Inorganic and Organometallic Polymers II - American Chemical Society

Chapter 11

Plasma Oxidation of Hydrocarbon Templates in Bridged Polysilsesquioxanes Porous Materials by Design Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0572.ch011

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Douglas A.Loy ,Kenneth J. Shea , Richard J . Buss , and Roger A. Assink 1

1

Sandia National Laboratories, Albuquerque, NM 87185-0367 Department of Chemistry, University of California, Irvine, CA 92717

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Hydrocarbon bridging groups in polysilsesquioxane network polymers were used as templates for porosity. Arylene- and alkylene-bridged polysilsesquioxanes were prepared by the hydrolysis and condensation of bis(triethoxysilyl)aryl monomers 1-4 and bis(triethoxysilyl)alkane monomers 5-9. The bridged polysilsesquioxane xerogels (X-1 through X-9) and monolithic gels (M-1 through M-3) were treated with inductively coupled oxygen plasmas at low temperature(≤100 °C). All of the bridged polysilsesquioxane gels save the microporous monolith M-3 were quantitatively converted to silica gels. Non-porous alkylene-bridged polysilsesquioxanes were plasma oxidized to porous silica gels whose pore size and shape appeared to be related to the length of the bridging group.

Sol-gel processing of tetraalkoxysilanes leads to the formation of porous silica gels (xerogels and aerogels). Pore structure in these silica gels can be manipulated by changing the reaction and processing conditions used for their preparation (1). However, the pore size distributions of these materials are too broad for application as molecular sieves. Another strategy for manipulating porosity is to build a material with molecular-sized building blocks (2-7). According to this strategy the properties, including porosity, of the resulting synthetic materials may be dictated by the size, length, and shape of the monomelic building blocks. Most of these efforts have afforded non-porous materials or demonstrated an inability to control the materials' pore distributions. For example, arylene-bridged polysilsesquioxanes are mostly microporous materials, yet the length of the arylene spacer or building block has no significant effect on the mean pore diameter (7). Unlike porosity in microcrystalline molecular sieves, porosity in sol-gel processed amorphous materials is more dependent on macromolecular-scale architecture than molecular-based structure. Bridging Groups in Polysilsesquioxanes as Pore Templates. In this study, the organic components of bridged polysilsesquioxanes were used as disposable pore templates as well as structural spacers. Arylene- and alkylene-bridged polysilsesquioxane gels were prepared by sol-gel processing bridged triethoxysilyl monomers (Figure 1) (6-11). The resulting dried gels were then treated with an oxygen

0097-6156/94/0572-0122$08.00/0 © 1994 American Chemical Society In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Hydrocarbon Templates in Bridged Polysilsesquioxanes 123

