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J. Phys. Chem. B 2001, 105, 5612-5617
ARTICLES Effect of Chemical Nature of 1,1-Salt on Structure of Polyelectrolyte Gel-Surfactant Complexes Artemi V. Mironov, Serguei G. Starodoubtsev, and Alexei R. Khokhlov* Physics Department, Moscow State UniVersity, Moscow 117234, Russia
Alexander T. Dembo and Kirill A. Dembo Institute of Crystallography, Russian Academy of Sciences, Moscow 117333, Russia ReceiVed: June 16, 2000; In Final Form: February 23, 2001
The effect of the chemical nature of a 1,1-salt on the structure of polyelectrolyte gel-surfactant complexes (PSCs) formed from poly(diallyldimethylammonium) chloride and anionic surfactants is studied. The anions of the salt can compete with the surfactant ions for the formation of ionic bonds with the gel, thus destroying the highly ordered crystal-like structure of the PSC. On the other hand, the cations of the salt can participate in the fitting of the crystal-like structure of nonstoichiometric complexes that contain an excess of surfactant ions. The kinetics study showed that the formation of a highly ordered structure in a PSC is a slow process that can be accelerated by the addition of sodium chloride.
Introduction Complexes formed by linear1-5 and chemically crosslinked6-10 polyelectrolytes with surfactants are the subject of intensive experimental and theoretical study. Polyelectrolyte gel-surfactant complexes (PSCs) exhibit many specific features as a result of the insolubility of chemically cross-linked networks and the elasticity of the charged polymer chains. The formation of a PSC is accompanied by the aggregation of the surfactant ions in the gel phase through hydrophobic interactions and by the collapse of the gel.6-8 Despite the irregular structure of the charged networks, their complexes with surfactants often form highly ordered supramolecular structures.9-19 In a number of studies it was demonstrated that the formation of a supramolecular lattice depends on the charge density and chemical nature of the swollen network, the length of the hydrophobic residue of amphiphilic molecules, and the concentration of the surfactant. The ion-exchange reaction between an anionic surfactant and a cationic network
[\-Z+ X-] + RY- + K+ f [\-Z+ RY-] + K+ + X- (1) results in an increase of the concentration of the surfactant ions in the gel.6,7 In eq 1, [\-Z+ X-] is a charged monomer unit of a network with its counterion and [RY]- and K+ are, respectively, the ions of the surfactant and its counterions in the solution. The driving force that moves the equilibrium of the reaction 1 to the right is the aggregation of the surfactant hydrocarbon groups as a result of hydrophobic interactions. From eq 1, it follows that (i) the saturation of the PSC is reached * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: +7 (095) 939 2988.
after a stoichiometric ratio between the number of the surfactant ions and the number of ions of the network inside the gel is achieved and (ii) the addition of an inorganic salt should result in partial or complete dissociation of the complex. The results of earlier studies have indeed demonstrated that the addition of salt can lead to partial or complete dissociation of a PSC.6-8 However, the structure of the stoichiometric complex might not be optimal from the viewpoint of steric fitting. If the distance between the charges along the chain is large enough, then the density within the surfactant microdomains, in the case of 1:1 complex, might be too low, thus preventing the formation of highly ordered structures. In this case, upon further increase of the concentration of surfactant in the external solution, it might be thermodynamically advantageous to incorporate extra surfactant molecules, together with their counterions, inside the gel-surfactant complex. In this way, the packing of the polymer and surfactant components in the PSC can be significantly improved.19 Schematically, the formation of nonstoichiometric complexes can be represented by the following reaction
[\-Z+ RY-]+ RY- + K+ f {[\-Z+ RY-]*[RY-K+]ν} (2) where ν characterize the fraction of surfactant ions that participate in the formation of the PSC together with their counterions. Further increases in the salt concentration lead to the precipitation of the surfactants, a decrease in their concentration in the solution, and the partial dissociation of the complexes.19 Despite the extensive investigation of PSCs, the role of the chemical structure of the ions participating in complex formation is not yet sufficiently clear. Our previous studies have shown that the phase of the PSC usually contains a small amount of
10.1021/jp002187m CCC: $20.00 © 2001 American Chemical Society Published on Web 05/26/2001
