Poly (propylene oxide)-Poly (ethylene oxide) Block Copolymer in

1 May 1994 - O M NaSCN. The influence of agarose on the clouding behavior and the diffusion of PE6400 as well as the effect of PE6400 on the gelation ...
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5508

J . Phys. Chem. 1994, 98, 5508-5513

Clouding and Diffusion of a Poly(ethy1ene oxide)-Poly (propylene oxide)-Poly (ethylene oxide) Block Copolymer in Agarose Gels and Solutions M. H. G. M. Penders? S. Nilsson,* L. Piculell,' and B. Lindman Physical Chemistry 1 , Chemical Center, University of Lund, Box 124, S-221 00 Lund. Sweden Received: December 10, 1993; In Final Form: March 16, 1994"

Light transmittance and N M R self-diffusion measurements were carried out on a poly(ethy1ene oxide)-poly(propylene oxide)-poly(ethy1ene oxide) block copolymer, PE6400 ( E O I ~ P O ~ ~ EinO gels ~ ~ )and , solutions of agarose in 1.O M N a S C N . The influence of agarose on the clouding behavior and the diffusion of PE6400 as well as the effect of PE6400 on the gelation behavior of agarose were studied. In the presence of agarose coils the cloud-point of PE6400 on cooling is decreased due to the incompatibility of the agarose/PE6400 couple. The cloud-point of PE6400 on heating in agarose gels is situated in the vicinity of the cloud-point of PE6400 in the absence of agarose, indicating a much less repulsive interaction between agarose in the gel state and PE6400. On cooling the mixtures, a large decrease in transmittance was observed a t a temperature above the gelation temperature of agarose. This temperature was identified as the critical micellization temperature (cmt) of PE6400. The large decrease in transmittance a t the cmt was caused by the formation of aggregates due to the strong incompatibility between agarose coils and hydrophobic impurities present in the PE6400 system. Above the cmt these impurities were solubilized in the PE6400 micelles. The observed self-diffusion of PE6400 in agarose/ 1.O M NaSCN systems is faster in the gel (more open structure) than in the solution state. The self-dissusion of the block copolymer in the presence of agarose is lowered compared to the agarosefree case due to the obstruction effect. The presence of 1.0 M NaSCN impedes the formation and growth of the PE6400 micelles on increasing temperature compared to salt-free solutions of PE6400 in D20.

Introduction In previous work' we reported the study on mixtures of nonionic clouding micellar systems of hexaethylene (C12E6) and octaethylene (C12EB) glycol mono (n-dodecyl) ethers in gels and solutions of agarose, a non-ionic gelling polysaccharide, without and with sodium thiocyanate (NaSCN). The large thermal hysteresis in the gelation behavior of agarose, presumably caused by theaggregation of agarose helices in thege1,24 makes it possible to compare mixtures in the solution state with mixtures in the gel state at the same temperature. Light transmittance and N M R self-diffusion measurements have proven to be very useful to study polymer-surfactant interactions and transport properties of CI2E6 and C12Es in agarose gels and solutions.' In the present paper wediscuss the results of light transmittance and Fourier transfer (FT) pulsed field gradient spin-echo (PGSE) IH N M R self-diffusion measurements vs temperature carried out on a poly [ethylene oxide]-poly [propylene oxide]-poly[propylene oxide] (PEO-PPO-PEO) block copolymer, PE6400 (E013P030E013),in gels and solutions of agarose in 1.0 M NaSCN. Triblock copolymers of the PEO-PPO-PEO type, commercially known as Pluronics, are important non-ionic surfaceactive agents and have received increasing attention during the past few year~.~-22They can form micelles in dilute aqueous solutions, provided that the ratio between the more hydrophobic PPO blocks and the more hydrophilic PEO blocks is suitable and that the temperature and concentration is sufficiently high. Furthermore, at elevated temperatures some Pluronics (e.g., PE6400), like other non-ionic surfactants that contain ethylene oxide segments, display clouding behavior due to the decreased aqueous solubility of the EO groups. The temperature at which the clouding takes place (cloud-point), depends primarily on the composition and the molecular weight but is also affected by the f Present address: Max Planck Institut fur Biophysikalische Chemie, Postfach 2841, 37018 Gattingen, Germany. Present address: Rogaland Research, Prof. Olav HanssensvPg 15, Box 2503 Ullandhaug, 4004 Stavanger, Norway. 0 Abstract published in Advance ACS Absfracrs, May 1, 1994.

