Fluorescence Spectroscopic Investigation To Identify the Micelle to Gel

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J. Phys. Chem. B 2009, 113, 5117–5127

5117

Fluorescence Spectroscopic Investigation To Identify the Micelle to Gel Transition of Aqueous Triblock Copolymer Solutions Sony George,† Manoj Kumbhakar,*,‡ Prabhat Kr. Singh,‡ Rajib Ganguly,§ Sukhendu Nath,‡ and Haridas Pal*,‡ Chemistry Department, UniVersity of Kerala, KariaVattom Campus, ThiruVananthapuram, Kerala 695581, and Radiation & Photochemistry and Chemistry DiVisions, Bhabha Atomic Research Centre, Mumbai 400085, India ReceiVed: NoVember 07, 2008; ReVised Manuscript ReceiVed: February 06, 2009

Steady-state and time-resolved fluorescence anisotropy measurements using probes coumarin 153 (C153) and 4-heptadecylumbelliferon (HUF) have been carried out to understand the micelle to gel transition of an aqueous triblock copolymer P123 ((EO)20-(PO)70-(EO)20) (EO ) ethylene oxide; PO ) propylene oxide) solution. Anisotropy results with a normal fluorescent probe, C153, do not show a characteristic change due to the micelle to gel transition. However, the probe HUF having a long hydrocarbon chain that helps its strong association with the micelle shows an increase in anisotropy above the sol-gel transition point. This difference has been explained as invoking a substantial contribution from the micellar structural fluctuations to the depolarization of HUF as its hydrocarbon chain is embedded in the micellar structure, which is not sensed significantly by the normal probe C153. That the extent of change in anisotropy for HUF upon gelation is not that large is possibly caused by the collective motion of the physically interconnected nodes, as observed from the dynamic light scattering studies, which acts in favor of a relatively faster depolarization in the gel phase. Similar studies in other copolymers, such as P85 ((EO)26-(PO)40-(EO)26) and F127 ((EO)100-(PO)65-(EO)100), further demonstrate the potential of probes latched with hydrocarbon chains in displaying a characteristic change for the micelle to gel transition which otherwise remains obscured for normal fluorescent probes. Introduction In addition to their widespread industrial applications, the ability to display a rich structural polymorphism has resulted in numerous investigations involving triblock copolymer systems.1-11 Triblock copolymers comprised of poly(ethylene oxide) ((EO)x) and poly(propylene oxide) ((PO)y) blocks have the general structural formula (EO)x-(PO)y-(EO)x, where x and y represent the number of EO and PO units, respectively. Above a certain temperature, the (PO)y block turns hydrophobic, and consequently, the triblock copolymers exhibit amphiphilic character. Apart from forming micelles, these triblock copolymer solutions also form a number of lyotropic liquid-crystalline phases with cubic, hexagonal, or lamellar structures at high copolymer concentrations. In general, an isotropic gel phase is formed at 53% micellar volume fraction, either due to hard sphere crystallization11-16 or due to micellar packing and entanglement.17,18 However, due to the substantial degree of hydration in the corona region consisting of the EO units, an aqueous solution of these copolymers can also form such an isotropic gel phase at much lower volume fraction than 53%.11-16 The stability of this gel phase is determined by two melting temperatures, namely, the lower and the upper melting temperatures. Even though considerable efforts have been made in elucidating the structures of different phases of block copolymer solutions, attempts to understand their microenvironments or * To whom correspondence should be addressed. E-mail: manojk@ barc.gov.in, [email protected] (M.K.); [email protected] (H.P.). Fax: 91-22-25505151 /25519613. † University of Kerala. ‡ Radiation & Photochemistry Division, Bhabha Atomic Research Centre. § Chemistry Division, Bhabha Atomic Research Centre.

the local friction offered by these microdomains are only modest. Understanding the local friction experienced by the solute molecules in confined geometries is essential to get a better appreciation of many physicochemical processes that transpire in these systems. Rotational diffusion of organic probe molecules solubilized in these confined systems is one of the convenient methods to investigate the characteristics of the microenvironments. A few such studies have been reported in the literature where different phases of various aqueous triblock copolymers have been examined with the aid of fluorescence anisotropy,19-27 Stokes shift,20 and solvation dynamics and lifetime28-32 measurements. According to the literature, the reorientation time of a neutral probe molecule (e.g., 2,5-dimethyl-1,4-dioxo-3,6diphenylpyrrolo[3,4-c]pyrrole, DMDPP, coumarin 153, C153, and coumarin 102, C102) increases drastically when there is a unimer to micelle transition for different copolymers (e.g., P123, (EO)20-(PO)70-(EO)20, and F88, (EO)109-(PO)41-(EO)109), because the solute molecule experiences more friction in the micellar environment compared to that in a homogeneous unimer solution.19-21 However, no significant increase has been observed in the reorientation time of these neutral probes and even for ionic probes such as coumarin 343 (C343) and rhodamine 110 (Rh110) in concentrated copolymer (P123 and F88) solutions at the micelle to gel transition temperature.20,21,26 These results also indicate that when probe molecules are solubilized in the corona region (i.e., for neutral probes) or at the surface region of the micelle (i.e., for ionic probes), they experience almost identical friction in both the micelle and gel phases, and hence, no drastic change is observed in the reorientation time of the solute on transition from the micelle to the gel phase. Recent excitation wavelength dependent

