Anchoring vs Bridging: New Findings on Polymer Additives in

Article pubs.acs.org/Langmuir

Anchoring vs Bridging: New Findings on Polymer Additives in Bicontinuous Microemulsions Simona Maccarrone,*,† Jürgen Allgaier,†,‡ Henrich Frielinghaus,† and Dieter Richter†,‡ †

Jülich Centre for Neutron Science JCNS, Forschungszentrum Jülich GmbH, Outstation at MLZ, Lichtenbergstr. 1, 85747 Garching, Germany ‡ Institute of Complex Systems, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany ABSTRACT: We show for the first time the effect of telechelic polymers as additives in bicontinuous microemulsions. We combined macroscopic observations of the phase behavior with microscopic measurements of the structure by small-angle neutron scattering (SANS) to recover the two elastic moduli, κ and κ,̅ namely the bending rigidity and saddle-splay modulus. On the basis of these results, we could classify the effect of telechelic polymers along with confinement, expressed as the ratio of the polymer end-to-end distance Ree and the oil−water domain size d. Their unique property to anchor at two points in the membrane (bridging) acts like a switch from antibooster to booster of the surfactant efficiency (between low and ultralow confinement). In the region of medium confinement, all telechelic polymers are in the bridging configuration and we have a maximum of the boosting effect, while at high confinement, the reversed behavior is found where anchoring and/or bridging do not play any role anymore.

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INTRODUCTION Amphiphilic end-capped polymers, also known as telechelic polymers, have been widely employed to obtain a significant enhancement of the viscosity, simply in water or together with surfactants.1−3 The capability of amphiphilic polymers to selectively adsorb on surfaces induces an effective interaction between two surfaces. This interaction is repulsive with simple amphiphilic chains with one adsorbing site called sticker.4 In the case of two or more stickers, the chains can form “bridges” between the two surfaces. Intuitively, this bridging produces an attraction between the surfaces,4,5 which competes with the repulsive force characteristic of chains with a single adsorbing group. The result of this competition is of relevant importance since their use as stabilizers for colloidal suspensions. What happens if, instead of colloids, we bridge membranes? Do telechelic polymers induce stability also in this case? In this sense, it is well-known that small amounts of amphiphilic diblock copolymer have a strong boosting effect increasing the efficiency of nonionic surfactants,6 proportional to the minimum quantity of surfactant needed to solubilize equal volumes of oil and water. Basically, the polymer decorates the surfactant membrane and increases its stiffness.7,8 The influence due to addition of diblock copolymer additives on the properties of the microemulsion depends on the geometry of the microemulsion itself. The confinement, expressed as the ratio of end-to-end distance of the polymer and the oil/water domain size Ree/d, can modify the pure polymer effect. If the domain size of the microemulsion d is bigger than the length of the polymer Ree, a predominant © 2014 American Chemical Society

boosting effect is observed. When the confinement increases, one passes from low to medium regime. Here while the polymer sits on the membrane having a stabilization effect, it also induces a repulsive interaction between membranes. This effect is similar to the steric repulsion or depletion between polymer-decorated colloids.9 Finally when d and Ree are comparable (high confinement), a reversed behavior is found.10 In this work, we want to use bifunctional (doubly endcapped) telechelic polymers for the first time as additives in bicontinuous microemulsions. We chose an amphiphilic triblock copolymer made by a hydrophilic middle block and two hydrophobic ends of relatively low molar mass, called stickers. The hydrophilic chain will try to pull the polymer away from the interface (entropic force) while the two stickers will try to stay inside the oil domain (enthalpic force). Since this kind of polymer is able to anchor at two points in the membrane (bridging), the interesting question is the interplay between bridging and confinement effects. As for the diblock copolymers,10 we combined macroscopic observations of the phase behavior with microscopic measurements of the structure by SANS. Polymers anchored to membranes modify their elasticity. In the mushroom regime (low polymer density on the membrane), the polymer coils do not interact and fluctuate independently. In this case, the radius of the polymer is proportional to the end-to-end distance of the coil in bulk. The effective bending Received: November 14, 2013 Revised: February 3, 2014 Published: February 6, 2014 1500

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rigidity and the effective saddle-splay modulus will depend linearly on the grafting density defined as σ = ρP a

NA δ V MW 1 − δ V

ΨC6E2 =

(1)

(3)

RW is the end-to-end distances of the water-soluble block. (The short stickers will not have a chain-like behavior; then it is reasonable to set the end-to-end distance of the oil-soluble part equal to zero.) In this case, the analytical values of Ξtribl and Ξ̅ tribl are not yet theoretically quantified so far.

