Nanostructure Changes with Krafft Transitions of Polyelectrolyte

Jun 7, 2007 - Shigeo Sasaki*. Department of Chemistry, Faculty of ... Yuko Nagai , Youhei Kawabata , and Tadashi Kato. The Journal of Physical Chemist...
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J. Phys. Chem. B 2007, 111, 8453-8458

8453

Nanostructure Changes with Krafft Transitions of Polyelectrolyte-Surfactant Complexes in Aqueous NaCl Solutions† Shigeo Sasaki* Department of Chemistry, Faculty of Sciences, Kyushu UniVersity, 33 Hakozaki, Higashi ku, Fukuoka 812, Japan ReceiVed: December 17, 2006; In Final Form: April 24, 2007

Nanostructure changes with the Krafft transition of complexes of poly(acrylicacid) with octadecyltrimethylammonium (PAA-OTA) in the aqueous solutions at various NaCl concentrations (Cs) from 20 to 400 mM, were studied making temperature-scanning small-angle X-ray scattering (SAXS) experiments and differential scanning calorimetric (DSC) measurements. For the PAA-OTA complex in the solution at a higher temperature than 25 °C, four SAXS peaks with a spacing ratio of 1:31/2:41/2:71/2, indicating the 2D hexagonal structure, were observed at Cs below 100 mM and two SAXS peaks with a spacing ratio of 1:2, indicating the lamella structure, were observed at Cs above 200 mM. For the complex in the solution at a lower temperature than 22 °C, a broad SAXS peak was observed at the scattering vector q ) 1.2 nm-1 when the Cs was less than 200 mM but not when Cs was 400 mM. Two peaks with a spacing ratio 1:2, indicating the lamella structure, were also observed for the complex in the solution at 8 °C. The DSC data demonstrated that the nanostructure changes were accompanied with the endothermic enthalpy change. On the basis of the experimental results, the salt concentration dependent nanostructures are discussed.

1. Introduction Self-assembly of polyelectrolyte and ionic surfactants of opposite charges, defined as polyelectrolyte-surfactant complexes (PSC),1 has attracted a great deal of attention, not only because of the fascinating physical mechanism to form the periodic structures,2,3 which can be detected by the small-angle X-ray scattering (SAXS) measurement, but also because of the potential applications as the drug delivery system and the antisoiling coating for textiles.4 The ordered nanostructures such as cubic, hexagonal, and lamella are also found in the surfactant solutions at concentrations higher than 40 w/w%, even if polyelectrolyte chains are absent.5 The surfactant molecules are concentrated in the electrostatic potential trough formed by the polyelectrolyte and are self-assembled to form the ordered nanostructures in the PSC. The ordered nanostructures in the PSC are due to the periodic arrays of the surfactant self-assembly induced by the polyelectrolyte, some of which are rarely observed in the absence of polyelectrolyte. The surfactant micelles composing the nanostructures are known to become the hydrated solid with decreasing temperature, which is called the Krafft transition.6 The lamella structures consisting of hydrated solid bilayers usually form in the absence of polyelectrolyte. Recently, we have found the rodlike hydrated solid in the complex of DNA with octadecyltrimethylammonium (OTA) at the high salt concentration,7 which forms the hexagonal structure indicated by the SAXS peaks with a spacing ratio of 1:31/2:2. At the low salt concentration, however, no hydrated solid has been found in the DNA-OTA complexes.7 The OTA molecules with counterions of Cl- and Br- in the aqueous solution are known to exhibit the structural transition from the cylindrical micelle to the planar hydrated solid with † Part of the special issue “International Symposium on Polyelectrolytes (2006)”. * E-mail: [email protected].

