PHASE SEPARATION IN.POLYMER SOLUTIONS

Poly ( 2 I 6 dimethy1-1 I 4 phenylene oxide)

i n toluene.

P.T. van E m e r i k

PHASE SEPARATION IN POLYMER SOLUTIONS

Poly(2,6-dimethyl-l,4 phenylene oxide)

in toluene

Proefschrift

ter verkrijging van de graad van doctor

in de technische wetenschappen aan de

Technische Hogeschool Twente te Enschede

op gezag van de rector magnificus prof.

dr.ir. P . J . Zandbergen, volgens besluit

van het College van Dekanen in het open-

baar te verdedigen op donderdag 22 maart

1973 te 16.00 uur

door

Piet Theodorus van Emmerik

geboren op 24 januari 1941 te Amsterdam

Dit proefschrift is goedgekeurd door de promotor

Prof , dr, C , A= Smolders

CHAPTER I . Xntroductton

CHAPTER I1 . . . . .

Liquid-liquid phase s'eparatìon of poly(2,6 dimethyl 1,4

phenylene oxìdel in binary mixtures with toluene and

ternary mixtures with toluene and ethyl alcohol . 1

Determination of the thermodynamic parameters of. poly-

(2,6 dimethyl 1,4 phenylene oxide)-toluene mixtures . 2 CHAPTER 'TV

The calculation of the cloudpoint curve with a concen-

t r a t i o n and temperature dependent free energy correction 3 parameter .

CHAPTER V

Liguid-liquid phase separation of concentrated polymer

so lu t ions s tud ied by electronmicroscopy . 4 CHAPTER V I

DifferenEial scanning calorimetry of poly ( 2 , 6 dimethyl 1,4 phenylene oxide)-toluene solutions . 5

CHAPTER V I 1

Liquid-liquid phase separation by nucleation and growth

of poly (2,6 dimethyl 1,4 phenylene +oxidel.-toluene solu-

t i ons . 6 CHAPTER V I 1 1

This thesis i s a co l l ec t ion of s i x articles which appeared

o r w i l l appear i n l i t e r a t u r e .

1

CHAPTER r . . . INTRODUCTION

Upon cooling a homogeneous polymer solut ion three types of

phase separatìon are possible:

a. c r y s t a l l i z a t i o n of t he polymer

b. crystal l ìzatSbn of the solvent

C. l iquid-lìquid phase separation

The l i m i t o f s t a b ì l i t y o f . a polymer s o l u t i o n i n which no

c rys t a l l ì za t ion o f t he polymer or the so lvent t akes p lace

upon coolìng, is called the cloudpoint curve . Upon enter- ing the thermodynamically meta-stable or instable area

comprised by this curve, l iquid- l iquid phase separat ion

takes place by nucleation and growth’ of one of the equi-

l ìbr ium phases , or when f o r some reason this does not hap-

pen, upon further cooling’by way of the spinodal decompo-

s i t i o n mechanism . The cloudpoint curve of many polymer- solvent systems has i t s maximum a t a r a t h e r low polymelr-

concentration (between 5 and 20 percent of weight polymer)

For normal in t e rac t ions between polymer segments and sol-

vent molecules this asymmetric cloudpoint curve finds i t s

o r ig in i n t he d i f f e rence i n s i z e of polymer and solvent

molecules”. The model descr ip t ion for such types of solu-

t i ons by Flory’ and Huggins’’, la ter modif ied by Koningsveld y i e lds an expression for the Gibbs free energy of mixing-

7

9

7

12

(AGm). #

BGm =

R = the gas

+o = volume

constant

f rac t ion of the so lvent

2

+i = volume fracticS’n of polymer component i _ j

4 = t o t a l polymer volume f r ac t ion

mi = the re la t ive chain length of polymer component i

g = an empirical free enthalpy correction parameter

For interaction parameters g independent of concen-

t ra t2on and temperature (non-polar type interaction] the

asymmetrec cloudpoint curve has been derived from the ex-

presseon of AGm . 7

Poly (2 ,Q dimethyl 1 p 4 phenylene oxide1 belongs to the class

of s lowly c rys t a l l i z ing polymers. It does no t c rys ta l l ize

Ln bulk by thermal treatment- only in the presence of cer-

ta in aromatic type organic l iquids , crystal l izat ion takes

place when phase separation i s brought about in a very di-

l u t e ~ o l u t i o n ’ ~ . Even then c rys ta l l iza t ion occurs very slow-

l y and t o an amount of a t maximum 20 percent of t h e t o t a l

polymer content, The s t i f f n e s s of t h e polymer chainp marked

a l so by the unusual high glass-transit ion temperature

(490K) seems t o be the reason for t h i s c r y s t a l l i z a t i o n be-

haviour .,

In the system poly(2,6 dimethyl 1,4 phenylene oxide)- tolu-

ene three phase separation curves can be detected experi-

mentally’, n e i t h e r of them shows a maximum i n s o l u t i o n s up

t o polymer concentrations of about 70 % of weight polymer.

Srnce crystal l izat ion of the polymer from so lu t ions of nor-

m81 Concentration can hardly occur, one of the three curves

must be the cloudpoint curve which deviates strongly from

the cloudpoint curves i n many oth.er polymer-sQ.lvent systems.

In Chapter Ifi 3 desc r ip tkm 2 s given how t o measure the

three phase separatìon curves, From the experiments it i s

concluded th4t upon c o o l h g a homogeneous poly (2,6 dìmethyl

1,4 phenylene oxidej-toluene solutfon a phase t rans i t ion

is i n i t i a t e d by l iquid-lìquid phase separation by nuclea-

t i o n and growth o r t he sp inoda l decomposition mechanism,

followed by crystal l izat ion of the concentrated phase.

Tn Chapter I11 a method is d iscussed to ob ta in the thermo-

dynamic parameters necessary to calculate the free enthal-

py of mixing. This can be done by combination of l i g h t

scattering experiments according to the method Scholte , with the melting point depression behaviour of t he system.

Prom the resul ts of these experiments it i s concluded that

the correction parameter i n the Flory-Huggins equation of

the free enthalpy of mixïng has an unusual dependence upon

concentration of the polymer and upon temperature.

Chapter I V g ives the descr ip t ion for the ca lcu la t ion of

the free enthalpy of mixing with a correction parameter

which i s concentration and temperature dependent. These

parameters are obtained by f i t t ing the c loudpoin t curve ob-

ta ined from t h i s model description with the experimental

cloudpoint curve.

A s a check on the va l id i ty of the values of the correct ion

parameter, the melting point curve i s calculated which a-

grees completely with the experimental melting point curve.

Knowledge of the pos i t ion of the cloudpoint curve and the

spinodal curvemakes it poss ib l e t o i nves t iga t e t he morpho-

14

4 u

l og ica l changes during l iquid-liquid phase separation of

poly(2 F6 dimethyl I,4 phenylene oxide] in solut ions of to lu-

ene and ca~rolactamr by e l ec t ron microscopy (Chapter v] d using the freeze etching technique15 as sample preparat ion

s tep . An es t imat ion of the s i z e of t h e c r i t i c a l r a d i u s for

nucleat ion is possible .

Chapter V I g ives the resu l t s o f an inves t iga t ion of t h e

phase separation phenomena of poly(2,6 dimethyl 1,4 pheny-

lene oxide]-toluene solutions studied by d i f fe ren t ia l scan-

ning calorimetry. With th i s ca lo r ime t r i c method one c m ais-

criminate between liquid-liquid phase separation and crys-

ta l l izat lon. Ear l ier conclusions about the three phase

separation curves are affirmed.

A survey of the nucleat ion and growth phenomena i n poly-

(2,Q dimethyl 1,4 phenylene oxide] in toluene solutions

during l iquid-liquid phase separation is given in Chapter

VII. The volume free energy driving phase separation is

calculated with the concentratTon and temperature depen-

dent free energy correction parameter. Together with these

values and the growth parameters during early stages of

nucleati .on, the activation energy of d i f fus ion and t h e

d i f fus ion coe f f i c i en t s can be calculated. Also an esti-

mate can be made of the radius of a c r i t i c a l n u c l e u s and of

t h e i n t e r f a c i a l free energy.

5

L'1,TERATUR'E

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

1 2 .

13..

1 4 .

15.

P.T. van Emmerik and C.A. Smolders

J. of Polym. Sc. C38, 73 (1972) P.T. van Emer ik and C.A. Smolders J. of Polym. Sc. C39, (1972) ( in p re s s ] P'.T. van Emmerik and C.A. Smolders Europ. Pol . J. ( ì n p r e s s ) P.T. van Emmerik, C.&. Smolders and W. Geymayer Europ. Pol . J. ( in p ress1 P.T. van Emer ik and C.A. Smolders Europ. Pol. 3 . ( i n p r e s s ] P.T. van Emmerik and C.A. Smolders submìtted t o Europ. P o l . 5.

P. J. Flory Principles of Polymer Science, Cornell Univ. Press 1953

M. Volmer and A. Weber Z. Phys. Chem. 119 , ' 277 (1925) J.W. Cahn Trans. M e t . Soc. AIME 242 , 166 (1968) H.G. Bunqenberg de Jong and H.R. Kruyt Proc. Kon. Ned. Akad. Amsterdarn, 32, 849 (1929) Kollofd Z. 50, 39 (1930) M.L. Huggins Ann. New York Acad. Sci . 4 3 , 1 (1942) R. Koningsveld Dissertation, Leiden (1967) Adv. Col lo id . In te r f . Sci. 2 , 151 '(1968) J . M . Barrales-Rienda and J.M.G. Fatou Kolloid Z.u.2. Polymere 244 , 317 (1971) T.G. Scholte Europ. Polym. J. 6, 1063 (19701 R. L. Steere J. Biophys. biochem. Cyt.3, 45 (19571

73

O B972 by J o h Wdey & Sons, Inc.

