Effect of temperature on structure and transport properties of hydrogels based on regenerated cellulose

Structure and transport properties of hydrogels

1833

4. Modifitsirovannyye kremnezemy v sorbtsii, katalize i khromatografii (Modified Silica in Adsorption, Catalysis and Chromatography) (Ed. G. V. Lisichkin) Moscow, 1986 5. V. A. KLIMOVA, Osnovnyye mikrometody analiza organicheskikh soycdinenii, Moscow, 1975

Polymer Science U.S.S.R. Vol. 29, No. 8, pp. 1833-1840, 1987 Printed in Poland

0032-3950/87 $ 10.00 + .00 © 1988 Pergamon Press plc

EFFECT OF TEMPERATURE ON STRUCTURE AND TRANSPORT PROPERTIES OF HYDROGELS BASED ON REGENERATED CELLULOSE* L. YE. BROMBERG, A. R. RUDMAN a n d B. S. EL'TSEFON All-Union Research Institute of Polymers for Medicine (Received 27 March 1986) Structure of hydrogels based on regenerated cellulose and transport of non-electrolytes have been investigated at various temperatures. When the temperature of the surrounding solution is raised from 273 to 318-328 K the hydration H of the hydrogel decreases slowly. The apparent partition coefficient of urea between water and the swollen membrane varies between 0-5 and 0"9 within the temperature interval 293-323 K in accord with the changes in H. Interaction of urea with the cellulose matrix becomes stronger above 328 K. PROPERTIES o f h y d r o g e l s b a s e d on r e g e n e r a t e d cellulose ( C H ) have excited c o n s i d e r a b l e interest because o f their a p p l i c a b i l i t y as m e m b r a n e s for h a e m o d i a l y s i s , for prel~arative s e p a r a t i o n s in b i o c h e m i s t r y etc. T h e i r t r a n s p o r t p r o p e r t i e s r e p r e s e n t the m o s t sensitive m e a s u r e which reflects the h y d r o g e l s t r u c t u r e and a r e also decisive in the t e c h n o l o g y o f m e m b r a n e p r o d u c t i o n . W a t e r p r e s e n t in a swollen h y d r o g e n (its c o n t e n t a n d structure) plays an i m p o r t a n t role in the t r a n s p o r t o f c o m p o u n d s in C H [1]. It has been shown [2] t h a t e q u i l i b r i u m swelling o f cellulose in w a t e r is m a r k e d l y influenced b y t e m p e r a t u r e . The m e c h a n i s m o f this p h e n o m e n o n has been however investigated o n l y over relatively n a r r o w t e m p e r a t u r e intervals a n d is not entirely clear. M o r e o v e r , d a t a on s t r u c t u r a l r e a r r a n g e m e n t s t h a t p r o c e e d in C H m e m b r a n e s at different t e m p e r a t u r e s a r e c o m p l e t e l y lacking. In this p a p e r we Investigate the influence o f t e m p e r a t u r e on t r a n s p o r t c h a r a c t e r i s t i c s o f C H m e m b r a n e s a n d on the u n d e r l y i n g changes in the s t r u c t u r e o f the cellulose m a t r i x a n d on its swelling in water. Hydrogels based on water-swollen CH membranes Diacell and Ultracell were investigated; the membranes were prepared by the viscose process according to [3]. The degree of swelling in water was determined by weighing [4] after removal of glycerol from the membrane. The equilib* Vysokomol. soyed. A29: No. 8, 1669-1675, 1987.

1834

L. YE. BROMBERGet at.

rium hydration H was calculated as H=(pm/pw) (1-wdw~)

where pm(Pw) is the density (g/cma) of the membrane (of water), wa(w.0 is the weight of the swollen (dry) specimen. The density p,, was determined pycnometrically. Diffusion experiments (at zero pressure drop across the membrane) were carried out in a thermostatted cell [5]; the flowrate of the circulating liquids was kept sufficienly high to ensure the effect of the stagnant liquid films adhering to both faces of the membrane to be negligible. Diffusive permeability P was calculated from standard formulae [6]. Hydraulic permeability of membranes, L, was determined according to [5]. Concentration of urea and of vitamin B j2 (both chemically pure) was measured spectrophotometrically [3, 5] in samples kept at 298 K for 1 to 2 hr. The apparent partition coefficient 7 was defined as the ratio of equilibrium concentrations in the gel and in the surrounding solution as in [7]. IR spectra of gels were registered on the instrument UR-20 (GDR), diffraetograms were obtained with the instrument DRON-3M. The crystallinity index K was calculated according to Nelson and O'Connor [8] and Hermans [9]. The modulus of elasticity of swollen membranes was determined with the dynamometer Adamel Lomargi (France) [4]. Phase behaviour of CH was studied by means of a mercury dilatometer It0]. The change of specific volume, A~, was calcu)ated from the expression [1] 1

