再生纤维素太赫兹光谱的实验与理论研究

采用低温溶解法制备了再生纤维素膜, 运用扫描电子显微镜、傅里叶红外光谱仪和X射线衍射仪对所制薄膜进行形貌和晶型表征。在此基础上, 结合太赫兹时域光谱和傅里叶变换红外光谱技术, 测量了再生纤维素膜的太赫兹光谱。据此, 详细指认了再生纤维素在太赫兹波段的特征峰, 指出再生纤维素的太赫兹透过率随波数的减少而增大的现象是由其无定形部分所导致。通过比较再生纤维素和脱脂棉在100～700 cm-1的光谱特性, 发现两者具有相似的峰形，但再生纤维素在此波段的特征峰相对于脱脂棉特征峰发生了不同程度的蓝移。据此, 提出了鉴别纤维素Ⅱ和纤维素Ⅰ_β同质异晶体的一种新方法。重要的是, 还采用CASTEP对纤维素Ⅱ晶体进行结构优化及光谱的理论研究, 并对再生纤维素的太赫兹特征峰进行了系统的归属。理论计算结果表明，再生纤维素在42和54 cm-1处的太赫兹特征峰分别是由平移振动和转动的晶格振动模式引起, 而位于68～238 cm-1间的太赫兹特征峰则与—CH₂OH基团的扭绞振动以及C—H及O—H的变形振动相关。此外, 351～583 cm-1范围内的吸收峰与C—O—C及吡喃环的骨架振动相关, 而611和670 cm-1两处的吸收峰则主要源于O—H的面外弯曲振动。结果不仅揭示了再生纤维素的物质结构与太赫兹光谱间的联系, 也为理论上研究部分结晶的聚合物及生物有机大分子等的太赫兹响应的物理原理提供了参考。

Experimental and Theoretical Study on Terahertz Spectra for Regenerated Cellulose

In t his work,regenerated cellulose films were prepared with an iced dissolution method,while the physical morphologies and crystal types of the products were systematically characterized with scanning electron microscope (SEM),Fourier transform in frared(FTIR),while X-Ray Diffraction (XRD) .The results demonstrate that the as-prepared continuous and uniform films are indeed cellulose Ⅱ,whose morphology and crystal type are significantly different from those of the degreased cotton .Moreover,Terahertz time domain system (THz-TDS) and FTIR were employed to measure the THz spectra of the regenerated cellulose films .Accordingly,the THz characteristic peaks for the regenerated cellulose films are experimentally identified for the first time .In addition,the increase of the THz transmittance with the decrease of the wavenumber is attributed to the existence of amorphous components in the regenerated cellulose films .Although the shapes of Far-IR spectra in the range of 100～700 cm-1 are similar,the absorption peaks of the regenerated cellulose films move to lower wavenumbers (blue shift) compared with those of the degreased cotton .Based on this,we developed a new approach to distinguish the allomorphism of cellulose Ⅱ and cellulose Iβ by Far-IR .Particularly,geometry optimization and IR calculation for the crystal structure of cellulose Ⅱ have been successfully processed by Density Functional Theory (DFT) using periodic boundary condition via CASTEP package .The calculated absorption peak positions are in good agreement with those experimentally measured .Consequently,the THz characteristic peak s of the regenerated cellulose films have been systematically and successfully assigned .Theoretical calculations reveal that the peaks at 42 and 54 cm-1 are assigned to the lattice vibration modes coupled with translational mode and rotational mode,respectively .Moreover,the absorption peaks in the range of 68～238 cm-1 are related with the torsion vibration of -CH2OH group and deformation vibration of C-H bond and O-H bond,while those in the range of 351～583 cm-1 are assigned to the skeletal vibration of C-O-C bond and pyranoid ring,and those at 611 and 670 cm1 are originated from the out-of-plane bending vibration of O-H bond .Each absorption peak is involved in more than single vibration mode .The THz spectra presented in this work,together with the theoretical simulations,indicate that the THz responses of regenerated cellulose are closely associated with both it s chemical constituents and molecular structure .These results will be helpful not only for better understanding the relations between the molecular structure o f the regenerated cellulose and its THz spectrum,but also for providing valuable information for future studies on the physical mechanisms of THz responses of other partially-crystalline polymers and organic biological macromolecules .