WO1990008169A1

WO1990008169A1 - Organic modified silicic acid heteropolycondensates

Info

Publication number: WO1990008169A1
Application number: PCT/US1990/000158
Authority: WO
Inventors: Bradley Keith Coltrain; William Edward Pascoe
Original assignee: Eastman Kodak Company
Priority date: 1989-01-19
Filing date: 1990-01-16
Publication date: 1990-07-26
Also published as: CA2045511A1; JPH04502779A; EP0454761A1

Abstract

Organic modified silicic acid heteropolycondensates can be upgraded by removal of residual silanol or hydrolyzable groups. The upgrading process is conducted by contacting the condensate with a supercritical fluid at elevated temperature and pressure. Upgraded products in the form of powders can be dissolved in an organic solvent and used to produce coatings which have improved oxygen permeability. Upgraded products have also been made in non-comminuted, solid form such as rods. These articles are useful as adsorbents.

Description

ORGANIC MODIFIED SILICIC

ACID HETEROPOLYCONDENSATES

Technical Field This invention relates to improved silicic acid heteropolycondensates which contain organic radicals. It also pertains to a method for their formation, and the use of such condensates.

Background Art

Silicic acid heteropolycondensates have been known in the art for quite some time. In general, these products are produced by a low or mild temperature process, which comprises hydrolysis and condensation. The process is performed using one or more silanes (having 2-4 hydrolyzable groups) as the starting material. The process is illustrated below using a triethoxysilane as the raw material on which the process is conducted. In the equations which follow, R is a non-hydrolyzable organic group, such as an alkyl or aryl group.

(1) nRSi(OC₂H₅)_{3 3ng Q} > nRSi(0H)₃ + 3nC₂H₅0H

(2) RSi - OH + HO - Si - R > RSi - 0 - SiR + H 0

I I I I ²

Equation (1) illustrates complete hydrolysis of the triethoxysilane. As shown, for each molar equivalent of trialkoxysilane which is completely hydrolyzed, three molar equivalents of water are consumed. The hydrolyzed product is comparatively unstable; it tends to rapidly undergo the condensation process illustrated by Equation (2). It is known in the art that condensation can begin to occur before all three alkoxide groups are hydrolyzed. Equation (2) is an oversimplification. It illustrates condensation between two silanol groups, each of which were formed in step (1). As shown by the unsatisfied valences, further condensation between other groups can occur. It is known in the art that such continued condensation results in products known as sol—gels. For the purpose of this invention, the term "sol— el" refers to a glassy solid mixture of organic-modified silicic acid heteropolycondensates formed by the hydrolysis and condensation of one or more silanes which contain one or two non—hydrolyzable groups (such as R in the equations above), and in which the remainder of silicon valences are satisfied by hydrolyzable groups. In general, these polycondensate products comprise a network of interconnected silicon-containing chains, wherein the silicon atoms are connected by oxygen. These products are polymeric; but because they are in the solid state, molecular weight determinations cannot be made. As shown by Equation (2), each condensation of two silanol groups results in the splitting out of one molecule of water. This will be discussed in more detail below. Linear polycondensates are produced from reactants solely composed of silanes having two hydrolyzable groups per molecule. On the other hand, starting materials which contain one or more silanes with three or four hydrolyzable groups yield crosslinked polycondensates.

Cross—linking can make it difficult for complete hydrolysis and/or condensation to take place, since groups within a three-dimensional polymeric network may not readily react because of steric hindrance. Thus, under conditions employed in prior art processes, crosslinked, silicic acid condensates can contain residual hydrolyzable groups (e.g., alkoxy radicals) or residual silanol groups. It is known in the art that silicic acid heteropoly¬ condensates (which are not organic modified, i.e., which do not contain organic groups) can be dried using fluids above their supercritical temperatures; Tewari et al, Materials Letters, Vol. 3, number 9, 10, pp. 363-367 (July, 1985), and Laudise et al, Journal of Non-Crvstalline Solids 79. pp. 155-164 (1986) pertains to the supercritical drying of alkoxide silica gels using systems containing ethanol. Monolithic non—organic modified silica gels have also been made using hypercritical solvent evacuation; Prassas et al, Journal of Materials Science 19. pp. 1656-1665 (1986).

Disclosure of Invention

A problem with prior art silicic acid heteropolycondensates is they contain residual silanol groups and residual hydrolyzable groups of the type discussed above. Such groups can have a deleterious effect on the materials of the prior art. We have discovered that this problem can be solved by removal of such groups by treating heteropolycondensates with a fluid in the supercritical state. We have also discovered that surprisingly, use of the supercritical liquid does not remove organic groups which are bonded to silicon atoms in such condensates by a silicon—to—carbon bond. We have also discovered that the organic modified silicic acid heteropolycondensates produced by our upgrading process have enhanced oxygen permeability, which makes them useful for forming films for use in oxygen permeable systems. We have also discovered that monolithic glassy, porous solids can be produced by a process which comprises contacting an organic modified silicic acid sol in a mold with a fluid above its critical temperature and pressure.

