Journal ofMaterials Chemistry A
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aCeramic Materials Engineering, University o
E-mail: [email protected] of Organic Chemistry, University of BcDepartment of Applied Mechanics and F
D-95440 Bayreuth, Germany
Cite this: DOI: 10.1039/c3ta13254d
Received 16th August 2013Accepted 18th October 2013
DOI: 10.1039/c3ta13254d
www.rsc.org/MaterialsA
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Selective cross-linking of oligosilazanes to tailoredmeltable polysilazanes for the processing of ceramicSiCN fibres
Octavio Flores,a Thomas Schmalz,b Walter Krenkel,a Lutz Heymannc
and Gunter Motz*a
An interesting alternative for the processing of non-oxide ceramic fibres at lower costs than that for the
current commercially available fibre types was developed by modifying different commercially available
liquid oligosilazanes (ML33 and HTT1800) into polysilazanes by selective cross-linking via the N–H and
Si–H groups with tetra-n-butylammoniumfluoride (TBAF) as a catalyst. Termination of the reaction with
calcium borohydride allows the processing of meltable solid polysilazanes (ML33S and HTTS) with
tailored chemical and thermal properties, to fulfil the requirements for the melt spinning of
mechanically stable and homogeneous polymeric fibres. The chemical and thermal stability of the
polysilazanes ML33S and HTTS were investigated by using GPC, DSC and rheological measurements.
These techniques indicate the dependency of the molecular weight and glass temperature on the
catalytical cross-linking conditions. Polymers with up to �10 000 g mol�1 show glass–liquid transition
(Tg) between 65 and 81 �C and viscoelasticity, which are essential properties for the melt-spinning
process. The thermal stability of ML33S is ensured up to 220 �C. In contrast the thermal stability of HTTS
is limited to 170 �C due to the presence of vinyl-groups. The viscoelastic behaviour of the polymer melts,
measured by oscillatory rheometry, and the sufficient thermal stability allowed the continuous
processing of stable green fibres by melt spinning in the temperature range of 110 to 130 �C. After fastelectron beam irradiation curing of the green fibres and pyrolysis of continuous amorphous ceramic
SiCN fibres from both ML33S and HTTS polysilazanes were successfully synthesized, while a defined Tgpoint influences the shape and the smoothness positively.
1 Introduction
Since the development of Si3N4/SiC ceramic bres for hightemperature applications by Verbeek and Winter1 and Yajimaet al.2 in the 1970s, great attention has been paid to the devel-opment of organosilicon polymers as a way to produce ceramicmaterials. These types of polymers are organometallic systemsprepared from monomers, such as chloro(organo)silanes,which allow the use of polymer techniques such as melt spin-ning, polymer inltration and pyrolysis (PIP), simple coatingtechniques or resin transfer moulding (RTM). Moreover it isalso possible to produce ceramics with tailored chemicalcomposition at lower temperatures, which is advantageous incomparison with conventional ceramic manufacturing frompowders.3–5 Suitable polymers can be cured into an unmeltablethermoset via catalytic or thermal treatment. By pyrolysis at
f Bayreuth, D-95447 Bayreuth, Germany.
ayreuth, D-95440 Bayreuth, Germany
luid Dynamics, University of Bayreuth,
Chemistry 2013
temperatures higher than 800 �C, the cross-linked materialsuffers an organic–inorganic transition, resulting in amor-phous/crystalline ceramics, the so-called Polymer DerivedCeramics (PDC).
The most common organosilicon polymers commerciallyavailable are polysiloxanes, polysilazanes, polycarbosilanes andpolysilanes. In this work, the attention was turned to non-oxidepolyorganosilazanes, which lead to SiCN ceramics with highthermal and chemical stability aer pyrolysis. They can be usedto produce ceramic bres,6–9 coatings,10–13 matrices in Ceramic-Matrix-Composites (CMCs)14–17 and porous components.18–21
Our work concentrated on the development of non-oxideceramic SiCN bres from polysilazanes, having not only higheroxidation stability, but also an affordable price in comparisonwith the commercially available non-oxide ceramic SiC bressuch as Hi-Nicalon, Tyranno or SCS-6.
The properties that an organosilicon polymer should have tobe appropriate for the production of non-oxide ceramic bresvia melt spinning are a sufficiently high molecular weight toprevent any volatilization of oligomers during pyrolysis, andalso meltability and viscoelastic properties to ensure the
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spinnability of the polymer before curing. The presence oflatent reactivity is crucial for the curing process, and sufficientceramic yield to reduce the shrinkage and the number of defectsduring ceramization to improve the mechanical stability of thenal ceramic bre.18,19One of themain challenges is to combineall these properties in the same material, which was achieved inthis work by increasing the molecular weight of commerciallyavailable liquid oligosilazanes by using a simple, controlled andselective chemical cross-linking reaction. The implementationof this strategy should lead to relatively cheap tailored poly-silazanes with the required properties for the processing of non-oxide ceramic SiCN bres.
