Second-Generation Polymeric Precursors for BN and - Deep Blue

REVIEW

Second-Generation Polymeric Precursors forBN and SiNCB Ceramic MaterialsThomas Wideman,1 Paul J. Fazen,1 Kai Su,1,2 Edward E. Remsen,3

Gregg A. Zank2 and Larry G. Sneddon1*1Department of Chemistry and Laboratory for the Research on the Structure of Matter, University ofPennsylvania, Philadelphia, PA 19104-6323, USA2The Advanced Ceramics Program, Dow Corning Corporation, Midland, MI 48686-0995, USA3Analytical Sciences Center, Monsanto Corporate Research, Monsanto Company, 800 North LindberghBlvd, St Louis, MO 63167, USA

Our recent work directed at the design, synth-esis, characterization and applications of newtypes of polyborazylene and polyborosilazanepolymers is reviewed with a focus on the use ofthese polymers as processable precursors to BNand SiNCB composites. A design strategy basedon the controlled functionalization of preformedpolymers with pendant groups of suitablecompositions and crosslinking properties hasbeen employed to yield second-generation di-pentylamine–polyborazylene (DPA) and pina-colborane–hydridopolysilazane (PIN–HPZ)polymers, which, unlike the parent polyborazy-lene (PB) and the borazine–hydridopolysilazane(B–HPZ) polymers, are stable as melts and canbe easily melt-spun into polymer fibers. Subse-quent pyrolyses of these polymer fibers thenprovide excellent routes to BN and SiNCBceramic fibers.# 1998 John Wiley & Sons, Ltd.

Keywords: ceramics; preceramic polymers;SiNCB composites; boron nitride; boron; silicon

Received 30 May 1997; accepted 7 July 1997

The development of efficient methods for theproduction of complex structural and electronicmaterials in usable forms is one of the most

important problems of modern solid-state chemis-try and materials science. The polymeric precursorapproach, in which a polymer is first formed intothe desired shape and then decomposed to the finalceramic material with retention of this shape, hasbeen shown to be an important new route forproducing many ceramics in processed forms.1

However, the design of new inorganic precursorpolymers with the required composition andappropriate chemical and physical properties re-mains a formidable scientific challenge. In thispaper, we review our recent work directed at thedesign, synthesis, characterization and applicationsof new types of polyborazylene and polyborosila-zane polymers, with a focus on the use of thesepolymers as processable precursors to BN andSiNCB composites.

HYBRID POLYBORAZYLENEPRECURSORS TO BORON NITRIDE

Boron nitride has a wide range of attractiveproperties, including high-temperature stabilityand strength, a low dielectric constant, largethermal conductivity, hardness and corrosion andoxidation resistance, leading to a number ofpotential applications as a structural or electronicmaterial.2–6 Boron nitride powders may be easilyobtained,7 but it has proven more difficult toprepare BN in more complex forms, especiallyfibers and coatings. Thus, there has been intenseinterest by many research groups on the develop-ment of polymeric precursors to BN.2

We have previously shown that borazine can bereadily dehydropolymerized to give a soluble

APPLIED ORGANOMETALLIC CHEMISTRYAppl. Organometal. Chem.12, 681–693 (1998)

# 1998 John Wiley & Sons, Ltd. CCC 0268–2605/98/100681–13 $17.50

* Correspondence to: Larry G. Sneddon, Department of Chemistryand Laboratory for the Research on the Structure of Matter,University of Pennsylvania, Philadelphia, PA 19104-6323, USA.Contract/grant sponsor: US Department of Energy, Division ofChemical Sciences, US Office of Basic Energy Sciences, US Officeof Energy Research.Contract/grant sponsor: US National Science Foundation.

polymer,polyborazylene(PB), in excellentyields(Eqn [1]).8,9

The combined analytical, spectroscopicandmolecular-weightdata indicate that the borazinering structure is retained in the polymer. Thepolymer hasalso beenshownto havea complexstructure,relatedto thoseof the organicpolyphe-nylenes,having linear, branched-chainand fused-cyclic segments with typical Mn = 500–900gmolÿ1 andMw = 3000–8000g mol.ÿ1 Furthermore,accordingto its powderX-ray diffraction spectra,the polymerappearsto havea layeredstructureinthesolid state.

Pyrolysis studiesshow that polyborazylene isconvertedto boron nitride in excellent chemical(89–99%)andceramic(84–93%)yields.Studiesofthe polymer-to-ceramicconversionsuggesta pro-cess involving a two-dimensional crosslinkingreactionsimilar to that shownin Scheme1.