Thus, this invention has several aspects. First, it pertains to a method of upgrading an organic modified silicic acid heteropolycondensate by removal of residual silanol and/or hydrolyzable groups, said method comprising contacting the polycondensate to be upgraded with a fluid above the critical temperature and pressure of the fluid, for a time sufficient to remove silanol or hydrolyzable groups from the condensate. In one embodiment, the hydrolyzable groups that are removed are alkoxide groups, and the supercritical fluid is the alcohol corresponding to the alkoxide radicals.

The products of the upgrading process are free or substantially free of silanol and hydrolyzable groups. For the purpose of this invention, "substantially free" means that no more than about 5% of the groups bonded to silicon are hydrolyzable or silanol groups.

In a second aspect, this invention comprises a preparative process. The preparative process of this invention comprises a hydrolysis/condensation reaction of the type exemplified by Equations (1) and (2) above, followed by an upgrading process which removes residual hydrolyzable and silanol groups from the polycondensate intermediate. The upgrading process of this embodiment is of the type described above.

The preparative and upgrading embodiments of this invention are directed to the preparation of organic modified, silicic acid heteropolycondensates products of two types which comprise a third embodiment of the invention, the first type of product are powders. In these powders at least about 90% of the repeating silicon-containing units have at least one organic group (R) bonded to the silicon atom.

In the third embodiment, the second type of products are porous, glassy monolithic solids. For the purpose of this invention, "monolithic" means cast or otherwise formed in a single piece. The monoliths of this invention can be in the form of rods or have another shape, e.g., lenticular. The monoliths of this invention are glass-like, (i.e., non-crystalline) opaque or transparent solid bodies. A typical rod has a diameter of 0.5 to 25 millimeters and a length of 10 to 125 millimeters. Products outside of the size range can be produced. In the monoliths, there are two main types of repeating units; viz, repeating units having the formulas Si=, and RSi≡ wherein R is a non—hydrolyzable, organic group bonded through carbon to the silicon atom. In the monoliths, from about 5 to about 35 mole percent of the repeating units have the formula RSi≡. Thus, the monoliths of this invention are mostly composed of repeating units having the formula Si=. They may have up to about 5% of repeating units with the formula RSi≡ wherein R has the same significance as above. The monoliths of this invention can be used as adsorbents, e.g., as substrates for chromatographic separations.

Although the process of this invention is primarily directed to the removal of hydrolyzable and/or silanol groups in crosslinked heteropolycon¬ densates, it is to be understood that the process can also be employed to remove such groups from linear polycondensates. Brief Description of Drawing

The Figure in the drawing is a plot, graphically comparing oxygen permeability of a non-monolithic product of this invention, with a similar material which has not upgraded by the process provided herein. Oxygen permeability values are on the "y" axis, while the oxygen concentration values are plotted along the "x" axis. As shown, an organic modified silicic acid heteropolycondensate (i) containing phenyl groups, and (ii) produced by the process of this invention, has a much greater oxygen permeability when compared to the permeability of a similar material not subjected to the upgrading process of this invention. However, the linearity of response with the product of this invention appears to be slightly less than with the comparison material. This slight decrease in linearity does not materially detract from the utility of the product.

Best Mode for Carrying: Out the Invention

In one embodiment, this invention comprises a method for upgrading an organic modified silicic acid heteropolycondensate by reducing the number of hydrolyzable groups and/or silanol radicals in the condensate. This method comprises heating said condensate at an elevated temperature, in contact with a fluid which is under high pressure, for a time sufficient to remove such groups. The process is conducted above the critical temperature and pressure of the fluid. The product can be recovered by separating the fluid and the volatile product(s) formed during removal of the hydrolyzable groups. As indicated above, for the purpose of description of this invention, the process of this embodiment is referred to as the "upgrading process" of this invention. The upgrading process can be conducted on a wide variety of organo odified silicic acid heteropolycondensates. The applicability of the upgrading process of this invention is independent of the method employed for producing the condensate to be upgraded, provided that the polycondensate to be treated contains hydrolyzable and/or silanol groups which can be removed.

Also as indicated above, a second embodiment of this invention is described herein as the "preparative process" or the "preparative process of this invention." The preparative method comprises: (a) formation of an organic modified silicic acid condensate by a reaction which comprises hydrolysis and condensation of an organosilane, followed by (b) removal of residual hydrolyzable and/or silanol groups with a supercritical fluid. Step (b) comprises the upgrading process of this invention.

