Phase equilibrium of polymer solutions in static conditions and in a flow regime

Polymer Science U.S.S.R. VoL 28, No. 12, lap. 2794-2799, 1986 Printed in Poland

0032-3950186 $10.00+,00 © 1987 Pergamon Journals Ltd.

PHASE EQUILIBRIUM OF POLYMER SOLUTIONS IN STATIC CONDITIONS AND IN A FLOW REGIME* S. A. VSmVKOV and A. P. SAFRONOV Urals Gorkii State University (Received 5 May 1985) The results of investigation of the phase equilibrium of systems with upper and lower critical dissolution temperatures and also crystalline phase separation in static and dynamic -conditions are presented. The role of the hydrodynamic field is considered in relation to the rate of shear, concentration and surface energy of the components.

ON FOgMATIONof fibres and films the polymer solution or melt is subject to considerable mechanical influences. This leads to profound change in the structure of the system and in some cases to phase transitions-crystalline or amorphous phase separation not observed in static conditions at these temperatures. The basic patterns of the influence of the mechanical field on the phase equilibrium of polymer melts have now been established [1]. For polymer-solvent systems the available experimental data are sparse and contradictory. Thus, for polymer solutions with amorphous layering improvement of the mutual solubility of the components during flow was found [2-6] expressed in a corresponding shift of the phase separation temperatures Tes o f the solutions. In references [7, 8] it is shown that the mutual solubility of the components in the shear field is less than in static conditions. As for solutions with crystalline phase separation phase equilibrium in the flow regime has been studied only for one system [9]. In this connexion new experimental information is necessary on the influence of the mechanical field on the phase equilibrium of polymer solutions. The aim of the present work is to study the phase equilibrium in the shear field of systems with amorphous (PS-di-(2-ethylhexyl)phthalate (DEHP), cellulose diacetate ( C D A ) - a c e t o n e water) and crystalline (PE-p-xylene) phase separation which allowed us to identify, having regard to the already known experimental data, some general patterns of the influence o f the mechanical field on the phase equilibrium of polymer solutions. We investigated LDPE (/~t~=2.3 x l0 s) purified by precipitation from 2% p-xylene solution with propanol at 348 K [10] and dried at 360-370 K to constant weight; PS (.~r = 3"33 x 106, 37/,/~r, = 1"06); CDA (/~r~=6.9 x 104) precipitated from acetone solution and dried at 350 K. The percentage of acetate groups determined by the technique in reference [11] was 52.1%. The solvents were purified as in reference [12]. The PE solutions in xylene were prepared at 370-380 K, CDA in mixed acetone-water solvent (weight ratio 1.6 : 1.0) at 290 K and PS in DEHP at 360 K over several days.

* Vysokomol. soyed. A28: No. 12, 2516-2520, 1986. 2794

Phase equilibrium of polymer solutions

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The phase separation temperature Tps in static conditions was determined by the technique in reference [13]. To determine Tps in the hydrodynamic fl~d we used a modified PBR-2 rotational plastoviscometer. The working unit of the instrument is made up of two coaxial cylinders in the gap (~ 5 mm) between which the test solution was placed. The inner cylinder was connected to the electric motor of the instrument, the outer fixed on a lifting stage, joined to the base by a pedestal and closed with a lid. At the bottom of the outer cylinder and in the lid were inspection windows situated one above the other through which into the gap between cylinders was directed a light beam modulated with the frequency ~ 120 Hz from an He-Ne laser. The transmitted light was fixed with a germanium photoresistor coupled to the V-3-6 a.c. millivoltmeter with sensitivity 2 x 10 -5 V per division. To control the time dependence of the intensity of the light of the laser beam we placed in its path a glass plate deflecting part of the light onto the photoresistance coupled to the M-1635 microammeter. A thermostatting jacket was sealed to the outer cylinder allowing us to make the measurements over the range 280-430 K. The temperature of the photoresistances was kept constant at 293 K. The polymer solution or melt was placed in the heated working unit of the instrument and the required speed of rotation of the inner cylinder set with the outer fixed immobile. The cooling rate was 5 deg/hr. The temperature dependence of the intensity of the light transmitted through the solution (melt) was measured. The temperature starting from which sharp fall in light transmission due to clouding of the system was observed was taken as Tps. The flow in all cases was laminar as judged from the value of the Reynolds' number. The phase state of the polymer formations released from the solutions was studied by polarization microscopy using the MIN-8 microscope and by the X-ray structural method. The diffractograms of the samples were obtained with the DRF-2.0 in Co K~ radiation.