A ~ = ~ [ ( V - Vo)-QVm(T-313)], where M is the sample mass, Q (K -x) is the thermal expansion coefficient of mercury [I1], V (Vo) is the dilatometer reading at T (at 313 K), Vm is the volume of mercury at 313 K. Temperature was controlled with an accuracy of _+0.l K and all experiments were repeated 3 to 10 times. The results were processed by the least-squares method on a computer (ES-1055). Diffusion in polymer membranes is described in terms of the " p o r e " mechanism a n d / o r as diffusiort of a permeant dissolved in the polymer matrix. In the first case the rate of permeation is in essence governed by the size of microchannels (stationary or fluctuating) and by the size of the permeant molecule. T r a n s p o r t in C H membranes C u p r o p h a n prepared by the c u p r a m m o n i u m process apparently obeys the " p o r e " mechanism up to 320-330 K [6, 12, 13]. The molecular volume o f the p e r m e a n t is o f secondary importance for its dissolution in the matrix, and physico-chemical characteristics of the m e m b r a n e and of the diffusant play the decisive role here. This type of permeation is typical, e.g., for t r a n s p o r t o f non-electrol~tes in polyether-polyurethane films [13]. These mechanisms in their pure f o r m are bowever seldom oncountered and diffusion is usually governed by a mixed mechartism. W a t e r present in swollen gels, in particular in CH, plays an important role in diffusional t r a n s p o r t [2]. The tempdrature dependence of hydration H is plotted in Fig. 1 for C H membranes Ultracell and Diacell. In both instances the quantity H is seen to decrease slowly to some critical value within the interval between the freezing point and 318-328 K, but begins to drop rapidly at still higher temperatures. Similar temperature dependences o f swelling were observed in [2] for C H gels with a moderate degree of polymerization and also for hydroxyethylcellulose membranes. One might assume [1 ] that structm al rearrangements which take place in C H samples swollen at different temperatures are associated with a variation in the content and structure of the crystalline phase. The quantity K

Structure and transport properties of hydrogels

1835

was 0.8 to 0.9 in membranes Diacell und Ultracell swollen at room temperature, remained constant within + 1 0 ~ in membranes swollen at higher temperatures, and a small increase was detectable only at the boiling point of water, in agreement with literature data: the difference in K between cellulose samples in contact with water at ambient temperature ,~nd at the boiling point was found to reach only 5,°/o [14]. Thus, within the temperature interval between 273 and 373 K the interaction of CH with water does not influence appreciably the ratio of the crystalline and amorphous phases. According to X-ray data, heating of our swollen membranes did not lead to the transition from the mesoform 11 to mesoform IV either. Internal stress preserved in the membrane after its formation from solution can also substantially influence its swelling in water. Relaxation of internal stress lowers the content of water in voids [15]. In our experiments with oriented membranes the structural anisotropy (which should be strongly affected by internal stress) was appreciably influenced by changes in temperature. Thus, the initial modulus E along the draw direction was 150 and 81 MPa in membranes swollen at 293 and 373 K, respectively. When measured perpendicularly to the draw direction, the values of E at these two temperatures were 17 and 37 MPa. These results lead us to assume th.at the sudden decrease of equilibrium swelling at 318-328 K is due to relaxation of internal stress (i.e., to a thermally induced increase in packing density) rather than to enhanced crystallinity of the cellulose matrix. Figure 2 shows the temperature dependence of specific volume of a Diacell membrane swollen in water at 313 K; the individual curves differ in the number of successive cycles heating/

/

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FIG. 1 FIG. 1. Temperature dependence of hydration H of Ultracell (1) and Diacell membranes (2) and of the partition coefficient 7 of urea (3) in the system Diacell-water. FIG. 2. Change of specific volume A ~ of a Diacell m e m b r a n e as a function of tempelature: successive cycles heating/cooling; see explanation in text.

