T H E STRUCTURE OF NITRATED CELLULOSE. I1 The X-Ray Examination of Kitroramie BY F R A N K DOCGLAS MILES A N D JAMES CRAIK
The first attempt to examine a range of nitrocelluloses of varying degrees of nitration was made by Herzog and Karay-Saabo.' The authors nitrated ramie fibre in various ways, preparing some samples by a drastic denitration of others, and concluded that any nitrocellulose containingfrom4.41to 13.31% of nitrogen was a mixture of cellulose trinitrate and unchanged cellulose since the X-ray diagrams showed mainly the diffractions of the latter substance, two diffraction spots due to unchanged cellulose occasionally persisting. They also formulated an elementary cell for the trinitrate which had, in two dimensions, the same dimensions as the cellulose cell usually accepted, but differed as to the third dimension. By these means, and the supposition that the nitrate groups were interculated in planes parallel to the face of the cell, they accounted for their observation that many of the cellulose spots did not change their position on nitration. A diagram given by these authors, obtained from a nitrated ramie containing 12.997~of nitrogen, was stated to be that of the trinitrate. Many exposures of nitrocellulose of various types which were made by us failed to yield a point-picture a t all resembling this but gave instead the characteristic picture of fibrous cellulose trinitrate, first obtained in a definite form by Andrem2 I n a later paper by von Naray Saabo and von Susich3 the diagram of the trinitrate identical with that due to Andress was reproduced and analysed. The layer lines are closer together than in cellulose and the periodicity in the direction of the fibre axis was given as 26.6 A. U. The composite nature of nitrocelluloses other than the trinitrate was still maintained and a diagram taken from a nitroramie of I 1 . 0 8 7 ~of nitrogen held to exhibit the diffraction of the trinitrate and of unchanged cellulose. Meyer and Mark (loc. cit., 603) in their discussion of the structure of cellulose and its derivatives refer to nitration and accept, in the main, the trinitrate-cellulose theory. They believe that the inhomogeneity in less nitrated material which causes the diffractions to be diffuse may be of two kinds. I n the first, microscopically visible positions of the fibre may be nitrated to different degrees; in the second, the nitration may be uneven inside the constituent particle or micelle. There can be little doubt that the accepted diagram for fully nitrated cellulose corresponds to a structure which is essentially that of the trinitrate, and is the final result of nitration, but the view that nitrocelluloses in general are mixtures of this substance with unchanged cellulose, either natural or mercerised, rests on no sure ground and is open to many serious objections from other 2. physik. Chem., 130, 616 (1927). and Mark: Ber., 61, 594 (1929). 2. physik. Chem., 134, 264 (1928).
* Meyer 3
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F R A S K DOUGLAS MILES AKD JAMES CRAIK
aspects of the chemistry and technology of these substances. Criticism has been made by Brunswig' and by us! in a preliminary account of the present results which has already been given. The question whether this theory is true is important and an answer to it is one of the objects of this paper. In a wider sense, however, the main object was to examine the structural changes which take place as cellulose is gradually nitrated, in nitric or nitro-sulphuric acid, throughout the possible range. Incidental to this was the examination of the point picture of the trinitrate, which proved to be obtainable only from material nitrated in particular ways which have not been described by the authors mentioned. I t appears to be impossible to prepare a complete series of jibrous nitrated celluloses on account of the loss of fibrous nature which is always to be observed when the nitrated acid is represented by a point lying within a certain area of the triangular chart of mixed acid composition. An account of these relations has been given in a previous communication. The limits of nitrogen content between which this disintegration is found vary considerably with the composition of the acid, and high concentrations of sulphuric acid have in particular a disturbing effect, but since in by far the great majority of nitrations very high proportions of either acid were not used, the limits may be taken to be 7 . j and 1o.jy0 of nitrogen. I t is convenient therefore to divide the nitrocellulose into three classes (i) those containing less than 7. j% nitrogen, showing unimpaired fibrous structure, insoluble in all solvents (ii) those containing from 7 . 5 to 10.57~ nitrogen showing a fibre structure more or less destroyed and a tendency to dissolve in acetone (iii) those containing more than I o . j x of nitrogen and including all the technically important varieties, the properties of which are well known.
Experimental Materials and Methods. Long-fibred refined ramie was used throughout. Cotton was in many ways the more attractive material but is of little use for X-ray work on account of its convolutions. The details of analysis of the ramie and of the method of nitration were the same as those given in a previous communication. Two hours was allowed for nitration at 20'. Five hours was found to be advisable at zoo. Many of the nitrated samples were denitrated by immersion in sodium hydrosulphide, the instructions given by Atsuk? being exactly followed. A transformer-driven Shearer tube was employed. Xost of the diagrams were obtained with iron radiation, the distance of the fibres from the plate being 2 . j o cm. From each specimen a bundle of parallel fibres about 0.3 mm. in diameter was prepared. This bundle was wrapped tightly round with a single ramie fibre, mounted on the goniometer head of the X-ray spectrometer and carefully centred. Z. ges. Schiess- und Sprengstoffe, 23, 337, 384 (1928). 123, 82 (1929). J. SOC.Chem. Ind., 45, 1336 (1925).
* Nature,
THE STRUCTURE O F NITRATED CELLULOSE
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The finished plates were varnished with a nitrocellulose lacquer which greatly facilitates examination, and measurements were made with an accurate steel rule between ink marks made on the lacquered surface with a very fine pen. Greater interest was attached as a rule to comparative than to accurate measurement. The comparison of two diagrams can be made precisely by holding them film to film before a viewing screen. The readings were all checked by this method of superposition and whenever comparison with the diffraction of cellulose, natural of mercerised, or the trinitrate, or mixtures of these were in question, diagrams of these bodies were obtained under the same conditions, so that the comparison was direct in nearly all cases.
Nitroramie of Class I containing less than 7.5% of nitrogen, unswollen. Seven specimens were produced by nitration in the mixed acids specified in Table I. The first four of these acids have almost the same sulphuric acid:
FIQ.I
nitric acid ratio and are the first members of the principal series employed in this work. Their compositions are situated near to the dotted line in Fig. I .
TABLE I Preparation and Denitration of Nitroramie Class I Mixed Acid NO. I 2
3 4
5 6 7
70 HzSO4 31.5 31.8 32 . i 33.2 48.5 39.0 39.3
72
HNOI 38.4 40.3 42. I 42 . o 20.3 31 . o 36.4
H20
+%HNO, 30.I 27.9 25.2
24.8 31.2 30.0 24.3
%
Nitrogen
3.41 4.56 5.85 6.45 2 . I8 3,37 7 , IO
FRANK DOU
