dendritic networks containing polyhedral oligomeric silsesquioxane (poss) and carborane clusters

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Dendritic Networks Containing Polyhedral Oligomeric Silsesquioxane(POSS) and Carborane Clusters

MANOJ K. KOLEL-VEETIL, DAWN D. DOMINGUEZ, TEDDY M. KELLER

Advanced Materials Section, Materials Chemistry Branch, Chemistry Division, Naval Research Laboratory,Washington, District of Columbia 20375

Received 22 October 2007; accepted 27 December 2007DOI: 10.1002/pola.22601Published online in Wiley InterScience (www.interscience.wiley.com).

Keywords: carborane; crosslinking; dendrimers; gelation; high temperaturematerials; nanoparticles; networks; POSS

INTRODUCTION

The assemblage of disparate inorganic clusters of excep-tional properties in a networked system raises the ex-pectation that such systems will retain some inherentindividual properties and inherit other properties,which are not expected on the basis of application of therule of mixtures to the component phases.1 The presenceof spatially radiating multiple organic functionalities ininorganic clusters of interest offers access to the con-struction of organic/inorganic hybrid dendrimers,wherein the structural and mechanical properties of theinorganic component can be well-tailored. Polyhedraloligomeric silsesquioxanes (POSS)2 and carboranes,3

two groups of inorganic nanoclusters with exceptionalproperties, have elicited increased interest in recenttimes in materials synthesis and application.

The benefit of POSS clusters, a hybrid betweensilica (SiO2) and silicone (R2SiO), is apparent in theenhancements they produce in use temperature, oxi-dation resistance, surface hardening and mechanicalproperties of polymers upon incorporation.4 POSSclusters, especially the ones that contain metals, havealso been known to cause reductions in flammabilityand heat evolution in a wide range of thermoplastic

and some thermoset polymeric systems.5 In addition,POSS polymers have found applications in photoresistcoatings for electronic and optical devices,6 interlayerdielectrics and protective coating films for semicon-ductor devices,7 liquid crystal display elements,8

magnetic recording media,9 optical fiber coatings,10

selective permeability membranes,11 binders forceramics,12 and as carcinostatic drugs.13

Carboranes possess astonishing chemical inertnessarising from their low nucleophilicity, high hydropho-bicity, and their electron-withdrawing properties,since they possess a highly polarizable r-aromaticcharacter.3 The chemical inertness of the best-knowncloso-carborane clusters, known as o-carborane, m-carborane, and p-carborane, arise from the perfectgeometrical and electronic shell closures of theircarborane skeletons. The versatility of carborane isself-evident from the diversity in carborane-containingmaterials such as conducting polymers,14 cancertreatment agents in medicine,15 precursors for highperformance fibers,16 supported organic catalysts,17

and precursors for supramolecular assembly geared to-ward the production of nanomaterials.18 In particular,polymers that contain linear siloxane and carboranegroups exhibit superior thermal and thermo-oxidativeproperties with potential for applications as high-tem-perature plastic, elastomeric, and ceramic materials,especially in the aerospace and defense industries forapplications in sealing assemblies of landing gears,flight control and fuel systems, and for cable insula-tions.19 The, as yet, unreported assemblage of POSS

Correspondence to: M. K. Kolel-Veetil (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 2581–2587 (2008)VVC 2008 Wiley Periodicals, Inc. §This article is a US Government work and, as

such is in the public domain in the United States of America.

2581

and carborane clusters in a single compound, a chal-lenging synthetic endeavor, makes the effort funda-mentally interesting and desirable.