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(5)

As oils, we chose n-alkanes CnH2n+2 in the hydrogenated form (n = 6, 8, 10, 12, and 14, from Sigma-Aldrich and Fluka), and we substituted H2O with D2O (from Chemotrade) for the bulk contrast method. We used linear-structure nonionic surfactant belonging to the poly(ethylene oxide) variety C2n+2En purchased from Bachem and Fluka (n = 2, 3, 4, and 5) without further purification, in its hydrogenated form. The triblock telechelic polymer is made by a hydrophilic middle block of PEO (of 4000 g/mol) and two hydrophobic ends called stickers of relatively low molar mass (alkyl chains of 12 carbon atoms). It was synthesized in our laboratory at the Forschungszentrum Jülich according to the work of Kaczmarski and Glass14 generating the terminal short hydrophobes by direct addition of dodecyl isocyanate to PEO. The radius of gyration of the polymer was evaluated neglecting the very small length of the stickers and considering just the PEO middle block:15 Rg2 = 4.08 × 10−2MW1.16 [Å2]. From this we recovered the end-to-end distance Ree = √6Rg = 60.8 Å. Phase Diagram Measurements. In a typical phase diagram from a nonionic surfactant that mixes equal amount of oil and water we can distinguish four regions. At low temperatures, we find an oil-in-water microemulsion coexisting with an upper oil excess phase (labeled by 2), while at high temperatures a water-in-oil phase coexists with a lower water excess phase (2̅). At intermediate temperatures and low surfactant concentrations, a three-phase coexistence occurs composed by an upper oil excess phase, middle phase microemulsion, and lower water excess phase (denoted by 3), while at higher surfactant concentration water and oil are completely mixed (phase 1). The phase boundaries draw a sort of fish and the critical point in which we have the phase coexistence is the fish-tail point (FTP). The phase diagram measurements were carried out in a thermal water bath. One test tube containing our microemulsion is put into the bath. In there, we observe by visual inspection how the light from a lamp is scattered or polarized by the sample varying the temperature. For this purpose a thermostat is used, eventually connected to a bigger one that helps to reach low temperatures. At each temperature the sample is stirred and then left for a while. The one phase regime appears completely clear while the 2 and 2̅ phases appear completely turbid after stirring as well as the three-phase regime but waiting for a certain time it is possible to see the phase separation (Figure 1). At the fish tail point the saddle-splay modulus becomes zero, and the thermal fluctuations (∼ln Ψ̃) are balanced by the polymer addition.16 Knowing the minimum surfactant concentration Ψ̃ from the measured phase diagram, it is possible to monitor its variation with the polymer concentration:

where ρP is the density of the polymer, a the thickness of the membrane, NA the Avogadro’s number, MW the molecular weight of the polymer chain, and δV the volume fraction of the polymer with respect to the total amphiphile. σ counts the number of polymers per membrane area. Qualitatively, the effect of diblock copolymer anchoring can be understood in terms of the entropy. When the polymers are homogeneously tethered to the membrane, both the membrane and neighboring polymers restrict the number of configurations that are accessible to the polymer. This should cause entropic repulsion suppressing the membrane fluctuations. Furthermore, diblock copolymers make saddle-shape deformations unfavorable. The renormalized curvature-elastic moduli can be expressed as sum of three terms: the first one being κ0 and κ0̅ the intrinsic moduli of the bare membrane, the second terms come from considering the influence of thermal fluctuations on shorter length scales,11 and the third terms linearly proportional to the grafting density σ have been calculated for a membrane with polymers in the mushroom regime without confinement. The analytical results for the sensitivity coefficients of the variation of κ and κ̅ with the presence of the polymer are Ξ = 0.214 and Ξ = 0.167 for ideal linear chains, respectively.12 The effect of telechelic or triblock polymers will be contained, as for diblock copolymers, in the third term of κ and κ:̅ κ κR 3 = 0 + ln(Ψ) + Ξ triblσR W 2 kBT kBT 4π (2) κ κR̅ 5 ln(Ψ) + Ξ̅ triblσR W 2 = 0̅ − kBT kBT 6π

