Propylene Oxide Block

J. Phys. Chem. 1995,99, 11981-11988

11981

Fluorescence Probe Studies of Ethylene Oxide/Propylene Oxide Block Copolymers in Aqueous Solution Richard J. Holland, Edward J. Parker,* Kathleen Guiney, and Frank R. Zeld BASF Corporation, 1419 Biddle Avenue, Wyandotte, Michigan’481 92 Received: May 22, 1995@

The aggregation properties of ethylene oxide/propylene oxide triblock polymers in water have been investigated using the fluorescence probe molecule, sodium 2-(N-dodecylamino)naphthalene-6-sulfonate(C12NS). A blue shift in the C I ~ N Semission maximum is observed as a function of block copolymer concentration and is associated with the formation of aggregates in solution. EORO block copolymer aggregation, as measured by the ClzNS probe technique, was found to occur at lower concentration as the molecular weight of the hydrophobe (PO) was increased, as the hydrophilic-lipophilic balance (HLB) was reduced and as the temperature was increased. These results are discussed in light of the currently accepted model for nonionic surfactant aggregation in water: the critical micelle concentration paradigm.

I. Introduction and Literature Review There is a controversy in the literature regarding the aggregation properties of ethylene oxide/propylene oxide (EOPO) block copolymers in water, which has not been resolved to date. Contradictory results were obtained in early measurements of the so-called “critical micelle concentration” (cmc) of these block polymers by surface tension and dye solubilization technique^.'-^ Light-scattering studies6-I8 and fluorescence probe experiment~’~-~l have not clarified EOPO block polymer aggregation. In one of the earliest investigations, Schmolka and Raymond’ measured the absorption of benzopurpurin dye in water at 540 nm as a function of EOPO block copolymer concentration. A break in the benzopurpurin absorption curve was characterized by these authors as the cmc. Values for the cmc obtained by this method were found to be extremely small, in the range 1473 parts per million (ppm), but were corroborated by adwater surface tension experiments carried out by Williams and coworkers. Saski and Shah2 used benzopurpurin absorption, iodine absorption, and du Nuoy ring surface tension measurements to determine the cmc of Pluronic L62, L64, and F68 surfactants. These workers obtained values 2-3 orders of magnitude higher in concentration than those reported by Schmolka et al.: L62 (24 000 ppm); L64 (22 000 ppm), and F68 (1000 ppm.) Anderson3 attempted to reconcile the Schmolka and Saski studies by using surface tension (Wilhelmy plate) and dye absorption (benzopurpurin, iodine) techniques. Surface tension measurements yielded cmc values of 0.3 ppm for L62 and 1.6 ppm for F68, significantly lower than Schmolka and Raymond’s results and 5-6 orders of magnitude lower than those of Saski and Shah. Anderson rejected the benzopurpurin and iodine absorption techniques as unreproducible. Prasad and coworkers4reinvestigated EOPO block copolymer aggregation using the same three techniques as Anderson. Critical concentrations (Cl) were obtained from the inflection points of the surface tension versus concentration curves for L62 (2.4 ppm), L64 (8.3 ppm), and F68 (15 ppm). Prasad et al. state that the CI values do not represent the cmc’s. Instead the authors used the benzopurpurin and iodine absorption techniques to arrive at cmc values (C2) in the percent range @Abstractpublished in Advance ACS Abstrucrs, July 1, 1995.