cooling them down.8 The surfactant molecules confined in the electrostatic potential trough owing to the rigid and anionic DNA molecules are forced to form the rodlike solid instead of the planar one in the PSC. At the low salt concentration, the charge density gap between the rodlike DNA and OTA micelle hinders the complexion of them. For compensating for the charge density imbalance, the rodlike DNA makes the micelle spheroid but not cylindrical at the low salt concentration. No Krafft transition has been found in DNA-OTA complexes at the low salt concentration, which indicates that the spheroidal solid of surfactant cannot morphologically form. This is because of no parallel array of alkyl chains in the spheroidal assembly of the surfactant molecules. As described above, the morphologies of micelle and the hydrated solid of OTA in the PSC are seriously affected by the electrostatic circumstances produced by the coexisting polyelectrolyte. It is interesting to reveal what morphologies of the surfactant self-assemblies are induced by the polyelectrolytes. An aim of the present study is the examination of structural transitions induced by changes of temperature and salt concentration of the PSC composed of a flexible polyelectrolyte such as PAA instead of the rigid and rodlike DNA. Nanostructure change of the PAA-OTA complex in an aqueous solution at NaCl concentration, Cs from 20 to 400 mM, was investigated using a temperature-scanning SAXS technique. For the PAAOTA complex in the solution at a higher temperature than 25 °C, the 2D hexagonal structure was observed at Cs below 100 mM and the lamella structure was observed at Cs above 200 mM. For the complex in the solution at a lower temperature than 21 °C, a broad peak was observed in the SAXS profile at a scattering vector q ) 1.2 nm-1 when the Cs was lower than 200 mM but not when the Cs was 400 mM. The lamella structure was observed for the complex in the solution at 8 °C. The differential scanning calorimetric (DSC) experiment demonstrated that the nanostructure changes with heating the complex

10.1021/jp0686644 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/07/2007

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were accompanied with the endothermic enthalpy change. On the basis of the experimental results, the salt concentration dependent nanostructures are discussed. 2. Experimental Section Sodium poly(acrylic acid) (PAA) was purchased from Wako Pure Chemical Industries, Ltd. and was used without purification. The molecular weight of PAA was about 500 000 according to the supplier. Octadecyltrimethylammonium chloride (OTAC) was purchased from Tokyo Kasei Kogyo Co., was purified by crystallization from an acetone-methanol mixed solvent, and was used. The complex of PAA with OTA was prepared with mixing equal amounts of 30 mM sodium PAA and 30 mM OTAC solutions. The PAA-OTA complex obtained as a highly viscous gumlike and white precipitate was rinsed thoroughly with water (more than 1000 times as large as the volume of the complex), was dried in vacuum, and was stored. Surfactant concentration much higher than the critical micelle concentration (0.3 mM) in the salt-free condition9 is not needed to form the complex. The complexes were immersed into large amounts (more than 100 times as large as the volume of the complex) of the aqueous solution of OTAC (1 mM) and NaCl (a given concentration). It needed more than 1 month’s shaking for the complex to achieve the equilibration at room temperature, which was confirmed by the reproducibility of SAXS profiles obtained on different days. All chemicals used were of analytical grade. Double-distilled water was used. The SAXS measurements for the PSC equilibrating with the NaCl solution were made with an SAXS spectrometer of BL45XU installed at SPring8 of Japan Synchrotron Radiation Research Institute, Hyogo, Japan. The scattering vectors, q ) (4π/λ) sin θ/2, where λ is a wavelength of the beam (λ ) 0.09 nm) and θ is a scattering angle (θ ) 0.08 ∼ 5°), ranged from 0.1 to 6 nm-1. The temperature-scanning SAXS profiles were obtained with increasing the temperature from 5 to 30 °C at a heating rate of 0.5 °C/min. The complex was incubated for 24 h at 5 °C before the SAXS measurements. The differential scanning calorimetry measurements were carried out with a DSC 120 calorimeter (Seiko Inc., Japan) at a heating rate of 0.5 °C/min. The complex equilibrating with the NaCl solution was taken out and put into an aluminum pan together with about 50 µL of the NaCl solution for the DSC measurement. The complex was also incubated for 24 h at 5 °C before the DSC measurements. 3. Results 3.1. Nanostructures of the Complexes at High and Low Temperatures. Figure 1 shows the SAXS profiles of the PAAOTA complexes in the solutions at CS from 20 to 400 mM and at temperatures higher than 25 °C. At CS below 100 mM, four scattering peaks with a spacing ratio of 1:31/2:41/2:71/2 are observed. Obviously, the ordered structures formed in the complexes are indexed as 2D-hexagonal. Increasing CS to 200 mM, the structures change to the lamella indicated by two scattering peaks with a spacing ratio 1:2. The SAXS peak and corresponding lattice spacing, d, obtained from the relation d ) 2π/q1, where q1 is a q-value of the first peak (a peak at the lowest q) shown in Figure 1, are listed in Table 1. It is interesting to examine a comparison of the d-value with twice the surfactant alkyl chain length in the all-trans state, LAT ) 4.8 nm (0.154 + 0.127 × 18 nm),10 which is a lattice spacing of the lamella or the 2-D hexagonal supramolecule composed of the fully stretched surfactant molecule. The d values less than 4.8 nm in the solution at Cs below 100 mM indicate that a part of the

Figure 1. SAXS profile of PAA-OTA complex at a temperature higher than 25 °C.

