Interaction of Surfactant Lamellar Phase and a Strictly Alternating

Sep 22, 2001 - The membrane consists of the nonionic surfactant penta(ethylene glycol) dodecyl ether (C12E5; >99%, Nikko Chemical Co. Ltd., Tokyo) and...
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Interaction of Surfactant Lamellar Phase and a Strictly Alternating Comb-Graft Amphiphilic Polymer Based on PEG Bing-Shiou Yang,† Jyotsana Lal,‡ Joachim Kohn,§ John S. Huang,† William B. Russel,† and Robert K. Prud’homme*,† Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544-5263, Intense Pulsed Neutron Source, Argonne National Lab, Argonne, Illinois 60439, and Department of Chemistry, Rutgers University, Piscataway, New Jersey 08854 Received April 16, 2001. In Final Form: August 1, 2001 We study the effect of hydrophobically modified polymers (hm-polymers) on the phase behavior and membrane elastic properties of the lyotropic lamellar (LR) phase of the nonionic surfactant penta(ethylene glycol) dodecyl ether (C12E5). The hm-polymer is a model comb-graft polymer with monodisperse PEG blocks (loops) of 6, 12, or 35 kg/mol connecting C18 stearylamide hydrophobes. The polymer is of interest because both its loops and hydrophobes are biocompatible. The membrane properties are extracted from the small-angle neutron scattering data based on the model of Nallet et al. The monophasic LR region exists over a narrower membrane volume fraction range but to a higher polymer concentration for hmPEGs with larger PEG spacers. The rigidity of the membranes increases by 40% with decreasing PEG spacer size from 12 to 6 kg/mol or increasing polymer concentration. For hmPEG polymers with 2-3 loops, the effect of finite chain lengths plays a role on the phase behavior and membrane properties.

Introduction Surfactants self-assemble into a host of different mesophase structures: vesicles, rods, lamellar sheets, and bicontinuous cubic arrays. The length scales of these structures are comparable to the sizes of many watersoluble polymers. Therefore, the phase behavior of mixtures of surfactants and polymers is intriguingly complex. It has been shown both theoretically1,2 and experimentally3-6 that the addition of even a relatively small amount of adsorbing polymers markedly affects the microstructure, elastic properties, and stability of the interface. In our previous study, we found the effect of randomly grafted hydrophobically modified polymers on the bilayer properties to depend on both the molecular structure of the polymer and polymer concentration.7 Our interest in specific interactions between hydrophobically modified polymers and surfactant or lipid bilayers is driven by the desire to form and stabilize multilamellar vesicles for delivery and controlled release of active compounds.8 We have observed that the addition of small amounts of hm-polymers drastically enhances and eases formation and stability of vesicles. However, the connection between vesicle formation and membrane elasticity, on one hand, and membrane elasticity and polymer anchoring on the membrane surface is not yet understood. In this study, we focus on the second problem: the relation between * To whom correspondence should be addressed. † Princeton University. ‡ Argonne National Lab. § Rutgers University. (1) de Gennes, P. G. J. Phys. Chem. 1990, 94, 8407. (2) Brooks, J. T.; Cates, M. E. J. Chem. Phys. 1993, 799, 5467. (3) Dexter, D. L. J. Chem. Phys. 1953, 21, 836. (4) Iliopoulos, I.; Olsson, U. J. Phys. Chem. 1994, 98, 1500. (5) Ligoure, C.; Bouglet, G.; Porte, G. Phys. Rev. Lett. 1993, 71, 3600. (6) Singh, M.; Ober, R.; Kleman, M. J. Phys. Chem. 1993, 97, 11108. (7) Yang, B.-S.; Lal, J.; Marques, C. M.; Richetti, P.; Russel, W. B.; Prud’homme, R. K. Langmuir 2001, 17 (19), 5834-5841. (8) Bernheim-Grosswasser, A. U. S.; Gauffre, G.; Viratelle, O.; Mahy, P.; Roux, D. J. Chem. Phys. 2000, 112, 3424.