L62 (24,000 ppm); L64 (22 OW),and F68 (1000 ppm), similar to the cmc results obtained by Saski and Shah. Prasad et al. speculated on the meaning of the two critical concentration values, C1 and C2. They suggested that “monomolecular micelles” form at Cl and that polymolecular micellization occurs at C2. A recent study of EOPO block copolymer aggregation which utilized classical aidwater interfacial tension measurements was performed by Wanka et These workers measured the critical micelle concentrations for Pluronic P104, P123, and F127 surfactants of 30, 15 and 40 ppm, respectively. The value for P104 is fairly close to that obtained by Schmolka and Raymond (44ppm). Two of the measurements were also corroborated by a parallel study of surface tension at the aiddecane interface: P104 (cmc = 45 ppm) and P123 (cmc = 30 ppm). There is no discussion in Wanka et al. of measurements which fix the cmc of EOPO block copolymers 3-5 orders of magnitude higher2,4than the values reported in their paper. In addition to these studies, fluorescent probe molecules have also been used to study the aggregation properties of EOPO block copolymers. Turro and ChungI9 used a number of fluorescent probe molecules to study the aggregation properties of PEO-PPO (0.8:1, the same EOPO ratio and M W as Pluronic L64 surfactant). Introduction of the probe molecule into a more nonpolar environment than water (as would be the case if the probe is situated in the interior of a micelle) results in a blue shift in the maximum (Ama) of the fluorescence spectrum. Studies carried out with pyrenecarboxaldehyde as a function of polymer concentration revealed three regions. For L64 concentrations up to 1000 ppm, the fluorescence emission maximum (Amax) is the same as that observed with water. With L64 concentrations between 1000 ppm and 100 OOO ppm, the A,, blue shifts and at concentrations above 100 000 ppm, A,, blue shifts sharply. Turro and Chung suggested that the transition between the first and second regions marks the onset of “monomolecular” micelle formation and that the transition between the second and third regions is due to the formation of block copolymer aggregates. According to this interpretation, the “cmc” of L64 (the inflection point between regions two and three) is 20 wt % (200 000 ppm). Light-scattering techniques have also been used to investigate EOPO block copolymer micellization. MankowichIo investi-

0022-365419512099-11981$09.00/0 0 1995 American Chemical Society

Holland et al.

11982 J. Phys. Chem., Vol. 99, No. 31, I995 TABLE 1: Structures of EO/PO Block Copolymers Studied Pluronic surfactant total mol wt EO mol wt PO mol wt wt o/o EO L35 P65 P7 5 P85 Pi05 P103 P104 F108

1900 3500 4100 4500 6500 4950 5900 14600

950 1750 2050 2250 3250 1700 2650 11350

950 1750 2050 2250 3250 3250 3250 3250

50 50 50 50 50 30 40 80

obtained from a Kontron Instruments Model S-200 GP Plotter 800. For ClzNS fluorescence spectra, an excitation wavelength of 303 nm was used, and emission was scanned from 550 to 350 nm at a rate of 5 nm/min. Experiments were conducted according to the following procedures. A 53.5 pM stock solution of ClzNS in methanol was prepared. A standard aliquot (40 pL) of C I ~ N Sstock solution was delivered to a cuvette and dried with nitrogen. Stock solutions of EOPO block copolymers in water were prepared and aliquots of these solutions were added to the cuvette, agitated, and allowed to equilibrate in the spectrometer (a total volume of 3 mL was added to the cuvette). Control experiments were also performed with water and methanol solutions of the Cl2NS probe, prepared in the same way. Surface tension measurements were carried out using a home built du Nuoy ring tensiometer. In this method a Sartorius Model 2432 analytical balance was modified by replacing the weighing pan with lead weights and suspending a du Nuoy ring from the weighing pan hook with a wire 5.1 cm in length. Approximately 40 mL of the sample to be tested was placed in a scrupulously clean 150 mL beaker. The beaker is raised and lowered under the ring using a hand-operated bellows driven by a large-capacity syringe. The difference between the dry weight of the ring and the maximum pull on the ring in solution is used in the calculation of the solution surface tension. Care was taken to allow surfactant solutions to reach equilibrium before recording the surface tension value. With distilled water and highly concentrated (> 1%) surfactant solutions, equilibrium was achieved fairly rapidly. However, very dilute surfactant solutions gave surface tension readings that changed over periods of 30 min to 5 h before a constant value was obtained. Control measurements with ultrapure water samples contained in the beaker for extended periods of time ('2.5 h) showed that the surface tension of water, as measured by this technique, remained constant at 71.4 dydcm over the entire storage period. This suggests that airborne impurities did not affect the surface tension measurements performed for extended periods of time,

