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The preparation and properties of a fibrous tube alkoxysiloxane derived from a tube silicate
Colloids and Surfaces, 63 (1992) 139-149
139
Elsevier Science Publishers B.V., Amsterdam
The preparation and properties of a fibrous tube alkoxysiloxane derived from a tube silicate B r u c e A. H a r r i n g t o n a n d M a l c o l m E. K e n n e y
Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA (Received 26 August 1990; accepted 26 November 1990)
Abstract The conversion of the tube silicate K 2CuSi4010 to the fibrous tube alkoxide (n-PrO) ~ 0.3(MeO) ~ o.3(HO) ~ 0.4 SiO 1.5 by treatment of the silicate with a solution composed of n-PrOH, MeOH, and HC1 is described. Also described are a glass devitrification synthesis of K2 CuSi4010, the structure and properties of the tube alkoxide, and the preparation, structure, and properties of a variant of this alkoxide in which the tubes are lightly cross-linked. In addition, the potential usefulness of the two alkoxides for the synthesis of ceramics, and the valuable sets of properties that can be expected from appropriately constituted tube polymers are discussed.
Keywords: Fibers; inorganic polymers; sol-gel ceramics; tube alkoxides; tube polymers; tube silicates.
Introduction Earlier, the preparation of a polymeric allyloxysiloxane from the layer silicate chrysotile by two different procedures was reported [1]. In the simpler of these procedures, chrysotile was treated with hydrochloric acid and then with allyl alcohol. Apparently the allyloxysiloxane obtained from these procedures had a framework similar to that of chrysotile. More recently, the preparation of a series of monomeric alkoxysilanes and oligomeric alkoxysiloxanes from monomeric and oligomeric metal silicates was reported [2-7]. In the procedures used in this work, the silicates were treated with solutions of hydrochloric acid and an alcohol. In each case, an alkoxide with a framework that was either the same as that of the parent silicate, or was similar to it, was obtained. The preparation of this latter set of compounds from silicates has established a new, general, silicate-based route to monomeric and oligomeric silicon alkoxides. In view of this previous work, it appears that it Correspondence to: M.E. Kenney, Dept. of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA. 0166-6622/92/$05.00
should be possible to use selected polymeric metal silicates to prepare polymeric alkoxysiloxanes having frameworks that are similar to those of their parent silicates. Included among these silicates are various members of the rare, but interesting, tube family [8] and various members of the sheet family. Here we describe work showing that the tube silicate K2CuSi4Olo [9] can be used to prepare a tube alkoxysiloxane. In addition, we describe the conversion of this alkoxide to a variant of it in which the tubes are lightly cross-linked. Finally, we discuss the potential usefulness of these tube alkoxides for the synthesis of ceramics and the valuable sets of properties that can be expected from appropriately constituted tube polymers.
Experimental Syntheses and preparations Synthesis of K 2CuSi401o A mixture of KOAc (7.59 g), Cu(OAc)2"H20 (7.72 g), (EtO)4Si (32.2 g), H 2 0 (105 ml), and EtOH
© 1992 - - Elsevier Science Publishers B.V. All rights reserved.
140 (145 ml) was stirred for 2 days, allowed to stand for 3 days, and evaporated to dryness under vacuum (80°C). The material remaining was ground, heated (200°C) for 25 h, and then heated more vigorously (590°C) for 18 h. The resulting solid was ground (15.4 g). A similar powder (30.4 g), prepared with an alternative calcination procedure (one in which the gel was subjected several times to a process in which it was heated (~ 600°C) and then ground), was melted (1200°C)in a platinum crucible. (This alternative calcination procedure was poorer because it led to the rapid evolution of a significant volume of gas that inflamed.) Part of the resulting melt was poured out onto a steel plate and formed into two disks (4.23 g; 4.61 g). The glass remaining in the crucible after the disks had been formed (15.0 g) was left in place (total yield of K2CuSi4Olo glass based on weight of the powder was 78%). The disks were placed on a fused silica plate and the crucible (with its glass) was placed on a hearth plate. With a controlled atmosphere furnace, the silica plate and the disks, and the hearth plate and the crucible were heated (750°C) under a slow flow of N 2 for 30 days. The solids formed by the disks were pried from the plates, and the solid formed by the glass in the crucible was broken away from the crucible. The disk products were crushed and the resulting granular material was converted by a particle reduction and sizing process involving grinding and screening (fume cupboard) into a moderate particle size fraction (less than 60 and greater than 270 mesh) and two other fractions. The moderate particle size fraction was separated into two fractions with a magnetic separator. The fraction composed of relatively magnetic blueviolet acicular crystals was retained (2.55 g, identified by X-ray powder diffractometry, estimated purity by light microscopy 98%; contained yield of K2CuSi4Olo based on weight of glass disks, 28%). The crucible solid was purified in the same manner (5.69 g, estimated purity 98%; contained yield of K2CuSi4Olo based on weight of glass in crucible ~ 37%).
B.A. Harrington,M.E. Kenney/ColloidsSurfaces63 (1992) 139-149 Synthesis of tube alkoxide A mixture of K2CuSi4Olo (less than 200 mesh, 258 mg), n-PrOH (25 ml), MeOH (50 ml), and an H C I - M e O H solution (8.7 N, 0.60 ml) was heated to reflux over 30 min and then was distilled slowly (12 drops per min) for 1 h. An H C I - M e O H solution (0.25 N, 21 ml) was added to the suspension obtained over a period of 3.5 h while the suspension was being distilled slowly (6 drops per min). The mixture remaining was distilled slowly (10 drops per min) for 120 min, and still more slowly (3 drops per min) for 15 rain. The resultant was filtered and the filtrate (yellow-brown) was discarded. The solid (the tube alkoxide) was washed (MeOH, 1:1 acetone-H20 solution, MeOH), left damp, and weighed (~ 327 mg). Samples of the solid in the reaction mixture that were taken before the mixture had been fully stripped of MeOH had a high ratio of methoxy to propoxy groups.
Preparation of product A The tube alkoxide was suspended in MeOH (~ 47 mg ml- 1) with the aid of sonication.
Preparation of product B Product A was spread on a Teflon-coated microscope slide and allowed to air dry.
Preparation of product C The tube alkoxide was suspended in 1,4-dioxane (~ 20 mg ml- 1) with the aid of sonication.
Preparation of product Dn The tube alkoxide was suspended in 1,4-dioxane (,,~0.2 mg m1-1) with the aid of sonication.
Preparation of product Dt The tube alkoxide was suspended in 1,1,2-trichlorotrifluoroethane (~ 0.2 mg ml- 1) with the aid of sonication.
Preparation of product E One to three drops of D d or D t were placed on an electron microscope specimen grid (copper, 200
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149
141
mesh, carbon coated, 200 A film) resting on filter paper. The grid was allowed to air dry briefly and then was inserted into the electron microscope and subjected to the vacuum of the microscope.
Properties of KzCuSi401o, tube alkoxide, and tube alkoxide products
Preparation of product F The tube alkoxide (~189 mg) was broken up, dried (,~ 75 Torr, 80°C, 1 h), and weighed (112 mg).