1-3-

L. YE. BROMBERG et at.

lg~

/slow cooling. The occurrence of an inclined plateau between 319 and 325 K in the first cycle gives evidence that the packing density increases within this temperature interval. A transition at 326 K (320 K) is observed during the second (third) cycle of the same specimen. We thus see that the increase in packing density in the first cycle is irreversible. Figure 2 also demonstrates that the coefficient of thermal expansion, given by the local slope of the curves, gradually decreases as the specimen is subjected to an increasing number of heating/cooling cycles. The reason for this is probably a disappearance of structural microdefects in CH during annealing. Such microdefects containing water and probably also dissolved air can substantially raise the thermal expansion of the gel. It is important that-according to the values of A~ measured at 293-310 K - t h e hydrated cellulose matrix at T>~313 K is already in a highly elastic state. This is supported by results reported for native cellulose and for hemicellulose [10]. Transitions similar to those observed in this work were found at 329 and 335 K for water-swollen samples of sulphite pulp [16]. Such transitions in polymeric hydrogels may be apparently attributed to anomalous changes in physical properties of water, in particular at 303, 318, and 333 K [16, 17]. These temperatures are characteristic for a change in the ratio of non-associated water molecules and molecules that exist in the form of clusters near the gel surface [10]. Since the coefficient of thermal expansion in these two forms of water differs by a factor of almost 9 [10, 17], these transitions can bring about a measurable change in thermal expansion of CH. The coefficient 7 characterizing partition between the gel and the surrounding medium is closely connected with the hydration H. The quantity 7 for vitamin Bt2, determined for our membranes at 293-323 K, and also in experiments with Cuprophane gels [12], was somewhat larger than H, demonstrating a specific interaction of vitamin Bt2 with hydrated cellulose. On the other hand, the activation energy U determined from the temperature dependence of diffusion coefficient of vitamin Bt2 (Fig. 3, line I)

InD.~Do o

IoS DM -2

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o

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-6.8

,

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Fro. 3 FIG. 4 FIG. 3. Temperature dependence of the diffusioncoefficientD M of vitamin B12 (1) and of urea (2) in a Diacell membrane. Fro. 4. Temperature dependence of the quantity In (D~Do).

Structure and transport properties of hydrogels

1837

was the same (21 kJ/mole) as for diffusion in pure water. Accordingly, the diffusion of vitamin B~2 in our membranes proceeds mostly in the water-filled regions. Values of y for urea (Fig. 1, curve 3) were practically constant over 293-323 K (con :idering the scatter of experinaental points) and the temperature dependence of this quantity mirrored the temperature dependence of H, indicating that the partitioning of urea takes place predominantly between water contained in the gel [20], similarly as in Cuprophane membranes [13, 141 and in poly(hydroxyethyl methacrylate) hydrogels [18]. ttowever, above 328 K the interaction between urea and the cellulose matrix becomes much stronger and at 371 K ?, is 3 to 5. A possible formation of stable carbamate cellulose dc~'ivatives under these conditions can be apparently dismissed [19]. Indeed, no ab:.orption bands near 1655 cm- ~, characteristic for associated amide bonds, were observed in IR spectra of our membranes equilibrated in boiling urea solutions and subsequently washed with distilled water. More probably, at higher tempelatures urea destroys the h)drogen bonds in the secondary structure of cellulose and itself forms hydrogen bonds of the type H

tt

I

I

C--OH...N--C--N...HO--C

I

II

I

H

O

H

(where C is cellulose) [19].

As expected, the increased interaction between urea and cellulose diminishes the diffusion coefficient D~a of urea in the membrane. The dependence of log DM on recipJ ocal absolute temperature is in Fig. 3 (curve 2). Between 293 and 328 K this dependence is represented by a straight line and its slope gives for the activation energy a value (U= 19 kJ/mole)in agreement with that for diffusion of urea in pure water. However, the dependence of log D~a on 1IT changes its character markedly at 328-333 K: DM begins to decrease and at 353 K attains a value of (4-6)x 10 -7 cm2/sec, indicating that the mechanism of diffusion in the hydrogel changes its character at 328-333 K. This is further supported by the data in Fig. 4 which gives the temperature dependence of log (DM/Do), where Do is the diffusion coefficient of urea in water, calculated from the Wilke-Chang equation [20]. The ratio D~a/Do remains constant at " 0 . 2 within the interval 293K~
DM~-1 --1 1 ...... x, In m-o/ p(l-~) /~(1-~)

(1)

L YE. BROMBEI~Get a~.

