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 THE MICRCSTRUCTURt Of CCILLICSC flBERS 
 
 June 1942 
 
 LiB 
 
 A r 
 ATLAWliH 
 
 BRANCH 1 
 
 
 
 No. R1432 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE 
 FOREST SERVICE 
 FOREST PRODUCTS LABORATORY 
 
 Madison, Wisconsin 
 
 In Cooperation with the University of Wisconsin 
 
THE "TRUCTURE OF CELLULOSE FIBERS^ 
 
 By 
 
 G. J. RITTFR, Senior D 
 
 This report is a brief review of the literature e miorostruoture 
 
 of cellulose fibers during the last three yea. 
 
 The first portion gives the -ally accepted cor.ceps regarding the 
 fiber struct I also the controversial concepts which were cf signifi- 
 cance at the beginning of 1930. 'eas and modification* of old ideas 
 about the fiber stricture are then discussed in connection with the various 
 papers that have appeared during 1938-1940, inclusive. 
 
 In a revi of the literature some articles are generally missed. 
 Accordingly, this reviewer - r, , at the outset, to acknow] - the liveli- 
 hood of his missing articles that should be included, and to beg the pardon 
 of any author who has been overlooked. 
 
 Generally Accepted Fiber Structures Pr v 38 
 
 A reading of the articles on the microstructure of cellulose fibers, 
 which appeared during the five years previous to 1938, seems to give rencral 
 agreement on the following points. 
 
 i vis ions of Cell Wall 
 
 The cell wall is divided into a primary and secondary layer and some- 
 38 a tertiary layer. The primary layer is adjacent the middle lamella, 
 secondary layer is adjacent the inner face of the primary; it is con- 
 siderably thicker than the primary layer even in thin-walled fibers and much 
 thicker than the primary layer in thick-walled fibers. The tertiary layer 
 is adjacent the inner face of the secondary layer, difficult to distinguish 
 in normal fibers but easily distinguished in some types of abnormal fibers; 
 its nrosence in normal fibers is sometimes questioned on account of the 
 difficulty of recognizing it. 
 
 .^Published in TAFFI Tech. Association Papers, Series XXV, June 1942 # 
 This paper forms a part of the report of the Committee on Cellulose i 
 Allied Substances of the Division of Chemistry an. - 'cal Technology 
 of the National Researc " 1. The previous report appeared in the 
 Par.^r Trade Journal 107, nos. 21, 23, 24 (1938), 
 
 R1432 
 
S leeye s 
 
 The secondary layer is divided into layers or sleeves. The number 
 of sleeves which have been reported vary from approximately 8 to 12 in wood 
 and as high as 30 in cotton. 
 
 Fib rils 
 
 /the 
 The primary layer of the wall and sleeves of the secondary layer appear 
 
 to have been dissected into long slender fibrils. The presence of fibrils, 
 to this reviewer's knowledge, has not been definitely demonstrated in the 
 tertiary layer. In some abnormal fibers a coil springlike structure loosely 
 associated with the secondary layer is discernible; some research workers 
 may consider that the coil-like structure demonstrates the presence of fibrils 
 in the tertiary wall. 
 
 Substruc ture of Fibrils 
 
 Microscopically visible substructures of the fibrils have been re- 
 ported. They comprise dermato somes, ellipsoids, fusiforms, and spherical 
 units. These are considered substructures of the fibrils because they are 
 each isolated by a treatment of the fibrils under rather carefully controlled 
 conditions. The treatment selected produces substructures of aporoximately 
 the same magnitude and shape. They can be produced from the fibers directly 
 if the treatment is less severe than that employed for dispersing the cellu- 
 lose. In dispersing the cellulose rather concentrated solvents are employed, 
 which quickly swell the fiber and disperse the cellulose without visibly 
 separating the cell wall into the fibrils and their visible substructures. 
 
 Sub-microscopic Str uctures 
 
 The largest submicroscopic structural unit of cellulose at the end 
 of 1937 was believed to be the micelle; since then the name "crystallite" 
 has been generally applied to the micelle on account of its crystal prop- 
 erties. The crystallite is assumed to be composed of long cellulose chains 
 which, in turn, are composed of anhydroglucose residues. 
 
 A preponderance of evidence favored orientation of the crystallites 
 in the primary wall to be approximately parallel with the perimeter of a 
 cross section of the fiber; also it favored orientation of the crystallites 
 in the secondary layer of the cell wall to be from to 15 degrees to the 
 lonr axis of normal fibers. 
 