While polyborazylene has proven to be anexcellent precursorfor the production of boronnitride coatings,films and shapedmaterials,thecrosslinkingreactiondepictedin Scheme1 occursat low temperatureandhaspreventedapplicationsof thepolymerrequiringmelt-processing.Thus,thekey to other more demandingapplicationsof thepolyborazylenepolymer, such as in the melt-spinningof fibers,is to controlthedehydrocouplingreaction that leads to the formation of boron–

nitrogen crosslinksbetweenthe polymer chains.Onestrategyby which this could be accomplishedis to reducethe numberof reactiveB–H or N–Hsitesby functionalizingthe polymer with suitablesubstituents.In this way it should be possibletoimprovethe processabilityof polyborazylenebothby increasingthe thermalstability of the polymerand by lowering its glass transition temperature(Tg). With theseaimsin mind,wehaveinvestigatedtwo new typesof ‘second-generation’polyborazy-lenes:(1) polyorganoborazylenes10 and(2) amine-functionalizedpolyborazylenes.11,12

Polyorganoborazylenes10

B-Alkylpolyborazyleneshave now been synthe-sizedby two different methods.First, B-alkylbor-azines, which can be readily obtained by thetransition-metal-catalyzed hydroborationof bora-zine with olefins(Eqn [2]),10 havebeenthermallypolymerized,asshownin Eqn [3].

B-Alkylborazines require longer thermal poly-merization reaction times than the parent com-

Scheme1 Proposedtwo-dimensionalcrosslinkingprocessfor theceramicconversionreactionof polyborazylene.

# 1998JohnWiley & Sons,Ltd. Appl. Organometal.Chem.12, 681–693(1998)

682 T. WIDEMAN ET AL.

pound.For example,samplesof B-ethyl- and B-propyl-borazinewere heatedat 70°C for weekswithout showing the change in viscosity orhydrogenevolutionwhich occurswithin 48h withborazine.After heatingfor onemonth,molecular-weightstudiesof thepolymerizedB-alkylborazinesindicated that these polymers have Mn valuessimilar to the parent polymer (Mn = 540 forpolyethylborazyleneandMn = 468 for polypropyl-borazylene).However,Mw is considerablylessthanthe parentpolymer (Mw = 926 for polyethylbora-zylene and Mw = 824 for polypropylborazylene).This is consistentwith a decreasein crosslinkingand chain branching,as would be expectedgiventhat one of the borazinyl B–H groups has beenremoved.

The secondmethodby which B-alkylpolybora-zylene polymers have been producedis by themodification of the preformed polyborazylenebackbone.Thus, the RhH(CO)(PPh3)3-catalyzedhydroborationof olefins by polyborazylene(Eqn[4]), producespoly-B-alkylborazinesin excellentyields.

Thedegreeof alkylationcanbecontrolled,froman averageof lessthanonealkyl groupper everytwo rings for the propyl-substituted polymer,B3N3H3(C3H7)0.4, to almost one alkyl group forevery borazinering in the ethyl-substitutedpoly-mer, B3N3.2H2.4(C2H5)0.9. Molecular-weight stu-dies showed increasedmolecular weights Mn =1349) comparedwith the crude starting polymer(Mn = 506),with incorporationof ethyl groupsintothe polymer.Densitymeasurementsindicatedthatthesepolymersarelessdense(1.42g mlÿ1) thantheparent polymer (1.56g mlÿ1, suggestingthat thealkyl substituentsincreasedinterchaindistances.

Studiesof the ceramicconversionreactionsofthesenew B-alkylated polyborazyleneshave de-monstratedthat the thermally-inducedcrosslinkingreactionsare reducedcomparedto those of thepolyborazylenepolymer. TGA studiesof the twopolymersunderargonshowtheonsetof their initialweight lossesare at higher temperaturesthan theparentpolymer,suggestingthe alkyl groupsdo, infact, inhibit interchain dehydrocoupling. Bothpolymers, however, show larger weight lossesduring pyrolysis than does polyborazylene.Pre-

dictedceramicyieldsbasedonlossof only thealkylgroupsand hydrogenwould be 72% for the ethylpolymerand63%for thepropyl polymer,but bothpolymers showedceramic yields near 50%. Thelower yieldsareprobablyattributableto volatiliza-tion of lower-molecular-weight species.Indeed,TGA–MSstudiesof thesepolymershaveshown,inadditionto the lossof hydrogenandhydrocarbons,the lossof somealkylborazinespecies.In thefinalstagesof the ceramic conversionboth polymersappearto loseonly hydrogen.

