intensity decays were well fit using a stretched exponential decay function in which the lifetime (tau) and the stretching exponent (beta) were the physically significant parameters. In maltose, the magnitude of k(TS0) was essentially constant in the glass and increased dramatically at the T(g); in maltitol k(TS0) increased moderately at T(g) = -50 degrees C and more dramatically in the melt at T(g) = +20 degrees C. The value of k(TS0) in maltose:maltitol mixtures was significantly smaller than that seen in pure maltose and maltitol, suggesting that specific interactions decreased the mobility of the mixed sugar matrix; this phenomenon was comparable to the antiplasticization seen in mixtures of small molecule plasticizers with synthetic polymers and starch. The extent of inhomogeneous broadening and dynamic heterogeneity were essentially constant in the glass and increased dramatically in maltose and more gradually in maltitol at the glass transition.
Shirke
Sonali
S
Department of Food Science, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08901-8520, USA.
Takhistov
Pavlo
P
Ludescher
Richard D
RD