 Controversial Aspects of the Fiber Structure Previous to 1938 
 
 Frevious to 1938 heated arguments had developed for and against the 
 presence of ! 'cross walls" in cellulose fibers. The proponents of the cross 
 
 R1432 -2- 
 
walls contended that Ice structures divided the cellulose fiber wall 
 into segments aonro-imately 40 microns in length* The structural units in 
 the cross walls were supposed to be oriented with their long axis parallel 
 with t rise p^rir - f the fiber. 7 -pe of arrangement would 
 restrain the fiber from shrinking and swel ( ransversely. If the fiber 
 were treated with a swelling i more drastic than water, the cross walls 
 
 re supposed to be responsible for the ballooning phenomenon which developed, 
 The presence of oross walls also limited the length of fibrils to the length 
 of the fiber segments between two sucn cross wal Is . No information as 
 regards the chemical composition of the cross walls was given. The idea of 
 cross walls was opposed by a large majority of research workers who ooi - 
 tended that the constrictions disclosed in the ballooning phenomenon were 
 caused by the primary wall fibrils. 
 
 orous controversic carried on ial nature 
 
 of the cementing material between the various microstructural units de- 
 scribed above. For example, advocates of the crosr.-vall concept admitted 
 that nothing was known about the composition of the crons walls. They 
 further advocated the presence of a mysterious materia] surrounding each of 
 the layers, sleeves, fibrils, dcrmatosomes, and micelles. Thus the largest 
 cellulose particle existing by itself was the micelle which was separated 
 from its adjacent micelles I mysterious cementing substan^ 
 
 Other re ..-search workers advocated the presence of a pectin-like 
 
 materia] surrounding the ellipsoidal particles. 
 
 icelluloses were also advocated as being present in ""he cell wall 
 but no definite stand was taken regarding its distribution among the various 
 microstructural units of the cell wall. 
 
 Articles Published Durinsr 1938-1940 Inclusive 
 
 Bailev, I. '7. Cell "'fell Structure of Higher Plants. Ind. Eng. 
 Chem. 30:40 (1938). The cell wall is divided into a primary and a seco.. - 
 ary layer. Each layer is composed of a coherent matrix of porous aniso- 
 tropic cellulose whose microstructural units gradually diminish to t 
 revolving power of the microscone (0.1 microns or less). Lignin and non- 
 cellulosic materials may be deposited in the Interstices of the cellulose 
 so as to form two continuous interpenetrating phases. One phase may be 
 removed without severim: the continuity of the other* The structure of the 
 secondary wall varies (l) in porosity in successively formed parts, (2) in 
 the arrangement of the aggregates of the chain molecules, (3) in the distri- 
 bution of noncellulosic materials, and (4) in the presence of noncellulosic 
 layers. The secondary portion of the wall can be dissected into layers, 
 fibrils, fusiform bodn^ , lermatosomes, and other substructures of varying 
 shapes and sizes. 
 
 Pre - ■■ .A. Sub:ricrcscopic structure and maceration pictures 
 of native cellulose fibers. Fapier-Fabr. 36:212 (1933). The author ces 
 
 R1432 - - 
 
the mioellar theory of Na'geli with the modification that the micelles are 
 held together by chain molecules which are a part of the two joined micelles. 
 
 Clark, S. F. Fine structure of the plant cell wall. Nature 142: ; 
 (1936). The principal idea of fiber structure conveyed by the article relates 
 to the crystallite structure. It is suggested that the cellulose chains are 
 arranged parallel in zones and nonparallel at intervals between the zones. 
 In this manner the crystallites or micelles are a more or less continuous 
 mass consisting of regularly and semiregularly arranged cellulose. Lignin is 
 believed to exist in the intermicellar spaces. The author divides the cell 
 wall into middle lamella, primary layer, and secondary layer. 
 
 Frey-Wys sling, .'-. The mice liar theory explained by the example of the 
 fine structure of fibers. ,r olloid-Z. 85:148 (1938). This article also 
 advocates the idea of the cellulose micelles and the intermediate substances 
 being attached to one another so as to form a continuous phase. The author 
 suggests that the term "micelle" be retained in the literature. Micelles 
 are about 60 A. wide, "-hereas the spaces between then 1 are 100 A. wide. 
 
