Langmuir 1993,9, 673-680
673
Growth of Silica Polymers in a Lamellar Mesophaset M. Dubois,*J Th. Gulik-Krzywicki,o and B. CabaneS Equipe mixte CEA-RP, Service de Chimie Moleculaire, CEN-Saclay, 91191 Gif sur Yvette Cedex, France, and C.G.M.,CNRS, 91191 Gif sur Yvette Cedex, France Received June 5, 1992. In Final Form: November 25, 1992 Silicic acid monomers have been polymerized within the lamellar phase of the system didodecyldimethylammonium bromidetwater. The locations of the polymers and their effect on the structure of the layers have been examined through neutron diffraction and electron microscopy. It has been found that the polymers condense on the polar head groups of the surfactantand cause a substantialdisorganization of the bilayers. The evolution of the structuresduring polymerization is determined by the texture of the mesophase. When the texture is made of liposomes, most of the silica remains coherent with the bilayers. When the texture is made of flat layers, the segregation is fast and complete. Introduction Materials which are dispersed at the nanometer scale can be made through the growth of inorganic polymers in a solvent. These processes are called “sol-gel” because the initial state is a solution and the final state a gel where the polymers have become bushy structures which trap the solvent and reaction byproducts in a macroscopic network.lv2 Later on the gel may be used as a host for active species, evacuated to yield a porous material, or collapsed to yield films or fiber^.^ The success of these operationsdepends on the structureof the polymers,which much be controlled. Usual sol-gelprocesses give inorganic polymers which grow as bushy isotropic structures. The resulting gels are three-dimensional materials made by cross-linking of these bushy structures. Our aim is to produce layered inorganic materials through a sol-gel process. This requires the use of a template which prevents the propagation of the polymerization in certain directions. Usual templates are rigid objects such as clay plateleh4p5 These templates have good mechanical stability, hence they will not be disturbed by the polymerization. However it will not be possible to eliminate them at the end of the reaction. Another possibility is to use fluid objects as templates;in this respect an attractive choice is the bilayers of soap-water mesophases. Organic polymers have already been polymerized in the surfactant bilayers and in the water layers of the lamellarmesophases;6J to our knowlege no polymerization of inorganic materials in mesophases has been reported so far. We chose this second option, using a template made with the lamellar phase of the cationic surfadant didodecyldimethylammonium bromide (DDAB), (C12H&N(CH3hBr. This soap-water mesophase is advantagous in many + This work used the neutron beams of ILL and the X-ray beam of LURE. Equipe mixte CEA-RP, Service de Chimie Moleculaire.
*
8 C.G.M., CNRS.
(1)Dubois, M.; Cabane, B. Macromolecules 1989,22,2526-2533. (2)Cabane, B.;Dubois, M.; Lefaucheux, F.; Robert, M. C. J. NonCryst. Solids 1990,119,121-131. (3)Brinker, C. J.: Scherer, G. W. Sol-Gel science; Academic Press: New York, 1990. (4)Blumstein, A.; Hertz, J.;Sinn, V.; Sadron, Ch. C. R. Hebd. Seances Acad. Sci. 1958.. 246., 1856. Blumstein. A. Bull. SOC.Chim. Fr. 1961, 899-906. (5)Brown, J. F.; White, D. M. J. Am. Chem. SOC.1960,82,5671C. (6)Hertz, J.; Reiss-Husson, F.; Rempp, P.; Luzzati, V. J. Polym. Sci. 1966,4,1275-1290. (7)Friberg, S.E.:Whon, Chang Sup. Macromolecules 1987,20,20572059.