10.1021/jp809826c CCC: $40.75  2009 American Chemical Society Published on Web 03/24/2009

5118 J. Phys. Chem. B, Vol. 113, No. 15, 2009 CHART 1: Chemical Structures of HUF and C153

rotational dynamic studies of coumarin 480 (C480) in a micelle as well as in the gel phase of P123 also corroborate the above literature findings.28,32 However, an increase in one of the components of the reorientation time for the cationic probe rhodamine 123 (Rh123) in 20% aqueous F127 ((EO)100-(PO)65-(EO)100) solution has been observed by Jeon et al.,22 who ascribed it to the micelle-micelle entanglement above the sol-gel transition temperature. Mali et al.26 showed later that the above observation is actually due to the unimer to micelle transition rather than the sol to gel transition, as initially suggested. In most of the literature reports, investigation of the micelle to gel transition has been attempted by temperature-dependent fluorescence anisotropy experiments where no clear transition could be observed in the anisotropy results.19-22,26 If the increase in the rigidity of the probe microenvironment is not sufficient to be reflected in the temperature-dependent anisotropy results, studying the concentration-dependent micelle to gel formation seems to be a viable alternative. This approach has also been attempted but with very limited success by Mali et al.,26 where the interfacial probe Rh110 was employed at different concentrations of P123. However, the uncertainty of the probe location in these heterogeneous media often impedes the conclusion from the fluorescence experiments to indirectly evaluate the surrounding microenvironment of the probe.33 Preferential solubilization of ionic probes, such as C343, Rh110, or Rh123, on the micellar surface is not dubious, but small partitioning of these probes in the bulk water phase cannot be ruled out. Similarly, neutral probes, such as coumarin 153 (C153), coumarin 102 (C102), etc., may reside either in the core or in the corona region of the micelle or may be distributed in both the locations. In fact, in the literature there have been arguments both in favor and against the solubilization of C153 dye in the core of the copolymer micelles.20,21,30,34 Thus, it is important to explore the micelle to gel transition using a fluorescence probe that has a conspicuous location in the micellar phase, and with this aim the present study has been undertaken. In this work we present a systematic investigation on the micelle to gel transition of triblock copolymer P123 solution using two coumarin probes, one a functionalized dye (with a long alkyl chain), 4-heptadecylumbelliferon (HUF), and the other a normal dye, C153. The structures of these dyes are shown in Chart 1. Structurally, HUF can be envisaged as equivalent to a surfactant molecule with a fluorescent moiety acting as its headgroup. There are several reports in the literature where such a probe has been extensively used to investigate the surface potential and interface polarity of surfactant aggregates from the variations in proton equilibrium.35-39 Following recent literature,30,40-53 it can be assumed that this functionalized probe (HUF) will be an integral part of the supramolecular assemblies when dissolved in low concentration in copolymer micellar solution, where the -C17H35 chain of HUF will dissolve in the core of the copolymer micelle and the polar chromophoric head of HUF will be projected out from the micellar core to the hydrated corona region (cf. Chart 2a).

George et al. CHART 2: (a) Qualitative Picture for the Probe (HUF) Location in the Micellar Phase and (b) Different Rotational Motions of a Probe for the Anisotropy Decay in a Micellea

a Wobbling in a cone (1), translational diffusion on the micellar surface (2), and overall micelle rotation (3).