polymer anchoring and membrane elasticity, specifically the role of polymer architecture. The hm-polymer architectures that have been studied previously in surfactant solutions can be classified into two groups. The first is comb-graft hm-polymers with pendent hydrophobic groups distributed along the hydrophilic backbone. Most of the studies have focused on polyacrylate, polyacrylamide, and cellulose derivatives.9,10 Depending upon the synthesis pathway, the hydrophobes are distributed statistically or in “bunches”.11 Typically, the degree of the hydrophobic substitution is between 0.1 and 5 mol %.12,13 The effective strength of the hydrophobic binding between the polymer and the surfactant membrane can be varied by adjusting the hydrophobe substitution level on the polymer and the hydrophobicity (e.g., the alkyl chain length) of the side chains.14 The second class is telechelic polymers with hydrophobes at both ends of a hydrophilic polymer backbone.15,16 Our studies have focused on comb-graft hm-polymers. For the random combgraft polymers, we have shown the changes in phase behavior and membrane elastic properties are independent of total chain molecular weight and chain polydispersity.7,17 But the importance of the distribution of block lengths between hydrophobes cannot be addressed with random copolymers. In this study, we introduce a new hm-polymer architecture which is a strictly alternating (9) Magny, B.; Lafuma, F.; Iliopoulos, I. Polymer 1992, 33, 3151. (10) Kramer, M.; Stefer, J.; Hu, Y.; McCormick, C. Macromolecules 1996, 29, 1992. (11) Candau, F. S. J. Adv. Colloid Interface Sci. 1999, 79, 149-172. (12) Yang, Y. S. D.; Steiner, C. A. Langmuir 1999, 15, 4335-4343. (13) Xu, B.; Li, L.; Zhang, K.; Macdonald, P. M.; Winnik, M. A. Langmuir 1997, 13, 6896. (14) Kabalnov, A.; Olsson, U.; Thuresson, K.; Wennerstrom, J. Langmuir 1994, 10, 4509. (15) Bhatia, S. R.; Russel, W. B. Macromolecules 2000, 33, 57135720. (16) Pham, Q. T.; Russel, W. B.; Lau, W. J. Rheol. 1998, 42, 159176. (17) Yang, Y.; Prud’homme, R. K.; McGrath, K. M.; Richetti, P.; Marques, C. M. Phys. Rev. Lett. 1998, 80 (12), 2729.

10.1021/la0105533 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/22/2001

Effect of Amphiphilic Polymer on Lamellar Phase

comb-graft associative copolymer derived from poly(ethylene glycol) (PEG) and an amphiphilic derivative of L-lysine. PEG acts as a hydrophilic spacer between the hydrophobic pendent chains attached to lysine. This polymer is identified as poly(PEGmb-lysine-stearylamide) (hmPEG) where mb is the molecular weight between hydrophobes (i.e., molecular weight of the PEG spacer). Within this molecular architecture, three variables may be changed independently: first, the molecular weight of the PEG spacer, mb, can be varied from 6 to 35 kg/mol; second, the copolymers can be prepared with hydrophobic pendant chains of varying molecular weight; and finally, the overall molecular weight of the copolymer can be changed by preparing copolymers with different degrees of polymerization while keeping the hydrophilic-hydrophobic sequence unchanged. This polymer is of interest as a carrier of drug compounds18,19 in addition to its ability to stabilize interfaces. Both the PEG hydrophilic steric loop and the hydrophobic anchoring lysine derivative are biocompatible which make it particularly attractive as a biomaterial. In this study, we present the phase behavior and membrane elastic constants of a surfactant lamellar phase containing a model hydrophobically modified polymer in which the hydrophobes on the polymer backbone are uniformly spaced. The effects of polymer architectures (hydrophilic PEG spacer size, polymer molecular weight, and number of loops), polymer concentration, and membrane volume fraction are investigated. The first section describes the materials and the experimental techniques. The next section describes phase behavior of the polymer/ surfactant solutions, and the following section presents results from neutron scattering experiments, which demonstrate the effect on membrane elastic constants of polymer interaction with the surfactant membrane bilayers. Experimental Section Systems and Materials. Figure 1 presents the structure and length scales for the mixture of hydrophobically modified polymer and surfactant lamellar phase. The membrane consists of the nonionic surfactant penta(ethylene glycol) dodecyl ether (C12E5; >99%, Nikko Chemical Co. Ltd., Tokyo) and 1-hexanol (>99%, Fluka), used as received. Pure C12E5/water mixtures have a wide LR phase region at 60 °C, spanning membrane volume fraction φ ) 0.005-1.20 Adding hexanol to the membrane reduces the membrane rigidity 21 and extends the wide lamellar range of the phase diagram to room temperature. The C12E5/hexanol molar ratio in all of our samples is fixed at 1:1.43, which reduces the membrane rigidity to approximately kBT.21 The solvent phase is 0.1 M NaCl(aq) which effectively screens the electrostatic interactions of the added anionic polymers; the Debye length lD is 1.0 nm for the NaCl solution at 0.1 M.22 For neutron scattering experiments, H2O is replaced by D2O (Cambridge Isotope Laboratories, Inc.) at the same volume fraction without appreciably affecting the phase behavior.7,23 The hydrophobically modified PEG is a neutral polymer and C12E5 is a nonionic surfactant, so the undulation force24 is the dominant long-range repulsion. The details of synthesis and characterization of hmPEGs as reported elsewhere25 involve synthesis of (a) lysine stearylamide (18) Nathan, A.; Zalipsky, S.; Ertel, S. I.; Agathos, S. N.; Yarmush, M. L.; Kohn, J. Bioconjugate Chem. 1993, 4, 54. (19) Vyavahare, N.; Kohn, J. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1271-1281. (20) Strey, R.; Schomacker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86 (12), 2253. (21) Freyssingeas, F.; Nallet, E.; Roux, D. Langmuir 1996, 12, 6028. (22) Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; Interscience: New York, 1976. (23) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982, 260, 10231028. (24) Helfrich, W. Naturforsch. 1978, 33a, 305.

