- Email: [email protected]
Structural properties of modified cellulose
552
KH. U. USMA~OV
78. JOSE-G6MEZ-IBANES and CHIA-TSUN-LIU, J. Phys. Chem. 65: 2148, 1961; 67: 1388, 1963. 79. F. BUECHE, Physical Properties of Polymers, Interscience Publishers, New Y o r k London, 1962 80. V. S. KLIMENKOV, V. A. KARGIN and A. I. KITAIGORODSKII, Zh. fiz. khimii 27: 1217, 1953 81. J. PRIGOGINE, A. MELLEMANS a n d V. MATHOT, The Molecular Theory of Solutions, Amsterdam, 1957 82. A . A . TAGER, A. SMIRNOVA and N. SYSUYEVA, Nauchn. dok]. vysshei shkely, K h i m i y a i khimich, tekhnologiya 1: 135, 1958 83. B. E. EICHINGER a n d P. J. FLORY, Trans. F a r a d a y Soc. 64: 2035, 1968 84. C. CANIBERTI a n d U. BIANCHI, P o l y m e r 7: 151, 1966 85. A. J. DAVENPORT, J. S. ROWLINSON and G. SAVILLE, Trans. F a r a d a y Soc. 62: 322, 1966 86. A. A. TAGER, L. V. ADAMOVA, M. V. TSILIPOTKINA a n d G. I. FLOROVA, Vysokomol. soyed. (in press) (To be translated in P o l y m e r Sci. U.S.S.R.) 87. A. F. I O F F E and N. N. SEMENOV, K u r s fiziki, p. 230, vol. 4, GTTI, 1932 88. V. N. ATHOT and A. DESMYTER, J. Chem. Phys, 21: 782, 1953 89. P. J. FLORY, R. A. ORWOLL a n d A. VRIJ, J. Amer. Chem. Soc. 86: 3515, 1964 90. J. PRIGOGINE, N. TRAPPENIERS and V. MATHOT, Disc. F a r a d a y Soc. 15: 93, 1953 91. J. HIJMANS, Physiea 27: 433, 1961 92. S. N. BHATTACHARYYA, D. PATTERSON and T. SOMEYNSKY, Physica 30: 1276, 1964 93. T. D. PATTERSON and A. A. TAGER, Vysokomol. soyed. A9: 1814, 1967 (Translated in P o l y m e r Sei. U.S.S.R. 9: 8, 2051, 1967) 94. D. PATTERSON, G. DELMAS a n d T. SOMCYNSKY, Polymer 8: 503, 1967 95. D. PATTERSON and G. DELMAS, Trans. F a r a d a y Soc. 65: 708, 1969
STRUCTURAL PROPERTIES OF MODIFIED CELLULOSE K~. U. US~A~OV Scientific Research I n s t i t u t e of the Chemistry and Technology of Cotton Cellulose
(Received 2 October 1970)
MODIFICATION of the properties of polymers and finished products made from them has become a most important trend in the chemistry and physics of this class of compounds. Modification normally produces materials with the requisite properties faster than the synthesis of new high molecular weight compounds. Numerous studies were therefore carried out in recent years in the field of modification of polymers. Results indicate that the properties of polymers are modified as a result of changes in chemical composition and physical structure of modified polymer materials. Furthermore, it is possible, in principle, that several structural * Vysokomol. soyed. A13: No. 2, 485-492, 1971.
Structural properties of modified cellulose
553
changes correspond to the same change in composition, each being determined by its own complex of properties. The role of structural changes in moclifieation is therefore of greater significance than the possible chemical conversions. We have tried in this paper to generalize the information accumulated concerning the supermolecular structure of modified cellulose samples. We will deal mabfly with cases of modification of natural cellulose and cellulose hydrate by grafting, crosslinking, partial replacement of functional groups and activation of cellulose for ester formation. In our investigations we used cotton fibres or cotton fabrics, cotton cellulose, various activated samples, derivatives of this cellulose and wood cellulose before and after activation, crosslinked and graft samples prepared from cotton cellulose and cellulose hydrate, polynose and cord fibres made by various firms with different physical and mechanical properties. We used electron microscopy and auxiliary methods of structural analysis and thermodynamics for our investigations. Results of many years' studies show that there is a regular correlation between many chemical and physico-chemical transformations in cellulose materials and the corresponding structural changes taking place in them. STRUCTURAL CHANGES TAKING PLACE DURING CROSSLINKING
It was found that during graft copolymerization of cellulose with several vinyl monomers many physical and chemical characteristics improve: cellulose fibres are easily coloured, their resistance to microorganisms, chemicals, and heat increases, the cellulose fabric becomes crease-resistant, etc. [1-3]. It should be pointed out that chemical transformations of cellulose are accompanied by structural changes which may take place both at the molecular and supermolecular levels [4-11]. It is shown that chemical combination of foreign cellulose macromolecules markedly alters the mutual packing of cellulose chains and this results in a variation of the shape and dimensions of structural elements and layers of the secondary wall of the cotton fibre. It was found, for example, [4-8] that a change is observed in the structure of transverse and longitudinal ultra-thin cotton fibre sections before and after grafting to it several vinyl polymers. We investigated several graft copolymers prepared from cellulose and polyaerylonitrile (PAN)--the weight increase was 12.5%: poly-2-vinylpyridine (PVP)--increase in weight 16%; polyvinylidene chloride (PVDC)--weight increase 14%; polymethacrylamide (PMMA)--increase in weight 56%; polymethaerylic acid (PMA)--increase in weight 54%; polystyrene (PS)--increase in weight 3.4%; 13.4% polyvinyltoluene (PVT)--weight increase 6.9; 14.1 and 27.8 ~o, etc. To obtain these samples e°Co y-radiation was used with a rate of 70 rad/sec. The irradiation doses were not more than 1 Mrad, at which cellulose does not
554
KH. U. Usm~.~ov
undergo considerable change. (See papers [1-3] for details of conditions of preparing these samples, their physical and chemical properties and other information). Figure 1 shows electron microphotographs of the surface structure of cotton cellulose PVT graft copolymers with different weight increases. The type of change in the surface structure of cellulose was shown as the degree of grafting of PVT increases: folds in the surface structure gradually disappear and the fibre surface becomes structurally homogeneous. When grafting PMMA the surface structure of the cotton fibre becomes slightly different. During fragmentation of the copolymer layers with high ordering are observed. This is apparently due to the fact that owing to grafting of synthetic polymers to cellulose, forces of adhesion (microfibril, fibril) increase. The position is somewhat different for PMA [8]. Although the folds in the surface structure of the cotton fibre are completely retained, owing to the considerable extent of grafting of the polymer, these folds are smoother. The fibrillar structure of this fibre is clearly seen in the form of large fragments of fibrillar layers. For PMA the fibrils forming layers of the secondary fibre wall, in contrast to the graft PMMA sample, are not elongated considerably. In places they go over to adjacent sections. It can hence be concluded that forces of adhesion of fibrillar formations in the lateral direction are less marked for PMA than in grafting PMMA. These results indicate that the surface structure of cotton fibres differs according to the type of vinyl polymer grafted to it. Furthermore, a general feature of all the cases studied is the fact that during graft copolymerization of cotton cellulose with vinyl monomers the steric arrangement of chains of the graft synthetic polymer in cellulose varies according to the conditions of copolymerization. When this process takes place from the vapour phase and prior to grafting, the fibre does not swell (PAN, PVP), and macromolecules are grafted to cellulose mainly on the surface of structural elements of the fibre, on microfibrils and fibrils. Results obtained during the investigation of graft cellulose samples with PVDC, PMMA and PMA [8] using swollen cellulose suggest that the structural changes of cellulose are most apparent under these conditions. In these samples the surface structure of the fibre and its sections with fibrillar packing (secondary wall) are modified more markedly. Hence it follows that in the case of grafting from solution the structural elements of cellulose in solution swell markedly, and consequently even those cellulose chains which are arranged inside the structural elements can take part in graft eopolymerization, since in this case they are more accessible to the grafting material. It is interesting to explain the structural changes of cotton cellulose according t o the degree of grafting of the same vinyl polymer. We examined fibres grafted with PVT with different weight gains (Fig. 1). An increase in. weight gain of PVT in cotton cellulose considerably influences the nature of fibrillation. With considerable weight gains fibrillation of the graft
Structural properties of modified cellulose
555
FIG. 1. Microphotographs showing replicas of the surface of cotton fibres grafted with PVT; weight gain 6.9 (a); 14.1 (b) and 27.89/00 (c). Fro. 2. Microphotograph of an ultra-thin cross section of a cellulose-PMA graft copolymcr (weight gain 549/oo). F l a . 3. Microphotographs of ultra-thin cross sections of cyanoethylated fibres: a - - i n i t i a l cotton fibre; b - - p r e v i o u s l y treated with sodium hydroxide; c - - w i t h a glycerine-water mixturo (1 : 1).