polymer concentration and the presence of cosolutes, e.g., salts14J6,20and surfactants.12 Compared to the regular non-ionic surfactants investigated previously,' Pluronics posses a higher molecular weight and display a strongly temperature-dependent micellization. At lower temperatures the monomer state prevails, whereas micelles are formed at higher temperatures. The study of Pluronics in combination with gelling polymers is important in view of potential applications (e.g., pharmaceutical formulations). Due to the combination of a gelling polymer and a clouding block copolymer with micellar properties, it is possible to probe both the influence of agarose (gels and solutions) on the clouding behavior of PE6400 and the effect of PE6400 (micelles/and or monomers) on the gelation behavior of agarose. As in our previous study,' we have here used added NaSCN, which increases the solubility of Pluronics16,20and impedes the formation of agarose gels,4,23to increase the miscibility of the agarose/Pluronics couple in water. The strongly temperature dependent micellization behavior of PE6400 enabled us to study the interaction of agarose with PE6400 monomers at lower temperatures and with PE6400 micelles at higher temperatures. With the FT-PGSE lH N M R technique the self-diffusion of PE6400 in agarose gels and solutions vs temperature was measured, which also provides information about the polymer network.

Experimental Section Materials. Agarose (type VIII, for isoelectric focusing, No. A-4905) was obtained from Sigma (St. Louis, MO) and used without further purification. NaSCN was of analytical grade. The agarose solutions were prepared by dissolving agarose in the appropriate solvent (water or salt solution) in sealed glass tubes, which were heated in boiling water with occasional shaking. Block copolymers PE6200 (molecular weight 2500, 20 wt % PEO), PE6400 (molecular weight 2900, 40 wt % PEO) and PE6800 (molecular weight 8600,80 wt % PEO), also known as Pluronics, were purchased from BASF Aktiengesellschaft. PEG 6000 and PEG 20000 (poly(ethy1ene glycol)) were obtained from

0022-3654/94/2098-5508$04.50/0 0 1994 American Chemical Society

Clouding and Diffusion of a Block Copolymer

1"he Journal of Physical Chemistry, Vol. 98, No. 21, 1994

TABLE 1: Influence of NaSCN (1.0 M) on the Celation of Agarose (1 wt %) and on Clouding of PPO and Some Pluronics (1 wt %). Symbols Explained in the Text system T,JoC T,,,J°C TcJ'C 41.0 76.8 agarose in water in 1.O M NaSCN 20.5 65.3 29.7 PE6200 in water in 1.0 M NaSCN 35.4 55-57 in water PE6400 70-7 1 in 1.0 M NaSCN in water >loo PE6800 in 1.0 M NaSCN >loo 52.1 PP0400 (20 wt 5%) in water in 1.0 M NaSCN 68.3 Serva (Germany) and PPO 400 from Fluka (Germany). All polymers were used without further purification. For the preparation of the samples, Millipore water was used for the light transmittance and optical rotation measurements and D 2 0 (99.8% purity, supplied by Merck or Dr. Glaser AG Basel) for the N M R self-diffusion studies. All solutions were prepared by weight. Methods. Light-transmittance measurements were carried out with a 5-cm path length cell in a Hitachi Perkin-Elmer (Model 124) double-beam spectrophotometer and optical rotation measurements at 435 nm were performed in a Jasco DIP-360 polarimeter. The temperature was controlled by the circulation of thermostatically regulated water through the jacketed cell. The transmittance (in %T) and optical rotation of the samples were recorded versus temperature on cooling and heating with a rate of 0.3 OC/min. From the transmittancevs temperaturecurves the cloud-points (T,) of the block copolymer on heating and cooling as well as the gel and melting temperatures of agarose were determined. The onset of a sharp decrease in transmittance due to the clouding of the blockcopolymer was taken as thecloud-point. The change in transmittance related to the gelation or liquefaction of agarose was usually more gradual. The gelation temperatures, Tg,were determined from the intersection of straight lines extrapolated from the higher temperature part ("constant" high transmittance) and the lower temperature part (large decrease in transmittance). A similar extrapolation procedure was used for the determination of the melting point, T,, of agarose gels. 'H N M R self-diffusion measurements were performed on a JEOL FX-60 spectrometer, operating at 60 MHz, using the FTPGSE technique, as described in more detail by S t i l b ~ With .~~ this technique one uses a 90'-7-1 80°-7-echo pulse sequence, with two added rectangular magnetic field gradient pulses of magnitude C,separation time A and duration time 6. The echo amplitude at time 27 is given byZS