Amine Modi®edPolyborazylenes11,12

While the polyorganoborazylenesdescribedaboveshowedadecreasein thedegreeof low-temperaturecrosslinkingreactions,theTg of thepolymerscouldnot be reducedbelow the onset of crosslinking.Thus, thesepolymers are not meltable. Further-more,becauseof inefficienteliminationof thealkylgroupsduringpyrolysis,highcarboncontents(2.0–6.5%) were found in the ceramicsderived frombothsyntheticroutes.Therefore,asecondapproachfor the formation of hybrid polyborazyleneswasinitiated.

Previouswork by Kimura and coworkers13–15

hasshownthatprocessableBN polymerprecursorscanbeobtainedfrom the reactionof B-trimethyla-minoborazine with laurylamine [Eqn [5]). Thelong-chainalkylaminegroupsimprovedthe plasti-city of the polymers,allowing them to be melt-spun.Furthermore,theaminescouldbeeliminatedeasilyduringpyrolysis,by transaminationreactionsto producelow carboncontents.Pyrolysis of thepolymer fibers under ammoniafurther servedtoremovecarbonfrom the materials,yielding 10mmBN ceramicfibersof excellentquality,with tensilestrengths of ca 1 GPa and elastic moduli of78GPa.13–15

Using a similar approach,we have now foundthatnewamine-modifiedpolyborazylenes,contain-ing aminogroupspendantto thepolymerbackbone,can be obtained readily by the reaction ofpolyborazylenewith secondaryamines,including


POLYBORAZYLENE AND POLYBOROSILAZANEPOLYMER PRECURSORS 683

diethylamine (DEA), dipentylamine (DPA) andhexamethyldisilazane (HMD).11,12For example,asshown in Eqn [6], the DPA-4 polymer wassynthesized by reacting polyborazylene withDPA in glyme solution at 75°C to give a clearpolymeric material in 91% yield with a(B3.0N3.12H3.9)(NPn2)0.47 composition.Otherreac-tion conditions and compositionsare shown inTable1.

In the case of the DEA- and DPA-modifiedpolymers, spectroscopicstudies,model reactionsand analysisof volatile by-productsindicate thatmodification of the polymer proceedsprimarilythroughdehydrocoupling.In contrast,modificationof the polymer with HMD occurred primarilythroughN–Si cleavagereactions,asshownin Eqn[7].

Molecular-weight studies show Mn = 450–800for the DEA- andDPA-modifiedpolyborazylenes,comparedwith Mn = 750 for the parentpolybora-zylene.TGA studiesof thesepolymersshowedareduction in low-temperaturecrosslinking reac-tions, with the onsetof decompositionandweightlossincreasingto ca100°C (Fig.1).Becauseof theintroduction of new crosslinkingreactionsinvol-ving the pendantHN-SiMe3 groups, the HMD-modifiedpolymersshowedanincreasein molecularweight (Mn = 1500). Likewise, TGA studiesshowed HMD-modified polymers to undergofurthercrosslinkingreactionsat low temperatures.

DSC studiesshowedthat the long-chainalkyl

groupsof thependantDPA groupsserveasefficientplasticizers,reducingtheTg of thepolymerto 90°Candmakingthe DPA-modifiedpolymersexcellentcandidatesfor melt-spinning. As shown in thescanning electron micrograph in Fig. 2, high-quality DPA-4 polymer fibers were achievedbythe continuousextrusionof polymermelts to givepolymerfibersthatareflexible,uniformandfreeofvoids.Pyrolysisof the curedpolymerfibersunderammoniato 1000°C producedclear,flexible boronnitride fibers of ca 30mm diameter.SEM micro-graphsFig.3 alsoshowthattheseceramicfibersaresmooth, uniform and dense. X-ray diffractionstudies,on both singleceramicfibers(Fig. 4) andbulk samples,and powder DRIFT spectra areconsistentwith the formation of turbostraticBN.A typical elementalanalysisof a bulk sampleisB1.00N1.03C0.00H0.20, whereasRutherfordbackscat-tering spectrographsof single fibers showed asurfacecompositionof B1.5� 0.3N1.0� 0.2O0.2� 0.1consistentwith the formation of a B2O3 surfacespeciesduring air-cure of the fibers. OxidationstudiesFig. 5 of crushedDPA-4 derivedBN fibersshowanonsetof oxidationtypical of boronnitride.