FIG. 1. Liquid ternary system: P,, polymer with chain length n; P,, polymer with chain length m (m 9 n); OP,, complete miscibility; OP, , limited miscibility; KLM, cloud point curve.

THERMODYNAMIC CONSIDERATIONS

The free enthalpy (Gibbs free energy) of mixing AG, of any closed system in stable equilibrium at constant pressure and temperature can be represented graphically for binary and ternary systems. In regions where A6m of the homogeneous solutions is higher than that of a combination of two phases the minimum free enthalpy of the system will be given by the phase separated system; the composition of the phases being given by the tangent phase of the AG, surface. When all sets of tangent points are projected on the base triangle, they represent the binodial which is the limit of stability of the homogeneous

Another set of characteristic points of the AGm surface are the inflection points which form the boundary of the concave part of the free energy surface. Its projection on the base triangle is called the spinodial. Phase diagrams of this type are characteristic for a nonsolvent (l)/solvent (2)/solute (3) system and have been extensively discussed by Tompa [6] .

phase.

It is clear that the composition diagram will have three regions: 1) A region where all sdutions are stable (AGm concave upwards). This is

the area outside the binodial. 2) A metastable region (AGm is still concave upwards, but fluctuations of a

certain composition and size, may initiate phase separation). This is the area between the binodial and the spinodial.

3) An unstable region (AGrn is concave downwards). Tbis is the area within the spino&d, where spontaneous phase separation always takes place by the sphodid mechhm.

A polymer solution actually represents a multicomponent system and a - graphical representation of AGm WOUH require more t.hm three hensions. Several authors [7,8] have s9zown that a kee-&ensiod diagram for a liquid ternary system brings out most of the essential features of the behavior .of polydisperse polymer solutions. A phase diagram for a polymer solution is shown in Figure 1. At the temperatures considered, OP1 and complete md limited 'miscibility, respectively. OX represents containing a certain mixture of PI and P2, which is the actual polymer sample, and the S Q ~ V ~ I I ~ . The plane TOX intersects the binodial surface; the re&lting curve is called the cloud point curve. The spinodlal must be situated below the cloud poht curve, touching the cloud point curve in the csticd p ~ h t ,

cloud point curve are situated between 2 and 15% by Liquid-fiquid phase separation seems to occur in most cases by nucleation am3 ~ Q W & which is fokwed by coalescence of the separated liquid phase to give two bulk liquid layers in the end. If nucleation and growth is a very ~ Q W process it is possible to cool a solution below the sphodid without extensive nucleation taking place. In that case liquid-licpid phase separation will be initiated by the spinodid mechanism.

For most polymer solvent systems the critical point and the

9

RESULTS

PPO-Toluene Mixtures

When a homogeneous mixture of PP0 .in toluene is cooled two transition temperatures will be found depending upon the cooling rate. The phase separation temperatures for different mixtures observed during cooling at 1 "C per 10 min are shown in Figure 2 as curve A, which is characterized by the experimental phenomenon of instantaneoùs complete demixing with- out formation of nuclei being detectable under the light microscope.

The transition temperatures observed during extremely slow cooling at 1°C each 48 hr are shown as curve B in Figure 2. When phase separation has set in and the solution is heated again after 24 hr of waiting, a third curve of transition temperatures wil l be found, curve C in Figure 2. This latter curve is found I independently of the rate of heating.

A different way to determine curve A is quenching a homogeneous mixture of PPO-toluene to a low temperature and to determine the time which elapses before phase separation becomes visible. At higher fmal temperatures this incipient precipitation time suddenly increases to high values as shown in Figure 3.

A 5 min time limit has been adopted for the estimation of the temperatures shown in curve A in 'Figure 2. The curves A and B are both cut off by the glass transition temperature line. Homogeneous solutions cooled to appropriate

10 20 30 40 50 60 - weight percentage of polymer FIG. 2. Phase separation curves of PPO-toluene solutions: (X) phase separation tempera- tures observed during cooling at 1°C per 10 min,'(n) phase separation temperatures observed during cooling at 1°C per 48 hr; (o) melting point curve; (- -) glass transition temperature line.

temperatures between curve B and C will remain homogeneous even after prolonged periods of at least 4 months.

Cooled at temperatures between B and A liquid-llcgmlid phase separation occurs by way of nucleation and growth (Fig. 4), nucleation O C C U ~ ~ after induction tinaes which approach W@ at temperatures near cum& B md which €d to zero when the temperature approaches curve A [S] .

The phase equilibrium curves of merent PPO-toluene mixtures have been determined. Wne to experimental difficulties the percentage of be determined itn the dilute phase only pig. 5). These equ%brim curves f%& h c w e B of Figure 2.

weight percent PP0

weight percent ethyl alcoh~l = l

If the total amount of BP0 + ethyl alcohol exceeds 40% of the total weight, Piquid-liquid p h e separation only occups, independent of the cooling rate. The W sitenation was always characterized by two bulk liquid fractions, a

FIG. 4. Liquid-liquid phase separation by nucleation and growth of a 30% of weight PWtohene solution at T = 48°C.

concentrated md a dilute one. If the solutions contained less than 40% of weight PP8 + ethyl dcohol, three curves could be observed in the Same way as with PPO-toluene &tures. . In Figure 7

weight percent toluene

weight percent ethyl alcohol = l

H2

10 20 30

P P 0

weight percentage of polymer + weight percentage

FIG. 6 . Phase separation curves of PPO-toluene-ethanol solutions: (v) cloud point curve; (0) melting point curve; (A) spinodial.

ethanol

class of slowly crystallizing polymers [ 101 . It cannot be crystallized by thermal treatment but may show crystallization when brought into contact with certain organic liquids [ 1 l] . Even then crystallization occurs very slowly.

When the system PPO-toluene is considered, the problem is how to interpretate the curves A, B, and C.

We believe that curve B represents the cloud point curve for the following reasons.

l) The end of the different liquid-liquid phase separation curves of the system PPO-toluene-ethyl alcohol in the WO-toluene plane is always situated on curve B.

2) When homogeneous mixtures of PPO-toluene are cooled to the appro- priate temperature between curves A and B phase separation occurs by

B4

. PP0

- weight percentage of polymer

15

l ethylalc. toluene

weight percentage of polymer+ toluene

FIG. 8. Phase separation curves of Pm-toluene-ethanol solutions. In each solution

wt 76 toluene - 7 wt %PP0 3

- -

(A) cloud point curve; (o) melting point curve; (V) spinodial.

4) Homogeneous solutions cooled rapidly at temperatures below curve A demix instantaneously but will become homogeneous again at temperatures corresponding to curve B when heated immediately after completion of phase separation.

Curve A represents '&e spinodal because 1) in homogeneous solutions of PP0 in toluene rapidly cooled to tempera-

tures on or below curve A nucleation has never been detected but structures well known for spinodial decomposition have been observed [4] .

2) homogeneous mixtures of PP0 in toluene demix instantaneously when cooled to any temperature below curve A (except when the quenching temperature is below the Tg of the mixture).

H6

t 1 4 0 1 50 50

ETHANQL TOLUENE I

I I I .

FIG. 10. Ternary liquid diagram for PPO-toluene: P, , polymer chain length n; Pz , polymer chain length m (m S n); S, solvent; PQ, cloud point curve; RS, spinodial curve.

PP0

ethanol toluene

FIG. 11. Cross sections at mferent temperatures of the phase diagram of the system PPO-tolueneethanol.

[3] J, J. van Aartsen and C, A, Smolders, Europ. Pojjm. J, 6,1105 (1970). [4] C. A. Smolders, J, J. van Aartsen, and A. Steenbergen, Kolloìd-Z. Z. Polym., 243,14

(1971)- [5] J. J. van Aartsen, Europ. Polym J., 6,919 (1970). [6] H. Tompa, Polymer Solutions, Buttworths, London, 1956. [7] R Koningsveld, Advan. Coll. Interface Scì., 2, 151 (1968). [8] G. Rehage, D. Maller, and O. Ernst, Makromob. Chem, 88, 232 (1965). [g] P. T. van Emmerik and C. A. Smolders, to be published.

[l01 J. Boon and E. P. Mag$, IUZ!ACInter. Symp., vol. II, Leiden, 834 (1970). [l13 F. E. Karasz and J. M. O'Reilly, J. Polym. Sci, B3,561(1956). [ 121 R. L. Scott, J . .Chem Phys., 17, 3,268 (1949). [ 131 P. T. van Emmerik and C. A. Smolders, J. Poljm Sci., C, 39, t,o.be published. [l$] S. P. Papkov and. S. G. Yefmova, Faserforsch Textil. tech. Sow$ Beitrage, 4, 118

(1967).

20

CHAPTER rrr

Determination of the thermodynamic parameters of poly-

( 2 , Q dimethyl 164 phenylene oxide)-toluene mixtures.

When homogeneous mixtures of PPO-toluene are cooled

phase t ransi t ion w611 b e i n i t i a t e d by l iqu id- l iqu id

phase separation! Depending upon temperature and com-

pos i t ion two mechanisms w i l l be possible , nucleat ion

and growth2 o r t h e so called spinodial decomposition 3 o

Spinodial decomposition can only be detected experimen-

t a l l y i f t h e c o n t r i b u t i o n of nucleat ion and growth t o

the phase separation during cooling can be ignored,

To t h i s purpose nucleation and growth should be a slow

processB which i s indeed the case f o r PPO-toluene' and

~ ~ ~ - c a p r o l a c t a m ' P 5 .

