Colloids and Surfaces A: Physicochemical and Engineering Aspects, 86 (1994) 137- 142 0927-1757/94/$01.00 0 1994 ~ Elsevier Science B.V. All rights reserved.

137

Adsorption and phase behaviour of Pluronic block copolymers in aqueous solution

Per Linse

Physical Chemistry 1, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden

(Received 14 June 1993; accepted 10 July 1993)

Abstract

The phase behaviour and the adsorption at solid surfaces of polymers containing ethylene oxide and propylene oxide

in aqueous solution have been modelled, and the predictions have been compared with experimental data. The basis

of the modelling is a mean-field lattice theory for multicomponent mixtures of copolymers with internal states occurring

in heterogeneous systems.

Key words: Adsorption; Modelling; Phase behaviour; Pluronic block copolymers

Introduction Table 1

Triblock copolymers of the PEO-PPO-PEO

type (PEO, poly(ethylene oxide); PPO, poly(pro-

pylene oxide) constitute a class of polymers with

amphiphilic character and are often referred to as

Pluronic polymers. In aqueous solution they are

surface active and tend to self-aggregate in a way

similar to short-chain surfactants [ 1,2].

Trademark and composition of each of the Pluronic block

copolymers studied

Trademark M,,oa EO contenta Composition

(wt.%)

On the basis of a mean-field lattice theory for

multicomponent mixtures of copolymers with

internal states occurring in heterogeneous systems

[3], the phase diagram and adsorption at solid

surfaces of Pluronic polymers in aqueous solution

have been examined. The Pluronic polymers con-

sidered in the present communication are compiled

in Table 1. The theory employed originates from

the lattice theory for polymer solutions in hetero-

geneous systems by Scheutjens and Fleer [4] and

a polymer model for describing clouding polymers

by Karlstriim [ 51.

PE6200

P75

P85

P98

P105 F127

“Data from manufacturer

Model

The polymer model by KarlstrGm takes into

account the fact that different conformations of

ethylene oxide groups differ in their dipole

moments and that the more polar conformations

are less numerous. The conformations are divided

into two classes or states, the one being more polar

with a lower energy and a lower statistical weight,

and the other being less polar (referred to as non-

polar), having a higher energy and higher statistical

weight. This categorization is also consistent, for

example, with 13C chemical shift measurements

[6]. At low temperatures, the former class domi-

nates and the effective polymer-solvent interaction

is favourable, whereas at higher temperatures the

SSDI 0927-7757(93)02602-B

1750 20 (EO),(PO),,(EO),

2050 50 (EO)z,(PO),,(EQ,,

2250 50 W),,(P%(EQ,,

2750 80 (EO)I,,(PQ,,(EO),X

3250 50 (EO),,(PO),,(E% 4000 70 (EO),,(PO),,(EO),,

138 P. Linse/Colloids Surfaces A: Physicochem. Eng. Aspects 86 (19941 137-l 42

latter class dominates, rendering the polymer-

solvent interaction less favourable. Furthermore,

the effective polymer-solvent interaction becomes

more unfavourable as the polymer concentration

increases. This description was also applied to

aqueous solutions of PPO which also display a

lower consolute point, although at much lower

temperature owing to PO being more hydrophobic

[3]. Figure 1 shows that the extension of the

Flory-Huggins theory with the polymer model

makes it possible to describe the phase separation

occurring in PPO-water systems. The two PO

parameters describing the polar-non-polar equilib-

rium and the PO-water interaction parameters

were essentially fitted to give the best agreement

between the experimental and calculated phase

boundaries.

The details of the unification of the polymer

model containing internal degrees of freedom and

the Scheutjens-Fleer lattice theory for hetero-

geneous systems have been described elsewhere

[3]. Although the formalism becomes more

involved, the physical picture remains simple. The

virtue of the internal degrees of freedom is that an

effective segment-segment interaction is obtained

without explicitly specifying a functional form of

5oo -

300’ ’ 3 ’ ’ L ’ ’ 1 0.0 0.2 0.4 0.6 0.8 1.0

XPPO

Fig. 1. Phase diagram for the PPO-water system: experimental

points from Malcolm and Rowlinson [7] for a molecular mass

of 400 g mol-’ (circles) and calculated phase boundary using

rppo= 7 (curve) [ 31. The two-phase region is above the curves. The unit of the abscissa is weight (experimental) and volume

(calculated) fraction. (Adapted from Ref. 3.)

x~_~,,__,,~,,~(T, 4). Instead, this dependence

emerges as a consequence of a physically plausible

model employing a restricted number of parame-

ters all of which have clear meanings.

