29Si MAS NMR investigation of the pyrolysis process of cross-linked polysiloxanes prepared from polymethylhydrosiloxane

Rafik Kalfat," Florence Babonneau,b NCji Gharbi*" and HCdi Zarrouk" "Laboratoire Chimie des Matiriaux, Dipartement de Chimie, Faculti des Sciences, 1060 Tunis, Tunisia bChimie de la Mutiire Condensie, U R A C N R S 1466, Uniuersiti Pierre et Marie Curie, 4 Place Jussieu, 75252 Puris, France

Polymethylhydrosiloxanes ( PMHSs) crosslinked with hexa-1,5-diene or hexane- 1,6-diol have been considered as potential precursors for Si- C- 0 systems. Their pyrolysis under an argon flow has been investigated mainly by 29Si MAS NMR spectroscopy. The two systems, with similar C/Si but different O/Si ratios, show different pyrolytic behaviour. This is essentially due to different graftings between the siloxane backbone and the alkyl chains, Si-C us. Si-0-C bonds. In both cases, degradation of the organic chains occurred at 600 "C; but in the case of PMHS cross-linked with hexanediol (D06) , all the C groups are lost, while for PMHS cross-linked with hexadiene (DE6), the C groups bonded to Si remain. As a consequence, the oxycarbide phase obtained at 1000 "C is much richer in C for DE6, compared to D06, but the amount of free C is also higher. These differences in composition strongly influence the nature of the samples obtained at 1500 "C: crystalline silicon carbide for DE6 and mixture of amorphous silica and silicon carbide for D06.

Silicon oxycarbide glasses have received increasing attention in the last ten years due to their high-temperature stability, good crystallization and oxidation resistance, and high mechanical strength.Ip4 Introduction of carbon into a silica network by direct solid-state reactions5 is very difficult, but it is quite easy by pyrolysing appropriate polysil~xanes.~ The use of polymers has also allowed the development of shaped objects such as silicon oxycarbide fibres7p9 and composites.lO,"

Several synthetic strategies have been followed to prepare the starting polymers: hydrolysis of chloro- or alkoxy-sil- anes,12-18 catalytic redistribution of oligo- and poly-sil~xanes,~~ and modification of polycarbosilanes.20 They have generated a variety of polysiloxane architectures containing Si- R groups (R=CH,, C6H5, CH2=CH, etc.), and also Si-H21-23 and Si- Si Commercial silicone resins have also been used.26 Pyrolysis of these polymeric precursors around 900-1000°C leads to a silicon oxycarbide network based on mixed SiC,04 - , units with 0 < x < 4, as demonstrated clearly in most of the studies by 29Si MAS NMR spectroscopy. Usually a free carbon phase is also present, from the decompo- sition of the organic moieties. The composition of the oxycar- bide phase as well as the free carbon content is clearly dependent on the nature of the R g r o ~ p , ~ , ~ ' , ~ ~ and on the O/Si and C/Si molar ratios21,29,30 in the starting precursors. The O/Si molar ratios mainly dictate the composition of the oxycarbide phase, and the C/Si ratio determines the amount of free carbon. Pyrolysis at T > 1500 "C will lead to a possible transformation of the silicon oxycarbide phase into silicon carbide, through carbothermal reactions depending on the amount of free carbon. This route to S ic has been explored by several

All these studies show that it is possible to tailor the architecture of the starting polysiloxane to influence its poly- mer-to-ceramic transformation and thus the structure of the final materials. Following this idea, polymethylhydrosiloxane (PMHS), has been cross-linked via hydrosilylation reactions with bifunctional hydrocarbon chains of various lengths, to produce polysiloxanes with various C and 0 content^.^' PMHSs reacted with hexa-1,Sdiene and hexane-1,6-diol have been pyrolysed to 1500°C: X-ray diffraction results show a crystalline silicon carbide phase in the case of the diene, and a mixture of amorphous silica and silicon carbide in the case

of the d i ~ l . , ~ The objective of this paper is to present a MAS NMR spectroscopic characterization of the pyrolysed products obtained from the two different cross-linked PMHSs in order to understand how they transform with temperature into two different inorganic structures.