When l iquid- l iquid phase separat ion has occurred in these

systems, solvent induced crystall izationb of the PP0 w i l l

g radual ly t ake p lace in the reg ions r ich in polymer. Upon

renewed warming up the mixture w i l l hecome homogeneous

a t a lower melt ing point of the c r y s t a l l i n e material, A s

a resul t three phase separat ion curves in the system PPO-

t o luene o r PPO-caprolactam can be determined: the spino-

d i a l A ( rapid cool ing) I the cloudpoint curve B (extreme-

l y slow cooling) and the melting point curve C (upon

warming) ( f i g m 1) .

21

15C

T k :

T 100

50

O 10 20 30 40 50 60 - weight percentage of polymer

Fig. 1. Phase separa t ion curves of PPO-toluene

s o l u t i o n s

A s p i n o d i a l

B c loud po in t cu rve

C m e l t i n g p o i n t c.urve

22

The amorphous phase separation range in the system PPO-

toluene is very broad, not showing a maximum up till

70 percent by weight of PPO, so it seems very interes-

t i n g t o compare the thermodynamic parameters of t h i s

system w i t h those obtained from systems like poly-

s tyrene - cyclohexane7, where rather narrow amorphous phase separation ranges are found.

OUTLINE OF THIS WORK,

I n a recent art icle Schol te8 d i scussed the poss ib i l i ty

of determining thermodynamic parameters of a concen-

t r a t e d polymer - solvent system by l i g h t s c a t t e r i n g experiments. In this way A V (which is the chemical

po ten t i a l o f t he so lven t i n so lu t ion minus the chemical

po ten t i a l of the pure solvent) can be calculated and

from Avo t h e Flory-Huggins interact ion parameter x can be evaluated.

In the o r ig ina l formula t ions F low and Huggins supposed

x t o be independent of the weight f ract ion w of the poly- m e r 9 and of the temperature T , bu t l a t e r i nves t iga t ions

proved t h a t f o r many systems t h i s i s not qu i te cor rec t .

Koningsveld'' has modified the Flory-Huggins treatment

by introduct ion of a weight and temperature dependent

parameter g(wbpT) into the expression for AGm,the free

enthalpy.of mixing, For a quant i ta t ive eva lua t ion of

this parameter g (w,T) quant i ta t ive vä lues of A V

(which is the chemical potential of t h e polymer i n so-

lu t ion minus t h e same i n .the pure state) as a functton

of w and T are needed, i n add i t ion t o quan t i t a t ive va lues

of Ap0 (w,T) D Light scattering experiments afford values

2 3

of .?.!i and h p o only.

However, i n t he t heo ry of melting point depression

the difference between the melting point of the pure

polymer solution has been related to the chemical po-

tential of the polymer-solvent system and quantitative

values of Ap at the var ious mel t ing points can be cal-

culated’’. When these values are combined with those

obtained by l i gh t s ca t t e r ing , quan t i t a t ive va lues of

A u over a range of w and T can be determined which en-

ables g t o be calculated over the same range of w and T.

A s a r e s u l t , i n a l l systems where a melting point curve

aw

e x i s t s , l i g h t s c a t t e r i n g measurements over a range of

concentration and temperatures including points of the

melting point curve can afford all parameters necessary

to ca l cu la t e t he f r ee en tha lpy of mixing AG over a range

of concentration and temperatures.

For the system PPO-toluene l ight scat ter ing experiments

have been performed below and including the melting

point curve (curve C, f i g . 1). These measurements are

possible here, since phase separation is i n i t i a t e d by

liquid-liquid phase decomposition (curve B) followed by

c r y s t a l l i z a t i o n i n t h e r e g i o n s r i c h i n polymer . 1

EXPERIMENTAL . The l ight scat ter ing experiments were performed with

a Sofica photogoniometer. T h e i n t e n s i t y of s ca t t e r ed

l i g h t w a s measured a t d i f f e r e n t a n g l e s , i n o r d e r t o

a l low extrapolat ion to zero scat ter ing angle .

24

The temperature was cont ro l led by means of c i rcu la t ing

water from a thermostat , The so lu t ions were made dust-

f r e e by f i l t r a t ion t h rough a commercial mi l l ipore f i l -

ter o

Refractive index increments w e r e measured with a d i f -

f e ren t ia l re f rac tometer

Experiments w e r e car r ied ou t wi th so lu t ions o f PP0 i n

toluene with concentrations up t o 24.5 per cen t by

weight of PPO.

PP0 w a s character ized by Mn = 25,000 and MW = 49 ,000 .

The toluene was Baker Analysed Reagent p.a.

%‘HEl?MODYW&MIC F(4R.FTUI;ATIONS o

To ca lcu la te the f ree en tha lpy of mixing AG the Flory-

Huggins equation i s used expressed i n weight f ract ions e 7

RT i Mi I

AG = free enthalpy of mixing of p A V grams of so lu t ion

wo = weight fraction of the so lven t

wi = weight fraction of the polymer component i

w = t o t a l weight fraction of t h e polymer

g = empi r i ca l co r rec t ion f ac to r

Mo = molecular weight of the solvent

Mi = molecular weight of t h e polymer component i

Different ia t ion of AG wi th r e spec t t o the number of moles

of the var ious components y ie lds the express ion for t he

chemical potentials:

25

Mn = number average m.w

and

Mi Mi

RT Mn M o - "i = lnwi + l-- w---(l-w) + (3)

The f ac tor

[g - ( 1 - w ) g J ( 4 ) represents the Huggins interaction parameter x. It can be calculated i f Avo i s known as a function of

w and T.

I f A s l i i s known, then

can also be calculated. Combination of eq 4 and 5

yields values for the interact ion parameter g .

With these g values (supposed t o be independent of m.w)

t he AG curve can be calculated with formula (1)

LIGHT SCATTERING.

From the general theory of l i g h t s c a t t e r i n g of so lu t ions

as appl ied to polymer solut ions12 'l3. Scholte' derived

an equation which relates t h e s c a t t e r e d l i g h t intensity,

extrapolated t o zero Scat te r ing angle, to the chemical

po ten t ia l s o f the so lu t ion , l ead ing to:

26

AI = s c a t t e r e d l i g h t i n t e n s i t y o f t h e s o l u t i o n minus

t h a t of t he pu re so lven t , r e l a t ive t o t he l i gh t

intensity using benzene, and measured perpen-

d i c u l a r t o t h e i n c i d e n t beam

= angle fac tor

= i n t e n s i t y of s c a t t e r e d l i g h t e x t r a p o l a t e d t o

zero angle 0

= re f rac t ive index of benzene

= WvogadroP S number

= wavelength i n vacuo

= Rayleigh f a c t o r of benzene

= density of t he so lu t ion

= refractive index increment

= weight average molecular weight

= molecular weight dis t r ibut ion

The r i g h t hand s i d e of eq 6 and 7 can be determined

experimentally and yields res-

pec t ive ly a t the concentration used.

27

When th i s has been done f o r a number of concentrations

upwards to the appropriate . value of w, g ives the depen-

dence of A V , on w a t cons tan t t empera ture ( in tegra t ion

constant zero since Ap0 = O a t w

With the va lues o f Au, x can now of equation 2 .

Integrat ion of 2 would g ive

= O ) .

be qalçulated by use

va lues for Ap f o r a l l

desired values of w if the in tegra t ion cons tan ts were

known. In t h i s ca se , however, Au $. O a t w = O and the

in tegra t ion cons tan t must be obtained independently

from other data.

MELTING POINT DEPRESSION.

The theory of melting point depression' offers a method

to ca lcu la te va lues o f A V f o r polymer-solvent solutions

a t t he me l t ing po in t of t h e c r y s t a l l i n e material.

The condition of equilibrium between crystall ine polymer ~

and t h e polymer u n i t i n s o l u t i o n y i e l d s

Ftu c - P, - v u - Ft; 0 - 1 (8) v u = chemica l po ten t ia l o f the c rys ta l l ine polymer C

repeat ing uni t

= t he 'same f o r t h e l i q u i d polymer u n i t

v1 = t he same f o r t h e polymer r epea t ing un i t i n so lu t ion . U

28

This equation can be written as

-Au = +AvMBU f +

f indicates fusion, M indicates mixing.

Hoffman14 has shown t h a t t o a good approximation

r u

QH: =

Tm -

TE =

AT =

-

may be wri t ten as

molar enthalpy of fusion

repea t ing un i t

(10 1

€or the pure polymer

melting point of t he so lu t ion

melting point of the pure polymer

T: - Tm

Combination of 9 and 10 yieldsLL:

AT. Tm

o 2 A P ~ , ~ = - AH: .)

(Trn)

and for I mole of polymer solut ion:

Since AT, T:, Tm and AHo can be determined experimentally,

Aum can be calculated for each solution a t i t s melting

point .

Combination of hpm values so obtained with l ight scat te-

r ing da ta for over a range of concentrations and a t

constant temperature enäbles the integrat ion constant

a t tha t t empera ture to be ca lcu la ted .

29

A numerical example fo l lows in the next sec t ion .

I n t h i s way quant i ta t ive values for both A V , and

Ap could be obtained in the concentration range

from O t o 24.5% of polymer and for temperatures

between 25OC and 65OC.

With equation 2 and 3 the interact ion parameter g

could be obtained in the same concentration and .

temperature ranges.

RESULTS.

Crystal l izat ion of PP0 from so lu t ion w i l l only be

induced af ter l iquid- l iquid phase separat ion has

occurred. The la t ter takes place a t lower temperatures

than the melting of the polymer crystals. Therefore it

is experimental ly possible here to obtain l ight scat-

tering data comprising concentrations and temperatures

of the mel t ing point curve. In fact most l i g h t s c a t -

t e r ing da ta , by l i m i t s set by the necessi ty of working

below about 65OC, o r ig ina t e from the region between the

melting point curve and the cloud point curve (curve C

and curve B, respec t ive ly , in f ig . 1). Even measurements

below the cloud point curve were possible here due t o

an induct ion per iod for the nucleat ion mechanism.

