Fibre Chemistry, Vol. 38, No. 1, 2006

THE LIQUID-CRYSTALLINE STATE OF CELLULOSE ACETATES: FROM PAST TO PRESENT. EXPERIENCE OF THE SARATOV SCHOOL* A. B. Shipovskaya and G. N. Timofeeva

UDC 547.458.82:532.783

Mesophasogenic solvents in the vapor state actively affect the structure of cellulose acetates of different morphological forms (fibre, film, bulk polymer). On the supramolecular level, this is manifested by initiation of orientation processes and on the conformational level by a change in the optical density, up to a change in the direction of rotation of the cholesteric helix to the opposite direction. The approach found creates conditions for formation of a given structure and three-dimensional organization of the polymer matrix for manufacture of materials with improved properties and new qualities.

Of the large number of polysaccharides, cellulose and cellulose derivatives are the most important in the total production volume of polymeric materials for industrial and domestic applications. This class of macromolecular compounds has been actively studied up to now both to obtain fundamental knowledge and for their practical use. Domestic scientists specializing in polymer science have made an enormous contribution to the study of the structure, composition, and properties of cellulose and its ethers. They include the internationally known scientists V. A. Kargin, Z. A. Rogovin, S. P. Papkov, P. V. Kozlov, A. A. Tager, E. Z. Fainberg, S. Ya. Fenkel’, L. S. Gal’braikh, V. E. Dreval’, M. V. Tsilipotkina, M. M. Iovleva, V. G. Kulichikhin, E. L. Akim, A. E. Chalykh, and others, as well as scientists from former Union republics: Kh. G. Usmanov, B. I. Aikhodzhaev, I. F. Kaimin’, R. G. Zhbankov, A. V. Bil’dyukevich, I. D. Atamanenko, M. T. Bryk, and others. The enormous amount of developments concerning the structural and physicochemical modification of cellulose and its derivatives allowed creating high-quality materials of the most varied applications: textile thread and medical fibres, protomembranes and membranes for ultra- and nanofiltration, films and filters, etrols, etc. To a great degree, these advances also belong to Saratov scientists, who have been working on natural polysaccharides for a long time. Professor S. A. Glikman began research in this direction. His monograph Introduction to the Physical Chemistry of High Polymers (1959) was one of the first publications on the study of phase separation in polymer—solvent systems. This direction has continued. Polysaccharides of plant and animal origin became the basic objects of investigation for many scientists at Saratov State University. The results of these studies are reflected, for example, in the candidate dissertations of V. M. Aver’yanova, The Structure of Acetylcellulose Solutions and Gels (1964); L. S. Gembitskii, Dynamic Optical Properties of Acetylcellulose Gels (1964), V. I. Klenin, Study of Supramolecular Particles in Acetylcellulose Solutions by the Turbidity Spectrum Method (1967); R. V. Kudashova, The Gelforming Capacity of Agaroid (1967); I. K. Doronina, The Nature of Carboxymethylcellulose Solutions and Gels (1969); I. I. Ryskin, Comparative Study of the Mechanism of Formation and Structure of the Solid Phase in Gel-forming Cellulose Triacetate and Secondary Acetate Systems in Benzyl Alcohol (1972); A. S. Buntyakov, Effect of Solvation of Functional Groups of Acetylcellulose Macromolecules in Solution on the Structure of Films (1972); Z. B. Komarova, Thermooxidative Transformations of Secondary Cellulose Acetate (1976); N. I. Panina, Study of Thermal and Mechanical Properties of Cellulose Acetate Systems (1977); G. N. Timofeeva, Physicochemical Study of Crosslinking during Phase Separation in the Cellulose ____________ *Based on proceedings of the memorial conference dedicated to the 100th birthday of Z. A. Rogovin (2005, Moscow). N. G. Chernyshevskii Saratov State University. Translated from Khimicheskie Volokna, No. 1, pp. 13-17, JanuaryFebruary, 2006. 0015-0541/06/3801-0017 © 2006 Springer Science+Business Media, Inc.

