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Langmuir 2005, 21, 10038-10045
Effect of Hydrophobically Modified Polymers on Shear-Induced Multilamellar Vesicles† Bing-Shiou Yang, William B. Russel, and Robert K. Prud’homme* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544-5263 Received April 22, 2005. In Final Form: August 3, 2005 The effects of polymer concentration, polymer molecular weight, and hydrophobe substitution level of modified poly(acrylic acid) polymers on the formation, size, and viscoelastic properties of shear-induced multilamellar vesicles (onions) are studied by rheology and light diffraction. The onions are close-packed, space-filling vesicles formed by shearing aqueous lamellar phases of C12E5 surfactant to produce phases with sufficient order and size uniformity (O(1-3 µm)) to diffract light. The addition of hydrophobically modified polymers enhances the rate of formation, uniformity, and stability independent of hydrophobe substitution level. Onion size decreases with increasing shear rate as observed for pure surfactant onion systems, but the shear-rate dependence is changed by the polymer. The onion phase has a plateau modulus that increases with polymer concentration but is independent of hydrophobe substitution level or molecular weight. The model presented by Panizza et al. that relates the plateau modulus of the onion phase to membrane rigidity and the compression modulus is consistent with independent measurements of membrane properties from SANS.
Part of the Bob Rowell Festschrift special issue. * To whom correspondence should be addressed. E-mail:
[email protected].
order and size uniformity.4,8 At high concentrations, these multilamellar vesicles may be so close-packed that the outer layers deform into a polyhedral structure similar to that of high internal phase ratio foams and emulsions.9 For even higher shear rates, the vesicles break and form a perfectly aligned lamellar phase that is essentially defectfree.8,10 The intermediate onion phases are metastable after shear is stopped, though the time scale for reversion to the planar structure has not been reported. The stabilization of onion against recoalescence back to the lamellar phase is a major focus of this work. We have previously characterized the effect of hydrophobically modified polymers (hm-polymers) that increase the rigidity of surfactant lamellar phases using smallangle neutron scattering.1,2,11,12 In this study, we demonstrate that multilamellar vesicles made by shearing hm-polymer-doped surfactant membranes form more rapidly and have greater stability than vesicles formed without polymers. This makes the polymer-modified vesicles (Figure 1) attractive candidates for encapsulation and controlled delivery applications. We systematically study the effect of polymer concentration, molecular weight, and hydrophobe substitution level of hm-polymers on the formation, size, and rheological properties of the onions. Section II describes the materials and the experimental techniques applied in this study. Section III presents the rheology and light diffraction results on the effects of polymer concentration, molecular weight, and hydrophobe substitution level on the onion properties. The rheological properties are compared to the theory of Pannizi8 and are found to correlate qualitatively with the predictions of the model. The final section summarizes the main conclusions and suggestions for future research.
(1) Yang, B.-S.; Lal, J.; Marques, C. M.; Richetti, P.; Russel, W. B.; Prud’homme, R. K. Langmuir 2001, 17, 5834-5841. (2) Yang, B.-S.; Lal, J.; Kohn, J.; Huang, J. S.; Russel, W. B.; Prud’homme, R. K. Langmuir 2001, 17, 6692-6698. (3) Rosoff, M., Ed.; Vesicles; Surfactant Science Series 62; Marcel Dekker: New York, 1996. (4) Diat, O.; Roux, D. J. Phys. II France 1993, 3, 9-14. (5) Lasic, D. D. Angew. Chem. 1994, 106, 1765. (6) Diat, O.; Roux, D.; Nallet, F. J. Phys. II 1993, 3, 1427. (7) Lasic, D. D. Biochim. J. 1988, 256, 1-11. (8) Panizza, P.; Roux, D.; Vuillaume, V.; Lu, C. Y. D.; Cates, M. E. Langmuir 1996, 12, 248-252.