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plasma to create silica gels (Scheme 1). Loss of the hydrocarbon bridging groups generates new porosity within the material. In porous materials, loss of the templates would be expected to occur in the pore walls with a net effect of coarsening or enlarging the pores. In non-porous materials, the loss of templates would generate porosity where none existed previously. The size and shape of the pores would be dependent both on the identity of the template and how the template groups are associated with each other in the matrix. For example, formation of nano-phases of alkylene-bridging groups could generate a nano-sized pore template surrounded by silicon-rich regions. Removing Pore Templates. In order to generate porosity through this approach, a method for removing the organic template without disturbing the inorganic framework must be used (Scheme 2). Organic phases in hybrid organic-inorganic materials have been used as templates for porosity before our effort. Chujo et. al. prepared nonporous nanocomposites of organic polymers and silica by a sol-gel method (12). These materials were then pyrolyzed at 600 °C in order to remove the templates and afford microporous silicas. Roger et. al. hydrolyzed non-porous tin-carboxylates to remove the organic pore templates leaving mesoporous tin oxides (13). We have focused on a low temperature oxygen plasma as a mild method for removing organic templates from bridged polysilsesquioxanes (14). In the bridged polysilsesquioxanes, the template is attached to two or more silicon atoms in the siloxane network through silicon-carbon bonds. Thermal gravimetric analysis of a phenylene-bridged polysilsesquioxane in air indicated that removal of the bridging group by thermal oxidation occurred only above 400 °C (10). We desired relatively mild conditions for template removal to minimize alteration of the existing siloxane framework. Plasma Oxidation of Pore Templates. Low temperature, inductively coupled oxygen plasma treatment was selected as a "mild" method for removing the hydrocarbon templates in the bridged silsesquioxane network materials. However, it was not clear that an oxygen plasma would penetrate very far into the silsesquioxane network before forming a plasma-resistant silica overlayer (15). Therefore, the first bridged polysilsesquioxanes treated in this study were porous arylene-bridged polysilsesquioxanes that had been ground into powders. Once the oxygen plasma technique demonstrated removal of the bridging groups in these materials, non-porous alkylene-bridged polysilsesquioxanes were also treated to determine if the size or length of the bridging group could be used in a predictable way to affect porosity in the resulting silicas. Finally, monolithic aerogels and a xerogel were plasma processed to determine if the plasma would penetrate deep into these gels. Experimental Arylene Monomer Synthesis. Bis(triethoxysilyl)arylene monomers 1-4 were synthesized from the aryl dibromides and tetraethoxysilane (TEOS) or chlorotriethoxysilane using Barbier-Grignard or organolithium chemistry (Scheme 3). An example of the preparation of 1 from 1,4-dibromobenzene and tetraethoxysilane is as follows: 1, 4-Bis(triethoxysilyl)benzene 1 (7). To a mixture of magnesium turnings (15.0 g, 0.62 moles) and TEOS (450 mL, 2 moles) in THF (300 mL) under nitrogen, a small crystal of iodine was added and the mixture brought to reflux. A solution of 1,4dibromobenzene (48 g, 20.4 mmol.) in THF (100 mL) was added drop wise over 2 hours [CAUTION: care must be exercised with the addition of dibromobenzene. Addition of more than 25% of 1,4-dibromobenzene to the reaction mixture before the

In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

124

INORGANIC AND ORGANOMETALLIC POLYMERS II

6H 0 2

(EtO) Si-|

-Si(OEt)

3

-Si-0+

3

Catalyst: H Q or OH"

Ν

X-N


N -

-(CH2) 2

5 -(CH )3-

-(CH ) -

-(CH ) -

-(CH2)i4-

6

7

8

9

2

2

6

2

1 0

Figure 1. Preparation of bridged polysilsesquioxanes X - l through X-9 from bis(triethoxysilyl) monomers 1-9.

Scheme 1. Oxidative removal of hydrocarbon template from bridged polysilsesquioxanes.

Si(OEt)

3

Si(OEt)

3

Monomer

Non-Porous Templated M a t e r i a l |

Porous Silica

Scheme 2. Organic template approach for creating porosity.


11. LOY ET AL.

Hydrocarbon Templates in Bridged Polysilsesquioxanes

125

induction period of Grignard formation could result in a violent exotherm!] Within 30 minutes of initiating the addition, the reaction became mildly exothermic. The reaction mixture was kept at reflux for 1 hour after the completion of the addition of dibromobenzene. The grey-green mixture was cooled to room temperature before the THF was removed in vacuo. Hexane (200 mL, distilled from CaH2) was added to precipitate any remaining magnesium salts in solution and the mixture was quickly filtered under nitrogen to afford a clear, light brown solution. Hexane was removed in vacuo and the remaining TEOS was distilled off leaving a brown oil. The residual oil was distilled (0.2 mm Hg, 130-5 °C) to give a clear colorless oil (1,42 g, 55%). *H NMR (300 MHz, CDC1 )^ 7.63 (s, 4H, ArH), 3.82 (q, 12H, J = 7.12 Hz, SiOC# CH ), 1.19 (t, 18 H, J = 7.11 Hz, SiOCH CJ/ ); C NMR (75.5 MHz, CDC1 ) dl34.36, 133.45, 59.09, 18.56; S i NMR (99.34 MHz, CDCI3) d -57.72; IR (fdm on NaCl) 3058, 2976, 2886, 1391, 1168, 1147, 1102, 1079, 961, 778, 706 cm- ; low resolution mass spectrum (EI, 70 eV) m/z 402 (M+), 357, 329, 297, 285, 147, 119, 107, 100, 91, 79; high resolution mass spectrum (EI, 30 eV) calcd. for C18H34O6S12: 402.1890, found: 402.1909.