VS − 0.0437VO − 0.0557VW VP + VS + VO + VW

EXPERIMENTAL SECTION

Materials. A systematic study on bicontinuous microemulsions with telechelic polymer additive was carried out varying the domain size d using different chain length oils and surfactants (large domains d for short-chain oils and long-tail surfactants).13 For every system, we established three values of polymer content δ defined as the mass fraction of polymer over the total amphiphile δ = mP/(mS + mP), considering also the pure system without polymer (δ = 0). For each δ three different surfactant contents were chosen not too far from the fish tail point γ = (mS + mP)/(mS + mP + mO + mW). The microemulsions were prepared mixing equal volumes of oil and water. The membrane volume fraction Ψ is defined as Ψ=

VS − 0.02VO VP + VS + VO + VW

(4) Figure 1. Phase diagram of C6-D2O-C10E4-C12E100C12 (from SANS samples). The FTP undergoes a shift in temperature from 17.4 °C for the pure system to 16.48 °C with δ = 2% and a shift to high surfactant concentrations from 0.063 to 0.074.

with Vi = mi/ρi where we took into account that a small percentage (2 vol %) of surfactant is solubilized in oil.13 Furthermore, the surfactant C6E2 experiences even solubility in water: 1501

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⎞ ⎛ 6π κ0̅ Ψ̃ = exp⎜ − Ξ̂σR W 2⎟ ⎠ ⎝ 5 kBT

(6)

Another important parameter that can be calculated from the phase diagram measurements is the spontaneous curvature that describes the tendency of the surfactant film to bend toward either the water (c0 > 0 by convention) or the oil:12

c0,eff = c0(T ) + Υσ(R W − R O)

(7)

It is evident that the addition of symmetric diblock copolymers (RW = RO) does not change c0 while adding asymmetric block copolymers means to favor a curvature toward the domain with the smaller polymer. So we expect an effect through a shift in temperature but not in the shape of the fish tail. In the case of the telechelic polymer the spontaneous curvature will not vanish

c0,eff = c0(T ) + ΥσR W

(8)

and the membrane will prefer to bend toward the oil domain. Strey in ref 17 showed that the mean curvature H of the amphiphilic film is a linear function of the temperature:

H ≈ μ(T − T̃ )

(9) −3

−1

−1

The coefficient μ is of the order of 10 Å K for all surfactants studied here and decreases only slightly with increasing surfactant chain length. The bicontinuous structure has “minimal surfaces”, characterized by a mean curvature H = 0 (and K < 0) that minimize the curvature energy.18 At the fish tail point not only κ̅ but also c0,eff vanishes. This implies that the presence of the polymer will be compensated by a change of the phase inversion temperature T̃ :

c0(T̃ ) = μ(T − T̃ ) = −ΥσR W

Figure 2. SANS spectra for sets of data at different confinement values: (top) C12-D2O-C6E2-C12E100C12 (intensities at δ = 0.01 and 0.02% are shifted of a factor 2 and 4, respectively); (middle) C6-D2OC8E3-C12E100C12 (intensities at δ = 0.01 and 0.02% are shifted of a factor 2 and 4, respectively); (bottom) C6-D2O-C10E4-C12E100C12. Straight lines are the corresponding Teubner−Strey fits. The exact values of b take a range from 0.13 to 0.18, and we finally agreed on the theoretical value of 0.15. The latter formula also holds for the coefficients ΞSANS, Ξ and Ξ̅ , and we therefore calculated the bare Ξ according to

(10)

