Langmuir 1992,8, 2666-2670
2666
Aggregation Behavior of Pluronic P-94 in Water P. Bahadur' and K. Pandya Department of Chemistry, South Gujarat University, Surat 395 007, India Received January 5,1992. In Final Form: May 26,1992 The aggregationbehavior of an ethyleneoxide-propylene oxidetriblock copolymer,PluronicP-94 (EO%P04rEO24, MW = 4600,40% poly(ethy1ene oxide)) in water was examined using several independent methods. Dynamic light scattering studies revealed the existence of micelles (Rh 90 A) along with monomers and clusters ( R h 20 A and >lOOOA, respectively). Only micelles with low polydispersity were seen at high temperature/concentration. While surface tension measurements at 40 O C gave a critical micelle concentration of -0.002 w t ?6 and molecular area at closest packing of 128 A2, viscosity results showed that micelles were spherical. Fluorescence spectra of solubilized pyrene conformed the presence of clusters,which dissolve at high concentration/temperature. Oscillatory shear measurements and phase diagramshowedreversible thermorheologicalbehavior in concentratedsolutions: An increasein temperature changesa Newtonian liquid to a solidlikegel which further dissolves at higher temperatures. This solution is again gellified with further increase in temperature ultimately leading to phase separation at cloud point.
-
-
Introduction Ethylene oxide-propylene oxide (EPE) block copolymers commercially known as Pluronics are nonionic surfactants with widely recognized applications in various fields.' The disputable aggregation in aqueous solution and unique thermorheological behavior of Pluronics has prompted us to study their solution behavior in aqueous solution.2-s Pluronics solutions exhibit some interesting features, viz., (i) anomalous behavior over a certain range of temperature, an effect shown, in general, by block copolymers in selective ~ o l v e n t swhich ~ * ~ has been found due to the presence of more hydrophobic minor component preferably diblock, present as impurity,618 (ii) micelle formation, strongly temperature and concentration dependent with a remarkable decrease in cmc with tem~erature,5*~1~ (iii) aggregation in presence of additives, drastic changes in solution behavior in the presence of additivedoor a very small amount of ionic surfactant like sodium dodecyl sulfate, SDS? and (iv) reversible thermorheological behavior, concentrated solutions of some Pluronics on increasing temperature form gels which break with further increase in temperature and gellifies again before reaching the cloud point.2J2 Pluronica P-85: L-64: and F-6811 showed different aggregation behavior in aqueous solution. This phenomenon strongly depends on PPO/PEO ratio in the Pluronics: while very hydrophilic copolymer, e.g., F-68, does not form micelles below 40 OC,ll L-64formed micelles above 30 OCSwith cmc 0.4 w t % at 40 "C which decreases in one
or two order with increase in temperature by 20 O C 9 and P-85 was micellar even at 15 "C with cmc < 0.1 wt %.2 This paper reports a study on dilute and concentrated solution of Pluronic P-94 by several independent methods to probe the effect of temperature on micellar and gelling behavior. P-94 contains 40% poly(ethy1eneoxide) (PEO) (similar to L-64)and molecular weight 4600 (similar to P-85). Experimental Section Materials. Pluronic P-94 (MW total = 4600,40% PEO) was HO --(CH,CH,O),,
--(CH,CHO)4,--(CHzCH,O)z4
I
-H
CH3