 Kess, Kurt. Recent results of the investigation of the structure of 
 the vegetable ce'l wall. Faoier-^abr. 57:28 (1939). The article reports 
 findings obtained by means of photomicrographs in which ultraviolet light 
 was employed as the light medium. The article indicates that the results 
 obtained, suggest a theory on fiber structure differing from that put forth 
 by Frey-Vys sling. 
 
 Bailey, I. W. The microfibrillar structure of the cell wall. Bui. 
 Torrey Bot, Club 66:201 (1939). This article conveys the idea of two 
 structural phases in the cell wall, one composed of cellulose and the other 
 composed of noncellulocic materials, each phase being continuous. The 
 author contends that the cellulose is regularly arranged in the microfibrils, 
 but that the microfibril arrangement fluctuates from layer to layer in the 
 secondary portion of the cell wall, thereby causing a phenomenon interpreted 
 as random crystallite arrangement. The remainder of the article is similar 
 to that published in Ind. Eng. Chem. 30:40 (1938). 
 
 Bailey, A. J., and Brown, R. ■!. Diameter variations in cellulose 
 fibrils. Ind. Eng. Chem. 32:57 (1940). The diameters of fibrils isolated 
 from various types of cellulose fibers were measured microscopically and 
 found to range from 0.928 to 0.955 micron. Fibers can be divided into 
 fibrils and hydrogel by mechanically agitating the fibers in water. 
 
 Hock, Chas. W. , and Seifriz, "". "icrodissection of paper pulp fibers. 
 Paper Trade J. 110, no. 5:31 (1940). Fibers frcm paper pulp were treated 
 with boiling water and phosphoric acid and examined microscoDically. The 
 examination revealed that fibers are made up of may fibrils arranged parallel 
 to the long axis of the fiber. These fibrils are wrapped in an outer layer 
 of transversely arranged fibrils and are bordered by an inner winding which 
 subtends the lumen. Balloon formation is due to uneven v/eakening of the 
 ir.terf ibrillar forces of the cuter wraoDin?:. 
 
 R1432 -4- 
 
... Lstenoe and nature of f i t 
 membranes. Pap^ ". " ". . •" : . X3« Papei 22:47! 
 (1939). Cellul lp fibers cent - diffc 1 g of lipnin 
 
 Lspersed .-lution- 
 
 due so ■• ' 1 was considered to be a fi rane similar to [ 
 ad" by Utt . The residue recovered from dispersing ur. .ed 
 
 pulps cor.;' of nor^ than half lignosulphonic acid and small amounts of 
 carbc - ron bleached pulps consisted of oompoucda of 
 
 p ard carbohydrate . 
 
 , r ■". . . ■ the inner structure of cellulose : 
 visible by embedd tances. a 22:3, 55 (1S40), The 
 
 article describes preliminary trials on Impregnating I ng metals 
 
 the microstructure of fibers. The treatment consists of two stages, 
 one to swell the fiber wall and the other a diffusion of a salt solution 
 into the fiber wall and deposition of - rhich le microsco] Lly 
 visible. This technique affords a means of measur: 
 fine capillary structure of fibers. 
 
 Kratky, 0. , Xair.z, . , and Treer, . Lcellar structure of native 
 cellulose. Holz Roh u. Werketoff 2:409 (1930). The cell wall is Impregna- 
 ted with a solution of a noble metal salt an tal salt subsequently 
 reduced to finely divided metal Which is deposited in the interstices 
 
 tween the cellulose micelles. An approximation of the size of the inter- 
 stices is 
 
 I rest on, R. D. Wall of the conifer tracheid as a single spiral 
 complex. Proc. Leeds Phil. Sec . 3, part 9:546 (1939). The mice liar 
 ■ngement of the primary layer is in ■ 1 leng : iber. 
 
 8 arrangement in the secondary layer shows - r dispersion. 
 
 Xundu, B, C., and Preston, R. D* Submicr ;cture of c^" 
 
 walls. Proc. Royal Soc. (London) 3128:214 (1940). The article conveys the 
 idea that the cellulose chains are arranged ler I e in the primary layer 
 of the cell wall in contradistinction to the circular transverse arrangement 
 around iber as advocated by other resear /hers. 
 
 r arr, Wanda '". Structure and comoosition nt cell membranes. 
 T'ature 146:133 (1940). The article is a brief ih of the author's 
 concepts given in previous articles, f ] se ellipsoids about 1.1 by 
 1,5 microns are the microstructural units of the cellulose membranes. They 
 are covered and oemented tc r by means of a pec - >:e materi 
 
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