Following the well-known two-step model,54-59 the fluorescence anisotropy decay of a probe in a micelle is mainly determined by the interplay of three different motions. They are (i) the wobbling motion of the micellized probe in a small cone, (ii) the lateral diffusion of the probe along the curved micellar surface, and (iii) the rotation of the whole micelle (cf. Chart 2b). In general, with normal fluorescence probes, the slow overall rotation of the micelle does not give any significant contribution to the observed anisotropy decay, because the major part of the anisotropy decay is caused by the other two motions (wobbling and lateral diffusion), which are much faster than the whole micelle rotation.19,26,34,45,46 In spite of this general trend, some estimation of reverse micellar sizes from a longer rotational time constant (employing probes with few nanosecond fluorescence lifetimes, e.g., ammonium salt of 1-anilino-8naphthalenesulfonic acid, ANS, and coumarin 500, C500) has been reported in the literature.60,61 It is expected that, due to the surfactant-like structure, HUF will experience an exceedingly large hindrance to its lateral diffusion owing to its greater anchoring to the micelle compared to a normal probe.58,59 Accordingly, the surfactant-like probe with a hydrophobic tail is often known to exhibit a nonzero residual anisotropy value.58,59 We expect that a small contribution from the micellar rotation to fluorescence depolarization with a long time constant may possibly be observed with HUF, though the same may not be possible with normal probe C153. The microenvironments for a probe residing in the corona or interfacial region are reported to be nearly alike in the gel and micellar phases.20,21,26 The rotational motion of a micelle in the gel phase will however be largely hindered in comparison to that in a simple micellar solution. This in effect is predicted to cause a significant change in the fluorescence anisotropy of HUF in the gel phase compared to that in the micellar phase, though such a change may not be expected with the normal C153 probe for which whole micelle rotation does not contribute much in the anisotropy decay.19,34 It is this proposition that motivated us to undertake the present study. Additionally, comparison of anisotropy results of HUF with those of C153 along with extension of this study to other block copolymers, such as P85 ((EO)26-(PO)40-(EO)26) and F127, of different hydrophilic ((EO)x) to hydrophobic ((PO)y) ratios is expected to provide more insight into the micelle to gel transition and give a kind of generalized picture in relation to the applicability of the present propositions and inferences. Motivation of the present work is also to characterize the influence of the surfactant probe on the aggregation behavior of the aqueous copolymer solutions. A qualitative structure of the micelle and gel phases of a copolymer solution is depicted in Chart 3.

Micelle to Gel Transition in Copolymer Solutions CHART 3: Schematic Representation of the Micelle to Gel Transition in Triblock Copolymers

J. Phys. Chem. B, Vol. 113, No. 15, 2009 5119 out by measuring the fluorescence decays for parallel (I|(t)) and perpendicular (I⊥(t)) polarizations with respect to the vertically polarized excitation light. The anisotropy decay function, r(t), was constructed using the following relation:63

r(t) )

Methods The copolymers P123, F127, and P85 were obtained from Aldrich, Sigma, and BASF, respectively, and used without further purification. Laser grade C153 and HUF dyes were obtained from Exciton and Sigma, respectively, and used as received. The nanopure water, having a conductivity of 440 nm) emission is mostly due to the anionic HUF form and the relatively weak emission from the keto tautomeric form of HUF does not complicate our inferences. The fluorescence decays of HUF as measured at 450 nm at pH 7 and 10.5 are shown in Figure 3. Since the pKa of HUF is about 9.5, at neutral pH, even with 373 nm excitation, we predominantly excite the neutral HUF species, which shows a fast decay component of ∼0.8 ns followed by a slow decay component of ∼6.0 ns. The fluorescence decay at higher pH is single exponential with a lifetime of ∼5.6 ns, which is very similar to that of the slow decay component at lower pH. Under

Figure 4. rss vs temperature (T) of C153 (squares) and HUF (circles) in 1% P123 solution. Lines are only a guide to the eyes.