gated an EOPO block copolymer having molecular weight of 7200-7700 (possibly Pluronic F87 surfactant) and found no evidence of micellization over the concentration range 0.21.0%. More recent studies suggest that aggregates do form, but at higher concentrations and higher temperatures."-'* There has been no attempt to reconcile these data, which suggest that high concentrations of EOPO block copolymer are required to form aggregates, with the extremely low cmc values reported in many of the surface tension and dye solubilization studies. This literature review shows that the micellization properties of EOPO block copolymers in water are poorly understood. Our investigation was, therefore, carried out to determine under what conditions EOPO triblock polymers associate in water. We report here studies of the aggregation properties of EO/ PO block copolymers using the fluorescence probe molecule, sodium 2-(N-dodecylamino)-6-naphthalenesulfonate(CI~NS). These concentration studies were carried out as a function of three key variables; hydrophobe (PPO) molecular weight, hydrophilic-lipophilic balance (HLB), and temperature. Tirrel and co-workersZ2recently studied the interaction of Cl2NS with poly(N-isopropylacryl) (PNIPAAM) in aqueous solution. C I ~ N has S the advantage of being an amphophilic probe, that is, it will self-associate in the presence of polymers when used at high concentrations (-40 pM). For the work reported here we use concentrations of 0.6 pM, where self-association does not occur. The fluorescence emission of Cl2NS blue shifts from -428 nm (in water) to 41 1 nm (in methanol) as the polarity of the solvent is reduced. This effect can be used to monitor the onset of aggregation of amphophilic molecules, since the probe emission will also blue shift (relative to emission of the 111. Results probe in water) if the Cl2NS experiences the nonpolar environment of a micellar core. The theoretical basis of the effects of The fluorescence spectrum of C I ~ N S (0.58 pM) in water is solvent polarity on the fluorescence spectroscopy of C12NS and shown in Figure 1 along with the spectrum for the probe related molecules has been discussed extensively by L i ~ p e r t . ~ ~ molecule in methanol at the same concentration. The emission maximum (Amax) in water was observed at 428 f 1 nm. This 11. Experimental Section is consistent with the fluorescence maximum observed by Schild and Tirre122for C I ~ N (429 S f 0.8 nm). Waggoner and Shyerz4 Commercial samples of EO/PO block copolymers, available have also observed that the emission maximum for 2-(Nfrom BASF Corporation under the registered trademark Pluronic, octadecylamino)naphthalene-6-sulfonate, the CIS analogue of were used without further purification (see Table 1 for the EO/ CIZNS, is in the same region of the spectrum (-427 nm). PO block copolymers used in these experiments). The sodium The fluorescence maximum measured for C12NS in methanol salt of 2-(N-dodecylamino)naphthalene-6-sulfonicacid (CIzNS) (411 nm) is also consistent with that reported by Schild and was obtained from Molecular Probes, Inc., Eugene, OR, and Tirrel.22 The blue shift in the C12NS/methanolspectrum relative used without further purification. Spectrograde methanol (Burto the ClzNS/water spectrum (see Figure 1) is due to the probe dick & Jackson) was used to prepare the Cl2NS stock solutions molecule experiencing a more nonpolar e n v i r ~ n m e n t . ~ ~ - ~ ~ and for studies of C I ~ N emission S in methanol. Ultrapure water having a resistivity greater than lo7 52 was obtained from a Three types of fluorescent probe experiments were conducted. Interlake Continental Deionizer. du Nuoy ring surface tension In the first set of experiments, EOPO block copolymers measurements were carried out on ultra pure water samples containing 50 wt % ethylene oxide were studied to determine using a home built tensiometer and a surface tension value of the effect of increasing the hydrophobe (PPO) molecular weight 7 1.4 dydcm was measured, reproducibly. on polymer aggregation. The second investigation involved Fluorescence measurements were conducted with a Kontron studies of EOPO block copolymers with the same hydrophobe Model SFh4 25 Spectrolfluorimeter equipped with 5 nm molecular weight, but different hydrophilic-lipophilic balance excitation and emission slits, a wavelength accuracy of 1.0 nm (HLB) values. In the final study we focused on the effect of and a thermostated cell holder (the cell temperature could be temperature on polymer aggregation. Each set of experiments fixed to within f 0.1 "C). Output from the spectrometer was is presented separately below.

J. Phys. Chem., Vol. 99, No. 31, 1995 11983

Ethylene OxidePropylene Oxide Block Copolymers INTENSITY (arbitrary-,-.-l--~l--l-~---,-,-~-l-. units)

-l--l-l-t

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Figure 1. Fluorescence spectra of the probe molecule, 2-(N-dodecylamino)naphthalene-6-sulfonicacid, sodium salt, in water and in methanol. 9

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Figure 2. Fluorescence emission maximum for a mixture of (CIZNS) and Plurafac B-25-5 surfactant as a function of B-25-5 concentration at 25 "C.