(9) Partal, P.; Kowalski, A. J.; Machin, D.; Kiratzis, N.; Berni, M. G.; Lawrence, C. J. Langmuir 2001, 17, 1331-1337. (10) Escalante, J. I.; Gradzielski, M.; Hoffmann, H.; Mortensen, K. Langmuir 2000, 16, 8653-8663. (11) Yang, B.-S.; Lal, J.; Mihailescu, M.; Monkenbusch, M.; Richter, D.; Huang, J. S.; Russel, W. B.; Prud’homme, R. K. In Neutron SpinEcho SpectroscopysFuture Aspects and Applications; Lecture Notes in Physics; Springer: New York, 2002. (12) Yang, B.-S.; Lal, J.; Mihailescu, M.; Monkenbusch, M.; Richter, D.; Huang, J.; S.; Kohn, J.; Russel, W. B.; Prud’homme, R. K. Langmuir 2002, 18, 6-13.
I. Introduction Surfactants may self-assemble into bilayers in the form of continuous lamellar1,2 or closed spherical lamellar structures, usually called multilamellar vesicles (MLVs), spherulites,3 or liposomes when composed of phospholipids.4 Vesicles have a wide range of applications in cosmetics and emulsions and for the encapsulation and controlled delivery of pharmaceuticals.5 Several methods have been developed to improve the performance of vesicles by controlling the size and polydispersity,6 encapsulation ratio (defined as the volume of solvent inside the vesicles relative to the total volume of solvent4,7), and stability.7 One of the most promising approaches to forming MLVs “onions” with high encapsulation ratios is the shearinduced process proposed by Roux’s group,4,6,8 which consists of shearing the planar lamellar phase under carefully controlled conditions. Roux and co-workers have constructed an “orientation” or “shear diagram” to describe the rheological response of a lyotropic lamellar phase under shear.4,6,8 At very low shear rate, the texture of the sample is inhomogeneous on macroscopic scales (>10 µm), whereas microscopic observation reveals a multidomain lamellar phase aligned parallel to the wall of the rheometer cell. At a threshold value of shear that is a function of the surfactant concentration, the bilayers break up because of an elastic instability4,6 and are transformed into vesicles. With increasing shear rate, a homogeneous vesicle phase is produced that can exhibit a high degree of translational †
10.1021/la0510836 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/18/2005
Hydrophobically Modified Polymers
Langmuir, Vol. 21, No. 22, 2005 10039 Table 1. Details of Hydrophobically Modified Polymers name (abbreviation)
Mw hydrophobe hydrophone M w/ length (kg/mole) Mn substitution (%)
HMPAA3%250kpoC14 HMPAA3%140kpoC14 HMPAA1.5%250kpoC14
Figure 1. Sketch of the anticipated structure of the hydrophobically modified polymer-doped shear-induced multilamellar vesicles (onions). The hydrophobic groups along the polymer backbone associate with the membranes forming polymer-doped membranes. The enlarged picture is the chemical composition of hm-polymer with the repeat units distributed randomly. The hydrophobe substitution level s and the number outside the repeat units represent molar percentages.
250 140 250
2 2 2
3 3 1.5
C14 C14 C14
levels, by 1H NMR. The compositions of the hmPAA samples are listed in Table 1. Onion Sample Preparation. The lamellar-phase samples were prepared by mixing surfactant (C12E5), alcohol (C6OH), and stock solutions of the hm-polymers by weight in 0.1 M NaCl(H2O). Samples are formulated according to our previous study on the phase behavior.1 In the following text, we define the membrane volume fraction φ as the volume of C12E5 plus C6OH divided by the total sample volume with the conversion from weight to volume fraction made with densities (kg/m3) of 996 (C12E5), 820 (C6OH), 1105 (D2O), and 998 (H2O), neglecting the effect of the added polymers on the solvent density. The volume fraction φ is fixed at 0.2 for all of our samples in this onion study. The onion samples were prepared by shearing the corresponding lamellar phase (with or without polymer) at a constant shear rate for 2 h after a constant stress was reached, with the formation process monitored through the time dependence of the viscosity (transient flow measurement). We have observed that this length of time is needed to form onions with optimal organization to produce the strongest diffraction patterns.16 Onion formation was conducted at 25 °C in the rheometer. This temperature was chosen because it is at least 7 °C away from any phase boundary in the lamellar/hm polymer phase diagram.1 After shear is stopped, light diffraction and dynamic oscillatory rheological measurements detect the dimension and rheological properties of the onions, respectively.