1838

rp,lOB crn

L.q,1013, cm

-03 27 ~ -0'5

4

9

25

7 I

t

310 a30 T,K 2.2 2.0 x "~ FIG. 6 FIG. 5 FIG. 5. The quantity [ln (DM/Do)]-1 as a function of x- t. FIG. 6, Temperature dependence of hydraulic permeability expressed as Lr/ in Ultracell (1) and Diacell membranes (2) and the equivalent pore radius rp as a function of temperature in a Diacell membrane (3); ra was calculated from mean values of parameters according to equation (2). with x=(1/H)- 1, a= V'}'/V'], B= V~/V'], and ~ = V~/V~j; V~' and V7 represent the free volume of the polymer matrix and of water contained in the membrane, V* is a volume parameter characterizing diffusion in the gel. The linear character of the dependence of In (D~/Do) on x - 1, observed in gels of different degree of swelling, was ascribed [18, 21,22] to the prevalence of the "pore" mechanism of diffusion in membranes of similar structure. The ratio DM/Do as a function of hydration H is plotted in coordinates of equation (1) as Fig. 5. The dependence of In (DM/Do) on x -~ is quite complex, with breaks at values of H corresponding to 323 and 333 K (indicated by arrows), and confirms the conclusion concerning the changed mechanism of diffusion within the investigated range of temperatures. It is important that the thermally induced variation in the hydrogel structure affects the hydraulic permeability with respect to dilute urea solutions. The temperature dependence of the quantity Lq (where r/is the dynamic viscosity of the solution) is plotted in Fig. 6 for the UltraceU (1) and Diacell (2) hydrogels. In both instances Lr/ does not change between 290 and 318 K, demonstrating viscous flow with no change in the hydrogel structure. Starting at 323 K the hydraulic permeability of the gels begins to decrease, apparently owing to an increase in packing density of CH, brought about by lowered hydration. Substantial dehydration with an ensuing fall of hydraulic permeability at elevated temperatures was reported [23] for propylene glycol monoacrylate hydrogels. The ratio of hydraulic (/5) and diffusive permabilities (P) of membranes varies strongly; in the case of transport through homogeneous gels this ratio characterizes the size of fluctuating pores. The equivalent pore radius, re, which describes the gel microstructure, can be calculated from the formula [24]

r~ = LDo 8q (1 - q)Z , Pt<

(2)

Structure and transport properties of hydrogcls

1839

where q = r s / r p (rs= 1.65 A is the Stokes radius of urea molecules), tc is a coefficient determined from h y d r o d y n a m i c considerations [25]. The q u a n t i t y rp, F l o t t e d against t e m p e r a t u r e in Fig. 6 for the Diacell hydro~el which has a h o m o g e n e o u s s t r u c t u r e [5], varies between 24 and 30 .~, in accord with data r e p o r t e d for C u p r o p h a n e gels [24]. The pore radius re increases m o n o t o n i c a l l y between 296 a n d 318 K, b u t begins to t'all rapidly within the interval 319-..325 K, a p p a r e n t l y because of the increased p a c k i n g density. A b o v e 325 K the q u a n t i t y P decreases m u c h m o l e rapidl3, t h a n L a n d leads to a rise in re (cf. e q u a t i o n (2)). In general, the t e m p e r a t u r e dependence of re agrees with the dilatometric curve (Fig. 2, curve 1). We note that between 325 a n d 370 K the radius rv increases while DM decreases. This a p p a r e n t c o n t r a d i c t i o n can be put aside by consideri~lg that the diffusion of urea proceeds not by the pore m e c h a n i s m but as a dissolution in the matrix of h y d r a t e d cellulose. The ~uthors express their gratitude to N. M. Kocherginskii for a helpful discussion of results and to N. N. K u z ' m i n for his help with the X-ray analysis.

