Dilute solution behaviour of styrene-ethylene oxide block copolymers in aqueous solutions

Eur. Polym. J. Vol. 24, No. 3, pp. 285-288, 1988

0014-3057/88 $3.00 +0.00 Copyright © 1988 Pergamon Journals Ltd

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DILUTE SOLUTION BEHAVIOUR OF STYRENE-ETHYLENE OXIDE BLOCK COPOLYMERS IN AQUEOUS SOLUTIONS P. BAHADUR and N. V. SASTRY* Department of Chemistry, South Gujarat University, Surat-395 007, India (Received 5 November 1986; in revised form 17 August 1987)

Abstract--Micellar behaviour of styrene (ethylene oxide) copolymers was studied in water and mixed solvent systems (tetrahydrofuran-water and 2-propanol-water) by using photon correlation spectroscopy (PCS), viscosity and turbidity methods. Detailed viscosity measurements were carried out to explain the solution behaviour of block copolymers in mixed solvents. The influence of temperature on intrinsic viscosity of micelles was examined and explained.

INTRODUCTION

Dilute solutions of block copolymers in selective solvents are known to form aggregates of colloidal dimensions. These aggregates are called "micelles" by analogy to conventional surfactant micelles. The micellar behaviour of block copolymers has been extensively reviewed [1]. Lundsted and Schmolka [2] considered the surfactant behaviour of ethylene oxide-propylene oxide block copolymers and explored various possible applications. Based on the enormous literature on these copolymers, several block copolymeric surfactants derived from poly(ethylene oxide) as the hydrophilic part and poly(~-butylene oxide [3], poly(vinyl pyridine) [4], poly(dimethyl siloxane) [5] and polystyrene [6-10] as the hydrophobic part have been synthesized and their surfactant properties have been reported. Bahadur and Riess [11] have reviewed the surfactant behaviour of hydrophilic-hydrophobic block copolymers in aqueous solutions. The micelization of hydrophobic-hydrophilic block copolymers in mixed solvents has been reported [12, 13]. The study of micelle formation in mixed solvents is interesting since the system offers variation in solubility parameter values and as well as dielectric medium. This paper presents the results on dilute solution behaviour of several styrene-ethylene oxide copolymers in mixed solvents and in aqueous solutions.

The solvents were of AnalaR grade. Water was triple distilled through all pyrex glass apparatus from alkaline permanganate solutions. Methods

Micellar solutions of block copolymers were made by simple dissolution at temperatures between 45 ° and 55°C. Solutions consisting of polymolecular micelles were bluish transparent to milky opalescent in appearance. Photon correlation spectroscopy (PCS)

PCS studies of the micellar solutions were made using Coulter N-4 (Coultronics, Inc.) which measures the translational diffusion coetficient (D) from the intensity fluctuations in the scattered light. The hydrodynamic radii of spherical micelles were calculated using the Stokes-Einstein equation R h = kT/6n~lD

where, k, T and ~/are the Boltzmann constant, temperature and viscosity of the solvent respectively. Viscosity

The viscosities of polymer solutions were measured using an Ubbelohde viscometer with proper thermostating with a temperature stability of +0.05°C. The flow times always exceeded 150sec and no kinetic energy corrections were necessary. The turbidity measurements were made at a wavelength of 470 nm and the results were fitted to the following equation HC/r = 1/(/l~w)app + 2A2C

Table

1. Molecular characteristics of styrene-ethylene oxide copolymers Copolymer % PS M.

EXPERIMENTAL

Materials

Block copolymer samples were synthesized in the laboratory of Professor G. Riess, ENS de Chimie Mulhouse, France; synthesis and characterization have been described [8]. The molecular characteristics of the copolymers are summarized in Table 1.

37 39 40 41

Di-block

30.4 10.8 26.3 15.6

13.800 38.900 14,100 23,600

Tri-block

44 48 49

*Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar-388 120, India. 285

21.0 35.3 21.0

7600 5200 8600

286

P. BAHADUR and N. V. SASTRY

PotyrnoLecuLor

Table 2. Hydrodynamic radii of block copolymer micelles in water at 25°C

mice~.Les

Copolymer

% PS

R h (nm)

39 41 40 37

Di-block 10.8 15.6 26.3 30.4

62.5 52.5 46.5 44.0

49

Tri-block 21.0

45.0

~

44 48

21,0 35.3

30.0 25.0

~- 0.3 i-4

0.6 0.5

/

0.4

BLuish ~ronsporent

CokourLess tronsporenl

0.2

where H is an optical constant, C is the concentration of copolymer and z is the turbidity of solution.