Experimental Samples were prepared by mixing precursors in stoichiometric proportions as shown in Table 1. Hexachloroplatinic acid was used as a catalyst [4 x

The pyrolyses were performed under an argon flow in a tubular furnace with a heating rate of 5°C min-' and a holding time of 5 h at the maximum temperature.

Solid-state NMR studies were carried out on an MSL 400 Bruker spectrometer using the MAS technique (spinning rate: 4 kHz). 29Si MAS NMR spectra were obtained at 79.5 MHz with a 50 kHz spectral width, a pulse width of 2 ps (0 30") and a relaxation delay of 60 s. Short flip angles along with relatively long recycle delays were used to try to overcome the problem of long 29Si relaxation times. The 13C C P MAS NMR experiments (100.62 MHz) were recorded with a contact time of 1 ms, a recycle delay of 6 s and a spectral width of 50 kHz. Tetramethylsilane was used as the external reference for all MAS NMR spectra. Peaks were labelled using the X, M,, D, and T, notation. X, M, D, T and Q refer to SiC,-,O, units with x=O, 1, 2, 3 and 4, respectively, where n is the number of bridging 0 atoms between two neighbouring Si. The simu- lations of the spectra were produced with the WIN-FIT program.37 Thermogravimetry (TG) studies were carried out with a Netzsch STA 409 instrument under an argon flow with a heating rate of 10°C min-l. The chemical analysis was performed by the 'Service Central d'Analyses du CNRS', Vernaison, France, for Si, C and H. Oxygen was estimated by difference.

mol (g PMHS)-l, in THF].

Results Characterization of the precursors

DE6 and DO6 samples were obtained by coupling PMHS with hexa- 1,5-diene and hexane- 1,6-diol, respectively, through hydrosilylation reactions. The schematic structures of these

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Table 1 Experimental conditions and nature of obtained products

reagent molar ratio

(reagent)/(Si-H) reaction time/min

nature of final product dried gels at 100°C

hexa- 1,5-diene 0 5 15 transparent gel white powder, DE6 hexane- 1,6-d1ol 0 5 90 transparent gel white powder, DO6

compounds are shown in Fig 1 The structural characterization of such samples has already been published36 and the NMR results will be summarized here for clarity

The 29S1 MAS NMR spectrum of DE6 [Fig 2(u)] shows a main peak at 6 -22 representing 290% of the total Si sites and due to D, clearly indicating as expected the replacement of Si-H bonds by Si-C bonds The peak at 6 6 6 (cu 5%) is due to M endgroups of the PMHS chains Two other small peaks are present at 6 -40 due to residual D2H sites from PMHS,39 and at 6 -57 due to T, sites, MeSi(O,,),(OH), related to hydrolysis of the DZH sites

The 13C C P MAS NMR spectrum of DE6 [Fig 3(u)] is dominated by four peaks due to CH3 -Si (6 0), C, ,, (6 18), C, ,, (6 23) and C, a, (6 33) sites Four peaks corresponding to C sp2 sites are present at 6 115, 125, 131 and 139, suggesting the existence of two types of residual vinylic groups those due to hexadiene that have reacted only on one side (6 115 and 139) and those resulting from the formation of unreactive double bonds in an isomerisation process promoted by the Pt

SI I 0

SI I 0

SI

SI I

DE6 SI

? 0 0 I I

CH3-St-O-CH2-CH2-CH2-CH2-CH2-CH2-0 - SI-CH~

0 0 I I SI SI

I a p a a ' p ' a ' I

DO6

Fig. 1 Schematic structures for DE6 and DO6 samples

Fig. 2 "S1 MAS NMR spectra of DE6 (a) and DO6 (b)