K2CuSi4Olo melts between 790 ° and 810°C. It reacts with concentrated HC1 within minutes to give a pale yellow-green solution and a white residue.
Method for determination of alkoxide content ofF A mixture of F (~ 50 mg), H 2 0 (~ 5 ml), Et3N (~0.05 ml), and dimethylformamide (,~10 ml) was heated (70 ° C) for at least 24 h in a vial closed with a septum, and the weights of n-PrOH and MeOH liberated were determined by gas chromatography (benzene-dimethylformamide reference solution). From these values the weight percentages of n-PrO and MeO in the polymer were calculated. The infrared spectrum of a residue from the treatment of F by this method showed no CH bands. Treatment of MeSi(O-n-Pr)3 by this method gave the anticipated amount of 1-propanol. No evidence indicating that the solvent had hydrolyzed was observed.
Instrumentation A Frantz L-1 isodynamic separator (S.G. Frantz Co., Inc.) was used in the magnetic separations, and a Branson Model 200 sonifier (Branson Sonic Power Co.) was used in the sonications. A Philips APD 3520 auto-diffractometer (Philips Electronic Instruments, Inc.) and a Rigaku DMAXB autodiffractometer (Rigaku USA, Inc.) were used to collect the X-ray powder data. For the gas chromatography work a Varian Vista 4600 gas chromatograph (Varian Associates, Inc.) fitted with a DB-5 FSOT capillary column (J&W Scientific Inc.) was used. A Model JEM-100SX Japan Electron Optics Laboratory transmission electron microscope (JEOL USA Inc.) equipped with a liquid N2 cooled sample chamber was used for the electron microscopy.
K2CuSi401o
Tube alkoxide The damp tube alkoxide is a fluffy light blue solid.
Products Product A Product A is an opaque, light blue suspension that does not separate for at least 7 days. Product B This product is light blue and is like a piece of rough paper. It gives an X-ray powder pattern with reflections at (d(A)(l/Io)) 14.3 (100), 7.05 (br, 36), and ~ 3.6 (br) as well as weak reflections for K2CuSi4Olo. Very probably the 14.3 and 7.05 A reflections are matching first- and second-order reflections. The 3.6 A reflection is probably composed of more than one line. Product C Product C is a translucent, very light blue gel that flows when agitated. Products Da and D, These products are transparent colorless colloidal suspensions. Product E Micrographs of E are shown in Figs 1, 2, and 3. These and other micrographs of E show that it is composed of matted, high-aspect-ratio fibers.
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B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149
Fig~1. Micrograph of E showinga fiber ,~60 ~tm long (lowerarrow) and a fiber with a kink (upper arrow).
Product F
This product is a fibrous, light blue solid. Light microscopy observations show that it is composed of colorless fibers (estimated abundance ~90%), K2CuSi4Olo (estimated abundance ~8%), and undevitrified glass (estimated abundance ~ 2%). It feels waxy when ground in an agate mortar and disperses in organic solvents such as dioxane, CHC13, and CC14. It does not dissolve in such solvents nor in water. F is extremely hydrophobic
and only after at least 48 h does it become wetted when in contact with water. As determined by the hydrolysis-gas chromatography method outlined above, F contains 18.3 wt.% n-PrO and 9.9 wt.% MeO. Mulls of it give infrared spectra with bands at: (Fluorolube) 3340 (m, br, OH), 2970 (m, CH), 2940 (sh, CH), 2880 (m, CH), 2855 (w, CH of OCH3) , 1480 (sh), 1470 (w), 1400 (w), 1385 (w); (Nujol) 1160 (s, SiOSi, SiOC), 1080 (s, SiOSi, SiOC), 970 (sh, SiOH), 840 (w), 610 (w), 470 (m)cm -1.
B.A. Harrington,M.E. Kenney/Colloids Surfaces 63 (1992) 139-149
Fig. 2. Higher magnification micrograph of E.
143
Fig. 3. Micrograph of E showing a multitude of small fibers diverging from a large fiber.
Discussion
K2CuSi401o Synthesis of K 2CuSi, O lo The devitrification synthesis of K2CuSi4Olo described here requires less elaborate apparatus than does the previously described hydrothermal synthesis of this silicate [9], and as a result it is more convenient. This conveniently makes the preparation of K2CuSi4Olo on the gram scale relatively simple. Work further extending that described here shows that K2CuSi,O~o can be prepared alternatively from a glass made by heating a 1:1:4 mole ratio mixture of K2CO3, CuCO3, and SiO2 [-10]. However, this synthesis gives a lower yield, probably because the glass is less homogeneous. Structure of K2CuSi401o The structure of the silicate ion in K 2 C u S i 4 O l o as determined by Kawamura and Iiyama [-9] is
illustrated in Fig. 4. As is apparent, this ion is, without ambiguity, a tube. (It could be argued that the silicate ions in Na2TiSi4Olo [11,12], NaKCuSi4Olo [13], and some other silicates, contrary to the common view, are not tubes but rather are elaborated ladders.) The rings in the silicate ion in K2CuSi4Olo contain, as inspection of Fig. 4 shows,
t3 oSi
©0
Fig. 4. Latticework tube ion in K2CuSi4Olo according to the data of Kawamura and Iiyama.
144 eight members, twelve members, and sixteen members, and accordingly are of stable sizes. The copper ions in K2CuSi4Olo coordinate with pendent oxygens of neighboring tubes and thus interconnect the tubes. Three-quarters of the potassium ions also coordinate with pendent oxygens of neighboring tubes and further interconnect the tubes. The remaining one-quarter of the potassium ions reside inside the tubes.
Tube alkoxide and products A - F Synthesis of tube alkoxide A property of K2CuSi4Olo that is essential to its usefulness in the synthesis of the tube alkoxide is its susceptibility to attack by acid-alcohol solutions. This susceptibility is like that of another copper silicate, CurSirOls • 6H20 [5,], and the zinc silicates Zn2SiO4 [14,], Ca2ZnSi20 7 [5-], and Zn4Si2OT(OH)2. H 2 0 [6-]. Presumably other copper and zinc silicates and various silicates formed by neighbors of these elements in the periodic table are subject to acid-alcohol attack and could also serve as alkoxylation substrates. The conditions used to convert K2CuSi4Olo to the tube alkoxide involve a low silicate-to-alcohol mole ratio, a low acid concentration, and a moderate reaction temperature. These conditions are like those used in the previously mentioned silicate-based route to monomeric and oligomeric alkoxides [2-7-]. Work beyond that described in this paper shows that solutions of HCI, EtOH and MeOH, and of HCI and MeOH can be used to prepare ethoxymethoxy and methoxy tube alkoxides. These alkoxides are similar to the propoxy-methoxy tube alkoxide described here [10,]. Presumably other tube alkoxides can be made as well. Preparation of A - F The sets of conditions used to prepare A - F are summarized in Scheme I. As is apparent they involve only mild suspension and drying treatments. However, even some of these sets of condi-
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149 tions are very likely to be sufficiently vigorous to cause some silanol-silanol condensation.
