Aspects of the rheological behaviour of polymer solutions in conditions of phase separation

1534

I . A . BARSUKOVet al.

REFERENCES 1. Y. IKADA, M. SUZUKI and Y. TAMADA, Amer. Chem. Soc. Polymer Preprints 24: 19, 1983 2. K. HAYASHI and K. MURATA, Kobunshi rombunshu 39: 179, 1982 3. A. S. HOFFMAN and B. D. RATHER, Radiat. Phys. Chem. 14: 831, 1979 4. A. RUCINSKA, Y. ROSIAK and D. PEKALA, Ibid. 24: 495, 1985 5. K. OTSUHATA, M. T. RAZZAK, R. L. CASTANARES, Y. TABATA, F. OHASHI and A. TAKEUCHI, Ibid. 25: 537, 1985 6. V. A. POSTNIKOV, N. Yu. LUKIN, B. V. MASLOV and N. A. PLATi~., Polymer Bull. 3: 75, 1980 7. L. Yu. BRAZHNIKOVA, S. I. POGODINA and L. P. KRUL', Vestsi Akad. Nauk BSSR, Ser. fiz.-energ, nauk, 4, 125, 1985 8. A. H. EL-SAYED, M. M. EL-DESSOUKY and S. A. EL-SHARABASY, Radiat. Phys. Chem. 27: 323, 1986 9. V. S. SAVOST'YANOV, D. A. KRITSKAYA and A. N. PONOMAREV, Vysokomol. soyed. B27: 66, 1985 (Not translated in Polymer Sci. U.S.S.R.) 10. A. L. IZYUMNIKOV, L. V. MINEYEV, V. A. MASLENNIKOV, L. S. SIDORINA, O. S. SAMSONOVA and A. D. ABKIN, Ibid. A30: 1030, 1988 (Translated in Polymer Sci. U.S.S.R. 30: 5, 1062, 1988) 11. T. J. SUEN, YUN YEN and J. V. LOCKWOOD, J. Polymer Sci. 31: 481, 1958

Polymer ScienceU.S.S.R. Vol. 31, No. 7, pp. 1534-1541, 1989 Printed in Poland

0032-3950/89 $10.00~ .00 © 1990PergamonPress pie

ASPECTS OF THE RHEOLOGICAL BEHAVIOUR OF POLYMER SOLUTIONS IN CONDITIONS OF PHASE SEPARATION* I. A. BARSUKOV, D. N. YEMEL'YANOV, and N. S. B O B Y K I N A

R. A. KAMSKII

Chemistry Research Instituteat the Gorkii Lobachevskii State University (Received 30 December 1987) Change in viscosity on exposure to shear deformation and temperature in the region of phase separation is examined for concentrated PS and PMMA solutions. Below the temperature of phase separation depending on the shear rate the viscosity of the system as a function of temperature has different values: at low shear rates the viscosity is higher. Viscosity abruptly diminishes with rise in the shear rate which characterizes the destruction of

the structure of a heterogeneous system and the phase separation process.

* Vysokomol. soyed. A31: No. 7, 1402-1407, 1989.

Raheological behaviour of polymer solutions

1535

THE diagram of the phasic state is a fundamental characteristic of polymer solutions determiniog the temperature-concentration regions of their processing and use [1], An important technological parameter is viscosity r/serving, on the one hand, for controlling the phase separation process of solutions and, on the other, determining the, choice of the ways of processing them [2, 3]. There is much evidence [2-7] that close to the binodal, as a rule, r/decreases. Thus, study of the temperature dependence of viscosity r/(T) of solutions of polymers with narrow MMD showed [4] that on passage of the solution to the biphasic region r/ decreases sharply. The sharp change in r/ coincides with the visually observed cloud points within the limits _+0.1 K. Similar results were obtained earlier in references [5, 61. Such behaviour of ~/ of polymer solutions on entry into the biphasic region may provide a method for determining the phase layering temperature Tph and constructing the phasic state diagram [4, 7, 8]. The method is of special interest for investigating systems in which Tph cannot be determined visually (so-called isorefractive components, coloured solutions, systems enclosed in non-transparent cells, etc.). However, it should be noted that the literature provides ample information indicating that on passage to the biphasic region a directly opposite effect is observed- the viscosity of the solution rises sharply [9, 10]. In reference [9] CTA solutions were studied in a mixture of a precipitant (ethyl alcohol) and solvent (methylene chloride). The authors isolated three states of the system: solution, gel and explicit phase separation. Transition through the solution-explicit phase separation boundary was accompanied by sharp rise in r/ending in the gelation of the system. Similar results were obtained by us in study of PAN solutions in the water-sodium rhodanide system [10]. The effect of fall or rise in r/on passing to the biphasic region may also show up in a single system depending on the concentration of the solution. In reference [6] the viscous properties of PS solutions in cyclohexane were studied in the phase separation region. As shown, the viscosity of dilute solutions falls in the critical region but that of concentrated solutions may rise. The character of the change in 71 during phase separation may also be influenced by the conditions of measurement [7, 8, 11]. In reference [7] it is shown that with rise in the strain rate ~ the intensity of the drop in viscosity decreases. Interesting results in the study of the viscous properties of blends of PMMA anti PBMA with limited com vatibility are presented in reference [11]. It was shown that for any MMs of the polymeric components the dependence of the greatest Newtonian viscosity on the composition of the blends in the region of the transition from a monophasic to a biphasic system is a maximum. For effective viscosity this dependence is characterized by the presence of a minimum. The brief literature review presented indicates the complexity and the equivocal nature of the behaviour of viscosity of solutions in the region of phase separation. As we see it, the experimental patterns observed by different authors may not contradict each other. Rather they are different aspects of one physical process the contradictory reflections of which are primarily due to the narrowness of the concentration intervals