1674 J Muter Chem, 1996,6( lo), 1673-1678

1 - 1 150 100 50 0 -50

b

Fig.3 13C CP MAS NMR spectra of DE6 (a) and after pyrolysis at 600°C (b)

catalyst 40 41 All these characterizations show that hydrosilyl- ation occurred to a large extent

The 29S1 spectrum of DO6 [Fig 2(b)] shows the presence of a main peak at 6 -58 corresponding to T, sites4, resulting from the reaction of alcohol functions with PMHS The small peak at 6 8 is due to end M sites No peak due to sites containing Si-H bonds are present It should be noted that, unlike DE6, the IR spectrum of DO6 shows the presence of OH groups36 Some of the T, sites could thus contain Si-OH groups

Solid-state 13C C P MAS NMR of DO6 [Fig 4(u)] shows one peak at 6 -3 6 assigned to the carbon atoms of methyl groups in T units belonging to the siloxane chain" and three peaks at 626, 33 and 62 due to Caa,, C,,, and C,,, carbon atoms of the aliphatic chains, respectively 43 The chemical shift values of C, ,, cannot differentiate assignment to HO-C, ,, or Si-OC, ,, sites

Pyrolysis of DE6 samples

TG analyses were performed up to 1400°C under an argon flow to study the thermal behaviour of the samples (Fig 5 ) DE6 is thermally resistant up to 300°C Further heating to 600°C causes a major mass loss (44%) Indeed, elemental analysis (Table 2) shows that at 600 "C the C/Si ratio has decreased from 4 to 2 This strongly suggests a degradation of the alkyl chains with just two C from the alkyl chains remaining in the pyrolysed sample It can be anticipated that they are the C,,, groups, and this will be confirmed by the following NMR study The other C groups, which have been transformed into volatile species, are responsible for the large mass loss The small mass loss observed between 620 and 800 "C ( 5 % ) should be due essentially to a loss of hydrogen the elemental analysis shows a sharp decrease in the H/Si ratio from 4 5 to 0 5 between 600 and 1000°C

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Fig. 4 13C C P MAS NMR spectra of DO6 (a) and after pyrolysis at 600°C (b)

10

-50

( b ) *-I --LLUX(-.XI-CLL -70

0 200 400 600 800 1000 1200 1400 TI'C

Fig. 5 TG traces of DE6 (a) and DO6 (b)

Table 2 Chemical analyses (mass%) of pyrolysed DE6 at different temperatures

molar T/"C Si (YO) C (YO) H (YO) 0 (YO.)" composition

25 26.49 48.91 8.88 15.72 SiC4.32H9.4001.04 600 36.51 30.90 5.89 26.7 SiC1.98H4.5301.28

1000 40.75 26.48 0.80 31.97 SiCl,52Ho.5501,38 1200 39.26 26.50 <0.20 34.24 SiCl.5701.53 1400 40.15 25.56 <0.20 34.29 SiC1.4801.51 1500 67.76 27.76 <0.20 4.28 SiCo.9600.11

"Oxygen content determined by difference, 0 (YO) = 100 - [ Si (%) + C (Yo)+H (Yo)].

The 29Si MAS NMR spectra were recorded on the samples pyrolysed up to 1500°C [Fig. 6(u)]. The 13C MAS NMR spectra were recorded using the cross-polarization technique only on the 600 "C sample, which contains appreciable amounts of protonated C sites (Fig. 3).

The I3C MAS NMR spectrum of the 600°C sample [Fig. 3(b)] is quite different to that of the starting DE6 sample [Fig. 3(u)]. In the aliphatic region, the spectrum is now domi- nated by one broader peak at 6 1 due to CH, groups bonded to Si. The peaks due the C atoms of the alkyl chains have disappeared in agreement with the elemental analysis and TG

1500 "C

1400°C

I200 "C

MK) "C J1, 600°C .-n

Fig. 6 29Si MAS NMR spectra of DE6 (a) and DO6 (b) pyrolysed at different temperatures

analysis, which suggests decomposition of these chains. The increase in the linewidth of the signal due to the siloxane sites can be related to a decrease in mobility of the siloxane chains in a network that becomes more rigid. In the C sp2 region, the sharp peaks due to residual vinyl groups have disappeared, but now a broad peak is present centred at 6 140 associated with spinning sidebands, which is certainly characteristic of aromatic carbons present in a free carbon phase.16 This phase is related to the decomposition of the organic moieties.