TABLE 1: SAXS Peak Positions and Corresponding Spacings, d, for PAA-OTA Complex at Different Cs and Different Temperatures Cs/mM

temp. region

q1/nm-1

d/nm

structure

20

below 8 °C below 24 °C above 25 °C

1.7 1.21 1.42

3.7 5.2 4.5

solid lamella Ia solid cylinder 2D-hexagonal micelle

50

below 11 °C below 24 °C above 25 °C

1.71 1.21 1.41

3.7 5.2 4.5

solid lamella I solid cylinder 2D-hexagonal micelle

100

below 11 °C below 21 °C above 22 °C

1.67 1.18 1.4

3.8 5.3 4.5

solid lamella I solid cylinder 2D-hexagonal micelle

200

below 17 °C below 20 °C above 21 °C

1.66 1.15 1.33

3.8 5.5 4.7

solid lamella I solid cylinder lamella micelle

400

below 20 °C above 21 °C

1.43 1.25

4.4 5.0

solid lamella II lamella micelle

a

Lamella I: interdigitated lamella; lamella II: orthorhombic lamella.

surfactant alkyl chain in the complex takes the gauche conformation because of the thermal motion11 and that the ionized polymer chains intervene in the ordered array of ionized surfactant head groups because of the strong electrostatic attraction among them. It is well-known that the van der Waals attraction between alkyl chains of surfactant molecules in excess of their thermal kinetic energies induces the Krafft transition from the micelle to the hydrated solid at a low temperature.6 For the purpose to examine the effect of Krafft transition, that is, the freezing alkyl chains on the nanostructures in the complex, the SAXS measurements were made for the PAA-OTA complex in the solution at about 8 °C, and SAXS profiles shown in Figure 2 were obtained. Two relatively weak but sharp peaks with a spacing ratio of 1:2 and one strong and broad peak at q of about 1.7 nm-1 are observed at CS below 200 mM. The peak positions and the intensities of the former sharp peaks decrease with increase in the temperature, but no change of the latter broad peaks is observed below the transition temperature. This indicates that two structures in the complex exist. The structure indicated by the sharp peaks is obviously indexed as the multilamella. The broad peaks are caused by less ordered structures composed of the self-assembled surfactants, the correlation length of which is characterized by the peak position. The relative intensity of sharp peak to that of the broad peak

Nanostructure Change of PSC with Krafft Transition

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Figure 2. SAXS profile of PAA-OTA complex at about 8 °C. Figure 4. Transition temperatures as functions of Cs for the PAAOTA complexes (closed circles) and 50 mM OTAC solution (open circles).

Figure 3. DSC curves of PAA-OTA complexes equilibrating with the solutions at NaCl concentrations from 20 to 400 mM.

increases with CS, and the broad peak is split into three peaks at CS ) 200 mM and disappears at CS ) 400 mM as shown in Figure 2. The d value of the lamella structure increases from 3.7 nm at CS ) 20 mM to 3.8 nm at CS ) 200 mM to 4.4 nm at CS ) 400 mM. The disappearance of the broad peak at q of about 1.2 nm-1 and the sudden increase in the d value of the lamella indicate the structural change in the complex between CS ) 200 and 400 mM. The positions of the primary SAXS peaks and the corresponding structures described above are listed in Table 1. 3.2. DSC Analyses of the Complexes. It is known that the Krafft transition of the surfactant molecule in the aqueous solution is thermodynamically endothermic12 and that the transition temperature increases with an increase in the salt concentration.13 Figure 3 shows the DSC curves of the PAAOTA complex equilibrating with the NaCl aqueous solution. One endothermic peak is observed at 22 °C for the PAA-OTA complexes in the solutions at Cs ) 400 mM; two endothermic peaks are observed around 6 °C and 22 °C at Cs ) 20, 50, and 100 mM; and more than two peaks are observed in a temperature region between 12 and 25 °C at Cs ) 200 mM. The fact that no endothermic peak is observed in a temperature region between 25 and 70 °C indicates that the transition at the temperature around 20 °C shown in Figure 3 is the Krafft transition. The Cs dependence of the transition temperatures for the PAA-OTA complexes and OTAC solutions is shown in Figure 4. The transition temperatures for the PAA-OTA