basic conditions (at pH 10.5) the observed fluorescence decay is that of deprotonated HUF. Although its population in the ground state is negligible at pH 7, as proposed from the steadystate results, the deprotonation of HUF in the excited state is quite reasonable, and it leads to a sizable contribution in the fluorescence decay. Accordingly, the fast and slow fluorescence decay components at pH 7 are assigned to the neutral and deprotonated species of HUF, respectively. As the emission band of the neutral species has significant overlap with that of the deprotonated species, we always observe a fast decay component arising from the neutral species even when measured at wavelengths greater than 440 nm (at the emission band of deprotonated HUF). It should be noted that, due to the involvement of the keto tautomeric form of HUF, a growth component in the fluorescence decays at wavelengths much above 500 nm is also expected, as discussed in the work of Cohen et al.70 A detailed study of HUF photophysics is beyond the scope of the present paper. Besides, the weak tautomer emission at longer wavelength does not affect the observations made in the present study, as already mentioned earlier. The overall trend in the fluorescence decays is quite similar irrespective of the P123 concentration used, except that there is a marginal decrease (∼5-10%) in the time constants at higher P123 concentration, as shown in Figure 3. Almost unaltered fluorescence decays were observed for C153 dye in P123 solutions at the different polymer concentrations used. Micellar Aggregation in the Presence of HUF. At this point it is of utmost importance to understand the influence of HUF on the aggregation behavior of P123, as HUF is also a surfactant-type molecule and is expected to participate in the formation of a kind of mixed micellar system.30,40-53 Thus, to investigate the influence of a surfactant probe (i.e., HUF) on the aggregation behavior of P123, we measured the steady-state anisotropy (rSS) of both C153 and HUF as a function of temperature to determine the critical micellar temperature (CMT). Rotational hindrance of a solute in the micellar phase is very high compared to that in the bulk water phase. Therefore, due to micellization above the CMT, a substantial increase in rSS is expected. Figure 4 indicates that the CMT is ∼291 K as measured with the C153 probe, which is reasonably close to 289 K as the CMT reported in the literature for 1% P123 solution.4,8,72 The small deviation could be due to the composition polydispersity of the copolymers, which also varies to some extent even for different batches.73 However, such a difference is not detrimental to draw the inferences in the present study, as our aim is to compare the relative CMT values estimated

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Figure 5. Fluorescence anisotropy decay of HUF and C153 in 5% P123 solution at ambient room temperature (∼298 K).

using normal probe C153 and surfactant probe HUF. As shown in Figure 4, the micellization starts (CMT) at around 286 K in the presence of surfactant probe HUF, which is ∼5 K lower than that estimated with C153. Since HUF is expected to form mixed micellar assemblies when dissolved in a low concentration in copolymer micellar solution, the present results indicate that the interaction between the surfactant probe HUF and P123 favors the micellar aggregation in the solution. Due to synergistic interaction, many cosurfactants are in fact reported to cause a decrease in the critical micellar concentration of copolymers, which results in the formation of mixed copolymer-surfactant micelles. Similar lowering of CMT by ∼3 K for F127 in the presence of SDS or TTAB (tetradecyltrimethylammonium bromide) as cosurfactant is also reported in the literature.74,75 In the present context, however, it is also important to know the extent of changes in the fluorescence lifetimes (τ0) of the probes for the temperature range studied, because a change in the τ0 values can also modulate the rSS values. This is because the rSS value is inversely related to the fluorescence lifetimes (τ0) of the probe, and the latter for many organic dyes is often seen to change quite significantly by changing the experimental temperature.63,71 For the present probes, C153 and HUF, however, it is observed that the changes in the τ0 values are quite insignificant for the studied temperature range. Consequently, our inferences based on the observed ∼65% increase in the rSS values do not seem to have any influence from the negligible changes in the τ0 values. Reorientation Dynamics of C153 vs HUF. With the preliminary information discussed so far on the photophysical properties of HUF in P123 micellar solution, we now intend to carry out a systematic study on time-resolved fluorescence anisotropy of the HUF probe and compare these results with those of C153 dye. Figure 5 shows the fluorescence anisotropy decays (r(t)) for both the probes in 5% P123 micellar solution. It is clearly indicated from Figure 5 that the reorientational motion of HUF is largely retarded in P123 micelles compared to that of C153. This is very much in accordance with the observation that the rSS value is also larger for HUF than for C153 in P123 micelles, as shown in Figure 4. The average rotational correlation time for C153 in 5% P123 solution is 2.04 ns [0.40 ns (19.3%) and 2.43 ns (80.7%)], which is quite less than the average rotational correlation time of 2.61 ns [0.64 (8.4%) and 2.79 (91.6%)] for HUF. The time constants for C153 dye are quite similar to those reported by Sen et al.32 with C480 as the probe in the micellar phase of the P123 solution. To be mentioned, unlike normal probe C153, HUF exhibits some residual anisotropy, though quite small (rR ≈ 0.04). Residual