Systems with amorphous layering. Figure la, presents the b o u n d a r y curves o f the P S - D E H P system determined in static and dynamic conditions. The system has an upper critical dissolution temperature (u.c.d.t.), i.e. layers on cooling. F r o m Fig. la, it will be seen that during flow the b o u n d a r y curve and u.c.d.t, shift to the region o f higher temperatures indicating worsening o f the miscibility o f the c o m p o n e n t s in a flow regime which agrees with the results o f references [7, 8] in which rise in Tps in the shear field was f o u n d for solutions o f PS in D E H P , decalin and cyclohexane. The worsening o f the miscibility o f the c o m p o n e n t s is related [7, 14, 15] to the uncoiling o f the macromolecular coils in the mechanical field a n d the orientation o f the chains in the direction o f the flow, which increases the degree o f association o f the m a c r o molecules, an assumption confirmed experimentally. In references [16, 17] the f o r m a tion o f associates o f the macromolecules in flowing PS solutions was recorded. However, the results presented contradict the findings o f the authors o f references [5, 6] w h o f r o m investigations o f the phase equilibrium o f the systems P S - D E H P , PS--decalin, etc. concluded that the miscibility o f the components improves during flow. P r o b a b l y this is connected with the fact that they in study o f phase equilibrium in static conditions used an optical m e t h o d but in dynamic the viscometric m e t h o d although it is k n o w n that the characteristic rise in viscosity on phase separation is observed when the mass o f the new phase f o r m e d is greater than is necessary for clouding. Consequently, Tvs determined viscometrically must be somewhat lower than Tps determined f r o m clouding (for systems with u.c.d.t.). Together with orientation processes in flow, processes o f degradation by the mechani-

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S.A. VsnwKovand A. P. SArRONOV

cal field of the seeds of the new phases, formed before macrolayering, may take place. If degradation predominate~ then improvement of the miscibility of the components in flow is possible. Such a phenomenon was discovered for the CDA-acetone-water system layering on cooling. rt

~K r Zg3

/+~+~+\ 2

303

7_83 -I+"'-+~+"'+-,.,I. 1

287

2

4

z/

6

8

10 c,%

FIG. 1. Boundary curves in static conditions (1) and flow regime (2) for the systems PS-DEHP (a) and CDA-acetone-water (b). Shear rate ~,=246 (a) and 170 (b) see -x. Figure lb, presents the boundary curves for CDA solutions determined in static and dynamic conditions. It will be seen that the application of a mechanical field reduces Tes, i.e. improves the miscibility of the components. Apparently, this phenomenon is connected with the fact that CDA has high kinetic rigidity. Therefore, in flow heavy additional uncoiling of the CDA macromolecules must not be observed. Consequently, in the system indicated the role of the hydrodynamic field essentially boils down to degradation of the seeds of the new phase which leads to improved miscibility of the components.

~K q30

3

35O I 20

I 60

cpe,%

FIG. 2. Boundary curves of the PE-p-xylene systemfor ~,=0 (1), 47 (2) and 246 (3) see -~.

System with crystalline phase separation. Figure 2 presents the concentration functions of Tps of PE solutions determined in static and dynamic conditions. The boundary curve corresponding to static conditions is typical of a system with crystalline phase separation [18]. The application of a mechanical field raises Tps and changes the form of the curve. However, the type of phase separation remains the same as in static conditions, i.e. crystalline. This is indicated by the anisotropy of the polymer formations released detected by the methods of polarization microscopy and X-ray structural analysis.