1536

I . A . B/d~SU'KOVet al.

o f the solutions or the shear rates chosen for c o m p a r i s o n i n which the m e a s u r e m e n t s w e r e made. The aim of the p r e s e n t w o r k is to study the viscous properties o f c o n c e n t r a t e d P S a n d P M M A solutions over a wide range of t e m p e r a t u r e s a n d shear rates which is of f u n d a m e n t a l interest in describing the properties in the phase layering region. The molar masses of PS and PMMA were 2"6 × l0 s and 8 × 10't respectively. As the solvent for PS we opted for cyelohexane (CH), the concentration of the solutions was varied between 630 vol.Yo in order to repeat the experiments of references [4, 6]. To prepare the PMMA solutions we used a mixed solvent (cyclohexane + toluene) since in such a solvent change in the viscosity during polymerization of MMA had earlier been studied [12].

[o9 ~[P~.~ec]

~/o

T,K 330

310 3

290

,

)

I

20 FIG. 1

qO ~, %

I

t

I

3.0

1

3'2

(/os/r),K -1 Fro. 2

FIG. 1. Phasic state diagrams of the systems PMMA-(CH+toluene) (1-3) and PS-CH (4, 5). CH : toiuene=31 : 69 (1), 29 : 71 (2) and 27 : 73 (3) vol.%, ¢ is the volumetric fraction of polymer in solution. Fxo. 2. Temperature dependence of the viscosity of PMMA--(CH+toluene) solutions. [PMMA] --30 vol.%. CH : toluene=27 : 73 (1), 29 : 72 (2) and 31 : 69 (3) vol.%. Empty circles correspond to strain rate 0"3; filled circles 5.3 see -1. Tph was determined by the turbidity method [6]. The phase diagrams of the systems studied are given in Fig. 1. Increase in the content of CH (a poor solvent for PMMA) in the cyclohexane + toluene mixture worsens the quality of the solvent giving a regular rise in T~a (Fig. 1, curves 1-3). The diagram of state of PS-CH is represented by curve 5: the empty circles were obtained by the turbidity method and the filled by the viscometric method (correspond to the moment of drop in viscosity). Curve 4 was plotted from the data of reference [13] for PS with close MM. To measure the viscosity of the solutions we used a hermetically sealed rotating viscometer with the GRB magnetic dynamometer [14] (the measurements were made for a constant shear rate j,) and the VPN-02 rotating viseometer (measurements at constant shear stress z), The outer cylinder of the working cell of the GKV instrument is made of glass which allowed us to control the change in optical density of the solutions and the phase separation process and also to isolate the syncretic fluid directly in the course of the measurement of viscosity. In all the experiments the solutions were thermostatted with an accuracy + 0"1°C. With approach to Tp~ the measurements were made through every 0-8°C which corresponds to the conditions of the experiment of reference [6].

Rheological behaviour of polymer solutions

1537

In the course of measurement of the viscosity of the solutions at T< Tph in some cases we observed release of synergetic fluid and, therefore, to conduct each repeat at the set temperature such systems were at first thermostatted with agitation at T> Tph to full homogenization. Figure 2 presents the functions log tl-1/T for the P M M A solutions and Fig. 3 for the PS solutions. Figure 3 also presents the results obtained in reference [6] when in one case drop in viscosity was observed (Fig. 3, curve 2) and in the other rise in ~/ on passing through the phase separation boundary (curve 1); curve 6 was plotted from the data o f reference [4]. A characteristic feature of the experimental data presented in Figs. 2 and 3 is the appearance in the region after phase separation of, as it were, two branches of the functions log rl-1/T: at low shear rates (~< 1 sec -1) the viscosity o f the system rises and that measured in the shear rate interval 1-253 sec -x falls as compared with q to Tph.