George et al. anisotropy for surfactant probes in other organized assemblies such as lipid membranes, vesicles, etc. is well reported in the literature.58,59,63 The anisotropy decay time constants [0.60 (9.3%) and 2.85 (90.7%)] measured at higher pH (∼10.5) for HUF in P123 solution, where the lifetime decay of HUF is clearly single exponential, matches well with that observed at neutral pH. This result indicates that the deprotonation process does not have any effect on the observed anisotropy. A detailed account of the anisotropy results along with the suitable models to describe various rotational motions of the normal as well as surfactant probes in micelles has been reported by the groups of Fayer58 and Periasamy.59 The observed fast and slow rotational times for normal probes were assigned primarily to the wobbling in a cone and the translational diffusion along the micellar surface, respectively. The micellar rotation time of P123, which is expected to be at least a few tens of nanoseconds,34 can have a very negligible contribution to the depolarization of a normal probe. In the case of a functionalized probe with a hydrocarbon tail (such as HUF, octadecylrhodamine B, etc.), following the literature reports,58,59 it is quite logical to assume that the translational diffusion along the micellar surface should be extremely hindered even though it may not be completely stopped. The slower anisotropy decay and the presence of small residual anisotropy for HUF in comparison to C153 in P123 micellar solution possibly support the above proposition. As a consequence of the sluggish translation motion, a small contribution from the overall micellar rotation with long time constants in r(t) is anticipated. Following the reports of Mitra et al., a long rotational correlation time of about 40 ns is observed for C500 and ANS and is attributed to the overall rotational motion of AOT reverse micelles.60,61 Moreover, the estimated rotational time constant for the P123 micelle is more than 100 ns at 298 K.19,34 These rotational times are substantially higher than even the slower rotational time constants of a few nanoseconds observed for the present systems. Hence, it seems not to be logical to attribute this slow reorientation time constant observed in the present cases to the rotational motion of the whole micelle. Possibly, it is rather due to the combined effect of several other depolarizing motions (as discussed later). This is anticipated because the micellar rotational motion in the physically rigid gel phase will be further restricted compared to that in the micellar phase, as the individual micelles will be tethered in an ordered structure in the gel phase. Therefore, it is beyond expectation for micellar rotation in the gel phase to contribute to the observed anisotropy decay, even if it might have a small contribution to depolarization in the micellar phase. Thus, the anisotropy decay of HUF in P123 solution seems not to be that straightforward to rationalize with the existing models. Micelle to Gel Transition of Aqueous P123 Solution. To comprehend different queries and also to verify whether micelle to gel transition can be sensed by the anisotropy measurements, we have carried out steady-state and time-resolved anisotropy measurements at varying P123 concentrations using both the probes. The rSS value for C153 effectively remains almost in a similar range (increases only marginally) in the P123 concentration region of 10-32% (w/v), without showing any break to indicate the micelle to gel transition (see Figure 6), although thistransitionisreportedtooccuraround28%P123concentration.4,8,43,72 No sharp change in the anisotropy values around the transition point is in accordance with the anisotropy results of other normal fluorescent probes reported in the literature.20-22,26 With HUF, the rSS values also apparently remain in a similar range (increase only marginally) for the micellar region but indicate a sudden

Micelle to Gel Transition in Copolymer Solutions

Figure 6. Steady-state fluorescence anisotropy of HUF (solid squares) and C153 (open squares) at different concentrations of aqueous P123 solution at ambient room temperature (∼298 K). Lines are only a guide to the eyes.

Figure 7. Steady-state fluorescence anisotropy (rSS) of HUF (solid squares) and C153 (open squares) in 30% P123 solution as a function of temperature (T). Lines are only a guide to the eyes. Inset: rSS vs T for HUF in 15% P123 solution, where the micelle to gel transition is not expected. This plot is in sharp contrast to that obtained with 30% P123 solution, where the micelle to gel transition is reported to occur at ∼290 K.

increase for solutions with >25% P123 concentration, indicating the changeover from the micelle to the gel phase. The distinct change in the observed rSS values for HUF around 27-28% P123 is thus attributed to the micelle to gel transition, which is very similar to the reported gelation concentration of P123. The absence of such a break in rSS with C153 indicates that the surfactant probes are only suitable to sense the gelation process, not the normal fluorescent probes. Though the exact reason for the observed change in the rSS values for the surfactant probe at the micelle to gel transition is not very clear yet, in the gel phase probably due to the micelle-micelle entanglement, HUF experiences a larger rotational restriction, leading to an increase in the rSS values. Steady-state anisotropy experiments were further extended to understand the temperature-dependent gelation of 30% P123. Figure 7 plots the rSS values of HUF and C153 as a function of temperature. Due to the inverse relation between temperature and viscosity, a gradual decrease in rSS is expected with an increase in temperature.21,26,63,76 It is obvious from Figure 7 that HUF shows an initial decrease in rSS with temperature similar to that with C153. However, the results with HUF show a kind of variation in the trend around 289 K, the gelation temperature reported for 30% aqueous P123 solution.4,8,43,72 Above the

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Figure 8. Normalized anisotropy decays of HUF (a) and C153 (b) in 25% (black) and 30% (red) aqueous P123 solutions. (c) Average rotational correlation times indicating larger rotational hindrance above 25% w/v P123 concentration for HUF (solid squares) unlike C153 (open squares). Lines are only a guide to the eyes.