Light scattering data have been obtained a t temperatures-

of 24.5OC, 34.5OC, 45OC, 55OC and 65OC for solut ions of

PP0 i n t o luene a t concen t r a t ions from w = 0.009 up t o

w = 0.245.

50.

40

30

20

10

(

va lues of PPO-toluene solutions

determined a t d i f f e r e n t t e m p e r a t u r e s

Q T = 24.5OC

V T = 34.5OC

'$ T = 45OC

A T = 5SoC

o T = 65OC

31

By use of equations 6 and 7 va lues for ( - Mn and (s)... were calculated, when ng w a s taken to be 1.502, X = 546 mp and RB = 16.3 x 10'' cm-'; values

f o r p and - have, been compiled i n t a b l e 1. dn dw Table 1 contains Avo values for var ious concentrat ions

and temperatures, obtained by graphica l in tegra t ion

Of (y) Mn an?Jo

versus w. The x parameters has been calculated

via eq. 2 . Note the negat ive x values. Fig. 2 values as a function of w a t d i f -

ferent temperatures. Because of d e t a i l s of the curves

a t h igher concent ra t ions the po in ts a t w = 0.009 have

not been included.

Table 2 conta ins the da ta used to ca lcu la te .Ap. The

va lues for Ap were ca lcu la ted from the mel t ing point

curve via equation 1 2 ahd with Tm = 513K (measured by

DSc) and AHo = 120 k c a l / m ~ l ' ~ .

The values were obtained by integrat ion of

from l i g h t s c a t t e r i n g d a t a g i v e n i n f i g . 2.

As an example how the in tegra t ion cons tan ts were ob-

ta ined we cons ider the da ta for a PP0 solut ion with

w = 0.153.

A t i ts melting point Tm = 65OC ( f i g . 1) the value of

A'm.p ' = -113.3 x l o l o erg/mol. From l i g h t s c a t t e r i n g

data a t T = 65OC w e obtain, by graphica l in tegra t ion

32

TABLE I

Chemical potentials and interaction parameters of PPQ-toluene solutions.

d

0 . 0 0 9

T O C

2 4 . 5

3 4 . 5

O .O48

6 5

2 4 . 5

3 4 . 5

45

0 . 2 0 4

O. 2 4 5

45

l ~ 6 5

4 5 * 5 5

6 5

45 ;c 5 5

6 5

1 7 5 . 2

1 9 1 . 4

409.0

4 5 8 . 5

600.0

6 5 1 . 9

7 4 2 . 7

1 4 7 4 . 4

% Light scattering experiments were p?rformed here at temperatures on or below the cloud point curve

3 3

from w = O t o w = 0.153, h u i o s = 454.0 x l o l o erg/mol.

The in tegra t ion cons tan t to be used for a l l o ther

( data a t tha t t empera ture i s the d i f fe rence between these two values and y i e lds

C. = (-113.3 - 454.0) x l o l o ergimol = -567.7 x l o l o erg/mol When the da t a fo r h p o and h p from t ab le s I and 2 a r e

inser ted in to equat ions 2 and 3 , t h e g values as given

i n t a b l e 2 and f ig . 3 are obtained.

The g values tabulated have been used to calculáte the

E values for Mo grams of solut ion for each w and T con- AG

sidered. Results are shown i n f i g . 4 and 5 and i n t a b l e 3.

For the ca lcu la t ion of the g values the da ta of w = 0.009

at different temperatures have not been used because for

w = O . O09 the mel t ing po in t was below T = 24.5OC.

34

TABLE I1

e r a c t i . and in :hemical n o t e n t i a l u t i o l PPO-to IS uene s

O. 153 3.204 3 o 245 0.009 O .O24 0.048 O. 103 I W 34.5 34.5 34.5

417.7

+ISO. l

1 ., 235 45

34.5

370 o 8

+933.2

1.122 45

34.5

288.0

i 50.4

0.970 45

34.5

116.7

-120.9

-237.6

-120.9

O. 709 45

119.0

-294.0

0.441 55

34.5

224-7

- 13.1 0.870 45

228.9

-984. T

0.621 55

234.6

-283.8

O. 488 65

235.8

-331 -5

0.436

I T O C 45 490 D 2

+77.2

1.094 55

500-0

-18-4

0.899 65

540. I

-27 s 2

O o 870

45

470 e 2

i57.2

1.026

55

476 o 4

-42,O

0.855 65

505.6

-61.7

o e 820

432 o 6

+ 19.6

Oi943 55

440 o O

- 78.4

O 790 6 5

294.2

-118.8

-413.0

-118.8

0.712 55

298.2

-220.2

0.576 65

307.2

-260. I

0.530

382.8

- 30-2 0.850

5 5

integr. s . 10

Ave 10

-10

-10

402 ., 2

-116.2

-518.4

-116.2

O. 727 65

404,8

-162.5

0.670

i n t e g r . c . 10

Avo 10’’’

-10

t T°C 454,O

-113.3

-567 D 3

-113-3

0.741

34 unit: e r g / m o l

35

1.3 -

1.2-

1 .l

1.0

0.9-

0.8-

0.7-

0.6-

0.5- 9

‘ 0.4

0.3

Q2-

0.1.

(

T=45 k

T= 55 “c T= 65 “C

l I

0.lO 0.2 o - W

I

0.3 O

Fig. 3. Plots of g vs c o n c e n t r a t i o n of PPO-toluene

s o l u t i o n s

A T = 34.5OC

o T = 45OC

V T = 55OC

o T = 65OC

36

TABLE 111

Values of - *' 1 0 5 for solutions of PPO in toluene RT D

-f W

+T 0 . 0 0 9 0 , 2 4 5 0 , 2 0 4 0 . 1 5 3 O . 1 0 3 0 . 0 4 8 0 . 0 2 4

34.5OC 1 8 3 5 5 3 0 - 3 0 5 - 366 - 2 7 9

4 5 O C

65OC

- 4 7 1 6 - 4 3 9 7 - 3 9 3 1 - 3 1 2 0 -2105 - 1 2 6 1 55OC

- 1 1 1 0 -1621 - 1 9 4 9 - 1 9 8 3 - 1 4 8 4 - 9 5 0 - 5 1 8

l - 1 3 8 3 - 5 2 5 4 - 4 9 6 5 - 4 5 7 9 - 3 6 4 6 - 2 3 1 5

37

1OOC

O

-2oc

L 1 0 -5

3.x) -5

AG Fig; 4 . Calculated E v a l u e s f o r d i f f e r e n t PPO-toluene

s o l u t i o n s a t T = 45Oc

38

AG Fig, 5. P l o t s of vs c o n c e n t r a t i o n of PPO-toluene

s o l u t i o n s a t d i f f e r e n t t e m p e r a t u r e s

A T = 34.5OC o T = 45OC

V T = 55OC o T '= 65OC

39

Liquid-liquid phase separation by nucleation and

growth, performed at the appropriate temperatures

between the cloud point curve B and the sp inod ia l A

of f ig . 1 w i l l occur af ter induct ion times which

are very short a t temperatures near curve A but

w i l l t ake a few hours a t temperatures near curve B . For th i s reason it has been possible to perform l ight

scattelling ,experiments a t temperatures not too far

16

.below the cloud point curve.

As can be seen from f i g . 3 t h e ($lwD values are high a t f i r s t and constantly decrease. For tempera-

t u re s which are f a r enough above the c loud po in t the

vallies of MWD a t any concentration are simply

proport ional to the temperature .

A t temperatures near the c loud point ( in this case

for va lues o f w la rger than 0 . 1 a t the experimental

temperatures mentioned above) the change of aw wD , (“1 i s more than propor t iona l to T , which means t h a t e x t r a

s c a t t e r i n g i n t h e neighbourhood of t h e i n s t a b i l i t y

region through the term 1 - w An eq 7, becomes very P (aI) e=o

important.

The sp inodia l i s character ized by (s)= O . The experimentally determined values of (2) a r e

aw W D

decreasing functions of w a t a l l temperatures being

measured a t down to va lues c lose to zero .

40

Near the spinodial , l ight scat ter ing experiments w e r e

no t poss ib le any more, Due to l iqu id- l iqu id phase

separat ion by nucleation and growth the s ca t t e r ing

in tens i ty increases enormously and the spinodial could

no t be de t ec t ed i n t h i s way.

When the curves in f ig . 2 are supposed to b e s t r a i g h t

l i n e s -for h igher concent ra t ions , l inea i r ex t rapola t ion

of (*) t o values of aw ~D - (3) - O y i e lds as the sp i - nodial composition €or T = 24.5, 34.5 and 45OC, respec-

t i v e l y , w = 0 , 2 2 , w = 0.27 andw = 0.33, in reasonable

agreement with the values that can be read from fig. 1

for the experimental spinodial curve.

The ca lcu la ted x values ( tab le I) show that x is s t rongly concentration dependent, but i s not very sens i t ive to

temperature changes. I n c o n t r a s t t o x values of most

aw MWD

other polymer-solvent systems, x shows negative values i n t h i s case, which means t h a t i n

x = g - (l-w) g 9 i s a dominant €&tor. aw

Previously negative x values have been found in t he sys t em PP0 - l - ~ h l o r o n a p h t a l e n e l ~ and several o the r po la r

systems”, In these systems there are s p e c i f i c polymer

d i luen t i n t e rac t ions which have n o t y e t been evaluated

quant i ta t ive ly .

The calculated g values shown i n f i g . 3 i n d i c a t e t h a t g

i s s t rongly w- and T- dependent.

