Influence of the polymer architecture on the high temperature behavior of SiCO glasses: A comparison between linear- and cyclic-derived precursors

Journal of Non-Crystalline Solids 356 (2010) 132–140

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Influence of the polymer architecture on the high temperature behavior of SiCO glasses: A comparison between linear- and cyclic-derived precursors P. Dibandjo a,*, S. Diré a, F. Babonneau b, G.D. Soraru a a b

Department of Materials Engineering and Industrial Technology, University of Trento, Via Mesiano 77, 38050 Trento, Italy Laboratoire de Chimie de la Matière Condensée de Paris, Université Pierre et Marie Curie-Paris6, 4 place Jussieu CC 174, Paris cedex 05, France

a r t i c l e

i n f o

Article history: Received 28 October 2008 Received in revised form 2 October 2009

Keywords: Glass ceramics Silicon carbide Powders Organic–inorganic hybrids

a b s t r a c t Two series of cross-linked polysiloxanes, precursors for silicon oxycarbide glasses, have been synthesized from a linear and a cyclic Si–H-containing siloxane having the same chemical formula (SiCOH4). The crosslinking has been achieved by hydrosilylation reaction with various amounts of divinylbenzene (DVB). A detailed structural characterization has been performed by 29Si and 13C MAS NMR, FT-IR and chemical analysis. As a result, two different structural models have been proposed for the two series of resins. The two resins have been pyrolyzed at 1400 °C and the resulting SiCO ceramics characterized by X-ray diffraction. It has been shown that the stability of the amorphous silica phase present in the SiCO ceramics is strongly influenced by the molecular organization of the starting precursors. The presence of siloxane rings in the cyclic-derived polysiloxane decreases the stability of the amorphous SiO2 and promotes the crystallization of cristobalite. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction One emerging application of polysiloxanes is as precursors for new ceramic and glasses such as silicon oxycarbide glasses (SiCO) as bulks, coatings or fibers [1–3]. SiCOs can be obtained from crosslinked polysiloxanes by a pyrolysis process in inert atmosphere (Ar, He or vacuum) at temperature exceeding 800–1000 °C. Ceramic materials obtained by the pyrolysis of a preceramic polymer are usually referred as PDC, Polymer-Derived Ceramics [4] and are now entering the commercial market either as precursors or as shaped components. A typical examples being the STARBladeÒ ceramic rotors made using the preceramic precursors from Starfire Systems Inc. Silicon oxycarbides display unusual properties, such as high temperature viscoelasticity [5], crystallization resistance [6] and photoluminescence [7,8]. These properties are related to a rather unique and complex nanostructure in which a silicon amorphous network co-exists with graphene layers and SiC nanocrystals [9,10]. The evolution of the nanostructure, as the material changes from the organic into the ceramic state, occurs via solid state radical reactions with formation of gaseous by-products such as methane and hydrogen [11]. The composition, the curing process, the pyrolysis atmosphere have been pointed out as variables that can control the nature and properties of the ceramic product. However, a still open question in this field is the role played by the polymer architecture, i.e. the different molecular arrangements of * Corresponding author. E-mail address: [email protected] (P. Dibandjo). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.10.006

the same structural units, in controlling the nanostructure and consequently the properties of the final ceramics. Indeed, few papers reported that the molecular organization of the preceramic polymer is important for the development of the ceramic nanostructure [12–15]. However, in most of these studies the starting polymers are not completely isomeric because along with the change of the polymer architecture an important modification in chemical composition also usually occurs. In the present study two preceramic polymer systems having the same chemical composition have been obtained starting from a linear and a cyclic polysiloxane. The two compounds are: polyhydridomethylsiloxane (PHMS) and 1,3,5,7-tetramethyl-1,3,5,7tetracyclotetrasiloxane (TMTS). Both precursors have the same chemical formula, SiOCH4, and are formed, apart from the three terminal –Si(CH3)3 units of the linear polymer, by the same repeating units: –½CH3 SiHOn . Thus, they seem the ideal choice to verify if polymer-derived SiCO glasses have memory of the architecture of the preceramic network from which they are derived. Indeed, the structure of the two precursors should be quite different: in one case the siloxane forms linear chains while in the other the Si–O bonds are part of well defined 4-member rings. PHMS and TMTS have been already used as precursors for silicon oxycarbides by the group of Yoshida and Schiavon [16,17] and by the group of Blum [18], however in these papers the influence of the molecular architecture on the structure of the resulting SiCO glass was not studied. The two silicon-containing compounds have been crosslinked with various amounts of divinylbenzene (DVB), via hydrosilylation reactions between the Si–H moieties and the vinyl groups of DVB, following a procedure proposed by Blum [18]. Accordingly, several

