退火条件对山梨醇玻璃焓松弛参数的影响

Effect of Annealing Conditions on Enthalpy Relaxation Parameters of D-Sorbitol Glass

采用差示扫描量热法(DSC)测定山梨醇样品经历不同时间(ta)等温退火后,以10K·min-1速率进行升温时玻璃化转变温度(Tg)前后的比热容(Cp(T)).将Gómez Ribelles(GR)提出的一种基于构型熵的现象学模型用于描述山梨醇玻璃的焓松弛行为,考察GR模型能否适用于小分子玻璃体系.结果表明,单组GR模型参数拟合的曲线均能较好重现对应热历史条件下的山梨醇体系的实验所得Cp(T)曲线,尽管并未找到不随热历史而变的一组参数作为材料常数,但与其它现象学模型应用于小分子玻璃时,其模型参数都随热历史变化而变化的特点相比,GR模型的某些参数基本保持不变.且在较长退火时间下拟合得到的模型参数普适性较好.同经历连续降温的山梨醇相比,等温退火过程得到的松弛极限态参数(δ)的平均值与Tg处比热容增量(ΔCp(Tg))的比值明显增大,但仍小于聚合物的值,表明GR模型提出的亚稳极限态对小分子玻璃的影响值得商榷.

We studied the enthalpy relaxation dynamics of amorphous D-sorbitol by differential scanning calorimetry （DSC）. A series of specific heat capacities （Cp（T）） for D-sorbitol were measured upon heating of 10 K·min-1. The samples were subjected to thermal treatment involving isothermal annealing with different annealing time （ta）. A new phenomenological model by Gómez Ribelles （the GR model） of enthalpy relaxation based on the evolution of configurational entropy was used to simulate the experimental data to verify the applicability of the GR model to small molecular glass. The results indicated that a single set of GR model parameters only reproduced the corresponding experimental Cp（T） curve of D-sorbitol fairly well. It failed to find a set of GR model parameters as the material constant independent of the thermal history. In contrast with other phenomenological models, some parameters of the GR model remained unchanged. The model parameter sets obtained under longer annealing time show better predictive ability. In contrast to the continuous cooling process for D-sorbitol, the ratio between the mean value of the limit state parameter （δ） and the increment of specific heat capacity at Tg （△Cp（Tg）） apparently increases, but the increase is less than that of polymers. The results brought into question if the metastable limit state introduced by the GR model applied to small molecular glass systems.