The peaks due to D (6 -22) and M (6 6) units are still present in the 29Si MAS NMR spectrum of the 600°C sample [Fig. 6(u)], showing clearly a retention of the siloxane back- bone. The linewidths have increased slightly, certainly because of an increasing rigidity of the network, responsible for a larger distribution of sites. The peak due to D2H units at 6 -39 has disappeared completely. Consumption of these units could be due to condensation reactions in the presence of traces of H20, such as physisorbed water. This reaction proceeds rapidly at moderate temperature (around 290 "C). Thus intra- or inter- molecular cross-linking of PMHS can occur according to the following successive reactions as reported by Hetem et

=Si-H + H 2 0 + SSi-OH + H2r = Si- OH + H- SiE + ESi- 0- Sif + H2

Small peaks are observed at 6 - 57 and - 66 due to T, and T, units respectively. The presence of these units may be due to hydrolysis of D,H units, eventually followed by condensation, according to the above equations. It could also be due to redistribution reactions involving Si-C and Si- 0 bonds that are known to take place at these temperatures in similar system^.^^.^^

The 29Si NMR spectrum of DE6 pyrolysed at 1000°C [Fig. 6(a)] shows a great change in the structure of the material. Broad peaks are observed in the spectrum, indicating an increase in disorder, associated with the polymer-to-ceramic transformation. The proportion of T units (29%) characterized by the peak at 6 - 70 is greatly increased. A new peak appeared at 6 - 110 (23%) due to Q units. The region between 6 20 and - 50 exhibits several broad components which could be simu- lated with three peaks at 6 7, - 13 and - 32 corresponding to M (4%), X (16%) and D (28%) units, respectively (Fig. 7). The redistribution reactions mentioned for the 600 "C sample have clearly proceeded to a larger extent at 1000°C. The spectrum at this temperature is characteristic of an oxycarbide network with a complete distribution of SiC,04-, units.

-

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A

0

- 0 ?? x 0 200 400 600 800 1000120014001600

5 5

P 1 ( a ) -1

1 I ' ' I I -----I I I 40 20 0 -20 -40 -60 -80 -100 -120 -140 6

Fig. 7 Simulation of the 29S1 MAS NMR spectrum of DE6 pyrolysed at 1OOO"C

Between 1000 and 1400 "C, the proportions of mixed SiC,O,-, units (O<x<4) decrease strongly in favour of SiO, and SIC, units Indeed, at 1400°C the material is mainly composed of SIC, (6 - 15) and SiO, (6 - 110) units, with respective percent- ages of 35% and 44% Such structural rearrangement of silicon oxycarbide at high temperature leading to a phase separation between silica and silicon carbide-rich phases has already been reported 23 34 In contrast, the spectrum at 1500 "C shows only one sharp peak at 6 - 18 corresponding to silicon carbide This result is in perfect agreement with the X-ray diffraction data which show only peaks due to crystalline S ic phases, mainly p with some traces of a (Fig 8)

All the 29S1 MAS NMR spectra have been simulated to extract the percentages of the various Si sites From these results, it is possible to evaluate the number of Si-0 and Si-C bonds per Si as has already been done in several papers (Fig 9) l7 The number of Si-0 and Si-C bonds per Si do not vary greatly until 1400 "C, showing that in this temperature range the changes in Si sites shown in the spectra are related mainly to redistribution reactions between S i r 0 and Si- C bonds Such structural rearrangements known for siloxanes in the 200-600°C range have already been found at higher temperatures for silicon oxycarbide phases 34 The slight increase in the number of S i r 0 bonds between 600 and 1000°C is certainly caused in this temperature range by the polymer-to-ceramic transformation which might occur with some Si-C bond cleavages An interesting feature is the total disappearance of Si-0 bonds at 1500 "C, due to carbothermal reduction of those bonds by the free carbon phase, the forma- tion of which was predicted from the I3C C P MAS NMR spectrum of the 600°C sample An evaluation of the free carbon content can be determined by a comparison of the