Some reviews report the numerous and versatile syntheticand analytical studies, which have been done in relation topolysilazanes.20–22 The most common method to synthesizeSiCN precursors is the ammonolysis or aminolysis of organo-silicon chlorides using ammonia and primary amines respec-tively. The resulting precursors are oen mixtures of liquidcyclic and/or linear polymers with complicated structures.Recently ammonolysis of inexpensive vinyltrichlorosilane (VTS)in tetrahydrofuran (THF) leading to the production of a poly-vinylsilazane (PVS) was described. This precursor has ahigh ceramic yield of �85 wt%, but its low molecular weight(�880 g mol�1), chemical/thermal stability and liquid state atroom temperature avoid the processing of green bres by meltspinning.23
Seyferth et al.24 and Duguet et al.25 performed signicantresearch to overcome this problem by cationic/anionic ring-opening polymerization of cyclosilazanes. Different linearpolysilazanes synthesized by this method could be very wellunderstood and characterized in terms of molecular andthermal properties. Because of molecular weights much higherthan 1000 g mol�1 and low dispersity the synthesized poly-silazanes would be an ideal precursor for the processing ofmechanically stable melt spun green bres for further pyrolysisinto ceramic SiCN bres. But as stated by Seyferth et al., linearorganosilicon polymers prepared by this method do not containsuitable functional substituents for thermally induced reactionsleading to sufficient cross-linking before pyrolysis. As a conse-quence, during the pyrolysis of the linear precursor, the polymerstructure suffers fragmentation instead of cross-linking withevolution of volatile silazane fragments as well as NH3, CH4 andH2. Almost no PDC is obtained from such precursors (ceramicyield < 10 wt%). In this context it became clear that the cross-linking step is crucial for obtaining PDCs with high ceramicyields. Therefore, a lot of synthetic and analytical studies weredone to investigate the chemical processes during thermallyinduced and particularly catalytic cross-linking. Especially in thecase of catalytic cross-linking the attachment of functionalgroups to the silicon or nitrogen atom is necessary, as the natureof these substituents determines the cross-linking reactions.Radical initiated polymerisation of vinyl monomers,26,27 hydro-silylation involving Si–H and vinyl groups28,29 and dehydrogen-ative couplings between Si–H and N–H groups, catalysed byorganometallic catalysts,30–35 are the most popular examples.
In applications such as coatings and CMC matrices, liquidoligosilazanes such as HTT1800 from Clariant GmbH
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(Germany), which have Si–H, N–H and Si–vinyl functionalgroups, are generally cross-linked with dicumylperoxide (DCP)by radical initiated polymerization and with Pt catalysts byhydrosilylation. These initiators and catalysts are very efficientin curing the precursor rapidly and producing a stable 3Dnetwork, which stabilizes the shape and ensures a higherceramic yield, in comparison with the former liquid oligosila-zane.36–39 But in the case of bres, these catalysts are notappropriate, since it is not possible to control the reaction andto have selective cross-linking. Once the cross-linking starts, nosolid thermoplastic with tailored properties is obtained, it leadsonly to insoluble and infusible materials.
Different from DCP and Pt, tetra-n-butylammoniumuoride(TBAF) is a nucleophilic catalyst, which is well-known in organicchemistry, but almost no work has been done in relation topolysilazanes. About 22 years ago, Corriu et al. reported aboutthe ammonolysis of dichloromethylsilane and the consecutivetransformation of the resulting polymers into SiCN ceramics at1000 �C.40 In this work, the authors pointed out that an increaseof the ceramic yield from 20% to 48% could be obtained, whenthe initially received poly- or oligosilazanes were treated withammonia and TBAF as a catalyst in an intermediate step. Byusing molecular model compounds they also investigated theformation of Si–N bonds by dehydrocoupling of Si–H and N–Hfunctions in the presence of this nucleophilic catalyst, but theydid not give any details about the reaction pathway and thestructural changes of the resulting SiCN polymers. Corriu et al.only described that the polymer product was unmeltable andinsoluble and hence not suitable for the processing of bres.40
Later it was also observed that uoride anions activate Si–Hgroups for the nucleophilic attack of N–H functions and soenable the formation of new Si–N bonds. Corriu et al. thereforepostulated a pentacoordinated intermediate, featuring ahypervalent Si atom.41–43
In all these studies the main focus of attention was onincreasing the ceramic yield in the nal product, but investi-gations to tune the physico-chemical properties of preceramicpolymers for special applications by special chemical modi-cations are rather rare. Therefore the purpose of the presentwork was to develop a controlled and selective cross-linkingreaction via the N–H and Si–H groups of commercially availableliquid organosilazanes with TBAF as a catalyst. By using calciumborohydride to terminate the reaction, tailored meltable solidpolysilazanes (ML33S and HTTS) were synthesized, which arechemically and thermally stable44–47 for the processing ofceramic SiCN bres via melt spinning. The resulting polymerssustain the processing temperature during the melt spinningprocess, have viscoelasticity for the formation of exible bres,and also suitable functional groups for subsequent curing totransfer the meltable bres into a thermoset and to allowpyrolysis to produce amorphous ceramic bres.