A preferred process within the preparative embodiment of this invention comprises: a method for preparing an organic modified silicic acid heteropolycondensate (i) having repeating silane units with the formula RSi≡ such that the free valences in said formula are interconnected by oxygen, and (ii) being characterized by being substantially free of hydrolyzable and silanol groups, said method comprising:

(A) forming an intermediate silicic acid condensate by reacting water with a silane at a low to mild temperature, and in the presence of a low boiling solvent for said silane, said silane having the formula R_χSi(0R' ^__χ, wherein R is an alkyl or aryl radical having up to about 20 carbon atoms, R* is an alkyl radical of up to about 4 carbon atoms, and X is equal to 1 or 2; (B) heating the reaction mixture thereby produced at a pressure and temperature above the critical temperature and pressure of said solvent, for a time sufficient to remove substantially all residual R'O— and silanol groups from said intermediate condensate, and form alkanol from said R'O— groups; and

(C) removing said alkanol and solvent from the organic modified silicic acid heteropolycondensate thereby produced.

In the particular embodiment set forth immediately above, the combination of process steps (A) and (B) comprise a preparative process of this invention. Step (B) is an upgrading process of the type referred to above. Step (C) comprises a product recovery step. Such a step is a desirable, but not a critical feature of the invention.

The third embodiment of this invention comprises an organic group—containing, porous monolithic glassy body containing the repeating units Si≡ and RS≡, wherein R is a stable organic group, such that (a) from about 5 to about 35 mole percent of said units are RSi≡, (b) said RSi≡ units are substantially uniformly dispersed throughout said body, and (c) said repeating units are interconnected by oxygen; said body having a surface area of from

2 about 500 to about 800 m /g, an apparent density of about 0.25 to about 0.40 g/ml, and being substantially free of residual silanol and hydrolyzable groups.

As part of this embodiment, this invention provides a process for producing a monolithic body of the type described above, said process comprising contacting a precursor polycondensate in a mold with a fluid above the critical temperature and pressure of said fluid, for a time sufficient to remove substantially all residual silanol and hydrolyzable groups from said sol, and subsequently venting and cooling to separate said body from said fluid and volatile by-products formed upon removal of said groups from said precursor polycondensate; said precursor condensate having been produced by hydrolysis and condensation of a mixture of silanes having the formula SiX and the formula

RSiX₃, wherein from about 5 to about 35 mole percent of said silanes in said mixture have the formula RSiX₃, and wherein each X in said formulae is a hydrolyzable group, and R is a non-hydrolyzable organic group.

The products of this invention are organic modified; i.e., they contain organic groups. They are polymeric in nature. Linear polymers produced by this invention generally are composed of repeating

R units having the formula —0—Si—0—. The crosslinked

R polymers are composed of repeating units having the R 0 formula —0- ι—0—, or —0— i—0—. It is not necessary

that the crosslinked products be solely composed of these groups. As set forth more fully below, the products of this invention may be modified by the inclusion of other silicon—containing radicals, albeit in comparatively minor amounts. Consequently, the products of this invention can be considered as heteropolycondensates derived from silanes having the formula R_χSiX^__y-, wherein X = 0 to 2, with the proviso that not all of the starting material is composed of silanes wherein X is equal to 0. As stated above, crosslinked, organic modified silicic acid heteropolycondensates, which are upgraded by a process of this invention, have the repeating unit RSiE, wherein the unsatisfied valences are interconnected through oxygen. As discussed, these repeating units can be formed by a hydrolysis and condensation reaction using one or more silanes having the formula:

RSiX₃

(I)

wherein R is an organic radical bonded to silicon through carbon, and the radicals indicated by X are alike or different and selected from hydrolyzable groups, such as alkoxides, (e.g., the alkoxide radicals referred to above), aryloxides, alkylalkoxide, alkoxyalkoxides, halides, and the like. The heteropolycondensates may also contain groups derived from one or more silanes having the formulas:

SiX, and R₂SiX₂

(II) (III)

wherein X and R have the same significance as above. When preparing powders according to this invention, only a relatively small number of the silicon containing units will be derived from silanes of Formulas II and III, or mixtures thereof. This is borne out in more detail by the following discussion. Turning first to the compounds of Formula II, they can be added to the reaction mixture to increase hardness or abrasion resistance in the product heteropolycondensates. It is preferred that the Formula II compounds be alkoxides, i.e., that they have the formula S^OR¹)^ It is also preferred that all of the alkoxide groups represented by X or (OR¹) be the same, because such tetraalkoxides are readily inexpensive and, in general, are more readily available. However, mixed alkoxides, i.e., silicon alkoxides having two, three, or four different alkoxide groups, can be used if desired.

