The bands due to stretching of OH groups of all four crystalline modifications of cellulose have been measured using a deuteration technique which enables crystalline regions to be studied independently of amorphous regions. Cellulose III prepared from cellulose I (IIII) was found to be different from cellulose III prepared from cellulose II (IIIII). Similarly different cellulose IV's were obtained from celluloses I and II (IVI and IVII). However, the infrared spectra suggest that only one of these, namely, IIII should be regarded as having a crystal structure distinctively different from those of cellulose I and II. Thus IVI and IIIII closely resemble celluloses I and II, respectively, while IVII shows a diffuse absorption and may well possess a disordered lattice. The bearing of these results on the crystal structures of celluloses I and II is discussed.
The unit cells of celluloses IIIII and II are different, although their infrared spectra are very similar. These facts suggest that hydrogen bonding in these two forms is of the layer type in the (101) plane, though the possibility of hydrogen bonding in the (101) plane cannot be entirely eliminated.
A method has been developed which enables the crystallinity of a cellulose, defined as the fraction of OH groups which are hydrogen-bonded in a regular crystalline manner, to be estimated by infra-red spectroscopy. The relative crystallinities determined by this method agree reasonably well with relative crystalhities determined by Hermans' X-ray method. The absolute crystallinities are, however, about one-third lower than those found by Hermans. Measurements on Fortisan and cotton micelIes prepared by acid hydrolysis show that these micelles contain appreciable amounts of amorphous OH groups. Estimates of the fraction of crystalline regions which can be deuterated with liquid D20 have been made by infra-red measurements.
The bands due to stretching of OH groups in celluloses I, II and IIII have been studied with plane polarized infrared radiation. The spectra show that all three crystalline modifications contain OH groups which make an angle much smaller than 55° with the chain direction, while other OH groups make angles greater than 55° with this direction. The former OH groups are attributed to intramolecular hydrogen bonds which join successive glucose residues in the cellulose chain. It is shown that ether oxygen atoms must participate in the hydrogen‐bonding systems in all three modifications. The symmetries of the cellulose chains and the relationship between the two chains in the unit cell are discussed in relation to the number of distinguishable OH groups. Structures are described which are in agreement with the infrared results and with the stereochemical considerations presented in Part III of this series. The frequencies of the bands due to intramolecular hydrogen bonds suggest that the chains in cellulose I have a different molecular form from those in celluloses II and IIII. This difference may not be very large, however, in relation to the accuracy with which polymer structures are normally determined.
A method has been developed for determining the accessibility of cellulose to liquid heavy water; the extent of the exchange reaction being measured by determining the change in the refractive index of the heavy water. It is shown that previous work using a slightly different method involved either large experimental errors or the use of unjustified assumptions. In this work an attempt has been made to avoid both these sources of error. A method has also been developed for determining the absolute crystallinity of a cellulose sample. In this method, cellulose is deuterated by D20 vapour until all the amorphous regions have been deuterated as shown by infra-red spectroscopy. The deuterium that has exchanged with the cellulose is then extracted with liquid H20 and the amount determined by measuring the refractive index of the mixture. This technique has advantages in that limitations are not imposed by the thickness or the orientation of samples as in spectroscopic work.
The interaction between cellulose and heavy water allows absorptions due to stretching of OH groups in crystalline and amorphous regions to be studied independently. Crystalline regions give several absorption bands in the 3600–3000 cm.−1 range and it is shown that all the bands are due to stretching of OH groups. Arguments are presented that support the idea that these bands can be interpreted in terms of vibrations of individual groups. It is shown that all the OH groups in crystalline regions of regenerated and bacterial celluloses are hydrogen‐bonded, and that crystal structures suggested by Peirce for these celluloses are incorrect. Suggestions are made about the types of hydrogen bond present in regenerated cellulose.
The relationship between infra‐red absorption frequencies and O……O distances in crystals is discussed. It is concluded that no reliable estimate of O……O distances in cellulose can be made from absorption frequencies on the basis of results available at present.
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