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Ññ , wt. %

L, % 160

a 9.0

b

2

1

1

140 3

4.5

2

3 120 4

0

for curve 1

2

4

6

8 t, min

25

50

75

100 t, min

100

for curve 2

σ, MPa 240

2 c

4

6 t, min

L, % 31 27

190

1 2

140

0

2

t, min 4

1

23 19

2 0

2

t, min

4

Fig. 1: a) Kinetic curves of sorption of NM vapors by cellulose acetates at 20°C (1 — fibre; 2 — powdered polymer; 3 — film); b) kinetics of spontaneous deformation (elongation—contraction) of acetate fibre in NM vapors at 25 (1), 30 (2), 40 (3), and 50°C (4); c) breaking strength σ and relative elongation at break l of acetate fibres as a function of treatment time in NM vapors at 20 (1) and 30°C (2). Acetate—Solvent System (1980); N. A. Fedyakova, Stereochemical Transformations in Manufacture and Processing of Cellulose Acetates (1991); in the doctoral dissertation by N. M. Ptichkina, Analysis of Phase and Extraction Equilibria in Polysaccharidecontaining Systems (2000), and others. The fundamental research of scientists of the Saratov school has been reflected in many publications and the applied developments are reflected in inventor’s certificates and patents. Against the background of this research, a new direction concerning the liquid-crystalline (LC) state of cellulose and its derivatives (G. N. Timofeeva), which began to be intensively investigated in the 1970s and is continuing to this day, began to be formed. Z. A. Rogovin, S. P. Papkov, and P. V. Kozlov were among the first to support this direction. In particular, Z. A. Rogovin invited G. N. Timofeeva to the annual conference on chemical fibre technology with the communication “The LC state of cellulose ethers.” The studies begun by G. N. Timofeeva were continued in the work of I. V. Fedusenko (candidate dissertation, Initiation of Orientation Processes in Cellulose Acetates by the Vapors of Low-molecular-weight Liquids, 1993) and A. B. Shipovskaya (candidate dissertation, Physicochemical Modification of Cellulose Acetates by the Vapors of Solvents Forming a Lyotropic Liquid-Crystalline Phase with the Polymer, 1996). We note that the possibility of realization of the LC phase in solutions and melts of semirigid-chain macromolecules such as cellulose ethers was first discussed in the Flory’s theoretical work [1, 2]. The effect of spontaneous elongation of cellulose ether films and fibres in a medium of specific liquids [3-5] and fibres and plastics on attaining the glass transition temperature during linear heating of the polymer [6-8] was the first direct evidence of formation of a nematic phase by cellulose derivatives. In solutions of cellulose derivatives, in aqueous solutions of hydroxypropylcellulose in particular, R. S. Werbowij and D. G. Gray were among the first to observe the LC state [9], identified the type of LC structure as cholesteric, and determined the pitch of the supramolecular helix and the specific optical rotation and the turning angle of one layer in the helix relative to another. An enormous number of studies of the LC state of cellulose and its derivatives from general theoretical positions then appeared in this area of polymer science. The fundamental causes of LC ordering were investigated, the physical properties 18

[α] 589 20 °C [α] 579 20 °C

[α] 589 20 °C Ññ , wt. %

Ññ , wt. %

t, min

Fig. 2: a) Change in the specific optical rotation of acetate films modified with NM (1) and TFAA vapors (2); b and c) specific optical rotation of solutions of cellulose diacetate in acetone—water mixture and cellulose triacetate in a mixture of ethylene chloride and ethanol vs. degree of absorption of TFAA (1), FA (2) and DMAA vapors (3) by the powdered polymer. 140