4 2 7 ) = A(0) exp[--27/T2 - y2G2Db2(A- 6/3)]

(1)

where T2 is the transverse relaxation time, and y the magnetogyric ratio for the proton. The self-diffusion coefficients D were determined by measuring the echo amplitude A as a function of 6 keeping G and A fixed. For all the experiments A = 140 ms and C = 16.7 mT/m or 40.0 mT/m, depending on the magnitude of the diffusion coefficient. The temperature control during the experiments was within 0.5 O C .

Results and Discussion Light Transmittance. For PE6400 (1 wt %) in water the cloudpoint lies at about 55-57 OC (see Table 1). In the presence of agarose (1 wt %) the cloud-point on cooling is lowered to 48-49 OC. The agarose (1 wt %)/PE6400 (1 wt %) sample in water is difficult to handle, since at temperatures below 42 "C the agarose starts to gel. On addition of sodium thiocyanate (NaSCN), however, the miscibility of the agarose/PE6400 couple can be

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Figure 1. Transmittance (in %T) vs temperature at a wavelength of 400 nm for (a) agarose (1 wt 96)in 1.0 M NaSCN/water, (b) PE6400 (1 wt %) in 1.O M NaSCN/water, and (c) agarose (1 wt %) PE6400 (1.O wt 9%) in 1.0 M NaSCN/water. Arrows indicate the direction of temperaturechange. Tsrepresents thegel-pointof agarose, Tmthe melting point of agarose, Tcthe cloud-point of PE6400, TOcthe cloud-point of the PE6400 on cooling, Tch the cloud-point of PE6400 on heating, and Ttr the "transition" temperature of the agarose/PE6400 system at which a large change in transmittance takes place.

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improved. NaSCN displays a "salting-in" effect according to the findings of H0fmeister.26.2~ The effects of NaSCN on agarose, on PPO 400, and on Pluronics (PE6200, PE6400 and PE6800) are presented in Table 1. The gel point of agarose is lowered drastically from 41 .O to 20.5 O C in the presence of 1.0 M NaSCN (see also Figure la).

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This decrease is in good agreement with results published The melting point T , of agarose is also lowered on addition of NaSCN. The presence of NaSCN raises the Tc of PE6200, PE6400, and PPO 400 and thus increases the solubility of PPO and Pluronic systems (see Table 1). The rise in cloud-point of the PE6400 system from 55-57 to 70-71 “ C is in accordance with the results shown by Pandya et a1.16 (see also Figure lb). The ‘‘saltingin” effect of NaSCN has also been observed in aqueous ethyl(hydroxyethy1)cellulose (EHEC)28and C12E6’ systems. In Figures 1 and 2 the influence of increasing PE6400 concentration on the phase behavior of agarose (1 wt %)/PE6400 mixtures in 1.0 M NaSCN is shown. Figure l a displays the transmittance (in %T) vs temperature for agarose (1 wt %) in 1.O M NaSCN in the absence of PE6400. On cooling, the transmittance (about 75%) of the agarose solution stays practically constant over a large temperature region from 80 to 25 OC. Lowering the temperature further results in a rapid decrease in transmittance due to the association of agarose helices (gelation process). The gelation temperature for agarose in 1.O M NaSCN ( Tg = 20.5 “C) remains practically unchanged on increasing or decreasing the rate of cooling and does not depend on the Pluronic concentration in the investigated range. The decrease in transmittance occurs over a broad temperature range (5-2 1 “C). On heating, the transmittance (about 8%) of the sample stays constant between 5 and 60 OC, reflecting the well-known hysteresis in the agarose gel sol transition. On further heating, the transmittance of the sample increases and above T , = 65.4 “ C a large increase in transmittance is seen. This phenomenon accompanies the “melting” of the agarose gel network. Although the transition is not sharp, we will for convenience refer to T , as the gel melting temperature which remains practically unchanged on increasing or decreasing the rate of heating. The gelation of agarose in 1.OM NaSCN is “thermoreversible” in the sense that the sol gel sol transition cycle (see Figure la) can be repeated by successive heating and cooling. In Figure 1b the transmittancevs temperaturecurve for PE6400 (1 wt %) in 1.O M NaSCN in the absence of agarose is presented. From this figure it can be seen that the decrease in transmittance, corresponding to the clouding of the solution, takes place at 7& 71 OC. The change in transmittance in the case of PE6400 is rather gradual in contradistinction to the sharp transition that has been observed for other non-ionic surfactants such as Cl2E6 and Cl2Eg.I No thermal hysteresis in clouding was seen. Figure I C contains the transmittance vs temperature curve for theagarose (1 wt %)/PE6400 (1 wt %) mixture in 1.0 M NaSCN.