Results and discussion

Recently, phase diagrams of aqueous solutions

of Pluronic polymers have been determined.

Figure 2(a) shows the phase diagram for Pluronic

P85 in aqueous solution determined by Mortensen

from neutron scattering experiments [S]. Similar

but more-detailed phase diagrams have been

obtained by Hvidt et al. [9]. The corresponding

calculated phase diagram for the closely related

Pluronic P105 in aqueous solution is shown in

340

I I I -I 0.0 0.1 0.2 0.3

polymer concentration

0.0 0.1 0.2 0.3

4 tot

Fig. 2. Phase diagram for an aqueous solution of (a) Pluronic

P8S (deduced from neutron scattering, adapted from Mortensen

[S] and (b) Pluronic P105 (calculated, adapted from Ref. 10).

P. LinselColloids Surfaces A: Physicochem. Eng. Aspects 86 (1994) 137-142 139

Fig. 2(b) [lo]. The phase diagram was obtained

with no parameters adjusted in the calculation; all

of them were predetermined by fitting calculated

phase diagrams of simpler systems to experimental

phase diagrams, as shown in Fig. 1. All the promi-

nent features, monomeric solution, micellar solu-

tion, solution of long rod-like aggregates, and a

two-phase region occurring at higher temperature

[9], were predicted in a qualitatively correct way.

In aqueous solution, Pluronic polymers were

found to adsorb on hydrophobically modified silica

surfaces, while pure PEO did not [ 111. Figure 3

shows the measured and calculated amounts of

Pluronic PE6200 adsorbed as a function of temper-

ature. At well below the cloud point, only minor

effects of temperature on the adsorbed amount

were observed. As the temperature approaches the

cloud point, however, there is a dramatic increase

in the adsorbed amount. After a temperature

resealing by about 10 K, in order to obtain the

same clouding temperature, it is clear from Fig. 3

that the model reproduces the salient small increase

of the adsorbed amount at low temperature, the

transition to larger adsorption some degrees below

r

t

I I

0 l 0

0 l 0 l

0

I I I I 290 300 310

TM

Fig. 3. Amount of Pluronic PE 6200 adsorbed from an aqueous

solution at a hydrophobized silica surface as a function of

temperature: experimental ellipsometry results at a weight

fraction of 1 x 10e3 (open circles) and calculated results at a

volume fraction of 2 x 10m3 (filled circles). The broken line

denotes the cloud point of the experimental system. The calcu-

lated data have been resealed (see text). (Reproduced with

permission from Ref. 3; copyright American Chemical Society,

1991.)

the cloud point, and the unlimited increase as the

cloud point is approached. In addition, by employ-

ing the reasonable lattice size of 4 A, the calculated

amount adsorbed virtually superimposes the exper-

imental one.

The model calculations provide further insight

into the nature of the adsorbed layer. Figure 4

displays the total segment density profiles at two

temperatures, 298 and 319.6 K, of which the latter

is 0.1 K below the cloud point. A prominent feature

is that, while the segment profile of primarily

adsorbed chains (squares) is fairly insensitive to

temperature changes, the segment profile of the

total excess in the surface region (circles) is

extended much further out at the elevated temper-

ature. The amount of segments primarily adsorbed

per lattice site and the corresponding excess quan-

tity are r= 2.3 and r,, = 2.4 at the lower temper-

ature (T=298 K) and 3.5 and 6.7, respectively, at

the higher one (T= 319.6 K). The reasonable lattice

size obtained from relating the experimental and

calculated adsorbed amounts and Fig. 4 strongly

support the notion of multilayer adsorption close

to the cloud point, which could be viewed as an

incipient phase separation phenomenon that takes

place in the surface region.

1.0

0.8

:s 0.6 s 2 & 0.4

0.2

0.0 0 5 10 15

I

Fig. 4. Calculated total volume fraction 4, of Pluronic PE 6200

vs. layer number i. Circles represent the total surface excess,

while squares represent primarily adsorbed copolymer mole-

cules. Filled and open symbols represent profiles at 298 K and

319.6 K respectively. The volume fraction is 2 x 10-j.

(Reproduced with permission from Ref. 11; copyright American

Chemical Society, 1991.)

