Probing the Structure of Pluronic PEOâPPOâPEO Block Copolymer

Langmuir 1996, 12, 4697-4703

4697

Probing the Structure of Pluronic PEO-PPO-PEO Block Copolymer Solutions with Their Apparent Volume and Heat Capacity Luc Meilleur, Alain Hardy, and Franc¸ ois Quirion* INRS-E Ä nergie et Mate´ riaux, 1650 boul Lionel Boulet, Varennes, Que´ bec J3X 1S2, Canada Received March 5, 1996. In Final Form: June 17, 1996X As the temperature of Pluronic copolymer solutions increases, the copolymer molecules evolve from a region with only monomers, monomer only, to a region where monomers are in equilibrium with micelles, monomer-micelle, to a region where only micelles are found, micelle only. In this article, we analyze the concentration dependence (0.1-10% w/w) of the reduced viscosity, ηred, the apparent specific volume, vp,φ, and the apparent specific heat capacity, cpp,φ, in terms of these regions at 5, 25, and 45 °C. Six Pluronics (F38, P85, P104, P103, L122, and L101) have a molecular weight around 5000 g‚mol-1 and two (F108 and P105) have a higher molecular weight. The method compares the apparent specific volume and heat capacity of the copolymer in solution with its additive values corresponding to the monomeric state (vmono and cpmono). If vp,φ ≈ vmono and cpp,φ ≈ cpmono, then all the copolymer molecules are in the monomeric state. If vp,φ J vmono and cpp,φ > cpmono, then the copolymer molecules are in equilibrium with micelles. Finally, if vp,φ > vmono and cpp,φ ≈ cpmono, then the copolymer molecules are all in the micellar state. At 5 °C, all the Pluronics studied are in the monomeric state between 0.1 and 10 g/mL. At 25 °C, the monomer only, monomer-micelle, and micelle only regions are all encountered depending on the molecular weight and the polyoxypropylene content. At 45 °C, F38 is still in the monomeric state and P85 molecules are in equilibrium with monomer only at low concentrations. All the other copolymers investigated form only micelles. The investigation of the apparent specific volume and heat capacity has proven quite sensitive, in particular for the identification of the concentration range where monomers are in equilibrium with micelles.

Introduction The aqueous solutions of Pluronic copolymers are known to have a critical micellization temperature, cmt, over which micelles are formed. Although the cmt is known to depend on the concentration of the copolymer in solution, most studies are performed with techniques that scan a property as a function of temperature. Recently, Chu1 reviewed the literature of such investigations where differential scanning calorimetry, fluorescence, expansibilities, and light scattering are used to probe the structure of Pluronic solutions. Recently, other studies came out, especially the fluorescence work by Alexandridis et al.2-5 and the phase behavior studies by Hecht et al.6 In an earlier study, Mortensen and Pedersen7 presented a very nice investigation of the structure of P85 solutions in the concentration-temperature diagram using small angle neutron scattering. They identified a region at low concentration and temperature where the P85 molecules exist only as monomers, a second region at intermediate temperature and concentration where the monomers are in equilibrium with their micelles, and a third region at high temperature where the copolymer molecules exist only in the micellar state. In a previous paper,8 we presented an exhaustive investigation of the phase * To whom correspondence should be addressed. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, August 15, 1996. (1) Chu, B. Langmuir 1995, 11, 414-421. (2) Alexandridis, P.; Athanassiou, V.; Hatton, T. A. Langmuir 1995, 11, 2442-2450. (3) Nivaggiopli, T.; Tsao, B.; Alexandridis, P.; Hatton, T. A. Langmuir 1995, 11, 119-126. (4) Alexandridis, P.; Nivaggiopli, T.; Hatton, T. A. Langmuir 1995, 11, 1468-1476. (5) Nivaggiopli, T.; Alexandridis, P.; Hatton, T. A.: Yekta, A.; Winnik, M. A. Langmuir 1995, 11, 730-737. (6) Hecht, E.; Mortensen, K.; Hoffmann, H. Macromolecules 1995, 28, 5465-5476. (7) Mortensen, K.; Pedersen, J. S. Macromolecules 1993, 26, 805812.