10 20 30 40 50 60 70 80 90 2Bldegrees

Fig.8 X-Ray diffraction patterns of (a) DE6 and (b) DO6 pyrolysed at 1500 "C under argon

I I 0

' t 0 200 400 600 800 loo0 1200 1400 1600

T/"C

Fig. 9 Variation of Si-X/Si ratios for DE6 (a) and DO6 (b) as a function of temperature

NMR and chemical analysis results assuming that the C atoms in the oxycarbide phase are bonded to 4 Si atoms Around 70-75% of the total C content is in a free C phase in the samples pyrolysed from 1000 to 1400"C, and this C reacts completely at 1500°C with the oxycarbide phase to give crystalline silicon carbide

Pyrolysis of DO6 samples

DO6 exhibits a mass loss (175%) at low temperature (180-420°C) that was not observed for DE6 [Fig 5(b)] As mentioned already, the IR spectrum shows the presence of OH groups and water in this sample, and thus this mass loss may be ascribed to the evaporation of adsorbed solvent In a way similar to that observed for DE6, a large mass loss (38%) is observed between 440 and 560°C due to the decomposition of the aliphatic chains This is confirmed by chemical analysis results, which show that only one carbon per silicon remains after pyrolysis at 600°C indeed, it corresponds to a total disappearance of the organic chain in agreement with NMR data (to be presented later) Then the small mass loss between 600 and 840 "C (3 5%) is due essentially to a loss of hydrogen, as confirmed by chemical analysis which shows a sharp decrease of H content from 600 to lOOO"C, from 4 0 to 0 6 mass%

The I3C NMR spectrum of DO6 has changed greatly between room temperature and 600 "C (Fig 4) Only one peak at 6 -3 7 remains in the spectrum after treatment at 600"C, corresponding to CH, - Si groups in T units This agrees with the drastic decrease in C and H contents as revealed by

Table 3 Chemical analyses (mass%) of pyrolysed DO6 at different temperatures

molar T/"C Si (YO) C (YO) H (Yo) 0 (%)o composition

25 2301 4064 882 2753 S1Cq13H10760210 600 4561 1645 404 3390 SiCo84H2490130

1000 4396 1366 063 41 75 SiCo,3H0400,67 1500 4585 1201 t o 2 0 4194 SIC0610161

"Oxygen content determined by difference, 0 (%) = 100- [Si (%) + C (%)+H (Yo)]

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Table 4 Composition of the silicon oxycarbide phases obtained at lo00 "C

sample C/Sl" O/Sl" sio, (%)b sico, (%)b sic,o, (%)b sic30 (%)b SIC, ( %)b

DE6 152 138 23 DO6 0 73 167 44

29 34

28 19

4 3

16 -

"From chemical analysis bFrom 29S1 NMR results

chemical analysis (Table 3) and corresponds to the high mass loss observed in the TG curve Note that no evidence for aromatic carbon at this temperature is present in this spectrum, compared to the spectrum of DE6 pyrolysed at the same temperature [Fig 3(a)]

The 29S1 spectrum at 600°C [Fig 6(b)] shows significant changes in the chemical structure of the starting polymer The peak corresponding to T, sites [H3C-Si(0, 5 ) 2 -OR] observed at 6 -58 in the spectrum before pyrolysis, has decreased strongly and is present only as a shoulder of a main peak at 6 -67 characteristic of T, sites [H,C-Si(O,,),] Almost all of the T, sites have thus been transformed into T3 sites This indicates clearly that a large number of the