1o,9~ (pa. sec]

a tog ~ [Pa.sec7

o

_o.;[ 9' 3"0

I

3.2

.,

.

[

3.1t (I03/T) /'(-'

Fro. 3

I

I

i

0

I

0

i

y log~,Esec"~ Fro. 4

FIG. 3. Temperature dependence of the viscosity of PS-CH solutions. [PS]=28.6 (1), 19.8 (2), 25 (3), 20 (4), 12 (5) and 11.7 (6) vol.yo. Empty circles correspond to strain rate 0.3, filled circles to 4-8 sec-1. Fro. 4. Viscosity of PS-CH solutions as a function of the strain rate. [PS]=25 vol. %. 1-293 K (T> Tph);2--288 K (T< Tph); 3--286-8 K (T< Tph). To define more clearly the splitting of the curves r/(T) recorded after phase separation we obtained the dependences of log t / o n ~ and log r/on ~" at T > Tph and T < Tph (Figs. 4 and 5). It will be seen that at T > Tph viscosity changes little with rise in ~. At T > Tph for a certain critical value 7or the viscosity falls sharply and then practically does not depend on 1; (in the measured shear rate range). The larger the magnitude A T = Tph -- T), i.e. the system is deeper in the region under the phase separation curve, the smaller the magnitude ~;or. With fall in ~ (direction of change in the strain rate is indicated by arrows) the viscosity values rise although the restoration curve differs from the curve of fall in r/with rise in ~. Thus, a kind of "hysteresis loop" is observed. Similar patterns

1538

I.A. BARSUKOVet aL

were observed on analyzing the dependence of r/on • (Fig. 5) obtained with the VPN-02 instrument. Study of the dependence of r / o n ~ and r / o n ~ also showed that fall in viscosity at ~>~cr or z>~T~r and rise in viscosity at 7<7~r or ~'rcr (or "CZ¢r the viscosity of the system drops (Fig. 6, curve 1) and with fall in shear stress ( ~ < ~ ) it rises with time (Fig. 6, curve 2).

toS

Epa.secJ

i

z

Io9 3

3 I 2

2 i

7

i

i

2

i

I .

3

log~ [Pa.]

FIG. 5

m

I

5

I

10 Time ~rain

I

15

I

20

PIG. 6

FIG. 5. Viscosity of PMMA-(CH+toluene) solutions as a function of shear stress. [PMMA] =43 vol.~/o. CH: toluene= 27:73 vol.Yo. 1-308 K (T> Tph); 2--300 K (T< Tph); 3--295 (T< Tph). FIG. 6. Time dependence of the viscosity of PMMA-(CH + toluene) solutions on exposure to stress. T=300 K (T< Tpb). [PMMA]=43 vol.~. CH : toluene=27:73 vol.~/o. Shear stress 680 Pa (~> r~,) (1) and 120 Pa (~<~or) (2). We obtained the following findings: the rise or fall in the viscosity of the solution after passage through the phase layering curve shows that the ambiguous patterns of change in viscosity of the solutions presented in the literature depending on temperature reflect the complex dependence of r/ on T, ~p and ~ and the measurement time. In our view, the experimental relations presented in Figs. 2-6 fully agree with the generally known notions of Papkov [1]. Transfer of concentrated PS and P M M A solutions through the binodal by cooling below UCTC leads to the appearance of a heterogeneous system in which portions of the low concentrated phase are distributed in a kind of matrix of the highly viscous phase forming a continuous skeleton of the system. The further p r o c e s s - p h a s e separation by densities-occurs very slowly (in our experiments ~ 10-30 hr), i.e. far exceeds in times the optimal duration o f the continuous measurements. The conditions are created for the formation of a quasinetwork structure of the solution. From the magnitude 7or (rcr) one may evaluate the shear strength of t h e structure formed. For the systems studied it is 102-103 Pa. On application of the shear field above a certain 7or the structure of the here rogeneous