Effect of 1,1-Salt on Structure of PSCs water and, hence, has a low dielectric constant.6,19 It can be expected that the electrostatic interactions in the ionic assemblies forming the PSC are of great importance and that the chemical structure of the ions participating in the formation of the structure of the PSC should strongly affect the properties of the PSC even when only monovalent ions are present in the system. The formation of a PSC is a complicated process that includes diffusion of the surfactant ions and mobile counterions into the swollen gel, conformational transition (collapse) of the network, and cooperative self-organization of the surfactant ions and oppositely charged polymer chains with the formation of the highly ordered structure. To date, there has been no systematic study of the kinetics of nanostructure formation in PSCs. The aim of this study is to demonstrate the role of the chemical structure of ions of low-molecular-weight 1,1-salts (sodium acetate, chloride, and iodide) that participate in the formation of PSCs of poly(diallyldimethylammonium chloride) (PDADMA) with the anionic surfactants sodium laurate (SL), sodium miristinate (SM), sodium dodecyl sulfate (SDS), and sodium dodecylbenzenesulfonate (SDBS). Another goal of this paper is to study the kinetics of formation of highly ordered structures in PSCs. Experimental Section Sample Preparation. Diallyldimethylammonium chloride (DADMACL) (60% aqueous solution), N,N′-methylene(bis)acrylamide (BIS), ammonium persulfate (PS), N,N,N′,N′-tetramethylethylenediamine (TEMED) SL, SM, SDS, and SDBS were obtained from Fluka Chemical Co. The gel samples were prepared as described elsewhere.19 The washed gel was cut into the disks with a diameter of 10 mm and a height of ca. 2 mm. The disks were dried at 37 and 90 °C to constant weight (ca. 3 mg) and kept until use. The dried samples were first swollen in closed vials containing water and salt; then, the calculated amount of the surfactant stock solution was added, and the samples were equilibrated at 25 °C for 2-4 weeks. The initial solution contained a 3-fold excess of the surfactant molecules with respect to the number of network cations. The initial surfactant concentration was 7.5 × 10-3 M. The total volume of the solution was 400 L/mol of PDADMA monomer units. Thus, in salt solution, the number of ions of the salt guest was more then 100-fold higher than the number of chloride counterions of PDADMA. We determined the preliminary state of the surfactant molecules in the solutions equilibrated with the gel samples. The presence of the surfactant micelles in the solution was estimated by the solubilization of the water-insoluble dye Sudan1. On the other hand, the solutions were transparent and did not contain any precipitate. The swelling ratio of the samples, F ) meq/mo, was estimated from the weights of the dry PDADMA network (mo) and the equilibrated PSC sample (meq). The composition of each complex was characterized by its water content β [β ) (meq mPSC)/meq, where mPSC is the weight of dried PSC] and by the ratio Q between the number of surfactant ions and the number of cations of the network inside the gel phase. The values of Q were calculated from a comparison of the weights of the dried samples before and after the interaction with the surfactants.19 X-ray Scattering. The scattering measurements were carried out on the small-angle X-ray scattering diffractometer AMUR-K (made at the Institute of Crystallography, Russian Academy of Sciences20) with a linear position-sensitive detector (produced at the Institute of Nuclear Physics, Siberian Division of the
J. Phys. Chem. B, Vol. 105, No. 24, 2001 5613 TABLE 1: Swelling Ratio (F), Water Content (β), and Composition (Q) of the Complexes Prepared with Different Surfactants in Water and Salt Solutions parameter
surfactant-free gel
SL
SM
SDS SDBS
F β Q
water
medium
66.7 0.98 0
3.7 0.51 0.95
4.2 0.42 1.15
3.1 12.0 0.32 0.43 0.85 1.25
F β Q
0.3 M NaAc
39.0 0.97 0
4.0 0.51 0.95
4.0 5.6 0.42 0.26 1.0 -
F β Q
0.3 M NaCl
23.8 0.97 0
7.2 13.9 0.70 0.44 1.0 0.90
5.4 0.28 1.81
2.3 0.29 2.8
F β
0.3 M NaI
4.6 -
-
6.4 0.48
5.7 0.4
-
8.1 0.39 3.2
