t~ structure of polyoxyethylene solutions with lower ... - Science Direct

Structure of polyoxyethylene solutions 7. K. A. MAKAROV, L. N. VOROB'EV, A. F. NIKOLAYEV and Ye. SYUDA, Vysokomol. soyed. B10: 757, 1968 8. K. A. MAKAROV, G. M. TOMASHEVSKAYA, Ye. M. DYN'KIN and A. F. NIKOLAYEV, Vysokomol. soyed. A13: 2739, 1971 (Translated in Polymer Sei. U.S.S.R. 13: 12, 3081, 1971) 9. V. V. KORSHAK, A. M. POLYAKOVA, V. F. MIRONOV and A. D. PETROV, Izv. Akad. Nauk SSSR, Otd. khim, nauk, 178, 1959 10. M. M. KOTON and T. M. KISELEVA, J. Polymer Sci. 52: 237, 1961 11. C. E. SCOTT and C. C. PRICE, J. Amer. Chem. Soc. 81: 2670, 1959 12. Y. MINOURA and Y. SAKANAKA, J. Polymer Sei. A-l, 7: 3287, 1970 13. Y. MINOURA, Y. SAKANAKA and Y. SUZUKI, J. Polymer Sci. A - l , 4: 2757, 1966 14. Yu. L. SPIRIN, Vysokomol. soyed. AI0: 1699, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 8, 1967, 1968) 15. Kh. S. BAGDASARYAN, Teoriya radikal'noi polimerizatsii (Theory of Radical Polymerization) Izd. "Nauka", 1966 16. N. S. NAMETKIN, S. G. DURGAR'YAN and V. G. FILIPPOVA, Dokl. Akad. l~auk SSSR 177: 853, 1967 17. M. KURATA and A. TERADA, J. Polymer Sei. A - l , 4: 2989, 1966 18. G. M. TOMASHEVSKAYA, Dissertation, 1971

T~

STRUCTURE OF POLYOXYETHYLENE SOLUTIONS WITH LOWER CRITICAL TEMPERATURES OF MIXING* A. A. TAGER, S. A. VSHIVKOV, V. 1~. AI~TDREYEVAa n d T. V. SEKACHEVA A. M. Gor'kii Urals State University

(Received 11 October 1971) The phase equilibrium, light scattering and viscosity of solutions of polyoxyethylene (POE) in water have been studied. The molecular weights, second virial coefficients and size and concentration fluctuations of the macromolecules and particles causing light scattering have been calculated. I t is shown that for a system with a lower critical temperature of mixing, as for those with an upper critical temperature of mixing, the R~o~ f ( T ) curves contain a sharp peak corresponding to the temperature of phase separation (Tp.g.) of the system. I t was found that as Tp.s. is approached structure formation occurs in homogeneous solutions of POE, as a result of worsening of the thermodynamic quality of the solvent. This is manifested as a gradual increase in light scattering, in the size of the light-scattering particles and in fluctuations of concentration. MANy p o l y m e r - s o l v e n t s y s t e m s h a v e n o w been f o u n d t o h a v e a lower critical t e m p e r a t u r e of m i x i n g (LCTM) [1-15]. Studies of the t h e r m o d y n a m i c p a r a m e t e r s o f m i x i n g of these s y s t e m s in t h e t e m p e r a t u r e region below t h e LCTM, i.e. in t h e region o f single-phase solutions, has s h o w n t h a t as t h e LOTM is a p p r o a c h e d t h e * Vysokomol. soyed. A16: No. 1, 9-14, 1974.

8

A.A.

TAGE~t et aL

t h e r m o d y n a m i c s t a b i l i t y o f t h e s y s t e m s t e a d i l y worsens [16]. This indicates a c o n t i n u o u s c h a n g e in t h e s t r u c t u r e o f t h e solutions. I t was t h e r e f o r e of i n t e r e s t to discover b y m e a n s o f l i g h t - s c a t t e r i n g a n d v i s c o s i t y m e a s u r e m e n t s w h a t changes o c c u r in t h e solutions as t h e L C T M is a p p r o a c h e d . F o r this p u r p o s e we selected t h e p o l y o x y e t h y l e n e ( P O E ) - w a t e r s y s t e m , w h i c h h a s a n LCTM. EXPERIMENTAL