complexes are substantially independent of Cs while those for the OTAC solutions increase with the Cs. 3.3. Temperature-Scanning SAXS Profiles of the Complexes. The nanostructures of PAA-OTA complex at high and low temperatures are different from each other as shown in Figures 1 and 2. The calorimetric behavior of the PAA-OTA complex exhibits several endothermic peaks in the temperature region between 5 and 25 °C as shown in Figure 3. For examining what changes in the nanostructures correspond to the endothermic peaks, temperature-scanning SAXS experiments were carried out for the PAA-OTA complexes equilibrating with the NaCl solutions at Cs ) 20, 50 100, 200, and 400 mM. The 3D SAXS profiles in the q-regions between 0.5 and 1.8 nm-1 are shown in Figures 5-7. Corresponding to the endothermic peaks at around 20 °C shown in Figure 3, the transitionally decaying and growing peaks are observed at around 20 °C in the temperature-scanning SAXS profiles. The decaying broad peaks at Cs ) 20, 50, 100, and 200 mM are positioned at lower q than the growing sharp peaks, while the decaying sharp peak at 400 mM is positioned at higher q than the growing sharp peak. The growing sharp peaks are due to the 2D hexagonal structures at Cs ) 20, 50, and 100 mM and the lamella structures at Cs ) 200 and 400 mM, which can be identified by higher order peaks shown in Figure 1. The peak decay indicates melting of the structures; less ordered structures are composed of the self-assembled surfactants at Cs ) 20, 50, 100, and 200 mM and the lamella structure at Cs ) 400 mM. The observation of endothermic peaks at the temperatures corresponding to the melting temperatures of the structures suggests that the less ordered and the lamella structures at lower temperatures than the Krafft transition are made of the solidlike alkyl chains of surfactant molecules. Figure 6 clearly shows that the lamella peak around q ) 1.7 nm-1 at Cs ) 200 mM weakens, shifts to lower q with an increase in temperature, and disappears. The sequential endothermic peaks at 12.1 and 16.6 °C shown in Figure 3 reflect the structural change mentioned above. The same temperature dependence of the structure is also seen for the lamella peak around q ) 1.7 nm-1 at Cs ) 20, 50, and 100 mM in Figures 5 and 6, although it is very faint. The endothermic peaks corresponding to the structure changes at Cs ) 20, 50, and 100 mM are also observed in DSC curves shown in Figure 3. The d value of the lamella structure gradually increases from 3.7 to 4.2 nm with an increase in temperature. The d-values are much

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Figure 5. 3D intensity of the temperature-dependent SAXS profile of the PAA-OTA complex at Cs ) 20 and 50 mM.

Figure 6. Temperature-scanning SAXS profiles of PAA-OTA complexes at Cs ) 100 and 200 mM.

shorter than LAT (4.8 nm),10 indicating that the lamella is interdigitated. The increase of d is due to a decrease in the interdigitated and frozen fraction of alkyl chains with heating. The lamella structure formed at 8 °C gradually melts with decreasing the frozen part of alkyl chains when the temperature increases. The melting temperature of the lamella increases with Cs as shown in Figures 5 and 6, in which the disappearance temperatures of the SAXS peaks at around q ) 1.7 nm-1 observed at 8 °C increase with Cs. This indicates that the interdigitated lamella structure in the complex is more stabilized by adding the salt. 4. Discussion It is well-known that the addition of monovalent electrolyte to ionic surfactants induces the structural transitions from micelles of high curvature in salt-free solutions to larger structures with cylindrical and planar geometries.14 In the PSC, the electrostatic potential trough made by polyelectrolyte takes very important roles in forming the micelles in an addition to the salt effects mentioned above. In the aqueous NaCl solution at a high concentration, say Cs ) 400 mM, the OTA micelles are cylindrical in the DNA-OTA complex as shown in the