Preliminary measurementsof the mechanicalpropertiesof crude30mm BN fibersderivedfromDPA-4 gave typical tensilestrengthsof 0.18GPaandelasticmoduli of ca 14GPa.Thesevaluesarelower than the poly(aminoborazine)-derived BNfibers reportedby Kimura;13,14 however,becausestrengthsdependstronglyon thefiberdiameterandprocessingparameters,significantincreasesin thestrengths of the DPA-derived BN fibers areexpectedwith the use of a more sophisticatedspinningapparatusand higher-temperaturesinter-ing.

The DPA-modifiedpolyborazylenesareparticu-larly attractiveprecursorsto BN fibers since theconvenient,high-yieldsynthesesof the[B3N3H�4]xpolymer8,9 and its precursor,borazine,16 provideefficient and economicalroutes to the polymer.

Table 1 Synthesisandcompositionof amine-modifiedpolyborazylenes

PolymerPB/Amine/Glyme

(g:ml:ml)Time

at 75°C (h) Composition

PB — — (B3.0N3.2H3.6)DPA-1 5.02:20:20 1 (B3.0N3.6H3.7)(NPn2)0.16DPA-2 4.97:20:20 15 (B3.0N3.0H3.5)(NPn2)0.23DPA-3 5.03:20:20 120 (B3.0N3.0H3.3)(NPn2)0.33DPA-4 2.55:25:25 192 (B3.0N3.12H3.9)(NPn2)0.47DEA 4.98:40:0 60 (B3.0N3.2H3.8)(NEt2)0.51HMD 4.98:10:30 120 (B3.0N3.0H3.6)[N(SiMe3)]0.29



Furthermore,it is expectedthatthepropertiesof thepolyborazylenepolymercannowbesystematicallytailored for usein different technologicalapplica-tions by modification with the wide variety ofdialkylaminesavailable. For example,the HMDpolymers({(R3Si)xB3N3H4ÿx)} y) are being inves-tigatedaspotentialprecursorsto oxidation-resistantBNSi hybrid ceramics.

NEW POLYBOROSILAZANES:PRECURSORS TO SiNCBCOMPOSITES

Comparedwith single-componentceramics,cera-mic compositesoften offer many advantagesforhigh-performanceapplications.For example,com-positeSiNCBceramicshaveanumberof enhancedpropertiescomparedwith traditional SiNC cera-mics, including higher thermal and oxidativestabilities, controllable thermal expansions,de-creased crystallinities and higher ceramic andchemicalyields. For somerecentreviews on thedevelopmentof polymeric precursorsto SiNCBmaterials,seeRefs 17 and 18. Likewise, SiNCBceramicshave been shown to function as self-

sealing barriers for the oxidation of carbon incarbon–carboncomposites(see,for example,Ref.19).

It has been our goal to develop new ‘single-source’ polyborosilazaneprecursorsthat wouldallow the formation of compositionallyhomogen-eous SiNCB composites in processed forms.Our initial work on first-generationSiNCB pre-cursors focused on borazine-basedpolyborosila-zanes, including both borazine-co-silazanebackbonepolymers and boron-modifiedhydrido-polysilazanes.

First generation SiNCB precursors:borazine-based polyborosilazanes

As shownin Scheme2, borazine-co-silazaneback-bonecopolymershavebeenobtainedby thethermalcondensationof borazine with two silazanes,1,1,3,3,5,5-hexamethylcyclotrisilazane(HCT) andtris(trimethylsilylamino)silane (TTS).20,21 Thefreshlypreparedpolymersaremeltableandreadilysoluble in ethers. Spectroscopicanalyses andmolecular-weight-distribution studies suggestboth series of copolymers have highly com-plex, branchedstructures,with multiple borazine-boron to silazane-nitrogenlinkages. As seen in

Figure 1 TGA studiesof amine-modifiedpolyborazylenes.



Table 2, elemental analysesof the B–TTS co-polymers are consistent with an approximate[[(-B3N3H4)NH]3SiH]x structural unit, while theB–HCT copolymers have compositionsrangingfrom (B3N3H4)1.00 (N)1.81 (SiMe2)1.57 (H)1.6 to(B3N3H4)1.00 (N)1.07 (SiMe2)1.28 (H)0.9. Molecular-weight studiesindicatehighly branchedstructuresand relatively low molecular weights withMn = 1000–2000for B–TTS and Mn = 3000–4000for B–HCT. The polymersalso have large poly-dispersities.