2 Experimental2.1 Precursors
All organosilazanes used in this work are sensitive to moistureand air. For that reason, the handling and investigations were
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Fig. 1 Chemical structures of liquid oligosilazanes ML33 and HTT1800.
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carried out in an inert gas atmosphere. Both educts, the liquidorganosilazanes HTT1800 and ML33, were purchased fromClariant GmbH (Germany). HTT1800 is synthesized by coam-monolysis of dichloromethylvinylsilane (H2C]CHSi(CH3)Cl2)and dichloromethylsilane (CH3SiHCl2) while ML33 is a coam-monolysis product of dichlorodimethylsilane ((CH3)2SiCl2) anddichloromethylsilane. The simplied chemical structures ofthese oligosilazanes are presented in Fig. 1.
2.2 Chemical cross-linking and characterization of thepolymers
Chemical cross-linking of the oligosilazanes ML33 andHTT1800 (Fig. 1), was performed using tetra-n-butylammo-niumuoride (TBAF, [CH3(CH2)3]4NF) as the catalyst, whilecalcium borohydride bis(tetrahydrofuran) (Ca(BH4)2$2THF) wasused as an inhibitor to stop the reaction. TBAF solution (1 M inTHF) and Ca(BH4)2$2THF were purchased from Sigma Aldrich.Preparation and handling of all chemicals were carried out inan atmosphere of dry argon, and dried (Na, benzophenone)solvents (THF, pentane). In a typical reaction 50 g of the cor-responding oligosilazane (HTT1800 or ML33) was dissolved inTHF (500 ml) and the corresponding amount of the catalyst,which varied from 0.15 to 1.5 wt%, was added drop wise to thevigorously stirred reaction mixture. Evolution of hydrogen gas(H2) started immediately and the mixture was stirred for a xedtime, which varied from 15 to 120 min, depending on theprepared batch. To stop the reaction Ca(BH4)2$2THF (1.5 moleq. concerning F�) dissolved in THF was added at once. Themixture was allowed to stir for another 5 min. Within this timethe gas evolution stopped completely. Aer ltration of thesolution to remove the residue (CaF2) of the termination reac-tion between the catalyst and the inhibitor, the solvent wasevaporated under vacuum and a colourless solid polysilazanewas obtained. The gases released during the catalytic reactionwere investigated by gas chromatography. For this measure-ment an Agilent 6890 N gas chromatograph was used. TheIR-spectra of the polymers were collected using a Bruker Tensor27 FT-IR spectrometer equipped with an ATR sampling unit.The measurement of the molecular weight of the polysilazaneswas performed with an Agilent 1100 HPLC (Institute of Macro-molecular Chemistry II at the University of Bayreuth). Thecalibration of the columns was done by using narrow poly-dispersed polystyrene standards. Dimethylacetamide (DMAC) +0.05M lithium bromide was used as the eluent because of better
This journal is ª The Royal Society of Chemistry 2013
dilution of the precursors in comparison with tetrahydrofuran(THF).
2.3 Thermal characterization
To investigate the conversion of the synthesized polysilazanesfrom thermoplastic to thermoset during the thermal treatmentand the resulting ceramic yield, thermal gravimetric analysis(TGA) was performed, using a Linseis L81 A1550 unit (Linseis,Germany). Samples of approximately 10 mg were heated from25 to 1400 �C with a heating rate of 5 K min�1 in a nitrogenatmosphere, the conditions being similar to those used duringthe pyrolysis of the cured bres to the ceramic SiCN bres. Tocharacterize the gases released during the pyrolysis of theprecursor, the TGA unit was coupled to an FT-IR spectrometerand samples of similar quantity as mentioned before were alsopyrolysed from 25 to 1400 �C with a heating rate of 10 K min�1.