Then from the phase diagram measurements we get the variation of c0 for the different polymer content as a function of σRW. Small-Angle Neutron Scattering (SANS). All the experiments were done on the small-angle neutron machine KWS2 at the FRJ-2 research reactor at the Forschungszentrum in Jülich. We used 1 mm quartz cells placed in the beam in a little furnace that kept the temperature constant for each sample. In order to cover the whole Qrange (from 0.002 to 0.2 Å−1), we measured each sample at two different detector distances (between 2 and 20 m) keeping the collimation apertures distance fixed at 20 m, due to the high scattering intensity. The incident wavelength of neutrons was λ = 7.3 Å (Δλ/λ = 0.1). The raw data were corrected for background and detector efficiency19 and afterward calibrated in absolute units (cm−1) by a Plexiglas standard. The SANS curves from bulk contrast samples (Figure 2) are well described by the Teubner−Strey formula:20 I(Q ) =

Ξ=

■

1 (ΞSANS + (1 − b)Ξ̅ ) b

(14)

RESULTS AND DISCUSSION Plots for all the three sensitivity coefficients are as a function of the inverse of the confinement d/Ree for a better visualization of the data. The results of Ξ for the triblock copolymer are illustrated in Figure 3. As far as we know, no theory for the

8πϕW ϕOΔρ2 /ξ ((2π /d)2 + ξ−2)2 − 2((2π /d)2 − ξ−2)Q 2 + Q 4 (11)

In this way, the scattering intensity is related to two structural parameters: the domain size d and the correlation length ξ. The former is related to the peak position and the latter to the peak width. The values of these two quantities are extracted from the spectra fitting and by the Gaussian random field approximation are connected with the bending rigidity κR,SANS.8,21,22 κR,SANS kBT

=

5 3 2π ξ 64 d

(12) 23

24

Recently, it was shown theoretically and experimentally that the bending rigidity obtained by SANS as in eq 1 is a mixture of the saddle-splay modulus and the bending rigidity. The general result reads κR,SANS = bκR − (1 − b)κR̅

Figure 3. Sensitivity coefficient of the bending rigidity for the triblock copolymers derived from SANS measurements as a function of the inverse of confinement.

(13) 1502

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elastic moduli κ and κ̅ was developed for this kind of polymer as additive in microemulsions. This makes the interpretation of our experiments harder. However, the data follow a clear behavior. At ultralow confinement the sensitivity coefficient should probably have constant negative values. In this region, κR̅ was not experimentally accessible, but since Ξ̅ has an almost constant value in the region of low confinement, we used the Ξ̅ value at d/Ree = 3.45 in order to get a rough estimation of Ξ via eq 13. Then in the region 2 < d/Ree < 3.5 the values start to increase. A physical explanation of this phenomenon can be the slow switching on of the bridging. This is corroborated by the findings of the droplet phase experiments: in fact, we expected traces of the bridging effect starting from 2.6 until the optimal bridging condition (minimum of the potential) at 1.3 of confinement where a maximum should occur.5 Similar evidence is supported by Monte Carlo simulation of loop and bridge formation between micelles by telechelic polymers.25 When d/Ree = 1.5, Ξ reaches a maximum value around 2.5, larger than the one found for the diblocks of 1.5. In this case, the high presence of the polymer at the interface would make the confining membranes stiffer. Then at high confinement, the reversed behavior is found. This observation confirms that, at this level, the anchoring, i.e., the different polymer topology, is not important anymore, and all the polymers follow a common behavior. The behavior of the sensitivity coefficient Ξ̂ of the saddlesplay modulus for the triblock copolymers is depicted in Figure 4. At very low confinement (d/Ree > 4) the addition of polymer

switching between these two regions might be interesting for applications. On the basis of these findings, we argue the following picture. At ultralow confinement all polymers are in the loop conformation, and the values of the sensitivities are negative; with the increasing of confinement, in the next region of low confinement regime, there will be a fraction of polymer chains able to bridge also opposite points. In a previous work with oilin-water droplet microemulsion,5 we showed that the addition of telechelic polymer leads to an effective attractive interaction starting to bridge droplets at d/Ree ≈ 2.6 with a maximum at d/ Ree ≈ 1.3 This would be the reason for the increasing values. The evidence for this effect grows with the fraction of polymers in the bridging conformation reaching a maximum value double as that one of the diblock copolymers.10 For high confinement the reversed behavior confirms a common feature of all kinds of polymers. In analogy to the diblocks, it is observed that the boundary of different regimes for Ξ and Ξ̂ do not occur at the same value of d/Ree, in particular when we pass from low to medium confinement. Also here an explanation could be based on the same consideration made for the diblock copolymers in the bicontinuous structure composed by channels and junctions.10 Indeed, the fraction of polymers in the channels would be most likely in the growing bridging conformation, and this will contribute more to κ as at this level of confinement, the junctions, contributing to κ,̅ disfavor bridging stronger than cylinders and only loops should be possible. So a stronger effect on κ̅ comes only with increased confinement. As already mentioned, for the triblock copolymers another important quantity can be calculated from the phase diagram measurements: the mean curvature H. In Figure 5 the sensitivity Υ of H is plotted vs the inverse of confinement.