The nanoscale dimensions of the POSS cluster(cage diameter of about 1.5 nm for Si8O12) with spa-tially radiating functionalities in the octameric cubicsilsesquioxane monomer, octasilane-POSS (1) (Fig.1),20 and of the carborane cluster (cage diameter ofabout 0.88 nm for m-carborane) in the vinyl-termi-nated carboranylenesiloxane, 2 (Fig. 1),21 present theopportunity for the production of dendritic inorganic–organic hybrid materials that contain dispersed POSSand carborane nanoclusters. In such a system, thecooperation of the individual properties of the twocluster species could present hitherto unavailableproperties such as, for example, (i) the neutron-cap-turing ability of a POSS-containing carcinostatictreatment membrane due to the presence of the neu-tron-capturing boron atoms in the carborane clusters,(ii) the gas separation/permeation ability of a carbor-ane-containing network polymer due to the presenceof the POSS clusters, and (iii) the thermo-oxidativestability of a POSS-containing photoresist due to thecooperative enhancement in such a property by thecarborane clusters, and so forth.

The ubiquitous hydrosilation reaction is a facilesynthetic strategy for producing dendritic networkpolymers from reactants containing Si��H bonds andorganic unsaturations. In the literature, there areseveral reports of the utilization of hydrosilation reac-tions involving POSS molecules for the creation ofnetwork polymers.20,22 In this communication, wedescribe the production of hydrosilated dendriticnetworked polymers containing both POSS and car-borane clusters by the hydrosilation reaction of vinyl-terminated carborane ligands and Si��H terminalgroups-containing POSS molecule catalyzed by theKarstedt catalyst. A schematic representation of theproduct is shown in Figure 2. A variant of the hydro-silated POSS/carborane-containing dendritic network,

the reinforced POSS-carborane network, was alsodeveloped wherein additional ��Si��O��Si�� cross-links were concomitantly available for reinforcementfrom a stoichiometric excess of the reacting POSS rea-gent, by the oxidation/hydrolysis/condensation of itsSi��H bonds mediated by the Karstedt catalyst in thepresence of Et3N/H2O cocatalysts.

EXPERIMENTAL

Materials and Instrumentation

The octasilane-POSS monomer and the Karstedt cata-lyst were procured from Hybrid Plastics and Gelest,respectively, and were used as received. Et3N (99.5%),CH2Cl2 (anhydrous, �99.8%) and toluene (anhydrous,99.8%) (all from Aldrich) were also used as received.Compound 2 was synthesized following a publishedprocedure.21 The reagents vinylmagnesium bromide(1.0 M in THF) (Aldrich) and 1,7-bis(chlorotetramethyl-disiloxyl)-m-carborane (Dexsil Corporation) used in thesynthesis of 2 were used without further purification.

Thermogravimetric (TGA) analyses were performedusing a SDT 2960 DTA-TGA analyzer under a nitro-gen flow rate of 100 cc/min. FTIR spectra of materialswere obtained from films on NaCl or as constituentsof KBr pellets with a Nicolet Magna 750 FTIR spec-trometer. Solution-state 1H NMR spectra wereacquired on a Bruker AC-300 spectrometer in C6D6

containing 1 wt % TMS. Dynamic mechanical analysis(DMA) was performed in a nitrogen atmosphere on aTA Instruments AR-2000 rheometer in conjunctionwith an environmental testing chamber for tempera-ture control. Measurements on rectangular solid sam-ples were carried out in the torsion mode at a strainof 2.4 3 10�4 and a frequency of 1 Hz. Samples wereprepared in aluminum molds with cavity dimensions65 mm 3 13 mm 3 4 mm by transferring flowable reac-tionmixtures into themolds to allow further gelation and

Figure 1. The polyhedral oligomeric silsesquioxane (POSS) monomer, octasilane-POSS (1), and the vinyl-terminated carboranylenesiloxane monomer (2).

2582 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 00 (2008)

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

concurrent expulsion of solvent, and subsequent heattreatment of the samples to 120 8C for 2 h. The storagemodulus (G0), loss modulus (G"), and damping factor(tan d) were determined as a function of temperature in25–450 8C range at a heating rate of 3 8C/min.