Russian Academy of Sciences21). The detector has a window of 10 × 100 mm; the range of measured angles is up to 8°. Monochromatization was achieved with a crystalline monochromator at a wavelength of 0.1542 nm. The sample holder was made of stainless steel and had windows made of poly(ethylenetherephthalate) (thickness ) 0.01 mm); the thickness of the samples was ca. 1 mm. The samples of the complexes did not have the same thicknesses along the window of the holder (10 mm); therefore, we could not calculate the desmeared curves. Results and Discussion Collapse and Composition of the Complexes. The values of the parameters F, β, and Q are listed in Table 1. First, for the initial PDADMA gel, there is a marked decrease of the swelling ratio, F, and the water content, β, in the order water, acetate, chloride and iodide. In the solution of 0.3 M sodium chloride, the F value is half as large as it is in acetate, whereas in the solution of sodium iodide, a further collapse of the gel is observed. Recently, it was shown that the swelling ratio, F, of PDADMA gel with different counterions in water decreases in the same order: acetate, chloride, and iodide.22 This result is explained by an increase in the ability of the anions to form ion pairs with the DADMA cations of the network. The interaction of the gel with the surfactants leads to a strong decrease of the parameters F and β of the gel in comparison with those of the gel swollen in water. The water contents of the PSCs with SL and SM are higher in solutions of sodium chloride than in solutions of acetate. The last observation is explained by the stronger tendency of chloride anions to compete with surfactant anions in binding the cations of the network in comparison with acetate anions. Thus, it can be expected that the PDADMA-SL complex formed in the presence of chloride anions is weaker than the complex obtained in the presence of acetate anions. In contrast, the addition of sodium chloride results in a marked decrease of β, i.e., in the deswelling of the gels formed by the PSCs containing SDBS. The last result is explained by the formation of a nonstoichiometric complex between PDADMA and SDBS (see below). In accordance with reaction 1, in the absence of added salt, all of the surfactants form stoichiometric complexes with the PDADMA gel (Table 1). The addition of salt should result in dissociation of the stoichiometric PSC because of the increase in the concentration of X - ions on the right side of eq 1. On the other hand, the addition of salt (in the region of low concentrations) should be favorable for the formation of nonstoichiometric complexes because the concentration of K+ ions on the left side of eq 2 increases. An analysis of the compositions of the PSCs shows that the direction of the reaction
5614 J. Phys. Chem. B, Vol. 105, No. 24, 2001
Figure 1. Dependence of complex composition, Q, on cs for the PDADMA-SDS complex.
depends on the chemical structure of the anion of the surfactant. For surfactants with carboxylic groups, the ratio Q is ca. 1.0 ( 10% corresponding to the formation of stoichiometric complexes. At the same time, SDBS and SDS form nonstoichiometric complexes in the presence of sodium acetate and chloride. The ability of salt to stabilize the nonstoichiometric complexes of SDBS and SDS with PDADMA is accompanied by a parallel ability to stabilize the surfactant aggregates out of the gel. Previously, it was shown19 that, at a concentration of sodium chloride higher than 1 M in SDBS solution, precipitate is formed. Therefore, the SDBS concentration in the solution falls, and the gel-surfactant complex partially decomposes. The decomposition of the complex is accompanied by a marked disordering of the lamellae in the PSC. Analogous behavior is also observed for PSCs containing SDS. Figure 1 shows that the composition of the complex, Q, as a function of the sodium chloride concentration, cs, passes through a maximum at cs ≈ 1.0-1.5 M. The addition of salt and the formation of nonstoichiometric complexes in PDADMA-SDS complexes does not influence the type of packing of the surfactant ions in the PSCs. Figure 2 shows SAXS profiles of the scattering curves obtained from PDADMA-SDS PSC in water and salt solutions. The relative positions of the observed maxima (1:31/2:41/2:71/2) manifest the formation of the hexagonal structure of the complex in water and salt solutions with concentrations up to 1 M. At a very high concentration of sodium chloride (2 M), the complete decomposition of the complex occurs, and instead of sharp peaks, monotonic decay of the X-ray intensity is observed. Effect of the Chemical Nature of the Salt on the Structure of the Complexes. The formation of highly ordered structures in complexes with different chemical compositions was studied using SAXS. Figure 3 shows SAXS curves obtained from stoichiometric complexes formed by SL in water (curve 1) and 0.3 M solutions of (2) sodium acetate, (3) chloride, and (4) iodide. The scattering curves obtained from the samples equilibrated in water show at least four overlapping sharp peaks and a well-resolved secondary peak. The highest peak is located at q ) 1.55 nm-1. This value corresponds to a d spacing of 4.05 nm, i.e., approximately twice the length of a fully straight surfactant molecule. The existence of sharp peaks shows the formation of a highly ordered structure in the PSC. At the same time, in solutions of sodium acetate and chloride, the scattering
Mironov et al.
