Shirke Sonali, Takhistov Pavlo, Ludescher Richard D
Department of Food Science, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08901-8520, USA.
J Phys Chem B. 2005 Aug 25;109(33):16119-26. doi: 10.1021/jp0521050.
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.
我们利用赤藓红B（四碘荧光素）在麦芽糖和麦芽糖醇非晶薄膜中的磷光，以0.8:10(4)的染料:糖摩尔比来监测这些基质在-25至110多摄氏度温度范围内的分子流动性。对发射峰频率、带宽（半高宽）和时间分辨强度衰减参数的分析提供了有关基质流动性热激活模式的信息，这些模式增强了三重态周围偶极弛豫速率以及系间窜越到基态的速率（k(TS0)）。在麦芽糖和麦芽糖醇中，可检测到的偶极弛豫在比玻璃化转变温度（Tg）低约50摄氏度的玻璃态开始；然而，弛豫速率虽然比通过介电弛豫测定的β弛豫的文献值慢3 - 4个数量级，但其活化能仅小2倍。在高于Tg的熔体中偶极弛豫进一步增强；熔体中的偶极弛豫速率几乎与通过介电弛豫测定的α弛豫速率精确成比例。强度衰减用拉伸指数衰减函数很好地拟合，其中寿命（τ）和拉伸指数（β）是具有物理意义的参数。在麦芽糖中，k(TS0)的大小在玻璃态基本恒定，在Tg时急剧增加；在麦芽糖醇中，k(TS0)在Tg = -50摄氏度时适度增加，在Tg = +20摄氏度的熔体中增加更为显著。麦芽糖:麦芽糖醇混合物中的k(TS0)值明显小于纯麦芽糖和麦芽糖醇中的值，表明特定相互作用降低了混合糖基质的流动性；这种现象与小分子增塑剂与合成聚合物和淀粉混合物中观察到的反增塑作用相当。在玻璃态下，非均匀展宽和动态非均匀性的程度基本恒定，在玻璃化转变时，麦芽糖中急剧增加，麦芽糖醇中增加较为缓慢。