$56
Km U. USM~WOV
cellulose considerably deteriorates, which is particularly marked with a weight gain of 27.8~. This may be explained by the fact that grafting PVT to cellulose reduces its hydrophilic nature. With an increase in the degree of grafting and with a gradual increase in its hydrophobic nature during fragmentation in water, the clear fibrillar structure of graft cellulose with PVT disappears especially with considerable weight increase. Our investigations indicate that the method of ultra-thin segments can be successfully used to observe the structural behaviour of cellulose. Figure 2 shows as an example an electron microphotograph of an ultra-thin copolymer section prepared from cellulose and PMA. It can be seen in the Figure that layers of the secondary wall of the graft fibre are evenly distributed. This typical pattern also characterizes other graft monomers and various degrees of grafting, which points to the uniform distribution of grafting in the fibre section. It may be concluded from the foregoing that grafting of various vinyl polymers to a cotton fibre has a variable effect on the surface structure and the structure of internal fibrillar sections of the cotton fibre. The degree of structural change in the fibre depends both on the type of polymer to be grafted and the degree of grafting, and on the conditions of obtaining the graft sample, i.e. whether grafting takes place from the vapour phase or in solution. When grafting takes place from the vapour phase, particularly in the case of small weight gains, the same cellulose macromolecules which are arranged on the surface of structural elements-mierofibrils and fibrils--ti~ke part in graft copolymerization. If grafting is preceded by swelling of cellulose in the monomer, or in solution, those macromolecules which are arranged inside the fibrils and microfibrils take part in the reaction. By studying dispersion of a graft cellulose sample some of its properties, e.g. its hydrophobic nature can be evaluated. Finally, a study of ultra-thin graft copolymer sections of cellulose provides valuable information concerning macrostructural changes in cellulose during graft copolymerization. In addition, from an examination by an electron microscope of the transverse and longitudinal copolymer sections the type of grafting (regularity or irregularity) in the fibre section can be determined. STRUCTURAL CHANGES IN ACTIVATION AND ESTERIFICATION OF CELLULOSE
It is now known that to increase the activity of natural cellulose in various chemical reactions--esterification, graft copolymerization, etc.--the supermolecular structure has to be loosened, in order to ensure full accessibility of the fibre to the effect of reagents. Methods of structural modification of cellulose recently developed, which ensure uniform reaction in the entire fibre volume, are therefore of considerable importance. Cotton fibres were therefore activated with sodium hydroxide, inclusion carried out with isoamyl alcohol and boiling in a glycerine-water mixture; the
Structural properties of modified cellulose
557
same samples and for comparison the initial cotton fibres were subjected to cyanoethylation (Table). The ester yield indicates the extent of accessibility of cellulose to cyanoethylation. The cotton fibre m a y undergo considerable changes even during preparatory physico-chemical treatment [12]. Typical signs of these changes in cotton cellulose during interaction with alkali are the increase in diameter, reduction in length and change in the shape of the fibre; from being twisted it becomes cylindrical and the transverse section becomes almost circular. Microscopic observations suggest that inclusion of cotton fibres with isoamyl alcohol after alkaline treatment intensifies swelling and therefore increases the fibre diameter even more. The form of the fibre during inclusion, compared with alkaline treatment, does not change substantially [13]. It was established that after treatment with a glycerine-water mixture the average diameter of the cotton fibre noticeably increases; however, the shape remains unchanged and the twisted condition is maintained. CrA~OET~C~LLVLOSE
~LD
AFTER ~ A ~ T
Type of treatment
Initial cotton cellulose Cotton cellulose treated with an 18~ sodium hydroxide solution Same with subsequent inclusion with isoamyl alcohol Cotton cellulose treated with a glycerinewater mixture
Nitrogen content in preparation, % of cyanoethylated products 2"8 5"0 7.5 3"2
Changes in the width of individual microfibrils were ~ 100 A for the initial (untreated) cotton fbre, after alkaline treatment ~ 150 A and finally, after iuclusion, 170 A [14, 15]. I t m a y be concluded that structural changes in cotton cellulose, dependent on alkaline treatment, are subsequently not only stabilized during inclusion with isoamyl alcohol, but are noticeably intensified. After inclusion the folded fibre surface changes considerably. Folds are almost absent. Fibrils are not observed either, owing to the strong swelling of the structural elements of cellulose [14]. Hence it follows that treatment of cotton cellulose with sodium hydroxide and the inclusion process cause intrafibrillar swelling. This is also suggested b y results of X-ray investigations [15]. Quite a different pattern was observed during the treatment of cotton cellulose with a glycerine-water mixture. Results of investigations indicate that
$58
K~. U, U s ~ o v
FIO. 4. Microphotographs of ultra-thin cross sections of an initial fibre (a) and a fibre crosslinked with acrolein (b). FIG. 5. The effect of preliminary swelling (treatment with a quaternary base) on the structure of initial fibre (a) and cotton fibre crosslinked with DMM (b) (ultra-thin sections). FIG. 6. Microphotographs of dense fibre sections before (a) and after crosslinking with cyanur chloride (b) in the swollen state. o n l y loose (interfibrillar a n d interlayer) sections of cellulose are s u b j e c t to t h e effect of swelling. As a consequence, fibrillar f o r m a t i o n s a n d layers s e p a r a t e . T h e d i m e n s i o n s o f i n d i v i d u a l microfibrils are t h e s a m e as those o f t h e initial c o t t o n fibre, i.e. ~ 100 A. T h e s t r u c t u r a l l y modified cellulose sampleh were c y a n o e t h y l a t e d . E l e c t r o n m i c r o s c o p e studies of t h e initial a n d c y a n o e t h y l a t e d ( w i t h o u t a c t i v a t i o n ) c o t t o n fibre indicate t h a t t h e s t r u c t u r e o f the t r a n s v e r s e section is
Sbruetural properties of modifiedcellulose
559