3

1 3

2

3

2

3

2 9

3

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Alkylene Monomer Synthesis. Ethylene monomer 5 was prepared by the hydrosilation of vinyltriethoxysilane with triethoxysilane in the presence of chloroplatinic acid. Propylene monomer 6 was synthesized by hydrosilating allyltriethoxysilane. The remaining monomers 7-9 were prepared by hydrosilating the corresponding a, ω-dienes (11). An example of the preparation of 1, 6bis(triethoxysilyl)hexane 7 by the hydrosilation of 1, 5-hexadiene (Scheme 4) is as follows: 1, 6-Bistriethoxysilyl)hexane 7 (11). To a mixture of 1, 5-hexadiene ( 100 g, 0.13 mol.) and chloroplatinic acid (20 mg) in benzene at room temperature (CAUTION: chloroplatinic acid is a cancer suspect agent and benzene is a carcinogen.) in a three-necked round-bottom flask equipped with stir bar, condenser with argon inlet, and rubber septa on the remaining two openings, triethoxysilane (100 mL, 0.27 mol.) was added by syringe (CAUTION: triethoxysilane is highly toxic!). Some bubbling was observed from the solid catalyst, which changed from orange to black. The solution was stirred at 50 °C and the reaction monitored by capillary gas chromatography. After four hours, the solution was cooled to room temperature; solvent and excess triethoxysilane were removed in vacuo, leaving a brown oil. Distillation afforded a clear colorless oil (33.9 g, 64 %). *H NMR (200 MHz, CDC1 ) δ 3.70 (q, 12 H, J = 7.02 Hz, SiOC# CH ), 1.35 (m, 4 H, C # C H C H S i ) , 1.22 (m, 4 H, CH C# CH Si), 1.12 (t, 18 H, J = 6.98 Hz, SiOCH C# ), 0.52 (t, 4 H, J = 7.81 Hz, CH CH Ctf Si); 13c NMR (75.5 MHz, CDC1 ) δ 58.17, 32.71, 22.57, 18.20, 10.31; IR (fdm on NaCl) 2975, 2927, 2887, 2735, 1484, 1443, 1391, 1366, 1295, 1261, 1168, 1167, 1105, 1082, 958, 782 cnr ; low resolution mass spectrum (EI, 70 eV) m/z 410 (M+), 364, 335, 321, 297, 291, 279, 253, 247, 163 (base peak), 135, 119, 107, 100, 91, 79. 3

2

2

2

2

3

2

2

2

2

2

2

2

3

3

1

Preparation of Gels. Bridged polysilsesquioxanes gels X - l through X-9 were prepared by sol-gel processing monomers 1-9 (Scheme 5). Typically, the monomer was dissolved in about half the required solvent needed to make up the final concentration. To this solution of monomer in a volumetric flask, a mixture of aqueous catalyst in solvent was added; additional solvent was added to achieve the final volume of solution. The monomer and catalyst solutions were thoroughly mixed by shaking die flask. The contents of the volumetric were then transferred to a polyethylene botde


126


Br-^Q-Br

*™\

S

(EtO) Si-HQ-Si(OEt) 3

3

Scheme 3. Barbier-Grignard preparation of 1,4-bis(triethoxysilyl)benzene 1.

(EtO) SiH

, . (EtO) Si^^


3

3

S i ( O E t ) 3

PhH Scheme 4. Hydrosilation of 1,5-hexadiene to synthesize 1,6-bis(triethoxysilyl)hexane 7.

Bridged Ethoxysilane Monomer Eto ,OEt EtO-SiHE3hSi-OEt EtO OEt %

H Hydrolysis/Condensation Polymer Gel Si-HSl-Si.-Or

o-k Aqueous Work-Up or Air-Drying

7

XEROGELS

\

)

Supercritical Extraction

[ AEROGELS

Oxygen Plasma

POROUS SILICA GEL Scheme 5. Preparation of bridged polysilsesquioxanes by sol-gel techniques followed by oxygen plasma treatment to generate porous silicas.