Impurities in tube alkoxide and A - F It is assumed that the tube alkoxide and A-E contain K2CuSi4Olo and glass impurities in about the same proportions as does F. These impurities cause the light blue color of the tube alkoxide and A, B, C, and F. Structure of tube alkoxide As indicated by the idealized postulated structure of the tube alkoxide shown in Fig. 5, the tube in this alkoxide is presumed to be similar to that in K2CuSi4Olo. However, most of the silicon atoms of the tube are postulated to have pendent n-PrO, MeO, or HO groups rather than pendent negatively charged oxygen atoms. These pendent n-PrO, MeO, and HO groups are postulated to be present in a ratio of about 3:3:4. The few remaining silicon atoms are presumed to carry bridging oxygen atoms which are thought to form both additional cross-links within the tube and cross-links between it and other tubes. The cross-links are not thought to distort the tube significantly. It is further postulated that the packing diameter of the tube is ~17/~, that a given tube is generally packed with other tubes in tight bundles, and that these bundles contain widely varying numbers of tubes. The packing arrangement of these bundles in cross-section is thought to be that shown in Fig. 6. In addition, it is postulated that the silanol groups on the surfaces of these bundles are hydrogen-bonded to alcohol molecules. Finally, it is postulated that the bundles are oriented in a random fashion, that the crosslinks between the tubes bridge both tubes belonging to a given bundle and tubes belonging to different bundles, and that the cross-links between tubes belonging to different bundles are relatively rare. Support for this view of the nature of the tube alkoxide comes from the parallels between the route used to make it and the previously mentioned route to monomeric and oligomeric alkoxides that
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149 suspend in M e O H )
A (suspension)
suspend in dioxane
(fluffy solid)
air dry, R T
B
)
(paper-like solid)
C )
tube alkoxide
145
(gel)
(moderate c o n e . )
suspendi, so,vent, (lowcone.) vacuum dry, 80 ° )
Dd and D t (colloids)
vacuum dry )
E (fibrous solid)
F (fibrous solid) Scheme I
b osi
Oo
©oR
Fig. 5. Postulated tube alkoxide idealized by the omission of intratube and intertube crosslinks where R is n-Pr, Me and H.
Fig. 6. Cross-section of postulated packing of alkoxide tubes in a bundle, planes in this bundle assumed to give rise to the intense X-ray line, and pertinent dimensions.
nearly or fully conserves the frameworks of the silicates [2-7]. Further support is provided by the combination of the synthetic relationships that exist between the tube alkoxide and A - F and the evidence for the proposed molecular and bundle structures of A - F (see below). Also providing backing for this view of the nature of the tube alkoxide is a calculation by
Harrington [10] on the packing diameter of the tube. This calculation is based on the assumption that the packing diameter is approximately the same as the packing diameter of a 16-membered siloxane ring with opposing n-PrO groups that project radially and are extended. As carried through, this calculation gives a value of 20 A, a value somewhat larger than the postulated value. However, this is not surprising because the calculation assumes that the propoxy groups are fully extended. If reasonable allowance is made for this, a value is obtained that is in a range near the postulated value. The reaction sequence that leads to the tube alkoxide is probably that shown in Scheme II. Evidence supporting part of this sequence is provided by the high ratio of methoxy groups to propoxy groups in the samples taken from the alkoxylation mixture before all the M e O H had been stripped from it. This high ratio supports the postulated introduction of the propoxy groups by transesterification. The existence of a tube polymer of this type is - SiO- M + MeOH
H +
+
, -
n--PrOH
SiOMe H +
, - SiO-n-Pr H
Scheme II
~ -= SiOH
H +
, - SiOMe
+
~ - SiOH H H +
, - SiO-n-Pr
+
, - SiOMe H
146
remarkable considering its high silanol content. Probably silanol-silanol condensation within the tubes is inhibited by the spacing of the silanol groups, while silanol-silanol condensation between the tubes of a given bundle is inhibited by steric hindrance arising from the presence of the alkoxy groups. Silanol-silanol condensation between the tubes of different bundles is clearly inhibited by the relatively small area of the bundle-to-bundle contacts. It is probably also inhibited by the sheath of alcohol molecules on the surfaces of the bundles and by steric hindrance from the alkoxy groups.
Structure of products A - F The alkoxide that composes the particles in A is postulated to have a molecular structure, a bundle structure, and a bundle-to-bundle arrangement generally like those of the tube alkoxide. However, it may differ from the tube alkoxide in having a higher concentration of hydrogen-bonded alcohol molecules on the surface of its bundles. Product B is postulated to have a molecular structure like that of the tube alkoxide except for a small decrease in the number of silanol groups per silicon and a corresponding small increase in the number of intratube, intrabundle, and interbundle siloxane cross-links per silicon. These additional cross-links are postulated to have little effect on the chemical composition, dimensions, and shape of the individual tubes. Product B is further postulated to have a bundle structure and a bundle arrangement generally like those of the tube alkoxide. Support for this view of the nature of B is provided by an analysis made of the X-ray data pertaining to it. In this analysis it is assumed that the 14.3 A line in the powder pattern of B can be interpreted as showing that the planes in the fiber bundles indicated in Fig. 6 are separated by 14.3 A. This leads, by geometry and algebra, to a value of 2 x 14.3/1.73 or 16.5 A for the effective diameter of the tube, a value in agreement with that proposed for the tube. The alkoxide in C is postulated to have molecular and bundle structures like those of the tube
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149
alkoxide. This alkoxide is further postulated to have a bundle arrangement that is quite open but still entangled. The surfaces of the bundles are postulated to have a lower concentration of alcohol molecules on them than the surfaces of the bundles of the tube alkoxide, and the bundles are postulated to entrap large numbers of dioxane molecules between them. Support for this view of the nature of this alkoxide comes from the fact that C is a translucent gel. The alkoxide in Do is postulated to be like that in C except that the bundles are more widely separated and less entangled. The alkoxide in D t is postulated to be like that in Do except for the substitution of trichlorotrifluoroethane for dioxane. Support for this view of the nature of the alkoxides in Do and D t is provided by the micrographs of E. These suggest that the alkoxides in D o and D t are composed of fibers with diameters that range from at least 8000 A down to 40 A, and lengths that can be at least 60 Ixm. They further suggest that the fibers bend, but as indicated by the large size of their minimum radius of curvature, are relatively stiff. In addition, they indicate that the fibers seldom kink or break but that they commonly split lengthwise. All of these properties are consistent with the postulated presence of bundled neutral tubes in the alkoxide. The ,~40 A diameter that is inferred for some of the fibers in Do and Dt suggests that bundles with as few as two to four tubes are present. Because the fibers appear to have a pronounced tendency to split lengthwise, this conclusion in turn suggests that fibers composed of separate single tubes are present in Do and D t. The large diameters that are inferred for some of the other fibers in Do and D t suggest that the alkoxylation of the parent silicate tubes takes place without full separation of the tubes since it is unlikely that isolated alkoxylated tubes would aggregate into large diameter fibers under the reaction conditions used (a mat or felt would be expected). Product E is postulated to have molecular and bundle structures like those of product B. It is further postulated to have a bundle arrangement
