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Original papers

Ceramics – Silikáty 59 (4) 341-347 (2015) 341

HYDROBORATION SYNTHESIS, CHARACTERIZATION ANDBIOLOGICAL ACTIVITY OF NOVEL CYCLIC PRECERAMIC

POLYMERS: TRANSITION TO SiBOC CERAMICS#BIRSEN S. OKSAL*, AYSEGUL GENCER**

*Department of Chemistry, Giresun University, Giresun, Turkey**Department of Chemistry, Akdeniz University, 07058 Antalya, Turkey

#E-mail: [email protected]

Submitted April 6, 2015; accepted October 2, 2015

Keywords: Silicon boron oxycarbide (SiBOC) polymers, SiBOC ceramics, Hydroboration reaction, Preceramic polymers, Antibacterial activity

Novel preceramic polymers were synthesized by hydroboration reaction and pyrolyzed under continuous nitrogen flow to obtain boron containing silicon oxycarbide (SiBOC) ceramics. The preceramic SiBOC polymers were synthesized with different cyclicsiloxane/borane molar ratios. Structure of the SiBOC polymers and transformation of them to the SiBOC ceramics investigated by Fourier Transform Infrared spectroscopy (FT-IR), Thermogravimetric analysis (TGA), X-ray Diffraction (XRD) technique and Scanning Electron microscope (SEM). Since the cyclicsiloxanes used in smaller sizes according to literature, variations were observed in structure of obtained polymers and ceramics. During the pyrolysis of polymers, loss of the carbon was little. The transformation from amorphous form to the β-SiC crystal was achieved above 1200°C and consequently thermally stable SiBOC ceramics were obtained at higher temperatures. Antibacterial activity of SiBOC polymers were evaluated by disc diffusion method against E. coli ATCC 25922- for the first time. The new polymers inhibited the growth of Gram-negative bacteria well enough.

INTRODUCTION

Preceramic polymers were first proposed for theproduction of silicon containing ceramics about 30yearsago[1].Themajoradvantageof thesepreceramicpolymers is to give anopportunity forworkingwith alower reaction temperature and preparing amorphousor crystalline powder depending on the work. Thesynthesizedpreceramicpolymersarepyrolyzedtotrans-formthepolymerstotheceramicmaterials.Preceramicmaterials shrink during pyrolysis due to the changesof the density from polymer materials (1 g∙cm-3) to ceramics(2-3g∙cm-3)and theconsistingofmass lossduring pyrolysis [2]. Theceramicmaterialsareimprovedbyusingboronand named as silicon boron oxycarbide (SiBOC) cera-mics [3]. In these ceramics, the addition of boron tothe structure results in an increase at the degradationtemperature which means an increase in the thermalstability[4,5].Theseceramicsalsobehavelikeahybridmaterialandthusofferthepossibilityofcombiningtheproperties of inorganic and organic materials in onematerial[2].SiBOCceramicscanbeproducedcheaplyfrom easily accessible poly(organoborosiloxanes) and obtained with shorter gelation time (t-gel) thanksto ultrasonic and microwave application [6, 7]. Thecommon methods used for the synthesis of boron-

containing polyorganosiloxane are sol-gel method andhydroboration reactions [8]. Hydroboration reactionis a three-stage reaction and alkenes are converted toalcoholsbytheanti-Markovnikovhydration.Inthefirststage,boronhydride is added to alkeneand thenalkylboron compounds consist at the end of the reactionwhich is following with the four-membered transitioncomplex. In the second stage, alkyl boron compoundsareoxidizedtoalkylborate.Inthethirdstage,theanti-Markovnikovalcohols areobtainedbyhydrolysis.Thefirst two stages of the reaction are used for obtainingpoly(organoborosiloxanes) [9, 10]. When the studies about cyclic SiBOCpreceramicpolymers are investigated, the following conclusionshavebeendrawn.Liebauetal.[11]synthesizedSiBOCpolymers via hydroboration reaction of 1,3,5,7-tetra-methyl-1,3,5,7-tetravinylcyclotetrasiloxanewith boranedimethyl sulfide (BH3.S(CH3)2) using different molarratios. In the mentioned study, the transformation of SiBOC polymers to ceramics occured at 1000°C under nitrogenatmosphereand theobtainedSiBOCceramicswereamorphousupto1200°C.Whenitwasreachedupto1300°Ctheβ-SiCcrystalphaseoccuredwithdegradationof the SiBOCmatrix. In another study, Schiavo et al.[12] synthesized poly (organoborosiloxanes) by thefollowingreactions.Hydrogenreleasingreactionofthe1,3,5,7-tetramethyl-1,3,5,7-tetracyclotetrasiloxane(D4H)

Oksal B. S., Gencer A.