previous experiment7 and are planar in the PAA-OTA complex as shown in the present experiment. In the solution at a concentration as low as 100 mM, the micelles are spheroidal in the DNA-OTA complex7 and are cylindrical in the PAA-OTA complex. It is true that adding salt has the effect to reduce the curvature of the micelle captured in the electrostatic potential trough made by the polyelectrolyte. The repulsive interaction among the head groups of surfactant molecules is reduced in the trough to stabilize the structures with the low curvatures. In comparing the micelle in the DNA-OTA complex with that in the PAA-OTA complex, we can say that the flexibility of polyelectrolyte or the amorphous geometry of potential trough promotes reducing the curvature of the micelle in the PSC. The repulsive interaction among the head groups in the soft potential trough made by PAA is more reduced than that in the hard potential trough made by DNA. At a temperature below the transition, the van der Waals attraction energy between alkyl chains of the surfactant molecules exceeds their thermally agitating energy. The endothermic transition observed for the PAA-OTA complex around 20 °C shown in Figure 3 corresponds to the Krafft transition, that is, the order-disorder transition of alkyl chain of the OTA

Nanostructure Change of PSC with Krafft Transition

Figure 7. Temperature-scanning SAXS profile of PAA-OTA complex at Cs ) 400 mM.

molecule. It is known that the Krafft transition temperature of the aqueous surfactant solution increases with an increase in the salt concentration.13 According to the thermodynamics, the transition-temperature TK is given by ∆H ) TK∆S, where ∆H and ∆S, respectively, are changes of enthalpy and entropy with the transition. The stacking of alkyl chains in an ordered manner is accompanied with counterion binding and dehydration of the head group of the surfactant molecule. An increase in Cs has the effect to reduce the contribution of counterion binding and dehydration to ∆S, because the changing amounts of binding counterion and dehydrating water molecules decrease with Cs. The ∆H due to van der Waals attraction between the alkyl chains is independent of Cs. Thus, TK of the OTAC micelle in the solution increases with Cs. This is not the case for the cylindrical micelles in the PAA-OTA complex at Cs below 100 mM as shown in Figure 4. This might reflect on stronger electrostatic constrained force exerting on the micelles in the PAA-OTA complex than that in the solution phase. The change in the degree of counterion binding to the OTA molecule and the dehydration of the head group, that is, the ∆S with the Krafft transition in the PAA-OTA complex, is not much influenced by the coexisting salt, since most of the counterions of OTA molecules are the ionized carboxylic acid groups of PA, the concentration of which is much higher than the salt concentration. In the aqueous OTAC solution at temperatures below the Krafft transition, two phases of coagel and gel exist, which are considered to be the lamella composed of the bilayer membrane.15 In the coagel phase, the alkyl chains of OTA molecule take the trans conformation to be in the orthorhombic parallel packing state.16 In the gel phase, the alkyl chain rotates around the axis of chain.16 It is considered that a thick water layer exists between the bilayers in the gel phases but not in the coagel phase. The mesophase in the PAA-OTA complex is identified as the coagel phase judging from the fact that the d-values shown in Table 1 are less than twice LAT (4.8 nm).10 It is interesting that the d-value of the lamella structure in the PAA-OTA complex at Cs ) 400 mM shown in Figure 7 gradually increases from 4.4 nm (q ) 1.43 nm-1) to 5.0 nm (q ) 1.25 nm-1) with an increase in temperature. This indicates that the alkyl chains in the PAA-OTA complex can slide over the packed chains to