Figure 4. Sensitivity coefficient of the saddle-splay modulus for the triblock copolymers calculated from the phase diagram measurements as a function of the inverse of confinement. Figure 5. Sensitivity coefficient of the mean curvature for the triblock copolymers derived from phase diagram measurements as a function of the inverse confinement.

should not influence the emulsification capability of the surfactant, and a negative limiting value should be reached in parallel with Ξ. We could not measure Ξ̂ in this region because of the dominating lamellar phase for the C12E5. The experimental points from the systems with big domain size (low confinement 2< d/Ree < 4) have a negative sign of the sensitivity that turn into positive in the next region of medium confinement (1< d/Ree < 2) increasing until a maximum of 0.98 is reached. Finally, at high confinement, a steep drop down is found. It is important to remark that the transition from low to medium regime seems to be not continuous. The sudden

For the low confinement regime, the resulting mean curvature is negative, meaning that the membrane is curved toward the oil domain. Here the polymer chain is pushing the anchored membrane to the oil domain. Thereby it stays away from the surface, provoking an antiboosting effect following from the observed behavior of Ξ and Ξ̂. Approaching d/Ree = 1.7, the values for the coefficient decrease crossing zero and becoming negative in the medium confinement regime. 1503

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In our picture, the fraction of polymers bridging between two opposite membrane points keeps the membrane curved toward the water domain (where the middle block is dissolved). This constraint brings back the fluctuating membrane every time it tries to “open” during its continuous rearrangement. The polymer will be in the vicinity of the membrane, rendering it more rigid (increase of κ and decrease of κ). ̅ The behavior of Υ adds one more point in establishing consistent picture. The surprising diverging behavior at high confinement could be justified by a “collapse” of the polymer that could cause consequently the collapse of the domain. A similar trend from positive to negative curvature was described in ref 26 with increasing adsorption tendency. The confinement brings the two anchoring points of the triblock copolymers closer together which could be interpreted as a larger tendency to adsorb. An attempt to explain the coefficients Ξ, Ξ̂, and Υ theoretically bases on the theory developed by Hiergeist27 for telechelic chains. The partition functions of a single telechelic polymer in a sphere and a cylinder were derived to be Zsphr

1 ∝ 3 R

Zcyl ∝

1 R3

⎛ π 2k 2R 2 ⎞ W ⎟ ∑ k exp⎜− 2 qR ⎠ ⎝ ∞

Figure 6. Theoretical coefficients Ξ, Ξ̂, and Υ as a function of the inverse of confinement parameter d/Ree. The dotted lines arise from the noncapped (unchanged) theory of Hiergeist, while the solid lines involve capping of the cylinder geometry. ∞

Z lids ∝

k=1

k=1

∫0

∑

⎛ π 2k 2R 2 ⎞ 1 W ⎜− ⎟ exp k2 qL2 ⎠ ⎝

(19)