loose (Fig. 3a). Layers of the secondary wall are separated by cavities. On the other hand, cyanoethylated cotton fibre samples which had previously been activated with sodium hydroxide or included with isoamyl alcohol have a dense structure of sections; large interfibrillar cavities are absent (Fig. 3b), the laminar microstructure disappears. This is due to the fact that cellulose fibres which have a loose structure are cyanoethylated. The marked accessibility of cellulose to the reagent increases cyanoethylation. The position is quite different in the case of activating cellulose with a glycerine-water mixture, followed by cyanoethylation (Fig. 3c). It should be assumed that since with this treatment only interfibrillar (interlayer) loosening takes place in the fibre and the microfibrils remain unaffected, mainly those microfibrils which are arranged on the surface of macrofibrils or their units will take part in the reaction. Thus, the structural mechanism of activation of cellulose was shown by electron microscopy. It follows from our results that the structure of cyanoethylated cotton fibres depends entirely on their initial physical structure. If a certain type of structural conversion--intrafibrillar or interfibrillar loosening--takes place in the fibre, this is also reflected by the fibre structure after chemical modification. STRUCTURAl CHANGES IN CROSSUNKING
Numerous investigations of cellulose materials treated by various reagents which form crosslinks lead us to several significant conclusions concerning the mechanism of modifying these materials at a supermolecular level [16-19]. Particular attention was given to the following problems: l) the general regularities in changes taking place during crosslinking in the cellulose structure; 2) to what extent the structural changes depend on the supermolecular structure of a certain cellulose preparation 3) whether the crosslinking reagent is distributed evenly enough, or is localized in certain parts of the fibre 4) the effect on the structure of crosslinked cellulose of the type of modifying reagent, and conditions and degree of modification. Investigations, the results of which formed the basis of the conclusions drawn, were carried out using cellulose preparations of different supermoleeular structures. Formaldehyde, dimethylolurea (DMU), dimethylolethyleneurea, dimethylolthiourea, acrolein, hexamethylenedi-isocyanate, diepoxide, epichlorohydrin (ECH) and cyanur chloride were used as modifying reagents. In addition to the bifunctional compounds mentioned, which erosslink cellulose [20, 21], some monofunctional analogues--monomethylolurea (MMU), monomethylo]thiourea, phenylisocyanate, propylene oxide were used. Cellulose preparations containing linear ~ n d crosslinked polymers, chemically uncombined with cellulose, were also investigated. A most general structural feature of cellulose preparations subjected to erosslinking is the marked increase in thickness of the structural elements and the absence of fibrillation on intensive ultrasonic irradiation. Another significant
560
K~. U. U s ~ o v
property is the noticeable reduction in dimensions of structural elements in proportion to the increase of crosslinking and the formation of a large number of fine splinter-like particles. It is evident that crosslinking begins and takes place most vigorously in the most accessible parts of cellulose, in the interlayer sections. Inter- and intrafibrillar ranges are then subjected to crosslinking, which increases the structural elements. Crosslinking of microfibrils inside the layers prevents fibrillation during mechanical or acoustic treatment of fibres. When a considerable number of crosslinks are formed in cellulose fibres, the possibility of mutual transfer of structural elements is greatly restricted. Accessible ranges, where crosslinking takes place very vigorously, are completely transformed into a three-dimensional reticular structure. As a result, brittle fracture of fibres takes place during dispersion with irregular distribution of load and not the breakdown of fibres along the natural interface, which results in the formation of a large number of fine particles. The physical and mechanical properties of fibres vary accordingly: elongation markedly decreases, strength and fatigue properties deteriorate, but the ability to recover after deformation as a result of the increase of elasticity and delayed elasticity markedly improves. Investigations of crosslinked fibres using ultra-thin sections show a noticeable increase in the dimensions of structural elements (ends of fibrils), compared with unmodified samples (Fig. 4a, b). Thus, an increase in the transverse dimensions of structural elements is one of the typical features of crosslinking. The supermolecular structure of the initial cellulose has a considerable effect on structural changes during crosslinking. I t can be stated that the more individual are the structural elements and the more different levels of supermolecular organization are contained in the material, the more marked are the structural changes after crosslinking. All the structural properties of crosslinked samples described are therefore most noticeable for natural cellulose, which has a clearly expressed fibrillar-laminar, heterogeneous structure. For cellulose hydrate fibres structural changes in crosslinking are clearly observed in standard polynose fibres, the structure of which is similar to the structure of cotton cellulose. I n the case of a cord fibre, where there is no developed fibrillar-layer structure [22], crosslinks are evidently formed in the loosest parts and in the intermediate zones between them and with different forms of molecular ordering. Consequently, a structureless mass is crosslinked with the more ordered ranges and can hardly be seen in the photographs. The same is observed with dispersed viscose silk preparations. I f the fibre has a dense homogeneous structure with a low content of structurally organized materials crosslinking does not cause any noticeable structural changes. This is typical of high-strength fibres, formed in concentrated acid. Investigations of ultra-thin fibre sections and the use of preliminary swelling before obtaining the sections suggest that crosslinking takes place fairly evenly in the fibre volume allowing for structural heterogeneity. This is confirmed by
Structural properties of modified cellulose
561
homogeneous etching of sections in solvents and in a gas discharge and by a uniform reduction in the degree of swelling in the fibre section (Fig. 5). It is relevant to deal here with the effect of the conditions of crosslinking cellulose on the structural changes. Conventional crosslinking conditions involve the impregnation of the fibre with aqueous solutions of reagents and crosslinking at increased temperature. These conditions ensure a fairly uniform process. However, if the process is carried out in an organic solvent, without causing swelling of cellulose, the reagents m a y be localized on the interface of structural fibre elements (e.g. modification with acrolein in ethyl ether). Changes in the structure and properties of cellulose during modification in the swollen state [23] are of special importance. No increase is observed in the thickness of fragments and several other typical structural featttres of crosslinked cellulose are retained. It is evident that crosslinking mainly takes place in the lateral direction of the structural elements, whereas in the axial direction these elements are separated by considerable distances as a consequence of strong swelling (fibre swollen in alkali is used). Swelling facilitates the access of reagents to the internal regions of microfibrils, and consequently t h e y are subject to intensive crosslinking. Here, as with grafting, the diameter of the structural elements increases several times (to 400-500 A, compared with 80-100 A for the