B.A. Harrington, M.E. Kenney/ColloidsSurfaces 63 (1992) 139-149 similar to that of B except for being more open, and to have a lower concentration of alcohol molecules on the surfaces of its bundles. Support for this view of E and information on the sizes of the bundles in E is found in the micrographs of it. The molecular structure of F is postulated to be like that of B except for a relatively small decrease in the number of silanol groups per silicon and a corresponding increase in the number of siloxane cross-links of all three types per silicon. Both the bundle structure and the bundle arrangement of F are postulated to be like those of B. However, the concentration of alcohol molecules on the surfaces of the bundles of F is postulated to be quite low. Substantiation for this view of F is provided by its waxy nature, hydrophobicity, and hydrolytic resistance. Additional substantiation is provided by the presence of bands in its infrared spectrum that are expected on the basis of its proposed nature. Further support for this view of F is given by an analysis made of the data on the weight percent of n-PrO and MeO in it. In this analysis it is assumed that, to a first approximation, the formula of F is (n-PrO)x(MeO)r(HO)zSi01. 5 where z = 1 - x - y. With this assumption, the values calculated from the alkoxide content data for x, y, and z are: x = 0.27, y = 0.26, and z = 0.47. These values are reasonably near the x, y, and z values postulated for the tube alkoxide, and hence for F, and thus support the formula given for the tube alkoxide and F. The values of x and y thus obtained would be increased if the inorganic impurities in F were taken into account, and decreased if instead the surface alcohol molecules on F were taken into account. Probably the effect of the inorganic impurities is more important. In short, it is postulated that the alkoxide in A, C, Dd, and D t is the tube alkoxide and that this alkoxide has the composition (n-PrO)~.o.3(MeO)~o.z(HO)~o.4SiO1.5. The differences between A, C, Dd, and D t are thought to be associated mainly with differences in bundle arrangement and differences in solvent. It is further postulated that the alkoxide in B, E, and F is a
147 lightly cross-linked variation of the tube alkoxide and has approximately the same composition as the tube alkoxide. The differences in B, E, and F (which are small) are thought to be associated mainly with bundle arrangement.
Potential of tube alkoxide and lightly cross-linked tube alkoxide As shown by the properties of F, the lightly cross-linked tube alkoxide hydrolyzes in the same way as tetraethoxysilane although more slowly. It is assumed that the tube alkoxide hydrolyzes like its lightly cross-linked analog. Because of this, the two alkoxides are attractive candidates for use as components in mixtures designed for the preparation of ceramics by the sol-gel process. The use of them in such mixtures would allow the preparation of preceramic bodies incorporating very fine silica fibers. If the tube alkoxide was used, the fibers could be oriented by shear or other techniques. These options open ways permitting greater control over the shape of preceramic bodies. In addition the two alkoxides could be used to make felt-like ceramics. By hydrolyzing the alkoxides under atmospheric pressure, low density, high surface area, felt-like xerogels could be made. By hydrolyzing gels composed of the tube alkoxide and an organic solvent such as dioxane and then removing the solvent at a temperature above its critical temperature, very low density, very high surface area felt-like aerogels could be made. Further, it is possible that by pyrolyzing the two alkoxides, felt-like silicon carbide products could be made. These various felt-like products could be of use for insulation and other purposes. The two alkoxides could also be used as the fiber components of organic composites. Here the small sizes and reactive surfaces of the fibers offer opportunities for making novel materials.
Properties of tube polymers The tube alkoxide is a tube polymer with a covalently bonded framework [15-17]. This type
148
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149
of polymer is not well known. However, m a n y such polymers are possible. F o r example, a tube alkoxide polymer with the tube framework in KCasSi2OTSirO15(OH)F, that is, the framework shown in Fig. 7 [18], is possible. Tube alkoxide polymers with other known and feasible silicate tube frameworks are also possible I-8]. Additional possible tube polymers include analogs of the tube alkoxides with alkyl instead of alkoxy pendent groups, and tube polymers with organic frameworks and various pendent groups or no pendent groups. With all these variations, tube polymers with tubes having widely varying Young's, bending, and twisting moduli can be envisioned 1-19]. The polymers containing these tubes will have widely varying sets of physical properties. Some of these sets could be quite valuable. Such polymers are of particular interest because they can be expected to be flexible and hence tractable, and to have moderate glass transition temperatures (assuming appropriate building blocks are used). Further, they can be expected to be resistant to thermally or chemically caused molecular weight degradation since cleavage of single bonds in their backbones will not result in backbone cleavage. Thus the polymers can be expected to be like chain polymers in that they can be expected to be flexible and have moderate glass transition temperatures, and to be like cross-linked polymers in that they can be expected to be resistant to thermally or chemically caused molecular weight degradation. In this they are, as is
oSi
©0
Fig. 7. Latticework tube ion in KCasSi2OTSirO15(OH)F according to the data of Scott 1-18].
apparent, like their similar but simpler relatives, the ladder polymers.
Conclusions A tube alkoxide with the composition (n-PrO),o.3(MeO),o.3(HO),o.4SiOl.s and a lightly cross-linked variant of it have been made. These alkoxides offer promise for use in the preparation of ceramics. They are representatives of one group of a very large class of potential polymers, the tube polymers. Such polymers, with appropriate building blocks, can be expected to offer valuable sets of properties.
Acknowledgments We thank O. Richard Hughes and Faruq Marikar for helpful discussions and Hoechst Celanese for financial support.
References 1 C. Bleiman and J.P. Mercier, Inorg. Chem., 14 (1975) 2853. 2 G.B. Goodwin and M.E. Kenney, Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem., 27 (1986) 107. 3 G.B.Goodwin and M.E. Kenney, in M. Zeldin, K.J. Wynne and H.R. Allcock (Eds), Inorganic and Organometallic Polymers, ACS Symp. Ser. 360, American Chemical Society, Washington, DC, 1988, pp. 238-248. 4 G.B. Goodwin and M.E. Kenney, in J.M. Zeigler and F.W.G. Fearon (Eds), Silicon-Based Polymer Science: A Comprehensive Resource, Adv. Chem. Ser., 224 (1990) 251-263. 5 G.B. Goodwin and M.E. Kenney, Inorg. Chem., 29 (1990) 1216. 6 M.E. Kenney and G.B. Goodwin, U.S. Patent 4,717,773, 1988. 7 G.B. Goodwin and M.E. Kenney, U.S. Patent 4,824,985, 1989. 8 J. Hefter and M.E. Kenney, Inorg. Chem., 21 (1982) 2810, and references cited therein. 9 K. Kawamura and J.T. Iiyama, Bull. Mineral., 104 (1981) 387. 10 B.A. Harrington, Ph.D. Thesis, Case Western Reserve University, 1990. 11 Yu.A. Pyatenko and Z.V. Pudovkina, Sov. Phys.-Crystallogr. (English Translation), 5 (1961) 540; Kristallografiya, 5 (1960) 563. 12 D.R. Peacor and M.J. Buerger, Am. Mineral., 47 (1962) 539.