II1.A. Effect of Hydrophobe Molecular Weight. Before undertaking a study of the aggregation properties of EOPO block copolymers as a function of molecular weight, we carried out studies of a "classical" nonionic surfactant, which is known to form micelles in aqueous solution, the alkoxylated alcohol, Plurafac B-25-5 surfactant (referred to as B-25-5) from BASF. The C I ~ N fluorescence S spectrum was measured as a function of B-25-5 concentration over the range 0.1 - 10 000 ppm (1.O%, see Figure 2). Notice that a fairly sharp blue shift in the emission spectrum (from -427-411 nm) occurs in the concentration range 4-50 ppm consistent with the onset of micellization in the system. For comparison, a surface tension study (du Nuoy ring) was also conducted with B-25-5 (see Figure 3) which shows that the aidwater interfacial tension reaches saturation at 13.5 ppm, a value that can be considered the critical micelle concentration. These results show that both the Cl2NS probe technique and the duNuoy ring tensiometer yield "cmc" values in the same concentration range, though the Cl2NS probe predicts a somewhat higher cmc (-50 ppm). The concentration dependence of ClzNS probe emission in block polymer solutions was found to be very different from that of the "classical" alkoxylated alcohol presented above. Figure 4 shows the Cl2NS probe emission as a function of Pluronic P105 concentration. The fluorescence maximum was found to shift from 426 nm at -20 ppm to 412 nm at 2000 ppm. This spectral shift is presented in Figure 5. This is a

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Figure 3. Surface tension as a function of concentration for Plurafac B-25-5 surfactant at 24 O C . I

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Figure 4. Fluorescence emission maximum for a mixture of (C12NS) and Pluronic P105 surfactant as a function of P105 concentration at 25 "C.

much broader transition region (almost 2 orders of magnitude) than is observed with B-25-5 (see Figure 1). If one decreases the size of the PO block to a molecular weight of 2050, as in Pluronic P75 surfactant, and carries out ClzNS fluorescence experiments, one observes four regions in the Amx versus concentration plot (see Figure 6 ) . There is an initial transition region between 1 and 10 ppm where Amax drops from 428 to 424 nm. The second region covers a broad concentration range (10-lo00 ppm) where Amax is constant at 424-423 nm. Next is a second transitional region (1000-

Holland et al.

11984 J. Phys. Chem., Vol. 99, No. 31, 1995

INTENSITY (arbitrary units)

I

Figure 5. Fluorescence spectra of ClzNS/Pluronic P105 surfactant mixtures at P105 concentrations of 20 ppm (lower curve) and 2000 ppm (upper curve).

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Figure 6. Fluorescence emission maximum for a mixture of (Ct2NS) and Pluronic P75 surfactant as a function of P75 concentration at 25 "C.

50 000 ppm) where A,, shifts from 423 to 41 1 nm, and finally there is the "limiting" concentration ( 1500 '1000

Figure 8 shows plots of I,, for C12NS fluorescence emission versus concentration for F108, P105, P104, and P103. The most significant difference in the concentration dependence of I,,, for Cl2NS is between F108, the most hydrophilic member of the series, and the three other block copolymers. F108 does not reach a limiting aggregation concentration (Imax = 412 nm) until a concentration of 5 wt % is approached. Differences between the other block copolymers are more subtle, but do show that as the HLB is reduced, the Lax versus concentration curve shifts to lower concentration. The data are summarized in Table 3. Both Figure 8 and Table 3 show clearly that a limiting concentration, (the point at which the polarity of the aggregate core sampled by the C12NS probe reaches a limiting value, also shifts to lower concentration as the HLB of the block copolymer is lowered: F108 (LAC = 50 000 ppm); P105 (2 000 ppm); P104 (1500 ppm); and P103 (1000 ppm). II1.C. Effect of Temperature. The dependence of Pluronic P105 block copolymer aggregation was studied over the range 25-55 "C. Plots of the ClzNS emission maxima as a function of P105 concentration are displayed in Figure 9. These curves show three significant trends. First, the limiting concentration drops steeply as the temperature is increased from 25 "C (2000 ppm) to 55 "C (50 ppm). Second, the transition region becomes sharper and narrower as the temperature rises; 50-2000 ppm at 25 "C vs 3-50 ppm at 55 "C. Third, the onset of the transition region occurs at lower concentration as the temperature is increased; 50 ppm at 25 OC vs 3 ppm at 35 "C. These results are summarized in Table 4. IV. Discussion The C12NS fluorescence emission experiments reported here suggest that ethylene oxide/propylene oxide (EOPO) triblock polymer aggregation depends on the molecular weight of the hydrophobe (propylene oxide), the hydrophilic-lipophilic balance (HLB) value and the solution temperature.