gelation temperature, the rotational hindrance experienced by HUF is markedly greater than that of C153, although they are quite similar below the gelation temperature. Therefore, temperature-dependent studies are also in accordance with the observed fact with concentration-dependent studies that the micelle to gel transition can be suitably indicated by the anisotropy measurements of a surfactant probe such as HUF. Time-resolved anisotropy measurements were carried out to further substantiate the observed changes in the steady-state anisotropy values due to gelation above 28% P123 concentration. Anisotropy decays of HUF and C153 in the micellar and gel phases are shown in Figure 8. As expected, the rotational correlation times were observed to be very similar in both the phases for C153 with an average rotational time (〈τr〉) around 2.60 ns (see Figure 8b). In contrast, though not very significant, a definite increase (of ∼18%) in the rotational hindrance for the gel phase compared to the micellar one is reflected for HUF. The average anisotropy values in the micellar (25% P123) and gel (30% P123) phases are 3.03 ns [1.07(15.7%) and 3.39 (84.3%)] and 3.52 ns [0.93(14.7%) and 3.97 (85.3%)], respectively. A close look at the time constant with a major contribution also indicates an increase in reorientational restriction upon gelation. An almost similar increase in the average rotational time from 2.91 ns (micelle) to 3.58 ns (gel) for HUF at pH ≈ 10.5 was also observed. However, the variation in the rR values (0.04 ( 0.02), which is quite low as such, was observed to be nonsystematic. The differences between the results with HUF and C153, where both the chromophores are of similar polarity (the groundand excited-state dipole moments are around 5.7 and 8.1 D for the HUF chromophore and around 5.5 and 9.5 D for C153, respectively77-79) and thus are expected to be located in similar corona regions,30,34-39 is probably related to their dissimilarity in the rotational motions that can cause the anisotropy decay. As discussed earlier and also following the literature reports, upon gelation the wobbling and the translational motions which govern the rotational dynamics of a normal probe (i.e., C153) in the corona region do not change significantly. The observed small increase in rotational hindrance for HUF upon gelation hints toward the contribution of additional depolarizing motions other than the wobbling and translational motions, which are sufficiently restricted in the gel phase. One of the probable candidates for this additional depolarizing motion is the slow

5124 J. Phys. Chem. B, Vol. 113, No. 15, 2009 rotational motion of the micelle. However, in the absence of a substantial increase in rR upon gelation there is doubt about its likely role in the observed effect during the micelle to gel transition. If micellar rotation had any participation in the anisotropy decay of HUF, a large increase in rSS and rR upon gelation was also expected, as micellar rotation was expected to be effectively frozen in the gel phase. Further, the decay rate constant corresponding to the micellar rotation could be much higher than the few nanosecond decay times observed. Thus, the observed small increase in the average rotational time constant for HUF upon gelation essentially echoes against the involvement of micellar rotation in the depicted micelle to gel transition. The other likely candidate for the additional depolarizing motion is the structural fluctuation of the micellar aggregates themselves, which is often called the tumbling motion. Such structural fluctuation is expected to be much faster than the micellar rotation and can be responsible for significant fluorescence depolarization. Recent NMR studies of biosurfactant surfactin in SDS and DPC (dodecylphosphocholine) micelles by Tsan et al. indicate a correlation time constant of 10.9 ns for the overall tumbling of the micelles at 288 K, which decreases to 6.4 ns at 308 K.80,81 These values are similar to those reported for other biosurfactant systems (such as 22residue motilin,82 30 N-terminal residues of HIV-I gp41,83 etc.) in SDS micelles. These tumbling time constants are in the range of the slow rotational relaxation time constant of a few nanoseconds observed in the present cases. Drawing an analogy from these results, we thus expect that the structural fluctuation of the micellar aggregates actually participates in the overall anisotropy decay of HUF. Following this proposition, an increase in the average rotational correlation time upon gelation is quite obvious, as in the gel phase the micellar structural fluctuations will be significantly retarded compared to those of the free micelle due to entanglement and packing of micellar units.17 This hypothesis is in accordance with the similar rR and increased 〈τr〉 values for HUF in the gel phase compared to those in the micellar phase. The absence of this additional depolarizing motion for C153 is perhaps related to the substantial contribution of the translational motion of the normal probe to the observed anisotropy decay, which is just negligible in the case of surfactant probe HUF. Besides, the reorientational dynamics of the EO block itself (which has a time constant of around a few nanoseconds84,85) can also contribute to the anisotropy decay. However, this motion alone cannot explain the observed increase in restriction upon gelation above the critical gelation concentration (cgc) or the critical gelation temperature (cgt) for HUF in comparison to C153. The gradual increase in 〈τr〉 of HUF from 5% to 25% P123 concentration (cf. Figure 8c) could be due to a sphere-to-rod shape transition of either the micelle or the micelle-micelle entanglement. Increased packing in the corona region due to the sphere-to-rod shape transition in the present case is ruled out following the report of Ganguly et al., which reveals a uniform micellar size and shape of P123 micelles in the above concentration range.43 However, micelle-micelle entanglement is reported at high copolymer concentrations.17 As HUF is proposed to experience additional tumbling motion of the micellar aggregates, the increase in the micelle-micelle entanglement with copolymer concentration can be responsible for the observed increase in the average reorientation time. It should be mentioned that a similar increase in reorientation time for the micellar surface probe Rh110 has also been observed by Mali et al.26 Additionally, an increase in shear viscosity due