The materials used for this study were two samples of POE with ~l~w=l'4× 106 and 5>( 106, as measured by light scattering. X-ray analysis showed that these POE samples have a crystalline structure, the melting point of POE crystals being 66 ° [17]. The solvent used was water. Phase-equilibrium, light-scattering and viscosity curves of the system were studied. The water was distilled water that had been redistilled and had a refractive index in agreement with the literature. The solutions and solvents were freed from dust particles by the method of reference [8]. Phase separation curves were obtained by the method of Alekseyev [18] and the phase separation temperature (Tp.s.) was found by the method of reference [8]. The temperature and angular dependence of light scattering by the POE solutions in the range of angles of 30-130 ° was studied by means of a visual, circular nephelometer [19], using unpolarized green light (~= 5460/~). Benzene, with the high Rayleigh scattering coefficient of 16-5× 10 -6 cm -1 [20], was used as the calibrating liquid. The refractive index increment was measured in an IRF-23 refractometer. An Ostwald viseometer with a capillary of diameter 0"6 mm was used for measuring the viscosity of the POE solutions. The effiux time for water was 104-4 sec at 20% and for toluene 70"5 sec at 25 °. The experiments were conducted with solutions in the region of Newtonian flow, demonstrated by special experiments. RESULTS AND DISCUSSION

Phase equilibrium curves. T h e c o n c e n t r a t i o n d e p e n d e n c e of Tp.s. is i l l u s t r a t e d in Fig. l. A c u r v e o f t h e m u t u a l m i x i n g of P O E f r a c t i o n s of lower m o l e c u l a r w e i g h t is g i v e n for c o m p a r i s o n . Because o f t h e v e r y high v i s c o s i t y of a q u e o u s solutions o f P O E o f h i g h m o l e c u l a r w e i g h t we were u n a b l e to s t u d y solutions of c o n c e n t r a t i o n a b o v e 4 % . D e s p i t e t h e c o m p a r a t i v e l y n a r r o w r a n g e o f c o m p o s i t i o n s howev e r , it is seen f r o m Fig. 1 t h a t t h e P O E - w a t e r s y s t e m h a s a n LCTM, i.e. t h e s y s t e m s e p a r a t e s into t w o layers w h e n it is h e a t e d , a n d this occurs a t lower t e m p e r a t u r e s t h e h i g h e r t h e m o l e c u l a r w e i g h t of t h e p o l y m e r , which is in a c c o r d w i t h i n f o r m a t i o n in t h e l i t e r a t u r e [5]. Rayleigh scattering of light by POE solutions. T h e t e m p e r a t u r e a n d a n g u l a r d e p e n d e n c e o f t h e excess light scattering, Rp0, o f a solution o f t h e P O E s a m p l e w i t h ~tTw~l.4× 10 e was studied. A solution of c o n c e n t r a t i o n 0.88 g/dl w a s u s e d for this p u r p o s e . F i g u r e 2 shows t h a t t h e t e m p e r a t u r e d e p e n d e n c e of t h e light s c a t t e r i n g is e x p r e s s e d b y a c u r v e w i t h a s h a r p m a x i m u m , c o r r e s p o n d i n g to Tp.8. o f a solution o f t h a t c o n c e n t r a t i o n . As was s t a t e d in reference [8], this is due to change in t h e m i c r o - i n h o m o g e n e i t y o f t h e s y s t e m , w h i c h increases v e r y s h a r p l y close to Tp.s. a n d is s h o w n b y a v e r y m a r k e d increase in t h e i n t e n s i t y of light scattering. T h e fall in R~0 is due to p h a s e s e p a r a t i o n in t h e s y s t e m . T h e c h a r a c t e r i s t i c p e a k in the c u r v e of l~o----f(T )

Structure of polyoxyethylene solutions

9

"~'-1

120 ~ 2 l/0 I 8

I

I /6"

c , 9,1dl

FZG, 1. Phase-equilibrium curves of the P O E - w a t e r system at M = 5 × (2) and 5 × ]0 6 (3).