Figure 2. X-ray intensity profiles for the PDADMA-SDS complex in (1) water and in (2) 0.5, (3) 0.9, and (4) 2.0 M solutions of sodium chloride.
Figure 3. X-ray intensity profiles for the PDADMA-SL complex in (1) water and in 0.3 M (2) sodium acetate, (3) sodium chloride, and (4) sodium iodide solutions.
curves show singlebroad maxima at q ) 1.48 and 1.34 nm-1, respectively, corresponding to characteristic sizes d ) 4.2 and 4.7 nm. The scattering curve obtained from the samples equilibrated in the solution of sodium iodide shows no maximum. Thus, no ordered PSC is formed in the latter case. The positions of the maxima of the main peaks on the scattering curves are shifted to lower q values in going from water to acetate and chloride salt solutions. In salt solutions, the corresponding d space size significantly exceeds twice the length of the fully straight surfactant molecules. Qualitatively analogous results (not shown) were obtained for PSCs with the SM surfactant. Thus, the addition of salt at rather high ionic strength results in destabilization of the highly ordered structures in the PSCs with carboxylic groups, SL and SM. The increase in the ability of the anions of salts to compete in binding with the cations of the network results in a decrease of the density of packing of low-ordered structures in the PSC until complete disordering of the surfactant ions in PSC occurs.
Effect of 1,1-Salt on Structure of PSCs
Figure 4. X-ray intensity profiles for the PDADMA-SDBS complex in (1) water and in 0.3 M (2) sodium acetate, (3) sodium chloride, and (4) sodium iodide solutions.
The scattering curves of the complexes of PDADMA with SDBS in water (curve 1), in solutions of (2) sodium acetate and (3) sodium chloride are shown in Figure 4. Each curve demonstrates a distinct main peak at q ) 1.9-2.0 nm-1 and a secondary peak at q ) 3.95-4.05 nm-1. The sharpness of the main peak and the equidistant position of the peaks manifests the formation of a highly ordered lamellar structure in the complexes, which was described in our previous study.19 The SAXS profile obtained from PSCs with SDBS in solutions of sodium iodide exhibits the existence of two broad maxima at q ) 1.8 nm-1 and ca. 0.2-0.7 nm-1. The position of the maximum at q ) 1.8 nm-1 corresponds to a d value of 3.5 nm, which is typical for surfactant aggregates in PSCs with minimal packing order.19 The broad peak at 0.2-0.7 nm-1 with maximum at ca. 0.4 nm corresponding to d space of ca. 16 nm was not previously observed. Obviously, the appearance of this peak is connected with the participation of iodide ions in complex formation. The results of the structural study of the complexes formed from PDADMA and SDS in water are described in refs 16 and 17. It was shown that the complex exhibits three maxima in the scattering curve. The main sharp peak at q ) 1.6 nm-1 corresponds to the characteristic distance d ) 3.9 nm. The relative positions of the four observed maxima (1:31/2:41/2:71/2) manifest the formation of a hexagonal structure. The SAXS profile of the PDADMA-SDS complex in water obtained in this paper (Figure 5, curve 1) is identical to those obtained in the previous studies. The profiles of the scattering curves for PDADMA-SDS complexes in solutions of sodium acetate and chloride also demonstrate a distinct main peak at q ) 1.6 nm-1 and secondary peaks (Figure 5, curves 2 and 3). The sharpness and relative positions of the three maxima (1:31/2:71/2) can be interpreted as indicating maintenance of the hexagonal structure. A comparison of the scattering curves obtained in water and in solutions of 0.3 M sodium acetate and chloride shows no marked difference between the curves. A comparison of the positions of the main peaks for PSCs with SDS and SDBS (1.6 and 1.95 nm-1, respectively, corresponding to d values of ∼3.9 and ∼3.2 nm) shows that the molecules of SDS, which are shorter than molecules of SDBS, form a more expanded supramolecular structure. This effect can arise from
J. Phys. Chem. B, Vol. 105, No. 24, 2001 5615
Figure 5. X-ray intensity profiles for the PDADMA-SDS complex in (1) water and in 0.3 M (2) sodium acetate, (3) sodium chloride, and (4) sodium iodide solutions.