S0743-7463(96)00200-4 CCC: $12.00

Table 1. Characteristics of the Pluronics, (CH2CH2O)n-(CH(CH3)CH2O)m-(CH2CH2O)n, Used during This Investigation pluronic

MW, (g/mol)

m

n

F38 P85 P104 P103 L122 L101 F108 P105

4700 4600 5900 4950 5000 3800 14600 6500

16 40 61 60 69 59 50 56

43 26 27 17 11 4 133 37

a

Tcp(1%),a °C

Tcp(10%),a °C

%H2O

>100 83.2 (85) 80.3 (81) 86.5 (86) 16.7 (19) 15.4 (15)

>100 85.3 (86) 78.3 (78) 51.4 (52) 12.8 (13) 12.4 (11)

0.41 0.73 0.56 0.71 0.66 0.41

Temperatures in parentheses are reported by BASF.

behavior of Pluronic L-122 in water and we demonstrated that the concentration dependence of the apparent specific volume and heat capacity of the copolymer can be used as a probe to micellization. That work was continued9 for other Pluronics (F38, F108, P85, P105, P104, P103, L122, and L101) at 5, 25, and 45 °C, and the results are presented here in light of the stimulating interpretation provided by Mortensen and Pedersen.7 Experimental Section This section describes the characteristics of the copolymers and the methodology for the determination of the apparent specific volume and heat capacity, vp,φ and cpp,φ, and the intrinsic viscosity and Huggins constant, [η] and kH, from the measurement of the density, volumic heat capacity, and viscosity of the Pluronic solutions. Block Copolymers. Pluronics are triblock copolymers of polyoxyethylene, POE, and polyoxypropylene, POP, with the general structure [CH2CH2O]n-[CH(CH3)CH2O]m-[CH2CH2O]n. Pluronic F38 (#WPTH-542B), F108 (#WPCH-525B), P85 (#WPAE592A), P105 (#WPWE-568A), P104 (#WPYE-526A), P103 (#WP(8) Camire´, C.; Meilleur, L.; Quirion, F. J. Phys. Chem. 1992, 96, 2360-2364. (9) Meilleur, L. Me´moire de Maıˆtrise, INRS-E Ä nergie et Mate´riaux, 1992, M.D.307, 87 pages.

© 1996 American Chemical Society

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Table 2. Specific Volume and Heat Capacity of Oxyethylene and Oxypropylene at Infinite Dilution in Water at 5, 25, and 45 °Ca 5 °C oxyethylene oxypropylene a

25 °C

45 °C

vp°, cm3‚g-1

cpp°, J‚K-1‚g-1

vp°, cm3‚g-1


vp°, cm3‚g-1


0.8200 0.8893

2.72 3.57

0.8391 0.9107

2.974 3.410

0.8552 0.9316

3.23 3.26

Specific volumes are from Harada et al.12 Heat capacities at 25˚C are from Riedl.13 See text for the heat capacity at 5 and 45 °C.

NC-566B), L122 (#WPYG-531B), and L101 (#WPEI-521C) were kindly provided to us by BASF Wyandotte Corporation, Michigan 48192. The last digit specifies the weight percent of POE in the copolymer, for instance 30% POE in Pluronic P103. The POE content was checked by 1H NMR, and it agreed with BASF specifications for all the Pluronics. The cloud point temperature, Tcp, was identified with the apparition of fog as the solutions were slowly heated. The water content in the commercial Pluronics was determined using Karl-Fisher titration. These values being relatively low, the concentrations of the Pluronics were not corrected for their humidity content. Table 1 summarizes these characteristics for the Pluronics investigated. All Pluronics were used as received, and their aqueous solutions were prepared by weight and investigated at 5, 25, and 45 °C except for F108 (25 °C), P105 (25 °C), and L101 (5 and 25 °C). Apparent Specific Volume and Heat Capacity. The apparent specific volume, vp,φ (cm3‚g-1), and heat capacity, cpp,φ (J‚K-1‚g-1), of the Pluronics in water were obtained from the densities, F (g‚cm-3), and volumic heat capacities, σ (J‚K-1‚cm-3), measured with a vibrating tube flow densimeter10 (Sodev 03-D) and a differential flow microcalorimeter11 (Sodev CP-C and DTC), respectively, thermoregulated at the experimental temperature with a stability of (0.001 °C. The densimeter was calibrated using the density of nitrogen and water at 5, 25, and 45 °C. The apparent specific volume of Pluronic in water was then obtained from the density of the solution and water, F and F0, and the weight fraction of Pluronic in the solution, Wp, with

