Needlelike and Spherical Polyelectrolyte Complex Nanoparticles of

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Needlelike and Spherical Polyelectrolyte Complex Nanoparticles of Poly(L-lysine) and Copolymers of Maleic Acid M. Mu¨ller,* T. Reihs, and W. Ouyang Leibniz Institute of Polymer Research Dresden, Hohe Str. 6, D-01069 Dresden, Germany Received July 6, 2004. In Final Form: September 27, 2004 We report on the bulk and surface properties of dispersions consisting of nonstoichiometric polyelectrolyte complex (PEC) nanoparticles. PEC nanoparticles were prepared by mixing poly(L-lysine) (PLL) or poly(diallyldimethylammonium chloride) (PDADMAC) with poly(maleic acid-co-R-methylstyrene) (PMA-MS) or poly(maleic acid-co-propylene) (PMA-P). The monomolar mixing ratio was n-/n+ ) 0.6, and the concentration ranged from 1 to 6 mmol/L. Subsequent centrifugation enabled the separation of the excess polycation, resulting in a stable coacervate phase further used in the experiments. The bulk phase parameters turbidity and hydrodynamic radius (Rh) of the PEC nanoparticles showed a linear dependence on the total polymer content independently of the mixed polyelectrolytes. This can be interpreted by the increased collision probability of the polyelectrolyte chains when the overlap concentration is approached or exceeded. Different morphologies of the cationic PEC nanoparticles, which were solution-cast onto Si supports, were obtained by atomic force microscopy (AFM). The combinations of PLL/PMA-MS and PDADMAC/PMA-MS revealed more or less hemispherical particle shapes, whereas that of PLL/PMA-P revealed an elongated needlelike particle shape. Circular dichroism and attenuated total reflection Fourier transform infrared (ATR-FTIR) measurements proved the R-helical conformation for the PEC PLL/PMA-P and the random coil conformation for the PEC PLL/PMA-MS. We conclude that stiff R-helical PLL induces anisotropic elongated PEC nanoparticles, whereas randomly coiled PLL forms isotropic spherical PEC nanoparticles.

Introduction Mixing solutions of oppositely charged polyelectrolytes (PELs) results in the formation of polyelectrolyte complex (PEC) dispersions. Pioneering work in that respect was performed by Bungenberg de Jong, Michaels, Kabanov and Dautzenberg.1-4 After mixing polycation (PC) and polyanion (PA) solutions, the final PEC dispersion in a beaker mostly consists of three components: (i) small soluble primary complexes consisting of a few PCs and PAs, (ii) dispersed colloidal particles of aggregated primary complexes (secondary complexes), and (iii) larger insoluble precipitate particles. The former exhibit typical sizes of a few nanometers, the second exhibit sizes of 100-300 nm, and the latter exhibit sizes of 0.1-1 mm. Especially the secondary complex particles (case ii) attract certain interest in polymer, colloid, and nanoscience. They are claimed to form a core/shell structure, according to which in the particle core a 1:1 charge stoichometry prevails and in the shell the excess polyelectrolyte component is located, giving the particle the defined charge sign. These PEC nanoparticles exhibit a remarkably low polydispersity,5-8 and it is increasingly better understood to * To whom correspondence should be addressed. Phone: 03514658-405. Fax: 0351-4658-284. E-mail: [email protected]. (1) Bungenberg de Jong, H. G. In Colloid Science; Kruyt, H. R., Ed.; Elsevier Publishing Company: Amsterdam, The Netherlands, 1949; Vol. II, pp 335-384. (2) Michaels, A. S.; Miekka, R. G. J. Phys. Chem. 1961, 65 (10), 1765. (3) Kabanov, V. A.; Zezin, A. B. Pure Appl. Chem. 1984, 56, 343. (4) Philipp, B.; Dautzenberg, H.; Linow, K. J.; Ko¨tz, J.; Dawydoff, W. Prog. Polym. Sci. 1989, 14, 91. (5) Buchhammer, H. M.; Petzold, G.; Lunkwitz, K. Langmuir 1999, 15, 4306. (6) Buchhammer, H. M.; Mende, M.; Oelmann, M. Colloids Surf., A 2003, 218 (1-3), 151. (7) Reihs, T.; Mu¨ller, M.; Lunkwitz, K. Colloids Surf., A 2003, 212 (1), 79-95. (8) Reihs, T.; Mu¨ller, M.; Lunkwitz, K. J. Colloid Interface Sci. 2004, 271 (1), 69-79.