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samples were synthesized starting from the linear and the cyclic polysiloxanes with increasing amount of DVB. The preceramic polymers have been pyrolyzed at 1400 °C in inert atmosphere to obtain the corresponding SiCO glasses which have been characterized by X-ray diffraction. In this paper we report a detailed characterization of the preceramic networks using multinuclear MAS NMR and FT-IR spectroscopies and these information will be related with the structural data collected on the corresponding SiCO glasses. We will show that silicon oxycarbide glasses have a noticeable memory of the molecular organization of the siloxane network from which they are derived.

2.3. Characterization methods 2.3.1. Chemical analysis Si, C, and O elemental analysis were performed by the Analytisches Labor Pascher, Remagen-Bandorf (Germany). 2.3.1.1. Determination of carbon content. Samples (1–3 mg) are oxidized in pure oxygen and the carbon dioxide produced is absorbed in 0.1 N NaOH solution and detected by conductimetry. 2.3.1.2. Determination of oxygen content. The sample, 1–3 mg, is pyrolyzed in a graphite capsule at 2000–3000 °C under vacuum. Then, the oxygen in the sample is converted to carbon monoxide which is detected by IR-analysis. The analyzer used is Balzers Exhalograph EAO-202.

2. Experimental section 2.1. Synthesis of the crosslinked siloxanes All the chemicals were purchased from Sigma–Aldrich and used as received. Two types of siloxane polymers were used: a linear polyhydridomethylsiloxane (PHMS, MW = 1900) and a cyclic 1,3,5,7-tetramethyl-1,3,5,7-tetracyclotetrasiloxane (TMTS, MW = 240). In a typical preparation the catalyst (platinum divinyltetramethyldisiloxane always 5 ppm relative to Si compound) and 0; 10; 50; 100; 200 wt% (calculated on the siloxane weight) of divinylbenzene (DVB, technical 80%, mixed isomers) were mixed together and then added to the siloxane to prepare the carbonenriched SiCO preceramic polymers without any addition of solvent. The resulting low-viscosity mixture was placed in a test tube which was covered. The cast solution was allowed to stand at room temperature (RT) and complete setting was observed overnight. After RT setting, both cyclic and linear-derived products become hard rubbery materials. Table 1 summarizes the studied compositions and reports the expected nominal chemical formula. The nominal chemical formula have been calculated considering, for TMTS the molecular formula: –½CH3 SiHO4 (with MW = 240) and for the PHMS the molecular formula: (CH3)3Si–[CH3SiHO]29– Si(CH3)3 (with MW = 1900).

2.3.1.3. Determination of silicon content. The oxidation of the sample in steel lined Teflon beaker is realized with nitric acid at 200 °C, followed by treatment with NaOH solution at 150 °C. Silicon is detected by ICPAES (Inductively Coupled Plasma Atomic Emission Spectroscopy) at 251.6 nm. 2.3.2. MAS NMR The MAS NMR experiments were performed with an AVANCE 300 Bruker using a 7 mn probe-head. The spinning rate was 4 and 5 kHz for 29Si and 13C MAS NMR experiments, respectively. All the 29Si NMR spectra were recorded using one pulse experiments with 90° pulses and 100 s as recycle delays, conditions that allow a quantitative assessment of the spectra. The experimental spectra were simulated using dmfit modeling software [19]. The observed Si units are designed according to the usual notation in silicon chemistry: M (SiC3O), D (SiC2O2) and T (SiCO3). The number that may be added to the unit symbol as a subscript (e.g., T2), indicates the number of oxo bridges bonded to the corresponding Si site. The symbol DH represents D units which have one hydrogen and one methyl group (i.e., HSiCO2 units). The 13C NMR spectra were recorded using CP (cross-polarization) experiments between 1 H and 13C nuclei with 5 ms contact time.

2.2. Pyrolysis

2.3.3. FT-IR Infrared spectroscopy was performed on a Nicolet Avatar 330 FT-IR (Fourier transform infrared spectrometer) from Thermo Electron Corporation (Waltham, MA). Powdered samples were incorporated into KBr pellets. The spectra were recorded in transmission geometry, and the background was subtracted by the usual procedure. An average of 32 scans with 2 cm1 resolution was taken for each specimen.