10 3 [2] (•); 1.4× l0 s

i n t h e r e g i o n o f T~.8. f o r p o l y m e r s y s t e m s w i t h a U C T M h a s b e e n o b s e r v e d b e f o r e [8, 12], b u t f o r s y s t e m s w i t h a n L C T M i t is s h o w n h e r e i n t h i s c l e a r w a y f o r t h e first time. tt

1~90"lO , o m -r

eoo i

35i-

jl

158

4

30

7O

llO ~°0

Fxo. 2. Temperature dependence of R~'o of an aqueous solution of P O E with M = 1,4 × 10' and c = 0 . 8 8 g/dl.

10

A.A. TAo~.~ e~ aL

Since the excess light scattering occurs in fluctuations of concentration we felt t h a t it would be of interest to calculate the size of these. For this we used equation (1), which is usually used for calculating the mean-square fluctuations in low-molecular liquids

R 0" (/fG)2=

(1)

2 V * ( d n 2 / d G ) 2,

where A is the wavelength of the light used and here equal to 5460 J~, V* a constant and equal to 10 -15 cm 3 and dn~/d~ the increment of the square of the refractive index. The variation with temperature of the mean square fluctuation of concentration of a POE solution (c=0-88 g/dl) is shown below. T, °C (JC) ~× 106, (g/cm3)~

30 1.0

40 1-8

70 3.3

I t is seen t h a t the size of the concentration fluctuation increases as the temperature is increased. I t m a y be supposed t h a t a temperature closer to Tp.s. the fluctuations will reach still greater values. Because of the experimental difficulty in determining refractive indices at high temperatures the concentration fluctuations close to Tp.6. were not calculated however. ~ESULTS OF MEASUREMENT OF I~GHT SCATTERING AND VISCOSITY OF AQUEOUS SOLUTIONS OF

POE T, °C

20 45 70

An × 10 4, crnS/g2/mole 2t~wX 10-'

6.0 4"8 3"0

1.4 2-4 4"0

(h~)~s., A

[r/], dl/g

1600 2300 2900

5.4 4.2 3"0

k'

0.23 0"43 0-65

(h2)~, J~

1530 1410 1260

The results indicate t h a t the scattering particles close to Tp.~. are not individual polymer molecules, but molecular aggregates. I n this connection it was of interest to find the size of the particles causing scattering and for this purpose the angular dependence of light scattering by the system was studied at three temperatures. The second virial coefficients and the size and molecular weight of the scattering particles were calculated by Zimm's double extrapolation method, using equation (2) Kc

1 = M P (0) +

where

Na'~4

(2)

11

S t r u c t u r e of p o l y o x y e t h y l e n e solutions

M is the weight of the light-scattering particle, P(O) the internal interference factor, A 2 the second virial coefficient and no the refractive index of the solvent. The calculated values are given in the Table, from which it is seen that for the P O E - w a t e r system A~ is positive and fairly large, and it decreases as the temperature is increased. These results indicate that in the range of temperatures studied water is a good solvent for P O E and its solvent power decreases as the temperature of phase separation is approached, resulting in increase in the size of the scattering particles (Table). This also causes the differences in the molecular-weight values calculated at different temperatures, which are usually found in polymer solutions in which association occurs [22, 23]. ~1/zo

/',,, ;I,;, A I0 ~ . . . o - . . - - ~ ~

2

~_..~

O'q

1200

I~

g

O'8

c, d/l#

G A~ ,ZO ~

FIG. 3

Fro. 4

FIG. 3. D e p e n d e n c e of ~sp/e for t h e P O E ( M = I . 4 × 10S)-water s y s t e m a t 20 ° (1); 40 ° (2); 60 ° (3) a n d 80 ° (4). Fro. 4. D e p e n d e n c e of t h e m e a n - s q u a r e e n d - t o - e n d distance of t h e p o l y m e r molecules on t h e second virial coefficient for t h e P O E ( M = 1.4× 106)-water system.