0.7

7

ÑPt , g/liter

30

0.7 10

ÑCh , mM

120

ÑÑa , mM

ÑNa , mM

130

50 0.8 ÑK , mM

0.8

5

3 2

6

10 Ññ , wt. %

Fig. 3. Results of filtration of human blood plasma through films formed from control CDA and CDA modified with vapors of a mixture of DMSO and water (10:90 ratio of components): 1) concentration of cholesterol (CCh), 2) protein (CPt), 3-5) potassium, calcium, and sodium (CK, CCa, CNa). The horizontal dashed lines indicate the CCh and CPt in the initial plasma from the patient. were measured and structural models were constructed, and correlations of the properties of the final products with LC structure with the conditions of processing the polymers [10], etc., were found. However, due to the high melting point, which is much higher than the thermal decomposition temperature, most cellulose derivatives cannot pass into the LC state in melts and only form lyotropic LC systems [9, 10]. In addition, the small group of solvents (including very aggressive solvents) in which the mesophase is realized, the necessity of using high concentrations of polymer and consequently preparing highly viscous solutions create difficulties in obtaining, identifying, and processing true LC systems. For this reason, despite the large number of cellulose ether—solvent systems experimentally characterized with respect to formation of the LC phase, this unique state was not widely used in practice. The above factors thus served as the stimulus for seeking other methods of realization of an ordered state in the polymer matrix. As a result of many years of studies of the nature of the LC state in solutions of cellulose and cellulose ethers, we found that acting on the structure of the polymer with vapors of mesophasogenic solvents, i.e., in which cellulose derivatives form a lyotropic LC phase, is one of the most effective methods of realizing a highly oriented state: for example, vapors of nitromethane (NM), trifluoroacetic acid (TFAA), dimethylsulfoxide (DMSO), dimmethylacetamide (DMAA), formic acid (FA), etc. Cellulose acetate fibres and films and the powdered polymer, used to fabricate fibres, filters, films for different applications, membranes, hollow fibres, etc., on the industrial scale, were investigated. Cellulose ethers were used to confirm the mechanisms established in other representatives of the class of natural polysaccharides: methyl- and ethylcellulose and a derivative of a polysaccharide of chitin animal origin, chitosan. 19

