School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States.
Biomacromolecules. 2013 Aug 12;14(8):2900-8. doi: 10.1021/bm400794e. Epub 2013 Jul 26.
The coefficient of thermal expansion (CTE) of cellulose nanocrystal (CNC) films was characterized using novel experimental techniques complemented by molecular simulations. The characteristic birefringence exhibited by CNC films was utilized to calculate the in-plane CTE of self-organized and shear-oriented self-standing CNC films from room temperature to 100 °C using polarized light image correlation. CNC alignment was estimated via Hermans order parameter (S) from 2D X-ray diffraction measurements. We found that films with no preferential CNC orientation through the thickness (S: ∼ 0.0) exhibited an isotropic CTE (∼25 ppm/K). In contrast, films with aligned CNC orientations (S: ∼0.4 to 0.8) had an anisotropic CTE response: For the highest CNC alignment (S: 0.8), the CTE parallel to CNC alignment was ∼9 ppm/K, while that perpendicular to CNC alignment was ∼158 ppm/K. CNC film thermal expansion was proposed to be due primarily to single crystal expansion and CNC-CNC interfacial motion. The relative contributions of inter- and intracrystal responses to heating were explored using molecular dynamics simulations.
采用新型实验技术并结合分子模拟，对纤维素纳米晶体（CNC）薄膜的热膨胀系数（CTE）进行了表征。利用 CNC 薄膜表现出的特征双折射，通过偏光图像相关法从室温到 100°C 计算了自组装和剪切取向的自支撑 CNC 薄膜的面内热膨胀系数。通过二维 X 射线衍射测量的 Hermans 取向参数（S）来估计 CNC 的取向。我们发现，在厚度方向上没有优先 CNC 取向的薄膜（S：∼0.0）表现出各向同性 CTE（∼25 ppm/K）。相比之下，具有取向 CNC 取向的薄膜（S：0.4 至 0.8）具有各向异性的 CTE 响应：对于最高的 CNC 取向（S：0.8），平行于 CNC 取向的 CTE 约为 9 ppm/K，而垂直于 CNC 取向的 CTE 约为 158 ppm/K。CNC 薄膜的热膨胀被认为主要是由于单晶膨胀和 CNC-CNC 界面运动引起的。通过分子动力学模拟探讨了晶体间和晶体内响应对加热的相对贡献。