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Figure 3. Self-diffusion measurements for PE6400. Self-diffusion coefficients D and/or hydrodynamic radii RH vs temperature are plotted for (a) PE6400 in DzO at several concentrations (in wt %), (b) PE6400 in 1.O M NaSCN/DzOat several concentrations (in wt %), and (c) PE6400 (3 wt %) in D20 and 1.0 M NaSCN/D20.

On cooling, there is a temperature T,, at which a strong decrease in transmittance takes place. This decrease in transmittance is not related to the gelation, nor to the coil helix transition of agarose in 1.O M NaSCN. Optical rotation measurements showed that the coil helix transition takes place at the gelation temperature Tg (see Figure la), which is situated at a lower temperature than Ttr. The temperature T,, can be identified as the critical micellization temperature (cmt) at which the micelle monomer (“polymer” coils) transition of PE6400 takes place (results concerning the transition are presented in Figure 3 and are also published el~ewhere~~J7). This temperature decreases on increasing the PE6400 concentration (see Figure 2) as expected for the cmt of PE6400.’3J7 At lower temperatures the PE6400

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The Journal of Physical Chemistry, V O ~98, . NO. 21, 1994 5511

Clouding and Diffusion of a Block Copolymer system consists mostly of monomers and at higher temperatures the micellar state prevails. The large decrease in transmittance on cooling at the cmt in the presence of agarose (no such change in transmittance is observed in the absence of agarose; see Figure 1b) may be caused by the formation of aggregates due to the large incompatibility between agarose coils and the hydrophobic impurities (mainly diblocks) present in the PE6400 system.6.13 To test this hypothesis, we filtered agarose/PE6400 samples at 25 "C-which is below the cmt for PE6400 concentrations smaller than 2 wt %-by using Millipore filter VM with a nominal size of 50 nm and again measured the transmittance versus temperature. This time we observed only a slight decrease in transmittance on cooling at the cmt, which evidences that the largest fraction of the hydrophobic impurities is successfully removed. Wealso found that the removal of the impurities by filtration is not very effective if a Millipore filter with a larger pore size (e.g., 100 nm) is used. The obtained results are in agreement with the experimental findings of Zhou and Chu,6 who demonstrated that the anomalous scattering of PE6400 samples caused by the hydrophobic contaminants was strongly reduced after filtration. According to Almgren et al.13 andZhou et a1.6 dynamic light scattering experiments show that the aggregates formed by the hydrophobic diblocks (about 3% of impurity in the PE6400 system13) have a hydrodynamic radius larger than 500 A. The fact that T,, stays the same after filtration-or in other words theconcentration of PE6400 triblocks in the agarose sample remains unchanged, since T,, (or cmt) is strongly dependent on the PE6400 concentration (see Figure 2)-proves that the filtration is very selective and removes mainly hydrophobic impurities (e.g., diblocks). At temperatures above cmt the solubility of the hydrophobic impurities is increased. According to Zhou and Chu,6 these impurities are either incorporated into the hydrophobic cores of the micelles or form mixed micelles, depending on their molecular characteristics. The strong increase in the solubility of hydrophobic impurities due to the presence of micelles is supported by recent model calculations based on a mean-field lattice theory to describe the effect of polymer impurities in the micellization of P l u r o n i c ~ . ~The ~ transmittance of the agarose coil/PE6400 samples at temperatures above cmt is higher than at temperatures below cmt. This can be explained by the fact that the interaction between agarose coils and PE6400 micelles (with solubilized diblocks) is less repulsive than the interaction between agarose coils and hydrophobic diblocks. A comparison with other types of Phonics, containing 30 PO groups/molecule, was made. For agarose/PE6200 and agarose/ PE6800 in 1.O M NaSCN, however, we found only a large decrease of the transmittance on cooling at T,, which corresponds to the coil helix transition of agarose. In the case of PE6800 (20 wt % PPO) the micelle monomer transition takes place at a higher temperature than in the case ofPE6400 (60 wt % PPO). Compared to the hydrophobicdiblocks in PE6400, the diblock impurities present in PE6800 contain relatively less PO units and are therefore less hydrophobic. Apparently, there is a much larger incompatibility between agarose and the more hydrophobic diblocks in PE6400 than between agarose and the less hydrophobic diblocks in PE6800. In the latter case the presence of impurities does not give rise to an increase in turbidity of agarose coil/PE6800 samples or to anomalous scattering of PE68006 at temperatures below the cmt. Since the diblock impurities in PE6200 (80 wt % PPO) are even more hydrophobic than the diblocks in PE6400 (60 wt 5% PPO) due to the higher PO content, one again might expect to find a strong decrease in transmittance on cooling for agarose/ PE6200 samples near the cmt caused by the large incompatibility betweenagarose and thestrongly hydrophobic diblocks. However, these samples are difficult to handle due to the decreased