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Figure 1. Sketch of the anticipated structure of the mixture of hydrophobically modified polymer and surfactant lamellar phase. The hydrophobic groups along the polymer backbone associate with the membranes forming polymer-doped membranes. The length scales are for φ ) 0.3 doped with hm PEG with spacers of mb ) 6 kg/mol (hmPEG6k-DP13 in Table 1). The enlarged picture is the chemical composition of hmpolymer with the repeat units distributed uniformly. R is the hydrophobe. Table 1. Details of Hydrophobically Modified Polymers name (abbreviation) hmPEG6k-DP3 hmPEG6k-DP13 hmPEG12k-DP2.5 hmPEG12k-DP5 hmPEG12k-DP6 hmPEG35k-DP2.5

mb Mn Mw (kg/mol) (kg/mol) (kg/mol) 6 6 12 12 12 35

18 78 29 64 71 86

42 112 48 106 113 138

Dp

Nb ξ (Å)

3 4 13 24 2.5 3 5 8 6 10 2.5 3

50 43 75 65 64 128

dihydrochloride, (b) bis(succinimidyloxycarbonyl PEG) (BSCPEG),26,27 and (c) poly(PEGmb-lysine-stearylamide), sequentially. Lysine stearylamide dihydrochloride was polymerized interfacially with BSC-PEG in the presence of potassium carbonate according to a published procedure.26 The structure of the synthesized polymers was characterized by 1H NMR. Molecular weights were determined by gel permeation chromatography and calculated relative to poly(ethylene oxide) standards. Six hmPEGs with different architecture parameters (weight-average molecular weight of polymer chain Mw, mb, and number of loops Dp) were made, and the details are shown in Table 1. The typical structure of poly(PEGmb-lysine-stearylamide) (hmPEG) is illustrated in the inset of Figure 1. The weight-average molecular weight of PEG spacers (mb) varies (6, 12, or 35 kg/mol), and the hydrophobe is RdC18H37. The number of hydrophilic loops varies (3, 5, 6, or 13) as calculated by Dp ) Mn/mb where Mn is the number-average molecular weight. We use the following nomenclature to identify these polymers: hmPEGmb-DPDp, that is, hmPEG6k-DP3 has 6 kg/mol PEG spacers and an average of 3 loops per chain. (25) Heitz, C.; Pendharker, S.; Kohn, J.; Prud’homme, R. K. Macromolecules 1999, 32, 6652. (26) Tanaka, F.; Edwards, S. F. J. Nonnewtonian Fluid. Mech. 1992, 43, 247. (27) Jones, M. J. Colloid Interface Sci. 1967, 23, 36.

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Sample Preparation. The samples were prepared by mixing surfactant (C12E5), alcohol (C6OH), and stock solutions of the hm-polymers by weight in 0.1 M NaCl(H2O or D2O). In the following, we define the “membrane volume fraction” φ as the volume of the C12E5 plus C6OH divided by the total sample volume. For conversion from weight to volume fractions, the following densities (g/cm3) have been used: 0.996 (C12E5), 0.820 (C6OH), 1.105 (D2O), and 0.998 (H2O), where we neglect the effect of the added polymers on the solvent density. Phase Behavior. The study of phase behavior was conducted in a thermal bath ((0.1 °C) with samples contained in sealed vials. The phase behavior of a sample was determined by visual inspection of transmitted light and scattered light and by observation of the samples between crossed polarizers. The lamellar phase (LR) is optically anisotropic (i.e., birefringent). The sponge phase (L3) is optically clear and isotropic. Coexisting lamellar phases, LR1/LR2, appear turbid under natural light and show an interface between two birefringent phases after centrifugation. Small-Angle Neutron Scattering (SANS). The small-angle neutron scattering experiments were performed at Argonne National Laboratory at Argonne, IL, with the time-of-flight smallangle diffractometer (SAD) over a q range from 0.005 to 0.35 Å-1. Samples were held in 1 mm path length quartz cells. The data are processed using the standard analysis algorithms developed for the SAD beam line,28 including the subtraction of scattering from the solvent as background and empty cell contributions. The removal of the incoherent scattering of the sample and the correction of the instrumental resolution are described elsewhere.7 Particular attention has been paid to maintaining the lamellar samples in a polycrystalline state to obtain a powder average. Prior to each neutron scattering run, samples were quenched to preserve a small domain size by first raising the temperature into the isotropic sponge phase regime and then quickly immersing the sample cell into a water bath at T ) 25 °C. Spectra obtained in duplicate runs indicated no sensitivity to details of quenching.