The glass transition temperature Tg and the cross-linkingtemperature Tc were measured by Differential Scanning Calo-rimetry using a DSC Q1000 (TA Instruments, USA) in a nitrogenatmosphere. To condition the precursor prior to the Tgmeasurement, DSC samples of about 5 mg were heated from�90 to 150 �C at a rate of 5 Kmin�1 and then cooled at the samerate. The curves of the second heating were then taken tocharacterize the glass transition temperature Tg of the poly-silazanes due to the fact that glass transition is inuenced bythermal history and the cooling rate.
2.4 Rheological characterization
To investigate the viscoelasticity of the polymers synthesizedaer the catalytical cross-linking with TBAF, dynamic shearexperiments were performed in a nitrogen atmosphere using anMCR 301 rheometer (Anton Paar GmbH, Germany), equippedwith a PTD 200 Peltier temperature control device and parallelplate geometry with a diameter of 25 mm and a nominal gap of1 mm. Samples of about 0.5 g were deposited on the lower plate.It was ensured that the melted material had no bubbles andthere was no excess material outside of both plates aer closingthe device. For each investigation, the precursor was heated upto the desired temperature with a subsequent holding time of5 minutes so that the rearrangement of the polymer moleculesis achieved. The strain amplitude tests were performed for eachpolymer (amplitude range between 0.01 and 100%, angularfrequency: 1 and 10 rad s�1). However, no dependence betweenthe dynamic rheological parameters and the strain amplitudewas observed. A strain amplitude of 1% and a gap of 1 mm werechosen to perform the dynamic rheological measurements.These parameters are typical for rheological measurements ofviscous organic polymers.
2.5 Spinning of bres
Green bres were spun in lab-scale melt spinning equipment.For each spinning test about 8 g of solid polysilazane was lledin the vessel of the spin equipment, evacuated to removebubbles, melted at temperatures between 110 and 130 �C andle at the desired temperature for 15 minutes to ensure aconstant temperature. By using a nitrogen over pressure endless
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bres were melt-spun through a spinneret having 7 holes, eachone with a diameter of 400 mm, and wound to a coil. Diametersbetween 50 and 80 mm for the continuous green bres wereachieved by adjusting the temperature of the melt, and conse-quently its viscosity, and the pull-off speed (from 100 to200 mm s�1). The collected endless bres were cured by usingelectron beam irradiation at BGS GmbH, Saal, at an accelerationvoltage of 10 MeV and a 600 kGy dose, based on previouswork,48,49 and subsequently converted into amorphous ceramicSiCN bres by continuous pyrolysis (bres under tension) in anitrogen atmosphere at 1100 �C.
Fig. 3 Gas chromatogram of the gaseous product from the reaction (a) ML33 +0.5 wt% TBAF and (b) HTT1800 + 0.5 wt% TBAF (in THF).
3 Results and discussion
Bearing in mind that both precursors ML33 and HTT1800contain N–H and Si–H groups, TBAF was employed as thecatalyst for cross-linking reactions involving just these func-tional groups, which are less reactive in comparison to Si–vinylgroups. TBAF catalysed reactions allow the formation of new Si–N bonds with the release of gaseous hydrogen (see Fig. 2a) andincrease the molecular weight of the precursor due to inter-molecular condensation of the polysilazane rings and chains.Other F� sources, like Me4NF or KF, were also tested, but due totheir low solubility or insolubility in organic solvents no reac-tion occurred. In contrast TBAF is highly soluble in THF andtherefore suitable for homogeneous catalysis.
Using TBAF as a catalyst to increase the molecular weight ofoligosilazanes has another important advantage in comparisonwith catalysts such as DCP and Pt. Additionally, TBAF offers thepossibility to tune the molecular weights, as termination reac-tions become available. In this step Ca2+ ions (which are addedvia the soluble complex calcium borohydride (Ca(BH4)2$2THF)as an inhibitor) capture the catalytically active F� ions and stopthe cross-linking process leading to the formation of insolubleCaF2, which precipitates from THF (Fig. 2b).
To investigate the TBAF catalysed reaction in detail at rstthe evolved gas was characterized by gas chromatography (GC).Fig. 3 displays the gas chromatogram which conrms theevolution of hydrogen. As a reaction via radicals seems unlikely,
Fig. 2 (a) TBAF catalysed cross-linking reaction of polysilazanes. (b) Terminationof the reaction using Ca(BH4)2$2THF as the inhibitor.
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a heterogenic formation of H2, from a proton of the N–H and ahydride of the Si–H function, can be adopted (see Fig. 2a).