100 1000 Concenlratlon (ppm)

10000

100000

Figure 9. Fluorescence emission maximum for a mixture of ( C I ~ N S ) and Pluronic PI05 surfactant as a function of PI05 concentration at four temperatures: (W) 55 "C; (+) 45 "C; (*) 35 "C; (0)25 "C. TABLE 4: Regions in the Emission Versus Concentration plots P105 as a Function of Temperature temp, "C 25 35 45 55

region I 428-424 nm 0.1-50

region I1 424 nm

region 111 424-412 nm

region IV 412 nm

50-2000 3 -400 3-100 3-50

2 2000

> 400

'100 >50

To discuss the effects of these three variables on EOPO block copolymer aggregation, we need to define two terms relating to the C I ~ N versus S concentration plots. The first term describes the broad range in concentration over which the C I ~ N emission S maximum blue shifts from its value in water (428 nm) to its "limiting" value (41 1-412 nm). This region we will label as the aggregation concentration range (ACR). It is likely that block copolymer aggregates begin to form in this region. However, changes in the size, shape, and, certainly, the polarity of these aggregates probably occur as the concentration of block copolymer is increased. The concentration at which the Cl2NS emission maximum reaches the saturation value of 41 1-412 nm we will designate the limiting aggregation concentration (LAC). This wavelength is equivalent to that observed with CIZNSin the nonpolar microenvironment of an alkoxylated alcohol micelle (see Figures 3 and 4) or in a 100% methanol solution (see Figure 1). The polarity of the aggregate interior does not appear to change after the LAC is reached (at least the polarity sampled by the Cl2NS probe). Therefore the LAC is similar to the classical "critical micelle concentration" (cmc) used in the literature to describe the onset of surfactant micellization in aqueous solution. We use the term "limiting aggregation concentration" instead of cmc to stress the point that in the case of EOPO triblock polymers in water, aggregation occurs not at a critical point or even in a critical range but rather over wide ranges of concentration. To analyze the impact of propylene oxide molecular weight on block copolymer aggregation, we will, therefore, focus our discussion on the limiting aggregation concentration (LAC). An increase in the molecular weight of the hydrophobe (PO) clearly lowers the LAC of EOPO block copolymers in water solution. The results in Table 2 show that limiting aggregation concentration values for block copolymers with 50% PO decrease from 50 000 ppm for W5, P85 (PO mol wt = 2050, 2250, respectively) to 2000 ppm for P105 (PO mol wt = 3250). Triblock polymers with lower hydrophobe molecular weights (L35, 950; P65, 1750) do not reach a LAC even at concentrations as high as 10 wt %. If these values are reported on a molar basis the effect is even more significant. The LAC values for W5 and P85 are

11986 J. Phys. Chem., Vol. 99, No. 31, 1995 12.2 and 11.1 pM, respectively. The LAC value for P105 is a factor of 40 smaller: 0.308 pM. According to Tanford?6 the critical micelle concentration of a nonionic amphiphile decreases by a factor of 10 when the alkyl chain length is increased by two carbon atoms. That the limiting aggregation concentration (which is analogous to the cmc) of EOPO triblock polymers should also decrease as the propylene oxide hydrophobe molecular weight increases is not surprising and is consistent with the hydrophobic effect. This conclusion is also supported by theoretical calculations carried out by Per Linse with Pluronic P105 and P95 surfactant^.^^ In addition, the size of the PO block in the EOPO/EO polymer must be sufficiently large for a limiting aggregation concentration to be reached. Our studies suggest that a PO block of 35 units (W5) is needed in a block copolymer containing 50% PO to observe a LAC of 50 000 ppm (5 wt % at 25 "C). To lower the LAC to 2000 ppm (P105, again with 50% PO) the PO block must contain 56 units. The LAC values reported here for EOPO block polymers are still significantly higher than cmc values for alkoxylated alcohols which, of course, require a far shorter alkyl chain as the hydrophobe to form micelles. The only systematic study of EOPO triblock polymers from which cmc values as a function of hydrophobe molecular weight can be obtained is that of Schmolka and Raymond.' They examined seven products in total with two different EO contents. At a fixed EO content of 50%, they obtained CMCs of 9.5 pM for L35, 9.1 pM for P75, and 8.1 pM for P85. When the EO content was decreased to 40%, they obtained the following cmc values: 8.6 pM for W, 5.6 pM for L64, 8.9 pM for P84, and 7.3 pM for P104. No clear trends can be seen in the cmc as the PO molecular weight was increased. These cmc values were determined using the benzopurpurin dye absorption technique. In the Introduction we mentioned Anderson's critique of this methodology and his conclusion that the technique does not yield reproducible re~u1t.s.~What is most disappointing about these measurements (and other benzopurpurin and iodine absorption studies in the literature) is the failure of the investigators to establish a link between dye absorption and aggregate formation. The observation of a break in the benzopurpurin absorption curve is evidence of a change in the system. It is unclear how this change is related to aggregate formation. Studies of the effect of block copolymer HLB on aggregation are also broadly consistent with models of alkoxylated alcohol micellization. It is well-known that increasing the ethylene oxide content (increasing the HLB) of an alkoxylated alcohol raises the cmc.28129 Cox used spinning drop tensiometry to measure the cmc of ethoxylated alcohols having a fixed carbon chain length and observed that a longer EO chain increased the cmc on a weight percent and a molar basis. Our studies show that the limiting aggregation concentration (LAC) in weight percent also increases as the block copolymer becomes more hydrophilic. If the LAC is calculated on a molar basis we observe the same trend: P103 (LAC = 202 pM); P104 (254 pM); P105 (308 pM); F108 (343 pM). Increasing the size of the hydrophilic block(s) increases the number of ethylene oxide/water hydrogen bonding interactions. As Cox has pointed out,28partial incorporation of the EO chain in the micellar interior requires the breaking of water/EO bonds, which increases the energy of the system and thereby raises the cmc. Although we observe an analogous trend with EO/ PO/EO triblock polymers, the limiting aggregation concentration of block polymers could also be affected by the size of the EO