Results and Discussion Phase Behavior. In our previous study of comb-graft poly(acrylate) polymers with the same lamellar surfactant system, we proposed a scaling model that defines the boundaries between homogeneous and biphasic solutions based on two criteria:7 (1) the surface coverage of chain adsorbed on the membrane surface must be less than the available membrane area (Figure 2a) and (2) the interlamellar spacing must be larger than twice the blob size (Figure 2b). In the previous study of hydrophobically modified poly(acrylate) (hmPAA), the area coverage was best described by a two-dimensional random walk of the chain segments between hydrophobes (i.e., blobs). To quantify these criteria, we regard anchored hmpolymer as a swollen planar coil of blobs composed of Nb ) 2Dp - 2 ) 2Mn/mb - 2 loops with average blob size ξ (Figure 3). The blob size is calculated from the “end-toend” distance ()61/2RG ) 0.76mb′1/2 with RG denoting the radius of gyration29) of each blob, with mb′ denoting the “effective” molecular weight between hydrophobes. The effect of finite chain length is observed for the shorter hmPEG polymers. For middle blobs, both ends of the hydrophilic loop are anchored onto the surfactant bilayer to form the blob. Therefore, the effective mb′ is half of the molecular weight of the hydrophilic loop (mb), mb′ ) mb/2. However, the dangling ends would produce blobs with twice the molecular weight compared to blobs with both ends anchored to the membrane surface; therefore, mb′ ) mb (Figure 3). At small Dp (e.g., the cases of hmPEG6k-DP3, hmPEG12k-DP2.5, and hmPEG35k(28) Thiyagarajan, P.; Epperson, J. E.; Crawford, R. K.; Carpenter, J. M.; Klipper, T. E.; Wozniak, D. G. J. Appl. Cryst. 1997, 30, 280-293. (29) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley and Sons: New York, 1989.

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Figure 2. Criteria for the limited miscibility of hm-polymers into the surfactant lamellar phase: (a) the surface coverage of chain segments between hydrophobes (i.e., blobs) must be less than the available membrane area; (b) the interlamellar spacing must be larger than the blob size.

Figure 3. Sketch of number of loops (Dp) on the effective molecular weight of spacer (mb′).

DP2.5), the fraction of ends becomes much greater and the end effect is important. The calculated ξ and Nb are shown in Table 1. The total membrane area (per unit volume) is

Amembrane ≈

2 2φ ≈ d δ

(1)

The polymer hydrophobic anchoring energy per anchor (∼1 kBT × 18 ) 18 kBT30) is large enough to localize the blobs on the membrane surface. The blobs on the surfactant membranes cover an area (per unit volume) of (30) Annable, T. B. R.; Ettelaie, R.; Whittlestone, D. J. Rheol. 1993, 37, 695.

Effect of Amphiphilic Polymer on Lamellar Phase

Acoverage ≈

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( )

CpNA # chains blobs area ≈ Nbξ2 (2) volume chain blob Mw

where NA is Avogadro’s number. This area coverage of blobs, we will show, describes the data better than our previous calculation of area coverage using the twodimensional radius of gyration of the chain of blobs. As will be discussed below, we believe this is because the hmPEG chain has only 3-24 blobs whereas the previous hmPAA chain has 100-220 blobs per chain. Therefore, criterion 1 (i.e., Acoverage ≈ Amembrane) from eq 2 suggests a phase boundary between LR/Lp and LR as

φ ≈ 1/2(Nbδξ2NA/Mw)Cp

(3)

The phase boundary between LR1/LR2 and LR follows from criterion 2 as

φ ≈ (δ/2ξ)

(4)