Using an alternative approach involving themodification of a performedpolysilazanewith aboron-containingpendant,borazine-substitutedhy-dridopolysilazaneswereachievedby thereactionofhydridopolysilazane(HPZ)22 with liquid borazineat moderatetemperatures(Eqn [8]).23,24As shown

in Table2, the borazine-modified hydridopolysila-zanescan be preparedwith relatively high mole-cular weights(Mn = 5000–24000),and a rangeofboron contents(2–17%).The amountof borazineincorporatedinto the hydridopolysilazanecan bereadily controlled. For example, polymers ofcompositions (B3N3H5)0.02 (HSi)0.30 (Me3Si)0.19(NH)0.26 N0.23 and (B3N3H5)0.07 (HSi)0.34(Me3Si)0.18(NH)0.13N0.28werepreparedby heatingHPZ in excessborazineat 73°C for 2.2 and7.0h,respectively.The spectroscopicdata for the newpolymers indicate that they have retained theirhydridopolysilazanebackbonesandaresubstitutedwith pendantborazinesby meansof a borazine-boron to polymer-nitrogenlinkage. Suchlinkagesmay haveformedby eitherhydrogenor trimethyl-silaneeliminationreactions.

Figure 2 SEM micrographs of DPA-4-derived polymerfibers.

Figure 3 SEM micrographsof DPA-4-derivedBN ceramicfibers.



Studiesof the ceramicconversionsreactionsofthe threeborazine-modifiedpolymersB–HCT, B–

TTS20,21 and B–HPZ23,24) demonstratedthat theyare eachconvertedto compositeSiNCB ceramicmaterialsin high ceramicandchemicalyields,butthat each polymer system has its own uniqueproperties.As shown in Tables3 and 4, the twocopolymer systemsyield ceramicshaving muchhigher boron contentsthan the higher-molecularweight B–HPZ polymers.Furthermore,evenwiththe similarities in their polymerelementalcompo-sitions,structuraldifferencesin thetwo copolymersystemsresult in different ceramic compositionsupon pyrolysis. Whereasthe B–HCT copolymersare convertedto SiNCB ceramic materials withretentionof Si andC to 1800°C, the facile lossof-SiMe3 groups from the B–TTS-basedceramicsyields low-carbon SiNB ceramics at 1400°C.Furtherlossof Si from the B–TTS-basedceramicsproducesdenseBN-rich materialsat 1800°C. Thehigher-molecular-weightB–HPZ-2polymers,how-ever, yield lower-boron-contentSiNCB ceramicswith typical compositions at 1400°C ofSi1.00B0.09N1.08C0.44. At 1800°C, the B–HPZ-2-derived ceramicsstill retain nitrogen efficiently,with typical compositionsof Si1.00B0.10N0.47C0.58.In contrast,the ceramicsderivedfrom unmodifiedHPZ are unstableto nitrogenloss,and are nearlypureSiC at 1800°C, with typical compositionsofSi1.00 C0.92 N< 0.03 (Tables3 and4).22,23,24

X-ray diffraction (XRD) studiesof the ceramicsobtained from all three types of borazine-basedpolymers showedthey are largely amorphousto1400°C. At 1800°C, crystallizationof b-Si3N4, b-SiC and elementalSi begins (Fig. 6). Although

Figure 4 X-ray diffraction spectrumof a single DPA-4-derivedBN ceramicfiber.

Figure 5 TGA analysisof theoxidationof crushedDPA-4-derivedBN fibersunderair at 5 °C minÿ1.



DRIFT spectrashowstrongabsorptionscharacter-istic of boron nitride, no crystalline boron-con-taining phases are observed to 1800°C. Incontrast, initial crystallization of a-Si3N4 inpolysilazane-derived ceramics, such as HPZceramics,is usuallyobservednear1400°C. Above1600°C, the ceramicsbegin to decomposewithloss of nitrogen, and both a-Si3N4 and b-SiCcrystallize.By 1800°C, nitrogenloss is completeand only crystallineb-SiC is usually observedbyXRD.22–24

The addition of boron to the silazanesystemsservesbothto inhibit crystallizationandto improve

the thermalstability of the silicon-basedceramics.Thedifferencesin thecompositionof theceramicsderivedfrom thesesystemsallow for significantlydifferent potential applications.Whereasthe B–HCT based ceramics could serve as effectiveprecursors to high-BN SiNCB ceramic matrixmaterials,the efficient elimination of both Si andC from theB–TTS-basedcopolymerssuggeststhatthey may be better suited as precursors foramorphousBN coatingson carbonand/orceramicfibers. The B–HPZ polymers serve as excellenthigher-molecular-weight precursors to lower-boron-contentSiNCB ceramics.