fraction of Pluronic was converted to c (g‚cm-3) using the density of the solutions, F. The Huggins constant reflects the intermolecular interactions between the Pluronic molecules. A value of 0.5 is expected for non-interacting hard sphere and attractive interactions; those responsible for micellization and phase separation generally result in higher values. It was noticed that some Pluronics, particularly P85, adsorb on the glass walls of the capillary with a dramatic effect on the reduced viscosity at low concentrations. For instance, between 0.01 and 0.04% P85 in water at 25 °C, the flow time of the solutions was smaller than that of water, resulting in a negative reduced viscosity. However, that effect is negligible in the range of concentration used (0.5-4 g‚cm-3) for the determination of [η] and kH.

Results and Discussion

where η is the viscosity of the Pluronic solutions. The weight

The first part of this section describes how the apparent specific volume and heat capacity of a Pluronic can provide information on the structure of these solutions. Then the concentration dependence of ηred, vp,φ, and cpp,φ of the eight Pluronics studied is discussed in terms of the three regions suggested by Mortensen and Pedersen:7 monomer only, monomer-micelle, and micelle only. Infinite Dilution versus Apparent Volume and Heat Capacity. The extrapolation to zero concentration of an apparent thermodynamic property provides a partial quantity known as the value at infinite dilution. These values can also be calculated by summing the contributions of the chemical groups of the molecule. For instance, the value at infinite dilution of Pluronics is the sum of the contributions of CH2CH2O and CH(CH3)CH2O. Harada et al.12 report the molar volume at infinite dilution of oxyethylene, CH, CH2, and CH3 at various temperatures while Riedl13 reports the molar volume and heat capacity of oxyethylene and oxypropylene at 25 °C. In both cases, these numbers are obtained at very low concentrations for molecules that do not have the ability to form micelles. Thus, values obtained by adding these contributions will correspond to Pluronic molecules in the monomeric state. Table 2 summarizes the value at infinite dilution of the specific volume and heat capacity of oxyethylene and oxypropylene at 5, 25, and 45 °C. Unfortunately, we could not find the value at infinite dilution for the heat capacity at 5 and 45 °C. Nevertheless, our experimental data at 5 °C indicate very weak interactions so that the extrapolation to null concentration results in good estimates of the value at infinite dilution. These values were extrapolated to get the contribution of oxyethylene (2.72 J‚K-1‚g-1) and oxypropylene (3.57 J‚K-1‚g-1) at 5 °C. With these values and the one reported at 25 °C, the value at infinite dilution of the heat capacity was extrapolated at 45 °C for oxyethylene (3.23 J‚K-1‚g-1) and oxypropylene (3.26 J‚K-1‚g-1). Micellization is generally enthalpic, and it may proceed over a wide range of concentration where monomers are in equilibrium with the micelles. During that transition, the apparent specific heat capacity shows a maximum with an amplitude proportional to the square of the

(10) Picker, P.; Tremblay, E.; Jolicoeur, C. J. Solution Chem. 1974, 3, 377. (11) Picker, P.; Leduc, P.-A.; Philip, P.; Desnoyers, J. E. J. Chem. Thermodyn. 1971, 3, 631-642.