respects. First of all, it forms at room temperature with water contents in the range 4 to 25 % ;a at the lower end of this range the thickness of the water layers is 10oO A and at the upper end it is 150 A; hence the confinement of the inorganic polymers can be modulated according to the water content of the mesophase. Next, it shows a good resistance to the addition of the other components such as alcohols and salts which are byproducts of sol-gel reactions. Finally, it is free of structural defects such as pores which would allow the polymers to cross from one water layer to another one;e hence it may be possible to produce independent inorganic lamellae. Still, any project of this type must confront three obstacles. First of all, the soap bilayers may inhibit the polymerization reaction (termination of the polar groups of the soap molecules) or make it more difficult (confinement). Alternatively, the polymerization can create mechanical constraints for the bilayers and disrupt their organization. Finally,the lamellarorder may be lost when the soap is washed out. This loss of the lamellar phase upon polymerization has indeed been a difficulty in previous attempts for making layered materials.lOJl In our case the inorganic polymers are made by the hydrolysis and condensation of silicon tetramethoxide. We have already studied this polymerization in the bulk through neutron and light scattering; it produces an array of branched polymers which entrap the solvent into a macroscopic ge1.192J2 In this work we have found that the lamellar phase of the water-DDAB system does allow such polymerization reactions to proceed in its water layers. However the late stage of polymerization shows a trend toward the segregation of polymers and expulsion of the templates. Finally, the direct washing of the templates with alcohol resulte in a condensation of inorganic lamellae; hence the final material has lost the lamellarstructure. In order to obtain a layered material it would be necessary to use a passivation of the inorganic layers. Materials The starting point in a sol-gel process is a solution of organometallic precursors. First these precursors are hydrolyzed into monomers which carry four to six reactive hydroxyls each. (8)Dubois, M.;Zemb, Th. Langmuir 1991,7 , 1352-1360. (9) Kekicheff, P.; Cabane, B.: Rawiso, M. J. Colloid Interface Sci. 1984, 102 (l),51-70. (10)Holtzcherer, C.; Wittmann, J.; Guillon, D.; Candau, F. Polymer 1990,31,1978-1985. (11)Laversanne, R. Macromolecules 1992,25,489-491. (12)Cabane, B.;Dubois, M.; Duplessix, R. J . Phys. (Paris) 1987,48, 2131-2137.
0743-7463/93/2409-0673$04.00/00 1993 American Chemical Society
674 Langmuir, Vol. 9, No. 3, 1993
Dubois et al.
Then the monomers condense to form branched polymers with an oxide skeleton and reactive hydroxyls as side groups. In this work the precursor was silicon tetramethoxide (TMOS). Through hydrolysisthis precursor yields silicic acid, which is the reactive monomer, according to the reaction
-
Si(OR), + 4H,O Si(OH), + 4ROH (1) The hydrolysis of TMOS was performed directly in water; initially the mixture was an emulsion but it became homogeneous within minutes as the hydrolysis removed the methoxy groups. Because the hydrolysis of TMOS is catalyzed by H+ ions, we used water at pH 3 as reaction medium. Since there was always a large excess of water, this hydrolysis was complete in a very short time. In the next stage the solution containing water, silica, methanol from the hydrolysis, and H+ions was used to prepare the lamellar phase of DDAB. The surfactant was simply dissolved in the solution and the lamellar phase formed immediately after manual agitation. Then the silicic acid molecules condensed spontaneously to form polymers according to the reaction
I
% MeOWH30
15
10
5
0
0
s
10
15
20
25
30
Sb
DDAB
Our observationsshow that the polymerizationproceeds faster
Figure 1. Phase diagram for the mesophases at room temperature. The bilayers were made of DDAB; the water layers contained water, methanol, and H+ ions (pH 3). The shaded area corresponds to lamellar phases which do not flow; the unshaded area corresponds to lamellar phases which flow spontaneously. In polymerizationexperiments it was found that the silica polymers could be contained within viscousmesophases whereas they segregated out of fluid lamellar phases.
Phase Diagrams In this section we examine the phase behavior of the system (observation through birefringence + neutron diffraction) at the onset of polymerization. Our starting point is the phase diagram of the pure DDAB/water system which was established recently (see Figure 1 of ref 8). At low DDAB concentrations (below 1.5%)the surfactant forms isolated vesicles in water. Between 1.5% and 3 % the system contains a mixture of unilamellar vesicles and multilamellarliposomes. Above 35% there is a lamellar phase where the surfactant chains are in liquid state if the temperature is above 17 "C. Finally, when the DDAB concentration exceeds 33% , the system separates out a collapsed lamellar phase which contains 75% DDAB. Effects of H+Ions. As mentioned above, the preparation of silicic acid from DDAB is catalyzed by H+ ions. These ions remain in water with silicic acid, and they also control the rate of condensation. The pH used for hydrolysis was 3; this set the ionic strength of the water used to make the lamellar phase. The effect of ions on the lamellar phase of DDAB is known.8 As the ionic strength is increased, the extent of the isotropic phases grows at the expense of the lamellar phase. At an ionic strength of 10-3 M, the proper lamellar phase starts at 4% DDAB. Thus the lamellar phase of DDAB can be used in the presence of the ions brought by the reaction if the DDAB concentration is between 4% and 33%. Effect of Methanol. Methanol is produced by the hydrolysis according to reaction 1;hence it is present in the water which was used to make the lamellarmesophase. We have observed two effects of methanol on the mesophase: the transformation of a viscous lamellar phase into a fluid one, and the replacement of the lamellar mesophase by an other phase. Figure 1shows the range