supplied by Serva, Germany,and was used as received. Double distilled water from Pyrex glass apparatus waa used. Fresh solutions were prepared by weight in double distilled water and filtered through 0.45-pm Millipore fiiters. Pyrene (99%)was purchased from Aldrich. Methods. The surface tension measurements were carried out by a drop volume method using an Agla micrometer syringe. Viscosity and density of solutions were measured using an Ubbelohde suspended level capillary viscometer and Kratlcy digital densitometer (Paar, Austria), respectively. Cloud pointa and gel fonnatiodbreakingpointa were determined from Pluronic P-94 solutions at different concentrations by taking the solution in the glass tubes which were immersed in a well-stirred heating bath. The solutions were stirred with a magnetic bar while being heated. The fiit appenrance of turbidity was taken as the cloud point. The cloud pointa were reproducible to f0.5 O C . Gel point and gel breaking point in concentratedP-94solutionweredetermined from the restriction (1)Lundsted, L. G.;Schmolka, I. R. In Block and Graft Copolymin movement of a small magnetic bar placed in a solution. The erization; Cereaa, R. J., Ed.; Wiley: New York, 1976; Vol. 2. (2)Brown, W.; Schillen, K.; Almgren, M.;Hvidt, S.;Bahadur, P. J. gelation temperatures were also determined at different P-94 Phy8. Chem. lSsl,%, 1860. concentrations from oscillatoryshearmeasurements when elastic (3)Almgren, M.;Alsins, J.; Bahadur, P. Langmuir 1991, 7,446. (4) Almgren,M.;VanStam,J.;Lindblad,C.;Li,P.;Stilba,P.;Bahadur, storage modulus (G') >, loss shear modulus (G"). Detaila of the measurements of diffusion coefficienta from P. J. Phys. Chem. 1991,95,6677. dynamic light scattering (DLS) and pulsed gradient spin echo (6)Almgren, M.; Bahadur, P.; Jansson, M.; Li,P.; Brown, W.; Bahadur, A. J. Colloid Interface Sci. 1992,151,167. nuclear magneticresonance (PGSE-NMR),oscillatory shear,and (6) T y r , 2.;Kratochvil, P. Adu. Colloid Interface Sci. 1976,6,201. fluorescence measurements were same as reported before.ao6 (7)Riese, G.;Bahadur, P.;Hurtrez, G. Block Copolymers,Encyclopedia on Polymer Science & Engineering, 2nd ed.; Wiley: New York, 1986; Results and Discussion Vol. 2, pp 324-434. (8) W o u , 2.;Chu,B. Macromolecules 1988,21,2648. Surface Tension. The surface tension-log polymer (9)Reddy, N. K.:Fordham, P. J.: Attwood, D.: Booth, C. J. Chem. concentration plot (Figure 1)shows an initial decrease in SOC.,Faraday Trans. 1990,86,1569. surface tension followed by a constant value with pro(10)Bahadur, P.; Pandya, K.; Almgren, M.;Li, P.; Stilbs, P. Colloid Polym. Sci., in press. gressiveincrease in concentration. The break point yields (11)Bahadur, P.; Li, P.; Almgren, M.;Brown, W. Langmoir 1992,8, a cmc of 0.002 w t % (4.4 X mol dm-3). The minimum 1903. surface tension value achieved was 33 mN m-l, which is (12) Wanka, G.;Hoffmann, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268.101. fairly low as compared to other nonionic8 in the series 0743-746319212408-2666%O~.OOl 0 Q 1992 American Chemical Society
Aggregation Behavior of Pluronic P-94 in Water
70
Langmuir, Vol. 8, No. 11, 1992 2667 Table I. Surface and Colloid Chemical Parameten for P-94(MW = 4,600, POE = 40%) in Aqueous Solutions
1t
~~~
HLB cloud point (1.0wt % ), "C cmc, w t % aredmolecde, A2
N Rh
13.P
76b 0.OOF 128e
36*
w
(NMR),A
Rh (DLS), A [?I, dL g-* Kli 0, mI.4
6,g of HzO/g of P-94
9V 0.ow 2.w 0.9ld, 1.4
Reportedvalue. R e p o d value is 76"C. From surface tension a t 40 "C. From static light scattering a t 40 "C. e From diffusion coefficient for 10.0 wt % a t 40 "C.f From DLS on extrapolation to zero concentration a t 40 "C.g From dilute solution viecosity measurements a t 40 "C. From density measurement a t 40 "C. 01
-4
-2
-3 log
c , ut
-I
0
I
Figure 1. Surface tension behavior of Pluronic P-94 in water as a function of concentration at 40 "C.
