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Articles Cylindrical Micelles of Wormlike Polyelectrolytes† R. Rulkens, G. Wegner,* and T. Thurn-Albrecht Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55028 Mainz, Germany Received August 5, 1998. In Final Form: November 30, 1998 We report on a study of the association behavior in solution of semirigid polymers consisting of a poly(p-phenylene) backbone with alternation of negatively charged and hydrophobic moieties. Small-angle X-ray scattering data of aqueous solutions of sulfonated poly(p-phenylene)s show aggregated polymer chains in the form of cylindrical micelles, in which the phenylene backbone is oriented parallel to the long axis of the micelle. The charge density and the length of the alkyl side chains influence the diameter and aggregation number of the micelles. Micelles with strand aggregation numbers between 3 and 11 are formed depending on the hydrophobic-hydrophilic balance. The observed behavior resembles the formation of supermolecular structures by biogenic semirigid polyelectrolytes.
Introduction Many biogenic polymers are electrolytes of wormlike, that is, semirigid, nature and have the tendency to selfaggregate into well-defined superstructures. The selfaggregation is intrinsic to the biological function. The number of chains, or strands, that form an aggregate depends on the type of interaction between the individual chains, in addition to other parameters such as solvent quality, temperature, and pressure. DNA, as the most prominent example, may exist as a double helix of two strands. An example of a biopolymer in which three strands form a helical superstructure is collagen. It is tempting to create synthetic polymers that model the complex behavior of such biogenic materials, at least with respect to the formation of well-defined superstructures. Synthesis offers the possibility to vary the charge density, hydrophobicity, and stiffness of the chains in a systematic way. Only a few studies that describe the synthesis and certain properties of semirigid polyelectrolytes are known to the authors.1 The aggregation behavior of these synthetic polymers was not studied. We have recently reported on the synthesis and characterization of semirigid charged polymers with a poly(p-phenylene) backbone.2,3 The polymers described vary with regard to charge density and hydrophobicity. Molecular weight and weight distribution are well-known * Corresponding author. † Presented at Polyelectrolytes ‘98, Inuyama, Japan, May 31June 3, 1998. (1) (a) Dang, T. D.; Arnold, F. E. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1992, 33, 912. (b) Gieselman, M. B.; Reynolds, J. R. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1992, 33, 931, 1065. (c) Foo, C.; Okamoto, T. K.; McBreen, J.; Lee, H. S. J. Polym. Sci. Polym. Chem. 1994, 32, 3009. (d) Pfeiffer, D. G.; Kim, M. W.; Kaladas, J. Polymer 1990, 31, 2152. Rau, I. U.; Rehahn, M. Acta Polym. 1994, 45, 3. (f) Chaturvedi, V.; Tanaka, S.; Kaeriyama, K. Macromolecules 1993, 26, 2607. (g) Wallow, T. I.; Novak, B. M. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1992, 33, 727. (h) Child, A. D.; Reynolds, J. R. Macromolecules 1994, 27, 1975. (i) Kowitz, C. Ph.D. Thesis, University of Mainz, Germany, 1996. (2) Rulkens, R.; Schulze, M.; Wegner, G. Macromol. Rapid Commun. 1994, 15, 669. (3) Rulkens, R.; Wegner, G.; Enkelmann, V.; Schulze, M. Ber. BunsenGes. Phys. Chem. 1996, 100, 707.
Figure 1. Sulfonated poly(p-phenylene)s 1-4. For R, R′, M, and n*, see Table 1. Table 1. Characteristic Data of Sulfonated Poly(p-phenylene)a 1 2 3 4
R, R′
M
n*
R (nm)
arad
CH3, C12H25 H CH3 CH3, C12H25
N(CH3)4 Na Na Na
38 27 105 38
3.1 ( 0.2 1.2 ( 0.1 1.0 ( 0.1 1.9 ( 0.1
11 4.4 2.9 6.7
a *n was determined from membrane osmometry experiments of the precursor polymers.3 n* is the degree of polymerization in terms of the number of phenylene units in the backbone, arad is the strand aggregation number, R (nm) is defined by eq 3; for M see Figure 1.