initial cellulose) and periodicity appears with an interval of 650 A. Similar structural changes are observed during the treatment of cotton with ECH and cyanur chloride. Thus, crosslinking of cellulose in the swollen state mainly results in the formation of intra- and interfibrillar crosslinks with retention of part of the liquid causing swelling of cellulose. Consequently, the elastic properties of the fibre noticeably increase during moistening. The type of crosslinking reagent has a certain effect (although less marked than the process conditions) on the structural changes of cellulose. Most effective are the reagents with a symmetrical molecule and identical functional groups not forming products of polymerization. F o r example, during crosslinking of CH20 ~ 1 ~o weight gain is sufficient for a sudden change in the structure and properties of cellulose. It m a y be conjectured that in this case cellulose contains about one crosslink per 10 glucose residues. During the modification of DMM, where various polymerization processes take place as well as crosslinking, with weight gains of 8-10% the sample contains one crosslink per 15-20 glucose residue. Finally, acrolein, which has different functional groups and readily forms homopolymers, is even less effective and weight gains in excess of 10% are necessary for a noticeable change in the structure and properties of modified fibres. An increase in the molecular weight of the reagent (diepoxide) markedly reduces the efficiency of crosslinking and localizes crosslinking in the surface layers of fibres. Interesting results were obtained when investigating the densest parts of modified fibres. It is shown that the length of dense sections after modification
562
Kg. U. USM~OV
with CH~O, DMM and with acrolein somewhat decreases compared with the initial length. When cellulose is modified in the swollen state the length of dense parts decreases several times, which is convincing evidence of the possible penetration of the modifying reagent in the dense parts of microfibrils (Fig. 6). The possibility of crosslinking of ordered parts was confirmed experimentally by their separation, treatment with CH~O and subsequent evaluation of swelling in alkali [24]. An analysis of X-ray photographs of crosslinked preparations and mechanical mixtures of cellulose with homopolymers shows [25] that a reduction in ordering m a y be due to the modification of structure (actual) and to the accumulation of polymerization products (apparent). In the first case a reduction is observed in the intensity of the 002 reflection (20--=22.4°), while in the second--"amorphons" scattering increases at 20= 18°. Thus, X-ray results confirm the conclusions derived b y electron microscope studies concerning the possibility of crosslinking in dense parts of cellulose. The density of modified cellulose varies slightly compared with the initial density; fibres crosslinked in the swollen condition are characterized b y a sudden drop in density, which proves a considerable structural loosening. Study of the structure of cellulose preparations modified b y mono-functional analogues of the compounds described showed that in this case no noticeable changes took place in structure or properties. The same can be said about preparations containing linear and crosslinked polymers, chemically uncombined with cellulose. Thus, the main structural changes during modification of cellulose with bifunctional reagents are due mainly to the formation of erosslinks and not to the accumulation of products of polymerization or monofunctional substitution. Translated by E. SEMERE
REFERENCES
1. Kh. U. USMANOV and U. AZIZOV, J. Polymer Sci., C, 579, 1964 2. Kh. U. USMANOV, Cellulo~a si Hirtie 14: 363, 1965 3. Kh. U. USMANOV, U. AZIZOV and M. U. SADYK0V, Sb. Radiatsionnaya khimiya polimerov (Radiation Chemistry of Polymers). p. 153, Izd. l~auka 1966 4. K. Kh. RAZIKOV, Kh. U. USMANOV and U. AZIZ0V, Proc. III Europ. Confer. Electr. Mier. Prague, A, p. 409, 1964 5. K. Kh. RAZIKOV and Kh. U. USMANOV,Vysokomol. soyed. 8: 387, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 3, 421, 1966) 6. K. Kh. RAZIKOV and Kh. U. USMANOV, Vysokomol. soyed. BP: 742, 1967 (Not translated in Polymer Sci. U.S.S.R.) 7. K. Kh. RAZIKOV, I. I. ISAMIYKHAMEDOVAand Kh. U. USMANOV, Sb. Soveshchanie po radiatsionnomu modifitsirovaniyu polimerov (Conferonce on Radiation Modification of Polymers). p. 60, Nauka, 1963 8. Kh. U. USMANOV, K. Kh. RAZIK0V et al., Mezhdunarodnyi simpozium po makro. molekulyarnoi khimii (International Symposium on Macromoleeular Chemistry). p. 165, Budapest, 1969 9. M. L, R01.IJEgS, A. T. MOORE et al., Amer. Dyestuff. Reporter 54: 36, 1965
Thermodynamics and morphology of phase transformations in polymers
563
10. R. M. LIVSHITS, T. S. SYDYKOV and Z. A. ROGOVIN, Cellulose Chem. Technol. 2: 3, 1968 l l. Z. A. ROGOVIN, T. S. SYDYKOV, R. M. LIVSHITS, M. V. SHABLYGIN, and N. V. MIKHAILOV, Vysokomol. soyed. B10: 46, 1968 (Not translated in Polymer Sci. U.S.S.R.) 12. K. Kh. RAZIKOV, E. D. TYAGAI, V. I. SADOVNIKOVA and Kh. U. USMANOV, Vysokotool. soyed. A l l : 1717, 1969 (Translated in Polymer Sei. U.S.S.R. 11: 8, 1948, 1969) 13. E. D. TYAGAI, et al., Vysokomol. soyed. B1O: 301, 1968 (Not translated in Polymer Sci. U.S.S.R.) 14. K. Kh. RAZIKOV, E. D. TYAGAI, P. P. LARIN and Kh. U. USMANOV, Vysokomol. soyed. A9: 393, 1967 (Translated in Polymer Sci U.S.S.R. 9: 2, 442, 1967) 15. K. Kh. RAZIKOV, E. D. TYAGAI, Yu. T. TASHPULATOV and Kh. U. USMANOV, Zh. prikl, khimii 41: 2706, 1968 16. G. V. NIKONOVICH, S. A. LEONT'EVA and Kh. U. USMANOV, Khimich. volokna, No. 6, 55, 1963 17. G. V. NIKONOVICH, S. A. LEONT'EVA and Kh. U. USMANOV, J. Polymer Sei. C16: 877, 1967 18. G. V. NIKONOVICH, S. A. LEONT'EVA and Kh. U. USMANOV, Vysokomol. soyed. A10: 2682, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 12, 3112, 1968) 19. M. ROLLINS, A. MOORE and V. TRIPP, Text. Res. J. 33: 117, 1963 20. It. TOVEY, Text. Res. J. 31: 185, 1961 21. A. SMITH, J. Soc. Dyers and Co]ourists 77: 416, 1961 22. G. V. NIKONOVICH, N. D. BURKHANOVA, S. A. LEONT'EVA and Kh. U. USMANOV, Cellulose Chem. Teehnol. 2: 231, 1968 23. G. V. NIKONOVICH, S. A. LEONT'EVA, V. P. SHATKINA, Kh. U. USMANOV, A. A. ~DYLOV and Yu. T. TASI-IPULATOV, Vysokomol. soyed. 7: 2132, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 12, 2337, 1965) 24. G. V. NIKONOVICH, S. A. LEONT'EVA and Kh. U. USMANOV, Struktura i modifikatsiya khlopkovoi tsellyulozy, 3, Tashkent, 1966 25. G. V. NIKONOVICH et al., Vysokomol. soyed. A10: 960, 1968 Translated in Polymer Sci. U.S.S.R. 1O: 4, 1118, 1968)
THERMODYNAMICS AND MORPHOLOGY OF PHASE TRANSFORMATIONS IN POLYMERS* S. YA. FRENKEL' a n d G. K. YELYASHEVICtt Institute of High Molecular Weight Compounds, U.S.S.R. Academy of Sciences (Received 5 October 1970)
ONE of the m a i n p r o b l e m s o f p o l y m e r science a t present is to develop a t h e o r y to give a q u a n t i t a t i v e description o f n u m e r o u s a n d varied experimental results which a t first sight often a p p e a r to be c o n t r a d i c t o r y . The m o d e r n t h e o r y of simple liquids gives a simple a n d a c c u r a t e description of the state of liquids at * Vysokomol. soyed. AI3: No. 2, 493-505, 1971.