control their size, charge sign, and colloid stability in dependence of the molar mixing ratio (n-/n+), concentration (cPEL), ionic strength, and pH value.6,8 An important application area of polyelectrolyte complexation is especially the macroscopic precipitate structures, which have already successfully been used as microcapsules (0.5-2 mm)9,10 for the encapsulation and immunoisolation of, for example, pancreatic insulin producing islet cells from nonhuman sources (e.g., pig). Further promising application aspects offer recently reported 3D structures, which were generated by precipitating PEC dispersions in an alcohol/water bath by an inkjet-like technique.11 In comparison to these microscopic precipitate structures, the nanoscopic PEC particles (100-300 nm), which shall be treated herein, are still underway to applications relevant for industry. Promising application directions of those nanoparticles are, for example, the flocculation of colloidal nanoparticles such as dyes,12-14 nanocarriers for biomedical and pharmaceutical purposes, or presumably latex analogous paints, where the enhancement of the solid content to values significantly higher than 1% under conservation of the colloid stability is actually still a challenge.8 Up to now, not much has been known from the literature on the influence of the PEL conformation (globular, flexible, or stiff) on the shape of the final PEC particle, (9) Schuldt, U.; Grasnick, G.; Karle, P.; Mu¨ller, P.; Lo¨hr, M.; Pelegrin, M.; Piechaczyk, M.; Rombs, K. v.; Gu¨nzburg, W. H.; Salmons, B.; Saller, R. M. Ann. N.Y. Acad. Sci. 1999, 875, 46-63. (10) Wang, T.; Lacik, I.; Brissova, M.; Anilkumar, A. V.; Prokop, A.; Hunkeler, D.; Green, R.; Shahrokhi, K.; Powers, A. C. Nat. Biotechnol. 1997, 15, 358. (11) Gratson, G. M.; Xu, M.; Lewis, J. A. Nature 2004, 428, 386. (12) Petzold, G.; Nebel, A.; Buchhammer, H.-M.; Lunkwitz, K. Colloid Polym. Sci. 1998, 276, 125. (13) Buchhammer, H.-M.; Oelmann, M.; Petzold, G. Melliand 2001, 82 (5), E104-E105. (14) Petzold, G.; Lunkwitz, K.; Schwarz, S. Chem. Eng. Technol. 2003, 26 (1), 48.

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Figure 1. Polycations and polyanions used for polyelectrolyte complex (PEC) formation: (a) poly(diallyldimethylammonium chloride) (PDADMAC); (b) poly(L-lysine) (PLL); (c) poly(maleic acid-co-propylene) (PMA-P; R1 ) methyl, R2 ) H); (d) poly(maleic acid-co-R-methylstyrene) (PMA-MS; R1 ) methyl, R2 ) phenyl).

which could be used for structuring or scaffolding on the nano or micro level.11 A nice example for templating elongated nanoscopic structures of polyelectrolyte complexes by inherently stiff polyelectrolytes was shown by Goessl.15 In that study, the authors reported the complex between stiff dendronized poly(styrene) bearing protonated peripherical amine groups and DNA. From high resolution atomic force microscopy (AFM) images before and after DNA complexation, stiff sections in elongated chain structures are visible, where the DNA wraps periodically around the dendronized polymer core, so that a defined regular pitch around 2 nm could be determined. We report herein on the bulk and surface properties of refined dispersions containing differently shaped PEC nanoparticles. The aim of this work is to show how the size and the shape of PEC nanoparticles can be influenced by polyelectrolyte structure and concentration. To do that, we mixed cationic poly(diallyldimethylammonium chloride) (PDADMAC) or poly(L-lysine) (PLL) with optionally the two polyanions sodium poly(maleic acid-co-propylene) (PMA-P) and sodium poly(maleic acid-co-R-methylstyrene) (PMA-MS). In contrast to other concepts, centrifugation was applied to separate the excess PEL as introduced therein.7,8 The bulk properties of PEC dispersions were characterized by turbidity, photon-correlation spectroscopy (PCS), and circular dichroism (CD) and the surface properties by atomic force microscopy (AFM) and attenuated total reflection Fourier transform infrared (ATRFTIR) spectroscopy. Experimental Section Materials. Poly(L-lysine) (PLL, Figure 1) was obtained from Sigma-Aldrich GmbH, Steinheim, Germany. The molecular weight was Mw ) 246 500 g/mol. Poly(diallyldimethylammonium chloride) (PDADMAC, Figure 1) was obtained from Aldrich Chemical Co., Inc., Milwaukee, WI, as a 20 wt % aqueous solution. The molecular weight was Mw ) 250 000-350 000 g/mol. The copolymers of maleic acid with R-methylstyrene (PMA-MS) or with propylene (PMA-P) (Figure 1) were prepared by hydrolysis of the corresponding copolyanhydrides with a solution of NaOH in an equimolar amount. The copolyanhydrides (Mw ≈ 25 000 g/mol) were obtained from Leuna AG, Schkopau, Germany. Preparation of Refined PEC Particles. The preparation of PEC nanoparticles is illustrated in Figure 2. According to the figure, PEC dispersions were prepared by direct mixing of the solutions of the oppositely charged PELs. The calculated amount of polyanion (PA) solution was added dropwise to a gently stirred solution of the polycation (PC) at (15) Goessl, I.; Shu, L.; Schlu¨ter, A. D.; Rabe, J. P. J. Am. Chem. Soc. 2002, 124, 6860.

