Thermal response in crystalline Iβ cellulose:: A molecular dynamics study
Thermal response in crystalline Iβ cellulose:: A molecular dynamics study
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DOI：
10.1021/jp072258i
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发表时间：
2007-08-02
影响因子：
3.3
通讯作者：
Mazeau, Karim
中科院分区：
文献类型：
--
作者：
Bergenstrahle, Malin;Berglund, Lars A.;Mazeau, Karim
采用GROMOS 45a4力场，通过分子动力学模拟研究了温度对纤维素I β晶体结构和性能的影响。在300 K时，模拟晶体与几组实验数据合理一致，包括晶体密度、相应的堆积和晶体晶胞尺寸、链构象参数、氢键、杨氏模量和室温下的热膨胀系数。在高温（500 K）下，纤维素链保持在片材中，尽管与室温结构相比在细节上存在差异。密度降低，而a和B细胞参数分别扩大7.4%和6%，c参数（链轴）略微收缩0.5%。细胞角α和β分为两个群体。羟甲基基团主要采用gt取向，从而改变了氢键模式。一个链内氢键O2'H2'中心点中心点O6消失，因此杨氏模量降低了25%。已经提出了低温和高温结构之间的过渡途径，其初始步骤是增加的层间分离，这允许每隔一个纤维素链围绕其螺旋轴旋转约30度。其次，所有羟甲基基团都改变了它们的取向，从tg到gg（旋转链）以及从tg到gt（非旋转链）。当温度进一步升高时，旋转的链返回到它们原来的取向和它们的羟甲基基团再次改变它们的构象，从gg到gt。建议约450 K的转变温度，然而，过渡似乎是渐进的比突然的。温度引起的晶胞尺寸和氢键模式的变化的模拟数据也与实验结果进行了比较。
The influence of temperature on structure and properties of the cellulose I beta crystal was studied by molecular dynamics simulations with the GROMOS 45a4 force-field. At 300 K, the modeled crystal agreed reasonably with several sets of experimental data, including crystal density, corresponding packing and crystal unit cell dimensions, chain conformation parameters, hydrogen bonds, Young's modulus, and thermal expansion coefficient at room temperature. At high-temperature (500 K), the cellulose chains remained in sheets, despite differences in the fine details compared to the room-temperature structure. The density decreased while the a and b cell parameters expanded by 7.4% and 6%, respectively, and the c parameter (chain axis) slightly contracted by 0.5%. Cell angles alpha and beta divided into two populations. The hydroxymethyl groups mainly adopted the gt orientation, and the hydrogen-bonding pattern thereby changed. One intrachain hydrogen bond, O2'H2'center dot center dot center dot O6, disappeared and consequently the Young's modulus decreased by 25%. A transition pathway between the low- and high-temperature structures has been proposed, with an initial step being an increased intersheet separation, which allowed every second cellulose chain to rotate around its helix axis by about 30 degrees. Second, all hydroxymethyl groups changed their orientations, from tg to gg (rotated chains) and from tg to gt (non-rotated chains). When temperature was further increased, the rotated chains returned to their original orientation and their hydroxymethyl groups again changed their conformation, from gg to gt. A transition temperature of about 450 K was suggested; however, the transition seems to be more gradual than sudden. The simulated data on temperature-induced changes in crystal unit cell dimensions and the hydrogen-bonding pattern also compared well with experimental results.