from the analysis of the uncharged precursor polymers.4 A persistence length of about 13 nm was determined for the precursor polymers, which may be taken as the lower bound of the persistence length of the polyelectrolytes under study. Saponification of the precursor polymers, in which sulfonate arylester substituents correspond to the later sulfonate groups, resulted in the poly(p-phenylene)s, which are shown in Figure 1. The meaning of R, R′, M, and n is listed in Table 1. Synthesis and characterization of 1-4 have been described in the literature under the same notation.2,3 In the following, we describe the structure of the aggregates that form spontaneously when these polyelectrolytes in the form of sodium or tetramethylammonium salts come in contact with water and form micelles. The major method of investigation is small-angle X-ray scattering (SAXS) in the semidilute concentration regime. SAXS is a well-established technique for structural (4) Vanhee, S.; Rulkens, R.; Rosenauer, C.; Schulze, M.; Ko¨hler, W.; Wegner, G. Macromolecules 1996, 29, 5136.
10.1021/la9809832 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/20/1999
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analysis of materials with heterogeneities on the length scale of a few nanometers to several tens of nanometers. SAXS has been used to investigate structures formed in solutions of charged rodlike chains by extracting information from the scattering profiles.5 Several concentration regimes have been identified in which structures appear or undergo changes. In our case, the experimental results are analyzed so as to obtain information on the shape, size, and packing of rodlike micelles formed in aqueous solutions of the polymers mentioned in Table 1. Small Angle X-ray Studies For reasons to become clear below, we restrict the discussion of the SAXS data to the case of a solution of rodlike particles. Under the condition Lq . 1, with L being the length of the rodlike particle and q the scattering vector, only the structure in a direction normal to the direction of the rod contributes to the scattering diagram. By approximating the micelles as infinitely long cylindrical particles, the scattered intensity can be written as a simple product of a particle form factor F(q) and a structure factor S(q), both referring to the structure within the plane normal to the rod:
I(q) ∝ |F(q)|2S(q)
(1)
The function F(q) describing the form factor of such cylindrical particles is known:6
I(q)q ) K
[
]
J1(qR) 2 S(q) qR
(2)
This expression is reduced in the limits of qR e 1 to the Guinier approximation
I(q)q ) A exp(- 1/2 Rc2q2)
(3)
Here K is a q independent contrast factor, which depends on the concentration, the scattering volume, and the scattering contrast of the particles in the solvent. Rc ) 2R is the radius of gyration of the cross-section area. J1(x) is the first-order Bessel function. Eq 2 will be valid for qdint , 1, with dint being a typical length scale for the internal structure of the rodlike particle. Being able to describe the form factor by this simple model, which contains only two parameters, i.e., the radius R and the factor K, we can separately determine the form factor |F(q)|2 from the experimental data and subsequently obtain the structure factor by dividing the intensity I(q) by |F(q)|2. Assuming a locally parallel packing of rods, qmax can be interpreted in terms of a reciprocal average distance between adjacent rods. If the rods pack in the form of a hexagonal lattice, qmax will be interpreted as a Bragg reflection of a lattice plane of distance dB:
dB ) 2π/qmax
(4)
The meaning of R and dB for hexagonal packing of rods is outlined in Figure 2a. As will be shown below, the observed rods are cylindrical micelles composed of arad strands. By neglecting any possible small contribution from isolated chains, it is possible to calculate the aggregation number arad from dB and the concentration of chains. Eq 5 may change with regard to the numerical factors contained if the packing symmetry changes from hexagonal to any other type. (5) (a) Koch, M. H. J.; Syers, Z.; Sicre, P.; Svergun, D. Macromolecules 1995, 28, 4904. (b) Castello, V.; Itri, R.; Amaral, L. Q. Macromolecules 1995, 28, 8395. (c) Luzzati, H.; Masson, F.; Mathis, A.; Saludjian, P. Biopolymers 1967, 5, 491. (d) Kirste, R. G.; Oberthu¨r, R. C. In Small Angle X-ray Scattering, 2nd ed.; Glatter, O., Kratky, O., Eds.; Academic Press: New York, 1983; p 373. (e) Bernal, J. D. Gen. Physiol. 1941, 25, 111. (f) Wintermantel, M.; Fischer, K.; Gerle, M.; Ries, R.; Schmidt, M.; Kajiwara, K.; Urakawa, H.; Wataoka, I. Angew. Chem. 1995, 107, 1606. (6) Glatter, O.; Kratky, O. Small Angle X-ray Scattering, 2nd ed.; Academic Press: London, 1983; p 32. (7) Strobl, G. Acta Cryst. 1970, A26, 367.