41

If l i q u i d - l i q u i d phase separation will take place the

minimum (or an inf lect ion point and an absolute

minimum) . I n f i g . 5 t h e AG Curves f o r w up t o w = 0.245 a r e

shown, AG being expresses in uni ts of RT. The AG

curve is con t r ibu ted t o by two fac to r s ,

MO

Mn wolnwo + - wlnw which is negative for each w, and

g wo w which has posi t ive values for each w. ( i n

f a c t t h e number average AG has been calculated)

As can be seen, g values increase with decreasing

temperature which means t h a t t h e s o l u b i l i t y of the

polymer i n t h e s o l v e n t becomes worse a t lower tempe-

ratures, e 'ventually giving rise t o phase separation.

For T = 34.5OC and 45OC t h e E versus w curve shows

a minimum but lack of information about the further

p a r t of t h e AG curve makes it impossible t o draw

AG

conclusions about the coexistence l ines.

From f ig . 1 i t , i s known t h a t a t T = 35OC and a t 45OC

solut ions with w larger than w = O . 15 and w = O - 2 0

are no t s t ab le and w i l l separate into other phases .

The AG curve for these two temperatures is not a t

variance with these experimental facts. For T = 55OC

and T = 65OC the AG curve does not show a minimum yet .

Solutions up t o w = 0.27 f o r T = 55OC and w = 0.33 f o r

T = 65OC a r e homogeneous and w i l l no t separa te in to

other l iquid phases, which is i n accordance with the

AG curve fo r these two temperatures.

1. P.T. van Emmerik, C.A. S m o l d e r s t o " b e published

2. M. Volmer , A. Weber - Z , Phys . Chem. 119 (192.5) 277 3. J a w , C a h n - A c t a M e t , 9 (1961) 795

J, Chem. Phys . 42 ( 1 9 6 5 ) 9 3 4, J,J. van Aartsen, C.A. Smolders - Europ. Polym,

J, 6 (1970) 1105 5 , CDA, Smolders, J.J. van A a r t s e n , A, S t e e n b e r g e n -

- Molloid Z.U. Z. P o l y m e r e 243 (1971) 14 6 . E.P. Magré, J. Boon - IUPAC Intern. Symp. L e i d e n

1970 pag. 835

7. T.G. Scholte - J. P o l p t . Sci. A2 8 ( 1 9 7 0 ) 8 4 1 8 , T.G, S c h o l t e - Europ. Polym. J. 6 (1970) 1063 9 o P. J, Flory - P r i n c i p l e s of P o l y m e r Chem. 1 9 5 3

C o r n e l l Univ. P r e s s

10 . R, Koningsveld - Adv. Koll, I n t e r f . Sci. 2 (1968) 151 11, J . B . H e l m s - Dissertation Groningen 1970 12 . H.C. Brinkman and J.J. Hermans - J, Chem. Phys .

1 7 ( 1 9 4 9 ) 5 7 4

13 . P . Debeije - J. Chem. Phys . 31 (1959) 680 1 4 , J , D , Hoffman - J, Chem. P h y s , 28 (1958) 1192 15, A . R a S c h u l t z and C.R. McCul lough - J. Polym Sc,.

A2 1 (1969) 1577 16. P O T D v- Emmerik, C.A. S m o l d e r s - t o be published 17 . F,E, Marasz and O. Mangaraj - P o l y m e r P r e p r i n t s

- 12 (1971) Div. of P o l , Chem. A,C.S.

18. R. Wessling - J. of Appl. Polym. Sc. 2 (1970)

43

CHAPTER IV

The calculation of the cloudpoint curve with a concen-

kration and temperature dependent free energy correc-

tion parameter.

SUMMARY

The free enthalpy correction parameter g in the Flory-Hug-

gins equation fo r the Gibbs free enthalpy of mixing in polymer

solutions is considered generally as a concentration and

temperature independent parameter. It has been extended here

with linear concentration and temperature dependent terms.

With these parameters,six different types of cloudpoint

curves can be predicted. Using the experimental cloudpoint

curve for solutions of pOly(2,6 dimethyl 1,4 phenylene

oxide) in toluene up to about 70% by weight of polymer, a

set of g-parameters is obtained, accounting for concentra-

tion and temperature dependence. With the parameters thus

obtained,the melting point curve has been calculated which

agrees very well with the experimental melting points for

this system.

44

INTRODUCTION I '

Liquid- l iquid phase separat ion in polymer solvent systems

have been descr ibed extensively by many au tho r s l r2 D Depen-

ding upon the na tu re o f t he polymer and the solventp cloud-

point curves have been reported with upper and lower cri-

t i c a l s o l u t i o n p o i n t s which may show a narrow o r wide a-

morphous phase separation range ., By assuming the o r ig ina l

Flory-Huggins equation for t h e free enthalpy of mixing

(AG,) t o be v a l i d f o r a l l these systems, cloudpoint curves

have been calculated but agreement with experimental curves

3

I

x

was general ly poor, Since then, ref ined theories have shown

t h a t t h e o r i g i n a l Flory-Huggins parameter x does not have a constant value; it may depend upon the molecular weight

of t h e polymer, t he t empera tu re o r t he t o t a l polymer con-

centration4.There are some advantages5 i n t h e u s e of weight

f r ac t ions (w) ra ther than volume f r ac t ions 4 in the F lory-

Huggins equation, al though the interaction parameter, than

labeled as xw, w i l l d i f f e r s l i g h t l y from x. By tak ing th i s in to account , a b e t t e r agreement f o r some

6

specific polymer-solvent systems could be obtained, In the

system poly ( 2 ,Q dimethyl-l , 4 phenylene oxide) Xx-toluene,

a cloudpoint curve has been measured which does not show

a maximum up t o 70 per cen t of weight polymer , Next t o 7

t h i s , a melting point curve could be measured on reheat ing

demixed solut ions, but never before l iquid- l iquid phase

separat ion had occurred, Light scattering* experiments

with PBO-toluene solutions, performed according to the

principles discussed by T.GO Scholte6 indicate t ha t

xThe free enthalpp of mixing AGm s tands for Gibbs free energy change !Xz PP0

..

45

the monotonouS rise of the cloudpoint curve as a function

of polymer concentration i s connected with an unusual de-

pendence of the interaction parameter g upon temperature

and polymer concentration, which must be due t o p o l a r i n -

t e r ac t ions between the polymer segments and the toluene

molecules.

Several authors have published theories which t a k e i n t o

account the polar interact ions between polymer molecules

and the so lvent '''l2. Thes.e theories are general ly very

complicated and requi re knowledge of numerous a p r i o r i

unknown parameters. Heil and pr au snit^'^ presented a semi- empir ica l equa t ion for AGm based on the l oca l volume frac-

t ion concept of Wilson". This segment-interaction equa-

t lon contains two adjustable parameters per binary mixture,

determined from binary vapour pressure data. When these

parameters are used t o c a l c u l a t e AGm vs w1 curves for poly-

mer-solvent systems a t temperatures lower than the c r i t i -

cal temperature, the cloudpoint curves obtained from the

AGm curves do not agree a t a l l with most types of experi-

mental cloudpoint curves. However, by extending the con-

s t a n t free enthalpy correction parameter with a l i n e a r

concentration dependent t e r m and assuming tha t bo th terms

may be simple functions of temperature, different types of

cloudpoint curves can be obtained. One of these types a-

grees with the system PPO-toluene. Using the experimental

cloudpoint curve of PPO-toluene a s a reference, a set of

g parameters can be obtained which i n t u r n can be used t o

calculate the mel t ing point curve for t h i s System-

46

THEORY

In order to calculate the equi l ibr ium conposi t ions of

a demixed polymer-solvent system, the free enthalpy

of mixing must be known.

The Gibbs f r e e energy of mixing i n a volume element AV

is given as follows' n 5 i n term of weight fractions:

MO A G ~ / R T = (W lnw0 + c - w i l m i + 9 w o w ) r p .AV O i Mi O

wo = weight fraction of the so lvent

Mo = molecular weight of the so lvent

wi = weight fraction of t he polymer component i

w = to ta l weight f rac t ion of t h e polymer

g = free enthalpy correction parameter

p = density of the so lu t ion

Mi = molecular weight of component i of t h e polymer

R = Universal Gas Constant

g is a dimensionless quantity which characterizes the in-

teraction energy per solvent molecule divided by kT (k =

Boltzman constant) and which reaches the value 0.5 ( f o r Mi

i n f i n i t e l y l a r g e ) a t t h e c r i t f c a l c o n c e n t r a t i o n a t which

phase separat ion f i rs t appears .

In a recent theory, Ruggins" suggests g t o be dependent

upon temperature and concentrat ion; the concentrat ion de-

pendence can be approximated by a polynomial i n wl, t he

polymer weight fraction. This concept has also been fol-

lowed by Roning~veld '~ , l ead ing to the fo l lowing expres-

s i o n f o r g:

47

In th i s formula each coef f ic ien t g; may be a temperature J

function of the form15:

I f i n a polymer-solvent system no po la r o r hydrogen bon-

ding i s t o be expected, n=O i n e q . ( 2 ) and g i s indepen-

dent of wl. I f i n t h o s e cases the temperature dependence

of g a l so i s not very marked, experimental values are i n

reasonable agreement with calculated values, this being

t rue for sys tems l ike polyethene-diphenylether and severa l

others .

D e l m a s e t succeeded in describing the occurrence of

16

a lower and upper c r i t i ca l solution point, by using the

second and t h i r d term of eq ( 3 ) as a temperature function

of g. Which terms must be taken depends upon the kind of

polymer-solvent system considered.

From u l t r a c e n t r i f u g e , l i g h t s c a t t e r i n g and c r i t i c a l m i s -

c i b i l i t y measurements performed with polymer s o l u t i o n s i n

which polar bonding exists between the polymer segments

and the solvent molecules, x, i s found to be dependent upon W1 and T 5,18

- A Types of cloudpoint 'curves obtained with a l i n e a r con-

cent ra t ion and temperature dependence of g.