Viscosity. Viscosities of P-94 solutions (in the concentration range 0-2 g dL-') were used to calculate intrinsic viscosity, [ql, and Huggins' constant, KH,using the equation
(280,or Tritons etc. Although a very small concentration (-0.002 wt %) is capable of reducing surface tension of water to a value by 36 mN m-l, P-94 possesses poor surface activity. Prasad et al.13in their studies on surface tension for different Pluronic solutions over a large concentration span (lW7-101 wt 9% ) have shown two inflections showing monomolecular micelles and polymolecular micelles, without showing any constant value in surface tension at high concentrations. A similar nature in the plot for F-68 has been observed by Hergeth et al.I4who showed the existence of dimers/trimers. Recent studies by Wanka et showed that surface tension-log C plots show concentration transition over approximately one concentration decade, where the surface tension values achieved constancy. Reddy et ale9based on surface tension of L-64 in commercial sample found a minimum which disappeared in purified samplesand showed plots similar to conventional surfactants with a striking difference that cmc changes from0.71toO.09gdL-lfrom27to4OoC. Sincethemicellar behavior of Pluronics strongly differs from one Pluronic to other (commercial samples further complicate this behavior) and also on temperature, the different pattern in surface tension-log C plots is not surprising. Area per molecule (A) of the adsorbed copolymer was calculated on application of the Gibbs' adsorption equation
r = - l d r
2.303RT d log C Where I' (converted to mol cm-2) is the surface excess concentration, 7 is the surface tension (in N m-l) at concentration C (in wt %), and if N is the Avogadro constant, interface area/molecule, A, is given by
A = 10"/I" (2) The calculated area (128 A2) from the slope just below the cmc is recorded in Table I. The high molecular area indicates that the PEO group determines A. The large A value arises from the poor packing at the a k w a t e r interface since the hydrated coiled chains sweep out large surface area. The soluble films of the PEO chains are known to penetrate the aqueous phase ae coils, the size of which increases with the chain length, thereby increasing A. (13)Prasad, K. N.;Luong, T.T.; Florence, A. T.; Paris, J.; Vaution, C.; hiller, M.;Puieieu, F. Colloid Interface Sci. 1979,69,226. (14)Hergeth, W.-D.;Bloes,P.;Schmutzler,K.; Warbwig,S.;Witkoweki, K.;W o h k i , L. Acta Polym. 1989,40,416.
%/c = 1111 + [VI2 KHC
(3)
Where qsp/cis reduced viscosity. The intrinsic viscosity, [VI, decreases from 0.09 dL gl to 0.05 dL g-l over a temperature range from 30 to 50 OC. This reduction at higher temperature is anticipated since micelles become more and more compact with increase in temperature, due to the dehydration of PEO chain. Assuming the micelles as spherical in shape, the micellar hydration, 6, was calculated from viscosity data using the Oncley equation
[?I = v ( D + 60,)
(4)
Where D and DO are the partial specific volumes of solute and water determined from density measurements. 6 is the weight of water in grams hydrating 1g of solute, and v is the viscosity increment equal to 0.025 for spheres. Hydration numbers, 6, of other nonionic surfactante in water have been reported earlier by some workers1"'*S1 and agree with those reported in literature. Hydration number, 1.4 g of H2O/g of P-94 is far higher than can be accountedfor solely by hydrogen bonding to ether oxygen of the PEO chain and indicates considerable mechanical entrapment of water by this chain. Temperature increase causes pronounced micellar dehydration. A rough indication of sphericity can be obtained from the Huggins constant. For solid uncharged spheres KH is approximately 2. In the present system KHranges from 1.45 to 2.75 (except 30 O C where KH is -0.2 due to the presence of significant proportion of unimers, clusters, etc.) indicating that micelles are spherical. The increased values of KH (Figure 2) with the increase in temperature may be due to the progressive aggregati~n.'~ The information on the micellar shape and volume of Pluronic P-94 was obtained by using Guth and Si"s equation20 qrel = 1
+ 2.54 + 14.1d2
(5)
and (15)Mnndal, A. B.; Ray, S.;Biswae, A. M.;Moulik, 5. P.J. Phya. Chem. 1980,84,856. (16)Birdi, K.S.B o g . Colloid Polym. Sci. 1986, 70, 23. (17)Robson, R. J.; Dennis,E. A. J. Phye. Chem. 1977,81,1027. (18)Oncley, J. L. Ann. N.Y. Acad. Sci. 1940,41,121. (19)Attwood, D.;Collett, J. H.;Tait, C. J. Int. J.Pharm. 1986,26,26. (20)Robito, D.C.; Thomas, I. L.J. Colloid Interface Sci. 1968,26,416.
Bahadur and Pandya
2668 Langmuir, Vol. 8, No. 11, 1992
o*zol
35%
L O
3.0
s 2.0
1.0
0
t
,
30
25
35
45
40
0
50
Temperature, " C
Figure 2. Plot of intrinsic viscosity and Huggins'constant vs temperature for Pluronic P-94 in water: A, intrinsic viscosity; 0 , Huggins' constant.