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Figure 2. Preparation protocol of PEC nanoparticles followed optionally by adsorption at a planar substrate. concentrations (cPEL) ranging between 0.001 mol/L e cPOL e 0.006 mol/L. The PECs reported herein were prepared at a molar mixing ratio of n-/n+ ) 0.6 and pH 6. In a second step, the obtained initial PEC dispersions were centrifuged at 11 000 rpm for 20 min (Centrifuge 5416, Eppendorf AG, Hamburg, Germany). After careful decantation of the supernatant phase, the remaining small milklike coacervate fraction containing the refined PEC particles was removed by syringe from the centrifugation tube and diluted to the initial or a desired volume.8 Methods. The turbidity of the PEC dispersions was determined by a Lambda 2 ultraviolet-visible (UV-vis) spectrometer (Perkin-Elmer, U.K.) at λ ) 500 nm (deionized water was used as the reference). Photon-correlation spectroscopy (PCS) data were recorded at a scattering angle of 90° with a Zetasizer 300 instrument (632.8 nm, 10 mW He-Ne laser, Malvern Instruments, U.K.). Using the Stokes-Einstein equation, the z-average hydrodynamic radius (Rh) could be calculated by measuring the z-average diffusion coefficient of the PEC particles. The polydispersity (PD) was determined by fitting the correlation function G(t) with a power series (log G(t) ) a + bt + ct2 + dt3 + et4) and ratioing the factors c and b, respectively, according to PD ) 2c/b2. The cationic charge of the particles was determined by the particle charge detector (PCD, Mu¨tek GmbH, Germany) on the basis of the titration with low molecular poly(ethylenesulfate) (PES). CD experiments were performed on a Jasco J810 spectropolarimeter (Gross-Umstadt, Germany) operating in double beam mode and using 1 mm quartz cuvettes. CD spectra were generated from the raw data by the Jasco software. Millipore water at pH 6 was used as the reference solution. The amount of R-helical conformation was approximated by software supplied by Jasco on the basis of the Yang approach, applying a linear combination of five independent CD spectra on protein/polypeptide samples with known conformations.21 Note that the quantitaive estimation is crucially dependent on the choice of the baseline. The AFM experiments were carried out on an ultramicroscope (Nanostation II, SIS GmbH, Herzogenrath, Germany) provided with an optical dark field microscope and an AFM device. AFM measurements were performed in the noncontact mode using tips from nanosensors (Darmstadt, Germany). AFM samples were prepared by solution-casting PEC dispersions onto Si supports followed by drying. Attenuated total reflection Fourier transform infrared (ATRFTIR) spectra on dried films of polyelectrolyte complexes were (16) Mu¨ller, M. In Handbook of Polyelectrolytes and Their Applications; Tripathy, S. K., Kumar, J., Nalwa, H. S., Eds.; American Scientific Publishers (ASP): Stevenson Ranch, CA, 2002 ; Vol. 1, pp 293-312. (17) Schulz, G. E.; Schirmer, R. H. Principles of Protein Structure; Springer: New York, 1985. (18) Susi, H.; Timasheff, S. N.; Stevens, L. J. Biol. Chem. 1967, 242, 5460. (19) Mu¨ller, M.; Buchet, R.; Fringeli, U. P. J. Phys. Chem. 1996, 100, 10810. (20) Sugai, S.; Ebert, G. Adv. Colloid Interface Sci. 1986, 24, 247. (21) Yang, J. T. Methods Enzymol. 1986, 130, 208-269.