Figure 6. X-ray intensity profiles for (1) the iodide salt of PDADMA in 0.3 M NaI solution, (2) the PDADMA-SDBS complex in 0.3 M NaI solution, and (3) the PDADMA-SDS complex in 0.3 M NaI solution.
different symmetries of packing of the complexes. Thus, the length of the surfactant hydrophobic tails is not the only factor that determines the characteristic size of the structure formed. The SAXS profile obtained from PSCs with SDS in solutions of sodium iodide shows two broad maxima at q ) 1.5 and 0.20.7 nm-1. The position of the maximum at 1.5 nm-1 corresponds to a d value of 4.2 nm, which is typical for surfactant aggregates in PSCs with poor packing,17 i.e., it is higher than twice the length of the surfactant molecule. A comparison of the position of the broad peak at 0.2-0.7 nm-1 with the maximum at ca. 0.4 nm is analogous to the peak observed for PDADMA-SDBS complex and corresponds to the same d spacing of ca. 16 nm. In an earlier study,16 it was shown that the scattering curve obtained from PDADMA (sodium salt) demonstrates monotonic decay of the intensity of X-ray scattering vs q. Figure 6 shows the scattering curve obtained from the iodide salt of PDADMA
5616 J. Phys. Chem. B, Vol. 105, No. 24, 2001 in 0.3 M NaI solution (curve 1) in comparison with scattering curves of PDADMA complexes with SDBS (curve 2) and SDS (curve 3). It can be seen that the monotonic decrease of the X-ray scattering intensity is observed for curve 1. Thus, the origin of the peak at low q ) 0.2-0.7 nm-1 (curves 2 and 3) is connected with the formation of complex associates, which include anions of both iodide and either SDS or SDBS. We now discuss the probable reasons for the formation of the large structural inhomogeneities in PSCs containing iodide anions. It is well-known that the addition of inorganic salts is favorable for the association of ionic surfactants and for their segregation from salt solution. In the case under consideration, the addition of the 1,1-salt sodium chloride stabilizes the structure of the highly ordered domains of nonstoichiometric PSCs of SDS and SDBS in PDADMA gels.17,19 On the other hand, the ions of iodide compete strongly with surfactant anions in the formation of ion pairs with the cations of PDADMA. The PDADMA network involved in PSCs can be considered as a cross-linked binary copolymer that consists of two types of monomer units, namely those that form salt bonds with the ions of the surfactants and those that form ion pairs with iodide anions. Through ion exchange, the chemical structure of the monomer units can change as well. Let us assume that the monomer units in such a system have a tendency to segregate. When the tendency toward segregation of the monomer units is very strong, a two-phase network should be formed. Actually, the formation of such a two-phase network, which consists of a collapsed PSC and a highly swollen part containing inorganic counterions, has been observed in systems where the number of charges in the networks exceeded the number of the surfactant ions.23 Usually, PSCs form a layer on the gel surface under such conditions.9 In the case of iodide anion, the PDADMA gel is in a collapsed state, and its swelling ratio is close to that of a PSC containing SDS or SDBS anions (Table 1). This particularity of the PDADMA-I - complex is favorable for the formation of microsegregated structures in the DADMAsurfactant-iodide complexes because the mechanical tensions in the gel in this case are low. On the other hand, the microsegregated structure in such system is expected to be somewhat more stable from an entropic point of view in comparison with macrophase separation. One possible example of the structure of a microsegregated PDADMA-SDS (SDBS)iodide complex is shown in Figure 7. Kinetics of Formation of Highly Ordered PSCs. Until now, there have been no systematic studies of the kinetics of formation of highly ordered structures in PSCs. Khandurina et al. assumed that the formation of lamellar structures in PSCs of poly(acrylic acid) and cationic surfactants occurs in parallel with the front diffusion of the surfactant into the gel.14 In their study, the formation of a lamellar structure could be