J. Phys. Chem. B, Vol. 111, No. 29, 2007 8457 change the orthorhombic angle owing to the thermal agitation. In this respect, the hydrated solid of OTA in the potential trough made by PAA is different from the coagel phase in the OTAC solution. It is also interesting that the d-value of the lamella structure in the PAA-OTA complex at Cs ) 200 mM shown in Figure 6 gradually increases from 3.7 nm (q ) 1.7 nm-1) to 4.2 nm (q ) 1. 5 nm-1) with an increase in temperature. A gradual increase in the d-value of the lamella structure from 3.7 nm (q ) 1.7 nm-1) is also observed for the PAA-OTA complex at Cs ) 20∼100 mM as shown in Figures 5 and 6. The lamella structure in the PAA-OTA complex at Cs below 200 mM is interdigitated and different from that in the complex at Cs ) 400 mM. This is inferred from the fact that the d-value of the lamella at Cs below 200 mM is much shorter than that of the lamella at Cs ) 400 mM. The interdigitated structure disappears when the d-value of the lamella exceeds about 4.2 nm. The morphology change in changing from the micelle to the hydrated solid with freezing the alkyl chains of surfactant could be less favored because of the energy barrier between the morphologies. It is noticeable that the planar structure of micelle in the complex at Cs ) 400 mM is preserved in freezing the alkyl chains at the Krafft transition. It is reasonable to say that the cylindrical geometry of micelles in the complexes at Cs below 100 mM can be also preserved in freezing the alkyl chains at the Krafft transition. If this is the case, then the broad SAXS peaks at q around 1.2 nm-1 shown in Figures 5 and 6 can be said to be due to the structures composed of rodlike solids of OTA molecules in the complex. The characteristic length of ordered array of the rodlike solid, 5.2 (∼2π/1.2) nm, is longer than the LAT value of OTA molecule. The cylindrical micelle composed of the liquidlike alkyl chains can change to the rodlike solid composed of the alkyl chains taking the trans-zigzag conformation packed in parallel with the neighboring chains arrayed along the rod axis. When the temperature decreases, the van der Waals attraction among the alkyl chains can induce the interdigitation of them, which might be accompanied by the assembly of cylindrical rods and the multilayer formation of flattened platelike structures. Judging from the SAXS profiles shown in Figure 2, there coexist the randomly arrayed rods and the multilayers of the flattened plates in the PAA-OTA complex at Cs below 200 mM when the temperature is low enough for the thermal motion of alkyl chains to freeze and to interdigitate them because of the van der Waals attraction among them. The coexisting two structures could be connected to form something like the mattress structure for the dry PSC proposed by Antonietti et al.17 For identifying the detailed structure, however, further investigations are needed. Acknowledgment. The SAXS experiments were performed at SPring 8 with the approval of Japan Synchrotron Radiation Institute. References and Notes (1) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (2) Khandurina, Yu, V.; Dembo, A. T.; Rogacheva, V. B.; Zezin, A. B.; Kabanov, V. A. Polym. Sci., Ser. A Ser. B 1994, 36, 189. (3) Sokolov, E. L.; Yeh, F.; Khokhlov, A.; Chu, B. Langmuir 1996, 12, 6229. (4) Antonietti, M.; Thu¨nemann, A. Curr. Opin. Colloid Interface Sci. 1996, 1, 667-671. (5) Balmbra, R. R.; Clunie, J. S.; Goodman, J. F. Nature 1969, 222, 1159. (6) Krafft, F. Ber. d. Deutch. Chem. Ges. 1894, 27, 1747, 1755. (7) Kawashima, T.; Sasaki, A.; Sasaki, S. Biomacromolecules 2006, 7, 1942-1950. (8) Kodama, M.; Seki, S. AdV. Colloid Interface Sci. 1991, 35, 1-30.

8458 J. Phys. Chem. B, Vol. 111, No. 29, 2007 (9) Klevens, H. B. J. Am. Oil Chem. Soc. 1953, 30, 74. (10) Tanford, C. The Hydrophobic Effect, Formation of Micelles and Biological Membrane; Wiley: New York, 1973. (11) Sokolov, E.; Yeh, F.; Khokholov, A.; Grinberg, V. Y.; Chu, B. J. Phys. Chem. B 1998, 102, 7091-7098. (12) Kodama, M.; Tsujii, K.; Seki, S. J. Phys. Chem. 1990, 94, 815-819. (13) Vautier-Giongo, C.; Bales, B. I. J. Phys. Chem. B 2003, 107, 53985403.

Sasaki (14) Swanson-Vethamuth, M.; Feitosa, E.; Brown, W. Langmuir 1998, 14, 1590-1596. (15) Tsuchiya, M.; Tsujii, K.; Maki, K.; Tanaka, T. J. Phys. Chem. 1994, 98, 6187-6194. (16) Kawai, T.; Uemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J. Colloid Interface Sci. 1985, 103, 56-61. (17) Antonietti, M.; Conrad, J.; Thu¨nemann, A. F. Macromolecules 1994, 27, 6007-6011.