95-

STUDY OF POLYSTYRENE-BLOCK-POLY(METHYL METHACRYLATE) MICELLES BY

SIZE EXCLUSION CHROMATOGRAPHY/ LOW ANGLE LASER LIGHT SCATTERING

.X-INFLUENCE OF COPOLYMERCONCENTRATIONAND FLOW RATE

_ATIEA GRUBI_IC-GALLOT and YVES GALLOT

Institut Charles Sadron, (CRM-E_P) (C_/RS-ULP)

6, rue BoussinEault, 67083 Strasbour, g, Cedex, France

JAN ST:;r_LA(_X

Dept. of Physical and Macromolecular Chemistry, Faculty of Science,

Charles University, Albertov 2030, Prague, Czech Republic

ABSTRACT

A size exclusion chromatography study of the micellar system

polystyrene-block-poly(methyl methacrylate] in the mixed solvent

1,4-dioxane/cyclohexane is reported. Good separation of the peaks of

micelles enabled direct determination of the weight-average

molecular-weights of micelles with a low angle laser light

scattering detector. The values obtained were found to be in

accordance with those determined independently by static light

scattering. Experiments with changing flow rate and concentration of

the injected sample solution show moderately fast unimer-micelles

re-equilibration in the course of the separation. A strong effect of

the solute trapping in the column, probably due to the adsorption of

the unimer on the packing, was observed.

*This manuscript has been published: Macromol. Chem. Phys. t95,

781-791 (1994).

173k

4

• INTRODUCTION

Block copolymers in selective solvents (i.e. thermodynamically

Eood solvent for one block and at the same time poor solvent for the

other block) form multimolecular associates - called micelles _

having a core formed by a block of low solubility and a protective

shell formed by a block of high solubility. Micelles are usually

formed via so-called closed association, which is characterized by

an equilibrium between the micelles (M), with a narrow molecular-

weiEht and size distribution, and the molecularly dissolved

copolymer - unimer (U):

K

nU m M (1)(

where n is the association number and K is the micellizationm

equilibrium constant. The equilibrium and the association number

strongly depend on the nature of the copolymer and on the quality of

the solvent for the individual blocks. The micelles/unimer weight

I)ratio also depends on the overall concentration of the copolymer

Size exclusion chromatoEraphy (SEC} is known to be a powerful

method for characterizing polymer molecules in solution. The

application of SEC to the characterization of individual components

for a micellar system is not straightforward: the separation of

micelles from unimer in a SEC column causes continuous disturbing

and subsequent re-establishment of the equilibrium (Eq. I). It is,

therefore, evident that SEC can provide information on the dynamics

of this equilibrium. A theoretical model describing the above

b

mentioned effects has recently been proposed by Proch_zka et

al. 2'3) The model predicts theoretical chromatograms fop various

values of n, the shape of which strongly depends on the relative

rates of SEC separation and of the unimer- > micelles association(

and dissociation. The model calculations predict that SEC

chromatoErams with two peaks (corresponding to unimer and micelles

respectively) may be obtained only for a relatively slow

unimer ) micelles equilibration (as compared with rates of(

separation processes).

SEC data on several block copolymer micellar systems were

3-9)reported in the last decade , majority of which prove the

influence of unimere/_> micelles re-equilibration on the results

obtained, confirming thus qualitatively the above theoretical model.

An additional phenomenon was observed in some of those studies,

which was a significant loss of the solute in the course of SEC

separation in the columns 4'9) . The explanation based on the

adsorption of the unimer on the column packing was proposed by Price

eta[. 4). _pa_ek I0) and Proch_zka et al. 9) offered an alternative

explanation of this phenomenon based on the steric trapping of

micelles formed from the unimer inside the pores of the Eel in some

systems. The utilization of SEC fop the determination of the

molecular-weight of micelles was attempted on two occasions. In one

study II), the value obtained agreed well with that determined by

static light scattering, while the aEreement was poor in the

other 4) In both cases the universal calibration method 12) was used• o

However, the application of the universal calibration to the

copolymer micellar systems is rather questionable. Even if full

175

, validity of this method is supposed for complex micellar particles

there are many effects (as stated above) which can influence the

' resulting retention volume besides "pure" steric exclusion

12)controlled by the hydrodynamic volume only

In this article we report upon the results of the SEC study of

polystyrene-block-poly(methyl methacrylate} (PS-PMMA] in the mixed

solvent 1,4-dioxane/cyclohexane where micelles with PMMA core and PS

shell are formed at higher contents of cyclohexane. The SEC

apparatus equipped with two detectors, differential refractometer

(DR) and low angle laser light scattering detector (LALLS} was used.