To account for the in te rac t ion energy due t o polar bonding

eq ( 2 ) can be used, in which w e have l imited ourselves to

n=l .

48

T h i s means that

g = go + g p 1 (4)

while both go and g1 may be a temperature function again.

A s it i s known t h a t p o l a r i n t e r a c t i o n depends s t rongly

upon temperature, the temperature dependency has been ta-

ken as follows:

"j - gj.1 -l- gje3T - (5 1

i n which q can be negative o r pos i t ive .

With eq (41 and (5) r AGm versus wp curves can be calculated

a t each temperature. From double tangent points of these

j - 3

&G versus wI curvesp the binodal curve i s found,

In general, polymer-solvent systems are t r e a t e d as quasi- m

bfnary systems and although the cloudpoint curve is not the

same as the b inodal in those cases, f o r polymers with a

r a t h e r narrow molecular weight distribution these curves

a r e comparable.

The following s i x types of cloudpoint curves can now be

obtained:

1, I f go f f (T) and q1 = O the c loudpoin t x rve w i l l show

a d i s t i n c t maximum a t values of w < 0,1 and w i l l have

a narrow amorphous phase separation range. (f ig. l a ) .

An example of this kind ts polyethene-diphenylether .

1

16

11. If go f f (T) and g, f f (T) the cloudpoint curve w i l l

show a very broad amorphous separat ion range; the

maximum of the CUTV@ is not well-defined. (f ig- lb)

An example of t h i s kind i s ce l lu losediace ta te - benzyl alcohol . 19

49

III. I f g f f ( T ) and g1 = f ( T ) , the cloudpoint curve w i l l O

show a broad amorphous separation range with a maximum

f o r w < 0 . 3 ( f i g . IC).

An example of t h i s k ind may be ethylene-vinyl acetate

(Elvax-40) -Caprolactam . 20

IV .

V.

V I .

I f go = f ( T ) and g1 = O , the cloudpoint curve w i l l show

a broad maximum a t values of w1 between 0 .05 and 0 . 2 0

( f i g . l a ) . An example of th i s k ind i s polystyrene-toluene 18 .

If go = f ( T ) and g1 9 f ( T ) , the cloudpoint curve w i l l show a broad amorphous separation range with a maximum

f o r w1 > O .7 ( f i g . le) . This combination of g values i s not very l ikely and experimental examples have not been

reported.

I f go = f ( T ) and g1 = f (T) , the c loudpoint curve w i l l show

a broad amorphous separation range with a maximum f o r

w1 between 0.3 and 0 .7 ( f i g . I f ) .

An example of th i s k ind is PPO-toluene . 7

The cloudpoint curves I t o V I represent the main types.

Since many combinations of values for g and g1 are pos-

s i b l e , t h e maxima of the cloudpoint curves may s h i f t t o

h i g h e r o r lower values of wl.and the height of the maxi-

mum may change t o h i g h e r o r lower temperatures. By i n t ro -

ducing proper values for g and gl, the AGm curves as a-

function of w at each temperature can be calculated for

any p a r t i c u l a r system.

O

O

1

d. t y p e I P

i 1 ! 1

c. t y p e m

_" _-_ -- z wetght pe%nt polymer 100

Fig. 2 AG/RT versus weight percent polymer; curves cal-

c u l a t e d w i t h eqs.1 and 4 making use o f eqs.11

and 12.

- B Calculation of the thermodynamic potentials

If phase separation occurs,the following conditions must

be obeyed 2

I

Avo = (chemical potent.ia1 of the solvent in phase 1)

-(chemical potential of the pure solvent) II

h p o = &he same in phase 2

A p l = (chemical potential of the polymer in phase 1)

-(chemical potential of the polymer in the pure

state) II

Apl = the same in phase 2.

Introduction of eq (4) in (1) and differentiation of (l)

with respect to the number of moles of each component,

assuming phV 1, , gives the expressions for the Hb thermodynamic potentials:

- = In(l-wl) + (l--)wl MO + (go+2g w - g l ) w l 2 RT Mn 1 1

The factor (go+2g1wl-g ) is to be compared wit.h the correc-

tion parameter x in the original Flory-Huggins equations’ 1

for Avo.

If go and g, are known as functions of T,Apo and

h p l can be calculated with eq ( 7 ) and ( 8 ) .

5 2

C Calculation of the melting point curve -

If in PPO-toluene solutions liquid-liquid phase separation

has taken place, the concentrated phase will crystallize

after a certain time 7 . Differential scanning calorimetry has shown that liquid-liquid phase separation always pre-

cedes crystallization and that the degree of crystallinity

is very small, When a demixed solution is heated againtit

will become homogeneous at its melting point.

At this temperature for each composition the following ex-

pression is valid:21

AT,Tm Aufru = AEuo - o 2 ‘Tm 1

hpfru = chemical potential of meltLng of one mole of

polymer segments O AHu = enthalpy of melting of one mole of polymer segments

Tm = melting temperature of the solution

= melting point of the pure polymer T; AT - - Tmo - *m*

the melting point curve can be calculated with eg ( 8 ) f ( 9 )

and 10 once the paraeters necessary to determine hpl and

further AHuo and Tmo are known.

The agreement of the calculated and measured melting point

curves has been used as a check on the validity of the

parameters obtained for g,

53

RESULTS

The calculations were based on the system PPQ-toluene; the

PP0 used is characterized by E = 25.000 and g = 49.000 Although this system is in fact a multi-component one, it

has been treated as a quasi-binary system. The thermodyna-

mic quantities have been calculated for M grams of solu-

. t.ion, this means that the factor eV = I in all calculati-

I1 W

O

MO ons.

To obtain proper values for go and glrAGm curves were cal-

culated at different temperatures starting for example with

go = 500 cal mol-’ and g1 = 1 5 0 0 cal mol-’

From the tangent points of these AG vs w1 curves,a cloud

point curve was obtained which in general had one common

point with the measured cloud point curve of PPO-toluene.

For that temperature and concentration the values of

g , Auo and hp1 were known. The same was done taking,other

values for go and g1 unti1,for a set of 4 concentrations,

the appropriate values for g, hpo and hp1 were known.

Sinceeqs.(4) and ( 5 ) must be obeyed, from these four values of

g, ~p~ and Apl the four unknown parameters could be obtai-

m

ned, leading to

and -

g1 - 1251.8 - 0.7 (T - 2 7 3 )

RT

With these values and eq (1) and (4),the AG/RT values have

been calculated attemperaturesfron! 278 K to T = 373 K.

54

The r e s u l t s are shown i n fig. 2 .

B y drawing tangent l ines along the BG/RT versus wLcurveslr

t he thermodynamlc po ten t i a l s hpo and A u 1 a t the concentna-

t i o n s of the tangent points can be obtained graphically,

The c a l c u l a t e d r e s u l t s f o r and A u l as a function of w O l p

obtalned from eq. (71 and (81 F a re r ep resen ted i n f i go 3 and

4. The complete cloudpoint curve obtained from f i g o 2 is

shown in f ig , 5 , together with the experimental c loudpoints ,

Hence the system PPO-toluene can be described as a type V I

cloudpoint curve with g and gl both functions of T.

To check the va l id i ty o f the parameters go and gl8 the m e l -

t ing po in t curve was calculated. with eg. (81 p ( 9 ) and ( 1 0 ) a

AH: values between 6 and 15 cal gram-’ have been reported 22 . In fig. 5 the calculated mel t ing point curve i s shown~ ob-

ta ined by taking AH: = 9cal gram I Ti = 513K and making

use of (11) and (121.

O

-1

55

Fig . 3 hvo/RT versus weight percent po1ymer;curves calcu-

l a t e d w i t h eqs .7 , 11 and 1 2 f o r p.AV/Mo.= 1.

; -010 - 020 Fig. 4 b p /RT versus weight percent po1ymer;curves cal- l

c u l a t e d w i t h &s. 8,11 and 1 2 f o r pAV/Mo = 1.

150

rc

f IOC

50

l

0-1 0-5 1.0 weight percent polymer

Fig . 5 Calcula ted and exper imenta l c loud po in t - and

me l t ing po in t cu rves

c a l c u l a t e d c l o u d p o i n t c u r v e

--- c a l c u l a t e d m e l t i n g p o i n t c u r v e

x expe r imen ta l c loud po in t s

O expe r imen ta l me l t ing po in t s

20 - we&& percent polymer F i g . 6 AG/RT versus weight percent polymer

x light s c a t t e r i n g r e s u l t s f o r T = 4 5 O ~

O c a l c u l a t e d r e s u l t s w i t h e q s . 1, 4 , II and P 2

57

DISCUSSION

It i s w e l l known t h a t t h e y f a c t o r I n many polymer-solvent

systems is concentration dependent. In this paper the ef-

feet upon A G m , h p and the cloudpoint curve of a l i n e a r

concentration dependence of g, i n combination with a simple

temperature dependence, has been d2scussed. It should be

r ea l i zed however t h a t i n many polymer-solvent systems g

may a l s o depend upon molecular weight of the polymer and

khe temperature dependence may be of a different kind.

Since these two effects are in genera l no t very s t rong ,

most polymer-solvent systems with a not too broad molecu-

l a r weight distribution can be quant i ta t ive ly descr ibed

according to types r t o VI. r€ g contains a concentration dependent t e r m , it is not equal anymore to t he cons t an t

x, term of Plory-Huggins. Following the. equations of Flory-Huggins, t he f ac to r x, is e q u a l t o q - (l-wl 2. I f ( 1 - w ) S outweighs g then x, w i l l become negat ive, which has been found in several systems . 23 . aw aw Since the calculat ion of Ayo i s ve ry s ens i t i ve fo r t he

value of 3 and, as i n many polymer-solvent systeins ( 1 - 6 v ] a g ,

has been neglected, it i s t o be expected that in general aw aw

t he agreement of the calculated and the experimental cloud-

point curves would be poor.