KH. U. USMA~OV
78. JOSE-G6MEZ-IBANES and CHIA-TSUN-LIU, J. Phys. Chem. 65: 2148, 1961; 67: 1388, 1963. 79. F. BUECHE, Physical Properties of Polymers, Interscience Publishers, New Y o r k London, 1962 80. V. S. KLIMENKOV, V. A. KARGIN and A. I. KITAIGORODSKII, Zh. fiz. khimii 27: 1217, 1953 81. J. PRIGOGINE, A. MELLEMANS a n d V. MATHOT, The Molecular Theory of Solutions, Amsterdam, 1957 82. A . A . TAGER, A. SMIRNOVA and N. SYSUYEVA, Nauchn. dok]. vysshei shkely, K h i m i y a i khimich, tekhnologiya 1: 135, 1958 83. B. E. EICHINGER a n d P. J. FLORY, Trans. F a r a d a y Soc. 64: 2035, 1968 84. C. CANIBERTI a n d U. BIANCHI, P o l y m e r 7: 151, 1966 85. A. J. DAVENPORT, J. S. ROWLINSON and G. SAVILLE, Trans. F a r a d a y Soc. 62: 322, 1966 86. A. A. TAGER, L. V. ADAMOVA, M. V. TSILIPOTKINA a n d G. I. FLOROVA, Vysokomol. soyed. (in press) (To be translated in P o l y m e r Sci. U.S.S.R.) 87. A. F. I O F F E and N. N. SEMENOV, K u r s fiziki, p. 230, vol. 4, GTTI, 1932 88. V. N. ATHOT and A. DESMYTER, J. Chem. Phys, 21: 782, 1953 89. P. J. FLORY, R. A. ORWOLL a n d A. VRIJ, J. Amer. Chem. Soc. 86: 3515, 1964 90. J. PRIGOGINE, N. TRAPPENIERS and V. MATHOT, Disc. F a r a d a y Soc. 15: 93, 1953 91. J. HIJMANS, Physiea 27: 433, 1961 92. S. N. BHATTACHARYYA, D. PATTERSON and T. SOMEYNSKY, Physica 30: 1276, 1964 93. T. D. PATTERSON and A. A. TAGER, Vysokomol. soyed. A9: 1814, 1967 (Translated in P o l y m e r Sei. U.S.S.R. 9: 8, 2051, 1967) 94. D. PATTERSON, G. DELMAS a n d T. SOMCYNSKY, Polymer 8: 503, 1967 95. D. PATTERSON and G. DELMAS, Trans. F a r a d a y Soc. 65: 708, 1969
STRUCTURAL PROPERTIES OF MODIFIED CELLULOSE K~. U. US~A~OV Scientific Research I n s t i t u t e of the Chemistry and Technology of Cotton Cellulose
(Received 2 October 1970)
MODIFICATION of the properties of polymers and finished products made from them has become a most important trend in the chemistry and physics of this class of compounds. Modification normally produces materials with the requisite properties faster than the synthesis of new high molecular weight compounds. Numerous studies were therefore carried out in recent years in the field of modification of polymers. Results indicate that the properties of polymers are modified as a result of changes in chemical composition and physical structure of modified polymer materials. Furthermore, it is possible, in principle, that several structural * Vysokomol. soyed. A13: No. 2, 485-492, 1971.
Structural properties of modified cellulose
553
changes correspond to the same change in composition, each being determined by its own complex of properties. The role of structural changes in moclifieation is therefore of greater significance than the possible chemical conversions. We have tried in this paper to generalize the information accumulated concerning the supermolecular structure of modified cellulose samples. We will deal mabfly with cases of modification of natural cellulose and cellulose hydrate by grafting, crosslinking, partial replacement of functional groups and activation of cellulose for ester formation. In our investigations we used cotton fibres or cotton fabrics, cotton cellulose, various activated samples, derivatives of this cellulose and wood cellulose before and after activation, crosslinked and graft samples prepared from cotton cellulose and cellulose hydrate, polynose and cord fibres made by various firms with different physical and mechanical properties. We used electron microscopy and auxiliary methods of structural analysis and thermodynamics for our investigations. Results of many years' studies show that there is a regular correlation between many chemical and physico-chemical transformations in cellulose materials and the corresponding structural changes taking place in them. STRUCTURAL CHANGES TAKING PLACE DURING CROSSLINKING
It was found that during graft copolymerization of cellulose with several vinyl monomers many physical and chemical characteristics improve: cellulose fibres are easily coloured, their resistance to microorganisms, chemicals, and heat increases, the cellulose fabric becomes crease-resistant, etc. [1-3]. It should be pointed out that chemical transformations of cellulose are accompanied by structural changes which may take place both at the molecular and supermolecular levels [4-11]. It is shown that chemical combination of foreign cellulose macromolecules markedly alters the mutual packing of cellulose chains and this results in a variation of the shape and dimensions of structural elements and layers of the secondary wall of the cotton fibre. It was found, for example, [4-8] that a change is observed in the structure of transverse and longitudinal ultra-thin cotton fibre sections before and after grafting to it several vinyl polymers. We investigated several graft copolymers prepared from cellulose and polyaerylonitrile (PAN)--the weight increase was 12.5%: poly-2-vinylpyridine (PVP)--increase in weight 16%; polyvinylidene chloride (PVDC)--weight increase 14%; polymethacrylamide (PMMA)--increase in weight 56%; polymethaerylic acid (PMA)--increase in weight 54%; polystyrene (PS)--increase in weight 3.4%; 13.4% polyvinyltoluene (PVT)--weight increase 6.9; 14.1 and 27.8 ~o, etc. To obtain these samples e°Co y-radiation was used with a rate of 70 rad/sec. The irradiation doses were not more than 1 Mrad, at which cellulose does not
554
KH. U. Usm~.~ov
undergo considerable change. (See papers [1-3] for details of conditions of preparing these samples, their physical and chemical properties and other information). Figure 1 shows electron microphotographs of the surface structure of cotton cellulose PVT graft copolymers with different weight increases. The type of change in the surface structure of cellulose was shown as the degree of grafting of PVT increases: folds in the surface structure gradually disappear and the fibre surface becomes structurally homogeneous. When grafting PMMA the surface structure of the cotton fibre becomes slightly different. During fragmentation of the copolymer layers with high ordering are observed. This is apparently due to the fact that owing to grafting of synthetic polymers to cellulose, forces of adhesion (microfibril, fibril) increase. The position is somewhat different for PMA [8]. Although the folds in the surface structure of the cotton fibre are completely retained, owing to the considerable extent of grafting of the polymer, these folds are smoother. The fibrillar structure of this fibre is clearly seen in the form of large fragments of fibrillar layers. For PMA the fibrils forming layers of the secondary fibre wall, in contrast to the graft PMMA sample, are not elongated considerably. In places they go over to adjacent sections. It can hence be concluded that forces of adhesion of fibrillar formations in the lateral direction are less marked for PMA than in grafting PMMA. These results indicate that the surface structure of cotton fibres differs according to the type of vinyl polymer grafted to it. Furthermore, a general feature of all the cases studied is the fact that during graft copolymerization of cotton cellulose with vinyl monomers the steric arrangement of chains of the graft synthetic polymer in cellulose varies according to the conditions of copolymerization. When this process takes place from the vapour phase and prior to grafting, the fibre does not swell (PAN, PVP), and macromolecules are grafted to cellulose mainly on the surface of structural elements of the fibre, on microfibrils and fibrils. Results obtained during the investigation of graft cellulose samples with PVDC, PMMA and PMA [8] using swollen cellulose suggest that the structural changes of cellulose are most apparent under these conditions. In these samples the surface structure of the fibre and its sections with fibrillar packing (secondary wall) are modified more markedly. Hence it follows that in the case of grafting from solution the structural elements of cellulose in solution swell markedly, and consequently even those cellulose chains which are arranged inside the structural elements can take part in graft eopolymerization, since in this case they are more accessible to the grafting material. It is interesting to explain the structural changes of cotton cellulose according t o the degree of grafting of the same vinyl polymer. We examined fibres grafted with PVT with different weight gains (Fig. 1). An increase in. weight gain of PVT in cotton cellulose considerably influences the nature of fibrillation. With considerable weight gains fibrillation of the graft
Structural properties of modified cellulose
555
FIG. 1. Microphotographs showing replicas of the surface of cotton fibres grafted with PVT; weight gain 6.9 (a); 14.1 (b) and 27.89/00 (c). Fro. 2. Microphotograph of an ultra-thin cross section of a cellulose-PMA graft copolymcr (weight gain 549/oo). F l a . 3. Microphotographs of ultra-thin cross sections of cyanoethylated fibres: a - - i n i t i a l cotton fibre; b - - p r e v i o u s l y treated with sodium hydroxide; c - - w i t h a glycerine-water mixturo (1 : 1).