II. Experimental Section Systems and Materials. The surfactant lamellar phase consists of the nonionic surfactant penta(ethylene glycol) dodecyl ether (C12E5; >99%, Nikko Chemical Co. Ltd., Tokyo) and 1-hexanol (>99%, Fluka), which were used as received. Adding hexanol to the membrane reduces the rigidity13 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 O(kBT).13 The solvent phase is 0.1 M NaCl(H2O). The presence of salt in the solution effectively screens the electrostatic interactions of the added anionic polymers, reducing the Debye length lD to 10 Å. Thus, the hydrophobically modified PAA (hmPAA) can be viewed as an almost neutral chain with an average number of 1900 (for Mw ) 140 kg/mole) or 3500 (for Mw ) 250 kg/mole) backbone monomers. With the electrostatic repulsion screened, the undulation force is the dominant long-range repulsion. The hmPAA’s were synthesized by grafting onto precursor poly(acrylic acid) n-tetradecylamine (C14) in the presence of dicyclohexylcarboiimide (DCC) using n-methylpyrrolidinone (NMP) as the solvent.14 This produces a random distribution of hydrophobic side chains along the PAA backbone.15 Precursor polymers were handled as follows. An aqueous solution of PAA (Aldrich; Mw ) 250 kg/mole, Mw/Mn ≈ 2) was freeze dried before use. Sodium poly(acrylate) polymer was converted to the acid from PAA by diluting the 25 wt % solution NaPAA (Polysciences, Inc.; Mw ≈ 140 kg/mole, Mw/Mn ≈ 2) with DI water and then passing it through a bed of Amberlite IR-120 Plus acidic cation exchanger (Supelco, Inc.) before freeze drying. After the grafting reaction, the modified polymers were precipitated from the NMP solvent by adding concentrated NaOH (40 wt % aqueous solution). The degree of polymerization remains the same as that of the precursor with the typical structure illustrated in the inset of Figure 1. Sodium acrylate neutralization of 75 ( 1.5 mol % was determined by elemental analysis, and hydrophobe substitution (13) Freyssingeas, F.; Nallet, E.; Roux, D. Langmuir 1996, 12, 6028. (14) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988, 20, 577. (15) Magny, B.; Lafuma, F.; Iliopoulos, I. Polymer 1992, 33, 3151.
Figure 2. Sketch of the setup of our homemade light diffraction apparatus. Light Diffraction Measurements. A homemade light diffraction apparatus, depicted in Figure 2, characterized the size of the onions. After shearing for 2 h, we carefully removed the sample from the rheometer and placed it into a 10 mm path length disposable cuvette (Fisher Scientific). The sample stays stable for over a month after sealing the top of the cuvette with Parafilm M (Structure Probe, Inc.) at ambient temperature (∼2022 °C, which is again more than 7 °C away from lamellar phase boundaries1). Light diffraction was observed by sending a laser beam (10 mW He-Ne laser; wavelength λ ) 0.6328 µm) through the cell and observing the diffraction pattern on a Mylar (Grafix Plastics) screen situated 60 mm from the cell. The image of the diffraction ring was captured with a CCD camera and a PC. The onion size D is calculated from the radius of the diffraction ring R and the distance between the sample cell and the screen S according to the following equation:
D)
λ 1 R 2n sin tan-1 2 S
[
( )]
(1)
Rheological Measurements. Both transient and oscillatory rheological measurements were conducted with a Rheometrics
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Figure 3. Time dependence of viscosity for the lamellar phase under a shear rate of 10 s-1 at hmPAA3%250kpoC14 concentrations of 0 (O), 0.2 ([), 0.5 ([), or 2 wt %(2) and a membrane volume fraction φ of 0.2. Fluids Spectrometer II (Rheometrics Inc., Piscataway, NJ) using a cone-and-plate geometry (50 mm diameter, 4° cone). A homedesigned trap was used to prevent solvent evaporation during the measurements. a. Transient Measurements. Starting from the lamellar-phase solution, the time dependence of the steady-state viscosity of the sample was measured within the 2 h shearing period, with the shear rate held constant in the range of 1.2-100 s-1. Then the onion sample was carefully transferred with a spatula into a transparent plastic cuvette cell for the light diffraction experiment. b. Oscillatory Measurements. After preshearing for 2 h at a constant shear rate, the storage (G′) and loss (G′′) moduli of the onion phase were measured as a function of frequency (0.01 to 100 Hz). Strain sweep experiments were initially performed to determine the linear viscoelastic region, where the modulus is independent of strain. All of the measurements were conducted within the linear region at 25 °C.
