X-ray diffraction study on the thermal expansion behavior of cellulose Iβ and its high-temperature phase

Oriented films of cellulose prepared from algal cellulose were hydrothermally treated to convert them into highly crystalline cellulose I beta. The lateral thermal expansion behavior of the prepared cellulose I beta films was investigated using X-ray diffraction at temperatures from 20 to 300 degrees C. Cellulose I beta was transformed into the high-temperature phase when the temperature was above 230 degrees C, allowing the lateral thermal expansion coefficient of cellulose I beta and its high-temperature phase to be measured. For cellulose I beta, the thermal expansion coefficients (TECs) of the a- and b-axes were alpha(a) = 9.8 x 10(-5) degrees C(-1) and alpha(b) = 1.2 x 10(-5) degrees C(-1), respectively. This anisotropic thermal expansion behavior in the lateral direction is ascribed to the crystal structure and to the hydrogen-bonding system of cellulose I beta. For the high-temperature phase, the anisotropy was more conspicuous, and the TECs of the a- and b-axes were alpha(a) = 19.8 x 10(-5) degrees C(-1) and at = -1.6 x 10(-5) C(-1), respectively. Synchrotron X-ray fiber diffraction diagrams of the high-temperature phase were also recorded at 250 degrees C. The cellulose high-temperature phase is composed of a two-chain monoclinic unit cell, a = 0.819 nm, b = 0.818 nm, c (fiber repeat) = 1.037 nm, and gamma = 96.4, with space group = P2(1). The volume of this cell is 4.6% larger than that of cellulose I beta at 30 degrees C. (C) 2010 Elsevier Ltd. All rights reserved.