Holland et al. blocks and by the triblock structure. These factors will require further investigation. The final part of our investigation involves the effect of solution temperature on block copolymer aggregation. As temperature is increased these studies show that the limiting aggregation concentration (LAC) of PI05 decreases (see Figure 9 and Table 4). In a recent review Meguro et al. reported a similar trend for the cmc's of ethoxylated aJcoh01s.~~ According to Meguro and c o - w o r k e r ~this , ~ ~ effect can be explained by examining the temperature dependence of one component of the total free energy of micellization (A&): the contribution of the hydrophilic head group to the free energy of micellization (called A&(-W)). A&(-W) is positive, since energy must be put into the system to break ethylene oxide/ water bonds when part of the EO chain is transferred to the micelle. As the temperature of the solution increases, A&( W) decreases (becomes less positive) because the EO chains are more readily dehydrated at higher temperature. The main contributor to the free energy reduction is the entropy term {A&(-W)}, which increases owing to a breakdown of the water structure at higher temperature. Since the total free energy of micellization (A&) is directly proportional to the logarithm of the C ~ C reduction , ~ ~of A&(-W) results in a more negative A& and therefore a lower cmc. Other studies in the literature also suggest that the cmc of EOPO block copolymers decreases as the temperature of the solution rises. Brown et al.I5 used static and dynamic light scattering techniques to investigate the aggregation of Pluronic P85 surfactant in water. At 25 "C, the authors reported an approximate cmc of 5 wt % for P85, a value which matches our experimental results (see Table 2). When the solution temperature was increased to 40 "C, micelles were observed at concentrations as low as 0.3 wt %,22 suggesting a significant drop in the cmc at higher temperature. Rassing and Attwood30 studied the aggregation of Pluronic F127 surfactant in water as a function of temperature by lightscattering and ultrasonic velocity techniques. The authors reported a decrease in the cmc of F127 from 17 500 ppm (1.75 wt %) at 10 "C to 75-80 ppm at 30 "C. Zhou and ChuI3 investigated the aggregation of Poloxamer 184 (L64) in water as a function of temperature using light scattering. In samples containing 2% block polymer no evidence of aggregation was observed at 21 or 25 "C. However, raising the temperature to 42.5 "C and above resulted in block polymer association. The authors also maintained that the room temperature (25 "C) micellization of Poloxamer 184 observed by other investigators" was due to the presence of an impurity fraction in the sample. Further studies by Zhou et a1.,I2 with Poloxamer 188 (F68) in water showed that the aggregation of this block polymer is also temperature dependent. No evidence of association was observed at 21.5 or 30 "C, but above 54.2 "C aggregation occurred. The authors also found that the critical micelle temperature (cmt), the lowest temperature at which Poloxamer 188 micelles form, decreases with increasing concentration. They suggest that dehydration of the EO chain results in a lower cmc at higher temperatures. Zhou and Chu conclude that the aggregation of EOPO block copolymers in water is very different from that of other nonionic surfactants (e.g., alkoxylated alcohols): "As mentioned above, the composition polydispersity could be appreciable even for a copolymer with a narrow distribution of molecular weight. Accordingly, no sharp cmc or cmt has been observed for block polymers. In practice, a certain cmc range with some notable uncertainty could be detected. Second, a large difference is