$56
Km U. USM~WOV
cellulose considerably deteriorates, which is particularly marked with a weight gain of 27.8~. This may be explained by the fact that grafting PVT to cellulose reduces its hydrophilic nature. With an increase in the degree of grafting and with a gradual increase in its hydrophobic nature during fragmentation in water, the clear fibrillar structure of graft cellulose with PVT disappears especially with considerable weight increase. Our investigations indicate that the method of ultra-thin segments can be successfully used to observe the structural behaviour of cellulose. Figure 2 shows as an example an electron microphotograph of an ultra-thin copolymer section prepared from cellulose and PMA. It can be seen in the Figure that layers of the secondary wall of the graft fibre are evenly distributed. This typical pattern also characterizes other graft monomers and various degrees of grafting, which points to the uniform distribution of grafting in the fibre section. It may be concluded from the foregoing that grafting of various vinyl polymers to a cotton fibre has a variable effect on the surface structure and the structure of internal fibrillar sections of the cotton fibre. The degree of structural change in the fibre depends both on the type of polymer to be grafted and the degree of grafting, and on the conditions of obtaining the graft sample, i.e. whether grafting takes place from the vapour phase or in solution. When grafting takes place from the vapour phase, particularly in the case of small weight gains, the same cellulose macromolecules which are arranged on the surface of structural elements-mierofibrils and fibrils--ti~ke part in graft copolymerization. If grafting is preceded by swelling of cellulose in the monomer, or in solution, those macromolecules which are arranged inside the fibrils and microfibrils take part in the reaction. By studying dispersion of a graft cellulose sample some of its properties, e.g. its hydrophobic nature can be evaluated. Finally, a study of ultra-thin graft copolymer sections of cellulose provides valuable information concerning macrostructural changes in cellulose during graft copolymerization. In addition, from an examination by an electron microscope of the transverse and longitudinal copolymer sections the type of grafting (regularity or irregularity) in the fibre section can be determined. STRUCTURAL CHANGES IN ACTIVATION AND ESTERIFICATION OF CELLULOSE
It is now known that to increase the activity of natural cellulose in various chemical reactions--esterification, graft copolymerization, etc.--the supermolecular structure has to be loosened, in order to ensure full accessibility of the fibre to the effect of reagents. Methods of structural modification of cellulose recently developed, which ensure uniform reaction in the entire fibre volume, are therefore of considerable importance. Cotton fibres were therefore activated with sodium hydroxide, inclusion carried out with isoamyl alcohol and boiling in a glycerine-water mixture; the
Structural properties of modified cellulose
557
same samples and for comparison the initial cotton fibres were subjected to cyanoethylation (Table). The ester yield indicates the extent of accessibility of cellulose to cyanoethylation. The cotton fibre m a y undergo considerable changes even during preparatory physico-chemical treatment [12]. Typical signs of these changes in cotton cellulose during interaction with alkali are the increase in diameter, reduction in length and change in the shape of the fibre; from being twisted it becomes cylindrical and the transverse section becomes almost circular. Microscopic observations suggest that inclusion of cotton fibres with isoamyl alcohol after alkaline treatment intensifies swelling and therefore increases the fibre diameter even more. The form of the fibre during inclusion, compared with alkaline treatment, does not change substantially [13]. It was established that after treatment with a glycerine-water mixture the average diameter of the cotton fibre noticeably increases; however, the shape remains unchanged and the twisted condition is maintained. CrA~OET~C~LLVLOSE
~LD
AFTER ~ A ~ T
Type of treatment
Initial cotton cellulose Cotton cellulose treated with an 18~ sodium hydroxide solution Same with subsequent inclusion with isoamyl alcohol Cotton cellulose treated with a glycerinewater mixture
Nitrogen content in preparation, % of cyanoethylated products 2"8 5"0 7.5 3"2
Changes in the width of individual microfibrils were ~ 100 A for the initial (untreated) cotton fbre, after alkaline treatment ~ 150 A and finally, after iuclusion, 170 A [14, 15]. I t m a y be concluded that structural changes in cotton cellulose, dependent on alkaline treatment, are subsequently not only stabilized during inclusion with isoamyl alcohol, but are noticeably intensified. After inclusion the folded fibre surface changes considerably. Folds are almost absent. Fibrils are not observed either, owing to the strong swelling of the structural elements of cellulose [14]. Hence it follows that treatment of cotton cellulose with sodium hydroxide and the inclusion process cause intrafibrillar swelling. This is also suggested b y results of X-ray investigations [15]. Quite a different pattern was observed during the treatment of cotton cellulose with a glycerine-water mixture. Results of investigations indicate that
$58
K~. U, U s ~ o v
FIO. 4. Microphotographs of ultra-thin cross sections of an initial fibre (a) and a fibre crosslinked with acrolein (b). FIG. 5. The effect of preliminary swelling (treatment with a quaternary base) on the structure of initial fibre (a) and cotton fibre crosslinked with DMM (b) (ultra-thin sections). FIG. 6. Microphotographs of dense fibre sections before (a) and after crosslinking with cyanur chloride (b) in the swollen state. o n l y loose (interfibrillar a n d interlayer) sections of cellulose are s u b j e c t to t h e effect of swelling. As a consequence, fibrillar f o r m a t i o n s a n d layers s e p a r a t e . T h e d i m e n s i o n s o f i n d i v i d u a l microfibrils are t h e s a m e as those o f t h e initial c o t t o n fibre, i.e. ~ 100 A. T h e s t r u c t u r a l l y modified cellulose sampleh were c y a n o e t h y l a t e d . E l e c t r o n m i c r o s c o p e studies of t h e initial a n d c y a n o e t h y l a t e d ( w i t h o u t a c t i v a t i o n ) c o t t o n fibre indicate t h a t t h e s t r u c t u r e o f the t r a n s v e r s e section is
Sbruetural properties of modifiedcellulose
559