11. LOY ET AL.


127

which was tightly sealed to prohibit evaporation. The solutions were clear and colorless. With the onset of gelation a blue tint appeared. Gelation times were dependent on the monomer and type and concentration of catalyst, but were typically less than 1 day. The final gels were glass-like in appearance. The gels were aged in sealed containers as long as several weeks before either air drying, supercritical drying, or simply crushing the gel in water before drying under vacuum at 100 °C. Arylene-bridged polysilsesquioxanes were prepared by reacting the arylene monomers (0.4 M) in tetrahydrofuran with an excess of aqueous ammonia (540 mol%) and water (>10 equivalents). The arylene-bridged gels were solvent-processed (7) and allowed to air dry. Alkylene-bridged polysilsesquioxane xerogels were prepared in tetrahydrofuran (0.4 M) with 10 mol% HC1 as catalyst. The gels were air-dried, affording xerogels with approximately 5-10% of the original "wet" gel volume. The alkylene-bridged gels were aged two weeks before being crushed in water then washed with water (2X) and ethyl ether (11). The gels were ground into powders and dried under vacuum at 100 °C before treating with plasma. The propylene monomer 6 takes months to gel using the conditions described above. The plasma treatments in this study were carried out on a porous propylene-bridged polysilsesquioxane made with acid catalyst at 1.2 M monomer 6 concentration. Ethylene- and decylene-bridged monolithic aerogels ( M - l and M-2) were prepared by extracting the wet gels with supercritical carbon dioxide (10). The phenylene-bridged polysilsesquioxane xerogel M-3 was prepared by slowly air drying the wet gel over several months. The resulting monolith was a transparent, colorless glass. Plasma Oxidation of Bridged Polysilsesquioxanes. Oxidations were carried out in an inductively-coupled rf plasma (Figure 2). In a typical experiment, a ground sample of decylene-bridged polysilsesquioxane X-8 was placed in a plasma of pure oxygen (30 Watts, 138 mTorr, 2.4 standard cubic centimeters per minute (seem) O2, sample temperature -50 °C). The plasma exhibited a strong carbon monoxide photoemission. After 16 hours of plasma exposure, no carbon monoxide emission was observed and the sample was removed. When enough sample was available, the plasma experiments were repeated three times. Solid state S i NMR spectra were obtained at 50.17 MHz with a Chemagnetics console interfaced to a Nicolet 1280 data station. Samples were spun at 4-5 kHz. Surface area analyses were carried out with a Quantachrome Autosorb-6 nitrogen porosimeter. Infrared spectra experiments were performed with a Perkin-Elmer 1750 FT-IR spectrometer. 2 9

Results Bis(triethoxysilyl)arylene and alkylene monomers 1-9 (0.4 M in tetrahydrofuran) were hydrolyzed in die presence of six or more equivalents of H 0 (Scheme 5). In all cases, clear colorless solutions or "sols" were formed. As hydrolysis and condensation progressed, the sols became more viscous, finally forming gels. Acid catalyzed polymerizations always afforded transparent gels. The base catalyzed arylene-bridged gels were transluscent, while alkylene-bridged gels were always opaque white. The polysilsesquioxane gels were further processed to afford xerogels or aerogels. The former are dried gels which have shrunk considerably during processing, while aerogels undergo little shrinkage and are thought to retain more of the original wet gel's structure. In tins study, xerogels are defined as gels that were air dried or processed by crushing in water. Gels that were extracted with supercritical carbon dioxide and generally exhibit pore distributions that extend well into the macropore regime are called aerogels. Solid state S i and C (CP MAS) NMR spectroscopy on the dried gels indicated the bridging groups were intact by the presence of the expected bridging 2

2 9

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INORGANIC AND ORGANOMETALLIC POLYMERS II 2 9

carbon resonances and the lack of S i NMR peaks arising from silica (7-11, 14). Nitrogen sorption analyses revealed the arylene-bridged polysilsesquioxanes to be mosfly microporous with surface areas ranging from 400-830 m /g (BET). The alkylene-bridged polysilsesquioxanes prepared with acid catalysts were non-porous (16). Monolithic aerogels of the ethylene-bridged (M-l) and decylene-bridged (M-2) polysilsesquioxanes prepared with base catalyst were porous with surface areas of 956 m /g and 547 m /g, respectively. 2