2

∞

⎛ π 2k 2R 2 ⎞ W ⎟ F (α ) dαα 3 exp⎜ − qR2 ⎠ ⎝

For the length of the cylinder we assumed a transition function switching between confinement and no confinement according to L = 2R + (1/2 + arctan(4(2R − RW)/RW)(1000 Å − 2R). The results are displayed in Figure 6 by solid lines. The dependencies now even look more similar to the experiments, while we neglected the absolute scales of the two functions. So far, we restricted ourselves to taking the morphologies of a sphere and a cylinder into account. This is exactly right in the limit of zero curvatures where all possible morphologies will lead to the same results. For finite curvatures that are needed to calculate the confinement effects, the whole ensemble of membrane morphologies displaying a bicontinuous microemulsion would be needed to be exact. This would put different weights on other morphologies than the two considered ones here, and so deviations of the scales (and possibly on the shapes) are to be expected. So the present model calculations are semiquantitative, but they give clear insight into what ingredients are needed to model the experiments. The very broad range in which microemulsions find applications corresponds often to the need of fulfilling many different requirements, and the use of the triblock copolymer would be a solution in a process with different stages that necessitate different properties. For example, cutting fluids can be made of emulsions29 and microemulsions.30 For purposes of recycling and volume reduction in the waste disposal several techniques of separating the components are applied. The skimming bases on the lower oil density that tends to float up, and air bubbles might possibly support the desired separation. While the telechelic polymer is able to stabilize the microemulsion at higher concentrations (medium confinement), the skimming might mean that the oil is continuously removed and there is excess water, which results in lower confinement. So the polymer can support the phase separation by the reduced emulsification efficiency, which leads to a faster phase separation. If the surfactant is chosen correctly, the floating up phase might contain all valuable chemicals (surfactant, polymer, and oil) that will allow for an easy reuse with only little material to be added other than water. Also, for the waste disposal it is desired to achieve a concentrated phase with most of the organic materials. Engineering experiments would be the next step to prove whether the telechelic polymer

(15)

(16)

with F(α) = (J1(α)Y0(α) − J0 (α)Y1(α))2 /(J0 2 (α) + Y0 2(α)) (17)

Here, the radius of the sphere and cylinder is R, the polymer size is RW, and the coordination number in 3-d space is q = 2d = 6. The Bessel functions of first and second kind are Jν and Υν. The free energy of a polymer decorated membrane of such shape would then be Fsphr/cyl = −kBTσ ln(Zsphr/cyl)

(18)

The factor σ accounts for the coexistence of many polymers in terms of the polymer number per membrane area. The coefficients κ, κ,̅ and c0 of the Helfrich free energy H = ∫ d2s(κ(c1 + c2 − 2c0)2/2 + κc̅ 1c2) are calculated by the comparison with the free energies obtained above for the first and second derivatives of the finite curvature c = 1/R (the principal curvatures are c1 and c2 and are identical for the sphere and different for the cylinder geometry). The corresponding coefficients Ξ, Ξ̂, and Υ are defined accordingly. In the limit of zero curvature the coefficients take the values Ξ = −1/2q = −1/ 12, Ξ̂ = −π/5, and Υ = 0. This method corresponds to most classical calculations of a particular polymer effect. The results of the different coefficients for finite curvatures are displayed in Figures 6, where we introduced the domain size to be d = 2R (for the sphere and cylinder). Already on this basis, the principal dependencies of the coefficients Ξ̂ and Υ are captured very well, but the function of Ξ looks quite unstructured. The weak point of the calculation is still that the cylinder is infinitely long, and if the curvatures become stronger, the polymer can escape by extending along the symmetry axis. In a real microemulsion there are no infinitely long domains, and so the introduction of lids makes sense. In this second step, we gave the cylinder reflecting lids, but the anchoring points were not allowed on the lids. In this sense, only an additional term for the partition function appears:28 1504