There is no limitation on the type of group bonded to the oxygen atoms in the alkoxide, provided, however, that the alkoxide groups derived therefrom are hydrolyzable under the reaction conditions employed. Thus, the organic radical can be a short or long alkyl chain, either branched or unbranched, unsubstituted or substituted with groups such as alkoxy, halogen (Cl, Br or I), or amine, or the like. More typically, they are short, unbranched, or slightly branched lower hydrocarbyl alkyl groups, i.e., alkyl groups that are solely composed of carbon and hydrogen, and which contain up to about 4 carbon atoms. Thus, for example, alkoxide groups within materials of Formula II are illustrated by the following non-limiting examples: methoxide ethoxide iso-propoxide tert—butoxide sec—butoxide

A most preferred alkoxide of this type is tetramethoxysilane; however, other compounds of this type, such as tetraethoxysilane, can also be used. Compounds of Formula II which are useful in this invention include aryloxides. Thus, compounds such as tetraphenoxysilane and diethoxydiphenoxy— silane can be used in this invention. Generally speaking, the aryloxy groups can be the phenoxy group or alkylphenoxy groups having up to about 14 carbon atoms. The alkyl substituents may be of the types discussed above. For powdered, i.e., non-monolithic, products the amount of Formula II compounds used in this invention is set forth in Table I.

One or more materials having Formula III can be added. These materials can be included within the reaction mixture to add linearity to the resultant heteropolycondensate, or to add more organic groups to the finished product. For example, with regard to using materials of Formula III to add organic groups, the combination of one molecule of Formula II and one molecule of Formula III contains two organic groups (indicated by R above). Hence, when it is desired to add a material of Formula II to improve hardness, a molar equivalent of a compound of Formula III can (optionally) be added so that the resultant product has as many organic groups as a condensate made solely from RSiX comρound(s). This can be a useful expedient when the organic group confers a highly desirous property on the heteropolycondensate such as enhanced oxygen permeability. Of the compounds of Formula III, the alkoxides having the formula R₂Si(0R')₂ are preferred. For non-monolithic products, typical amounts of R₂SiX₂ compound(s) added to the reaction mixture are set forth in Table I.

In the Table, the Column "General Use Range" shows that compounds of Formulas II and III need not be included in heteropolycondensates produced by the method of this invention. However, when these optional ingredients are employed, they are generally used in an amount up to about the higher value in Column 5.

Thus, for example, when one or more materials within Formula II are employed, they are used in an amount up to about 10 mole percent based on the total amount of silane employed within the reaction mixture. Turning now to column 5, when one or more compounds of Formula II is utilized, it is preferred that it be employed in a range of from about 1 to about 5 mole percent. Similarly, when a compound of Formula III is utilized, it is usually employed in an amount up to about 25 mole percent, or preferably within the range of from about 10 to about 20 mole percent. The Table clearly indicates that when one or more silanes (other than molecules within Formula I are employed) that the amount of Formula I material is generally about 60 mole percent, or greater.

As stated above, the amounts set forth in Table 1 are generally useful when it is desirable to prepare organic modified silicic acid heteropoly¬ condensates in the form of a powder or similar comminuted material. When it is desired to prepare a sol-gel product in the form of a rod or similar non—comminuted article, then one employs a silane starting mixture which contains compound(s) of Formula II in an amount of at least about 65 mole percent, more preferably at least about 70 mole percent. The remainder of the silane mixture is preferably one or more compounds of Formula I. However, a small amount, say up to about 5 mole percent, can be selected from compounds of Formula HI.

We have made rods using ethoxides, such as a mixture of such compounds as exemplified by Example III.

It is to be understood that the monolithic products of this invention are not limited to rods or rod-like shapes. The shaped articles of this invention generally conform to the shape of the vessel in which the reaction mixture is subjected to the hypercritical drying step illustrated by Example 3- Use of vessels (with other than a tubular shape) to hold the reaction mixture will result in different shaped products, e.g., spheres. The rods or other shaped products can be comminuted after preparation by hammering, grinding or similar techniques, if desired.

As pointed out above, the silanes of Formulas I and III all contain at least one organo group bonded to silicon through a carbon bond. In general, there is no limitation on the size or type of the organic group employed. Preferably, the group is "stable"; i.e., it does not undergo an untoward amount of decomposition during the preparative or upgrading process of this invention. More preferably, the organic group is relatively inexpensive. Thirdly, it is also preferred that the organic group be not so bulky as to unduly retard the process by steric hindrance, or by entering into an untoward amount of undesirable side reactions.