(12) Harada, S.; Nakajima, T.; Komatsu, T.; Nakagawa, T. J. Solution Chem. 1978, 7, 463-474. (13) Riedl, B. Ph.D. Thesis, De´partement de Chimie, Universite´ de Sherbrooke, 1984.

vp,Φ ) (1/F) - (1 - Wp)(F - F0)/(WpFF0)

(1)

The measured volumic heat capacity of the solution was converted into a massic heat capacity, cp (J‚K-1‚g-1) using the density measurements. The apparent specific heat capacity of Pluronic was then obtained from the massic heat capacity of the solution and water, cp and cp0, and the weight fraction of Pluronic in the solution, Wp, with

cpp,Φ ) cp + (1 - Wp)(cp - cp0)/Wp

(2)

For each measurement, the sample was pre-equilibrated two degrees below the experimental temperature while being gently stirred. They were then introduced in the densimeter and microcalorimeter at a flow rate of 0.4 and 0.7 cm3‚min-1, respectively. This procedure ensures that the samples have all the same thermal history; i.e., they are warmed to the experimental temperature prior to the measurements. This is particularly important for the solutions which are close to phase separation or gelation. Under these conditions, the maximum experimental error on vp,φ and cpp,φ decreases exponentially from (0.01 cm3‚g-1 and (1 J‚K-1‚g-1 at Wp ) 0.1% to smaller than (0.001 cm3‚g-1 and (0.1 J‚K-1‚g-1 at Wp ) 1%. Intrinsic Viscosity and Huggins Constant. The viscosity, η (cP), was determined only for the monophasic systems with an Ostwald viscosimeter. The viscosity and the density of water, η0 and F0, and its flow time, t0, were used to calibrate the viscosimeter at the experimental temperature. The intrinsic viscosity, [η] (cm3‚g-1), and the Huggins constant, kH, were obtained from the plot of the reduced viscosity, ηred (cm3‚g-1), versus the concentration, c (g‚cm-3), according to

ηred ) [(η/η0) - 1]c-1 ) [η] + kH[η]2c + ...

(3)

Structure of Pluronic PEO-PPO-PEO Block Copolymers

Langmuir, Vol. 12, No. 20, 1996 4699

Figure 1. Comparison between the apparent specific volume and heat capacity of Pluronics at a concentration of 10% in water at 5 (9), 25 ([), and 45 °C (2) with the values calculated from group additivity at infinite dilution (full lines).

Figure 2. Intrinsic viscosity and Huggins constant of the Pluronics in aqueous solutions determined from the extrapolation and the slope of the reduced viscosity versus concentration in the range 0.5-4% at 5 (9), 25 ([), and 45 °C (2).

enthalpy of micellization.14 If the concentration of Pluronic in solution is in that range, the apparent specific heat capacity of the Pluronic will be much higher than the expected value in the monomeric state. The maximum in the heat capacity versus concentration has the same origin as the maximum observed in DSC experiments (heat capacity versus temperature), i.e. the enthalpy of micellization. A consequence of micellization is the loss of hydrophobic hydration, which results in a positive contribution to the volume and a negative one to the heat capacity15 of the copolymer molecules. Armstrong et al.16 investigated many Pluronics using differential scanning calorimetry and scanning densitometry, and they always observed very little differences between the heat capacity before and after the maximum caused by micellization. On the other hand, the volume in the micellar state was always higher than that in the monomeric state. Other DSC17 and expansibility18 investigations, as well as our previous work,8 support these observations. On this basis, we know that the apparent specific volume of a Pluronic in solution will correspond to its value calculated at infinite dilution if it is in the monomer only region. It will have a significantly higher value if it is in the micelle only region. And it will have an intermediate value in the monomer-micelle region. The apparent specific heat capacity will be close to the value calculated at infinite dilution both in the monomer only and micelle only regions. In the monomer-micelle region the apparent