2

2

Plasma Treatment of Bridged Polysilsesquioxanes. Without any carbon source in the plasma chamber, the inductively coupled oxygen plasma appeared violet However, when the oxygen plasma was generated in the presence of bridged polysilsesquioxane gels, an intense, light blue carbon monoxide emission was observed. The plasma color has proven to be a useful indicator for the completion of the oxidation process. Once the hydrocarbon removal was complete, the plasma reverted to a violet oxygen emission-dominated plasma. Powdered samples placed into oxygen plasmas did not undergo any apparent physical changes other than the appearance of a thin, light brown discoloration on the surface of the powders that persisted even after the carbon monoxide emission had disappeared. The powder underneath the brown surface was white. No evidence of residual hydrocarbon could be detected in the bulk samples and it is likely that the brown discoloration arose from "color centers" generated by ion or vacuum UV photon bombardment. Weight lossesfromthe plasma treatments were recorded (Table 1). Weight losses for the gels are difficult to compare with calculated values unless the degrees of hydrolysis and condensation in the precursor material and the degree of condensation in the resulting silica is determined. We are presently measuring these values by solid state C and S i NMR spectroscopy. As one would expect, weight losses for materials with large molecular weight contributions from the bridging groups or templates correspond more closely with the calculated values. 1 3

2 9

Plasma Oxidation of Monolithic Gels. In order to determine if structures more massive than micron-sized granules in the powdered gels could be plasma treated to remove the bridging groups or templates, we plasma treated a series of monolithic gels (M-l, M-2, M-3) possessing different mean pore sizes. After plasma treatment die porous ethylene-bridged aerogel M - l had lost 22 % of its volume and 16% of its mass. Surprisingly, the M - l suffered only from a single crack which ran through the middle of the gel. It retained the faint blue transluscence of the original ethylenebridged aerogel. Scanning electron microscopy of the aerogel before and after the plasma treatment revealed littie change in the gel's structure. Yet, no ethylene-bridging groups could be detected by infrared spectroscopy. The oxygen plasma had penetrated all the way through the monolith! Infrared spectroscopy also confirmed that only silica remained after the plasma oxidation of a mesoporous decylene-bridged polysilsesquioxane aerogel M-2. The opaque, white decylene-bridged monolith lost volume during plasma oxidation and, unlike the ethylene-bridged gel, suffered extensive shallow cracking over its exposed surface. The last gel to be plasma treated was a transparent, colorless phenylene-bridged xerogel M - 3 . Once the mostly microporous gel was exposed to the plasma, a white coating formed on the majority of the exposed surfaces. The remaining surfaces remained transparent, but turned dark brown. Scanning electron microscopy revealed that the silica coating extended slighdy less than 1 micron into the bulk material. We can conclude from the results with the monoliths that an oxygen plasma can be used to convert monolithic polysilsesquioxane gels to silica as long as there is substantial meso- and macroporosity for the plasma to penetrate.


11. LOY ET AL.


Infrared Analysis. Loss of the hydrocarbon template in powdered samples was also monitored by obtaining a series of infrared spectra on material withdrawn from the plasma chamber. In this experiment a porous phenylene-bridged polysilsesquioxane powder was placed in a watch glass in die plasma apparatus. At regular intervals, the plasma was stopped and samples of the powder removed for analysis. The aromatic (Γ Η stretching band at 3060 cm" was used as the diagnostic peak. The plot of absorbance over time in the oxygen plasma indicates that nearly all of the phenylene spacer was gone within 15 minutes of exposure to the oxygen plasma (Figure 3). Plasma treatment was continued, however, until the carbon monoxide emission had disappeared, typically requiring an additional 4-12 hours. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0572.ch011