Rheological behaviour of polymer solutions

1539

system is destroyed with the release of synergetic fluid and sharp fall in the viscosity of the system, i.e. the phase separation process under the influence of the mechanical field is greatly accelerated. With the removal of the shear field because of the large volume of the highly concentrated phase the viscosity of the system rises. However, r/is not completely restored which is connected with the presence of the separated synergetic fluid. As noted, the literature provides no generalizations of the experimental dependences of the viscosity of polymer solutions in the phase separation region. A general approach to this problem was proposed by Papkov [1] according to which this problem is linked with the position of the system relative to the polymer-solvent phase diagram. However, Papkov did n ot specify the regions of concentrations of the polymer in solution for which the transition through the binodal is accompanied by drop or rise in viscosity. In our view, to predict the rheological behaviour of polymer solutions on passing through the binodal it is necessary to link clearly the position of the system (~0, T) in the phase diagram with the initial presence of the solution in a particular structurorheological state. Systematic investigation of the rheology of homogeneously polymerized vinyl monomers and their models-solutions of amorphous polyacrylates in their monomersrevealed four conversion regions in which the system is in different structuro-rheological states: visco-Newtonian (I), structuro-viscous (II), highly elastic (III) and glassy (IV). The transition from one state to another comes about in the regions of critical concentrations and leads to qualitative change in the complex of structuro-rheological parameters [15, 16]. We shall analyze the rheological behaviour of PS and PMMA solutions during the phase transition to heterogeneous colloidal systems successively passing from dilute to highly concentrated solutions through the states I, II and III. The region of dilute solutions (to ,~3 vol. %) corresponds to state I. In this structuro-rheological state the macromolecules are in the form of weakly interacting coils. Deformation of such solutions causes flow which in a sufficiently wide shear stress interval obeys the Newtonian law. With fall in temperature or with worsening of the quality of the solvent the relative viscosity of the solutious decreases [5, 6, 12, 17]. In the critical region the intensity and asymmetry of light scatter sharply grow. Viscosity either drops or remains constant [5, 6]. Passage of the polymer solution present in state I to the heterogeneous state leads to the formation of systems with disperseunbound structure. A coil-globule type transition of the macromolecules is observed. In state II the macromolecules are in the form of associates. On deformation nonNewtonian flow of the mass is observed as a result of the destruction of these primary structural formations. The PS and PMMA solutions studied in this work at T> Tph are in state II. With worsening of the quality of the solvent or with fall in temperature the relative viscosity of the solution does not fall as in region I but rises and this to a higher degree the higher the concentration of the polymer [6, 12, 17]. With the passage of such a solution to a heterogeneous system a dispersion forms consisting of aggregates of interacting globular particles. This transition is accompanied by sharp intensification of the viscosity anomaly and the appearance of dearly marked thixotropy.

1540

I.A. BARSUKOVet al.

In region l I I the concentration of the polymer is such that a fluctuation network forms embracing the whole volume of the solution. The advent of a network is the cause of the appearance of elastic properties and intensification of the viscosity anomaly. The introduction into such solutions of a non-solvent raises elasticity, strength, the rupture strains and viscosity [18]. PS solutions with M = 5 x l0 s possesss high elasticity starting from ~ 30 ~o concentration. In the case of rigid chain polymers such as nitrocellulose, PVS and PAN, etc. the spatial network may appear during transition o f the solution through the binodal f r o m state II. A system consisting of the highly concentrated spatial skeleton and portions of the low concentrated phase incorporated in it capable of heavy reversible strains ceases in practice to flow [9, 10]. As emphasized by Lipatov [19] the spatial network in this case results from the interaction of the disperse particles of the highly concentrated phase. There is little point in describing the properties of such systems simply as highly viscous systems consisting of two phases, one of which is dispersed in the other and in which phase separation does not occur only because of the kinetic conditions. The authors with to thank V. P. Budtov for useful discussion of the work. Transited by A. CRozY