The character of the effect of mesophasogenic solvents on the structure of the polymer, whether in the form of film, fibre, or powder, is identical: the sorption curves are extremal (Fig. 1a) [11, 12], which indicates structural changes in the system [13]. Spontaneous elongation of acetate fibres and films in vapors (Fig. 1b) [14, 15], the new (not previously described in the literature) opposite effect of spontaneous contraction of elongated fibres in the same conditions (i.e., in vapors of a mesophasogenic solvent) [14], induced optical anisotropic in acetate films [15], decrease in angles of disorientation [14], potentiation of the intensity and number of reflections in diffractograms [16], increase in the physicomechanical properties of fibres and films (Fig. 1c) [16-18], etc., are an incomplete list of the properties acquired as a result of such modification which indicate initiation of orientation processes in the polymer matrix. The effect of vapors of active media obeys the “dose— effect” rule with the greater effect of small amounts — no more than 6-8 wt. %) — solvent vapors on the structure of the polymer. The effect of mesophasogenic solvent vapors is so large that it is accompanied by a change in not only the supramolecular structure of the cellulose ether but also the three-dimensional organization of the macromolecules [12, 15, 16, 19-21]. Even small amounts of absorbed modifier vapors create conditions for a change in the optical activity of the polymer and wide variation of the specific optical rotation [α] (Fig. 2). In addition, the polymer, which has a dextrorotatory coil with respect to the plane of polarization in the initial state, can change to the opposite direction of rotation (Fig. 2b, c). The polymer modified in this way serves as the basis in fixation for obtaining a special, controllable structure that effectively works not only due to the different pore diameter but also due to the three-dimensional packing of stereoisomers of different optical activity. Films and filters formed in a different stage of modification differ sharply in separatory and filtering properties, selectivity, hydrodynamic and gas permeability, ability to block UV rays, etc. [16, 21]. A high degree of dilution of an active solvent with water (up to 90-99%) does not reduce the intensity of the effect of vapors of binary mixtures on the structure of cellulose acetate and in many cases potentiates this effect, while preserving the effectiveness of small amounts of sorbate. We note that the content of pure solvent in the vapors of the mixture is very low and absorption of it by the polymer is even lower and is no more than 10-4-10-5% of the mass of the polymer [21]. The combined effect of vapors of a mixture of water and small amounts of a specific solvent also change the structure of the polymer on the fine conformational level. Biofilters and medical membranes with the unique ability to retain excess cholesterol (or bilirubin) while passing protein fractions in filtration of blood plasma from patients with hypercholesterolemia (or hyperbilirubinemia) in the required quantities (Fig. 3) were obtained with this method [20, 22]. The problem of manufacturing filters and membranes with such selectivity has been pressing in world medicine up to now [23]. It is pertinent to note that cholesterol and steroids, as well as cellulose ethers, are optically active substances and membranes formed from a polymer with an altered steric structure can selectively separate these components. Using this approach to structural modification of cellulose acetates on the stereomeric level resulted in a sorbent for cigarette filters that differs from the best world analogs by its high retention of such harmful components of tobacco smoke as phenol, resins, nicotine, ammonia, etc. [24]. And finally, recent studies have shown that phase separation in the cellulose acetate—mesophasogenic solvent system, including formation of the LC phase, is also correlated with a change in the steric structure of the polymer [25]. Phase diagrams involving optically active polymers, cellulose ethers in particular, are only limitedly included in the large number of studies of phase separation in polymers systems published in the world literature [26]. In addition, the information on the phase and physical states of the same substances from the class of natural polysaccharides is occasionally ambiguous and contradictory, as for the cellulose trinitrate—glycerin trinitrate system, for example. The fact that natural polymers can vary the optical rotation and direction of twisting of the cholesteric helix within wide limits due to the steric structure can perturb and not allow attaining thermodynamic equilibrium in the polymer—solvent system. Up to now, in plotting the cellulose acetate—solvent phase diagrams, it has not been considered that this polysaccharide, like any natural polymer, can be characterized by a different set of conformational states that differ from each other by the energy characteristics and thermodynamic parameters, which can also be reflected in the position of the boundary curves in the temperatureconcentration field of the phase diagram. Our approach to controlled monitoring of the structural changes in a polymer system and parallelly, the optical activity can also be very useful for solving this problem. In conclusion, we note that the mechanism of the change in the direction of rotation of the cholesteric helix of cellulose acetates will allow executing such transitions in other polysaccharides of both plant (methyl-, ethylcellulose [27]) and animal origin (chitosan [28, 29]), and in biopolymers. For example, it is possible to solve the biochemical problem of conversion of native DNA from dextrorotatory to levorotatory to evaluate the causes of such transitions and their possible consequences. 20

Research in this direction is continuing. Many years later, we want to thank our prominent scientists, including one of the authors of this article, who have in their time obtained good direction and international support: Z. A. Rogovin, S. P. Papkov, and P. V. Kozlov. Their monographs on chemical fibre and film technology are still the reference book for polymer scientists. Beginning in 2000, all research has been conducted with the financial support of the Russian Federation Basic Research Fund (projects 00-03-33058a, 03-03-33049a). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