Further information about the reaction mechanism isprovided by FT-IRmeasurements. Fig. 4 shows the FT-IR spectraof both starting oligosilazanes (ML33 and HTT1800) and cross-linked polysilazanes (ML33S and HTTS; reacted with 0.25 wt%TBAF or 1.50 wt% TBAF for 90 minutes). As expected the pres-ence of the structural elements (see Fig. 1) can be approved ingeneral agreement with previously published results.33,50–54 Thespectra of both starting liquid oligosilazanes (ML33 andHTT1800) and cross-linked polysilazanes (ML33S and HTTS)exhibit bands at �3380 and �1160 cm�1 from the N–Hstretching (ns(NH)) and N–H (ds(NH)) bonding mode. Thesymmetric C–H stretching at �2950 cm�1 (ns(CH3)) and thesymmetric C–H deformation band at �1250 cm�1 (ds(CH3)) areused as the internal standard when comparing starting andcross-linked silazanes, as the CH3 groups should remainunchanged during the catalytic reactions with TBAF (Fig. 4). Thestrong band at �2110 cm�1 can be assigned to the symmetricSi–H stretching (ns(SiH), Fig. 4).
The differences between the FT-IR spectra are important tocharacterize the reactions during cross-linking. For ML33S thebands concerning the N–H groups (ns(NH)) and ds(NH), at
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Table 1 GPC data for ML33, HTT1800 and cross-linked samples (various reactiontimes and catalyst concentrations)
PrecursorCatalyst(wt%)
Time(minutes)
Mw
(g mol�1) Mw/Mn State
ML33 0 0 2495 2.6 LiquidML33S_1 0.15 90 5636 3.5 Liquid to solidML33S_2 0.25 90 9937 4.3 Meltable solidML33S_3 0.5 15 5728 3.1 Liquid to solidML33S_4 0.5 30 6052 3.2 Liquid to solidML33S_5 0.5 60 7778 4.0 Meltable solidML33S_6 0.5 90 14 680 7.0 Meltable solid
HTT1800 0 0 4836 6.5 LiquidHTTS_1 0.15 90 6584 9.0 Liquid to solidHTTS_2 0.25 90 8010 9.4 Meltable solidHTTS_3 0.5 15 5128 7.8 Liquid to solidHTTS_4 0.5 30 5636 7.5 Meltable solidHTTS_5 0.5 60 6093 7.7 Meltable solidHTTS_6 0.5 90 10 470 11.3 Meltable solid
Fig. 5 GPC curves of (a) ML33 or (b) HTT1800 and their respective catalyticallycross-linked polysilazanes, depending on the catalyst content (constant reactiontime: 90 min).
Fig. 4 FT-IR spectra of (a) ML33 and ML33S or (b) HTT1800 and HTTS poly-silazanes (constant reaction time: 90 minutes).
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�3380 and �1160 cm�1 became signicantly smaller (dottedcurve, Fig. 4), a trend which is also observed for the Si–H band(ns(SiH)). This is an indication that the number of N–H and Si–Hfunctionalities decreases during the TBAF catalysed dehydro-genation reaction, which is in agreement with the postulatedmechanism (Fig. 2a).
Finally the progression of the molecular weights (Mw)depending on the reaction time and catalyst concentration wasmonitored using GPC measurements. If the cross-linking reac-tion occurs as postulated (Fig. 2a) an increase of the molecularweight must be detectable. At rst, reaction times longer than2 h and catalyst concentrations higher than 1.5 wt% led to non-soluble and especially non-fusible polymers in both the ML33Sand HTTS systems. In the next step the reaction time waslimited between 15 and 90 minutes before adding the inhibitorand the catalyst concentration was regulated between 0.15 and0.5 wt%. Depending on these parameters, viscous liquid tomeltable solid polysilazanes were successfully synthesized. TheGPC data for the reaction of ML33 and HTT1800 at a constantreaction time of 90 minutes but three different catalystconcentrations of 0.15, 0.25 or 0.50 wt% are displayed in Fig. 5.All collected data for both polysilazanes (ML33S and HTTS) aregiven in Table 1.
As expected, an increase of reaction time as well as catalystconcentration led to higher molecular weights (Mw) for bothpolymer types (Fig. 5 and Table 1) due to the condensation ofthe silazane molecules by the formation of new mainly inter-molecular Si–N bonds during the TBAF catalysed reaction. The
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GPC data provide another piece of evidence for the abovementioned reaction mechanism.
At this point it is important to stress that only the possibilityof a termination reaction, using Ca2+ ions, enabled the tuningand optimisation of the Mw values to obtain meltable polymersand to tailor the melting behaviour and the melt viscosity. Onlythe combination of both reactions leads to suitable polymers forthe melt spinning process.