Synthesis of Hydrosilated POSS-CarboraneDendritic Network

In a typical reaction, 0.250 g of 2 (0.540 mmol; MW ¼460.36 g/mol) was taken in a reaction vial with 0.138 gof 1 (0.135 mmol; MW ¼ 1018 g/mol) and was mixedthoroughly using a mechanical stirrer. This yielded aSi��H:Si-vinyl ratio of 1:1 in the reaction mixture. Tothe mixture, 5 mL of either CH2Cl2 or toluene wasadded and the solution was thoroughly mixed to yielda clear solution. At this point, 20 lL (2.2 lmol of Pt)of a 2.0–2.4 wt % Pt Karstedt catalyst solution inxylenes was added dropwise with concomitant stir-ring. The solution became warmer and appeared yel-low in color. Subsequently, the solution in the vialwas heated for 30 min (the CH2Cl2 solution, at 40 8C,and the toluene solution, at 70 8C). The reaction mix-tures, which were still flowable at this point, weretransferred to Teflon molds and left at ambient con-ditions for the evaporation of the solvent and their

subsequent gelation into films. The films were heatedfurther at 120 8C for 2 h to facilitate the completeexpulsion of any residual reaction solvent.

Synthesis of Hydrosilated POSS-CarboraneDendritic Network with Additional StructurallyReinforcing ��Si��O��Si�� Bonds (ReinforcedPOSS-Carborane Network)

In a typical reaction, 0.250 g of 2 (0.540 mmol; MW ¼460.36 g/mol) was taken in a reaction vial with 0.276g of 1 (0.270 mmol; MW ¼ 1018 g/mol) and was thor-oughly mixed. This yielded a Si��H:Si-vinyl ratio of2:1 in the reaction mixture. To the mixture, 5 mL oftoluene was added and the solution was thoroughlymixed to yield a clear solution. At this point, 20 lL(2.2 lmol of Pt) of a 2.0–2.4 wt % Pt Karstedt catalystsolution in xylenes was added dropwise with concomi-tant stirring. The solution became warmer andappeared yellow in color. The reaction mixture wasinitially heated at 70 8C for 30 min to ensure the com-pletion of the hydrosilation reaction. The mixtureremained as a liquid at this point. After cooling toroom temperature, 24 lL (0.14 mmol) of Et3N wasadded to the reaction mixture. Subsequently, 24 lL ofdistilled water (1.1 mmol) was added to the mixture

Figure 2. Schematic representations of (a) the dendritic inorganic/organic hydrosi-lated networked system formed from octasilane-POSS (1) and the vinyl-terminatedcarboranylenesiloxane (2) and (b) of the additional ��Si��O��Si�� bonds-containingportion formed in the ‘‘reinforced POSS-carborane’’ network (some connecting linkershave been omitted or shortened for clarity).

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and after thorough mixing, the resulting reactionmixture was heated at 100–125 8C in a Teflon molduntil gelation occurred to form a tough film (�30 min).The films were heated further at 120 8C for 2 h to facili-tate the complete expulsion of any residual reaction sol-vent and/or cocatalysts.

RESULTS AND DISCUSSION

The synthetic strategy for the production of thehydrosilated dendritic system is similar to the oneused in the previously reported elastomeric networkformation from 2 and branched siloxane cross-linkers.21 The choice of the reaction solvent was dic-tated by the solubility of 1. Compound 1 is readilysoluble in toluene and CH2Cl2. The progress of thereaction can be monitored by FTIR and solution 1HFT-NMR spectroscopy. The FTIR spectra on NaCldisks of the reaction stock at various times during thereaction showed the gradual disappearance of boththe Si��H absorptions of 1 (2140 cm�1 (mSi��H)) (Fig. 3)and the vinyl absorptions of 2 (1596 cm�1 (mCH¼¼CH2

))(Fig. 3) indicating the completion of the reaction. Sim-ilar disappearance of the resonance peaks for thevinyl protons of 2 at 5.68–6.16 ppm in the 1H FT-NMR spectra suggested the completion of the reac-tion. The reaction was observed to be finished within30 min. The film obtained from toluene was found tobe visually less porous or rough than that obtainedfrom CH2Cl2. It was found to be somewhat flexible atvery low strains and of reasonable optical clarity,implying the absence of any extensive phase separa-tion of the components. The porous nature of the filmformed from CH2Cl2, a result of the competing dy-namics of solvent evaporation and gelation, made itless transparent than the film from toluene. However,the spatial disposition of the POSS and carboranecomponents from the hydrosilation sites in this film isexpected to be similar as in the film from toluene.