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miscibility of the agarose/PE6200 couple in 1.O M NaSCN. The cloud-point on cooling of PE6200 (ca. 25 "C) and the gelation temperature of the agarose/PE6200 sample in 1.O M NaSCN (ca. 21 "C) are namely close to each other. At temperatures above 20 "C thecmc is rather low and the micellar state of PE6200 predominates,ls meaning that the strongly hydrophobic impurities are solubilized in the cores of the PE6200 micelles. In the caseof poly(propy1ene oxide) (100 wt % PPO) containing 30 PO groups there is no temperature at which agarose and PPO will mix in 1.O M NaSCN/H20 due to the strong hydrophobic character of the PO group. PPO 400 (20 wt %), containing 7 PO groups, is soluble in 1.0 M NaSCN/H20 at temperatures below 68.7 OC (see Table 1). (PPO with a higher molecular weight is only sparingly soluble in water at room t e m p e r a t ~ r e . ~ ~ ) The agarose (1 wt %)/PPO 400 (20 wt %) couple, however, is not miscible in 1.OM NaSCN/H20 due to a large incompatibility between agarose and PPO. The gelation temperature TBon cooling remains practically unchanged on addition of PE6400 to agarose as was seen from optical rotation measurements. This has also been found for PEG 6000 and PEG 20000, non-ionic surfactants like Cl2Eg and C1ZE6l and other low molecular additives.31 From Figures 1 and 2 and Table 1 it follows that in the presence of agarose coils (1 wt %) the cloud-point of PE6400 (1 wt %) in 1.O M NaSCN on cooling decreases from 70-7 1 to 63-64 "C due to an "incompatibility" effect (repulsive coil-micelle interactions). We found similar results in the case of agarose/C12E6 and agarose/ ClzE8 systems.l On addition of agarose (1 wt %) we observed a lowering of the cloud-point of non-ionic surfactant (1 wt %) from 70 to 59 "C in the case O f in 1.0 M NaSCN and from 78.9 to 69.7 "C in the case of C1& in water. The observed decrease in cloud-point of PE6400 (or non-ionic surfactants like ClzEg and C&6') on cooling in the presence of agarosecoilsmay beinterpretedin termsof energy and/or entropy contributions. In the former case, short-range pair interactions between sugar units of agarose, PE6400 micelles and water molecules play an important role. In the latter case the decrease in cloud-point can be explained by the fact that the polymer segment density decreases near the surface of the PE6400 micelle, due to the loss in configurational entropy experienced by a polymer close to a surface. This "depletion" gives rise to a net attraction between the micellar particles. In Figure IC a hysteresis in clouding of PE6400 (1 wt %) can be inferred. The cloud-point of the Pluronic on heating Tch(72.1 "C) is situated at a higher temperature than the cloud-point obtained on cooling, Tw (63-64 "C) and has approximately the same value as the cloud-point of PE6400 in 1.O M NaSCN in the absence of agarose (70-71 "C). The hysteresis is also apparent at other Pluronic concentrations (see Figure 2). Evidently, the interaction between agarose coil and PE6400 micelles is more repulsive than the gel-micelle interaction, since in the latter case theagarosegelnetwork leaves morespaceavailable for the micelles than in the coil state. The hysteresis in clouding has also been observed in aqueous agarose/ClzEs samples and in agarose/ClzEs systems in 1.O M NaSCN.' No hysteresis is found if the cooling and heating cycle is restricted to temperatures above Tg,where agarose remains in the coil state throughout the measurement. At Pluronic concentrations larger than 3 wt % the miscibility of the agarose/PE6400 couple in 1.0 M NaSCN is strongly decreased. In those cases T , of PE6400 lies close to the Tgof the agarose/PE6400 system. Self-Diffusion. In Figure 3 the results of FT-PGSE 1H N M R measurements concerning the self-diffusion of PE6400 in DzO and in 1.O M NaSCN/D20 are presented. At low concentrations the self-diffusion coefficient D increases initially with the temperature to a maximum (see Figure 3). At higher temperatures there is a decrease in D which is due to micelle formation. Parts a and b in Figure 3 indicate that the maximum in D shifts