Better interpretation of the experimental elution curves was thus

achieved and the molecular-weight of the micelles was directly

determined. Additional information obtained by standard static light

scattering measurements is reported as well.

EXPERIMENTAL

Copolymer: Polystyrene-block-poly(methyl methacrylate) {PS-

PMMA) was synthesized by anionic polymerization. The polystyrene

block was prepared first by polymerizing styrene in tetrahydrofuran

(TIE} at low temperature [-70°C) using phenylisopropylpotassium as

initiator. In order to avoid the attack of the ester group of methyl

methacrylate by the PS carbanions, l,l-diphenylethylene was

introduced prior to the addition of methyl methacrylate to decrease

the nucleophilicity of the active sites 13). A copolymer with 64 wt.M

of PS according to elemental analysis was prepared; its weight-

average molecular weight {%) and number-average molecular-weigh t

176

: r

(M.) determined by size exclusion chromatography / low angle laser: n

light scattering, are: Mw = 96000, M = 92000.n

Solvents and preparation of solutions: Cyclohexane and 1,4-

dioxane CRhone-Poulenc, for analysis) were distilled over sodium

wire. 1,4-dioxane is a good solvent for both copolymer blocks;

cyclohexane is a precipitant for PMMA blocks and a e solvent for P_

blocks (at the temperature of 34.9°C), and micelles with PMMA cores

are formed in mixtures of these solvents rich in cyclohexane. Both

solvents have quasi identical refractive indexes: n = 1.420dloxane

and ncyclohexan, = 1.424_ The copolymer was always dissolved in a

given solvent mixture for a day at the temperature of 25°C prior to

analyses.

Size exclusion chromatoEraphy (SEC): Waters 150 C "apparatus

with two detectors coupled on line - low angle laser light

scattering (LALLS) photometer (Chromatix CMX-100) and a standard

Waters differential refractometer (DR) was applied. The principle of

apparatus as well as the data treatment were described in details by

14)Grubi_ic-Gallot For the characterization of the copolymer sample

five columns in series, packed with PL Eel, havin E upper

permeability limits of 106 , 105 , 104 , 103 , and 5.102, respectively,

and THF as the eluent with a flow rate of 1 mL/min were used. The

SEC study of the micellar system was done with one coltuml packed

with _-styragel (105). Different 1,4-dioxane/cyclohexane mixtures

were employed as the mobile phase at a temperature of 25°C. The

experiments were carried out at various flow rates in the range from

0.5 to 3.0 mL/min, and the concentration of injected solutions

(2OO_zi) was between 2.5 and 30 mE/mL.

177

" Static light scattering: SEH-633 apparatus from SEMATECH

" (A=632.8 nm) was used. The weight-average molecular-weights of

micelles were obtained at 25°C using the relation:

= 1/1_ + 2A2c (2)KC/Ro w

where Mw is the weight-average molecular-weight of scattering

particles, K is the optical constant, R° is the Rayleigh ratio

extrapolated to zero angle, c is the copolymer concentration and A2

is the second virial coefficient. The estimates of the critical

micelle concentration 1) (CMC) were obtained from the shape of the

dependencies Kc/R vs. c. The principle of molecular-weight and CMCO

determination from the static light scattering data is available in

the reviews of Tuzar and Kratochvil I'15)

RESULTS AND DISCUSSION

Static light scattering

The results of the static light scattering measurements are

shown in Fig. I where the dependencies of Kc/R (extrapolated toO

zero angle) on the overall concentration of the copolymer c are

given for different solvent mixtures. Fig. 1 corresponds

qualitatively to the light scattering results reported on various

1,15)copolymer micellar systems in many papers The interpretation

of the results in Fig. 1 is straightforward: in the solvent mixtures

1,4-dioxane/(0-60)vol.% cyclohexane the copolymer is dissolved

molecularly (unimer). The diminishing slope of Kc/R vs. c witho

178 .