In the system PPO-toluene, po lar in te rac t igns occur between

t h e benzene r ings in PP0 and toluene. This kind of inter-

act ion a lso takes place with other solvents l ike chloro-

form, nitrobenzene etc., in genera l wi th those so lvents

with a dfpole moment or wi th so lvents tha t can be po lar i -

zed eas i ly .

From our experiments f t can be concluded that solvents with

a large dipole moment are very good so lvents ‘ for PPO. If

the solvent molecules do no t have a dipole moment and can-

not B e p o l a r f z e d , o r i f t h e p o l a r i n t e r a c t i o n between the

solvent molecules themselves i s strong, the system PPO-

solvent behaves l ike one described as type I: o r Ir, a t

rather high temperatures.

W2th l igh t sca t te r ing exper iments , quant i ta t ive va lues for

t h e thermodynamic parameters of PPO-toluene have been ob-

t a ined for concentrations up t o w1 = 0.245. Figure 6 shows

the AG/’RT curves €or T = 45OC calculated with the parame-

ters obtazned from l igh t Sca t te r ing , toge ther wi th the cal-

culated curve obtained above, Figures 7 and 8 represent the

g and x values respect ively, Light scat ter ing experiments for polar systems are d i f f i c u l t t o i n t e r p r e t s i n c e l i g h t

scat ter ing theory has not been adapted f o r correct ions

n.ecessairy f o r t h e a t t r a c t i v e f o r c e s o€ t he polymer segments

8

with the solvent molecules. From f igu re 7 and 8, it can be

concluded t h a t € o r wl> O . 1 t h e c a l c u l a t e d v a l u k f o r x and g agree qua l i ta t ive ly , For low concentrat ions, the

l i gh t s ca t t e r ing r e su l t s dev ia t e s t rong ly from the newly

calculated values .

W

59

I

F i g . 7 x as a funct ion of weight percent polymer x ' l i g h t s c a t t e r i n g r e s u l t s f o r T = 45OC

0 . c a l c u l a t e d r e s u l t s f o r T = 45OC

i" -1.0-

-2.0-

-3.0

10 ' 20- - weight percent polymer F ig . 8 Free e n t h a l p y c o r r e c t i o n p a r a n e t e r g versus weight

percent polymer

x l i g h t s c a t t e r i n g r e s u l t s for T = 45 OC

O c a l c u l a t e d r e s u l t s ' f o r T = 45 OC

60

1, P.J . Plory; Pr inciples of Polymer Chem, E953

Cornell University Press

2 , H, Tompa; Polymer $olutions,London Buttemorths Sc,

P u b l , 1956.

3 , S.P. Papkov, S.G. Yefimova; Faserf. u. Text i l tech .

Sowj. B e i t r - . 118 (1967) D

4 , A.J, Staverman in: Encyclopedia of Physics, Springer

Verlag 1962 p. 456.

5. T.G, Scholte; J. Polymer Sci A2 2, 841 (1970) . 6 . T.G. Scholte; Europ. Polymer J. g r 1063 (1970)- 7, P.T, van Emmerik, C.A. Smolders; Journa l of Polymesr.Sc,

Part C. 38 ( i n press) ., S. P O T , van Emmerik, C.A. Smolders; Joulcnal of Polymef Sc.

P a r t C. 39 ' ( i n p re s s ) m

9. T ,G , Scholte; J. Polymer S c i , P a r t A2 2 . b 1533 (1971) o 10. T.G, Schol te ; Publ icat ion no. 9 2 9 , Central L a b - ,

Dutch Sta te Mines, Geleen ( 1 9 7 1 ) -

11, MOL, Huggins; J. Arn. Chem, Soc. 86, 3535 (19641, 12 o A. Mbster; S t a t i s t i s c h e Thermodynamik

Springer-Verlag Berlin (1956) . 13. F.J, H e i l and J , M , Prausnitz;

A.I,Ch,E. Journa l 12, 678 (1966) . 14. G.M. Wilson; J. Am. Che,. Soc. Er 127 (1964) 15, R. Koningsveld; J. Pol. Sc i , Part C 16-3, 1775 (1967) . 16. R, Koningsveld; Dissertation, Leiden 1967.

17. C. D e l m a s , D, Pat te rson , T. Somcynsky;

J. Polymer S c i 57, 79 (1962) .

61

18. G. Rehage, D. Maller, 0.Ernst; Makromol Ch. 88, 232 (1965).

19. E. Mardles; Kolloid Z. 49, 2 (1929).

20. C.A. Smolders, J.J. van Aartsen, A. Steenbergen;

Kolloid-2.u.Z Polymere 243, 14 (1971).

21. J.D. Hoffman; J. Chem. Phys. 28, 1192 (1958). 22. J.M. Barsales-Rienda and J.M.G. Fatou;

Kolloid-2.u.Z. Polymer 244, 317 (1971). 23. R. Wessling; J. of App. Polymer Sci. 14, 2263 (19701.

6 2

Liquid-liquid phase separation of concentrated polymer

so lu t ions s tud ied by electronmicroscopy-

Abstract D

Depending upon quenching temperature and composition chosen

l iquid- l iquid phase separat ion in polymer solut ions w i l l

occur by one of the following mechanisms: nucleation and

growth o r the spinodal decomposition.

Both mechanisms occur in the system poly(2,Q dimethyl

1,4 phenylene oxide)-toluene and can be made v i s i b l e by

electronmicroscopy using the freeze etching method as

sample preparation technique .,

63

Introduction.

Liquid-liquid phase separation in homogeneous solutions

will in principle proceed along either one of the following

mechanisms: nucleation and growth or the spinodal decompo-

sition . Which of these mechanisms will be followed depends on quenchingettemperature and solution concentrations chosen

and on the,rate of cooling. The initia1,structures connec-

ted with these mechanisms can be characterized as follows:

in the first case nucleii will be formed which will grow

until equilibrium is reached , in the second case inter- 2

connected structures can be formed . In both cases these primary structures can be destroyed

by after-effects like coalescence.

The morphological changes during these phase separations

have been widely investigated in many liquids3 but they

could be visualised in inorganic glass-forming solutions

only . In these systems low angle X-ray studies have been

1

1

6

performed7 which support the concept of spinodal decompo-

sition. However, direct observation of the onset of the

demixing process with other than inorganic glass-forming

systems have not yet been performed, due to experimental

difficulties in preparing suitable replicas which can be

studied by electronmicroscopy.

Recently in PPO*-caprolactam and in PPO-toluene solutions

the mechanism of liquid-liquid phase separation has been

fL From here PP0 will stand for poly(2,6 dimethyl 1,4

phenylene oxide).

64

s tud ied u s ing l i gh t s ca t t e r ing methods8 and normal micros-

0

The purpose of the p resent work w a s t o s tudy t he first '

s tages of phase separation in PPO-solutions, while using

the freeze e tch ing method to ob ta in su i t ab le r ep l i cas fo r

e l ec t ron microscopy.

The freeze-etching method.

I n f957 the f reeze e tch ing method has been introduced for

the study of b io log ica l materials

The purpose of t h i s method w a s t o c o o l a sample of biologi-

If

cal material extremely rapidly in order to prevent damage

of t h e s t r u c t u r e by c r y s t a l l i z a t i o n of w a t e r . By e tching

away t h e ice fqact ion a t l o w temperature (-100 C) i n h igh

vacuum t h e s t r u c t u r e of t h e material was made vis ible through

repl ica technique and e lec t ron micuoscopy.

In our s t u d i e s t h e same procedure has been followed t o

s tudy the onset of l iquid- l iquid phase separat ion of PPO-

so lu t ions , A schematic view of sample treatment is presen-

t e d i n f i g u r e f .

O

6 5

I

F i q u r e 1.

Schematic view of t h e f r e e z e e t c h i n g method

I. Q u e n c h i n g t h e s a m p l e i n t h e s a m p l e h o l d e r i n l i q u i d

n i t r o g e n .

11. C u t t i n g t h e s ä m p l e u n d e r l i q u i d n i t r o g e n t o clear the

i n n e r s t r u c t u r e .

111. E t c h i n g a n d t h e p r e p a r a t i o n o f t h e r e p l i c a i n vacuum.

I V . Chemica l separa t ion of t h e r e p l i c a .

Prel iminary resul ts for the system WO-caprolactam,

Light sca t te r ing measurements performed with PPO-capro-

lactam solutions could be explained by accepting a spi-

nodal decomposition mechanism during l iquid-liquid phase

separat ion,

Observations made under the light microscope supported

the occurence of t h i s mechanism,'which_ was fur ther s tu-

8

d i e d i n 1968 i n Graz with electron microscopy using the

freeze e tching method. Some r e s u l t s a r e shown in f i gu res

2 a t o d o I n f i g u r e 2a a 10 % of weight PPO-caprolactam

so lu t ion is shown, which had been made homogeneous by

hea t ing t o 160°C and suhseauently quenched i n l i q u i d n i -

trogen, The rate of cooling i s so l a rgep t ha t t he so lu -

t i o n w i l l t ransform into the glassy state without phase

separation occurring.

If a homogeneous so lu t ion w a s kept a t 9 0 G d-uring 60 se-

conds before quenching i n l i q u i d N a r egu la r s t ruc tu re

which w a s believed t o be the sp inodal pa t te rn , was obser-

ved (f igure 2b) .

In f i gu re 2c t h e r e s u l t i s shown of a homogeneous so lu t i -

on being cooled a t 90 C f o r 30 second-s; t h i s i n d i c a t e s

that formatfon of nuc le i a t that temperature could not

always be avoided. Upon quenching a PPO-caprolactam solu-

t i on a t higher temperatures the spinodal ,pattern disappea-

red and numerous small nuc le i w e r e observed (figure 2d) .