Figure 4 shows the phase diagram in terms of membrane volume fraction φ versus polymer concentration Cp for hmPEG6k-DP13 (Figure 4a) and hmPEG12k-DP2.5 (Figure 4b). In both hmPEG systems, the reduction of the monophasic lamellar area with increasing polymer concentration is observed. We first compare the upper phase boundaries between LR1/LR2 and LR for the 6 and 12 kg/mol spacers, which should depend only on the average blob size between hydrophobes (ξ) based on criterion 2 (eq 4). The upper phase boundary for the hmPEG12k-DP2.5 system occurs at a surfactant volume fraction about 50% higher than that of the hmPEG6k-DP13 system in the Cp range from 0.5 to 2 wt %. This is consistent with the prediction of the model (φ12k/φ6k ) (43.03/75.06) ) 0.57). However, at Cp ∼ 0.5 wt % and φ ≈ 0.5, eqs 1 and 2 indicate that the blobs cover only 0.3% of the bilayer surface. Therefore, the blobs are more likely to form a monolayer between two surfactant bilayers due to low surface coverage (inset figure in Figure 4). This suggests that the proper criterion for the phase boundary is d > ξ at low Cp and d > 2ξ at higher polymer concentration. The lower phase boundary between LR/Lp and LR should vary linearly with Cp with slope proportional to (Nbξ2/Mw), based on criterion 1. The dotted lines in Figure 4 show that criterion 1 conforms well with the experimental data with values for the prefactors given in the caption, which differ only modestly from the expected values of 1/2(Nbδξ2NA/Mw) ) 0.029 (Figure 4a) and 0.025 (Figure 4b). The maximum amount of polymer that can be incorporated into the surfactant lamellar phase is Cp ) 3.0 and 2.3 wt % for hmPEG12k-DP2.5 and hmPEG6k-DP13, respectively. In our previous study on hydrophobically modified poly(acrylate) (hmPAA), we found the smaller the average blob size (ξ), the larger the Cp that could be incorporated, up to 10 wt % hmPAA. We can compare previous hmPAA data with 3 mol % hydrophobe substitution, which results in average blob size between hydrophobes of ξ ) 26 Å,31 to the hmPEG6k-DP13 with a blob size of ξ ) 43 Å. The maximum amount of polymers that can be incorporated for the hmPAA was 4.3 wt %,31 whereas for the hmPEG6k-DP13 it is 2.3 wt %. As observed for the hmPAA, the smaller the blob size, the larger the range of membrane volume fractions over which homo(31) Yang, Y.; Prud’homme, R. K.; Richetti, P.; Marques, C. In Supramolecular structure in confined geometries; Manne, S., Warr, G. G., Eds.: ACS Symposium Series 736; American Chemical Society: Washington, DC, 1999.

Figure 4. Phase diagram of hmPEG-doped membrane systems: (a) with hmPEG6k-DP13 and (b) with hmPEG12k-DP2.5. One phase lamellar (O); two phases (2). The region in gray is the theoretically predicted region of single-phase behavior of the polymer/lamellar system. The criterion for close-packed polymer blobs filling the membrane surface [criterion 1] is given by the line (s ‚ s), and the criterion for intermembrane blob packing [criterion 2] is given by the two lines depending on whether two layers of blobs or a single layer is assumed (d ∼ 2ξ, s ‚ ‚ s; d ∼ ξ, - - -). The best fits of the expected functional forms based on criterion 1 are φ ) 0.029Cp + 0.05 (a) and φ ) 0.025Cp + 0.05 (b).

geneous one-phase solution can be made.31 For example, at Cp ) 1 wt %, hmPEG6k-DP13 can be incorporated into the surfactant membranes with interlamellar spacing d from 55 to 120 Å (0.20 e φ e 0.43) while the homogeneous polymer-doped phase of the hmPEG12k-DP2.5 (ξ ) 75 Å) system only exists at 120 Å < d < 180 Å (0.13 e φ e 0.20). Membrane Properties. The elastic properties of the lyotropic lamellar phase LR can be characterized by two fundamental smectic elastic constants: bilayer mean bending modulus κ and layer compression modulus B h 32 reflecting interactions between bilayers. The static elastic properties of the membranes were measured by SANS. The scattering intensity from a lamellar phase produces a Bragg peak of q0 with a powerlaw singularity, I(q) ∝ |q - q0|-1+η, where the Caille´ constant η is defined as33

η)

q02 8πxκB h /d

(5)

where d ()2π/q0) is the interlamellar spacing. Structural and thermodynamic properties can be obtained from the positions and shapes of the Bragg peaks, respectively.34 (32) Roux, D.; Safinya, C. R.; Nallet, F. Micelles, Membranes, Microemulsions and Monolayers; Springer-Verlag: New York, 1994. (33) Caille`, A.; Heb, C. R. C. R. Acad. Sci. B 1972, 274, 1733.