Si - 0 - C bonds have been cleaved, leading to the formation of new rSi-O-Si= linkages This is in agreement with the previous results (chemical analysis, TG analysis and I3C NMR spectra) which show a complete degradation of the organic chains the siloxane network has indeed been transformed into a polymethylsilsesquioxane The peaks at 6 -21 and - 110, due to new D, and 4, units, result from redistribution reactions as already mentioned for DE6 The same phenomenon of peak broadening mentioned for DE6 is also observed in the case of DO6 by pyrolysis up to 1000 "C [Fig 6(b)] The number of D (6 -37) and Q (6 - 106) units has increased to the detriment of that of T (6 -70) units Distribution reactions between Si-0 and Si-C bonds are still occurring The composition of the oxycarbide phase is as follows 44% Q units, 35% T units, 19% D units and 3% M units, which is quite close to what was observed previously for silicon oxycarbide phases derived from gels based on T units 30

At 1500°C, the 29S1 NMR spectrum [Fig 6(b)] shows a broad peak at 6 -17 due to SIC, units and an intense one at 6 - 110 due to SiO, units showing that the material is mainly made of a mixture of SiO, (73%) and SIC (22%) phases This result is in agreement with the X-ray diffraction pattern [Fig 8(b)] Some oxycarbide sites are still present, charac- terized by a broad peak around 6 - 35 ( 5 % )

The evolution of the number of Si-C and Si-0 bonds per Si does not show any remarkable changes us pyrolysis tempera- ture, even at T = 1500°C [Fig 9(b)]

Discussion The difference in pyrolytic behaviour of the two cross-linked PMHSs is clearly related to the difference in their composition C/Siz4 for both but O/Siz1 for DE6 and z 2 for D06, as well as in their architecture the grafting of the organic chains is made with Si-C bonds for DE6 and Si-0-C bonds for DO6 At 600"C, degradation of the organic chains has occurred, leading to a complete loss of the C groups in the case of D06, while for DE6 the C groups bonded to the siloxane backbone remain, so that C/Si z 2 for DE6 and z 1 for DO6 This large difference in composition clearly influences the nature of the oxycarbide phases obtained at 1000°C and reported in Table4 Mutin et have already shown for Si -C- 0 precursors with different compositions that the environment of the Si sites in the derived oxycarbide phase can be described as a purely random distribution of Si-0 and Si-C bonds, and thus that the structure depends on the O/Si molar ratio In the present examples, O/Si= 1 38 for DE6 and 1 67 for D06, and indeed the compositions of the oxycar- bide phases show trends very similar to that predicted by

Mutin, much richer in C for DE6 (Coxy/Si = 0 40) than for DO6 (Cox,,/Si = 0 22) Comparison between the 29S1 NMR results and elemental analysis shows the presence of a free carbon phase, Cfree/Si= 1 12 for DE6 and 0 51 for DO6

The high-temperature (1000 < T < 1500) behaviour of the silicon oxycarbide phase can usually be divided into two steps at T < 1500 "C, structural rearrangement involving Si - C and Si-0 bonds occurs, leading to a strong decrease in the mixed Si units, SiC,O, -, (0 < x < 4) A clear phase separation occurs between silica and silicon carbide-rich regions This has been observed in the present samples At T 3 1500 "C, the Sir-C-0 system is known to be unstable46 48 Carbothermal reaction between silica and carbon to produce S ic is well known The extent of such reactions is governed by the amount of C and 0 present in the pyrolysed samples In the case of DE6, this reaction clearly has occurred at 1500°C and leads to the crystallization of silicon carbide, with an almost total consump- tion of Si-0 bonds In contrast, silica is still present in DO6 pyrolysed at 1500"C, due to a much lower amount of free carbon, compared to the 0 content that certainly prevents carbothermal reactions from occurring to a large extent

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Paper 6/022351, Received 1st April, 1996

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