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(10) Maccarrone, S.; Byelov, D.; Auth, T.; Allgaier, J.; Frielinghaus, H.; Gompper, G.; Richter, D. Confinement effects in block copolymer modified bicontinuous microemulsions. J. Phys. Chem. B 2013, 117, 5626−5632. (11) Peliti, L.; Leibler, S. Effects of thermal fluctuations on suystems with small surface-tension. Phys. Rev. Lett. 1985, 54, 1690−1693. (12) Eisenriegler, E.; Hanke, A.; Dietrich, S. Polymers interacting with spherical and rodlike particles. Phys. Rev. E 1996, 54, 1134−1152. (13) Sottmann, T. Dissertation, Georg-August Universität zu Göttingen, Göttingen, 1997. (14) Kaczmarski, J. P.; Glass, J. E. Synthesis and solution properties of hydrophobically-modified ethoxylated urethanes with variable oxyethylene spacer lengths. Macromolecules 1993, 26, 5149−5156. (15) Kawaguchi, S.; Imai, G.; Suzuki, J.; Miyahara, A.; Kitano, T.; Ito, K. Aqueous solution properties of oligo- and poly(ethylene oxide) by static light scattering and intrinsic viscosity. Polymer 1997, 38, 2885− 2891. (16) Gompper, G.; Kroll, D. M. Membranes with fluctuating topology: Monte Carlo simulations. Phys. Rev. Lett. 1998, 81, 2284− 2287. (17) Strey, R. Microemulsion microstructure and interfacial curvature. Colloid Polym. Sci. 1994, 272, 1005−1019. (18) Helfrich, W. Z. Elastic properties of lipid bilayers - theory and possible experiments. Naturforscher 1973, 28c, 693−703. (19) Pedersen, J. S.; Posselt, D.; Mortensen, K. Analytical treatment of the resolution function for small-angle scattering. J. Appl. Crystallogr. 1990, 23, 321−333. (20) Teubner, M.; Strey, R. Origin of the scattering peak in microemulsions. J. Chem. Phys. 1987, 87, 3195−3200. (21) Pieruschka, P.; Safran, S. A. Random interfaces and the physics of microemulsions. Europhys. Lett. 1993, 22, 625−630. (22) Pieruschka, P.; Safran, S. A. Random interface model of sponge phases. Europhys. Lett. 1995, 31, 207−212. (23) Peltomäki, M.; Gompper, G.; Kroll, D. Scattering intensity of bicontinuous microemulsions and sponge phases. J. Chem. Phys. 2012, 136, 134708. (24) Holderer, O.; Frielinghaus, H.; Monkenbusch, M.; Klostermann, M.; Sottmann, T.; Richter, D. Experimental determination of bending rigidity and saddle splay modulus in bicontinuous microemulsions. Soft Matter 2013, 9, 2308−2313. (25) Testard, V.; Oberdisse, J.; Ligoure, C. Monte Carlo simulations of colloidal pair potential induced by telechelic polymers: Statistics of Loops and Bridges. Macromolecules 2008, 41, 7219−7226. (26) Breidenich, M.; Netz, R. R.; Lipowsky, R. Adsorption of polymers anchored to membranes. Eur. Phys. J. E 2001, 5, 403−414. (27) Hiergeist, C. S. PhD Thesis, Potsdam-Teltow, 1997. (28) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics; Oxford University Press: New York, 1988, and lecture notes of W. Briels: http://cbp.tnw.utwente.nl/PolymeerDictaat/index.html. (29) Sheng, P. S.; Oberwalleney, S. Life-cycle planning of cutting fluids - A review. J. Manuf. Sci. Eng. 1997, 119, 791−800. (30) Dicharry, C.; Bataller, H.; Lamaallam, S.; Lachaise, J.; Graciaa, A. Microemulsions as cutting fluid concentrates: Structure and dispersion into hard water. J. Dispersion Sci. Technol. 2003, 24, 237−248.

is an intelligent additive in microemulsion/emulsion based cutting fluids.

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CONCLUSION We have characterized telechelic polymers as additives in microemulsions by means of phase diagram measurements and SANS. We obtained three sensitivities for the polymer effect on the bending rigidity, the saddle-splay modulus, and the mean curvature. The choice to extend the investigation to a telechelic polymer proved that the introduction of one more degree of freedom for the possible conformations of the polymer opens one more mechanism on the confinement axes. The negative values of Δκ raise with a continuous transition while Δκ̅ seems to jump between negative and positive values. This sharp transition may have an important impact for eventual applications because this polymer acts like an intelligent booster or antibooster on the selected domain size.

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AUTHOR INFORMATION

Corresponding Author

*Tel +49-89-289-13805; Fax +49-89-289-10799; e-mail s. [email protected] (S.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Thorsten Auth from ICS-2, Forschungszentrum Jülich, for fruitful discussions about the possible theoretical interpretations.

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REFERENCES

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dx.doi.org/10.1021/la404354w | Langmuir 2014, 30, 1500−1505