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Figure 5. Scattering intensity I(q) (O) of C12E5/C6OH/0.1 M NaCl(D2O)/hmPEG6k-DP13 at φ ) 0.20 and Cp ) 1 wt %. I(q) from the model of Nallet et al. (ref 35) (s) with the fitting parameters of d ) 131.95 Å, η ) 0.31, and δ ) 24 Å.

In this study, a model proposed by Nallet et al.35 was used to obtain η, d, and δ, with the details addressed elsewhere.7,36 The Caille´ constant η and the interlamellar distance d are extracted from the structure factor S(q), while form factor P(q) yields the membrane thickness δ. Figure 5 exemplifies the fit of an experimental scattering curve to the Nallet model (C12E5/C6OH/0.1 M NaCl(D2O)/ hmPEG6k-DP13 with φ ) 0.2 and Cp ) 1 wt %). The fitted membrane thickness δ of 24 ( 2 Å remains unchanged for all the samples studied, which is consistent with our previous observation.36 The second-order peak in the hmPEG system at q ) 0.09 Å-1, which was not observed in the bare membrane and hmPAA systems,7 indicates the better orientation of the lamellar phase with the addition of hmPEGs. In the following, the “normalized spectra” are shown after dividing the intensities I(q) by their respective peak values I(q0) and subtracting the scattering vector q by the peak positions q0. Effect of Polymer Architectures. Three polymer architecture parameters (Mw, mb, and Dp) were varied by examining all six hmPEGs shown in Table 1. The following aspects will be discussed: (a) At constant number of loops Dp, what is the effect of loop size mb? (b) At constant molecular weight Mw, what is the effect of loop size mb? (c) At constant loop size mb, what is the effect of number of loops Dp? (a) Same Dp, Different mb. Figure 6 shows the SANS spectra of hmPEG-doped lamellar systems for hmPEGs with 3 loops but different spacings between hydrophobes (mb ) 6, 12, and 35 kg/mol). The normalized spectra for systems with 6 and 12 kg/mol spacing hmPEGs are shown in the inset figure. For the two hmPEGs with smaller mb (6 and 12 kg/mol), two features are observed. The first is the reduction in the width of the Bragg peak with decreasing molecular weight of the PEG spacer. This indicates a decrease of the Caille´ constant η (eq 5) and, therefore, an increase in rigidity (i.e., the product κB h ).7 The rigidity (κB h ) arises naturally from the Caille´ theory where it describes the stiffness of the membrane stack to deformation. It encompasses both single-membrane contributions from κ and intermembrane interactions from (34) Joannic, R.; Auvray, L.; Lasic, D. D. Phys. Rev. Lett. 1997, 78, 3402. (35) Nallet, F.; Laversanne, R.; Roux, D. J. Phys. II France 1993, 3, 487. (36) Yang, B.-S.; Lal, J.; Mihailescu, M.; Monkenbusch, M.; Richter, D.; Huang, J. S.; Russel, W. B.; Prud’homme, R. K. In Lecture Notes in Physics: Neutron Spin Echo Spectroscopy - future aspects and applications; Springer: New York, 2001.

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Figure 6. SANS spectra of hmPEG-doped lamellar systems with hmPEGs of 3 loops. The polymer concentration and membrane volume fraction are fixed at 0.5 wt % and 0.2, respectively. The molecular weights of the PEG spacers between hydrophobes are 6 (hmPEG6k-DP3, O), 12 (hmPEG12k-DP2.5, 2), and 35 (hmPEG35k-DP2.5, 0) kg/mol. In the inset, the normalized spectra are for the systems with hmPEGs of 6 and 12 kg/mol spacers.