Alkyl radicals of the straight- or branched- chain type represent one type of organic group which can be present within the silanes employed in this invention. More preferably, the alkyl radicals contain from about 1 to about 20 carbon atoms, and most preferably from 1 to about 10 carbon atoms. Of particular interest are the lower alkyl radicals in which the radicals contain up to about 6 carbon atoms. Examples of lower alkyl radicals which may appear within the silanes employed in this invention are methyl, ethyl, isopropyl, n-butyl, &££-butyl, texi-butyl, n— entyl, and n—hexyl. Aryl radicals represent another type of organic group which may be bonded to silicon in the silanes employed in this invention. Of particular interest are the hydrocarbyl aryl radicals which contain from about 6 to about 20 carbon atoms, more preferably from 6 to about 14 carbon atoms, and even more preferably from 6 to about 10 carbon atoms. Specific examples are phenyl, naphthyl, biphenyl, and the like. (Hydrocarbyl radicals are solely composed of carbon and hydrogen.) The radical(s) indicated by R in the above formulas may also be an alkenyl radical, e.g., a straight- or branched-chain radical containing from 2 to about 20 carbon atoms, and more preferably, 1 carbon—to-carbon double bond. Specific examples of alkyl radicals of this type are the ethylenyl, allyl, and vinyl groups.

To produce the organic modified silicic acid heteropolycondensate, the starting component (or mixture of starting components) is contacted with water. Preferably, the silane or mixture of silanes is first dissolved in an organic solvent. Suitable solvents are alcohols, preferably the lower alcohols having from 1 to about 4 carbon atoms and exemplified by ethanol, ethanol, isopropanol, and isobutyl alcohol. The solvent may also be selected from ketones, particularly from lower dialkyl ketones. Solvents of this type include acetone and methyl isobutyl ketone. In addition, the solvent may be an ether, such as diethyl ether or another lower dialkyl ether. Alternatively, the solvent may be an amide, such as dimethylamide. A skilled practitioner will recognize that the above list of solvents is illustrative but not limiting, and that other solvents of the general type described above can also be utilized in this invention. Moreover, a mixture of solvents can be used. Preferably, the solvent is a lower alcohol.

The starting material(s), preferably in an organic solvent, is mixed with water in order to cause the reaction to proceed. Preferably, the amount of water is at least the quantity required to hydrolyze the hydrolyzable groups that are present.

For example, if the silane starting material used in the preparation process of this invention is composed of one or more compounds of Formula I, i.e., compounds having the formula RSiX₃, then at least three moles of water is used, even though 1.5 mole equivalents of water are theoretically required to be added. Although three moles of water are required to hydrolyze each of the hydrolyzable groups, Equation 2 shows that in each condensation between silanol groups one mole equivalent of water is formed. This mole of water formed in the overall hydrolysis- condensation process is available for the hydrolysis step. Hence, in this example, the amount of water to be added to the reaction mixture is two thirds of the amount required to hydrolyze the hydrolyzable groups within the materials to be reacted.

It is not necessary that an exactly stoichiometric amount of water be used. Thus, an excess of water can be employed. However, the amount of water added should not be too great since the monomers are not water miscible. Furthermore, if the amount of water is beyond a desired amount, the more reactive materials or groups will be more completely hydrolyzed and condensed before the less reactive groups have a chance to react. Thus, if an excess of water is employed, only up to about a fivefold molar excess (500 mole %) is preferably used.

It is preferred that the water be slowly added to be reaction zone. Accordingly, it is preferred that the water be added as a slow stream or in a drop-wise fashion. Addition of the water in this manner helps prevent unchecked hydrolysis and/or condensation of the more reactive groups within the material being reacted.

The preparative process of this invention can be conducted with water alone, or in the presence of a catalyst. Suitable catalysts are acidic or basic substances. Examples of suitable acid catalysts are strong, non-oxidizing, inorganic acids, such as hydrochloric acid, sulfuric acid, acetic acid, and the like. These preferred acids have a pK of at least about 5.

Alternatively a catalyst may be a non—oxidizing alkaline substance, such as sodium hydroxide, potassium hydroxide, or a lower alkylamine, such as triethylamine having a pK, of at least 5.

It is preferred that their reaction be conducted in the presence of a catalyst. It is also preferred that an acidic catalyst be employed. When a catalyst is used, it is usually employed in an amount up to about 5 weight percent based on the total weight of the reaction mixture.

The hydrolysis and condensation reactions within the preparative process of this invention are normally carried out at mild to slightly elevated temperatures. Generally, reaction is conducted at a temperature within the range of from about 10^βC to about 130°C. Preferably, the reaction is conducted at a temperature within the range of from about 25°C to about 65°C.