O

3,

O

67

Figure 2.

Poly(2,6 dimethyl 1,4 phenylene oxide’) (10 % ) in capro lac ta l t t .

a) Q u e n c h e d d i r e c t l y i n l i q u i d n i t r o g e n . . I

b ) Quenched 60 sec a t 9 0 O C , t h e r e af ter i n l i q u i d n i t r o g e n .

c ) Quenched 30 sec a t 90 OC, t h e r e after i n l i q u i d n i t r o g e n . ,

d) Quenched 10 min. a t 1 2 0 OC, t h e r e a f t e r i n l i q u i d n i t r o g e n .

68

Since thermodynamic d a t a of the system PPO-caprolactam w e r e

unknown at t h a t t i m e , i n t e r p r e t a t i o n of t h e s e r e s u l t s w a s

v e r y d i f f i c u l t and could only be given i n a q u a l i t a t i v e

way.

The system PPO-toluene shows t h e same phase separation

behaviour as PPO-caprolactam . 9,l.O For PPO-toluene the cloudpoint curve has been measured

and a r a t h e r accurate estimate €or t h e p o s i t i o n of the sp i -

nodal could be given’, which made it worthwhile t o s tudy

the onse t of l iquid- l iquid phase separat ion by electronmicros-

copy more thouroughly .

Morphological characterist ics.

In liquid-liquid phase separation the formation and growth

of s tab le concent ra t ion f luc tua t ions o f l imi ted s i z e has

been the sub jec t of nucleat ion theory 12 13

The r e s u l t of t h i s mechanism w i l l be the formation of

numerous small droplets of one of the equilibrium phases.

Growth of these droplets , coalescence and aging phenomena

can eventual ly lead to two bulk l iquid phases, If t h e l i q u i d

i s very viscous and the growth of nucleii is slow the sepa-

ration process can be stopped a t any t i m e by rapid cooling

and an inves t iga t ion of t h e crft ical s i z e o f n u c l e i i is

poss ib le ,

In the spinodal decomposition mechanism the so lu t ion is

uns tab le wi th respec t to in f in i tes imal concent ra t ion f luc tua-

t i o n s and the only barrier to s epa ra t ion i s d i f fus ion .

The kinetics of this type of phase change has been extensi-

vely discussed by Cahn 14,15. In this theory the decomposition

process is initiated by a rapid growth of certain components

of composition fluctuations, and fluctuations with a cer-

tain wave number, B,, will show maximum growth rate. The

selective amplification of these spatial components may

result in the formation of a periodic,mozaic-like texture.

By comparison between theories f o r critical opalescence

and spinodal decomposition the dimension most frequently

. ,

realized in early stages of phase separation in polymer

solutions has been given by 16

Z = range of molecular interaction

T = phase separation temperature (T

70

Experimental

Homogeneous so lu t ions of PPO-toluene w e r e made by weighing

appropriate amounts of t h e components i n small glass tubes

which w e r e flushed- with nitrogen and sealed under vacuum.

A f t e r complete dissolut ion in a thermostate bath a t 160 C

the tubes w e r e brought into a bath a t the , des i r ed tempera-

ture during an appropriate ' t i m e .

A f t e r breaking the tubes par t of the contents was ca re fu l ly

brought into the sample holder which w a s h e l d a t t h e same

temperature on a hea t ing s tage . The sample and t h e sample

holder w e r e then quenched i n l i q u i d n i t r o g e n t o s t o p the

phase separation process.

+fter c u t t i n g a t -196 C, t h e sample was brought i n the

freeze etching apparatus (EPA 1 0 0 , Leybold) where f r e e z e

etching took place a t -196 OC and a pressure of 10 t o r r

for periods between 15 and 9 0 minutes. A ,carbon replica

was prepared i n t h e normal way, which, a t t he end- of the

procedure w a s separated from t h e sample by d isso lv ing the

latter in chloroform a t room temperature. The r e p l i c a w a s

deposited upon a g r i d and studied with transmission elec-

t ron microscopy.

O

O

-6

7 1

Results and Discussion

In the system PPO-toluene three phase separation curves

can be detected, the melting point curve, the cloudpoint

curve and the spinodal . If a homogeneous PPO-toluene solu- t i o n is cooled l iquid-liquid phase separation always pre-

ceeds t he c rys t a l l i za t ion , t he l a t t e r t ak ing p l ace on ly i n

the concentrated polymer phase.

Homogeneous solut ions cooled a t the appropriate temperature

between the cloudpoint curve and the spinodal demix by the

nucleat ion and growth mechanism. A s a typ ica l f ea tu re o f

t h i s system, nucleat ion occurs af ter a cer ta in induct ion

time T which approached zero a t temperatures close to the

spinodal.

Por solut ions up t o 50 % of weight PP0 in t o luene t he induc-

t i o n times have been determined a t d i f fe ren t t empera tures .

9

Por each concentration the values of In T versus 1 / T proved

to g ive a s t r a i g h t l i n e dependency, in the temperature range

between the c loudpoint curve and the spinodal ( f igure 3 ) . A s a consequence the induction times a t each. temperature are

known and t h e demixing process could be stopped by rap id

cooling a t times below o r above the induct ion time. I n such

a way so lu t ions could be s tud ied in which nuc le i were n o t y e t

formed, had j u s t been formed and a t later s tages of t h e de-

mixing process. . .

I n s o l u t i o n s k e p t a t an appropriate demixing temperature du-

r i n g a time t T nei ther regular s t ructures nor nuclei could

be detected, indicat ing that for this system indeed a def i -

n i t e t i m e i s requi red before nuc lea t idn s ta r t s .

3.00 i10 320 330 m (K-1) T

F i a u r e 3 ,

I n T v e r s u s lOOOJT ; T i s t h e i n d u c t i o n time i n minutes

of a 30 % of weight poly ( 2 , 6 dimethyl I ,4 phenylene ox ide) - t o l u e n e s o l u t i o n as a f u n c t i o n of tempera ture .

F i a u r e 4 .

L i q u i d - l i q u i d phase s e p a r a t i o n by nuclea t ion and growth of

a 30 % of weight pohy(2,6 dimethyl 1,4 pheny lene ox ide ) -

t o l u e n e s o l u t i o n q u e n c h e d a t 53 C. O

O - Tcloudp t 6o T s p i n o d a l = 38 OC; r = 100 minutes .

7 3

Figure 5.

Liquid-liquid phase separation of a 30 % of weight poly

(2,6 dimethyl 1,4 phenylenen oxide)-toluene solution quenched

at a temperature (T = 4OoC) just above the spinodal .

Figure 6.

Spinodal decomposition of a 30 % poly(2,6 dimethyl 1,4

phenylene oxide) -toluene solution quenched at T = O OC.

Figure 7

Spinodal decompositionpatterns of a 30 % of weight poly

(2,6 dimethyl 1,4 phenylene oxide)-toluene solution

quenched at T = -1 5 OC.

7 4

The resul ts obtained for a system where nucleii had just been

formed is shown i n f i g u r e 4.

Measurements of t-he growth rate of nuc le i i w i th t he l i gh t

microscope i n d i c a t e t h a t a reasonable estimate of the growth

r a t e a t tha t par t icu lar t empera ture for a 30 % of weight solu-

t i o n amounts t o

7 :t = O . 15 pm min w -I Since quenching takes place i n about 0 . 5 sec the nucleus can

grow a t utmost 10-20 a during the quenching t i m e . Eence the nuc le i i thus de tec ted must be of about c r i t i ca l s i ze .

In thosecaseswhere homogeneous mixtures of PPO-toluene a r e

cooled a t temperatures below tlze spinodal two phenomena have

been observed,

1. A t f i n a l quenching temperatures not too far above or

below the spfnodal temperature many nuc le i i are formed

while passing the metastable area. Cooling to a tempera-

t u r e j u s t above the spinodal gives the pat terns of f igure

5. Hence it is clear that the speed of passing the meta-

s tab le reg ion de te rmines the s t ruc ture in the l iqu id

mixture . x

x The i r r egu la r shapes o f t he d rop le t s i n f i gu re 5 mLght

be due t o artefacts introduced during sample preparation,

o r might be caused by c r y s t a l l i z a t i o n of these concentra-

ted areas . Differential scanning calorimetry17 has shown t h a t after

l iquid-liquid phase separation has set i n by e i t h e r o f

the two former mentioned mechanisms, the concentrated

phase w i l l c r y s t a l l i z e if kept a t the quenching tempera-

t u r e f o r a ce r t a in t i m e .

9

7 5

The spinodal s t ructure obtained when cool ing to a

temperature not far below Tspinodal, i s shown i n f i g u r e

6 . The interconnected concentrated areas are b u i l t up

out of numerous very small nucleii which may contain

also c r y s t a l l i z a t i o n e f f e c t s .

2. If homogeneous so lu t ions were quenched a t temperatures

f a r below the spinodal the metastable region was passed

quickly and, apart from a few nuclei i , in terconnected

s t ruc tu res were formed. This i s shown i n f i g u r e 7.

I n f i gu re 7 the concentrated areas have been printed

equally black by photographic means i n o r d e r t o make a

crude estimate of the spinodal dimension D

By using eq. 5 , ' taking Z = 200 a and T, = 311 K t he character is t ic dimensions of the spinodal wavelength for

m'

a 30 % of weight PPO-toluene so lu t ion can be calculated.

A t T = 258 K, Dm,calc. = 0,16 p, the experimental

values as determined by electronmicroscopy ' a r e i n the

same order of magnitude.

76

CONCLUSIONS

The f reeze e tch ing method o f f e r s a p o s s i b i l i t y t o make t h e