~e.~Q. i [

o.~

I RESULTS AND DISCUSSION

Micelle formation in water Styrene--ethylene oxide copolymers are typical hydrophobic hydrophilic block copolymers. In water, a selective solvent for poly(ethylene oxide) but a precipitant for polystyrene, spherical polymolecular miceUes with an insoluble polystyrene core surrounded by a flexible outer shell of poly(ethylene oxide) were detected for several copolymers by PCS and ~H-NMR methods [10]. Table 2 presents the hydrodynamic size of micelles as a function of the molecular characteristics of copolymers. The micelle size was found to decrease with increase in styrene content. This dependence emphasizes that the micelle size also depends on the soluble part of the copolymer. The intrinsic viscosity, [r/], of micelles is deduced from the concentration dependence of reduced viscosity by following the procedure developed by Huggins. Figure 1 shows a typical Huggins plot for Cop-39 in water at various temperatures. The slope

0'31. .~e• 0.2 . ~'e""""e~/" / ' ' ~ 4 5 " C 01 0.3

-----/

40*C

0.2 ~ ~

0.1

/

35"C

..4r.~/

~

(12

30"C 0.2 0.1 o

o.I

I

I

o.2 0.3 C (gdt -1)

I

0.4

I

0.5

Fig. I. Reduced viscosity vs concentration plot for Cop-39 micelles in water.

i 20

II

40

i

60

t

80

i

100

VoL % THF Fig. 2. Intrinsic viscosity vs composition of tetrahydrofuran in water, curve for Cop-40 at 30°C.

of the plot was always positive. It can also be seen that the intrinsic viscosity decreased with rise in temperature. This result is in strong contrast to styrene-isoprene block copolymer micelles [14], which showed a maximum over a temperature range. This behaviour perhaps may be due to deterioration in selectivity of water with respect to poly(ethylene oxide) at high temperatures. Thus the micelles are expected to have the soluble outer shell of poly(ethylene oxide) less extended at high temperatures resulting in a decrease in overall hydrodynamic volume.

Micelle formation in mixed solvents Water-tetrahydrofuran mixture. Cop-40 formed polymolecular micelles in water whereas it was molecularly dissolved in tetrahydrofuran, which is a good solvent with respect to both blocks. However, the solution behaviour of Cop-40 in mixtures of w a t e r - T H F of various compositions was interesting. The intrinsic viscosities of copolymer solutions are shown in Fig. 2 as a function of the proportion of tetrahydrofuran. From the results, it is deduced that compact polymolecular micelles are formed up to 45 vol% of THF. This is characterized by a decrease in intrinsic viscosity as the compactness of copolymer micelles increases with increase in selectivity of solvent. The solution in this region exhibited a bluish transparent tint, characteristic of micelles. As the concentration of T H F increased in the mixed solvent system, the selectivity of the solvent decreases and polymolecular micelles are dissociated into unimolecular micelles before forming molecular solutions. It is thought that the mixed solvent system virtually becomes a good solvent at 8 0 o T H F for both blocks of the copolymer and that micelles were totally absent. Solutions in this region were colourless and transparent. Figure 3 presents the dependence of the Huggins coefficient on the composition of the mixed solvent system. Two maxima were seen before the final decrease in the value of the coefficient in THF-rich regions of the solvent system. Such a distinct depen-

Block copolymers in aqueous solutions 6.5

287

1.2

1.1

6.0

1 0 % PrOH

o

'1.0

0,9

4.0

0.8

IE ()

2.0

/

0.7

0.6 x tO

0.5

0,4

0 20

I 40

I 60

I *, I 80 loo

0.3

VoL % THF 0.2

Fig. 3. Dependence of Huggins constant (K') for Cop-40 on the composition of tetrahydrofuran in water at 30°C.