increasing content of cyclohexane indicates a decrease in thermo-

dynamic quality of the solvent mixture. In mixtures with a higher

content of cyclohexane, the presence of micelles in the solutions is

evident. Curve 3 (Fig. i) exhibits a shape typical of associatinE

systems: in agreement with the closed association mechanism the

significant decrease in Kc/R with concentration (for lowero

concentrations) reflects the onset of micellization. After the

initial decrease, the value of Kc/R ° changes only little, since the

concentration of the unimer is almost constant and small in this

region and the measured values correspond to %hose of micelles. In

the solvent mixtures 1,4-dioxane/80 vol. X cyclohexane and 1,4-

dioxane/85 vol. X cyclohexane only the second (horizontal) part of

dependency Kc/R vs. c was obtainable (curves 4 and 5 in Fig. I),o

due to the shift of the beginnin E of the micellization to lower

copolymer concentrations. Extrapolating the Kc/R value from thiso

region (from horizontal parts of dependencies) to c = 0, the weight-

average molecular-weight of all scatterinE particles in a given

solvent mixture, i.e. so-called particles molecular-weight (M(P})w

was obtained. This value can be considered as weight-average

molecular-weight of the micelles (_(M)) because the equilibriumw

concentration of the unimer is negligible (in comparison with the

concentration of the micelles} in this region and the contribution

of micelles to the measured value is much higher due to their high

molecular weiEht. The values of _(M) for different solvent mixturesw

are summarized in Tab. I where also correspondinE values of critical

micelle concentrations [CMC) resultinE from static light scattering

measurements are given. CMC represents the concentration of the

179

copolymer at which micelles are just detectable by a given method.

It is apparent from Fig. I that at higher cyclohexane contents only

upper estimates of CMC were available. The decrease in CMC with

increasing cyclohexane content in the solvent mixture is evident,

this trend being generally observed with a decrease in thermodynamic

quality of the solvent in the copolymer micellar systems. Accordin E

to the closed association model, the equilibrium concentration of

the unimer in the micellar system is very close to the value of CMC;

(p)_ _(M) is acceptableit is thus evident that the approximation, Mw - w '

for all tested solvent mixtures.

Tab. I: Weight-average molecular-weight of polystyrene-block-poly-

(methyl methacrylate) micelles (_(M)) determined by static lightw

scattering (SLS} and by size exclusion chromatography/low angle

laser light scattering (SEC/LALLAS}, polydispersity index of

micelles (_(M}/_(M}) determined by SEC/LALLS values of criticalw n

micelle concentration (CMC) determined by SIS and association number

(n) from SEC/LALLS for various 1,4-dioxane/cyclohexane solvent

mixtures.

Content of SIS SEC/LALLS

cyclohexane

in vol.% CMC in g/mL 10-6.M (M) 10-6.M (M) M(M)A_(M) nw w w n

70 Ixl0 -4 1.8 -- -- --

80 < Ixl0 -5 8.5 7.9 1.19 82

85 < 3x10 -6 29 31 1.19 323

180

.°

Size exclusion chromatography

The effect of solvent composition in the region 0-85 vol._ of

cyclohexane was investigated using a concentration of injected

sample (100_L) of I0 mE/mL and a flow rate of I mL/min. The

copolymer was always dissolved in the same solvent mixture as that

used for the SEC mobile phase. The shapes of the resultinE

chromatograms can be seen in FiE. 2. The concentration profiles (DR

response) with two peaks were obtained for solvent mixtures

containin E 80 vol. X and 85 voI._ of cyclohexane (FIE. 2d, e). The

peaks with a lower elution volume (V =9.8 mL in FiE. 2d, V = 9.5 mLe e

in FiE . 2e), for which very high LALLS detector responses were

obtained, were ascribed to micelles. The peaks at a hiEher elution

volume (V = 11.5-12.3 mL) correspond to the unimer in the case ofe

all studied solvent mixtures. It should be noted that for all unimer

peaks a sufficient LALLS response can be achieved by usin E a high

detector sensitivity and molecular-weiEht characteristics of the

unimer calculated from these responses are in Eood aEreement with

the values determined for the copolymer in THF (see Exptl. part).