The films were characterized by thermogravimetricanalysis in air and in N2 (Fig. 4). Char yields of theproduct, formed from the toluene reaction, at 1000 8Care 77% in N2 and 91% in air. Similar char yieldswere obtained for the product formed from CH2Cl2. Aperusal of the literature revealed that these charyield values were greater by at least 35% than anyreported char yields of inorganic–organic hybrid ma-terial derived from a POSS system.23,24 In addition,the highest reported decomposition temperature(defined as the temperature at 5% weight loss) forPOSS-derived inorganic–organic hybrids falls in the320–340 8C range, while the corresponding range forthe POSS-carborane hybrid of this study is 525–575 8C.

Figure 3. FTIR spectra of octasilane-POSS (1) (left) and vinyl-terminated carbora-nylenesiloxane (2) (right). The Si��H (mSi��H) absorption at 2140 cm�1 of 1 and thevinyl (mCH2¼¼CH) absorption at 1596 cm�1 of 2 used to monitor the progression of thereaction are indicated.

Figure 4. The TGA thermograms of the reinforceddendritic network containing both hydrosilated andhydrolysis/condensation bonds (solid line in air anddouble dash-single long dash line in N2) and of thedendritic network containing only the hydrosilatedbonds (dash-dot line in air and dash line in N2) formedfrom octasilane-POSS (1) and the vinyl-terminatedcarboranylenesiloxane (2).



The construction of the dendritic network isbelieved to have resulted from a b-addition of theSi��H groups of 1 to the vinyl groups of 2, as in thehydrosilation reaction between 1 and octavinyldime-thylsiloxy-functionalized POSS reported by Laineet al.25 In addition, Pt-mediated hydrosilations ofalkenes and alkynes are well-known to proceed with theinitial formation of b-trans products.26 Further, thesterical encumbrance of the siloxyl groups aroundthe POSS cluster should limit the attack of the Si��Hgroups to the b-carbons of the vinyl groups. The degreeof crosslinking in both the products appears to be highdue to the small amount of sol fraction (less than 5%),which was obtained on Soxhlet extraction of the respec-tive products in either CH2Cl2 or toluene. This is notsurprising when considering the �82% reactive groupinvolvement in the crosslinking reaction leading to thehydrosilated product from 1 and octavinyldimethylsi-loxy-functionalized POSS.25

To improve the structural rigidity of the POSS-car-borane dendritic network polymer a variant of thenetwork polymer containing additional ��Si��O��Si��linkages for reinforcement was developed using a sim-ilar reaction strategy. A stoichiometric excess of 1 wasused in the reaction mixture to make availableunreacted Si��H bonds of 1 for conversion to��Si��O��Si�� linkages. Siloxane chemistry is repletewith mechanistic studies of the base-catalyzed hydro-lysis of Si��H to Si��OH and the ensuing condensa-tion of Si��OH and Si��H groups to ��Si��O��Si��bridges.27 Metallic catalysts such as the Pt-based Kar-stedt catalyst have also been known to promote suchhydrolysis and condensation of Si��OH and Si��Hgroups at moderate reaction conditions in the pres-ence of a base such as triethylamine (Et3N) andwater.28 The Karstedt catalyst was proposed to oxi-dize Si��H to Si��OH, whereas the Et3N cocatalystwas proposed to mediate the condensation of Si��OHto Si��O��Si. Such reactions have been recently uti-lized in the formation of amphiphilic membranes thatare crosslinked and reinforced by POSS.23