Ethylene OxidePropylene Oxide Block Copolymers often noted between the cmc values determined by different methods because their sensitivity to unimers and micelles and the nature of the average quantity may be considerably different”. ] Zhou and Chu adopt the hypothesis put forward by Prasad et a1.4 and also supported by Turro et that EOPO block copolymer aggregation proceeds through three ranges as concentration is increased. At low concentration, monomolecular micelles exist. This region is followed by a transition region where changes in micellar structure occur. Finally, at high concentration, intermolecular micelles form. Zhou et al. identified a unique feature of EOPO block copolymer aggregation, the broad cmc range, but they attributed it to block polymer polydispersity. In contrast our research shows that these features are directly related to the size of the PO hydrophobe and its interaction with water. In addition, Zhou et al. uncritically accept the surface tension and dye solubilization data as evidence for “monomolecular micelles”, a conclusion that is highly questionable. Still, Zhou and Chu have shown in the case of both Poloxamer 18413 and 18814that the temperature of the water solution has a significant effect on the aggregation properties of EOPO block copolymers.

V. Conclusion The aggregation properties of ethylene oxidelpropylene oxide triblock polymers in water have been investigated using the fluorescence probe molecule, sodium 2-(N-dodecylamino)naphthalene-6-sulfonate (C I 2NS). In experiments with EOPO block polymers, a blue shift in the ClzNS fluorescence emission maximum relative to that observed in water (428 nm) is observed which indicates that the probe molecule is situated in a less polar environment and suggests that these triblock polymers form aggregates in solution at certain concentrations. This interpretation is supported by parallel Cl2NS probe and surface tension measurements carried out with an alkoxylated alcohol (Plurafac B-25-5 surfactant) and is also consistent with C12NS studies conducted by Schild and T i ~ ~ e l . ~ ~ Studies of the concentration dependence of the CUNS emission maximum in EOPO triblock polymer solutions reveal that the polarity of the microenvironment sampled by the probe changes over a very wide concentration range. This suggests that aggregate formation in EOPO block polymer solutions does not fit the “critical micelle concentration” (cmc) model traditionally used to describe the micellization properties of surfactants. As Mukerjee has pointed out,33334the phrase “critical micelle concentration” is misleading, since the physical property (surface tension, dye solubilization, etc.) which is used to determine the cmc, “changes over a narrow concentration range, rather than a single c~ncentration”.~~ It is our view that the search for a “sharp” cmc in studies of EOPO triblock polymer aggregation studies has led to a confused set of interpretations: cmc values ranging from the part per billion3 to the percent range2-4$’9 have been put forward because a “break” in a plot of some physical property versus concentration has been observed. In contrast, our work shows that there is nothing “critical” about the ClzNS probe emission of EOPO/EO polymers as a function of concentration. This strongly suggests that block copolymer aggregation occurs over a broad concentration range, rather than at a “sharp” cmc as previous investigators have proposed. We have defined two regions in the C12NS A,,, versus concentration plots which describe the process: a transition region which we call the “aggregation concentration range” and a saturation region characterized as the “limiting aggregation concentration”. We have also investigated the effect of three classical variables used in surfactant chemistry on block copolymer aggre-