342 Ceramics – Silikáty 59 (4) 341-347 (2015)

withboricacid,sol-gelreactionofvinyltriethoxysilanewith boric acid and hydrosilation reactions of theD4Hwith sol-gel reaction’s product.As a result, blackamorphousSiBOCmaterialswereobtainedbypyrolysisofpoly(organoborosiloxanes)at1000°C. Only two study have been seen about biologicalperformance of SiOCpreceramic polymers in the lite-rature. The result of one of these studies confirmed agoodbiologicalperformanceofSiOCceramicwithbothhardandsofttissuecellmodels.Inthesecondstudy,theresults indicatedgoodbiologicalperformanceofSiOCceramicsandconfirmedcitocompatibility[13,14]. Best ofourknowledge, in the literature, there areno study about biological performance of SiBOC pre-ceramicpolymersandceramics.Furthermore,thereareno available data in literature concerning antibacterial activityofSiOCandSiBOCpreceramicpolymers.So,itisnotknownhowisantibacteriyelactivitymechanismoftheSiOCorSiBOCpreceramicpolymers.Inmanyyears,biologicalroleofboronhasbeenthesubjectofanumberofbiologicalstudies.Itisproventhatanumberofboron-containing compounds possess strong antibacterial activity specifically against the enteric group of gram-negativebacteriabothinvitroandinanimalinfections.Also,theeffectoftheboroncompoundstocellsisdefinedasanabnormal formofgrowth inacell envelope [15,16].So,inourstudyantibacterialactivitiesofobtainedSiBOCpreceramicpolymerswereexplainedonthebasisofboronfeature. In this study, novel SiBOC preceramic polymersweresynthesizedbyusing1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane(siloxane)/boranedimethylsulfide(BH3.S(CH3)2)(borane)withdifferentmolarratios(3:4,1:2and1:4)byhydroborationreaction.Theantibacterialactivity test ofpreceramicpolymers at 3:4,1:2 and1:4molarratioswasperformedbydiscdiffusionmethodforthe first time.The novel preceramic polymerwith 3:4molar ratio was pyrolyzed at 700°C, 1200°C, 1400°C topreparenovelSiBOCceramics.Duetofurthercross-linking in 3:4 preceramic polymer by formation ofintermolecularB–Clinkages,onlyitwaspreferedforthepyrolysisamongthepreceramicsamples.Theobtainedpreceramic polymers and ceramics were characterizedby Fourier Transform Infrared spectroscopy (FT-IR),Thermogravimetric analysis (TGA), X-ray Diffraction(XRD) technique and Scanning Electron microscope(SEM).

EXPERIMENTAL

SynthesisofSiBOCpreceramicpolymers

The synthesis of the preceramic polymers werecarried out under nitrogen atmosphere using standardSchlenktechnique.3:4molarratiosofSiBOCpreceramicpolymerwaspreparedaccordingtothefollowingroute[11]:Firstly,theproperamountof1,3,5-trimethyl-1,3,5-

trivinylcyclotrisiloxane (cyclic siloxane)was dissolvedin 10 ml of hexane. The Solution was cooled to 0°Cand stirred for 30 min. Then, the proper amount ofBH3.S(CH3)2wasdissolvedin10mlofhexaneandaddedslowlyintotheprevioussolution.Thefinalsolutionwasstirred for 20 h at room temperature and solvent wasremovedbydistillationunderreducedpressureat40°C.Finally,thewhitepowderswereobtainedafterdryinginthevacuumovenat room temperature.Cyclicsiloxane/boranemolarratiosof1:2and1:4ofpreceramicpolymerswere prepared according to the same procedure. Thesynthesizedpreceramicpolymerswerenamedas3:4,1:2and 1:4 samples.