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Of the synthesized meltable solid polysilazanes withdifferent molecular weights, only the two polymers with thehighest molecular weight for each type of precursor, ML33S andHTTS, were selected (marked in italics in Table 1) for the pro-cessing of green bres and later ceramic SiCN bres. The reasonfor this choice is the experience that polymers with highermolecular weights possess better viscoelasticity and allow theprocessing of mechanically more stable green bres. Thereforethe ML33S_2 precursor charge (contains no vinyl groups) wastreated for 5 hours at 220 �C under vacuum to remove oligomerswith low molecular weights (Fig. 6). The resulting precursorcharge ML33S_2t possesses a higher molecular weight(Mw: 24 130 g mol�1 andMw/Mn: 9.3) and is still meltable for theprocessing of ceramic bres. But additional cross-linking at atemperature of 220 �C by condensation of NH3 which also leadsto higher molecular weights cannot be excluded. However, evenhigher temperatures result in increased thermal cross-linking,which leads to the reduction of the viscoelastic properties and,consequently, the spinnability of the polysilazane. Moredetailed physical and thermal properties of the selected poly-silazanes (ML33S_2, ML33S_2t, ML33S_6, HTTS_2 andHTTS_6), such as glass temperature, ceramic yields and gasesreleased during the conversion of the polymer to the amor-phous ceramic, were collected by TGA coupled with FTIR(Fig. 6–8) and DSC investigations (Fig. 9).
The TGA curves displayed in Fig. 6 and 7 indicate that aerthe cross-linking of the ML33 and HTT1800 oligosilazanes withTBAF the ceramic yield increases signicantly, especially in thecase of ML33. The ceramic yield from ML33S attained about74 wt% in comparison with just 29 wt% for ML33 (see Fig. 6).
Fig. 7 TGA for HTT1800 and meltable HTTS polysilazanes.
Fig. 6 TGA for ML33 and meltable ML33S polysilazanes.
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The cross-linking reaction with TBAF and subsequent termi-nation with the inhibitor (Ca(BH4)2$2THF) not only increase theceramic yield (as for other catalysts such as DCP and Pt also),but also allow controlling of the properties of the resultingpolymers, in particular meltability (see Fig. 9) and viscoelasticity(see Fig. 10 and 11), which are important properties for brespinning.
Pyrolysis of all precursors consists of three well denedsteps. In the rst step up to about 400 �C the thermoplasticpolymer is transformed to a thermoset. Because these poly-silazanes are a mixture of oligomers and polymers, they suffercross-linking withmass loss, mainly caused by evolution of non-cross-linked volatile oligomers and ammonia (see Fig. 8).
Depending on the functional groups on the polysilazanechains, four different reactions can happen during a thermaltreatment, leading to a cross-linked material: cross-linking viavinyl polymerization, hydrosilylation between Si–H and Si-vinylgroups, dehydrocoupling between Si–H and N–H groups andtransamination.28,55,56
In ML33S and HTTS transamination (see Fig 8) and dehy-drocoupling lead to mass loss. Further dehydrocoupling is moreprobably a consequence of thermal cross-linking reactions,because TBAF is completely terminated by the inhibitor. Testswere also performed by cross-linking ML33 and HTT1800 withTBAF without addition of the inhibitor. Although the polymersare slightly meltable aer synthesis, if the temperature is raisedto more than 100 �C, the viscosity increases rapidly due to thepresence of the catalyst, leading nally to a thermoset.
Looking at the chemical structures, it can be expected thatML33S is less reactive at higher temperatures than HTTS, due tothe lack of vinyl groups. Therefore at about 200 �C HTTS_2 andHTTS_6 have a lower mass loss than ML33S_2 and ML33S_6because of the additional cross-linking via vinyl groups (seeFig. 1). ML33S_2t shows no signicant mass loss up to about300 �C due to the previous removal of volatile oligomers. Fig. 6and 7 show also that the differences in ceramic yields strongly
Fig. 8 TGA coupled with FTIR of released gases during the pyrolysis of ML33Sand HTTS polysilazanes.
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Fig. 10 Rheological behaviour of polysilazanes at 110 �C depending on theoscillatory frequency.
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depend on the loss of volatile oligomers at temperaturesbetween 200 and 400 �C.
Between 400 and 700 �C the largest mass loss occurs for bothtypes of precursors, when the polymers change to amorphousSiCN ceramic characterised by degradation of organic substit-uents and release of mainly CH4, other volatile hydrocarbons,volatile Si containing decomposition products, H2 and smallquantities of NH3 (Fig. 8). At temperatures higher than 700 �Cthe mass loss is almost complete.