P. J. Flory, Proc. Roy. Soc. A, 234, No. 1, 60 (1956). P. J. Flory, Proc. Roy. Soc. A, 234, No. 1, 73 (1956). T. G. Majury and H. J. Wellard, in: Int. Symp. on Macromol. Chem., Abstr., Rome-Milan-Turin (1954), p. 354. N. G. Bel’nikevich, L. S. Bolotnikova, et al., Vysokomolek. Soedin., 18B, No. 7, 485 (1976). N. G. Bel’nikevich, L. S. Bolotnikova, et al., Vysokomolek. Soedin., 20B, No. 1, 37 (1978). B. A. Fomenko, L. P. Perepechkin, et al., Vysokomolek. Soedin., 11A, No. 9, 1971 (1969). A. T. Kalashnik and S. P. Papkov, Vysokomolek. Soedin., 18B, No. 6, 455 (1976). O. A. Fridman, N. I. Naimark, et al., Vysokomolek. Soedin., 24A, No. 3, 512 (1982). R. S. Werbowij and D. G. Gray, Mol. Cryst. Liquid Cryst. Lett., 34, No. 4, 97 (1976). V. G. Kulichikhin and L. K. Golova, Khim. Drevesiny, No. 3, 9 (1985). G. N. Timofeeva, N. V. Protsenko, and I. V. Fedusenko, Khim. Volokna, No. 2, 13 (1989). A. B. Shipovskaya and G. N. Timofeeva, Vysokomolek. Soedin., 43A, No. 7, 1237 (2001). A. Ya. Malkin and E. A. Chalykh, Diffusion and Viscosity of Polymers [in Russian], Khimiya, Moscow (1979). G. N. Timofeeva and E. V. Tolkunova, Vysokomolek. Soedin., 28A, No. 4, 869 (1986). G. N. Timofeeva, I. V. Fedusenko, et al., Vysokomolek. Soedin., 37B, No. 6, 1093 (1995). A. B. Shipovskaya, G. F. Mikul’skii, and G. N. Timofeeva, Zh. Prikl. Khim., 77, No. 1, 152 (2004). G. N. Timofeeva, Z. D. Tul’guk, et al., RF Patent No. 1171580; Byul. Izobr., No. 29 (1985). A. B. Shipovskaya and G. N. Timofeeva, in: Proceedings of the International Conference Composite-2004 [in Russian], Izd. SGTU, Saratov (2004), p. 72. A. B. Shipovskaya and G. N. Timofeeva, Vysokomolek. Soedin., 45B, No. 1, 101 (2003). A. B. Shipovskaya, N. V. Evseeva, and G. N. Timofeeva, Zh. Prikl. Khim., 76, No. 9, 1553 (2003). A. B. Shipovskaya and G. N. Timofeeva, Kolloidn. Zh., 66, No. 5, 693 (2004). A. B. Shipovskaya, G. N. Timofeeva, and O. V. Osipova, RF Patent No. 2174140; Byul. Izobr., No. 27 (2001). F. Diederich and B. Peterson, Angew. Chim., 33, 1621-1624 (1994). G. N. Timofeeva and A. B. Shipovskaya, RF Patent No. 2223971; Byul. Izobr., No. 5 (2004). A. B. Shipovskaya, O. F. Kazmicheva, and G. N. Timofeeva, Structure and Dynamics of Molecular Systems [in Russian], 11th ed., Vol. 2, Izd. Kazansk. Un-ta., Kazan’ (2004), p. 334. A. E. Chalykh, V. K. Gerasimov, and Yu. M. Mikhailov, Phase Diagrams of Polymer Systems [in Russian], Yanus-K., Moscow (1998). A. B. Shipovskaya, E. S. Sveshnikova, and G. N. Timofeeva, in: Problems in the Rheology of Polymer and Biomedical Systems [in Russian], Izd. Sarat. Un-ta, Saratov (2001), p. 74. A. B. Shipovskaya, O. F. Kazmicheva, and G. N. Timofeeva, Structure and Dynamics of Molecular Systems [in Russian], Vol. 2, Izd. Instituta Fiz. Molekul i Kristallov UNTs RAN, Ufa (2002), p. 293. V. I. Fomina, N. A. Solonina, et al., in: Current Prospects in the Study of Chitin and Chitosan, Proceedings of the VII International Conference [in Russian], Izd. VNIRO, Moscow (2003), p. 367.

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