Khatiwada Bal K, Blum Frank D
Department of Chemistry , Oklahoma State University , Stillwater , Oklahoma 74078 , United States.
Langmuir. 2019 Sep 3;35(35):11482-11490. doi: 10.1021/acs.langmuir.9b01847. Epub 2019 Aug 23.
The heat capacities of very small adsorbed amounts of poly(methyl methacrylate) on high-surface-area silica (Cab-O-Sil) were measured using temperature-modulated differential scanning calorimetry (TMDSC) using a quasi-isothermal method and interpreted via different models. The composition-dependent heat capacities of the adsorbed samples were measurably less than those predicted with a simple mixture model. A two-state model, composed of tightly and loosely bound polymer, fits the data better with heat capacities of the tightly bound polymer found to be 70-80% (glassy region) and 70-94% (rubbery region) of that of the bulk polymer at the same temperatures. The amount of tightly bound polymer was estimated to be about 1.2 mg/m (about 1 nm thickness) in both the glassy and rubbery regions, consistent with heat flow measurements. The data sets were also extensive enough to model them with a more detailed layered gradient model, including a nonzero heat capacity for the polymer at zero adsorbed amount, which increased based on an exponential growth function to bulk polymer value of the heat capacity away from the surface. More importantly, this gradient model mimicked the experimental dependence on adsorbed amounts in the tightly bound adsorbed amount region (approximately 1 mg/m). This model provided, for the first time, an experimental estimate of the heat capacity of the polymer adsorbed closest to the surface. The fractional heat capacity of the adsorbed polymer closest to the silica surface, relative to bulk polymer, increased with temperature from 0.3 (well below) to 0.8 (well above the bulk ). It was also possible to estimate the exponential growth parameter of the development from the initial heat capacities to the bulk heat capacity as 0.4 to 0.6 mg/m, identifying a distance scale (0.3 to 0.5 nm) consistent with the notion of a transition from tightly bound to loosely bound polymer.
使用温度调制差示扫描量热法（TMDSC）的准等温方法测量了高比表面积二氧化硅（Cab-O-Sil）上吸附量极少的聚甲基丙烯酸甲酯的热容，并通过不同模型进行了解释。吸附样品的组成依赖性热容明显小于简单混合模型预测的值。由紧密结合和松散结合的聚合物组成的双态模型能更好地拟合数据，发现在相同温度下，紧密结合聚合物的热容为本体聚合物的70 - 80%（玻璃态区域）和70 - 94%（橡胶态区域）。在玻璃态和橡胶态区域，紧密结合聚合物的量估计约为1.2 mg/m²（约1 nm厚度），这与热流测量结果一致。数据集也足够广泛，可以用更详细的分层梯度模型进行建模，包括零吸附量时聚合物的非零热容，该热容基于指数增长函数增加到远离表面的本体聚合物热容值。更重要的是，这个梯度模型模拟了紧密结合吸附量区域（约1 mg/m²）中吸附量的实验依赖性。该模型首次提供了最接近表面吸附的聚合物热容实验估计值。最接近二氧化硅表面的吸附聚合物的分数热容相对于本体聚合物，随温度从0.3（远低于）增加到0.8（远高于本体）。还可以估计从初始热容到本体热容发展的指数增长参数为0.4至0.6 mg/m²，确定了与从紧密结合聚合物到松散结合聚合物转变概念一致的距离尺度（0.3至0.5 nm）。