B.A. Harrington, M,E. Kenney/Colloids Surfaces 63 (1992) 139-149 13 J.M. Martin Pozas, G. Rossi and V. Tazzoli, Am. Mineral., 60 (1975) 471. 14 G.B. Goodwin, Ph.D. Thesis, Case Western Reserve University, 1987. 15 J. Hefter and M.E. Kenney, J. Am. Chem. Soc., 103 (1981) 5929. 16 J. Hefter and M.E. Kenney, in J.S. Falcone, Jr. (Ed.),
149 Soluble Silicates, ACS Symp. Ser. 194, American Chemical Society, Washington, DC, 1982, pp. 319-328. 17 L.M. Johnson and T.J. Pinnavaia, Langmuir, 6 (1990) 307. 18 J.D. Scott, Can. Mineral., 14 (1987) 515. 19 B.A. Harrington, R.L. Mullen and M.E. Kenney, Macromolecules, 19 (1986) 2888.
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Elsevier Science Publishers B.V., Amsterdam
The preparation and properties of a fibrous tube alkoxysiloxane derived from a tube silicate B r u c e A. H a r r i n g t o n a n d M a l c o l m E. K e n n e y
Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA (Received 26 August 1990; accepted 26 November 1990)
Abstract The conversion of the tube silicate K 2CuSi4010 to the fibrous tube alkoxide (n-PrO) ~ 0.3(MeO) ~ o.3(HO) ~ 0.4 SiO 1.5 by treatment of the silicate with a solution composed of n-PrOH, MeOH, and HC1 is described. Also described are a glass devitrification synthesis of K2 CuSi4010, the structure and properties of the tube alkoxide, and the preparation, structure, and properties of a variant of this alkoxide in which the tubes are lightly cross-linked. In addition, the potential usefulness of the two alkoxides for the synthesis of ceramics, and the valuable sets of properties that can be expected from appropriately constituted tube polymers are discussed.
Keywords: Fibers; inorganic polymers; sol-gel ceramics; tube alkoxides; tube polymers; tube silicates.
Introduction Earlier, the preparation of a polymeric allyloxysiloxane from the layer silicate chrysotile by two different procedures was reported [1]. In the simpler of these procedures, chrysotile was treated with hydrochloric acid and then with allyl alcohol. Apparently the allyloxysiloxane obtained from these procedures had a framework similar to that of chrysotile. More recently, the preparation of a series of monomeric alkoxysilanes and oligomeric alkoxysiloxanes from monomeric and oligomeric metal silicates was reported [2-7]. In the procedures used in this work, the silicates were treated with solutions of hydrochloric acid and an alcohol. In each case, an alkoxide with a framework that was either the same as that of the parent silicate, or was similar to it, was obtained. The preparation of this latter set of compounds from silicates has established a new, general, silicate-based route to monomeric and oligomeric silicon alkoxides. In view of this previous work, it appears that it Correspondence to: M.E. Kenney, Dept. of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA. 0166-6622/92/$05.00
should be possible to use selected polymeric metal silicates to prepare polymeric alkoxysiloxanes having frameworks that are similar to those of their parent silicates. Included among these silicates are various members of the rare, but interesting, tube family [8] and various members of the sheet family. Here we describe work showing that the tube silicate K2CuSi4Olo [9] can be used to prepare a tube alkoxysiloxane. In addition, we describe the conversion of this alkoxide to a variant of it in which the tubes are lightly cross-linked. Finally, we discuss the potential usefulness of these tube alkoxides for the synthesis of ceramics and the valuable sets of properties that can be expected from appropriately constituted tube polymers.
Experimental Syntheses and preparations Synthesis of K 2CuSi401o A mixture of KOAc (7.59 g), Cu(OAc)2"H20 (7.72 g), (EtO)4Si (32.2 g), H 2 0 (105 ml), and EtOH
© 1992 - - Elsevier Science Publishers B.V. All rights reserved.
140 (145 ml) was stirred for 2 days, allowed to stand for 3 days, and evaporated to dryness under vacuum (80°C). The material remaining was ground, heated (200°C) for 25 h, and then heated more vigorously (590°C) for 18 h. The resulting solid was ground (15.4 g). A similar powder (30.4 g), prepared with an alternative calcination procedure (one in which the gel was subjected several times to a process in which it was heated (~ 600°C) and then ground), was melted (1200°C)in a platinum crucible. (This alternative calcination procedure was poorer because it led to the rapid evolution of a significant volume of gas that inflamed.) Part of the resulting melt was poured out onto a steel plate and formed into two disks (4.23 g; 4.61 g). The glass remaining in the crucible after the disks had been formed (15.0 g) was left in place (total yield of K2CuSi4Olo glass based on weight of the powder was 78%). The disks were placed on a fused silica plate and the crucible (with its glass) was placed on a hearth plate. With a controlled atmosphere furnace, the silica plate and the disks, and the hearth plate and the crucible were heated (750°C) under a slow flow of N 2 for 30 days. The solids formed by the disks were pried from the plates, and the solid formed by the glass in the crucible was broken away from the crucible. The disk products were crushed and the resulting granular material was converted by a particle reduction and sizing process involving grinding and screening (fume cupboard) into a moderate particle size fraction (less than 60 and greater than 270 mesh) and two other fractions. The moderate particle size fraction was separated into two fractions with a magnetic separator. The fraction composed of relatively magnetic blueviolet acicular crystals was retained (2.55 g, identified by X-ray powder diffractometry, estimated purity by light microscopy 98%; contained yield of K2CuSi4Olo based on weight of glass disks, 28%). The crucible solid was purified in the same manner (5.69 g, estimated purity 98%; contained yield of K2CuSi4Olo based on weight of glass in crucible ~ 37%).
B.A. Harrington,M.E. Kenney/ColloidsSurfaces63 (1992) 139-149 Synthesis of tube alkoxide A mixture of K2CuSi4Olo (less than 200 mesh, 258 mg), n-PrOH (25 ml), MeOH (50 ml), and an H C I - M e O H solution (8.7 N, 0.60 ml) was heated to reflux over 30 min and then was distilled slowly (12 drops per min) for 1 h. An H C I - M e O H solution (0.25 N, 21 ml) was added to the suspension obtained over a period of 3.5 h while the suspension was being distilled slowly (6 drops per min). The mixture remaining was distilled slowly (10 drops per min) for 120 min, and still more slowly (3 drops per min) for 15 rain. The resultant was filtered and the filtrate (yellow-brown) was discarded. The solid (the tube alkoxide) was washed (MeOH, 1:1 acetone-H20 solution, MeOH), left damp, and weighed (~ 327 mg). Samples of the solid in the reaction mixture that were taken before the mixture had been fully stripped of MeOH had a high ratio of methoxy to propoxy groups.
Preparation of product A The tube alkoxide was suspended in MeOH (~ 47 mg ml- 1) with the aid of sonication.