4.0
I
47'C
53.4.C
1.0
0
0.5
1.5
1.0
[ P-94
1,
2.0
g d1-l
Figure 3. Plots of V vs C for Pluronic P-94 in water at different temperatures: A,30 O C ; A, 35 OC;+, 40 OC;0 , 4 5 "c;0 , 5 0 O C .
v = $IC
(6)
where qrel is the relative viscosity, $ is the volume fraction occupied by the particles, C is the concentration of P-94 solutions in g/mL, and V is the effective specific volume of the Pluronic including hydrated water. The plota of effective specific volume V against concentration C are linear (Figure 3) with no or little change in V, except at low concentrations and at low temperatures where the monomers of P-94 and clusters from diblock impurities are likely to be present. The effective volume remains constant over almost all of the concentration range studied which indicates that the micelles are spherical. Similar behavior has been observed by Saito and Sat@ and El Eini et al.22who explained this behavior due to the spherical micelles. Dynamic Light Scattering(DLS).The dataobtained by the Laplace inversion of correlation functions from dynamic light scattering (DLS) for 10, 20, and 35 wt % solutions of P-94 at 20.0,30.0,40.0,47.0, and 53.4 "C are shown in Figure 4. The relaxation time distribution shows a pattern closely similar to P-85.2 At 20 OC, usually monomers (shortrelaxationtime),micelles (relaxationtime of the order of 102 ps), and clusters (long relaxation time) were observed. At high concentration or temperature, both the peaks due to monomers and clusters disappear, leaving the micelle peak alone with decreasing polydispersity. Higher concentration and temperature favor micelle formation and thus increase micellar concentration and (21)Saito, Y.;Sato, T.J. Phye. Chem. 1985,89, 2110. (22)El Eini. D.I. D.;Barry, B.W.;Rhodes, C. T.J. Colloid Interface Sci. 1976,54,
Figure 4. Relaxation time distribution obtained by Laplace inversion of dynamic light scattering data, 7A(7)versus log ?/pa, for P-94 at high concentrations (C1 10wt 9% ) and at temperatures between 20 and 55 O C . Peaks attributed to monomers and micelleaare indicated in addition to the peaks at low concentration which probably reflect clusters of diblock contaminant. Measurements are at an angle of 1 3 0 O .
I
I
10
20
[P-91],
30 Wl
b0
'/.
Figure 5. Reduced diffusion coefficients(Dqd T )from DLS data for Pluronic P-94 as a function of concentration at 40 O C . therefore the monomers and clusters disappear (asall the
added P-94 forms the micelles and dissolves the clusters). The low polydispersity in micelles has been found in polydispersed block copolymers earlier.' The calculated values of the hydrodynamic radius, Rh, of monomers, micelles, and clusters from the diffusion coefficient using the Stokes-Einstein equation are shown in Figure 4. (7)
The
Rh
values are not absolute and are concentration
Langmuir, Vol. 8, No. 11, 1992 2669
Aggregation Behavior of Pluronic P-94 in Water
Id,
I
I
20
30
I
I
I
I
I
I
I
LO 50 Tmporoturo,'C
m
60
Figure 6. Plots of self-diffusioncoefficient of Pluronic P-94 in water (10 w t %) obtained from PGSE-NMR and DLS as a function of temperature: 0,PGSE-NMR; 0 , DLS.
/
-3
0
I
10 O 0
O
o
o
O
0
0
O
0
O
20
0
Tempsmture ( "C
Figure 9. Storage modulus, G' at 1 radh from oscillatoryhear measurementsas a functionof temperaturefor differentPluronic
P-94 concentrations.
* * 10
0
30
20
40
tP941, 8 dl-1
Figure 7. Partial phase diagram of Pluronic P-94 in water: 0, gel point; 0,gel breaking point; 0, gel point (from oscillatory
shear measurements). Id
'61
1
-A
ro
A , 10
M
LO 50 Tempmture I tI
60
70
0
90
Figure 8. Storage and loes moduli, G' and G", respectively, from oscillatoryshear measurementsas a function of temperature for a Pluronic P-94 solution of 26.0 w t %
.