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Figure 3. (a) Hydrodynamic radii of PEC dispersions of PLL/PMA-P (9), PLL/PMA-MS (0), PDADMAC/PMA-P (b), and PDADMAC/ PMA-MS (O) as a function of total polymer content, cPOL (n-/n+ ) 0.6). (b) Turbidity of PEC dispersions of PLL/PMA-P (9), PLL/PMA-MS (0), PDADMAC/PMA-P (b), and PDADMAC/PMA-MS (O) as a function of total polymer content, cPOL (n-/n+ ) 0.6). performed on Si internal reflection elements (IREs), which were coated by casting five drops of the PEC dispersion on the IRE surface, respectively. The coated IRE was mounted in the in situ ATR cell,16 and a gentle N2/D2O gas stream was allowed to flow over the film surface.

Results and Discussion In this paper, we focus on polyelectrolyte complex (PEC) morphology in dependence of the polyelectrolyte (PEL) components and conformation. For that, we investigated the following four PECs (A-D) as combinations of the semiflexible polycation PDADMAC, the stiff R-helical PLL, and the semiflexible copolyanions PMA-MS and PMA-P: PDADMAC/PMA-MS (A), PLL/PMA-MS (B), PDADMAC/ PMA-P (C), and PLL/PMA-P (D). The paper is structured as follows. First, the bulk properties of refined PEC particles are shown, and then, the morphology of solutioncast layers of the nanoparticles onto Si supports is described. Bulk Properties. As shown in Figure 3a, the hydrodynamic radii of the refined polyelectrolyte complex particles of A-D depend on the polyelectrolyte type and the total polymer content. All four systems showed slightly different particles sizes but the same trend: With increasing total polymer content, the particles became larger. This can be explained with respect to the overlap concentration of the polycation and polyanion solutions: the PELs come closer together, the collision probability of the polyelectrolyte chains is increased when the overlap concentration is approached or exceeded, and complexation gets more and more favorable. Furthermore, from Figure 3a, a certain trend can be obtained: the PMA-MScontaining PEC particles were slightly larger compared to the PMA-P-containing ones, which shows up more significantly for those PECs with PLL as the polycation component and not that much for those with PDADMAC. The turbidity measurements shown in Figure 3b reveal in analogy to those on the particle size that there is approximately a linear dependence on the polymer content, too. However, elevation of PEL mixing solution concentration reflects both increasing particle size as well as particle number. Additionally, the turbidity values are strongly influenced by the chosen polyanion. For both polycations PDADMAC and PLL, the higher values were found when complexed with PMA-MS compared to PMAP. Presumably, PMA-MS containing polyelectrolyte complex particles are built up by a more densely packed core, which scatters the light to a higher extent due to the higher difference in refractive index between the water and the