dence of the Huggins constant on the solvent composition is attributed to the presence of polymolecular micelles up to the first maxima (corresponding to 40% T H F ) and the dissociation of micelles in T H F rich regions. A similar explanation has been given by Gallot et al. [13] for polystyrene-poly(4-vinylN-ethyl pyridinium bromide) copolymer in (watermethanol 0.1 M LiBr mixtures). Water-2-propanol mixture. The micellar behaviour of Cop-40 in water-2-propanol mixtures was studied by turbidity and viscosity measurements. Figure 4 shows the effect of concentration of 2-propanol on the turbudity behaviour of Cop-41. Addition of propanol caused marked decrease in turbidity and increase in the intercept of the plots. This result may be explained by assuming that the solvent system becomes less selective causing destablization of polymolecular micelles. Detailed viscosity studies were made on Cop-40 micelles in water-2-propanol mixtures of various compositions. The dependence of intrinsic viscosity of micelles on proportion of alcohol is shown in Fig. 5. The intrinsic viscosity showed and initial increase (up to about 20% 2-propanol) before a gradual decrease with increase in propanol concentration. The variations in intrinsic viscosity can be explained as follows. An initial increase in [r/] with increasing propanol concentration may be ascribed to the partitioning of propanol molecules from the solvent phase to the micelle, thus forming a situation similar to mixed micelles. Similar mixed micelle formation between ionic surfactants and 2-propanol has been reported by Bahadur et al. [15] Above 20% propanol, the solvent system becomes less selective and, as a result, micelles are destabilized to dissociate into unimers up to a propanol content of 60%, beyond which the solvent system becomes a fairly good solvent for both blocks and the copolymer

0.1

I 0.01

I 0.02

[ 0.03

I 0.04

I 0.05

[ 0.06

I 0.07

C(gmt -1)

Fig. 4. Turbidimetric behaviour of Cop-41 micelles in aqueous solution with various 2-propanol concentrations at 25°C. 0.6

0.5

0.4

E

0.3

0.2 0.1 0

I 20

I I 40 60 VOt. % PrOH

I J 80 90

Fig. 5. Intrinsic viscosity vs composition o f 2-propano] in

water, plot for Cop-40 at 30°C. molecules are molecularly dissolved forming true solutions. There is a decrease in intrinsic viscosity as the hydrodynamic volume of unimers is less than that of polymolecular micelles. Acknowledgements--The authors thank Professor G. Riess, Mulhouse, France, for his courtesy in supplying copolymers as gift samples and for valuable comments. Mr J. J. Saimbi is thanked for a few viscosity experiments. Part of this work was supported by a grant from CSIR, India [No. 5(68)/85 EMR-II].

288

P. BAHADUR and N. V. SASTRY REFERENCES

I. G. Riess, P. Bahadur and G. Hurtrez. In Block Co-

polymers Encyclopedia Polymer Science and Engineering, Second Edition, Vol. 2, pp. 324-434. Wiley, New York (1985). 2. L. G. Lundsted and I. R. Schmolka. In Block and Graft Copolymerization, (Edited by R. J. Ceresa), Vol. 2. Wiley, New York (1976). 3. J. Szymanowski, J. Myszkowski, K. Szafranaik and J. Nowicki. Tenside Detergents 19(1), 14 (1982); Ibid. 11 (1982). 4. P. Marie and Y. Gallot. Makromolek. Chem. 180, 1611 (1979); Ibid. 183, 2961 (1982); Ibid. 185, 205 (1984). 5. M. J. Owen, T. C. Kendrick, B. M. Kingston and N. C. Lloyd. J. Colloid Interface Sci. 24, 135 (1967). 6. K. Nakamura, R. Endo and M. Takeda. J. Polym. Sci., Polym. Phys. Edn 14, 1287 (1976). 7. G. Riess and D. Rogez. ACS Polym. Prepr. 23, 19 (1982).

8. G. Riess, J. Nervo and D. Rogez. Polym. Engng Sci. 8, 634 (1977). 9. Huynh-Ba-Gia, R. Jerome and Ph. Teyssie. J. Polym. Sci., Polym. Chem. Edn 18, 2391 (1980). 10. P. Bahadur, N. V. Sastry and G. Riess. Paper presented in the 6th International Symposium on Surfactants in Solution, New Delhi (1986). 11. P. Bahadur and G. Riess. Tenside Detergents Communicated. 12. J. Selb and Y. Gallot. Makromolek. Chem. 181, 2605 (1980). 13. J. Selb and Y. Gallot. Makromolek. Chem. 182, 1491 (1981). 14. P. Bahadur and N. .V Sastry. J. Macromolec Scil,-CHEM. A23, 1007 (1986). 15. P. Bahadur, P. H. Kothwala and T. N. Nagar. Colloids Surfaces 14, 59 (1985).