REFERENCES

1. S. P. PAPKOV, Fiziko-khimicheskiye osnovy pererabotki rastvorow polimerov (Physical Chemical Bases of Processing Polymer Solutions). p. 363, Moscow, 1971 2. V. N. KULEZNEV, Smesi polimerov (Polymer Blends). p. 303, Moscow, 1980 3. A. Ya. MALKIN and S. G. KULICHIKHIN, Reologiya v protsessakh obrazovaniya i prevrashcheniya polimerov (Rheology in the Processes of Formation and Conversion of Polymers). p. 240, Moscow, 1985 4. B. A. WOLF and M. C. SEZEN, Macromolecules 10: 1010, 1977 5. E. V. FRISMAN and SYUI NAO, Vysokomol. soyed. 3: 285, 1961 (Not translated in Polymer Sci. U.S.S.R.) 6. A. A. TAGER, V. Ye. DREVAL' and K. G. KHABAROVA, Ibid. 6: 1593, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 9, 1766, 1964) 7. B. A. WOLF, Pure Appl. Chem. 57: 323, 1985 8. A. Ya. MALKIN, S. G. KULICHIKHIN and R. Z. MARKOVlCH, Vysokomol. soyed. A28: 1958, 1986 (Translated in Polymer Sci. U.S.S.R. 28: 9, 2177, 1986) 9. L M. MALOFEYEVA and V. M. AVER'YANOVA, Protsessy studneobrazovaniya v poli, mernykh sistemakh (Processes of Gelation in Polymeric Systems). issue, 2, p. 43, Saratov, 1977 10. I. A. BARSUKOV, I. Ye. SMETANINA, D. N. YEMEL'YANOV, R. A. KAMSKII and T, N. PODMOGAYEVA, Vysok0moh soyed. B28: 368, 1986 (Not translated in Polymer Sci. U.S.S.R.) 1i. D. N. YEMEL'YANOV, V. A. MYACHEV and V. Ye. DREVAL', Tez. dokl. Vscsoyuz. konf. po smesyam polimerov (Summaries of Reports to All-Union Conference on Polymer Blends). p. 149, Ivanovo, 1986 12. I. A. BARSUKOV, I. Ye SMETANINA, T. N. PODMOGAYEVA, D. N. ¥EMF.,L'YANOV and L. I. AMENITSKAYA, Fiziko-khimicheskiye osnovy sinteza i pcrerabotki polimerov (Physical Chemistry Bases of Polymer Synthesis and Processing). p. 81, Gorkii, 1986 13. A. R. SCHULTZ and P. J. FLORY, J. Amer. Chem. Soc. 74: 4760, 1952 14. R. A. KAMSKII, U.S.S.R. Pat. 1265545. Byull. izobr., No. 39, 144, 1986 15. D. N. YEMEL'YANOV and I. Ye. SMETANINA, Vysokomol. soyed. B, 824, 1979 (Not translated in Poluymer Sci, U.S.S.R.)

Phase equilibrium of polyether mixtures

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16. I. Ye. SMETANINA and D. N. YEMEL'YANOV, Novoye v reologii polimerov (New Developments in Polymer Rheology). p. 320, Moscow, 1981 17. A. A. TAGER, V. Ye. DREVAL', G. O. BOTVINNIK, S. B. KENINA, V. I. NOVITSKAYA, L. K. SIDOROVA and T. A. USOL'TSEVA, Vysokomol, soyed. A14: 1381, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 6, 1551, 1972) 18. T. I. ZATSEPINA and A. A. TRAPEZNIKOV, Ibid. 3: 113, 1961 (Not translated in Polymer Sci. U.S.S.R.) 19. Yu. S. LIPATOV, Kolloidnaya khimiya polimerov (Colloidal Chemistry of Polymers). p. 344, Kiev, 1984

Polymer Science U.S.S.R. Vol. 31, No. 7, pp. 1541-1546, 1989 Printed in Poland

0032-3950/89 $10.00+ .00 O 1990 Pergamon Press pie

INFLUENCE OF THE MECHANICAL FIELD ON THE PHASE EQUILIBRIUM OF POLYETHER MIXTURES AND THE CELLULOSE DIACETATE-ACETONE-WATER SYSTEM* S. A. VSHIVKOV, L. A. PASTUKHOVA and R. V. TITOV Gorkii State University of the Urals (Received 30 December 1987)

In the shear field and static conditions the authors have studied the phase equilibriam of systems polyethylene glycol-polypropylene glycol and cellulose diaeetate-acetone-water. Addition of water to acetone at high temperatures raises and at low temperatures reduces its dissolving capacity in relation to cellulose diaeetate. A mechanical field leads to worsening of the compatibility of the polyethers and improvement of the solubility of cellulose acetate in wateracetone mixtures. DURING use and processing polymer blends and solutions are subject to diverse mechanical agents which may promote change in the mutual solubility of the components most clearly manifest close to phase separation [1-7]. This m a y lead to undesirable changes in the physicomechanical properties of polymeric systems. To predict such changes information is needed on the influence of the mechanical field on the position of the boundary curves characterizing crystalline and amorphous phase separation. In the present work in a shear field and in statistical conditions phase equilibrium of a commercial fibre-forming system cellulose diacetate (CDA)-acetone-water and also mixtures of polyethylene glycol (PEG) and polypropylene glycol (PPO) modelling the behaviour of the polymeric components in the mechanical field was studied. We investigated CDA (37/',=7.8 x 105), PEG (37/',=800) and PPG (/VI,=3100) [8]. CDA was reprecipitated from acetone solution and dried at 350 K. The percentage of acetate groups determined by the technique of reference [9] was 52.1. Distilled water and acetone were purified bY distilla. * Vysokomol. soyed. A31; No, 7, 1408-!41!, !989,