Scheme2 Synthesisandpossiblestructuresof borazine–silazanecopolymers.

Table 2 Polyborosilazanesynthesisandcomposition

PolymerSilazane/Borane

(g:g)Time (h)/Temp

(°C) Tg (°C) Composition Mn (g molÿ1)

B–TTS 3.30:5.90 210/90 — (B3N3H4)1.00(N)1.17(SiMe3)1.16(SiH)0.34 1356B–HCT 3.43:4.61 120 — (B3N3H4)1.00(N)1.67(SiMe2)1.49(H)1.5 3201HPZ — — 87.0 Si1.00N1.04C1.16H4.69 25800B–HPZ-1 5.16:8.00 2.2/65 — Si1.00N0.77C1.09H4.44(B3N3H5)0.025 24000B–HPZ-2 26.00:21.68 2.2/73 — Si1.00N0.99C1.17H4.61(B3N3H5)0.045 23300B–HPZ-3 1.52:3.24 7.0/73 — Si1.00N0.79C1.02H3.93(B3N3H5)0.14 13300B–HPZ-4 1.51:3.15 17.5/73 — Si1.00N0.94C1.04H3.41(B3N3H5)0.56 5492PIN–HPZ-1 7.99:7.98 2/60 120.9 Si1.00N0.96C1.00H4.25(C6H12BO2)0.06 6110PIN–HPZ-2 7.98:8.00 8/60 110.9 Si1.00N0.87C0.83H4.01(C6H12BO2)0.13 2655PIN–HPZ-3 8.00:7.99 24/60 102.3 Si1.00N0.90C0.86H3.93(C6H12BO2)0.18 2205DEB–HPZ-1 6.60:6.67 8/100 78.3 Si1.00N0.73C1.04H4.18(C4H13B3N3)0.07 3710DEB–HPZ-2 7.96:8.00 24/100 49.7 Si1.00N0.84C1.04H4.25(C4H13B3N3)0.18 3260DEB–HPZ-3 7.50:7.63 70/100 23.0 Si1.00N0.76C0.94H4.29(C4H13B3N3)0.35 2730



Second-generation SiNCBprecursors

WhereastheB–TTS,B–HCTandB–HPZpolymersmay prove to be suitable precursorsto SiNCBmatrix materials and/or coatings, many otherpotential applicationswill require much greatercontrolof structureandreactivity thanis at presentavailable in thesesystems.Becauseof the latentreactivity of the B–H groups on the pendantborazinyl rings, thesepolymerscontinueto reactand build molecularweight in their melts by thecrosslinkingreactionillustratedin Scheme3. Thesepolymersare thereforenot suitablefor processesthat require stable melt viscosities, e.g., in thegenerationof polymerfibersby melt-spinning.

Table 3 Polyborosilazane-derivedceramiccompositionsat 1400°C

PolymerBulk ceramicconversion

(wt%) Composition Density(g/mlÿ1)

B–TTS 37.8 Si1.00N5.52C0.21B5.26 1.7B–HCT 54.3 Si1.00N3.89C0.64B2.78 1.7B–HPZ-2 70.0 Si1.00N1.08C0.44B0.09 2.3PIN–HPZ-3 59.1 Si1.00N1.02C0.67B0.15O0.26 2.3HPZ 57.0 Si1.00N0.97C0.41 2.6

Table 4 Polyborosilazane-derivedceramiccompositionsat 1800°C

PolymerBulk ceramicconversion

(wt%) Composition Density(g/mlÿ1)

B–TTS 26.1 B1.00N0.86C<0.01Si1.01 2.3B–HCT 36.2 Si1.00N2.81C0.73B2.70 2.0B–HPZ-2 51.1 Si1.00N0.47C0.58B0.10 —PIN–HPZ-3 53.7 Si1.00N0.91C0.68B0.18O0.28 2.45HPZ 42.4 Si1.00N0.03C0.94 —

Figure 6 PowderX-ray diffraction spectraof the ceramicsderivedfrom pyrolysis at 1800°C of (a) B–TTS, (b) B–HCTand(c) B–HPZ-2polymers.