The small peaks at V = 13.8-14.7 mL (at the permeation limit of thee

column) correspond to the so-called SEC system peaks commonly and

not fully accurate called "impurity peaks". These peaks, well-known

in SEC, are caused by several effects from which the selective

solvati0n of the solute being most important, as described by Berek

16)et al. The last is the most probable explanation of the system

peaks in Fig. 2.

A Eood unimer-micelles resolution is evident from FiE. 2d, e.

The peaks of the micelles resultin E from the LALLS detector are

181

• sharp, almost symmetrical and their shapes correspond to those

recorded by the DR detector. The determination of weight-average

molecular weight (_(M}) and polydispersity index (_(M)/_(M)) of thew w n

micelles from LALLS and DR detectors responses led to the results

listed in Tab. I. Good agreement between the values of _(M)w

determined by SEC-LALLS and by static light scattering is evident.

The increase in _(M) with increasing cyclohexane content in thew

solvent mixture is in general agreement with many papers where an

increase in molecular-weight with decreasing thermodynamic quality

of the solvent is reported. Low values of the polydispersity index

testify to significant molecular-weight uniformity of the micelles

confirming thus the closed association mechanism of their formation.

From the areas below the the DR peaks, a significant loss of

the solute in the column is apparent if cyclohexane content is

higher than 65 vol. Z. This trapped solute can be completely eluted

by a zone of good solvent. In Fig. 3 the elution curve is given

which resulted from an injection of 100 gL of 1,4-dioxane on the

column just after the analysis of I00 _L of the copolymer sample

with a concentration I0 mg/mL in 1,4-dioxane/80 vol._ cyclohexane in

the eluent of the same composition. The injection of I00 gL of 1,4-

dioxane was sufficient "to clean" the column quantitatively in the

case of all solvent mixtures where the loss of solute was observed.

We suppose that the loss of the solute is caused by a trapping of

the unimer form of the copolymer on the column packing. The results

do not allow an unambiguous interpretation of the trapping mechanism

but an adsorption of the unimer on the gel seems to be probable

since the eluent is a thermodynamically poor solvent for the

!82

copolymer. A siEnificant adsorption of the micelles present in the

injected sample is less probable: the structure of micelles

(insoluble PMMA blocks concentrated in the core protected by soluble

PS blocks) enhances the resistivity of these particles from an

adsorption. On the basis of the above assumption, the interpretation

of the elution curve in FiE. 3 is as follows: 1,4-dioxane, as a good

solvent, desorbs the unimer from thecolumn packing. Released unimer

moves faster than a zone of 1,4-dioxane and being surrounded by the

mobile phase of low thermodynamic quality it can be adsorbed again

at some extent. As the 1,4-dioxane zone moves in the column the

concentration of unimer in front of this zone (represented by a drop

in the DR response below the base line) increases continuously. As a

consequence of the described processes, an asymmetric concentration

profile as in FiE. 3 (DR response) may result. The elution curve

given by the LALLS detector in FiE. 3 reflects an interestinE fact:

the hiEh response of this detector proves the presence of copolymer

associates (micelles) in the eluate. This can be explained by the

formation of micelles from the released unimer when its

concentration exceeds that of CMC in front of the 1,4-dioxane zone.

A releasing (desorption) of micelles from the column packinE cannot

explain this phenomenon because this process should be accompanied

with a rapid micelles decomposition caused by 1,4-dioxane. Neither

the molecular-weiEht nor the weight fraction of this micelles in the

eluate were available from the chromatogram in Fig, 3, because of a

bad unimer-micelles resolution. A similar effect (formation of

micelles from unimer released from the column packing) was observed

in a system of polystyrene-block-poly(hydrogenated butadiene)-block-

183

I

9)polystyrene in 1,4-dioxane/heptane by Proch_zka et al.

In all injected micellizing solutions the unimer( ) micelles

equilibrium was significantly shifted in favour of the micelles.

This fact is evident from the light scattering measurements (Fig.