observed 2-3 days after the components were mixed. We have found that, for the systems investigated in such studies, the formationof highly ordered structures in NPSCs can be a very slow process. The diffusion of the surfactant into the gel is accompanied by the collapse of the gel.6,7 In particular, the kinetics of the collapse of a PDADMA gel in the presence of surfactants with alkyl chain lengths of C11-C13 was studied in ref 18. For the samples with the same characteristic size as in this paper, gel collapse was practically completed within 2-3 days after the addition of excess surfactant to the swollen gel. In contrast, the process of self-organization and the formation of highly ordered structures in PSCs is much slower. Figure 8 shows SAXS profiles obtained from PSCs of PDADMA and SDBS in (a) water and (b) 0.3 M sodium
Mironov et al.
Figure 7. Scheme of microsegregation in the PDADMA-SDS (SDBS)-iodide complexes.
Figure 8. X-ray intensity profiles for the PDADMA-SDBS complex (a) in water and (b) in 0.3 M sodium chloride solution at different time of observation (2, 14, and 49 days).
chloride at different times of observation. It can be seen that the intensities and sharpness of the main and secondary peaks significantly increase with time. This increase is observed even after two weeks of observation of the complexes, i.e., much longer than the characteristic time of the collapse of the gel. Thus, the formation of highly ordered structures in PSCs can be a very slow process, which is finally completed within several weeks after the components are mixed. A comparison of the kinetics of self-organization in PSCs in water and in 0.3 M NaCl solution (Figure 8a,b, respectively)
Effect of 1,1-Salt on Structure of PSCs shows that the addition of salt accelerates the formation of highly ordered structures in PSCs. At the same time, the d spacing values for nonstoichiometric complexes that are formed in the presence of salt have somewhat higher values.19 For example, the value of d corresponding to the position of the main peak in the SAXS profile of the PSC in 0.3 M NaCl is 3.25 nm (Figure 8b, 49 days), whereas the analogous value for the PSC in water is 3.15 nm. A similar tendency is also observed for the other systems studied (PSCs formed by SDS and SL). This increase in d can be explained by the lower density of packing in the hydrophilic PSC regions containing the ions of polymer, surfactant, and salt. Moreover, the addition of salt leads to the screening of the electrostatic interactions in the PSC at high ionic strength. The lower density of packing of the final highly ordered structure and the screening effect of the salt both should lead to enhanced molecular mobility in the PSC and, hence, to acceleration of the self-ordering processes. Conclusions This study has demonstrated that the chemical structure of the ions of inorganic 1,1-salts (sodium acetate, chloride, and iodide) plays an important role in the formation of PSCs of PDADMA with anionic surfactants. The anions of the salt (especially iodide anions) can compete with surfactant ions in the formation of salt bonds with the charges of the polyelectrolyte network. As a result, the highly ordered structures in the complexes are destroyed. On the other hand, at intermediate concentrations of salts with weak competitive anions (sodium chloride and acetate), the addition of salt is favorable for the formation of nonstoichiometric complexes containing large excesses of surfactant ions over the number of the charges of the network. Such complexes are observed only for sulfo- and sulfonic-containing surfactants, whereas for surfactants with carboxylic groups, only stoichiometric complexes are formed. The stability of the latter decreases with increasing salt concentration. A SAXS study demonstrated the appearance of a peak at low q values in the scattering curves obtained from the complexes containing ions of both SDS (or SDBS) and iodide. The position of the peak corresponds to a characteristic size of the ordered elements in the gel of d ≈ 102 nm. Such elements in gel structures have not previously been observed in PSCs. They probably reflect microsegregation in the mixed complex.