of existence for the lamellar phase in the presence of methanol and H+ ions. In the presence of methanol the extensionof the lamellarphase is reduced, and above 30 % methanol in water this phase is replaced by an isotropic solution of DDAB in the water/alcohol mixture. Within the lamellar phase there are three ranges which differ according to the texture of the sample. At DDAB concentrations between 3% and 5 % the phase is a dispersion of liposomes in water.8 At higher DDAB concentration, the phase consists of densely packed liposomes (see below); it is slightly opalescent and it does not flow. Finally, at high DDAB and methanol concentrations the phase is clear and fluid; it is made of stacked bilayers (see below). The boundary between the viscous phase and the fluid one depends on temperature: at temperature above 50 O C all lamellar phases are fluid. Finally, fluid phases can be turned into viscous ones by vigorous shaking of the sample. This confirms that the differencebetween viscous and fluid phases is a matter of texture rather than structure. Effect of D2O. D2O was used in some of the lamellar phases because it provides a good contrast in neutron scattering between the water layers and the surfactant bilayers. However the substitution of HzO byD2O reduces the extension of the lamellar phase. For a complete replacement, the lamellar phase starts at 5 % DDAB and ends at 10%. Fortunately, a full replacement of H2O by D2O is not necessary. What is needed is a substitution which matches out the contrast of silica. This is obtained with 60% DzO in H2O. In this case the upper limit of the lamellar phase is at 15% DDAB; beyond this point the collapsed phase separates out. Lamellar Phases with Polymerized Silica. At the end of the polymerization the samples were examined accordingto their optical and mechanical properties.Some samples remained transparent and birefringent;they were labeled as pure lamellar phases, and this was confirmed through diffraction (see below). These phases were also mechanically rigid, indicating that the silica formed a continuous network through the sample. Other samples became turbid or opaque; this was taken as an indication of phase separation. Figure 1shows the location of either type according to concentrations of DDAB and TMOS. This diagram is similar to the initial phase diagram in the presence of methanol; indeed the hydrolysis of TMOS releases methanol in a volume which is approximately equal to the TMOS volume. The samples which remained
HO-Si
+ Si-OH
-
Si-0-Si
+ HOH
(2)
in the lamellar phase than in the bulk. This appear to be a catalytic effect of the surfactant heads groups. Indeed the bulk polymerizationof TMOS was found to be accelerated by a factor of 4 in the presence of N(CH3)4+Br. A typical reaction time is a few hours for a lamellar phase containing 20% TMOS by volume in water. In summary silicic acid may be polymerized within the water layers of DDAB mesophase. However the presence of other speciescannot be avoided H+ions which are necessaryto catalyze the hydrolysis and methanol which is a byproduct of this hydrolysis. Therefore it is necessaryto examine how the lamellar phase behaves in the presence of these additives. This is presented in the next section.
Langmuir, VoZ. 9, No. 3, 1993 675
Silica Polymers in a Lamellar Mesophase
OOnm
Figure 2. Freeze-fracture electron micrograph of an isotropic solutionof DDAB in water (0.2% of DDAE3). Notice the presence of a heterogeneous population of well-dispersed vesicles.
Figure 3. Freeze-fracture electron micrograph of sample at the lower boundary of the lamellar phase of the DDAB-water system (2% of DDAB). Noticethe coexistenceof very large, multilayered structures and unilamellar vesicles.
pure lamellar phases are all located in the region of the viscous lamellar phases of (Figure 1). Samples which were prepared as fluid lamellar phases ended as phase separated mixtures. Electron Microscopy A very thin layer of the sample (20-50 pm thick) was placed on a thin copper holder and then either rapidly quenched in liquid propane or squeezed between the holder and a thin copper plate before being quenched in liquid propane.13J4 Both types of preparations were fractured in vacuo, better than 10-6Torr, with a liquid nitrogen cooled knife in a Balzers 301 freeze-etching unit. In the case of “squeezed” samples, the fractures were obtained by removing the copper plate with the cold knife. The replication was done using unidirectional shadowing, at an angle of 35O,with platinum-carbon, 1 to 1.5 nm of mean metal deposit. The replicas were washed with organic solvents and distilled water and were observed in a Philips EM 301 electron microscope. The contrast in images is related to the depth fluctuations of the metal deposit. Isotropic Phase. Diluted DDAB samples (