specific heat capacity will be much higher than the calculated value at infinite dilution. When put together, these pieces of information allow us to identify the region corresponding to any Pluronic solution. This is shown in Figure 1, where vp,φ and cpp,φ of Pluronic (F38, P85, P104, P103, L122, L101) at a concentration of 10% in water are compared with the calculated values at infinite dilution at 5, 25, and 45 °C. F38 remains in the monomer only region up to 45 °C. From 5 to 25 °C, the 10% solutions of P85, P104, and P103 switch from the monomer only to monomer-micelle region. For the same temperature change, the solutions of L122 and L101 go from monomer only to micelle only. At 45 °C, all the Pluronics at a concentration of 10% are in the micelle only region. These data also show that the apparent specific heat capacity is more sensitive to the micellization than the apparent specific volume. For instance, the apparent volume of 10% P85 is still close to its value at infinite dilution while the apparent specific heat capacity has almost doubled. The sensitivity of these properties to micellization will become more evident in the next section, where the concentration dependence is discussed. Intrinsic Viscosity and Huggins Constant. Figure 2 reports the intrinsic viscosity and the Huggins constant obtained from the concentration dependence of the reduced viscosity. These numbers do not provide much information by themselves, but they reflect general trends with respect to temperature and POE content. The intrinsic viscosity is related to the hydrodynamic volume, and it decreases with the POP content, suggesting a tighter packing for the more hydrophobic Pluronics. The same effect is also observed when the temperature increases. The tighter packing could be a consequence of intramolecular folding or micellization, and it is in accordance with the increase of the apparent specific volume of the Pluronic molecules due to the release of hydrophobic hydration. The kH values reported here reflect the interactions in the concentration range used for the analysis, i.e. 0.5-

(14) See for instance: Desnoyers, J. E.; Caron, G.; DeLisi, R.; Roberts, D.; Roux, A.; Perron, G. J. Phys. Chem. 1983, 87, 1397-1406. (15) See for instance: Huot, J.-Y.; Jolicoeur, C. In The Chemical Physics of Solvation, part A, Theory of Solvation; Ulstrup, J., Ed.; Elsevier: Amsterdam, 1985; pp 417-471. (16) Armstrong, J. K.; Parsonage, J.; Chowdhry, B.; Leharne, S.; Mitchell, J.; Beezer, A.; Lo¨hner, K.; Laggner, P. J. Phys. Chem. 1993, 97, 3904-3909. (17) Leboeuf, D. Me´moire de Maıˆtrise, De´partement de Chimie, Universite´ de Sherbrooke, 1990. (18) Williams, R. K.; Simard, M.-A.; Jolicoeur, C. J. Phys. Chem. 1985, 89, 178-182.

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Figure 4. Concentration dependence of the apparent specific volume ([) and heat capacity (9) of Pluronic F108 at 25 °C.

Figure 3. Concentration dependence of the apparent specific volume and heat capacity and of the reduced viscosity of Pluronic F38 at 5 (9), 25 ([), and 45 °C (2).

4%. At 5 °C, the kH values are all around 0.5 up to a POP content of 60%, and they start to increase slightly for the more hydrophobic Pluronics. At 25 °C, F38 and P85 both behave as non-interacting monomers. The values for P104 and P103 are abnormally low, maybe because the system is undergoing micellization in that concentration range. At 45 °C, F38 is still in the monomer only region while P85 and P104 show strong attractive interactions in accordance with the presence of micelles in that region. In this section we have concentrated our analysis on the structure of a solution at a concentration of 10% in water (vp,φ and cpp,φ) or in the concentration range 0.54% ([η] and kH). In the following sections, we will discuss in more detail the concentration dependence of the structure of Pluronic solutions. The apparent specific volume and heat capacity and the reduced viscosity of F38, F108, P85, P105, P104, P103, L122, and L101 are presented in Figures 3-10, respectively. As seen in Table 1, most Pluronics have a molecular weight around 5000