J. Phys. Chem. B, Vol. 105, No. 24, 2001 5617 The study of the kinetics of the formation of the highly ordered structures in PSCs has shown that the processes of selfordering in PSCs can be very slow. The addition of a 1,1-salt such as sodium chloride can significantly accelerate the formation of highly ordered structures in PSCs. Acknowledgment. The authors are thankful to the Russian Foundation of Basic Research (Grants 97-03-32770a and 0003-33108a) for financial support. References and Notes (1) Hayakawa, K.; Kwak, J. J. Phys. Chem. 1982, 86, 3866. (2) Hayakawa, K.; Santerre, J. P.; Kwak, J. J. Biophys. Chem. 1983, 17, 175. (3) Dubin, P. L.; Oteri, R. J. Colloid Interface Sci. 1983, 95, 453. (4) Dubin, P. L.; Rigsbee, D. R.; McQuigg, D. W. J. Colloid Interface Sci. 1985, 105, 509. (5) Goddard, E. D Colloid Surf. 1986, 19, 301. (6) Khokhlov, A. R.; Kramarenko, E. Yu.; Makhaeva, E. E.; Starodubtzev, S. G. Macromolecules 1992, 25, 4779. (7) Khokhlov, A. R.; Starodubtzev, S. G.; Makhaeva, E. E.; Vasilevskaya, V. V. AdV. Polym. Sci. 1993, 109, 123. (8) Makhaeva, E. E.; Starodubtsev, S. G. Polym. Bull. 1993, 30, 327. (9) Khandurina, Yu. V.; Rogacheva, V.B.; Zezin, A. B.; Kabanov, V. A. Polym. Sci. 1994, 36A, 184. (10) Okuzaki, H.; Osada, Y. Macromolecules 1994, 27, 502. (11) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 4554. (12) Antonietti, M.; Conrad, J.; Thunemann, A. Macromolecules 1994, 27, 6007. (13) Antonietti, M.; Conrad, J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1869. (14) Khandurina, Yu. V.; Alexeev, V. L.; Evmenenko, G. A.; Dembo, A. T.; Rogacheva, V. B.; Zezin, A. B. J. Phys. II France 1995, 5, 337. (15) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 380. (16) Sokolov, E. L.; Yeh, F.; Khokhlov, A. R.; Chu, B. Langmuir 1996, 12, 6229. (17) Dembo, A. T.; Yakunin, A. N.; Zaitsev, V. S.; Mironov, A. V.; Starodoubtsev, S. G.; Khokhlov, A. R.; Chu, B. J. Polym. Sci. B: Polym. Phys. 1996, 34, 2893. (18) Le Minh, Thanh; Makhaeva, E. E.; Starodoubtsev, S. G. Polym. Sci. 1993, 4, 476. (19) Mironov, A. V.; Starodoubtsev, S. G.; Khokhlov, A. R.; Dembo, A. T.; Yakunin, A. T. Macromolecules 1998, 31, 7698. (20) Mogilevsky, L. Yu.; Dembo, A. T.; Svergun, D. I.; Feigin, L. A. Kristallografia 1984, 29, 587. (21) Aultchenko, V. M.; Baru, S. E.; Sidorov, V. A.; Savinov, G. A.; Feldman, I. G.; Khabakhpashev, A. G.; Yasenev, M. V. Nucl. Instrum. Methods 1983, 208, 443. (22) Starodubtsev, S. G.; Khokhlov, A. R.; Sokolov, E. L.; Chu B. Macromolecules 1995, 28, 3930. (23) Starodubtsev, S. G. Vysokomolec. Soed. 1990, 31B, 925.
