荧光光谱跟踪结冷胶水溶液的溶液-凝胶转变

FLUORESCENCE SPECTRUM CHARACTERIZATION FOR SOL-GEL TRANSITION IN GELLAN GUM AQUEOUS SOLUTIONS

将异硫氰酸荧光黄(FITC)标记在结冷胶分子链,用荧光光谱跟踪了结冷胶水溶液凝胶化过程中FITC荧光强度和各向异性比的变化.结果表明在结冷胶的凝胶化转变中,FITC的荧光相对强度和各向异性随温度降低而增大,在某一温度荧光相对强度和各向异性比对温度的曲线出现了明显的转折点,这个转折点的温度低于流变温度扫描曲线中G′=G″的温度.利用荧光的方法确定物理交联体系的关于重均聚合度和凝胶分数的相关临界指数γ和β.γ和β不符合Flory-Stockmayer和逾渗模型的预测.

In order to establish the experimental techniques for monitoring the physical sol-gel transition without disturbing the system mechanically,the steady state fluorescence spectrum was adopted to study the sol-gel transition in aqueous solutions of fluorescence isothiocynate（FITC） labeled gellan gum upon cooling.During the sol-gel transition,the fluorescence emission intensity and the anisotropy were monitored against temperature.Both the fluorescence emission intensity and anisotropy were increased with decreasing temperature,and an obvious inflexion appeared at certain temperature T_i in the curves of fluorescence intensity and anisotropy versus temperature,except for 1.0 wt% of FITC labeled gellan gum solution.As to 1.0 wt% of FITC labeled gellan gum solution,though both fluorescence emission intensity and the anisotropy were increased with decreasing temperature,no inflexion was found in the curves of fluorescence intensity and anisotropy versus temperature,implying that the 1.0 wt% of polymer concentration was too low to form a physical gel form the solution.Rheological temperature ramp method was also used to monitor the sol-gel transition process of the aqueous gellan gum system,the temperature T_c where storage modulus equaled to loss modulus was higher than T_i due to difference in testing objects：T_c reflected a change in macroscopic viscoelastic properties during cooling process while T_i showed a variety in microenvironment.Critical exponents about weight-averaged polymerization degree and gel fracture γ and β were evaluated from the corrected fluorescence intensities,the values of γ and β were inconsistent with the prediction of Flory-Stockmayer model or percolation model.