George et al.

Figure 9. Correlation function vs time plots at different P123 concentrations recorded at a 90° scattering angle at 303 K. The solid lines represent the fitting curves to the data.

Figure 10. Diffusion coefficient of the faster moving species vs P123 concentration obtained at 303 K. Lines are only a guide to the eyes.

to such micellar entanglement with an increase in copolymer concentration has also been reported by Ganguly et al.43,86,87 Besides micelle-micelle entanglement, an increased reorientation time could also be due to the crowding of EO blocks in the corona region as a result of an increased micellar aggregation number with P123 concentration, although in the present systems the micellar size remains almost similar.43 Such an increase in the aggregation number with P123 concentration has been demonstrated by the double-electron resonance and neutron scattering experiments by Ruthstein et al.88 and Soni et al.,89 respectively. Unlike HUF where micelle-micelle entanglement as well as reduced mobility due to increased aggregation number is responsible for the increase in 〈τr〉 even in the micellar phase from 2.61 to 3.03 ns, it is only the latter effect that is expected to cause the observed increase in 〈τr〉 for C153 (2.04 and 2.63 ns at 5% and 32% P123 concentration) with P123 concentration (cf. Figure 8c). It should be noted that the observed small increase in rSS with an increase in copolymer concentration, shown in Figure 6, is also related to these factors. Further, the absence of a very sharp change for the micelle to gel transition in both concentration- and temperature-dependent rSS studies could be due to the microscopic gelation that starts initially and then extends throughout the solution with an

Micelle to Gel Transition in Copolymer Solutions

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Figure 11. Steady-state fluorescence anisotropy of HUF as a function of the P85 concentration at ambient temperature (a). rSS values of HUF at different temperatures in 30% P85 solution (b). Lines are only a guide to the eyes.

Figure 12. Steady-state fluorescence anisotropy of HUF as a function of the F127 concentration at ambient temperature (a). rSS values of HUF at different temperatures in 25% F127 solution (b). Lines are only a guide to the eyes.

increase in the copolymer concentration or the temperature of the solution, on approaching the cgc or cgt. Structural Dynamics of P123 Aggregates by DLS. At this point it is worth exploring the structural dynamics of copolymer aggregates by DLS studies as a function of the copolymer concentration. The intensity correlation function vs time plots from the DLS measurements performed at 303 K and a 90° angle as a function of the P123 concentration are shown in Figure 9. At low P123 concentrations the presence of micelles as the only scattering species is reflected from the singleexponential decay of the correlation function. The nature of the decay becomes biexponential at higher copolymer concentration due to the presence of the bridged network like micellar clusters, as observed in the other block copolymer systems.90-93 This change in the nature of the decay of the correlation function is evident from the decay plots for the 5% and 25% P123 solutions shown in Figure 9. Such a bridged network like micellar cluster at higher copolymer concentrations further substantiates the existence of the micelle-micelle entanglement, as proposed earlier, and corroborates well with increased rotational time. In 30% solutions, where a thermoreversible gel is formed, the correlation function undergoes a further change with a significant decrease in the fraction of the slower moving micellar clusters. To understand the concentration dependence of the correlation function plots further, we have calculated the diffusion coefficient of the scattering species by analyzing the plots on the basis of single-exponential and biexponential equations at low and high copolymer concentrations, respectively. In Figure 10 we have shown the variation of the diffusion coefficient of the faster moving species with the copolymer volume fraction, φ.

