The crystallinity of acid modified fibers prepared from viscose rayon and cotton cellulose is determined by x-ray diffraction and iodine adsorption methods. The results are analysed and interpreted in the light of the definition of each method. In addition, the role of low fractions as well as the effect of vacuum drying on the measured crystallinity are discussed. The literature shows a lack of consistency between crystallinity determined for cellulosic fibers using different methods. The present work is an attempt to analyse the crystallinities of hydrocellulose residues determined by the x-ray diffraction and the iodine adsorption methods.
Results and DiscussionThe crystallinity percentage of cotton cellulose, viscose rayon, and their hydrolysis residues determined from x-ray (Cr,) as well as from iodine adsorption (Cr12) are shown in Tab. 1. The calculated crystallinity change percentages dCr, and dCrI, are plotted as a function of the hydrolysed fibers in Fig. 1.These results show that: (i) both ACr, and dCrI, increase rapidly in the initial stages of hydrolysis and soon attain a constant value with both fibers studied, (ii) in case of viscose rayon, the change of ACr,, and ACr, in the earlier stages of hydrolysis, the constant value attained, and the stage at which this value is reached, depend on the hydrolytic conditions, (iii) almost no effect of acid concentration on the change of crystallinity is observed with cotton cellulose, and (iv) with all samples studied dCrI, is greater than-ACr,. Different attempts have been made to differentiate between crystalline and non-crystalline components of cellulosic fiben-"), as well as to determine their quantitative ratio. The results obtained indicate that there is no sharp defined borderline between the crystalline and the so-called amorphous portions of cellulosic fibers. A system like cellulose probably contains every gradation between the state of perfect three dimensional order which is termed crystalline and the other extreme of randomly oriented chains with no order. Accordingly, when the glycosidic linkages of cellulose molecules are subjected to acid hydrolysis, the rate of action of acid solution on cellulose is not the same in the different parts of the fibers and, as a result, a complex mixture of particles of various lateral order of arrangement is f~r m e d '~-~' ' . Thus, it is expected that the use of different methods to determine the crystallinity of the complex hydrocellulose residues would give different results.In order to analyse the results shown in Fig. 1 and shed more light on the fine structure of the hydrocelluloses examined, it must be considered that the x-ray method of assessing the fine structure depends upon the quantity and the size of the larger units of submicroscopic