1 x3a ) ( 2 )
1/2
rad
dB
cA1/2
(5)
cA is the number of chains crossing a unit area perpendicular to the direction of the chains and is related to the weight per volume concentration c (in g/nm3) by:
cA )
cNALrep Mrep
(6)
with the Avogadro number NA ) 6.022 × 1023 mol-1, and Lrep and Mrep being the length and the molecular weight of the repeat unit, respectively.
Results and Discussion All of the polymers could be cast from aqueous solutions to form homogeneous films. These films gave rise to wideangle X-ray diffraction patterns. Figure 2b shows such a X-ray diffraction pattern of 3-a as a typical representative with the incident beam in the film plane. The film was cast from a dilute solution in salt-free water on a Teflon surface followed by very slow evaporation of the water. The final films dried at room temperature contained 2-5 wt % of water. The sixfold symmetry of the main Bragg reflection in the diffraction pattern indicates a hexagonal packing. The corresponding layer distance determined from the meridional reflections is 12.8 Å. The fact that the diffraction pattern shows many orders of a meridional reflection associated with the layer distance demonstrates that the film is highly oriented with the molecules extending in the layer plane. A diffraction pattern obtained with the primary beam perpendicular to the layer plane (not shown here) exposed only Debye rings of the same Bragg values. By using eq 5 it follows from the magnitude of dB and c that the primary structure of the hexagonal array does not consist of individual chain molecules but of aggregates, each composed of three strands. These aggregates can be interpreted as cylindrical micelles. In Figure 2a a sketch is included demonstrating how several chains of polymer 3 could be packed to form a cylindrical micelle. To investigate whether the cylindrical aggregates (micelles) exist also in solution, SAXS experiments over a large concentration range were performed. For these experiments, scattering data were obtained with a Kratky camera using a slit collimation. The data, therefore, had to be desmeared.7 Figure 3 shows the desmeared SAXS profile of 4-Na for a series of different concentrations as an example. A maximum in scattering intensity is clearly observed, which shifts to higher values of q with increasing concentration. A more detailed analysis of the scattering pattern is demonstrated in Figure 4, which depicts the SAXS profiles of 2 wt % aqueous solutions of all the polymers mentioned in Table 1. The data are plotted in terms of the Guinier approximation already mentioned as eq 3. The fat line through the data points is a fit according to the model on which eq 2 is based. S(q) ) 1 is assumed. The lines describe the experimental values very well, except where I(q) becomes dominated by the maximum in S(q). Having obtained the parameters K and R and therefore |F(q)|2 as outlined above, S(q) can be determined by dividing the experimental scattering profile by |F(q)|2. In that way, the position of the interference peak qmax is determined with higher precision than by directly reading off a value from a plot of I(q). qmax was obtained for all samples 1-4 at various concentrations. dB is obtained from the maximum of S(q) according to eq 4. According
4024 Langmuir, Vol. 15, No. 12, 1999
Rulkens et al.
(a)
(b)
Figure 2. (a) Model of the hexagonal column at superstructure of a sulfonated poly(p-phenylene) forming strands of radial aggregation number arad ) 3. The Bragg spacing dB is observed as layer reflection from cast films. (b) WAXS profile for 3-Na from a cast film exposed with the film surface parallel to the X-ray beam.
Figure 3. Experimental SAXS profiles for 4-Na at concentrations c in aqueous solution.
to eq 5, the dependence of dB on c should show a slope of -0.5 when plotted on a double logarithmic scale. Figure 5 proves that point. The linear relation holds over a very large concentration range. In the case of polymer 3-Na it extends to the solid
Figure 4. Experimental SAXS profiles for 4-Na (c ) 18.57 g/kg), 3-Na (c ) 22.52 g/kg), 2-Na (13.03 g/kg), and 1-TMA (c ) 23.6 g/kg). The calculated profiles (s) indicate the respective least-squares fits according to eq 3.
film. The fact that the data of all polymers fall on parallel lines is a strong hint that they adopt the same type of
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Langmuir, Vol. 15, No. 12, 1999 4025
Figure 5. Double logarithmic plot of dB vs polymer concentration cA. ()) 1-TMA; (4) 2-Na; (O) 3-Na; (0) 4-Na. CA+ ) 1 chain/ nm2; dB+ ) 1 nm. The lines indicate least-squares fits with slope -0.5 according to eq 5 and the aggregation number arad as the only variable.