The formation of an organic modified silicic acid heteropolycondensate by hydrolysis/condensation is preferentially conducted at atmospheric pressure. However, it is to be understood that other pressures can be employed. Thus, for example, it is possible to use slightly lower or higher pressures, say, pressures within the range from about 0.5 to 5 atmospheres. The formation of organic modified silicic acid heteropolycondensates in this invention is conducted for a time which affords a desired state of reaction. In a preferred embodiment the process is conducted for from about 1 minute to 24 hours. When a catalyst is employed, the reaction can be completed in a shorter time.

After a sol—gel is prepared as discussed above, it can be upgraded or improved by use of the upgrading process of this invention. For the upgrading, it is not necessary to remove any solvent that was employed in the preparative step. Stated another way, the upgrading step can be performed using the reaction mixture produced in the preparation step. To upgrade an organic modified silicic acid heteropolycondensate according to this invention, the condensate to be upgraded is treated with an extractant fluid at a supercritical temperature and pressure above atmospheric, so that silanol groups and hydrolyzable groups are removed from the polycondensate. By-products which are produced, e.g., water and alcohol, become admixed with, absorbed, or dissolved in the extractant fluid. Subsequently, the pressure is reduced and the fluid removed from the condensate so that the by-products are separated with the extractant fluid.

The operating temperature is at or preferably somewhat above the critical temperature of the fluid which is used, e.g., methanol or ethanol. The operating pressure is also above the critical pressure of the fluid.

The upgrading is conducted at a temperature within the range of from about 200^βC to the decomposition temperature of the heteropolyconden¬ sate, more preferably at a temperature of from about 250°C to about 350°C. It is to be understood that temperatures outside of this range can be used. The operator will select a temperature which is sufficient to give a reasonable process rate, but not so high as to cause an intolerable amount of unwanted decomposition.

The upgrading process is also conducted at an elevated pressure. In general, one uses a pressure higher than about 1000 pεi. Thus, the upgrading is usually conducted at a pressure of from about 1000 to about 5000 psi.

The upgrading may be conducted in the presence of a small amount of water to remove any hydrolyzable groups which may still remain in the polycondensate. Thus, one usually employs from about 0.5 to about 1.5 times the amount of water theoretically required to hydrolyze all of the hydrolyzable groups in the starting silanes. A skilled practitioner will understand that greater or lesser amounts of water can be used. In general, one uses enough water to conduct the desired amount of hydrolysis, but not so much water as to cause the process to proceed in an undesirable manner.

The upgrading step is conducted for a time sufficient to give the desired amount of hydrolysis or condensation. In general, one uses a reaction time of rom about 3 to about 20 hours. The reaction time is not a truly independent variable, but is dependent at least to some extent on the other reaction conditions employed. For example, higher reaction temperatures generally require shorter reaction times.

A skilled practitioner will recognize that the processes of this invention involve a considerable number of process and composition variables to select from. Consequently, the invention includes an ability to "tailor—make" or "fine—tune" a heteropolycondensate composition so that it has a desired set of properties. This offers considerable advantage when it is desired to produce a composition having preselected performance criteria. When a practitioner wishes to produce a heteropolycondensate according to the teachings of this invention, he or she can be guided by the description above and the examples which follow.

Example 1

In a 250 mL, 3-neck, round-bottom flask equipped with a mechanical stirrer and reflux condensor were placed 40 mL (0.17 mol) of phenyltriethoxysilane, 6.6 mL (0.031 mol) of dichlorodiphenylsilane, 2.3 mL (0.010 mol) of tetraethoxysilane, and 49 mL of ethanol. This mixture was stirred at 60°C in a constant temperature bath while 10.8 mL (0.60 mol) of 0.15 M HCl was added dropwise. The resulting solution was stirred at 60°C for 1 hour, allowed to stand overnight at ambient temperature, and then stirred an additional 4 hours at 60^βC.

The procedure described above can be modified to produce a variety of compositions simply by varying the monomers. The amount of ethanol which is added may be in equal volume to the silane monomers. The amount of water added, in the form of 0.15 M HCl, for example, may be equal to the number of moles of hydrolyzable groups on the silane monomers. In some instances, the organic modified silicic acid heteropolycondensates produced by the above process were isolated from the ethanol solution in which they were made. This was accomplished by pouring the ethanol solution of the condensate into an excess of water. The condensate separated as a viscous oil which could be collected, dried (in a vacuum oven), and dissolved in methylene chloride. To produce a material with a high enough molecular weight to be isolated in this manner, a suitable reaction time is employed. In general, three hours of stirring at 60°C after addition of the aqueous HCl was sufficient.