I). As already stated the equilibrium concentration of the unimer in

the copolymer micellar system is very close to the value of CMC. It

may be deduced from Tab. I that these values are several orders of

magnitude smaller than the weight concentration of micelles in the

injected samples. In the solvent mixture 1,4-dioxane/70 vol. X

cyclohexane, the injected sample contains ca 99 wt.X of copolymer in

the form of micelles. However, the resulting SEC chromatogram (Fig.

2b) shows only the unimer peak, the area of which represents a great

part of the copolymer injected on the column because the adsorption

effect is not very important in this solvent mixture (the tailing of

this peak reflects a release of weakly adsorbed unimer from the

column packing). This observation agrees with the theoretical

conclusions on the unimer( ) micelles re-equilibration in the course

• _2,3)of the SEC experlment (see Introduction). As soon as the

micelles are separated from the unimer in the column as a result of

different hydrodynamic volumes, the equilibrium is disturbed in the

zone of the micelles. Consequently, a certain amount of micelles

dissociates to re-establish the micellar equilibrium in this zone.

An additional dissociation is caused by a dilution of micelles in

the column given by a spreading of the zone of the micelles. The

described processes can lead even to the total disappearance of

micelles from the system as evident in Fig. 2b. The same

interpretation can be applied to Fig. 2c. However, the drop in the

184

flow rate of I mL/min and an injection volume of 100 _L were

applied. The equilibrium for all injected samples was shifted

significantly in favour of micelles. The values of _(M) resultingw

from LALLS detector (only from the chromatograms a, b and c in Fig.

5 the determination was possible) were found to be unchanged with

the concentration of the injected sample, being within the limits of

experimental errors equal to the value given for this solvent

mixture in Tab. I. This is in accordance with the constant elution

volume of the peaks of the micelles in Fig. 5. This fact stands for

another proof of the closed association mechanism of the formation

of micelles. Decreasing relative fraction of the copolymer detected

in the form of micelles with the decrease in concentration of the

injected sample is evident from Fig. 5. Assuming that the

equilibrium concentration of the unimer is almost constant above

CMC, the following qualitative explanation can roughly be proposed.

During the SEC separation, the same amotmt of micelles tends to

dissociate to form the unimer in an original equilibrium

concentration regardless of the concentration of the micelles in the

separating zone. For a lower concentration of micelles in a column

zone, this dissociation will cause a more significant decrease in

the resulting peak of micelles. It is clear that the process

described is complicated by other effects, e.g. by a spreading of

the micelles zone and mainly by the concentration dependence of the

• 17)rate of micelles dissociation . At a lower concentration of the

micelles this rate can be significantly slowed down in comparison

with the separation of the unimer from the zone of the micelles. The

detection of a trace of micelles even at lower copolymer

185

concentration (Fig. 5d} suggests this assumption. Moreover, a

substantial complication of the interpretation also arises due to

the unimer adsorption on the column packing. The influence of this

phenomenon on all above mentioned processes is difficult to

evaluate. An interesting finding results from Fig. 5: the fraction

of the detected unimer is almost proportional to the concentration

of the injected sample. Contamination of the copolymer sample by a

significant amount of non-micellizinE macromolecules can be ruled

out (SEC analysis of copolymer in THF gives a single symmetrical

peak and no homopolymer has been detected]. Thus this fact might be

interpreted only as a result of a complicated process in which the

above contributions are involved.

CONCLUSION

SEC results on the micellizing block copolymer systems show

that the rate of the re-establishment of the unimer _ micelles( ,

equilibrium in the zone of micelles is not negligibly small in

comparison with the rate of the SEC separation processes. The shapes

of chromatograms are in a qualitative accordance with those

predicted for a moderately fast re-equilibration in the theoretical

model 3}, at least in the case of the solvent mixture 1,4-dioxanel80

I vol._ cyclohexane which was used for the detailed studies. TheI

separation of peaks of unimer and micelles achieved for studied

system seems to be more pronounced than it should result from the

theoretical predictions. This may be a result of the high complexity

of processes involved in a real chromatographic separation

{dissociation of micelles, adsorption of unimer, spreading of zones,

1

etc.) which must have been simplified or even omitted in the

theoretical model by a reason of mathematical solvability.