particle. This might be caused by additional hydrophobic interaction in PMA-MS-containing PECs compared to PMA-P ones. Morphology of Solution-Cast PEC Films. In Figure 4, AFM images of solution-cast dispersions of the PECs PDADMAC/PMA-MS (a), PLL/PMA-MS (b), PDADMAC/ PMA-P (c), and PLL/PMA-P (d) are shown. Thereby, the spherical caps of the PECs PDADMAC/ PMA-MS and PLL/PMA-MS seem to be the strongest curved ones with diameters ranging between 120-190 and 230-380 nm, respectively, whereas the PEC PDADMAC/PMA-P formed rather flattened spherical caps with diameters ranging from 190 to 470 nm. From this, we conclude for the particles a state between solid- and liquidlike, being more solidlike for the PMA-MS-containing PECs (a and b) compared to the PEC PDADMAC/ PMA-P (c). Furthermore, it would be interesting to study the influence of both substrate surface tension (varied by substrate material) or particle surface tension (presumably variable by comonomers) on the final particle shape at the substrate. However, in contrast to those isotropic particles, the PEC particles of PLL/PMA-P (Figure 4d) were significantly different: the PEC PLL/PMA-P showed bundles of elongated needlelike nanoparticles. The length of the individual anisotropic particles from the PEC PLL/PMA-P was difficult to determine from Figure 4d due to partial fusing and merging. They ranged between l ) 150 and 230 nm with a width of approximately w ) 40 nm, resulting in aspect ratios around 4.75. However, the correlation of this length with the calculated R-helical PLL contour length of LPLL ≈ 180 nm (for a given polymerization degree of DP ) 1200 and rise per residue of r ) 0.15 nm17) is rather speculative, since PLL might not be fully in the R-helical conformation, as will be pointed out in the circular dichroism data. What is now the reason that PEC particles of PLL/ PMA-MS form spherical shapes and those of PLL/PMA-P form elongated anisotropic shapes? To answer this question, we checked for the conformation of PLL within the PEC, since it is well-known that PLL can form defined secondary structures such as R-helixes, β-sheets, or random coils depending on the temperature, pH, or ionic strength.18-20 Fortunately, these structural changes can be recorded by circular dichroism (CD) and FTIR measurements, as is shown in the next paragraph. Conformation of Solution-Cast PEC Films. In Figure 5, CD spectra of PLL/PMA-P polyelectrolyte complex particles are shown, from which a doublet at 206/

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Figure 4. AFM micrographs of solution-cast nanoparticles of the PEC PDADMAC/PMA-MS (a), PEC PLL/PMA-MS (b), PEC PDADMAC/PMA-P (c), and PEC PLL/PMA-P (d) at Si supports (cPEL ) 0.002 mol/L, n-/n+ ) 0.6 before centrifugation).

Figure 5. Circular dichroism spectra on the dispersions of the PECs PLL/PMA-MS (solid line) and PLL/PMA-P (dashed line) in the range between 180 and 250 nm.

222 nm can be obtained, indicating diagnostically the R-helical conformation of PLL. In contrast, the PLL/PMAMS complex particles did not show this doublet and a negative intensity around 195 nm occurred which can be assigned to the random coil conformation of PLL. The semiquantitative analysis21 reveals that within the PLL/PMA-P PEC particles at least 45% of the PLL was in the R-helical conformation and 0% R-helical content was obtained in the PLL/PMA-MS ones. Obviously, in the PMA-P-containing PEC particles, the stiff R-helical PLL (R-PLL) rod could work as a template on the macromo-

Figure 6. ATR-FTIR spectra of solution-cast films of the PECs PLL/PMA-P and PLL/PMA-MS. The sample films were exposed to a N2/D2O stream.

lecular level for the elongated structures consisting of complexed R-helical PLL on the supramolecular level, whereas, in the PMA-MS-containing PEC particles, the macromolecular random coil conformation of PLL resulted in globular or spherical supramolecular structures (Figure 3b). This is supported by ATR-FTIR spectroscopy on the solution-cast PEC films, giving directly information on the conformation of PLL within the PEC deposited on the surface. In Figure 6, ATR-FTIR spectra of solution-cast films of the PECs PLL/PMA-P and PLL/PMA-MS are shown, which were exposed to a D2O/N2 stream above the sample. By D2O, both a proton exchange from N-H to