Antibacterial activity test

AmericanTypeCultureCollection(ATCC)qualitycontrol strain recommended by CLSI (Clinical andLaboratory Standards Institute) [17] and disc diffusionmethod were used to determine the antibacterialproperties of all the prepared preceramic polymers.E. coliATCC25922-wasusedastestedbacterialstrain. E. coli bacteria species was first inoculated intoMueller-Hinton Agar (Merck KGaA, D-Darmstadt) and incubated at 37°C over-night and checked for purity.Thenbacteria suspensionwasprepared in0.9%NaClsolution by means of 0.5 McFarland (1 × 108 cells per ml, BioMérieux, Marcy I’Etoile, France) standarddensity. Prepared bacteria suspension was inoculatedonto Mueller-Hinton Agar platesbysterileswab.30and40mgofallpreceramicpolymerswerepreparedunderpressure(10ton)asdiscsin10mmdiameter,thentheywere placed on inoculatedMueller-Hinton Agar plates one by one.An inoculated Mueller-Hinton Agar plate withoutpreceramicpolymerwasusedtotestthesterility.Allplateswereincubatedat37°Cfor24h.Thediametersoftheinhibitionzonesweremeasuredinmillimetres.

Preceramic polymer pyrolysis

SiBOC preceramic polymer of 3:4 molar ratiowas converted into SiBOC ceramic by pyrolysis under continuous nitrogen flow at 700°C, 1200°C, 1400°C.Thepyrolysiswas carriedout in aLENTON tube fur-nace using alumina and ceramic crucibles. Pyrolysis temperature (700°C, 1200°C or 1400°C) was reachedusingaheatingrateof10K∙min-1.Thesampleswereheld1 h at pyrolysis temperature and cooled to room tem-perature.ObtainedthreeSiBOCceramicswerestoredindessicatoratroomtemperatureuntilfurtheranalysis.

Characterizationtechniques

Thestructuralevolutionfromthepreceramicpoly-mer to the ceramic was investigated by FT-IR using a Bruker Tensor 27 spectrometer operating between 4000 and 400 cm-1.Thermogravimetricanalysiswasperformed

Hydroboration synthesis, characterization and biological activity of novel cyclic preceramic polymers: Transition to SiBOC ceramics


with TA Instruments TGA Q500. This analysis wascarriedoutwithaheatingrateof10K∙min-1up to themaximumtemperatureundernitrogen.X-raydiffraction(XRD) patterns were recorded on a PANanalyticalX’Pert Pro MPD using CuKα, 2θ = 15 - 95°. Themicrostructural analysis was carried out using scanning electron microscope (SEM, LEO 440) operating at 20.00 kV.

RESULTS AND DISCUSSION

Thehydroborationreactionsofsiloxaneswithbo- rane were given in Figure 1, 2 and 3 with differentmolar ratios of 3:4, 1:2 and 1:4, respectively. TheobtainedSiBOCpreceramic polymers and 3:4SiBOCceramics characterized by Fourier Transform Infraredspectroscopy(FT-IR)wereintroducedinFigure4and5.Thepyrolyzedpreceramicpolymers(SiBOCceramics)at 700°C, 1200°C, 1400°C were also introduced inFigure5namedas3:4-700,3:4-1200,3:4-1400. Thespectraof3:4,1:2,1:4sampleswereintroducedinFigure4,together.Inthespectrumof1:4sample,theabsorptionpeaksat2264cm-1 and 543 cm-1assignedtotheB–HstretchingvibrationaresharperthanB–Hpeaksoftheotherspectra.Theseresultsindicatethatamountofboranein1:4samplewasmorethantheothersamples.In

Figure4,theabsorptionpeaksat668cm-1 and 1194 cm-1 assignedtotheB–CstretchingvibrationandtheabsenceofthepeakofC=Cbond(1680-1620cm-1) demonstrate theformationofbindingbetweenboraneandsiloxaneinall samples [11]. Inthespectrumof3:4sampleinFigure5,theab-sorptionpeaksat2964cm-1and2876cm-1 areassignedtotheC–Hstretchingvibration.Theabsorptionpeaks at 1259 cm-1and786cm-1areassignedtotheSi–C;at671 cm-1 and 1194 cm-1 are assigned to the and B–Cstretchingvibration.Theabsorptionpeaksat1047cm-1

and 424 cm-1 are assigned to the Si–O stretchingvibration. B–H stretching vibration at 2258 cm-1 and 549 cm-1 de-creased in3:4-700,3:4-1200 samples anddisappeared in 3:4-1400 sample spectra in Figure 5.Inthespectraof3:4-700,3:4-1200,3:4-1400samples,C–Hstretchingvibrationhavecompletelydisappeared.Disappearing of C-H bond peak in 3:4-1400 sample