structure. This assumption is corroborated by the observation that all polymers form highly birefringent lyotropic liquid crystal phases in this concentration range. The data in Figure 5 indicate for polymer 3-Na that it maintains a hexagonal type of packing from the highest concentration in the solid state to the lowest concentration investigated, which was as low as 6 g/kg. The parallel offset of the lines in Figure 5 is now interpreted as a consequence of the change in the aggregation number of the different polymers. The aggregation number arad was calculated from dB and cA. The resulting data, together with the values for the radii R of the micelles, are listed in Table 1. A further proof of the consistency of the data interpretation comes from the correlation between R determined from |F(q)|2 and the aggregation number arad determined from the position of the Bragg peaks, which leads to the models shown in Figure 6. From the SAXS and diffraction data and from information on crystal structures of model compounds,3 we constructed models of the supermolecular assemblies. They represent an educated guess on the basis of the X-ray scattering measurements and are presented in Figure 6. The micellar structures are shown in a projection along their longitudinal axis. Only one repeat unit of the polymers is shown. For clarity, not all dodecyl chains are drawn in the case of polymers 1 and 4. The radius of the columns corresponds to the circles included in the drawings. Assuming all polymers are within the measured cross-radius and taking the aggregation number arad, polymer densities in the columns ranging between 0.3 and 0.5 kg/L are calculated. Therefore, a considerable amount of water must be contained around the strands of the polysulfonates 1-4. It is clearly shown that changing the charge density and hydrophobicity of a stiff backbone changes the aggregation phenomena of these materials. Whereas the hydrophobic backbones and the dodecyl chains tend to (8) Liu, T.; Rulkens, R.; Wegner, G.; Chu, B. Macromolecules 1998, 31, 6119. (9) (a) Wittmann, J. C.; Lotz, B.; Candeau, F.; Kovacs, A. J. Polym. Sci., Polym. Phys. Ed. 1982, 20, 1341. (b) Hashimoto, T.; Shibayama, M.; Kawai, H.; Watanabe, H.; Kodaka, T. Macromolecules 1983, 16, 361. (10) (a) Maasen, H.-P.; Yang, J. L.; Wegner, G. Macromol. Chem. Macromol. Symp. 1990, 39, 215. (b) Yang, J. L.; Wegner, G. Macromolecules 1992, 25, 1786.
Figure 6. Proposed model of the aggregation of PPP in the form of strands. A projection along the columnar axis is depicted. The radial aggregation number arad and the cross-radius R have been taken from the SAXS results. The position of the backbones and side chains is based on assumptions taking the hydrophilichydrophobic balance into account. In the case of 1-TMA and 4-Na only half of all side chains are shown to account for the density of the side chain elements per unit length of the column.
minimize the interaction with water molecules and, therefore, favor micellation, the repulsive interaction of the charges opposes micellation. Thus, the decrease of aggregation number with increasing charge density and decreasing alkyl side chain density can be understood. In contrast to DNA, in the polysulfonates 1-4 the hydrophobic interactions are fully responsible for holding the linear molecules together in a strand. It is thus not too surprising that addition of a water-compatible organic solvent such as methanol changes the situation in such a way that the equilibrium between individual macromolecules and aggregated macromolecules is shifted toward observable concentrations of the free macromolecules. The observation of this aggregation-deaggregation equilibrium in aqueous solutions of methanol is the subject of another publication.8 Hence, the sulfonated poly(para-phenylene)s 1-4, in a simple way, mimic the formation of supermolecular assemblies, as they are built up in many biopolymers by the hydrophobic-hydrophilic balance. Structures such as the ones shown here are likely to be an essential factor in learning more about the complex behavior of biologically occurring polyelectrolytes. The fact that amphiphilic polymers form micelles is not surprising at all. Many examples of micelle-forming random or block copolymers are known.9 Formation of lyotropic liquid crystal phases composed of block copolymers that are organized in cylindrical micelles has been described.10 What is new in the case presented here is that the polymer backbone extends along the long axis of the cylindrical micelles and that the cylindrical aggregates are stable with regard to the aggregation number over a surprisingly large concentration range. LA9809832