5512 The Journal of Physical Chemistry, Vol. 98, No. 21, 1994

to lower temperatures at increasing concentration of PE6400 in accordance with the results found by Almgren et a1.13 This implies that the cmt is situated at a lower value at higher PE6400 concentrations.l3 This also holds for other Pluronics (see, e.g., refs 6, 7, 11, and 18-22). A comparison of parts a and b of Figure 3 shows that the self-diffusion coefficient of PE6400 is increased on addition of NaSCN (1 .O M). This effect is enhanced at higher temperatures. Figure 3c demonstrates that due to the addition of NaSCN (1.0 M) the hydrodynamic radius RH of PE6400 is decreased compared to the salt-free case in the temperature regime between 30 and 45 OC. The hydrodynamic radii RH have been estimated by using the Stokes-Einstein relation for spheres:

RH = k e T / 6 ~ @

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60 20 30 Tf'C 40 Here ke is Boltzmann's constant, Tthe absolute temperature and x 10" qo the viscosity of the medium (in this case D20 or 1.OM NaSCNI 14 I I I I I I I kl D20). Apparently the addition of NaSCN impedes the formation PE6400 and growth of the PE6400 micelles on increasing the temperature. in 1.0 M NaSCN LSimilar results have been found for the C&/1.0 M NaSCN t - 9 system.] In the latter case it was demonstrated that the growth of the ClzE6 micelles on increasing the temperature from 20 to 50 "C was suppressed on addition of NaSCN. The results of the N M R measurements concerning the selfdiffusion of PE6400 in aqueous agarose/ 1.O M NaSCN gels and solutions vs. temperature are given in Figure 4. Figure 4 shows a decrease in D of PE6400 (1 and 3 wt %) in aqueous agarose agarose solution (1 wt %)/PE6400/1.0 M NaSCN gels and solutions compared to Do,representing the self-diffusion coefficient of PE6400 in 1.O I I I I I 1 I I M NaSCNlD20 in absence of agarose. The slower self-diffusion of PE6400 (it also holds for non-ionic surfactants like C & and 20 30 TPC 40 50 60 CIzE81) in the agarose system it caused by the obstruction of I I I I I I agarose. This obstruction effect was also observed by Johansson I cl I et al., who studied the self-diffusion of glucose and sucrose,32 poly(ethy1ene glycol) (PEG),33J4C12E8, and C12E635in K+-KI carrageenan gels and Na+-K-carrageenan solutions at 25 "c. According to their findings DIDOfor C I ~ (RH E ~ is 2.9-3.2 nm), PEG 2834 (RHis 1.4 nm), and PEG 3978 (RHis 1.6 nm) are about 0.60, 0.72, and 0.62, respectively, at 1 wt % K+-Kcarrageenan gel (mainly coils present) and 0.45, 0.65, and 0.58 for 3 wt % ' K+-K-carrageenan gel (mainly helices p r e ~ e n t ) . ~ ~ - ~ ~ We found a higher value of DlM-about 0.85-for PE6400 (3 wt %) in 1 wt % agarose11 .OM NaSCN gel at 25 OC (see Figure 4c). At this temperature the PE6400 block copolymer (molecular weight 3000, RH = 1.5-1.6 nm) in 1.OM NaSCN consists mainly of unimers or small micelles (see Figure 3d). The higher D/Do value might be explained by the fact that the network structure 20 30 40 50 60 TPC of the aggregated agarose gel is more open (larger mesh-size) in comparison with the less aggregated K+-K-carrageenan gel. Figure 4. Self-diffusion coefficients of PE64OO vs temperature in agarose (1 wt %)/ 1.O M NaSCN gels and solutions and in 1.0 M NaSCN/D20 On increasing the temperature from 25 to 53 OC the DID0 for solutions without agarose: (a) 1 wt % PE6400, (b) 3 wt % PE6400, and PE6400 (3 wt %) in 1 wt % agarose/ 1.O M NaSCN gel decreases (c) DID0 vs temperature for 3 wt % PE6400. from about 0.85 to 0.62 (see Figure 4c). DIDOfor PE6400 in the agarose solution state was lowered from about 0.72 to 0.52 (such as, e.g., ClzEs, PEG, glucose, and sucrose) in K+-Kby increasing the temperature from 33 to 53 OC. The decrease carrageenan gels and in Na+-K-carrageenan solutions at 25 OC. in DIDOis probably due to the fact that at higher temperatures, Their findings of faster diffusion in the gel state are supported apart from the obstruction effect, also micellar formation and by theoretical considerations about the obstruction effect34 and growth of PE6400 plays an important role for the agarose/ hard-sphere Brownian dynamic ~imulations.3~ PE6400/1.0 M NaSCN system. Similar results have been observed for the self-diffusionof micelles in agarose/C&6/ Summary and Conclusions 1.0 M NaSCN.1 As for Cl2Eb and ClzEs,I the self-diffusion of PE6400 in the With light transmittance and FT-PGSE IH N M R self-diffusion agarose gel (1 wt %) is faster than in the solution. The faster measurements vs temperature we monitored the interactions, selfdiffusion, and micellization of the Pluronic PE6400 in agarose/ diffusion of the block-copolymer in the gel may be explained by theincreaseofdistances between theagarosechains in thenetwork, 1.0 M NaSCN gels and solutions as well as the influence of and thus the available volume fraction for PE6400, due to the agarose on the clouding behavior of PE6400. formation of double helices. A similar result has also been found From the former technique it follows that on addition of PE6400 by Johansson et a1.32-35 in their self-diffusion study of cosolutes to agarose there is a transition temperature on cooling at which