Preparation of product B Product A was spread on a Teflon-coated microscope slide and allowed to air dry.
Preparation of product C The tube alkoxide was suspended in 1,4-dioxane (~ 20 mg ml- 1) with the aid of sonication.
Preparation of product Dn The tube alkoxide was suspended in 1,4-dioxane (,,~0.2 mg m1-1) with the aid of sonication.
Preparation of product Dt The tube alkoxide was suspended in 1,1,2-trichlorotrifluoroethane (~ 0.2 mg ml- 1) with the aid of sonication.
Preparation of product E One to three drops of D d or D t were placed on an electron microscope specimen grid (copper, 200
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149
141
mesh, carbon coated, 200 A film) resting on filter paper. The grid was allowed to air dry briefly and then was inserted into the electron microscope and subjected to the vacuum of the microscope.
Properties of KzCuSi401o, tube alkoxide, and tube alkoxide products
Preparation of product F The tube alkoxide (~189 mg) was broken up, dried (,~ 75 Torr, 80°C, 1 h), and weighed (112 mg).
K2CuSi4Olo melts between 790 ° and 810°C. It reacts with concentrated HC1 within minutes to give a pale yellow-green solution and a white residue.
Method for determination of alkoxide content ofF A mixture of F (~ 50 mg), H 2 0 (~ 5 ml), Et3N (~0.05 ml), and dimethylformamide (,~10 ml) was heated (70 ° C) for at least 24 h in a vial closed with a septum, and the weights of n-PrOH and MeOH liberated were determined by gas chromatography (benzene-dimethylformamide reference solution). From these values the weight percentages of n-PrO and MeO in the polymer were calculated. The infrared spectrum of a residue from the treatment of F by this method showed no CH bands. Treatment of MeSi(O-n-Pr)3 by this method gave the anticipated amount of 1-propanol. No evidence indicating that the solvent had hydrolyzed was observed.
Instrumentation A Frantz L-1 isodynamic separator (S.G. Frantz Co., Inc.) was used in the magnetic separations, and a Branson Model 200 sonifier (Branson Sonic Power Co.) was used in the sonications. A Philips APD 3520 auto-diffractometer (Philips Electronic Instruments, Inc.) and a Rigaku DMAXB autodiffractometer (Rigaku USA, Inc.) were used to collect the X-ray powder data. For the gas chromatography work a Varian Vista 4600 gas chromatograph (Varian Associates, Inc.) fitted with a DB-5 FSOT capillary column (J&W Scientific Inc.) was used. A Model JEM-100SX Japan Electron Optics Laboratory transmission electron microscope (JEOL USA Inc.) equipped with a liquid N2 cooled sample chamber was used for the electron microscopy.
K2CuSi401o
Tube alkoxide The damp tube alkoxide is a fluffy light blue solid.
Products Product A Product A is an opaque, light blue suspension that does not separate for at least 7 days. Product B This product is light blue and is like a piece of rough paper. It gives an X-ray powder pattern with reflections at (d(A)(l/Io)) 14.3 (100), 7.05 (br, 36), and ~ 3.6 (br) as well as weak reflections for K2CuSi4Olo. Very probably the 14.3 and 7.05 A reflections are matching first- and second-order reflections. The 3.6 A reflection is probably composed of more than one line. Product C Product C is a translucent, very light blue gel that flows when agitated. Products Da and D, These products are transparent colorless colloidal suspensions. Product E Micrographs of E are shown in Figs 1, 2, and 3. These and other micrographs of E show that it is composed of matted, high-aspect-ratio fibers.
142
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149
Fig~1. Micrograph of E showinga fiber ,~60 ~tm long (lowerarrow) and a fiber with a kink (upper arrow).
Product F
This product is a fibrous, light blue solid. Light microscopy observations show that it is composed of colorless fibers (estimated abundance ~90%), K2CuSi4Olo (estimated abundance ~8%), and undevitrified glass (estimated abundance ~ 2%). It feels waxy when ground in an agate mortar and disperses in organic solvents such as dioxane, CHC13, and CC14. It does not dissolve in such solvents nor in water. F is extremely hydrophobic
and only after at least 48 h does it become wetted when in contact with water. As determined by the hydrolysis-gas chromatography method outlined above, F contains 18.3 wt.% n-PrO and 9.9 wt.% MeO. Mulls of it give infrared spectra with bands at: (Fluorolube) 3340 (m, br, OH), 2970 (m, CH), 2940 (sh, CH), 2880 (m, CH), 2855 (w, CH of OCH3) , 1480 (sh), 1470 (w), 1400 (w), 1385 (w); (Nujol) 1160 (s, SiOSi, SiOC), 1080 (s, SiOSi, SiOC), 970 (sh, SiOH), 840 (w), 610 (w), 470 (m)cm -1.
B.A. Harrington,M.E. Kenney/Colloids Surfaces 63 (1992) 139-149
Fig. 2. Higher magnification micrograph of E.
143
Fig. 3. Micrograph of E showing a multitude of small fibers diverging from a large fiber.
Discussion
K2CuSi401o Synthesis of K 2CuSi, O lo The devitrification synthesis of K2CuSi4Olo described here requires less elaborate apparatus than does the previously described hydrothermal synthesis of this silicate [9], and as a result it is more convenient. This conveniently makes the preparation of K2CuSi4Olo on the gram scale relatively simple. Work further extending that described here shows that K2CuSi,O~o can be prepared alternatively from a glass made by heating a 1:1:4 mole ratio mixture of K2CO3, CuCO3, and SiO2 [-10]. However, this synthesis gives a lower yield, probably because the glass is less homogeneous. Structure of K2CuSi401o The structure of the silicate ion in K 2 C u S i 4 O l o as determined by Kawamura and Iiyama [-9] is
illustrated in Fig. 4. As is apparent, this ion is, without ambiguity, a tube. (It could be argued that the silicate ions in Na2TiSi4Olo [11,12], NaKCuSi4Olo [13], and some other silicates, contrary to the common view, are not tubes but rather are elaborated ladders.) The rings in the silicate ion in K2CuSi4Olo contain, as inspection of Fig. 4 shows,
t3 oSi
©0
Fig. 4. Latticework tube ion in K2CuSi4Olo according to the data of Kawamura and Iiyama.
144 eight members, twelve members, and sixteen members, and accordingly are of stable sizes. The copper ions in K2CuSi4Olo coordinate with pendent oxygens of neighboring tubes and thus interconnect the tubes. Three-quarters of the potassium ions also coordinate with pendent oxygens of neighboring tubes and further interconnect the tubes. The remaining one-quarter of the potassium ions reside inside the tubes.
