Infrared spectrum and crystal structure of cellulose

Abstract

Infrared absorption spectra of oriented cellulose fibers have been observed using polarized radiation. Natural fibers are in general too thick but swelling them in NaOH solution followed by pressing, washing, and drying is an effective way of obtaining a suitable thickness in the 650–3100 cm. −1 region. The dichroism observed for the bands which can be assigned has been interpreted on the basis of the crystal structure of cellulose. The CH stretching band is polarized perpendicular to the fiber axis, in accord with the previously proposed model of the crystal structure (Meyer and Misch). Because both the CH 2 symmetric and antisymmetric stretching bands are polarized parallel, a modification of the previously proposed model is necessary. In the region of CH and OH deformation frequencies (1200–1500 cm. −1 ) nine bands are observed, of which four are polarized parallel and five, perpendicular, to the fiber axis. Deuteration effect of amorphous cellulose on the spectrum in this region was examined. Probable assignments are given for these bands. In the region of CO and CC stretching frequencies (800–1200 cm. −1 ) all the bands are polarized parallel to the fiber axis. Observations were also made on deuterated cellulose, i.e. , cellulose which had been swelled in NaOD. Of the six OD stretching bands observed, five show parallel dichroism, indicating that the OH bands in cellulose are arranged predominantly parallel to the fiber axis. The OH out-of-plane deformation band at 650 cm. −1 is polarized perpendicular to the fiber axis.