Heteropolycondensates made according to the process illustrated by this Example can be upgraded by heating them at an elevated temperature and pressure as (discussed above and) illustrated by the following Example:

Example 2 Phenyltriethoxysilane (60 ml, 0.25 mol) was polymerized according to the procedure set forth in Example 1 using 46.7 ml of ethanol and 13.4 ml (0.75 mol) of water. No acid catalyst was employed.

The ethanolic solution of the silicic acid heteropolycondensate produced was transferred to a stainless steel reaction vessel and dried by heating above the critical temperature of ethanol as in Example 3.

This was accomplished by raising the temperature of the solution at a rate of 100°C/hr. to 280°C and a pressure of 2000-2500 psi. At that point the reactor was vented and swept with nitrogen. The product was allowed to slowly cool to ambient temperature over a time span of about 16 hours. After removing the sample tube from the reactor, a powder was obtained. This powder was found to be soluble in organic solvents, such as

CHCl or toluene. A portion of this powder was dissolved in CDC1₃ and a 29SiNMR spectrum obtained. This spectrum clearly showed that >95% of the silicon atoms had fully condensed and had no residual silanol bands. Silicon atoms with one remaining —OR group were barely detectable above baseline noise. Peak positions in 29SiNMR spectra —82 ppm vs. Si0₂ 112 ppm— indicated that the phenyl groups remained. This fact was confirmed by infrared spectroscopy with bands at 3050, 2920, 1600, 1435, 730, 690 cm , and as broad band at 1100 cm due to Si—0—Si. These bands clearly confirm the presence of the phenyl group.

The phenyl group-containing silicic acid heteropolycondensate produced by the process of this Example is a product of this invention.

A method of upgrading an organic modified silicic acid heteropolycondensate, such that a product in the form of a rod is obtained, is illustrated by the following example. Esaro le 3 In a 250 ml, 3-neck, round—bottom flask equipped with a mechanical stirrer and reflux condensor were placed 30 ml of tetramethoxysilane, 8.6 ml of phenyltriethoxysilane, and 30 ml of methanol. The flask was placed in a 60°C constant temperature bath and 26.5 ml of distilled water were added dropwise with stirring. After the H₂0 addition, stirring was maintained for two hours at 60°C before removing from the bath. The reaction mixture contained 15 mole percent phenyltriethoxy¬ silane based on the total moles of silane employed.

The solution of organic modified silicic acid heteropolycondensate was added to a glass tube 25.2 cm long with a 7 mm inner diameter. The tube was placed in a stainless steel reactor and charged with N₂ to about 800 psi. The temperature of the vessel was then increased at a rate of 100'C/hr. to 240-250°C with a resulting pressure of 2000 psi. This created a supercritical fluid (above the critical point of methanol). At this point, the reactor was vented and swept five times with ₂ and then slowly allowed to cool to ambient temperature. The product was isolated in the form of a rod which readily separated from the walls of the glass tube. The rod was slightly opaque.

The process was repeated using the following starting compositions which contain about 15 and 5 mole percent respectively^' of phenyltrimethoxysilane.

25% Phenyl 23.2 ml tetramethoxysilane 12.6 ml phenyltriethoxysilane 30 ml methanol 15.1 ml H₂0 5% Phenyl 30 ml tetramethoxysilane 2.47 ml phenyltriethoxysilane 30 ml methanol 15.1 ml H₂0

The following demonstrates the utility of upgraded organic modified silicic acid heteropolycondensates of this invention in oxygen sensing.

Example 4 To demonstrate utility of the non-monolithic products of this invention, stock solutions were prepared by dissolving one gram of organic modified silicic acid heteropolycondensates in 10 mL of CH₂C1₂ in which one milligram of PtOEP was added. Light was excluded from the stock solution. In this study, an optical fiber spectrometer (Guided Wave Model 200) was used to measure the phosphorescence intensity. This commercial instrument is equipped with a bifurcated fiber with one input fiber in the center axis to guide light to the sample and six output fibers along the edge to collect the scattered light and guide it back to the spectrometer. The fibers are terminated in a stainless steel probe. A polycarbonate or polymethacrylate barrel was screwed with dichloromethane. The 25 mil thick plate places the dye in the cross section of the exciting beam and the field of view defined by the numerical aperture of the optical fibers, maximizing the observable phosphorescence. The PtOEP/sol—gel was first drop coated on the circular glass plate. This coating was then processed at 60^βC to 120^βC for one to four hours. After cooling to room temperature, this glass plate was "glued" to the barrel. During the measurement, the fiber was kept in a glass bottle which is connected to two flowmeters. The two flowmeters were used to control the ratio of compressed nitrogen and air partial pressure, while keeping the total pressure constant. The dye/polymer was excited with a tungsten lamp and an Ealing interference filter which passed wavelengths <550 nm. The phosphorescence was monitored at 642 nm with a bandwidth of 10 nm. Phosphorescence lifetimes were estimated from recorded decay curves by nonlinear curve fitting. Visible absorption spectra were taken with an HP-4850A UV-VIS spectrometer.