Since the spinnability of green bres depends on the thermalstability of the precursor,57 DSC measurements were performedto obtain more details about the glass temperature of eachpolymer at temperatures lower than 300 �C. Between 60 and85 �C the DSC curves (Fig. 9) indicate a shi upward in the heatow for ML33S_2 (Tg ¼ 65 �C), HTTS_2 (Tg ¼ 73 �C) andML33S_2t (Tg ¼ 81 �C), representing the glass transition Tg,typical for amorphous polymers, when the polymer soens intoa viscous liquid.
As expected, the glass temperature of ML33S_2t is higherthan for ML33S_2, which is consistent with its higher molecularweight. It is also noticeable that the molecular weight distri-bution values of the polysilazanes approach those of fourdifferent types of commercially available silicone resins (poly-siloxane) investigated by Takahashi et al.58 In a similar way, thechemical structure of the polysilazanes is non-linear andcomposed of a mixture of oligomers, branched and cyclicpolymers.56
Due to the complex structure of the polysilazanes in thiswork the DSC curves show weaker shis (representing the Tg) incomparison with amorphous organic polymers. The higher themolecular weight and polydispersity of the polymer the higheris the degree of cross-linking in the polymer and the shiupward in the DSC tends to disappear. Although ML33S_6 andHTTS_6 are also meltable when heated up to 200 �C no clearglass transition is observed in the DSC measurement.
Investigations by Duguet et al.25 complement the discussionabove. The authors synthesized linear polysilazanes by cationic/anionic ring-opening polymerization. All polymer samplespresented clear DSC signals of glass and also melting transitiontemperature. These results support the assumption that areduced number of branched and cyclic molecules (oligomersand polymers) in the polysilazane in comparison with the linear
Fig. 9 DSC heating traces of ML33S and HTTS polysilazanes.
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species leads to a thermal transition signal more similar tolinear organic polymers.
Furthermore the rheology of the studied polysilazanes wasmeasured to analyse their viscoelasticity, which is very impor-tant for a stable melt-spinning process. Based on oscillatoryrheological measurements, the frequency dependence of thestorage modulus (G0), loss modulus (G0 0) and complex viscosity(|h*|) at 110 �C have been plotted for ML33S_2, ML33S_2t andHTTS_2 as indicated in Fig. 10.
As stated by Duperrier et al.,45–47 the dynamic moduli G00 andG0 are closely correlated to the polymer spinnability, because themelt spinning process requires a polymer with an appropriateviscosity-to-elasticity ratio (tan d > 1) to allow the ow of thepolymer melt through the spinneret and stretching of the breat a constant pull out velocity without breaking the laments.
According to the Cox–Merz rule59 the shear rate dependenceof the steady shear viscosity h (for example the extensionalviscosity in a capillary) and the frequency dependence of thecomplex viscosity |h*| for the polysilazanes in this work areassumed here to be equivalent. In this case, the oscillatoryrheological measurements can be used to predict the spinn-ability of the precursor.
The curves shown in Fig. 10 indicate that the polysilazanesML33S_2, ML33S_2t and HTTS_2 exhibit similar rheologicalbehaviour and resemble the typical behaviour of amorphouspolymers with low molecular weight. The viscosity dominatesthe most frequency range (G00 > G0), having a terminal zone atlow frequencies and a direct transition to the glassy zone. Athigh frequencies the value of the storage modulus G0 exceedsthat of the loss modulus G0 0, showing a dominant elasticity(particularly observed for ML33S_2t at frequencies higher than100 s�1).
In Fig. 10, shear thinning of the complex viscosity occurswith increasing frequencies, being more noticeable inML33S_2t. Considering that the rheological curves are obtainedat 110 �C for the three polysilazanes, it is expected that highermolecular weights lead to higher complex viscosity. Choosing aspecic value for the complex viscosity (which can be connectedto melt-spinning tests of the polymers) will represent a differentprocessing temperature to be used during the melt-spinningprocess, depending on the respective precursor.
During the processing of bres, the thermal stability of thepolymer is a critical property. Thus, a temperature sweep test
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was performed by means of oscillatory shear at a frequency of10 rad s�1 and an amplitude of 1%. As can be seen in Fig. 11, ifthe temperature increases, the complex viscosity of ML33Sreduces and the loss modulus G0 0 is dominant over the storagemodulus G0 (observed by a loss tangent >10�) during the
Fig. 11 Rheological behaviour of polysilazanes at a frequency of 10 rad s�1
depending on the temperature.
Fig. 13 SEM micrographs of the surface and cross-section of amorphous ceramic
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complete temperature range tested. The rheological behaviourof HTTS is close to that of ML33S up to about 170 �C, where thecurve of the complex viscosity passes through a minimum andincreases again to a value of about 104 Pa s. At about 170 �C thestorage modulus G0 crosses the loss modulus G0 0 in the gel pointand the elastic part becomes dominant (observed by a loss
Fig. 12 (a) Melt spinning equipment and (b) melt spun green fibres/pyrolysedfibres.