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N-D of the amide groups of PLL and a replacement of bound H2O by D2O takes place, resulting in a slight shift of the amide I band to the amide I′ band (∆ν ≈ 5-10 cm-1), a large shift of the amide II band to the amide II′ band (∆ν ≈ 100 cm-1), and a very large shift of the δ(OH) band from 1650 to 1200 cm-1. This simplifies the assignment of the amide I′ band significantly, since the R-helical amide I′ component shifts to wavenumbers below 1640 cm-1 22,23 and could be better distinguished from the random component appearing above 1644 cm-1 and the interfering δ(OH) is completely absent. On the basis of the assignment of PLL amide components given herein,22,23 the narrow peak around 1640 cm-1 in the PLL/PMA-P sample can be assigned to the R-helical structure, whereas the broad peak around 1650 cm-1 for PLL/PMA-MS can be assigned to the random conformation. These experimental positions were slightly higher than those in the given reference, since solid D2O hydrated films were used instead of D2O solutions, for which the amide I′ components are shifted to lower wavenumbers due to higher hydrogen bonding.24 This result is in accordance with the CD spectra (Figure 5) on the same PECs in solution, suggesting a conservation of the secondary structure at the surface with respect to that in solution. Up to now it was not fully understood how PMA-P induces the R-helical conformation and PMA-MS induces the random coil conformation. We speculate that PMA-P could form a more extended conformation, allowing R-helicalization of PLL in the complex. A comparable result was observed by Shinoda,25 who complexed PLL with poly(acrylic acid) (PAC), claiming a stoichometric left-handed superhelix of PAC around the right-handed R-helix of PLL. Whereas this reference describes the (1:1) complex on the molecular level, the supramolecular structure and the final anisotropic particle shape may be also described as consequences of nematic interactions. In a certain relevance to this, we reported recently consecutively adsorbed polyelectrolyte multilayers (PEMs) of R-helical PLL and poly(vinyl sulfate), which showed nematic-like preferential alignment along parallel surface textures, as evidenced by dichroic ATR-FTIR spectroscopy and AFM measurements.26,27 Since the structure of a PEM is somewhat (22) Chirgadze, Y. N.; Brazhnikov, E. V. Biopolymers 1974, 13, 17011712. (23) Jackson, M.; Haris, P. I.; Chapman, D. Biochim. Biophys. Acta 1989, 998, 75-79. (24) Susi, H.; Timasheff, N.; Stevens, L. J. Biol. Chem. 1967, 242 (23), 5460-5466. (25) Shinoda, K.; Hayashi, T.; Yoshida, T.; Sakai, K.; Nakajima, A. Polymer J. 1976, 8 (2), 202. (26) Mu¨ller, M.; Keβler, B.; Lunkwitz, K. J. Phys. Chem. B 2003, 107 (32), 8189-8197. (27) Mu¨ller, M.; Reihs, T.; Keβler, B.; Adler, H.-J.; Lunkwitz, K. Macromol. Symp. 2004, 211, 217.

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closely related to those of PECs, we take this as an additional argument for the capability of stiff R-helical PLL to work as a template for the needlelike morphology of PEC particles. In contrast, PMA-MS might form preaggregated structures in solution due to hydrophobic interactions via the phenyl rings, which presumably prevents PLL from folding into the R-helix upon complexation. Associated structures of copolymers of maleic acid in solution have been already reported for poly(maleic acid-co-styrene) by Garnier28 in that respect, who claimed a zipperlike pairing of the phenyl residues of adjacent chains resulting in macrocoils. We expect a similar association trend for PMA-MS in solution. For further studies on the induction of R-helical PLL in dependence of polyanion structure, beside simulations with respect to charge interaction, also molecular modeling on the atomistic level could be of certain value. Conclusions (1) Stable cationic PEC nanoparticles were prepared by mixing polycation (PDADMAC and PLL) and polyanion (PMA-P and PMA-MS) solutions at concentrations ranging between cPOL ) 0.001 and 0.01 mol/L and at a molar mixing ratio of n-/n+ ) 0.6. By centrifugation of the PEC dispersion, the excess polycation could be removed. (2) For all polyelectrolyte combinations, the bulk phase parameters turbidity and hydrodynamic radius of the PEC nanoparticles increased linearly with cPOL. This can be interpreted by the increased collision probability of the PEL chains when the overlap concentration is approached or exceeded. (3) Solution-cast PEC nanoparticles of PDADMAC/ PMA-MS and PLL/PMA-MS revealed more or less hemispherical shapes, those of PDADMAC/PMA-P revealed flattened hemispheres, and those of PLL/PMA-P showed novel elongated or needlelike polymer particle shapes. (4) CD and ATR-FTIR measurements gave evidence that PLL in the needlelike PEC PLL/PMA-P was in the R-helical conformation, whereas PLL in the spherical PEC PLL/PMA-MS was in the random coil conformation. From that, we conclude that polyanions can induce different PLL conformations in the PEC, which further can cause different nanoparticle shapes. (5) Hence, simple wet chemical PEL-based concepts can be used to generate defined nanostructures of polymer particles in solution and at the surface. Acknowledgment. This work was funded by Deutsche Forschungsgemeinschaft (DFG, SPP 1009, MU 1524/1-1 and SFB 287, B5). LA0483257 (28) Garnier, G.; Duskova-Smrckova, M.; Vyhnalkova, R.; van de Ven, T. G. M.; Revol, J. F. Langmuir 2000, 16, 3757.