The slope of a plot of [I(N₂)/I(0₂)-1] vs. P is proportional to the product of the lifetime λ. and the permeability P .

The drawing in the Figure shows that an organic modified silicic acid heteropolycondensate made from phenyltriethoxysilane according to the method of Example 2, is significantly more permeable to oxygen than a polycondensate material made from the same silane but which was not upgraded by the process of this invention. As shown by the Figure, the material made by the process of Example 2 has some nonlinear response in permeability to oxygen relative to oxygen concentration over the range examined.

During the course of work conducted in the development of this invention, it was noted that the photostability of PtOEP in heteropolycondensates is not permanent, and that the phosphorescence of incorporated PtOEP decreased with time when exposed to light and oxygen. It was also observed that the PtOEP incorporated in a heteropolycondensate of Example 2 had a relatively slow degradation rate. This demonstrates another advantage of this invention. Films and other articles made from the materials of this invention may be used in other applications besides oxygen sensing. For example, they may be used wherever enhanced oxygen permeability is required, e.g., in gas separating applications, or in the construction of devices or materials to be applied to animal or human skin or other external human membranes. Thus, they may be used in the fabrication of contact lenses, for example.

It is to be understood that the materials of this invention need not be solely composed of silicic acid heteropolycondensates such as described above. They may also be modified to contain other metal oxide units in the polymeric matrix. For such modification, the raw material composition used in the preparative process of this invention may be modified to contain up to about 80 mole percent, and more preferably up to about 50 mole percent of one or more hydrolyzable materials, such as the alkoxides of titanium, molybdenum, lead, geranium, zironium or vanadium. Modified condensates comprising metal oxide units derived from such materials have utilities similar to those discussed above. Because the method in accordance with this invention removes residual silanol groups and hydrolyzable groups, it improves the properties of silicic acid condensates. For example, because the method of this invention improves the oxygen permeability of such condensates, it makes such materials more useful in applications such as oxygen sensing, where it is necessary to have utilize a substance that is permeable to oxygen. Also, since the upgrading method of this invention does not remove organic groups bonded to silicon through Si-C bonds, it can be used to improve the oxygen permeability of organic modified silicic acid heteropolycondensates without decomposition of their organic substituents. In addition, as provided by this invention, use of the supercritical fluid with an organic modified heteropolycondensate in a closed mold provides a new type of glassy porous solid. Such materials with organic groups bonded to silicon atoms by Si-C bonds have never been made before. They are useful in many applications, for example as absorbents.

Claims

Clairos :

1. A process for upgrading an organic modified silicic acid heteropolycondensate by removal of residual silanol or hydrolyzable groups therein, said method comprising contacting said polycondensate with a fluid above the critical temperature and pressure of said fluid for a time sufficient to remove silanol or hydrolyzable groups from said condensate.

2. The process of claim 1 wherein said residual hydrolyzable groups are alkoxy radicals and said fluid is the alcohol corresponding to said alkoxy radicals.

3. The process of claim 2 wherein said fluid is selected from the class consisting of methanol or ethanol.

4. The process of claim 1 wherein said organic modified silicic acid heteropolycondensate is substantially composed of repeating units having the formula RSi≡, wherein R is a stable, organic group.

5. The process of claim 4 wherein R is phenyl.

6. An organic group—containing, porous monolithic glassy body containing the repeating units Si= and RSi≡ wherein R is a stable organic group, such that

(a) from about 5 to about 35 mole percent of said units are RSi≡;

(b) said RSi≡ units are substantially uniformly dispersed throughout said body; and

(c) said repeating units are interconnected by oxygen; said body having a surface area of from

2 about 500 to about 800 m /g, an apparent density of about 0.25 to 0.40 g/ml, and being substantially free of residual silanol and hydrolyzable groups.

7. A process for producing a monolithic body of claim 6, said process comprising contacting a sol in a mold with a fluid above the critical temperature and pressure of said fluid, for a time sufficient to remove substantially all residual silanol and hydrolyzable groups from said sol, and subsequently cooling and venting to separate said body from said fluid and volatile by-products formed upon removal of said groups from said sol; said sol having been produced by hydrolysis and condensation of a mixture of silanes having the formula SiX^ and the formula RSiX₃, wherein from about 5 to about 35 mole percent of said silanes in said mixture have the formula RSi∑₃, and wherein each X in said formulae is a methoxy group, and R is a non-hydrolyzable organic group.

8. The process of claim 7 wherein said fluid is methanol.