Translated by M. KUBiN REFERENCES I. N. 1. NIKOLAYEV, Diffuziya v membranakh (Diffusion in Membranes). p. 232, Moscow, 1980 2. K, M. CHITUMBO, Doctoral Thesis, 79 pp., Uppsala University 1975 3. A. S. R Y A B C H E N K O , T. A. V Y S O T I N A , I,. I. TKACHENKO, N. A. VENGEROVA, B. S. EL'TSEFON, S. G. OSININ, V. M. IRKLEI, N. N. BEGICHEV and Yu. G. KOZLOV, Khim.farmatsev, zh., No. 11, 107, 1978 4. N, A. VENGEROVA, A. R. RUDMAN, B. S. EL'TSEFON, T. A. VYSOTINA, Ye. A. DEVIRTS z',nd I ~. N. BROVKINA, Khim.-farmatscv, zh., No. 10, 1246, 1984 5. N. A. VENGEROVA, T. A. VYSOTINA, T. M. SELINA, T. N. TARASOVA, B. S. EL'TSEFON, L. S. REIFMAN and V. N. GOMOL1TSKII, Khim.-farmatsev. zh., No. 7, 82, 1980 6. P. C. FARRELL and A. L. BABB, J..Biomed. Mater. Res. 7: 275, 1973 7. M. C. COLLINS and W. F. RAMIREZ, J, Phys. Chem. 83: 2294, 1979 8. M. L. NEI.SON and R. T. O'CONNOR, J. Appl. Polym. Sci. 8: 1311. 1964 9. P. H. HERMANS and A. WEIDINGER, Makromol. Chem. 44: 24, 196l 10. M, V. RAMIAH and D. A. I. GORING, J. Polymer Sci., No. 11, 27. 1965 11. M. AIZAWA and S. SUZUKI, Bull. Chem. Soc. Japan 44: 2967, 1971 12. C.K. COLTON, K. A. SMITH, E. W. MERRILL and P. C. FARRELL, J. Biomed. Mater. Res. 5: 459, 1971 13. G. M. ZENTNER, J. R. CARDINAL and S. W. KIM, J. Pharm. Sci. 67: 1352, 1978 14. H. J. MARRINAN and J. MANN, J. Polym. Sci. 21: 301, 1956 15. S. P. PAPKOV and E. Z. FAINBERG, Vzaimodeistviye tselyulozy i tselyuloznykh materialov s vodoi (Interactiota of Cellulose and Cellulose Materials with Water). p. 237, Moscow, 1976 16. J. KUBAT, Svensk Papperstidning 72: 731, 1969 17. W. DROST-H.~NSEN, Proc. Ist lnternat. Syrup. on Water Desalination, Vol. 1, p. 382, Washington, 1965 18. S. WISNIEWSKI and S. W. KIM, J. Membr. Sci. 6: 299, 1980 19. T. P. STARUNSKAYA, PhD Thesis, p. 140, VNIIVproekt, Leningrad, 1984 20. C. R. W1LKE and P. CHANG, AIChE J. 1: 264, 1955 21. H. YASUDA, C. E. LAMASE and A. PETERLIN, J. Polym. Sci. A-2, 9: 1117," 1971 22. H. YASUDA, L. D. IKENBERRY and C. E. LAMASE, Makromol. Chem. 125: 108, 1969

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B . E . KRISYUK et aL

23. M. F. R E F O J O , J. Appl. Polym. Sci. 11: 407, 1967 24. E. KLEIN, F. F. HOLLAND and K. EBERLE, J. Membr. Sci. 5: 173, 1979 25. A. VERNIORY, R. DUBOIS, P. D E C O O D T , J. P. GASSEE and P. P. LAMBERT, J. Gen. Physiolog. 62: 489, 1973

Polymer ScienceU.S.S.R. Vol. 29, No. 8, pp. 1840-1846, 1987 Printed in Poland

0032-3950/87 $10.00+ .00 © 1988 Pergamon Press pie

RATE OF SPIN EXCHANGE BETWEEN OXYGEN AND NITROXIDE RADICALS IN POLYOLEFINS MEASURED BY CONTINUOUS SATURATION EPR SPECTRA* B. E. K.RISYUK,YE. I. YUDANOVA,A. V. KUL1KOVand A. A. PoPov Department of the Institute of Chemical Physics, U.S.S.R. Academy of Sciences (Received 27 March 1986)

Rate constants of the spin exchange between nitroxide radicals and oxygen in oriented and isotropic samples of polypropylene, high density and low-density polyethylene were measured over the temperature range of 290-373 K by the method of continuous saturation of the EPR spectrum of the spin probe, and used to calculate oxygen permeability. The results agree with literature data for isotropic samples, but are smaller by 1.5 to 2.5 times upon orientation.

MANY chemical reactions of oxygen in polymers (e.g. oxidation of alkyl to peroxide groups) proceed in the diffusion-controlled regime where the rate constant depends on the collision frequency of the reaction centre with oxygen, which is in turn given by the product of solubility s and diffusion coefficient D of oxygen. The permeability sD of isotropic polymeric films is usually determined by traditional methods [1] that are however less suitable for oriented polymers containing many defects. In this paper we measure sD by a novel technique [2], where the quantity ske proportional to sD is obtained from the saturation curves of EPR spectra of nitroxide radicals ~/~N6 introduced into the polymer (ke is the rate constant of spin exchange between 02 and ) N ( ) ) . Two polypropylene (PP) samples, one with a broad and the other with a narrow molecular weight distribution, were investigated together with low density and high density polyethylene films (LDPE and HDPE). Orientation was accomplished by elongation in the thermostatted chamber of an Instron Tester. Characteristics of investigated polymers and conditions employed in sample preparation are collected in Table 1. 2,2,6,6-tetramethyl-4-acetamidopiperidine-l-oxyl was used * Vysokomol. soyed. A29: No. 8, 1676-1681, 1987.