B h . This result parallels our observations for random combgraft hmPAAs. Second, the scattering at small q is greater with larger mb (12 kg/mol). The small q limit, I(qf0), reflects the layer compression modulus B h , which depends on the bilayer/bilayer interaction.7 The stronger the repulsion between bilayers, the more fluctuations in the intermembrane spacing are suppressed. Since I(qf0) ∝ 1/B h , reduced scattering at low angles corresponds to an increase of B h with decreasing mb, though a quantitative characterization is not possible since the low angle limit is not reached. Performing scattering to lower q will probably not resolve the question because scattering from grain boundaries will become significant. For the mb ) 35 kg/mol system, the appearance of two Bragg peaks at q1 ) 0.022 Å-1 and q2 ) 0.056 Å-1 reflects scattering from two coexisting lamellar phases in Figure 6. The peak positions determine the interlamellar distances (d ) 2π/q) as d1 ) 114 Å and d2 ) 300 Å, while that for the system without polymer is d0 ) 159 Å. The blob size ξ of hmPEG with spacers of 35 kg/mol is 128 Å, which is comparable to d1 ) 114 Å but much smaller than d2 ) 300 Å. The addition of polymer produces one bilayer spacing that is approximately 2ξ (see Figure 2b) and one with a spacing smaller than the bilayer spacing without polymer. After centrifugation at 3000 rpm for 1 h, a phase boundary between two birefringent phases appears, confirming the biphasic behavior. The phase separation can be explained by criterion 2: the average interlamellar spacing is not sufficiently greater than the blob size. When the average interlamellar spacing is close to or below the blob size, the system is forced to phase separate into two coexisting lamellar phases LR1/LR2, one of which can accommodate the polymer while the other is compressed and devoid of polymer.7 Similar behavior is also predicted theoretically by Brooks et al.2 We compare the neutron scattering spectra of the two coexisting lamellar phases (Figure 6) of the hmPEG35k-DP2.5 system with those of the corresponding bare membrane systems, that is, d1 ) 114 Å at φ1 ) 0.20 and d2 ) 300 Å at φ2 ) 0.08, calculated by φ ) δ/d. The width of the Bragg peak at d1 ) 114 Å is similar to that of the bare membrane system at φ1 ) 0.20, while the peak width at d2 ) 300 Å is narrower than that of the bare membrane system at φ2 ) 0.08. Both the spacings and the relative stiffening of the LR phase with large spacing indicate that polymers are concentrated in

Effect of Amphiphilic Polymer on Lamellar Phase

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Figure 7. The normalized SANS spectra of hmPEG-doped lamellar systems with hmPEGs of molecular weight around 113 kg/mol (hmPEG6k-DP13 (O) and hmPEG12k-DP6 (2)). The polymer concentration and membrane volume fraction are fixed at 0.5 wt % and 0.2, respectively. The data have been normalized with respect to the positions and intensities of the scattering peaks.

the lamellar phase with d2 ) 300 Å but are excluded from the phase with d1 ) 114 Å. (b) Same Mw, Different mb. The effect of the molecular weight between hydrophobes (mb) at fixed molecular weight of polymer chain (Mw) is demonstrated in Figure 7 with the normalized SANS spectra of hmPEG6kDP13 (mb ) 6 kg/mol; Mw ) 112 kg/mol) and hmPEG12kDP6 (mb ) 12 kg/mol; Mw ) 113 kg/mol), at Cp ) 0.5 wt % and φ ) 0.2. The breadth of the Bragg peaks and the scattering intensity at low angles increase as the molecular weight between hydrophobes increases, indicating that the Caille` constant η and the compressibility both increase. Similar behavior was observed in our previous study on the hmPAA systems. We found that the rigidity and compression moduli of membranes increase with increasing hydrophobe substitution level (i.e., decreasing molecular weight between hydrophobes) at fixed Mw, Cp, and φ.7 (c) Same mb, Different Dp. Here, we study systems with the same molecular weight of hydrophilic segments (i.e., mb) but with a different number of loops. In Figure 8, parts a and b depict the behavior with hmPEGs of mb ) 6 and 12 kg/mol, respectively. In Figure 8a, rigidity and compression modulus increase with increasing number of loops (Dp). This behavior is different from the hmPAA systems in which the membrane properties are insensitive to overall polymer molecular weight and polydispersity.7 As mentioned earlier, the effect of finite chain length is observed with the hmPEG, because of the modest number of loops per polymer chain (Figure 3). In Figure 8a, hmPEG6k-DP3 actually has a larger effective blob size than the hmPEG6k-DP13, even though both polymers have the same PEG spacer molecular weight, because dangling end loops have a higher effective molecular weight than the internal loops between hydrophobic anchors. The number of ends becomes more significant for the chains with only 3 loops. In Figure 8b, hmPEG12kDP5 and hmPEG12k-DP6, which have a 20% difference in number of loops, have a similar rigidity and compression modulus. Effect of Polymer Concentration. The SANS spectra of the lamellar phase doped with hmPEG6k-DP13 (mb ) 6 kg/mol; Dp ) 13) at φ ) 20% under different polymer concentrations are shown in Figure 9. The addition of polymer reduces the width of Bragg peak, indicating a decrease of the Caille´ constant η and, therefore, an increase in rigidity (i.e., the product κB h ). The reduction of the

Figure 8. The normalized SANS spectra of hmPEG-doped lamellar systems with hmPEGs with the same spacing between hydrophobes but a different number of loops: (a) spacing ) 6 kg/mol, Dp ) 3 (O) and 13 (2); (b) spacing ) 12 kg/mol, Dp ) 5 (]) and 6 (/). The polymer concentration and membrane volume fraction are fixed at 0.5 wt % and 0.2, respectively.