The reaction was performed in toluene because ofthe necessity to heat the reaction stock to a tempera-ture around 100–125 8C for causing the oxidation ofSi��H to Si��OH bonds and the further hydrolysis/condensation of the Si��H and Si��OH bonds of 1.The completion of the initial hydrosilation reactionand the subsequent oxidation/hydrolysis/condensationof the Si��H bonds of 1 were verified by the disap-pearance of the Si��H stretching vibration in thesolid-state FTIR spectrum of the product obtained ina KBr disk. The film was observed to be stiff andslightly opaque, suggesting the structural reinforce-ment of the product and the possibility of some mac-roscopic phase separation of the components in com-parison to the films containing only the hydrosilatedbonds. Char yields at 1000 8C of the gelated productwas determined by thermogravimetric analyses(Fig. 4). The values are 81% in N2 and 99% in air.

The enhanced char yield values presumably arosefrom the structural reinforcement of the films by theadditional ��Si��O��Si�� linkages.

Dynamic mechanical analysis was attempted from25 to 450 8C on rectangular solid samples formedfrom both networks. After a few oscillations at lowtemperatures, the elastomeric POSS-carborane den-dritic network samples with only the hydrosilatedbonds failed catastrophically. Data acquired on sev-eral POSS-carborane network samples revealed thatthe room temperature storage modulus (G0) of the ma-terial was 5–10 MPa. In contrast, the dynamic me-chanical properties of the reinforced POSS-carboranedendritic network were evaluated over the entire tem-perature range and the results are presented in Fig-ure 5. The reinforced network exhibited a G0 of120 MPa at room temperature. The higher G0 value ofthe reinforced hybrid is evidence of the increase innetwork rigidity resulting from the reinforcing��Si��O��Si�� linkages. From room temperature to�300 8C, the G0 value of the reinforced network grad-ually increased as the sample was heated. Theobserved increase in G0 is attributed to the occurrenceof some additional crosslinking on increase of thetemperature, which further strengthens the network.Above 300 8C, the reinforced network exhibited stor-age and loss (G@) moduli decreases and a tan dincrease typical of a material undergoing a glass torubber transition. Above the glass transition tempera-ture (342 8C), the storage modulus of the reinforcedhybrid is �30 MPa reflecting the decrease in materialrigidity. The reinforced network underwent a changefrom a pale yellow to an off-white plaster-like appear-ance during the DMA analysis. The color changeprobably reflects the change in the viscoelastic prop-erties of the material. Two tan d maxima are observedin the G" and tan d plots in the high temperatureregion (300–450 8C). The appearance of two tan d

Figure 5. The storage (G0) l and loss (G@) * moduliand damping factor (tan d) n of the POSS-carboranereinforced network as a function of temperature.



peaks implies that the reinforced network has aheterogeneous morphology.

The mechanical properties of both sets of networkscan be possibly improved by increasing their cross-linking density by forming them under pressure usingtechniques, such as warm-pressing by sandwichingthem between aluminum foils at temperatures suchas 200–250 8C under a pressure of around 1 MPa.Improvements in the oxygen-barrier properties ofPOSS-derived films have been reported when formedusing such a curing technique.29 The improvementshave been attributed to the increase in crosslinkingdensity in the films when compared with similarsolvent-cast films.

CONCLUSIONS

The first example of an inorganic/organic hybrid den-dritic network system containing both POSS and car-borane clusters has been realized by the hydrosilationstrategy and are herein reported. It is anticipated thatthis will provide an avenue for harnessing the excep-tional individual properties of POSS and carboraneclusters for material applications from a single system.As corroborative evidence, the thermal and thermo-oxidative properties of these products are observed tobe far superior to that of any existing POSS-derivedinorganic–organic hybrid polymers, because of the co-operative enhancement in such properties by the car-borane clusters. This bodes well for their applicationas a new generation of advanced high temperaturematerials including as photoresistive coatings for elec-tronic and optical devices and as membranes for gas-separation and neutron-absorption.

The authors thank the Office of Naval Research forits financial support of this work.

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dendritic networks containing polyhedral oligomeric silsesquioxane (poss) and carborane clusters

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