Acknowledgement. ,

The authors are grateful to Prof. K. Proch6zka from Charles

University for many valuable discussions and to J. P. iingelser from

I.C.S. for his valuable assistance in the synthesis of the copolymer

sample. One of the authors (J.S.) is grateful to the French Ministry

for Research and Technology for granting the post-doctoral

fellowship at the Institute Charles Sadron.

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1187 (1984)

8. R. Xu, Y. Hu, M.A. Winnik, G. Riess, M.D. Croucher, J. Chromatogr.

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9. K. Proch_zka, B. Bedn_f, Z. Tuzar, M. Ko_i@_k, J. Liq. ChromatoEr.

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I0. P. _pa_ek, J. Appl. Polym. Sci. 32, 4281 (1986)

11. C. Price, A. L. Hudd, C. Booth, B. Wright, Polymer 23, 650 (1982)

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13. D. Freyss, M. LenE, P. Bempp, Bull. Soc. Chim. Ft. 1964, 221 (1964)

14. A. Halbwachs, Z. Grubi_ic-Gallot, Makromol. Chem., Rapid Co,m,n.

7, 709 (1986)

15. Z. Tuzar, P. Kratochv£1, "Surface and Colloid Science Series

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187

• °

I I 1 I I_5 -

o-_12- -

,_ 9- -

_ 6 " --

0,15-

0,10-

O,O5-..... 5

I I I I I

1 l 3 4- 5

c.103(g/mL)

Fig. I. Concentration dependence of Kc/R ° from static liEht

scattering measurements for solutions of polystyrene-block-

poly(methyl methacrylate) in the mixtures 1,4-dloxane/cyclohexane at

25°C. Vol._ of cyclohexane: 0(I); 60 (2); 70 (3); 80 (4); 85 (5).

188

I i I I i I I I i

(a) (b) Co)WIn

0

..... _--_A....I I I I I I I I I

16 12 8 16 12 8 16 12 8

' V. (mL)

(d)

II i

II

" ! i "! I

. ! ! .I e_ IAI I

16 12 8 16 12 8

Ve(mL)

Fig. 2. SEC curves o£ polystyrene-block-poly(methyl methacrylate) in

the mixtures 1,4-dioxane/cyclohexane at 25°C. Vol._ of cyclohexane:

0-65(a); 70 (b); 75 (c); 80 (d); 85 (e). Concentration c = 10 mg/mL;

injection volume: I00 _L; flow rate: I aL/=in. Full line - DR

response; dashed line - LALLS response. (Ve: elution volume)

189

Y

I i I I

dJ

° !" _

I I16 12 8

V. (mL)

Fig. 3. SEC curve resulting from an injection of 100 _L of 1,4-dioxane on the column just after the analysis of 100 _L of

polystyrene-block-poly(methyl methacrylate) with a concentration of10 mg/mL in 1,4-dioxane/80 vol._, cyclohexane in the eluent of thesame composition. Full line - DR response; dashed line - LALLS

response. (Ve: elution volume}

I I i I I I

I I I I I I i16 12 8 16 12 8

V=(mL)

_I I I I I I I

¢)

16 12 8 16 12_ 8v, (too

Fig. 4. SEC curves (DR responses) of polystyrene-bJock-poly(methylmethacrylate) in the mixture 1,4---dioxane/8Ovol.Z cyclohexane at25°C. Concentration c = 10 mg/mL, injection volume: 100 gL. Flowrates (mL/min): 3 (a); 2 {b}; I (c); 0.5 {d). (Ve: elution volume)

190

--T_I"_ _ i.--=-

(a) (b) (c)

-..__u__...__.u__ __.X__L____L_

_6 _2 8 _6 _2 8 _6 _ 8%(mL)

i i I I I I

(d) (e)f_c0 "

tL

, I I I I ! I16 12 8 16 17- 8

Ve(mL)

Fig. 5. SEC curves (DR responses] of polystyrene-block-poly(methylmethacrylate) in the mixture 1,4-dioxane/8Ovol.Z cyclohexane at

25°C. Flow rate: I _L/min; injection volume: I00 _L. Concentration

of the samples injected (in mg/mL): 30 (a); 20 (b); I0 (c); 5 (d);2.5 (e). (V : elution volume)

e

191