Scheme3 Low-temperaturecrosslinkingreactionsof B–HPZpolymers.



As in the caseof the polyborazylenepolymers,the key step neededto retard the crosslinkingreaction of the polyborosilazane polymers is toremoveor block any reactiveB–H groups.Withthis aim in mind we have now investigatedthedesignof new second-generationhydridopolysila-zanesderivedfrom monofunctionalboranes(Eqn[9]).25 Sincethesenew borane-modifiedpolymersdo not contain reactiveB–H groups,they cannotundergothecrosslinkingreactionsoccurringin theborazine-basedpolymers.

Our initial studieswith boranereagentssuchas9-borabicyclo-[3,3.1]nonane (9-BBN) and cate-cholboranedemonstratedthatthesemonofunctional

boranesare not sufficiently stable to achieve aprocessablepolymer.However,asshownin Fig. 7we have now identified severaldifferent mono-functionalboranes,includingpinacolboraneandB-diethylborazine,which areboth reactivewith HPZandthermallystableat thetemperaturesneededformelt-processing.Pinacolborane,which is uniqueamong the O–B–O heterocyclesin that it is notproneto disproportionation, is commerciallyavail-able or can be prepared from the reaction ofBH3�THF with pinacol. B-Diethylborazinecan beobtainedreadily on a largescalevia the rhodium-catalyzedborazine/ethylenereaction.10

As summarized in Table 2, boron-modifiedpolymers,PIN–HPZandDEB–HPZ,were readilypreparedby heatingsolutionsof HPZ dissolvedineachboraneat60–100°C.Forexample,in atypicalpreparationof PIN–HPZ-3,a 8.00g (90.4mmol ofN–H sites) sample of HPZ dissolved in 7.99g(62.5mmol)of PIN–Hwasheatedat60°C for 24h.After vacuumevaporationof thevolatile materials,the modified polymer was removed from thereaction vessel under an inert atmosphereandisolated as a clear, colorless solid. Elementalanalysis of PIN–HPZ-3 is consistent with an(HSi)0.33 (Me3Si)0.15 (NH)0.24N0.13 (PIN)0.09 com-position. The DEB–HPZ polymerswere preparedin a similar fashion,but requiredlonger reactiontimes (up to 29 days), and higher temperatures(100°C). Both polymer systemswere soluble incommon organic solvents, including ethers andhydrocarbons.

The degree of modification can be easily

Figure 8 DRI chromatogramsof (a)HPZ,(b) PIN–HPZ-1,(c) PIN–HPZ-2and(d)PIN–HPZ-3.

Figure 7 Monofunctionalboranes:pinacolborane(PIN), andB-diethylborazine(DEB).



controlled by the duration of the reaction, withtypical boron content ca 1–3%. Increasesin theboroncontentwereaccompaniedby stoichiometricincreasesin the carbon and hydrogen contents

owing to the alkyl groupson the boranerings. Inaddition, the DEB–HPZ polymers showed theexpectedincreasein nitrogencontentowing to theboranering nitrogens.Oxygen content,as deter-

Figure 9 PowderXRD spectraof PIN–HPZ-3pyrolyzedto (a) 1400°C, (b)1600°C and (c) 1800°C. (The Al2O3 impurity comesfrom the pestleandmortarusedto grind thesample.)

Figure 10 TGA studiesof HPZ, B–HPZ,PIN–HPZ-3andDEB–HPZ.



mined by difference,paralleledchangesin boroncontentin thePIN–HPZpolymers.

In eachsystem,hydrogenwasevolvedin amolarratioof almost1:1,with theamountof incorporatedborane suggesting that dehydrocoupling is thedominant means of modification. Spectroscopicstudies,modelreactionsandanalysisof thevolatileby-productssuggest,as was proposed23,24 for theB–HPZ polymer, that boranemodification of thepolymer occursvia the creationof B–N linkages.SEC/DRI chromatogramsfor HPZ and modifiedHPZ weretypically multimodal,indicatinghetero-geneouschain/cagestructuresand/orcompositionalheterogeneity(Fig. 8). As canbeseenin bothFig. 8andTable2, increasingfunctionalizationdecreasedpolymerweight,suggestingchaindegradations.