Figure 9. SANS spectra of hmPEG6k-DP13-doped lamellar systems with different polymer concentrations: Cp ) 0 wt % (- - -), 0.2 wt % (O), 0.5 wt % (2), 0.8 wt % (]), and 1.0 wt % (/). The membrane volume fraction is fixed at 0.2.

intensity at low angles with increasing polymer concentration corresponds to an increase of B h since I(qf0) ∝ 1/B h .7 Figure 10 summarizes the polymer concentration dependence of the Caille` constant η for all the samples within the single lamellar phase region, for φ ) 20%. From the SANS spectra and fitting of η, the rigidity and compression modulus increase with increasing polymer concentration. Similar behavior was also observed in the hmPAA systems.7 Conclusion In this study, we demonstrate the effects of a strictly alternating comb-graft amphiphilic polymer based on PEG (hmPEG) on the phase behavior and membrane properties

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Figure 10. Polymer concentration dependence of membrane rigidity (η) for systems with hmPEG6k-DP3 (O), hmPEG6kDP13 (2), hmPEG12k-DP5 (b), and hmPEG12k-DP6 (×). The membrane volume fraction φ is fixed at 0.2 for all the samples.

of the surfactant lamellar phase using direct observation of the phase behavior and small-angle neutron scattering: (a) For lamellar phases doped with hmPEG with larger PEG spacers, the monophasic LR region exists at a narrower membrane volume fraction φ range but a larger polymer concentration Cp range compared to that of smaller spacing hmPEG. (b) Under the condition of either equal number of loops or equal total molecular weight, the smaller the PEG spacer, the larger the rigidity and compression modulus of the polymer-doped surfactant membranes. (c) For fixed PEG spacing, the rigidity and compression modulus of the doped membranes are larger with more loops. This is in contrast to our previous study with hydrophobically modified poly(acrylate) polymer (hmPAA) which showed dependencies on loop sizes but not on the total number of loops. Since the hmPAA polymers had from 50 to 110 loops per chain whereas the hmPEG polymers have from 2.5 to 13 loops, we suggest that the different behavior may be attributed to the finite size effect of few numbers of loops. (d) Rigidity and compression of membranes increase with increasing polymer concentration.

Yang et al.

(e) The criteria for the phase boundaries for polymer and lamellar phase systems has been developed based on a model of the polymer interaction with the surfactant membrane. To form a homogeneous monophasic hmpolymer/lamellar phase mixture, (1) the surface coverage of chain segments between hydrophobes (i.e., blobs) must be less than the available membrane area and (2) the interlamellar spacing must be larger than twice the blob size. At low polymer surface coverage, the blobs are more likely to form a monolayer between two surfactant bilayers. This suggests that the proper criterion for the phase boundary at low surface coverage is d > ξ instead of d > 2ξ for higher surface coverage. We have proposed the existence of the transition from membranes separated by two layers of blobs (one attached to each membrane surface) to membranes separated by a single layer of blobs. This is suggested by the macroscopic phase behavior. Neutron scattering studies that would focus on the polymer conformation on the membrane surfaces would provide additional insight. Our previous studies on the hydrophobically modified poly(acrylate) (hmPAA) systems found the membrane rigidity (the product κB h ), compression modulus B h , and bilayer mean bending modulus κ to be independent of molecular weight, polydispersity, and hydrophobe length for hmPAAs. The rigidity and compression moduli of membranes increase with increasing polymer concentration and with decreasing blob size between hydrophobic anchoring sites.7 For the model comb-graft hm-polymers, hmPEGs, the rigidity and compression modulus also increase with polymer concentration and blob size of hmpolymers. As mentioned above, the dependence on the number of loops is a consequence of the finite chain effects for the hmPEGs. Overall, it appears that the uniformity of spacing is not critical for controlling the phase behavior of mixtures of hm-polymer with concentrated surfactant lamellar phases. This new hmPEG polymer is a potential and attractive candidate for biomedical applications of these concepts because of its biocompatibility. Its interaction with phospholipid membranes is currently under study. LA0105533