The high values of Mw indicate not the weight of individual molecules b u t of associations of molecules, and the lowest value of 1.4 × 10 6, obtained under conditions when the system is far from Tp.s. , is closest to the true molecular weight of this sample. Fiscosity of solutions. Figure 3 shows the dependence of reduced viscosity on concentration, described b y the Huggins equation

G

c,

(3)

where k' is the Huggins viseometric constant. The figures in the Table indicate good correlation between the values of A~ [~/] and k'. As the temperature is increased A 2 and [0] decrease, and k' increases, i.e. the relationships characteristic of solutions of polymers with flexible chains are found. The mean-square end-to-end distances of the chains (h~)~ were calculated b y

12

A . A . TAGER et al.

means of the F o x - F l o r y equation [19], using the above values of [7]

[,z]=

(4)

M~

taking ~ - 2 . 1 × 1021. The value ~¢w= 1.4 × l0 s was used in equation (4) for this calculation, assuming that this is closer to the true molecular weight of the POE. The values of (~2)~ obtained from equation (4) are in agreement with (h2)~s. at temperatures sufficiently remote from Tp.s. (Table). The difference between these values increases as Tp.s.is approached, the values of (h)Ls. -2 increasing with increase in temperature, while (hS)~ decreases. We fell that the differences between the absolute values of (~2)~. and (~2)~ are not chance differences but are due to the fact that from viscosity measurements we are determining the size of the molecular coils, but from light-scattering measurements the size of associations. This is supported by calculations of the unperturbed dimensions of POE molecular coils by the two methods. Figure 4 shows the dependence of (~2)~ on A2. Extrapolation of the straight-line graph to A 2 = 0 (a-solvent) gives (~2)~= 1000 ~x. For POE Mark and Flory [24] put forward the equation ()~2)~/'/M= 0.60 ( ± 0.06) ×

10 -16

cm 2,

(5)

from which, for Mw~--1.4 × 108, we obtain (h2)~=920 ~. These values are in agreement within the limits of experimental error. Thus the unperturbed end-to-end distance of the molecule of our sample of POE is ~ 1000 A. In a solution of POE in a good solvent (water) the size of the coils a n d ' t h e value of [t/] at 20 ° are greater than the values under ~-conditions, which is in accord with generally accepted views [19]. As the temperature is increased and thus as the thermodynamic affinity of water for POE decreases the coils become compressed and smaller, but even at 70 ° they do not decrease in size to the unperturbed dimensions. The mean-square end-to-end distances (h2)~s., calculated from light-scattering measurements at temperatures close to Tp.s. are in all instances greater than (~2)~, indicating that th.e light-scattering particles are of a supermolecular nature, i.e. that associations are present. The size of these associations increases steadily as Tp.s. is approached. At temperatures sufficiently far from Tp.~. of a solution of a given concentration the associations fall apart and the size of the light-scattering particles becomes close to the size of the molecular coils, as calculated from viscosity measurements. The explanation given here indicates that in the present instance the intrinsic viscosity is evidently insensitive to formations of supermolecular order that axe shown up by tlae light-scattering method. This conclusion was reached previously by Klenin, Frenkel' and their collaborators [25, 26] in studies of aqueous solutions

Structure of polyoxyethylene solutions

13

o f polyvinylalcohol. This is possibly e x p l a i n e d b y t h e fluctuating n a t u r e of t h e associations a n d hence b y the existence of a relationship b e t w e e n t h e i r " s e t t l e d life", i.e. t h e r a t e of t h e i r f o r m a t i o n a n d b r e a k d o w n , on the one h a n d a n d the t i m e scale of the e x p e r i m e n t on the other. T h u s t h e i n f o r m a t i o n o b t a i n e d indicates t h a t phase s e p a r a t i o n in the polym e r - s o l v e n t s y s t e m is m a d e r e a d y in the h o m o g e n e o u s solution, where associations are f o r m e d e v e n a t v e r y high dilutions. T h e results o b t a i n e d for this s y s t e m are in good a g r e e m e n t with its t h e r m o d y n a m i c characteristics, n a m e l y with the fact t h a t the P O E - w a t e r s y s t e m has negative values of the excess e n t h a l p y a n d e n t r o p y , i.e. H ~ < 0 and S E < 0 [2, 27, 28]. Consequently the t h e r m o d y n a m i c criteria for the existence of an LCTM are satisfied. This kind of v a r i a t i o n in e n t h a l p y a n d e n t r o p y could be b r o u g h t a b o u t b y i n t e r a c t i o n b e t w e e n the p o l y m e r a n d w a t e r molecules, resulting in t h e i r m u t u a l orientation, which is a c c o m p a n i e d b y decrease in e n t r o p y . The decrease in e n t r o p y resulting f r o m o r i e n t a t i o n of molecules of different t y p e s is e v i d e n t l y greater t h a n t h e e n t r o p y of disorientation resulting f r o m b r e a k d o w n of t h e P O E c r y s t a l lattice. T h e r e f o r e the t o t a l e n t r o p y S E < 0. T h e strong e n e r g y i n t e r a c t i o n between the P O E a n d w a t e r molecules indicates the occurrence of solvation due to h y d r o g e n bonding b e t w e e n the w a t e r molecules a n d the o x y g e n a t o m s in the P O E chain. T h e r e f o r e the s t r u c t u r e of dilute aqueous solutions of P O E can be r e p r e s e n t e d in the form of a highly c o n c e n t r a t e d , contin u o u s n e t w o r k , f o r m e d b y i n t e r a c t i o n of swollen molecular coils, w i t h i n a n d bet w e e n which there are molecules of water. The f o r m a t i o n of such ~ highly concent r a t e d , strong n e t w o r k is e v i d e n t l y the reason for the well k n o w n f a c t of quenching of t u r b u l e n c e in w a t e r b y t h e a d d i t i o n of small q u a n t i t i e s of P O E [29]. Translated by ]~. O. PHILLIPS REFERENCES