Studiesof theconversionsof polymerto ceramicshowedthat the modified polymersyield SiNCBceramicscontainingapproximately1–3%boronat1400°C,with thehighestboroncontentin thePIN–HPZ-derived samplesTable 3. At 1800°C, thePIN–HPZ-derived ceramics exhibited improvedthermal stability with nitrogen contentsof up to23%, comparedwith the ceramicsobtainedfromunmodifiedHPZ,whichusuallyretainlessthan4%nitrogenat this temperature(Table4).Furthermore,while the DEB–HPZ-derived ceramics showedcrystallization propertiessimilar to the ceramicobtainedfrom unmodifiedHPZ, the PIN–HPZ-3-derivedceramicswereamorphousto 1600°C, andat 1800°C showedonly weak diffraction from b-SiC (Fig. 9). Thus,the higher-boron-contentPIN–HPZ-basedceramicshavepropertiesequivalenttoor better than those obtained from the B–HPZpolymer. However, unlike the B–HPZ polymers,whichshowinitial weightlossesdueto crosslinkingbelow80°C,23,24TGA studies(Fig.10)of thePIN–HPZ and DEB–HPZ samples showed a broadweightlossoverthe200–800°C temperaturerange,very similar to that of the parentHPZ. TGA–MSanalysesof the volatiles evolvedduring pyrolysisshowedno differencesbetweenthe modified andunmodified polymers. Clearly, the crosslinkingreactionthat resultsin the low-temperatureweightlossin theB–HPZpolymershasbeensuppressedinthePIN–HPZandDEB–HPZpolymers.

With aTg of 87°C andanonsetof weightlossatca 200°C, the parentHPZ polymer hasexcellentrheology for melt-spinning.As seenin Table 2,thermalmechanicalanalyses(TMAs) of the PIN–HPZ andDEB–HPZpolymersshowedonly slightchangesin the Tg with modification. Most im-portantly, in contrastto the B–HPZ polymers,thePIN–HPZandDEB–HPZpolymerswerefound to

Figure 11 Scanningelectronmicrographsof (a)polymerfiberderived from PIN–HPZ-3, (b) polymer fiber derived fromDEB–HPZ and (c) ceramic fiber derived from the 1200°Cpyrolysisof PIN–HPZ-3.



bestableasmelts,andthusbeexcellentcandidatesfor melt-spinning.Using a crude ram extruder itwas then possible to extrude approx. 30–40mmpolymer fibers continuouslyat a rate of ca 60mmin.ÿ1 Scanningelectronmicrographsof thePIN–HPZ-3 polymer fibers show that they are smoothand uniform (Fig. 11a). After a brief cure withCl3SiH, pyrolysis under an argon atmosphereto1200°C produced black SiNCB ceramic fibers(Fig. 11c). The ceramic fibers were denseandrelatively uniform but, becauseof the defectsinduced by the crude spinning conditions, theywerefound to be too weakfor mechanicaltesting.With the use of a more sophisticatedspinningapparatusand curing proceduresit should bepossibleto generatefiberswith enhancedmechan-ical properties.Thesestudiesarein progress.

CONCLUSIONS

The work describedabove has demonstratedaviabledesignstrategyfor theformationof meltablepolymeric precursorsto both boron nitride andSiNCB ceramicsbasedon the controlled functio-nalization of preformed polymers with pendantgroupsof suitablecompositionsand crosslinkingproperties.Both thedipentylamine–polyborazylene(DPA) and pinacolborane–hydridopolysilazane(PIN–HPZ)polymers,unlike the parentpolybora-zylene(PB) andthe borazine–hydridopolysilazanepolymers(B–HPZ),arestableasmeltsandcanbeeasily melt-spuninto polymer fibers. Subsequentpyrolyses of these polymer fibers then provideexcellentroutesto BN andSiNCB ceramicfibers.The easeof synthesisof both polymer systemssuggeststhat newhybrid polymerswith a rangeofsubstituentsappendedto the polyborazyleneorhydridopolysilazanebackbonesshould now bereadily achieved,allowing even greater controloverpolymerandceramicproperties.

Acknowledgments We thank the US Departmentof Energy,Division of Chemical Sciences,the Office of Basic EnergySciences,the Office of Energy Researchand the NationalScienceFoundationfor their supportof this research.

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Second-Generation Polymeric Precursors for BN and - Deep Blue

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