dependent. However, these provide an indication of a decrease in the hydrodynamic size of micelles with increase in temperature, except at the highest temperature (53.4 "C)where an increase in R h was observed. The micelles
seem to become more and more compact due to dehydration with increase in temperature. Increase in R h at 53.4 "C is perhaps due to a large increase in micelle aggregation number at this temperature which is not far away from the cloud point (76 "C). The hydrodynamicradius of micelles at 40 "Cwas also determinedby extrapolatingreduced diffusioncoefficients (DqdT) as a function of concentration (Figure 5). The extrapolated value ( D q o l T ) was ~ used in the StokesEinstein equation to obtain Rh which yielded a value of 92 A. This hydrodynamicradius of P-94 is comparableto those of Pluronic L-64 (80 A) and P-85 (80 A) at 40 OC. PGSE-NMR.In Figure 6 are shown D values obtained from PGSE-NMR and DLS at 10 wt % P-94 as a function of temperature. The two methods yield D values in good agreement except at 20 "C. The difference in D values at this temperature is perhaps due to the presence of monomers, which are responsible for an increase inD value from PGSE-NMR. The diffusion coefficient, D, values remain more or less same up to 50 "C showing not much change in micelle size occurring in this temperature range. The D values at 40 "Cwere used to calculate hydrodynamic radius, Rh, which is close to the value obtained from DLS data. The hydrodynamic radii also calculated from D (PGSE-NMR) values at 10 wt % P-94 solutions showed a progressive increase in size with increase in temperature; the Rh values at 26.5, 32.0, 43.0, and 52.0 "C were 82.0, 89.0, 104.0, and 115.0 A, respectively. Rheology. The gel formation and breaking temperatures of concentrated P-94 solutions determined visually are shown in Figure 7. While solutions (> G’). With an increase in temperature, a sharp rise of over an order of magnitude in these parameters shows a transition from a Newtonian liquid to a solid like a gel (G’2 G”). Figure 8 also shows qualitatively the dissolution of gel (and its re-formation at higher temperatures). This rheological behavior was reversible as observed from heating and cooling scans. Recent SANS ~ t u d i e s l ~ *the ~ ~structures on of gels formed by Pluronics in aqueous solution have shown that the formation of gels is associated with the close packing of the micelles with weak interparticle interactions and by crystallization in a body centered cubic lattice. Figure 9 shows the storage modulus with different P-94 concentrations as a function of temperature. It can be seen that the storage modulus increases rapidly at the gel temperature and approaches a saturation value on further increase in the temperature. It shows also that the gel temperature shifts toward a lower value for higher P-94 concentration. Gel temperatures obtained from oscillatory shear measurements are also shown in Figure 7 where a good agreementis seen from the data obtained by two different methods. Fluorescence. Fluorescence spectra of 5 X 10-6 M pyrene in 1 w t % P-94 at different temperatures are recorded in Figure 10. The formation of excimers by pyrene is clearly shown by the spectrethe excimeremission decreaseswith increase in temperature. This is in contrast to an expected behavior where a growth of micelles is usually anticipated with increase in temperature. These observationscan be explained by considering the fact that P-94 at low concentration (- 1 wt % ) and temperature
Acknowledgment. P.B. is thaukfultoProfeeeorsMats Almgren and Wyn Brown for providing facilities for fluorescenceand dynamiclight scattering measurements. The authors are grateful to Dr. Soren Hvidt for providing oscillatory shear measurementa and fruitful comments. The authors also thank Dr. Puyong Li for making PGSENMR measurements. Financial support from CSIR, India is gratefully acknowledged.
(23) Morteneen, K.;Brown,W.; Norden, B. Submitted for publication in Phys. Rev. Lett.
Regiatry No. (E0)PO)(block copolymer),106392-12-5; HnO, 7732-18-5;pyrene, 1’29-00-0.
Conclusion Dynamic light scattering measurements reveal the existence of unimers and clusters at low concentrations/ temperatures in Pluronic P-94 solution in water. Micelles with low polydispersityusually form at higher temperature; micellar size increases with increase in temperature particularly above 45 “C. PGSENMR yielded micelle diffusion coefficients in good agreement with DLS d a t a The Pluronic exhibited marked surface activity (cmc = 0.002 w t % and area/molecule = 128A2,determined from surface tension measurements). Viscosity data reflect an aggregation pattern and sphericity of micelles. Fluorescenceprobing results further support that P-94 preparation is polydisperse; the more hydrophobic content present forms clusters at low temperature/concentrationas o b served for another Pluronic L-64.3 Interesting reversible thermorheological behavior of concentrated P-94 solution is depicted from oscillatory shear measurements.