g‚mol-1 except for F108 and P105 (Figures 4 and 6), which were used to investigate the effect of molecular weight at 25 °C. F38 and F108 Solutions. Figure 3 shows the concentration dependence of the apparent specific volume and heat capacity and of the reduced viscosity of Pluronic F38 at 5, 25, and 45 °C. As was mentioned in the above section, F38 remains in the monomer only region up to 10% and 45 °C. The apparent specific volume and heat capacity are futureless with null slopes indicating a lack of interactions between the Pluronic molecules. What may sometimes look like a trend at low concentrations is not significant within the experimental uncertainty. Figure 4 shows the apparent specific volume and heat capacity of Pluronic F108 at 25 °C, this time up to a concentration of 18%. The first observation is that an increase of the molecular weight of a Pluronic enhances the interactions leading to micellization. The F108, which is three times larger than F38, is in its monomer only region below 5% and in the monomer-micelle region between 5 and 20%, as reflected by the maximum in the apparent specific heat capacity. This is in good agreement with the fluorescence study by Alexandridis et al.,19 who report a cmc of 4.5% at 25 °C. As in most commercial products, Pluronic F108 is a mixture of molecules having a distribution of molecular weight.4 The large concentration range corresponding to the monomer-micelle equilibrium may be a consequence of the polydispersity of critical micelle concentrations. As mentioned earlier, the volume is less sensitive to micellization than the heat capacity. For instance, the volume at 5% is still at its value in the monomer only region while the apparent specific heat capacity has already significantly increased. P85 and P105 Solutions. The molecule of Pluronic P85 has longer POP and shorter POE units than F38, and its solutions show phase separation above 83 °C. This increased hydrophobicity is also expected to enhance the micellization of P85 with respect to F38. Figure 5 shows the apparent specific volume and heat capacity of P85 up to 10%. At 5 °C and up to 10%, the P85 molecules are in the monomer only region. At 25 °C, the micellization of P85 is reflected by an increase of the apparent specific volume around 10% and the onset of the maximum in the heat capacity around 7%. Additional measurements up to 20% (Figure 6) show that Pluronic P85 is undergoing micellization from 7 to higher than 20%. Mortensen and Pedersen7 used SANS measurements to investigate the structure of P85 in D2O. As shown in (19) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414-2425.


Langmuir, Vol. 12, No. 20, 1996 4701

Figure 6. Concentration dependence of the apparent specific volume and heat capacity of Pluronic P85 in water (9) and D2O (0) and of P105 in water ([) at 25 °C.

Figure 5. Concentration dependence of the apparent specific volume and heat capacity and of the reduced viscosity of Pluronic P85 at 5 (9), 25 ([), and 45 °C (2).

Figure 6, the trend of the apparent specific heat capacity and volume of P85 in D2O at 25 °C is similar to that in water with only a slight shift of the micellization to lower concentrations. Using Mortensen’s temperature dependence of the critical micelle concentration, we calculated a cmc of 7.8% at 25 °C, in good agreement with our value of 7% but slightly higher than the cmc reported by Alexandridis et al.,19 4%, and Brown et al.,20 5%. According to the phase diagram of Mortensen, the monomer-micelle region of P85 extends up to about 25%, also in accordance with our observation. It is known14 that the shape of the maximum in the heat capacity is related to the conversion of monomers into micelles. Assuming that the concentration at the maximum, 15% for P85 at 25 °C, corresponds to half (20) Brown, W.; Schillen, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991, 95, 1850-1858.

conversion of the monomers into micelles and using 0.89 cm3‚g-1 for the specific volume of P85 in the micelles, one obtains a micelle volume fraction of 6.7%, in fair agreement with the value of 7.4% deduced from Mortensen’s data. The critical micelle concentration of nonionic surfactant generally decreases with temperature.14 From 25 to 45 °C, the micellization range of P85 shrank, and it has shifted to concentrations lower than 0.1%. The presence of micelles is supported by the apparent specific volume, which remains significantly higher than its value at infinite dilution even at the lowest concentration studied, i.e. 0.1%. This is in accordance with the very low cmc, 0.014%, reported by Alexandridis et al.19 According to Mortensen and Pedersen,7 all the P85 molecules are in micelles over 32 °C. In fact, our data indicate that the monomer-micelle equilibrium region extends to about 4% P85, as observed from the trailing part of the maximum in the specific heat capacity. At higher concentrations, the apparent specific heat capacity slowly decreases to values only slightly below that of P85 at infinite dilution, suggesting that the apparent specific heat capacity of P85 molecules in the monomeric state is not that different from that of the micellar state. The apparent specific volume and heat capacity of Pluronic P105 are compared to those of P85 at 25 °C in Figure 6. Increasing the size of the Pluronic shifts the micellization to lower concentrations. In that case, the monomer-micelle equilibrium region begins around 0.1% and extends to 15%. This is in fair agreement with the cmc of 0.3% reported by Alexandridis et al.19 at 25 °C. P104 and P103 Solutions. Except for the absolute values, the trends of the apparent specific volume and heat capacity of P104 and P103 are almost the same. They are both in the monomer only region up to 10% at 5 °C. At 25 °C, they both have a monomer-micelle region that begins at low concentrations (≈0.1%) and extends to concentrations higher than 10%. However, the maximum in the apparent specific heat capacity occurs at a lower