SiCN fibres from (a) ML33S_2t, (b) HTTS_2 and (c) ML33S_6.
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tangent <10�), makingmelt spinning of the polymer impossible.This behaviour is another piece of evidence for the thermalinduced cross-linking via the vinyl groups and limits thethermal stability of the HTTS precursor types.
Based on the molecular, thermal and rheological analysis ofthe polysilazanes produced by controlled cross-linking,continuous green bres were processed via melt spinning. Forthe processing of the green bres, a complex viscosity of about5 � 103 Pa s was chosen to ensure a continuous viscoelasticowability of the meltable polymer (by using the parametersmentioned in the experimental procedure for the lab scale meltspinning equipment), which implies processing temperaturesof about 50 K higher than their glass temperatures, and thediameter of the green bres was further controlled by the pull-off speed. In the case of ML33S_6 and HTTS_6 the temperaturewas gradually increased from 100 �C as the glass temperaturewas not found in the DSC measurements to a processingtemperature which allowed the spinning of bres.
Innite green bres with homogeneous surface were spunfrom melts of ML33S_2, ML33S_2t and HTTS_2. Aer curing bythe electron beam irradiation, the colourless polymeric bresare pyrolysed in a nitrogen atmosphere at 1100 �C resulting inblack amorphous ceramic SiCN bres. This transition from thepolymer to the ceramic can be noticed in the black coil inFig. 12. Aer pyrolysis ceramic SiCN bres with a smoothsurface and constant diameter were manufactured fromML33S_2, ML33S_2t (Fig. 13a) and HTTS_2 (Fig. 13b). Further-more the SEM micrographs of their cross-section conrm theirdense core without noticeable defects or pores.
In contrast, green bres made from ML33S_6 have anirregular diameter. Its spinnability is poor and the cross-sectionof the ceramic amorphous bre present small pores and defects(Fig. 13c). The spinnability of HTTS_6 was nearly impossibleand no bres could be drawn. Even if the polymer is a meltablesolid, the material sticks in the spinneret during the meltspinning process and if the temperature is raised, almost nochange is observed in its owability. For temperatures higherthan 180 �C, the precursor cross-links and blocks the holes ofthe spinneret. These results indicate that the synthesizedpolysilazanes ML33S_2, ML33S_2t and HTTS_2 are promisingcandidates for the processing of non-oxide ceramic SiCN bresat lower costs than the current commercially available products.
4 Conclusions
The developed selective cross-linking reaction of the commer-cially available oligosilazanes ML33 and HTT1800 using tetra-n-butylammoniumuoride (TBAF) as a catalyst and calciumborohydride as the inhibitor and the dehydrocoupling offunctional Si–H and N–H groups led to the solid and meltablepolysilazanes ML33S and HTTS. Depending on the catalystconcentration and reaction time before termination poly-silazanes were synthesized with tailored chemical and physicalproperties, such as viscoelasticity, dened glass temperatureand thermal stability especially regarding the melt spinningprocess. As expected the polymer batches with high molecularweights (�10 000 g mol�1), a dened glass transition
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temperature (Tg) between 65 and 81 �C and a viscoelastic meltbehaviour (ML33S_2, ML33S_2t, HTTS_2) enabled the spinningof green bres with a regular shape and less defects. Whereasthe ML33S batches possess temperature stability up to 250 �C,further thermal cross-linking of the HTTS batches due to poly-merization via the vinyl groups started already at 180 �C.Therefore melt spinning of this type of precursor is onlypossible at considerably lower temperatures. Curing of allpolymer bre types was possible with an electron beam doseless than 600 kGy. The quality of the resulting pyrolysed ceramicSiCN bres depends on the used green bres. This means thatthe selective cross-linking is the crucial step for the processingof ceramic SiCN bres from commercially availableoligosilazanes.
The processing of thick ceramic bres with diametersaround 100 mm is a very unique challenge because brittleness ofthe bres depends on the diameter. Especially for this appli-cation the newly developed catalytic process offers high poten-tial for the production of thick ceramic SiCN bres on the basisof an optimized polymer.
Acknowledgements
The authors thank Marietta Bohm from the Institute ofMacromolecular Chemistry II, University of Bayreuth, forthe GPC measurements and Ute Kuhn from the PolymerEngineering Institute, University of Bayreuth, for the DSCmeasurements. We also thank Deutsche For-schungsgemeinscha (DFG) for supporting this work within theproject SiMet.
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