loose (Fig. 3a). Layers of the secondary wall are separated by cavities. On the other hand, cyanoethylated cotton fibre samples which had previously been activated with sodium hydroxide or included with isoamyl alcohol have a dense structure of sections; large interfibrillar cavities are absent (Fig. 3b), the laminar microstructure disappears. This is due to the fact that cellulose fibres which have a loose structure are cyanoethylated. The marked accessibility of cellulose to the reagent increases cyanoethylation. The position is quite different in the case of activating cellulose with a glycerine-water mixture, followed by cyanoethylation (Fig. 3c). It should be assumed that since with this treatment only interfibrillar (interlayer) loosening takes place in the fibre and the microfibrils remain unaffected, mainly those microfibrils which are arranged on the surface of macrofibrils or their units will take part in the reaction. Thus, the structural mechanism of activation of cellulose was shown by electron microscopy. It follows from our results that the structure of cyanoethylated cotton fibres depends entirely on their initial physical structure. If a certain type of structural conversion--intrafibrillar or interfibrillar loosening--takes place in the fibre, this is also reflected by the fibre structure after chemical modification. STRUCTURAl CHANGES IN CROSSUNKING
Numerous investigations of cellulose materials treated by various reagents which form crosslinks lead us to several significant conclusions concerning the mechanism of modifying these materials at a supermolecular level [16-19]. Particular attention was given to the following problems: l) the general regularities in changes taking place during crosslinking in the cellulose structure; 2) to what extent the structural changes depend on the supermolecular structure of a certain cellulose preparation 3) whether the crosslinking reagent is distributed evenly enough, or is localized in certain parts of the fibre 4) the effect on the structure of crosslinked cellulose of the type of modifying reagent, and conditions and degree of modification. Investigations, the results of which formed the basis of the conclusions drawn, were carried out using cellulose preparations of different supermoleeular structures. Formaldehyde, dimethylolurea (DMU), dimethylolethyleneurea, dimethylolthiourea, acrolein, hexamethylenedi-isocyanate, diepoxide, epichlorohydrin (ECH) and cyanur chloride were used as modifying reagents. In addition to the bifunctional compounds mentioned, which erosslink cellulose [20, 21], some monofunctional analogues--monomethylolurea (MMU), monomethylo]thiourea, phenylisocyanate, propylene oxide were used. Cellulose preparations containing linear ~ n d crosslinked polymers, chemically uncombined with cellulose, were also investigated. A most general structural feature of cellulose preparations subjected to erosslinking is the marked increase in thickness of the structural elements and the absence of fibrillation on intensive ultrasonic irradiation. Another significant
560
K~. U. U s ~ o v
property is the noticeable reduction in dimensions of structural elements in proportion to the increase of crosslinking and the formation of a large number of fine splinter-like particles. It is evident that crosslinking begins and takes place most vigorously in the most accessible parts of cellulose, in the interlayer sections. Inter- and intrafibrillar ranges are then subjected to crosslinking, which increases the structural elements. Crosslinking of microfibrils inside the layers prevents fibrillation during mechanical or acoustic treatment of fibres. When a considerable number of crosslinks are formed in cellulose fibres, the possibility of mutual transfer of structural elements is greatly restricted. Accessible ranges, where crosslinking takes place very vigorously, are completely transformed into a three-dimensional reticular structure. As a result, brittle fracture of fibres takes place during dispersion with irregular distribution of load and not the breakdown of fibres along the natural interface, which results in the formation of a large number of fine particles. The physical and mechanical properties of fibres vary accordingly: elongation markedly decreases, strength and fatigue properties deteriorate, but the ability to recover after deformation as a result of the increase of elasticity and delayed elasticity markedly improves. Investigations of crosslinked fibres using ultra-thin sections show a noticeable increase in the dimensions of structural elements (ends of fibrils), compared with unmodified samples (Fig. 4a, b). Thus, an increase in the transverse dimensions of structural elements is one of the typical features of crosslinking. The supermolecular structure of the initial cellulose has a considerable effect on structural changes during crosslinking. I t can be stated that the more individual are the structural elements and the more different levels of supermolecular organization are contained in the material, the more marked are the structural changes after crosslinking. All the structural properties of crosslinked samples described are therefore most noticeable for natural cellulose, which has a clearly expressed fibrillar-laminar, heterogeneous structure. For cellulose hydrate fibres structural changes in crosslinking are clearly observed in standard polynose fibres, the structure of which is similar to the structure of cotton cellulose. I n the case of a cord fibre, where there is no developed fibrillar-layer structure [22], crosslinks are evidently formed in the loosest parts and in the intermediate zones between them and with different forms of molecular ordering. Consequently, a structureless mass is crosslinked with the more ordered ranges and can hardly be seen in the photographs. The same is observed with dispersed viscose silk preparations. I f the fibre has a dense homogeneous structure with a low content of structurally organized materials crosslinking does not cause any noticeable structural changes. This is typical of high-strength fibres, formed in concentrated acid. Investigations of ultra-thin fibre sections and the use of preliminary swelling before obtaining the sections suggest that crosslinking takes place fairly evenly in the fibre volume allowing for structural heterogeneity. This is confirmed by
Structural properties of modified cellulose
561
homogeneous etching of sections in solvents and in a gas discharge and by a uniform reduction in the degree of swelling in the fibre section (Fig. 5). It is relevant to deal here with the effect of the conditions of crosslinking cellulose on the structural changes. Conventional crosslinking conditions involve the impregnation of the fibre with aqueous solutions of reagents and crosslinking at increased temperature. These conditions ensure a fairly uniform process. However, if the process is carried out in an organic solvent, without causing swelling of cellulose, the reagents m a y be localized on the interface of structural fibre elements (e.g. modification with acrolein in ethyl ether). Changes in the structure and properties of cellulose during modification in the swollen state [23] are of special importance. No increase is observed in the thickness of fragments and several other typical structural featttres of crosslinked cellulose are retained. It is evident that crosslinking mainly takes place in the lateral direction of the structural elements, whereas in the axial direction these elements are separated by considerable distances as a consequence of strong swelling (fibre swollen in alkali is used). Swelling facilitates the access of reagents to the internal regions of microfibrils, and consequently t h e y are subject to intensive crosslinking. Here, as with grafting, the diameter of the structural elements increases several times (to 400-500 A, compared with 80-100 A for the