J. Phys. Chem., Vol. 99, No. 31, 1995 11987 gation: the hydrophobe molecular weight, the hydrophiliclipophilic balance (HLB), and the temperature. We find that EOPO triblock polymers, far from behaving in an anomalous fashion, aggregate in a manner that is consistent with classical predictions of how these variables affect nonionic surfactant association in solution. As the molecular weight of the hydrophobe (PO) is increased for block copolymers with the same EOPO ratio, the limiting aggregation concentration (LAC) decreases significantly in accordance with the hydrophobic effect.26 An increase in the hydrophilic-lipophilic balance value for block copolymers with the same hydrophobe molecular weight was found to increase the LAC, and this trend is also consistent with studies reported in the literature for ethoxylated alcohols.28 Finally, we found that the LAC for EOPO block copolymers decreased with increasing temperature. One would expect that dehydration of the EO chains of the block polymer at higher temperature would reduce the free energy of micellizationz9and thus lower the limiting aggregation concentration. Additional work needs to be carried out to further characterize the aggregation properties of EOPOEO block polymers and to relate this to gel formation at higher concentration^.^^^^^ Efforts should focus on the transition region (the “aggregation concentration range”) to obtain information about the size and growth of aggregates. Investigations of diblock polymer aggregation by the techniques reported here should help to elucidate the impact of molecular architecture and steric factors in solution on EOPO block copolymer association. The studies reported here are, therefore, a first step in our attempt to understand the aggregation properties of a unique class of nonionic surfactants: ethylene oxidelpropylene oxide block copolymers. References and Notes (1) Schmolka, I.; Raymond, A. J. Am. Oil Chem. Soc. 1965,42, 1088. (2) Saski, W.; Shah, S. J. Pharm. Sci. 1965, 54, 71. (3) Anderson, R. Phar. Acta Helv. 1972, 47, 304. (4) Prasad, K.; Luong, T.; Florence, A.; Paris, J.; Vaution, C.; Seiller, M.; Puisieux, F. J. Colloid Interface Sci. 1979, 69, 225. (5) Wanka, G.; Hoffmann, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268, 101. (6) Pandya, K.; Bahadur, P.; Bahadur, A. Tenside 1994, 31, 182. (7) Reddy, N.; Fordham, P.; Attwood, D,; Booth, C. J. Chem. Soc., Faraday Trans. 1990, 86, 1569. (8) Bahadur, P.; Pandya, K. Langmuir 1992, 8, 2666. (9) Linse, P.; Malmsten, M. Macromolecules 1992, 25, 5434. (10) Mankowich, A. J. Phys. Chem. 1954, 58, 1027. (1 1) Al-Saden, A.; Whateley, T.; Florence, A. J. Colloid Interface Sci. 1982, 90,303. (12) Zhou, Z.; Chu, B. J. Colloid Interface Sci. 1988, 126, 171. (13) Zhou, Z.; Chu, B. Macromolecules 1988, 21, 2548. (14) Attwood, D.; Collett, J.; Tait, C. Int. J. Pharm. 1985, 26, 25. (15) Brown, 0. W.; Schillen, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991, 95, 1850. (16) Zhou, Z.; Chu, B. Macromolecules 1994, 27, 2025. (17) Brown, 0. W.; Schillen, K.; Hvidt, S. J. Phys. Chem. 1992, 96, 6038. (18) Almgren, M.; Bahadur, P.; Jansson, M.; Li, P.; Brown, W.; Bahadur, A. J. Colloid Interface Sci. 1992, 151, 157. (19) Turro, N. Chung, C. Macromolecules 1984, 17, 2123. (20) Nakashima, K.; Anzai, T.;Fujimoto, Y. Langmuir 1994, IO, 658. (21) Lianos, P.; Brown, W. J. Phys. Chem. 1992, 96, 6439. (22) Schild, H.; Tirrel, D. Langmuir 1990, 1676. (23) Lippert, E. 2. Elektrochem. 1957, 61 962. (24) Waggoner, A.; Stryer, L. Proc. Natl. Acad. Sci. U.S.A. 1970, 67, 579. (25) BASF Corporation, Performance Chemicals Product Brochure, 1991, 4. (26) Tanford, C. The Hydrophobic Eflect,2nd ed.; John Wiley & Sons: New York, 1980; p 66. (27) Linse, P. J. Phys. Chem. 1993,97, 13896. (28) Cox, M. J. Am. Oil Chem. SOC. 1989, 66, 367. (29) Meguro, K.; Ueno, M.; Esumi, K. In Nonionic Surfacrants Physical Chemistry; Schick, M., Ed.; Marcel Dekker, Inc.: New York, 1987; pp 125-6. ~

11988 J. Phys. Chem., Vol. 99, No. 31, 1995 (30) Rassing, J.; Attwood, D. Int. J. Pharm. 1983, 13, 45. (31) Reference 12, pp 174-175. There is an inconsistency in the text. Zhou and Chu do not report the data shown in Figure 1 correctly in the text, but they do state the trend (that the CMT decreases with increasing Poloxamer 188 concentration) properly. (32) Schild, H.; Tirrel, D. Langmuir 1991, 7, 665. (33) Mukerjee, P. Adv. Colloid Interface Sci. 1967, I , 246.

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