Phase equilibrium o f p o l y m e r solutions

2797"

Figure 3 gives the Trs values of the PE melt and solutions as a functions of the shearrate ~. Only for dilute solutions (c< 1%') in the range of shear speeds investigated is there no change in T~. For more concentrated solutions with rise in the shear speed rise iv, Tps by 10-30 K is observed. For solutions with c = 30 and 40 ~o extremal dependence of Tps on ~ is found indicating the extremal dependence of the rate of crystallization of PE on the rate of deformation of the solution. A similar dependence was seen in re-. ference [9] for the polycaproamide-caprolactam system. Apparently, this is a common phenomenon associated as indicated in reference [9] with the development in the system of two oppositely directed processes: uncoiling of the macromolecules and their orientation in flow (which promotes crystallization) and the destruction by the mechanical field at high shear speeds of the seeds of the new phase, which impedes crystallization. Consequently, in different ranges of shear speeds one may expect the hydrodynamic field to have a different influence on the miscibility of the components. For low shear speeds orientation processes may predominate promoting phase separation and at high speeds processes of seed destruction which prevents separation of the system into phases. From the results reported in the present and earlier work [7, 8] one may also isolate other features in the behaviour of systems in the hydrodynamic field. It is well known

37O

tOO

200 ~,,sec-~

Flo. 3. D e p e n d e n c e o f Tps o f P E melt and solutions in p-xylene o n the shear rate for [PE] = 1 (1), 5 (2), 10 (3), 20 (4), 40 (5), 60 (6), 30 (7) a n d 100 (8)Yo.

IATI,K

40

h

a 40m

2

2Ol 6'

2

20

5O

IOD CpE, %

q

8

c,%

Fro. 4. C o n c e n t r a t i o n dependence o f IAT[. a: ~,=47 (1) a n d 246 (2) sec -a. P E - p - x y l e n e b: 1 -~, = 0 ' 9 5 × 104 sec _1 [8], P E O - w a t e r ; 2 - ~,= 246 s e c - 1, P S - D E H P ; 3 - }, = 3 × 104 s e c - 1 [8], P M M A - b u t a n o l ; 4 - 7 = 2.1 x 104 see - 1 [8], PS-cyclohexane; 5 - 7 = 170 sec - 1, C D A - a c e t o n e - w a t e r .

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that formation and destruction of the seeds of the new phases are largely determined by their surface energy. Therefore, this parameter was compared with the result of the influence of a mechanical field on the Tps values. For binary polymer-solvent systems, as a first approximation, it may be assumed that the value of the intephasic surface energy is proportional to the difference in the surface energies of polymer and solvent trl 2 according to the Antonov rule [19]. The calculated values of tr12 using the data of references [20, 21] at temperatures close to Tes are given in the Table where it will be seen that for try2 ,~0.01-0-03 J/m 2, the miscibility of the components worsens in flow but for 312 ~ 10 -4 J/m 2 it improves. Consequently, for such minor differences in the surface energy o f the polymer and solvent, one may expect rise in the lower critical dissolution temperature (l.c.d.t.) and fall in the u.c.d.t, of the systems in the flow regime.

.VALUES OF 0"12 AND THE INFLUENCE OF THE HYDRODYNAMIC FIELD ON THE COMPATIBILITY OF THE COMPONENTS

System PS-DEHP PS-decalin PS-cyclohexane PMMA-butanol PEO-water CDA-acetone-water

tr12 x 103 at 293 K, J/m 2 12"6 15.3 [7] 20.2 30.5 32. 0* 0.3

Solubility in shear field Diminishes

Increases

* T = 363 K.

The effect of the hydrodynamic field on Tes depending on the concentration is of an extremal character as shown by Fig. 4 giving the data for a number of systems in the coordinates A T - c where AT is the difference between Tps in static and dynamic conditions. The maximum effect is seen in the region of moderately concentrated solutions. This applies both to the amorphous and crystalline phase separation. Apparently in dilute solutions the polymer chains are more or less isolated and may change in flow their conformations without interacting with each other. Therefore, the mechartical field does not influence the compatibility of the components. With increase in the polymer concentration in the system a fluctuation network forms which still does not hamper the course of the orientation processes. In this case the interaction between macromolecules is considerable which leads to increased viscosity and rise in the shear stress for constant y and the hydrodynamic field significantly influences the Tes values. With subsequent rise in the concentration further increase in the frequency of the network impedes the orientation processes. This weakens the influence of the mechanical field on phase equilibrium. With increase in the shear speed ~the position of the maximum shifts to the region of lower concentration of polymer. Translated by A. CRozY

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