SiOSiO

SiO S+

3n + 4n H3B–

B B

B

O

OSi

Si SiO

n-hegzan

n

SiOSiO

SiO S+

1n + 2n H3B– n-hegzan

n

B B

B

O

OSi

Si SiO

H

H

H

SiOSiO

SiO S+

1n + 4n H3B– n-hegzan

B B

B

O

OSi

Si SiO

HH

H H

H

H

Figure 1. Synthesis of poly(organoborosiloxane) preceramicwith3:4siloxane/boranemolarratioviahydroborationreaction.



4000 3500 3000 2500 2000 1500 1000 500

3/4

3/4-700

3/4-1200

3/4-1400

Wavenumber (cm-1)

Tran

smitt

ance

(a.u

.)

32482964

2258

1463

1259

1193

1047 786

873

671

424

2876

549

1194

4000 3500 3000 2500 2000 1500 1000 500

3:4

1:4

1:2

2264

543

1195

668

Wavenumber (cm-1)

Tran

smitt

ance

(a.u

.)

Figure5.FTIRspectraofpoly(organoborosiloxane)preceramicwith3:4siloxane/boranemolarratioandthesamplesformedbyits pyrolysis at 700, 1200 and 1400°C.

Figure4.FTIRspectraofpoly(organoborosiloxanes)with3:41:2 and 1:4 siloxane/borane molar ratios.



supportstoformβ-SiCcrystallization[12]. XRD patterns collected from the samples formedbypyrolysisof3:4 sampleat700°C,1200°C,1400°CarereportedinFigure6.Thespectraof3:4-700and3:4-1200samplesshowsharppeaksat2θ=28°(002)whichare assigned to the presence of crystallineB(OH)3. In the spectrum of 3:4-1400, the appearance of the fourpeaks at 2θ = 35.7°(111), 41.3°(200), 60.1°(220), and71.8°(311)provetheexistenceofβ-SiCcrystallization.Inaddition,abroadhalowithamaximumcenteredat 2θ ~ 23° is associated with amorphous silica in allspectra. As shown in XRD spectra, B(OH)3 occured until 1400°C and combinated to the silica network in

the structure, then at 1400°C β-SiC crystal structureoccurred[18-20]. TheTGAcurvesof3:4,1:2and1:4sampleswereshowninFigure7,Figure8andFigure9,respectively.Thesecurvesindicatethatweightlossin1:4sampleislower than1:2sampleand theceramicyields increasewith rising of boron amount. However, the curve of3:4sampleinFigure7indicatethatweightlossdoesn’tincreasewithfurtherdecreasingofboronamount.Accor-ding to these results, organoborosiloxane structures in 3:4 sample are more interconnected than the othersamples.Moreover,obtainedstructurein3:4sampleisexactlyapolymer,sothestructureshowsmoreresistantagainst weight loss. Acording to the TGA curves ofall samples it canbe said that theweight lossesoccurbetween 300 and 400°C due to releasing of organicgroups and H2 [11]. ScanningElectronmicroscope(SEM)imagesof3:4and3:4-1400sampleswereshowninFigure10and11,respectively.WhentheSEMimagesareevaluated,itisobservedthat3:4samplehasanamorphousstructureand3:4-1400samplehasacrystalstructurewith1micron-

sizecrystals.TheseresultsprovethatthecrystalformofSiBOCceramicsareachievedat1400°C. AntibacterialactivitiesofSiBOCpreceramicpoly-mersof3:4,1:2and1:4molarratiosweredeterminedagainst the bacterial strain (E. coli ATCC 25922) by discdiffusionmethodasmentionedabove.TheresultsoftheantibacterialactivitytestwereshowninTable1

(111)

(220

)

(311

)

(002)

(200

)

20 30 40 50 60 70 80 90

3/4-700

3/4-1200

3/4-1400

2θ (°)

Inte

nsity

(a.u

.)

60

70

80

90

100

110

0 200 400 600 800 1000Temperature (°C)

Weight

620.77°C

Deriv. weight

Wei

ght (

%)

-0.02

0

0.02

0.04

0.06

0.08

0.10

0.12

Der

iv. w

eigh

t (%

/°C

)

482.76°C343.79°C

%31.62

108.19°C

736.17

70

80

90

100

110

0 200 400 600 800 1000Temperature (°C)

Weight

610.68°C

Deriv. weightWei

ght (

%)

-0.02

0

0.02

0.04

0.06

0.08

0.10

0.12

Der

iv. w

eigh

t (%

/°C

)538.12°C

325.29°C

% 28.71

196.