III. Results and Discussion Onion Formation. Figure 3 shows the transient viscosity of the bare and hm-polymer-doped membrane systems at different polymer concentrations for a shear rate of 10 s-1. The viscosity increases with time to a steady state that depends on the polymer concentration (Figure 3) and shear rate (Figure 4). The sample is sheared for 2 h after the attainment of steady state and then carefully transferred to a disposable cuvette cell for the light diffraction measurement. A diffraction ring is observed, indicating an isotropic and spherical structure with a characteristic size, and often accompanied by a second ring or second-order peak attesting to the quality of the order. The membrane volume fraction φ ) 0.2 corresponds to an interlamellar spacing d of around 150 Å,1,2 whereas the radius of the diffraction ring indicates a characteristic size on the order of a few micrometers, consistent with earlier confirmation that these spherical objects are multilamellar vesicles.4,8,16 Our previous study using simultaneous shear and light diffraction (Rheovisiometer RVM3, Rheocontrol) found the viscosity to reach steady state when the onions begin to diffract light strongly.16 Therefore, we use the rheological transition to follow the formation of onions. Figure 3 shows that the addition of polymer increases the rate of formation and the steadystate viscosity of the onion phase, reducing the time (16) Yang, Y.; Richetti, P.; Marques, C. M.; Herve, P.; Prud’homme, R. K. Personal communication with the Rhodia Complex Fluids Lab.
Figure 4. Time dependence of viscosity for the polymer-doped lamellar phase under different shear rates of 5 (9), 10 (2), 30 ([), 60 (*), and 100 s-1 (O) at Cp ) 0.2 wt % and φ ) 0.2 for (a) hmPAA3%140kpoC14 and (b) hmPAA3%250kpoC14.
required from 500 s in the absence of hm-polymer to 60 s with 2 wt % added polymer. The increased viscosity with increasing polymer concentration may be due to one or more effects: (1) at higher polymer concentrations, the polymer may bridge between adjacent vesicles or (2) the higher density of adsorbed polymer may increase the effective volume fraction of the vesicles, which leads to an increase in viscosity as is seen for emulsions.17 A study (17) Princen, H. M.; Kiss, A. D. J. Colloid Interface Sci. 1989, 128, 176-187.
Hydrophobically Modified Polymers
Figure 5. Time dependence of viscosity for the hmPAA1.5%250kpoC14-doped lamellar phase under different shear rates of 5 ([), 10 (2), 30 ([), 60 (*), and 100 s-1 (O) at Cp ) 0.2 wt % and φ ) 0.2.
of bridging and sliding forces with a surface forces apparatus would indicate the relative importance of mechanisms 1 and 2, whereas cryo-TEM might determine changes in the overall volume fraction of the vesicle phase.18 The effects of shear rate and molecular weight of hmPAAs on onion formation are shown in Figure 4 for a relatively low hm-polymer concentration (Cp ) 0.2 wt %). For hmPAA3%140kpoC14 (Figure 4a) and hmPAA3%250kpoC14 (Figure 4b), the onion phase forms faster at higher shear rate but results in a lower steady-state viscosity. This inverse correlation between rate and viscosity is also seen for the formation of onions in pure surfactant systems4 and is more prominent at lower shear rates (