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Clouding and Diffusion of a Block Copolymer a strong decrease in transmittance is seen. This temperature, which is strongly dependent on the PE6400 concentration, is identified as the critical micellization temperature of PE6400, and the large decrease in transmittance is ascribed to formation of aggregates of hydrophobic impurities (mainly diblocks) which, below the cmt, are no longer solubilized in PE6400 micelles. Owing to their strong incompatibility with agarose, the hydrophobic impurities thus act as sensitive solubilization indicators, making possible a simple and accurate determination of cmt in the agarose/ Pluronics mixtures. The repulsive interaction between agarose and PE6400-or in other words the incompatibility of the agarose/PE6400 couple-explains the decrease in clouding temperature of PE6400 on cooling in the presence of agarose coils. We also observed a hysteresis in clouding of PE6400, meaning a difference in cloudpoint of the Pluronic system on heating and cooling. This is caused by the fact that the interaction between agarose coil and PE6400 micelles is more repulsive than thegel-micelle interaction. In the latter case the agarose gel network leaves more space available for the micelles than in the coil state. From the self-diffusion measurements it follows that the selfdiffusion of PE6400 in agarose is faster in the gel (more open structure) than in the solution state. The self-diffusion of PE6400 in the presence of agarose is lowered compared to the polymerfree case due to the obstruction effect. The self-diffusion results of PE6400 in 1.0 M NaSCN are different from those in the salt-free case. Addition of NaSCN seems to impede the formation and growth of the PE6400 micelles on increasing the temperature.

Acknowledgment. This work was financially supported by grants from the Swedish Institute, the Wenner-Gren Center Foundation, and the Swedish Natural Science Research Council. References and Notes (1) Penders, M. H. G. M.; Nilsson, S.; Piculell, L.; Lindman, B. J. Phys. Chem. 1993, 97, 11332. (2) . . Dea. 1. C. M.: McKinnon, A. A.; Rees, D. A. J . Mol. Biol. 1972,68,

153. (3) Arnott, S.; Fulmer, A.; Scott, W. E.; Dea, I. C. M.; Moorhouse, R.; Rees, D. A. J. Mol. Biol. 1974, 90, 269.

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