Tube alkoxide and products A - F Synthesis of tube alkoxide A property of K2CuSi4Olo that is essential to its usefulness in the synthesis of the tube alkoxide is its susceptibility to attack by acid-alcohol solutions. This susceptibility is like that of another copper silicate, CurSirOls • 6H20 [5,], and the zinc silicates Zn2SiO4 [14,], Ca2ZnSi20 7 [5-], and Zn4Si2OT(OH)2. H 2 0 [6-]. Presumably other copper and zinc silicates and various silicates formed by neighbors of these elements in the periodic table are subject to acid-alcohol attack and could also serve as alkoxylation substrates. The conditions used to convert K2CuSi4Olo to the tube alkoxide involve a low silicate-to-alcohol mole ratio, a low acid concentration, and a moderate reaction temperature. These conditions are like those used in the previously mentioned silicate-based route to monomeric and oligomeric alkoxides [2-7-]. Work beyond that described in this paper shows that solutions of HCI, EtOH and MeOH, and of HCI and MeOH can be used to prepare ethoxymethoxy and methoxy tube alkoxides. These alkoxides are similar to the propoxy-methoxy tube alkoxide described here [10,]. Presumably other tube alkoxides can be made as well. Preparation of A - F The sets of conditions used to prepare A - F are summarized in Scheme I. As is apparent they involve only mild suspension and drying treatments. However, even some of these sets of condi-
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149 tions are very likely to be sufficiently vigorous to cause some silanol-silanol condensation.
Impurities in tube alkoxide and A - F It is assumed that the tube alkoxide and A-E contain K2CuSi4Olo and glass impurities in about the same proportions as does F. These impurities cause the light blue color of the tube alkoxide and A, B, C, and F. Structure of tube alkoxide As indicated by the idealized postulated structure of the tube alkoxide shown in Fig. 5, the tube in this alkoxide is presumed to be similar to that in K2CuSi4Olo. However, most of the silicon atoms of the tube are postulated to have pendent n-PrO, MeO, or HO groups rather than pendent negatively charged oxygen atoms. These pendent n-PrO, MeO, and HO groups are postulated to be present in a ratio of about 3:3:4. The few remaining silicon atoms are presumed to carry bridging oxygen atoms which are thought to form both additional cross-links within the tube and cross-links between it and other tubes. The cross-links are not thought to distort the tube significantly. It is further postulated that the packing diameter of the tube is ~17/~, that a given tube is generally packed with other tubes in tight bundles, and that these bundles contain widely varying numbers of tubes. The packing arrangement of these bundles in cross-section is thought to be that shown in Fig. 6. In addition, it is postulated that the silanol groups on the surfaces of these bundles are hydrogen-bonded to alcohol molecules. Finally, it is postulated that the bundles are oriented in a random fashion, that the crosslinks between the tubes bridge both tubes belonging to a given bundle and tubes belonging to different bundles, and that the cross-links between tubes belonging to different bundles are relatively rare. Support for this view of the nature of the tube alkoxide comes from the parallels between the route used to make it and the previously mentioned route to monomeric and oligomeric alkoxides that
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149 suspend in M e O H )
A (suspension)
suspend in dioxane
(fluffy solid)
air dry, R T
B
)
(paper-like solid)
C )
tube alkoxide
145
(gel)
(moderate c o n e . )
suspendi, so,vent, (lowcone.) vacuum dry, 80 ° )
Dd and D t (colloids)
vacuum dry )
E (fibrous solid)
F (fibrous solid) Scheme I
b osi
Oo
©oR
Fig. 5. Postulated tube alkoxide idealized by the omission of intratube and intertube crosslinks where R is n-Pr, Me and H.
Fig. 6. Cross-section of postulated packing of alkoxide tubes in a bundle, planes in this bundle assumed to give rise to the intense X-ray line, and pertinent dimensions.
nearly or fully conserves the frameworks of the silicates [2-7]. Further support is provided by the combination of the synthetic relationships that exist between the tube alkoxide and A - F and the evidence for the proposed molecular and bundle structures of A - F (see below). Also providing backing for this view of the nature of the tube alkoxide is a calculation by
Harrington [10] on the packing diameter of the tube. This calculation is based on the assumption that the packing diameter is approximately the same as the packing diameter of a 16-membered siloxane ring with opposing n-PrO groups that project radially and are extended. As carried through, this calculation gives a value of 20 A, a value somewhat larger than the postulated value. However, this is not surprising because the calculation assumes that the propoxy groups are fully extended. If reasonable allowance is made for this, a value is obtained that is in a range near the postulated value. The reaction sequence that leads to the tube alkoxide is probably that shown in Scheme II. Evidence supporting part of this sequence is provided by the high ratio of methoxy groups to propoxy groups in the samples taken from the alkoxylation mixture before all the M e O H had been stripped from it. This high ratio supports the postulated introduction of the propoxy groups by transesterification. The existence of a tube polymer of this type is - SiO- M + MeOH
H +
+
, -
n--PrOH
SiOMe H +
, - SiO-n-Pr H
Scheme II
~ -= SiOH
H +
, - SiOMe
+
~ - SiOH H H +
, - SiO-n-Pr
+
, - SiOMe H
146
remarkable considering its high silanol content. Probably silanol-silanol condensation within the tubes is inhibited by the spacing of the silanol groups, while silanol-silanol condensation between the tubes of a given bundle is inhibited by steric hindrance arising from the presence of the alkoxy groups. Silanol-silanol condensation between the tubes of different bundles is clearly inhibited by the relatively small area of the bundle-to-bundle contacts. It is probably also inhibited by the sheath of alcohol molecules on the surfaces of the bundles and by steric hindrance from the alkoxy groups.