140 P. LinseiColhids Swfxes A: Plzysicochern. Eng. Aspects 86 ( 1994) 137-l 42

It is anticipated that the EO and PO segments

in the adsorbed layer are segregated. From the

analysis of the volume fraction profiles of the EO

and PO segments separately, it was found that the

PO segments are preferentially located close to the

surface, whereas the EO segments are mainly

located further away from the surface.

The adsorption of a number of EO and PO

containing polymers from aqueous solution on

hydrophilic silica has also been examined [ 121. In

general, the saturation value of the adsorbed

amount was much lower (about 0.4 mg m-“) than

that for a hydrophobically modified silica surface

(cf. Fig. 3). The adsorption of different PEO-

PPO-PEO block copolymers and a PEO polymer

behaved rather similarly at the silica surface. Only

the most hydrophobic Pluronic P75 displayed a

significantly lower adsorbed amount. The corre-

sponding calculated adsorption isotherms are

shown in Fig. 5. The calculated results agreed well

with the experimental ones, with the exception that

the amount of Pluronic P75 adsorbed was too low.

Again the use of a lattice size of 4 A gave a

quantitative agreement of the amount adsorbed.

The volume fraction in the layer adjacent to the

surface was at most 0.15 as compared to about 0.8

for the hydrophobically modified surface (cf.

Fig. 4).

0.8 ,_” :’

0.6

L.$

0.4

0.2

Fig. 5. Calculated adsorption isotherms on silica for PEO

(Two = 455), Pluronic F127. Pluronic F98 and Pluronic P75.

(Adapted from Ref. 12.)

Also for the hydrophilic surface the distribution

of different segments within the adsorbed layer was

examined. As can be seen in Fig. 6, the EO seg-

ments are preferentially adsorbed, and located in

the proximity of the surface. Furthermore, since

EO is more soluble than PO, the EO segments are

also preferentially located in the outer part of the

adsorbed layer. The PO segments, on the other

hand, interacting poorly as they do with both

solvent and surface, are located primarily in the

middle part of the adsorbed layer.

In both adsorption studies, two polymer-surface

interaction parameters were fitted to give the over-

all best result, whereas the other parameters involv-

ing the polymer-water interaction where obtained

from simpler systems as previously described.

Conclusions

The Scheutjens-Fleer lattice theory extended

with a polymer model with internal degrees of

freedom is able, qualitatively and in some cases

semiquantitatively to describe adsorption phen-

omena occuring in aqueous solutions of EO and

PO containing polymers. In addition. the versatile

theory is able to predict phase diagrams, to provide

information of self-assembling and to predict the

modulation of forces between surfaces due to the

6

d +%

4

d 2

2

0 I I I

0 10 20 30 40

I I I

Fig. 6. Calculated EO-PO volume fraction ratio &o,,/q!+o,, for

Pluronic F127 vs. layer number i. The dotted line indicates the

corresponding ratio in the bulk solution. The volume fraction

is 2 x lo-“. (Adapted from Ref. 12.)

P. LinselColloids Surfaces A: Physicochem. Eng. Aspects 86 (1994) 137-142 141

presence of polymers. Besides the thermodynamic 2

results and gross structural elements, where com- 3

parison with experimental results is possible, the 4

theory also provides a detailed picture of the

molecular arrangement which still has to be 5

assessed by future experiments. In particular, it has 6

been shown that the amount adsorbed and the 7

preferential location of the EO and PO segments

in the adsorbed layer differ for the adsorption at 8

hydrophobically modified and hydrophilic silica 9

surfaces. 10

11

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142

Discussion

Speaker: P. Linse

Questioner: J.A. Waters

Q. With adsorption onto the hydrophilic substitute, you indicated only little dependence on temperature.

Presumably the competition between water and polymer adsorption is very much in favour of the water.

Did you extend your studies to temperatures as high as 80°C where the association with water is induced‘?

A. No.

Speaker: P. Linse

Questioner: E. Kiss

Q. Is there any experimental evidence for the conformational change of PEO due to the increase of

temperature‘?

A. Yes, we have measured the 13C shift which depends on the dihedral angle. By fitting a shift for the

polar and non-polar state, we were able to describe the variation of the experimentally determined 13C

shift of PEO in different solutions at different concentrations and temperatures.