1

Table 1. Silica Yields from Plasma Oxidation of Bridged Polysil sesquioxanes

Sample

Pore Template Bridging Group

X-5 X-6 X-7 X-8 X-9 M-l M-2 M-3

ethylene propylene hexylene decylene tetradecylene ethylene decylene phenylene

% Weight Remaining after Plasma Theoretical % Experimental 66.4» 76.0 59.8 50.5 41.0" 87.7 51.2» 80.0»

104.5 94.5 73.4 56.5 46.0 104.5 56.5 76.6

e

e

e

a

a) Sample oxidation was performed only once, b) Plasma experiment was performed twice, c) Experiment was performed three or more times. Theoretical % weight calculated based on fully condensed silsesquioxane oxidizing to Q silica (Scheme 1). 3

2 9

Solid State NMR Analyses. Solid state S i NMR confirmed conversion of the bridged polysilsesquioxanes to silica. The Τ resonances (7, 9, 17) in the solid state S i CP MAS NMR spectrum of phenylene-bridged polysilsesquioxane X - l lie at -61, -70 and -78 ppm (Figure 4A). No Q resonances, representing Si-C bond cleavage to silica during the sol-gel preparation of the material, are evident. After plasma treatment (Figure 4B), the Τ resonances were greatly attenuated (< 5% remaining) and Q resonances at -90, -101 and -108 ppm indicate that silica is the major constituent in the product. Similar S i NMR analyses of the arylene-bridged polysilsesquioxanes X-2 through X-5 before and after plasma treatment indicated near complete conversion to silica. 2 9

2 9

Nitrogen Sorption Porosimetry. Nitrogen sorption measurements of porous arylene- and alkylene-bridged polysilsesquioxanes and the post-plasma silica gels show coarsening of the existing porosity (Table 2). In general, mean pore diameters increased while surface areas decreased. In effect, the plasma treatments opened up the walls of existing pores to form mesoporous silicas. Interestingly, plasma treatment of non-porous alkylene-bridged polysilsesquioxanes (X-7, X-8, and X-9) generated pores that appear to increase in size with increasing spacer length. However, the


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Mass Flow

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C^Gas Figure 2. Glass Rf oxygen plasma apparatus for removing template groups.

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90

110 130 150

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Figure 3. Decrease in phenyl C-H absorbance (3060 cm ) in infrared spectrum of X - l with time in oxygen plasma.



11. LOY ET AL.


1—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι— -40

-60

-80

-100

-120

-140

2 9

Figure 4. S i MAS NMR spectra of phenylene-bridged polysilsesquioxane xerogel before oxygen plasma treatment, A, and the resulting silica after plasma treatment, B.


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increase is small enough to warrant further characterization of the pore distributions. In addition, there was a significant increase in size of the hysteresis between the adsorption and desorption isotherms with die increase in length of the alkylene bridging groups. The silica gels prepared from the plasma treatment of the propylene-bridged gels exhibited almost no hysteresis. Thus, as the length of the bridging group increased, the size of the hysteresis increased. Hysteresis in a gas sorption isotherm is commonly attributed to bottlenecks in the pore structure. An increase in the hysteresis in these isotherms may be due to separation of a hydrocarbon nano-phase in the bridged polysilsesquioxanes with increasing template length and flexibility. After the plasma treatment, the hydrocarbon templates are replaced by pores whose dimensions mirror those of the original hydrocarbon nano-phase. Table 2. Nitrogen Sorption Surface Areas and Mean Pore Diameters of Bridged Polysilsesquioxanes Before and After Treatment with an Oxygen Plasma *

Sample

Bridging Group

After Plasma Treatment Before Plasma Treatment Surface Mean Pore Surface Mean Pore Area(m /g) Diameter (À) Area(m /g) Diameter (À) 2

X-l X-2 X-3 X-4 X-5 X-7 X-8 X-9 M-2

phenylene biphenylene terphenylene 1, 3, 5phenylene ethylene hexylene decylene tetradecylene decylene