The dry copolymer volume fraction value was calculated using the specific volume of the copolymer as 0.93.94 Below the gelation concentration the species can be considered as diffusive micelles. In the gel, in the absence of any micellar diffusion, it must be associated with some internal motion within the system. The diffusion coefficient increases steadily with the copolymer concentration up to 25% but shows a significantly faster rate of increase at the gelation concentration. Below the gelation concentration, the variation in the diffusion coefficient can be explained on the basis of the hard sphere model with repulsive intermicellar interaction.10 The sudden increase in the diffusion coefficient of the faster moving species associated with gelation has also been observed in other block copolymer micellar systems.93 In view of the improbable diffusive motion of the micelle in the gel phase, the observation of the correlation function of the faster moving species was attributed to the collective motion of the physically interconnected nodes (“gel mode”).93 The nonlinear nature of Γ vs q2 plots also suggests that the scattering species are not diffusive in nature in the gel phase. In the solution phase, however, Γ was found to vary linearly with q2, because of the presence of the diffusive micelles as the scattering species. These DLS results thus highlight the change in the dynamics of the aggregates present in the aqueous P123 solutions as a function of the P123 concentration up to the gelation concentration. It should be noted that both motions, the tumbling motion of individual micelles and the overall collective gel mode of the interconnected micelles in the gel phase, can lead to fluorescence depolarization. Following these results on the structural dynamics of dilute and moderately concentrated copolymer solutions, it is expected

5126 J. Phys. Chem. B, Vol. 113, No. 15, 2009 that the collective motion of the micelles in the gel phase of P123 will assist the depolarization process for HUF. In the present system we propose that the additional depolarizing motion in the gel phase is not a true translational, tumbling, or collective motion of the nodes; rather it is a combined complex motion as a whole of the otherwise physically rigid interconnected micelles in the gel phase. It should be further noted that the gel mode basically acts against the expected influence of a substantial decrease in the rotational mobility on the depolarization of HUF in the gel phase. In essence we thus observe only a small increase rather than a large increase in the steadystate and time-resolved anisotropy of HUF. Micelle to Gel Transition of Aqueous P85 and F127 Solutions. The general observation of increased rotational rigidity upon gelation, the main aim of the present investigation, is unambiguous in both steady-state and time-resolved anisotropy studies with the HUF probe. Therefore, for qualitative examination of the sol to gel transition of copolymer solutions, simple steady-state anisotropy measurements are equipped enough to provide reasonable insight. To ensure the applicability of anisotropy measurements of HUF to indicate the gelation behavior of copolymer solutions, we have extended our steadystate anisotropy experiments to other triblock copolymers with different EO to PO block length ratios. The EO/PO ratios for P85 and F127 are 1.3 and 3.1, respectively, whereas it is only 0.6 for P123. The results of a systematic steady-state investigation employing HUF in P85 and F127 are shown in Figures 11 and 12, respectively. The required concentration of triblock copolymer for the formation of the gel phase generally decreases with an increase in the EO/PO ratio.4 The cgc values at room temperature for P85 and F127 are reported to be about 25% and 18% (w/v), respectively.4,43,72,95 A comprehensive change in the rSS values due to gelation above 24% (w/v) P85 and 17% (w/v) F127 aqueous solution is indicated in Figures 11a and 12a, respectively, and these are in good correspondence with the reported values. Observed changes in the present systems can also be visualized with a logic similar to that argued with P123. Thus, the results with P85 and F127 solutions further ascertain our proposition of using HUF as a suitable indicator to characterize the micelle to gel transition. In addition to that, temperature-dependent rSS studies, as shown in Figures 11b and 12b, also confirm increased rotational restriction above 299 and 295 K for 30% (w/v) P85 and 25% (w/v) F127 solutions, respectively. These values closely match with the cgt values reported for these triblock copolymers.4,8,43,72,95 Therefore, assimilation of all these results establishes the suitability of surfactant probe HUF over the commonly used fluorescence probes to realize the micelle to gel transition process. Conclusions A fluorescence spectroscopic investigation has been carried out to realize the micelle to gel transition of aqueous triblock copolymer P123. It is observed that with proper selection of the probe it is possible to clearly realize the micelle to gel transition of block copolymer solutions. Anisotropy results with a normal fluorescent probe (C153) which resides in the corona region of the micelle, however, do not exhibit any significant change due to the micelle to gel transition above the cgc or cgt. The special coumarin probe having a long hydrocarbon chain (HUF) that anchors the probe strongly with the copolymer micelles clearly shows a small but definite increase in the anisotropy above the cgc or cgt. On the basis of the possible rotational motions responsible for the anisotropy decay in micellar media, we propose that this increase in anisotropy upon

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