Structure of products A - F The alkoxide that composes the particles in A is postulated to have a molecular structure, a bundle structure, and a bundle-to-bundle arrangement generally like those of the tube alkoxide. However, it may differ from the tube alkoxide in having a higher concentration of hydrogen-bonded alcohol molecules on the surface of its bundles. Product B is postulated to have a molecular structure like that of the tube alkoxide except for a small decrease in the number of silanol groups per silicon and a corresponding small increase in the number of intratube, intrabundle, and interbundle siloxane cross-links per silicon. These additional cross-links are postulated to have little effect on the chemical composition, dimensions, and shape of the individual tubes. Product B is further postulated to have a bundle structure and a bundle arrangement generally like those of the tube alkoxide. Support for this view of the nature of B is provided by an analysis made of the X-ray data pertaining to it. In this analysis it is assumed that the 14.3 A line in the powder pattern of B can be interpreted as showing that the planes in the fiber bundles indicated in Fig. 6 are separated by 14.3 A. This leads, by geometry and algebra, to a value of 2 x 14.3/1.73 or 16.5 A for the effective diameter of the tube, a value in agreement with that proposed for the tube. The alkoxide in C is postulated to have molecular and bundle structures like those of the tube
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149
alkoxide. This alkoxide is further postulated to have a bundle arrangement that is quite open but still entangled. The surfaces of the bundles are postulated to have a lower concentration of alcohol molecules on them than the surfaces of the bundles of the tube alkoxide, and the bundles are postulated to entrap large numbers of dioxane molecules between them. Support for this view of the nature of this alkoxide comes from the fact that C is a translucent gel. The alkoxide in Do is postulated to be like that in C except that the bundles are more widely separated and less entangled. The alkoxide in D t is postulated to be like that in Do except for the substitution of trichlorotrifluoroethane for dioxane. Support for this view of the nature of the alkoxides in Do and D t is provided by the micrographs of E. These suggest that the alkoxides in D o and D t are composed of fibers with diameters that range from at least 8000 A down to 40 A, and lengths that can be at least 60 Ixm. They further suggest that the fibers bend, but as indicated by the large size of their minimum radius of curvature, are relatively stiff. In addition, they indicate that the fibers seldom kink or break but that they commonly split lengthwise. All of these properties are consistent with the postulated presence of bundled neutral tubes in the alkoxide. The ,~40 A diameter that is inferred for some of the fibers in Do and Dt suggests that bundles with as few as two to four tubes are present. Because the fibers appear to have a pronounced tendency to split lengthwise, this conclusion in turn suggests that fibers composed of separate single tubes are present in Do and D t. The large diameters that are inferred for some of the other fibers in Do and D t suggest that the alkoxylation of the parent silicate tubes takes place without full separation of the tubes since it is unlikely that isolated alkoxylated tubes would aggregate into large diameter fibers under the reaction conditions used (a mat or felt would be expected). Product E is postulated to have molecular and bundle structures like those of product B. It is further postulated to have a bundle arrangement
B.A. Harrington, M.E. Kenney/ColloidsSurfaces 63 (1992) 139-149 similar to that of B except for being more open, and to have a lower concentration of alcohol molecules on the surfaces of its bundles. Support for this view of E and information on the sizes of the bundles in E is found in the micrographs of it. The molecular structure of F is postulated to be like that of B except for a relatively small decrease in the number of silanol groups per silicon and a corresponding increase in the number of siloxane cross-links of all three types per silicon. Both the bundle structure and the bundle arrangement of F are postulated to be like those of B. However, the concentration of alcohol molecules on the surfaces of the bundles of F is postulated to be quite low. Substantiation for this view of F is provided by its waxy nature, hydrophobicity, and hydrolytic resistance. Additional substantiation is provided by the presence of bands in its infrared spectrum that are expected on the basis of its proposed nature. Further support for this view of F is given by an analysis made of the data on the weight percent of n-PrO and MeO in it. In this analysis it is assumed that, to a first approximation, the formula of F is (n-PrO)x(MeO)r(HO)zSi01. 5 where z = 1 - x - y. With this assumption, the values calculated from the alkoxide content data for x, y, and z are: x = 0.27, y = 0.26, and z = 0.47. These values are reasonably near the x, y, and z values postulated for the tube alkoxide, and hence for F, and thus support the formula given for the tube alkoxide and F. The values of x and y thus obtained would be increased if the inorganic impurities in F were taken into account, and decreased if instead the surface alcohol molecules on F were taken into account. Probably the effect of the inorganic impurities is more important. In short, it is postulated that the alkoxide in A, C, Dd, and D t is the tube alkoxide and that this alkoxide has the composition (n-PrO)~.o.3(MeO)~o.z(HO)~o.4SiO1.5. The differences between A, C, Dd, and D t are thought to be associated mainly with differences in bundle arrangement and differences in solvent. It is further postulated that the alkoxide in B, E, and F is a
147 lightly cross-linked variation of the tube alkoxide and has approximately the same composition as the tube alkoxide. The differences in B, E, and F (which are small) are thought to be associated mainly with bundle arrangement.
Potential of tube alkoxide and lightly cross-linked tube alkoxide As shown by the properties of F, the lightly cross-linked tube alkoxide hydrolyzes in the same way as tetraethoxysilane although more slowly. It is assumed that the tube alkoxide hydrolyzes like its lightly cross-linked analog. Because of this, the two alkoxides are attractive candidates for use as components in mixtures designed for the preparation of ceramics by the sol-gel process. The use of them in such mixtures would allow the preparation of preceramic bodies incorporating very fine silica fibers. If the tube alkoxide was used, the fibers could be oriented by shear or other techniques. These options open ways permitting greater control over the shape of preceramic bodies. In addition the two alkoxides could be used to make felt-like ceramics. By hydrolyzing the alkoxides under atmospheric pressure, low density, high surface area, felt-like xerogels could be made. By hydrolyzing gels composed of the tube alkoxide and an organic solvent such as dioxane and then removing the solvent at a temperature above its critical temperature, very low density, very high surface area felt-like aerogels could be made. Further, it is possible that by pyrolyzing the two alkoxides, felt-like silicon carbide products could be made. These various felt-like products could be of use for insulation and other purposes. The two alkoxides could also be used as the fiber components of organic composites. Here the small sizes and reactive surfaces of the fibers offer opportunities for making novel materials.
Properties of tube polymers The tube alkoxide is a tube polymer with a covalently bonded framework [15-17]. This type
148
B.A. Harrington, M.E. Kenney/Colloids Surfaces 63 (1992) 139-149
of polymer is not well known. However, m a n y such polymers are possible. F o r example, a tube alkoxide polymer with the tube framework in KCasSi2OTSirO15(OH)F, that is, the framework shown in Fig. 7 [18], is possible. Tube alkoxide polymers with other known and feasible silicate tube frameworks are also possible I-8]. Additional possible tube polymers include analogs of the tube alkoxides with alkyl instead of alkoxy pendent groups, and tube polymers with organic frameworks and various pendent groups or no pendent groups. With all these variations, tube polymers with tubes having widely varying Young's, bending, and twisting moduli can be envisioned 1-19]. The polymers containing these tubes will have widely varying sets of physical properties. Some of these sets could be quite valuable. Such polymers are of particular interest because they can be expected to be flexible and hence tractable, and to have moderate glass transition temperatures (assuming appropriate building blocks are used). Further, they can be expected to be resistant to thermally or chemically caused molecular weight degradation since cleavage of single bonds in their backbones will not result in backbone cleavage. Thus the polymers can be expected to be like chain polymers in that they can be expected to be flexible and have moderate glass transition temperatures, and to be like cross-linked polymers in that they can be expected to be resistant to thermally or chemically caused molecular weight degradation. In this they are, as is
oSi
©0
Fig. 7. Latticework tube ion in KCasSi2OTSirO15(OH)F according to the data of Scott 1-18].
apparent, like their similar but simpler relatives, the ladder polymers.
Conclusions A tube alkoxide with the composition (n-PrO),o.3(MeO),o.3(HO),o.4SiOl.s and a lightly cross-linked variant of it have been made. These alkoxides offer promise for use in the preparation of ceramics. They are representatives of one group of a very large class of potential polymers, the tube polymers. Such polymers, with appropriate building blocks, can be expected to offer valuable sets of properties.
Acknowledgments We thank O. Richard Hughes and Faruq Marikar for helpful discussions and Hoechst Celanese for financial support.
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149 Soluble Silicates, ACS Symp. Ser. 194, American Chemical Society, Washington, DC, 1982, pp. 319-328. 17 L.M. Johnson and T.J. Pinnavaia, Langmuir, 6 (1990) 307. 18 J.D. Scott, Can. Mineral., 14 (1987) 515. 19 B.A. Harrington, R.L. Mullen and M.E. Kenney, Macromolecules, 19 (1986) 2888.