2

440 822 622 756

23 25 19 34

367 525 576 450

28 38 46 53

956 8.7 1.3 1.8 547

197 NA NA NA 34

617 583 582 647 254

214 32 34 38 72

•Surface areas are derived from the BET model (18). Conclusions Arylene and alkylene-bridged polysilsesquioxanes were treated with low temperature oxygen plasmas to remove the hydrocarbon components leaving porous silicas. Plasma oxidation was particularly efficient in removing the organic components from both porous and non-porous, powdered gels. Similarly, macro- and mesoporous monolithic aerogels were converted quantitatively into silica gels, but a microporous phenylene-bridged monolithic xerogel exhibited only surface etching on the order of 1 micron depth. Solid state S i NMR and infrared spectroscopy were used to confirm the loss of the hydrocarbon bridging groups. We were successful in generating new porosity with the plasma treatment of the non-porous alkylene-bridged polysilsesquioxanes. There appears to be an effect on the dimensions of the resulting pores' size and shape from the length of the alkylene-bridging group used as evidenced by the increase in the isotherm hysteresis. 2 9


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133

Acknowledgements We would like to acknowledge Linda McLaughlin for the use of her porosimeter, Gary Zender for scanning electron micrographs, Duane Schneider for NMR spectroscopy, surface area analysis, and chemical synthesis, and Brigitta Baugher for surface area measurements and infrared spectroscopy. Edward Russick performed the supercritical CO2 extractions. This research was supported by the United States Department of Energy under Contract No. DE-AC04-94AL85000.


Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Brinker, C. J.; Scherer, G. W., Sol-Gel Science; Academic Press: London, 1990. Agaskar, P. A. J. Am. Chem. Soc. 1989, 111, 6858. Kaszynski, P.; Michl, J. J.Am.Chem.Soc. 1988, 110, 5225. Day, V. W.; Klemperer, W. G.; Mainz, V. V.; Miller, D. M. J.Am.Chem Soc. 1985, 107, 8262. Dines, M. B.; DiaGiacomo, P. M. Inorg. Chem. 1981, 20, 92. Shea, K. J.; Loy, D. Α.; Webster, O. W. Chem. Mater. 1989, 512. Shea, K. J.; Loy, D. Α.; Webster, O. W. J. Am Chem. Soc. 1992, 114, 6700. Loy, D. Α., Ph.D. Dissertation, University of California at Irvine, 1991. Small, J. H.; Shea, K. J.; Loy, D. A. J. Non-Cryst. Solids 1993, 160, 234. Loy, D. Α.; Russick, E.; Shea, K. J. "Better Ceramics Through Chemistry V," edited by M. J. Hampden-Smith, W. G. Klemperer, and C. J. Brinker (Mater. Res. Soc. Symp. 271 Proc. Pittsburgh, PA 1992), p. 699. Oviatt, H.; Shea, K. J.; Small, J. H. Chem Mater.1993,5,943. Chujo, Y.;Ihara,E.; Kure, S.; Susuki, K.; Saegusa, T. Makromol. Chem., Macromol. Symp., 42/43 (Int. Symp. Ring Opening Cyclopolym., 6th, 1990), 303-12. Roger, C.; Hampden-Smith, M. J.; Brinker, C. J. "Better Ceramics Through Chemistry V," edited by M. J. Hampden-Smith, W. G. Klemperer, and C. J. Brinker (Mater. Res. Soc. Symp. 271 Proc. Pittsburgh, PA 1992), p. 51. Loy, D. Α., Buss, R. J., Assink, R. Α., Shea, K. J., Oviatt, H. Polymer Preprints 1993, 34, 244. Stability of polysiloxane-polyimides to oxygen plasmas was attributed to formation of a silica overlayer by Yilgor, I., Yilgor, E., Spinu, M. Polymer Preprints1987, 23, 84. The absence of porosity in the alkylene-bridged polysilsesquioxanes may be related to nanophase separation and will be discussed in more detail, elsewhere. Τ resonances in the Si NMR spectra arise from silicon species with one substituent attached through a carbon-silicon bond and three substituents attached with silicon-oxygen bonds: RSi(OH)3 = T , [RSiO ] = T . Brunauer, S.; Emmett, P.H.; Teller, E. J. Am Chem Soc. 1938, 60, 309. 29

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Inorganic and Organometallic Polymers II - American Chemical Society

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