4702 Langmuir, Vol. 12, No. 20, 1996


concentration, and it is sharper for the more hydrophobic P103. Once again, our results are in accordance with the cmc reported by Alexandridis et al.19 for P104, 0.3%, and P103, 0.07%. At 45 °C, the two systems are in the micelle only region. L122 and L101 Solutions and Dispersions. These two systems undergo phase separation around 16 and 12 °C at 1 and 10% in water. The measurements reported at 25 and 45 °C were thus obtained with dispersions instead of solutions of Pluronics. Only the L101 at 45 °C was not stable enough to be investigated using flow microcalorimetry. The phase separation in Pluronic solutions does not involve a significant amount of heat,8,21 so that it does not interfere with the interpretation of our (21) Deng, Y.; Yu, G.-E.; Price, C.; Booth, C. J. Chem. Soc., Faraday Trans. 1992, 88, 1441-1446.

Meilleur et al.


results. The apparent specific volume and heat capacity of L122 and L101 at 5, 25, and 45 °C are shown in Figures 9 and 10, and they are concordant with our earlier work8 at 10, 22, and 35 °C. At 5 °C, L122 and L101 are in the monomer only region. At 25 °C, one can still observe the trailing of the maximum in the apparent specific heat capacity corresponding to the monomer-micelle region, suggesting the presence of monomers at low concentrations, probably in the waterrich phase of demixed systems. At 45 °C, the trend of the thermodynamic properties suggests that L122 is in its micelle only region over all the concentration range investigated. Conclusions This article reports the investigation of the concentration dependence of the apparent specific volume and heat capacity of eight Pluronics (F38, F108, P85, P105, P104,


Figure 9. Concentration dependence of the apparent specific volume and heat capacity and of the reduced viscosity of Pluronic L122 at 5 (9), 25 ([), and 45 °C (2).

P103, L122, and L101) in water at 5, 25, and 45 °C. These properties were used with success to probe the structure of Pluronic solutions in light of the three regions, monomer only, monomer-micelle, and micelle only, reported by Mortensen and Pedersen.7 The micellization behavior of Pluronics (F38 and P85) was enhanced by increasing the size of the copolymer (F108 and P105) while keeping the composition the same. The apparent specific heat capacity has proven quite sensitive to the micellization and in particular to the identification of the range of concentration corresponding to the equilibrium between monomers and micelles. For instance, it is suggested that the monomermicelle region of P104, P103, L122, and L101 solutions still exists at low concentrations and 25 °C while other studies report that the micellization is completed. As the

Langmuir, Vol. 12, No. 20, 1996 4703

Figure 10. Concentration dependence of the apparent specific volume and heat capacity and of the reduced viscosity of Pluronic L101 at 5 (9) and 25 °C ([).

Pluronics become bigger and more hydrophobic, the region of equilibrium between monomers and micelles becomes sharper and is shifted to lower concentrations. The intrinsic viscosity and Huggins constant obtained for these systems corroborate the trends deduced from the thermodynamic properties. Acknowledgment. We wish to thank BASF for the Pluronic samples and the Natural Sciences and Engineering Council of Canada for its financial support. We are also grateful to Ge´rald Perron for his technical support throughout this investigation. LA960200H

Probing the Structure of Pluronic PEOâPPOâPEO Block Copolymer

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Probing the Structure of Pluronic PEOâPPOâPEO Block Copolymer