initial cellulose) and periodicity appears with an interval of 650 A. Similar structural changes are observed during the treatment of cotton with ECH and cyanur chloride. Thus, crosslinking of cellulose in the swollen state mainly results in the formation of intra- and interfibrillar crosslinks with retention of part of the liquid causing swelling of cellulose. Consequently, the elastic properties of the fibre noticeably increase during moistening. The type of crosslinking reagent has a certain effect (although less marked than the process conditions) on the structural changes of cellulose. Most effective are the reagents with a symmetrical molecule and identical functional groups not forming products of polymerization. F o r example, during crosslinking of CH20 ~ 1 ~o weight gain is sufficient for a sudden change in the structure and properties of cellulose. It m a y be conjectured that in this case cellulose contains about one crosslink per 10 glucose residues. During the modification of DMM, where various polymerization processes take place as well as crosslinking, with weight gains of 8-10% the sample contains one crosslink per 15-20 glucose residue. Finally, acrolein, which has different functional groups and readily forms homopolymers, is even less effective and weight gains in excess of 10% are necessary for a noticeable change in the structure and properties of modified fibres. An increase in the molecular weight of the reagent (diepoxide) markedly reduces the efficiency of crosslinking and localizes crosslinking in the surface layers of fibres. Interesting results were obtained when investigating the densest parts of modified fibres. It is shown that the length of dense sections after modification
562
Kg. U. USM~OV
with CH~O, DMM and with acrolein somewhat decreases compared with the initial length. When cellulose is modified in the swollen state the length of dense parts decreases several times, which is convincing evidence of the possible penetration of the modifying reagent in the dense parts of microfibrils (Fig. 6). The possibility of crosslinking of ordered parts was confirmed experimentally by their separation, treatment with CH~O and subsequent evaluation of swelling in alkali [24]. An analysis of X-ray photographs of crosslinked preparations and mechanical mixtures of cellulose with homopolymers shows [25] that a reduction in ordering m a y be due to the modification of structure (actual) and to the accumulation of polymerization products (apparent). In the first case a reduction is observed in the intensity of the 002 reflection (20--=22.4°), while in the second--"amorphons" scattering increases at 20= 18°. Thus, X-ray results confirm the conclusions derived b y electron microscope studies concerning the possibility of crosslinking in dense parts of cellulose. The density of modified cellulose varies slightly compared with the initial density; fibres crosslinked in the swollen condition are characterized b y a sudden drop in density, which proves a considerable structural loosening. Study of the structure of cellulose preparations modified b y mono-functional analogues of the compounds described showed that in this case no noticeable changes took place in structure or properties. The same can be said about preparations containing linear and crosslinked polymers, chemically uncombined with cellulose. Thus, the main structural changes during modification of cellulose with bifunctional reagents are due mainly to the formation of erosslinks and not to the accumulation of products of polymerization or monofunctional substitution. Translated by E. SEMERE
REFERENCES
1. Kh. U. USMANOV and U. AZIZOV, J. Polymer Sci., C, 579, 1964 2. Kh. U. USMANOV, Cellulo~a si Hirtie 14: 363, 1965 3. Kh. U. USMANOV, U. AZIZOV and M. U. SADYK0V, Sb. Radiatsionnaya khimiya polimerov (Radiation Chemistry of Polymers). p. 153, Izd. l~auka 1966 4. K. Kh. RAZIKOV, Kh. U. USMANOV and U. AZIZ0V, Proc. III Europ. Confer. Electr. Mier. Prague, A, p. 409, 1964 5. K. Kh. RAZIKOV and Kh. U. USMANOV,Vysokomol. soyed. 8: 387, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 3, 421, 1966) 6. K. Kh. RAZIKOV and Kh. U. USMANOV, Vysokomol. soyed. BP: 742, 1967 (Not translated in Polymer Sci. U.S.S.R.) 7. K. Kh. RAZIKOV, I. I. ISAMIYKHAMEDOVAand Kh. U. USMANOV, Sb. Soveshchanie po radiatsionnomu modifitsirovaniyu polimerov (Conferonce on Radiation Modification of Polymers). p. 60, Nauka, 1963 8. Kh. U. USMANOV, K. Kh. RAZIK0V et al., Mezhdunarodnyi simpozium po makro. molekulyarnoi khimii (International Symposium on Macromoleeular Chemistry). p. 165, Budapest, 1969 9. M. L, R01.IJEgS, A. T. MOORE et al., Amer. Dyestuff. Reporter 54: 36, 1965
Thermodynamics and morphology of phase transformations in polymers
563
10. R. M. LIVSHITS, T. S. SYDYKOV and Z. A. ROGOVIN, Cellulose Chem. Technol. 2: 3, 1968 l l. Z. A. ROGOVIN, T. S. SYDYKOV, R. M. LIVSHITS, M. V. SHABLYGIN, and N. V. MIKHAILOV, Vysokomol. soyed. B10: 46, 1968 (Not translated in Polymer Sci. U.S.S.R.) 12. K. Kh. RAZIKOV, E. D. TYAGAI, V. I. SADOVNIKOVA and Kh. U. USMANOV, Vysokotool. soyed. A l l : 1717, 1969 (Translated in Polymer Sei. U.S.S.R. 11: 8, 1948, 1969) 13. E. D. TYAGAI, et al., Vysokomol. soyed. B1O: 301, 1968 (Not translated in Polymer Sci. U.S.S.R.) 14. K. Kh. RAZIKOV, E. D. TYAGAI, P. P. LARIN and Kh. U. USMANOV, Vysokomol. soyed. A9: 393, 1967 (Translated in Polymer Sci U.S.S.R. 9: 2, 442, 1967) 15. K. Kh. RAZIKOV, E. D. TYAGAI, Yu. T. TASHPULATOV and Kh. U. USMANOV, Zh. prikl, khimii 41: 2706, 1968 16. G. V. NIKONOVICH, S. A. LEONT'EVA and Kh. U. USMANOV, Khimich. volokna, No. 6, 55, 1963 17. G. V. NIKONOVICH, S. A. LEONT'EVA and Kh. U. USMANOV, J. Polymer Sei. C16: 877, 1967 18. G. V. NIKONOVICH, S. A. LEONT'EVA and Kh. U. USMANOV, Vysokomol. soyed. A10: 2682, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 12, 3112, 1968) 19. M. ROLLINS, A. MOORE and V. TRIPP, Text. Res. J. 33: 117, 1963 20. It. TOVEY, Text. Res. J. 31: 185, 1961 21. A. SMITH, J. Soc. Dyers and Co]ourists 77: 416, 1961 22. G. V. NIKONOVICH, N. D. BURKHANOVA, S. A. LEONT'EVA and Kh. U. USMANOV, Cellulose Chem. Teehnol. 2: 231, 1968 23. G. V. NIKONOVICH, S. A. LEONT'EVA, V. P. SHATKINA, Kh. U. USMANOV, A. A. ~DYLOV and Yu. T. TASI-IPULATOV, Vysokomol. soyed. 7: 2132, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 12, 2337, 1965) 24. G. V. NIKONOVICH, S. A. LEONT'EVA and Kh. U. USMANOV, Struktura i modifikatsiya khlopkovoi tsellyulozy, 3, Tashkent, 1966 25. G. V. NIKONOVICH et al., Vysokomol. soyed. A10: 960, 1968 Translated in Polymer Sci. U.S.S.R. 1O: 4, 1118, 1968)
THERMODYNAMICS AND MORPHOLOGY OF PHASE TRANSFORMATIONS IN POLYMERS* S. YA. FRENKEL' a n d G. K. YELYASHEVICtt Institute of High Molecular Weight Compounds, U.S.S.R. Academy of Sciences (Received 5 October 1970)
ONE of the m a i n p r o b l e m s o f p o l y m e r science a t present is to develop a t h e o r y to give a q u a n t i t a t i v e description o f n u m e r o u s a n d varied experimental results which a t first sight often a p p e a r to be c o n t r a d i c t o r y . The m o d e r n t h e o r y of simple liquids gives a simple a n d a c c u r a t e description of the state of liquids at * Vysokomol. soyed. AI3: No. 2, 493-505, 1971.