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14

A.A.

TAGER e$ al.

12. V. N. KUZNETSOV, V. B. KOGAN and M. S. BILESOVA, Vysokomol. soyed. A l l : 1330, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 6, 1509, 1969) 13. A. H. LIDDEL and F. L. SWINTON, Disc. F a r a d a y Soc. No. 49, 115, 1970 14. V. McANDREYEVA, A. A. ANIKEYEVA, S. A. VSHIVKOVA and A. A. TAGER, Vysokomol, soyed. B12: 789, 1970 15. A. A. TAGER, Vysokomol. soyed. AI3: 467, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 2, 531, 1971) 16. A. A. TAGER and L. V. ADAMOVA, T r u d y po khimii i khimicheskoi tekhnologii (Transactions in Chemistry and Chemical Technology). P a r t 2, p. 112, Gor'kii, 1972 17. P. H. GEIL, Polimernye monokristally (Polymer Single Crystals). p. 469, Izd. "Khim i y a " , 1968 (Russian translation) 18. V. F. ALEKSEYEV, Zh. russk, fiz.-khim obshch. 7: 56, 1875; 9: 208, 1877 19. V. N. TSVETKOV, V. Ire. I~SKIN and S. Ya. FRENKEL', S t r u k t u r a makromolekul v rastvorakh (Structure of Macromoleeules in Solution). Izd. " N a u k a " , 1964 20. M. I. SHAKHPORONOV, Metody issledovaniya teplova dvizheniya molekul i stroyeniya zhidkostei (Methods of Investigation of the Thermal Motion of Molecules and the Structure of Liquids). Izd. MGU, 1963 21. V. Ye. ]~SKIN and S. L. MAGARIK, Vysokomol. soyed. 2: 806, 1960 (Not translated in P o l y m e r Sci. U.S.S.R.). 22. I. Ya. PODDUBNYI, Ye. G. ~.RENBURG, Ye. P. CHERNOVA-IVANOVA and T. T. K A RTASHEVA, Dokl. Akad. N a u k SSSR 148: 384, 1963 23. N. G. ELIAS and R. BAREISS, Makromolek. Chem. 21: 53, 1967 24. J. E. M A R K and P. J. FLORY, 5. Amer. Chem. See. 87: 1415, 1965 25. V. I. I(LENIN, O. V. KLENINA and V. V. GALAKIONOV, Vysokomol. soyed. 8: 1574, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 9, 1734, 1966) 26. N. K. KOLNIBOLOTCHUK, V. I. KLENIN and S. Ya. FRENKEL', Vysokomol. soyed. A12: 2257, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 10, 2558, 1970) 27. M. L. LAKHANPAL, H. G. SINGH, H. SINGH and S. C. SHARMA, I n d i a n J. Chem. 6: 95, 1968 28. M. L. LAKHANPAL, V. KAPOOR, R. K. SHARMA and S. C. SHARMA, I n d i a n J. Chem. 4: 59, 1966 29. G. I. BARENBLATT, V. N. KALASHNIKOV and A. M. KUDIN, Zh. prikl, mekhanika i tekh. fiziki 5: 118, 1968