55°C

85.3

7°C

413.

04°C

815.59

70

80

90

100

110

0 200 400 600 800 1000Temperature (°C)

Weight

77.09°C

Deriv. weight

Wei

ght (

%)

-0.02

0

0.02

0.04

0.06

0.08

0.10

0.12

Der

iv. w

eigh

t (%

/°C

)

572.61°C

257.47°C

% 28.58

671.72

Figure6.XRDspectraofSiBOCceramicsformedbypyrolysisofpoly(organoborosiloxanes)with3:4siloxane/boranemolarratio at 700, 1200 and 1400°C.

Figure8.TGAandDTGcurvesofpoly(organoborosiloxane)preceramicwith1:2siloxane/boranemolarratio.





as the diameters of inhibition zones.The results showthattheantibacterialactivityof1:4sampleishigherthanothersand3:4samplesisthelowestinboth30mgand40mgpreceramicpolymers.Thesecasesoriginatefromdifferentpolymerizationofthesamples.Forinstance,1:4sampleisnotapolymerandpolymerizationof1:2sampleis lowerthan3:4sample.Antibacterialactivitiesof the

samples increase with decrease of the polymerization.Because,withoutpolymerization,releaseofboronatomfrom the structure canbemore easily.Boron atoms inall sampleshaveoccupied three coordination sites andoneemptycoordinationsite.Nitrogenoftheproteinscaneasilycoordinatewithboron’semptycoordinationsite.Besides,stericeffectofpolymericstructureof3:4sample

Figure10.SEMimagesofpoly(organoborosiloxane)preceramicwith3:4siloxane/boranemolarratioinimages1-6at10µmand1µmsizeswithdifferentmagnifications.

b)

d)

f)

a)

c)

e)



Figure11.SEMimagesofSiBOCceramicsformedbypyrolysisofpoly(organoborosiloxane)with3:4siloxane/boranemolarratioat1400°C(images1-6at10µmand1µmsizeswithdifferentmagnifications).

b)

d)

f)

a)

c)

e)

Table1.Antibacterialactivitiesof30mgand40mgpreceramicpolymerswith3:4,1:2and1:4molarratiosagainstthebacterialstrain(E.coliATCC25922)testedbasedondiskdiffusionmethod.

Molar ratios 30 mg 40 mg

3:4 19 mm 23 mm 1:2 21 mm 26 mm 1:4 26 mm 32 mm



inhibits the reaction between B atoms and proteins ofbacteriacells,thusantibacterialactivityofpolymeric3:4preceramicsample is lower.Theantibacterialactivitiesof thepreceramicpolymerswereevaluated in termsofsampleamount.When40mgpreceramicpolymerwasusedforthediscpreparation,antibacterialactivityofthepreceramic polymer was found to be higher owing tomore boron amount.

CONCLUSIONS

Inthisstudy,novelSiBOCpreceramicpolymersandSiBOCceramicshavebeensynthesizedbyhydroborationreactionwithdifferentmolarratiosofcyclicsiloxaneandborane.Reducingofringsizeinthecyclicvinylsiloxanesaltered the structure of obtained preceramic polymersandceramics.AccordingtotheTGAresults,carbonlosswas littleduring thepyrolysisofpreceramicpolymers,thus obtained ceramics shouldprovidebetter electricalresistance. 3:4 molar ratios of cyclic siloxane/boranepreceramic sample showed better polymerization than1:2 and 1:4molar ratios of preceramic samples.As isseen fromXRDandSEManalysis, the transformationfromtheamorphousstructuretotheβ-SiCcrystalformwasachievedabove1200°Candconsequentlyathighertemperatures thermally stable SiBOC ceramics wereobtained. Antibacterial activity of preceramic samplewas examined for the first time onSiBOCpreceramicpolymers. The best antibacterial activity among thepreceramicswasobserved in1:4 samplewhichhasnopolymerization.

Acknowledgments

The authors gratefully acknowledge Scientific Research Projects Unit of Akdeniz University (2012.02. 0121.006) for financial support.

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