Fragments (3–5 mm in size) of the siloxane resins were pyrolyzed using an alumina tubular furnace (Lindberg/Blue) under 150 mL/min of flowing argon. The samples were heated at 5 °C/min up to 1200 and 1400 °C and maintained for 1 h at the maximum temperature. Cooling to room temperature was performed by turning off the furnace power. The oxycarbide fragments were milled in an agate mortar and the powders analyzed by XRD.

Table 1 Composition of the starting TMTS–DVB and PHMS–DVB precursors. Sample

DVB/Si precursor ratio

Bonds molar ratio 3

Expected chemical formula

Mass (g)

Moles (10

PHMS-0 PHMS-10 PHMS-50 PHMS-100 PHMS-200

4.6:0 0.56:4.6 1.78:3.53 3.03:3.03 5.91:2.90

2.4:0 4.3:2.4 13.7:1.8 23.3:1.6 45.4:1.5

1:0 1:0.12 1:0.55 1:0.93 1:2.06

SiC1,13O0,97H4,32 SiC1,71O0,97H4,90 SiC3,58O0,97H6,78 SiC5,83O0,97H9,08 SiC10,89O0,97H14,09

TMTS-0 TMTS-10 TMTS-50 TMTS-100 TMTS-200

0:4.05 0.44:4.05 1.42:2.80 2.68:2.64 4.63:2.30

0:16.9 3.4:16.9 11.0:11.7 20.6:11.0 35.5:9.5

1:0 1:0.10 1:0.47 1:0.94 1:1.87

SiC1O1H4,00 SiC1,50O1,0H4,50 SiC3,35O1,0H6,35 SiC5,68O1,0H8,68 SiC10,34O1,0H13,34

)

Si–H:C@C

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2.3.4. XRD X-ray diffraction measurements were done at the European Synchrotron Radiation Facility (ESRF) using the ID31 high resolution powder diffraction station, operating in capillary Debye Scherrer geometry with a wavelength 0.4562 Å. 3. Results The synthesis of the precursors is based on crosslinking polyhydridomethylsiloxane (PHMS) or cyclic 1,3,5,7-tetramethyl1,3,5,7-tetracyclotetrasiloxane (TMTS) with different amounts of divinylbenzene (DVB) using a hydrosilylation reaction in presence of a very efficient platinum catalyst. Typically, each formulation is converted overnight at room temperature into a rubbery hard material. It is worth to emphasize that DVB serves simultaneously as: (a) reactive cosolvent, (b) crosslinking agent, and (c) carbon content enhancer for the resulting hybrid materials. The hydrosilylation reaction leads to the formation of a new Si–C bond and transforms the starting DH units (CSiHO2) into D units (C2SiO2) in the crosslinked resin. A side reaction, which could occur if residual water is present in the siloxane precursors or if the reacting system is not moisture tight (like in the present case), is the dehydrocoupling of the Si–H bonds. In this case, the DH units will be converted into tri-functional T units (CSiO3). Dehydrocoupling of the Si–H bonds is expected to be the only active mechanism in the crosslinking of the pure PHMS and TMTS siloxanes without DVB addition (PHMS-0 and TMTS-0 samples). 3.1. Characterization of the precursors resins 3.1.1. Chemical analysis The elemental compositions obtained from the chemical analysis are reported in Table 2. As expected, carbon content increases by increasing the DVB/Si precursor ratio. The O/Si atomic ratio, (see last column of Table 2) is close to the expected value (O/ Si = 1) for the samples with a DVB/Si ratio P50%. Below this value, the O/Si ratio is higher, suggesting that, when the DVB is completely absent (PHMS-0 and TMTS-0 samples) or present in low amount (PHMS-10 and TMTS-10 samples), the dehydrocoupling reactions play an important role in the crosslinking process. The comparison of the O/Si values for DVB/Si 6 10% shows that the TMTS-derived cyclic precursors (O/Si = 1.37–1.40) is more inclined toward dehydrocoupling reaction than the linear PHMS-derived one (O/Si = 1.22–1.25). 3.1.2. MAS NMR 29 Si MAS NMR spectra of the different formulations and compositions are presented in Fig. 1. The quantitative analysis obtained

Table 2 Elemental chemical analysis of the PHMS–DVB and TMTS–DVB resins.

a b

Sample

Sia (wt%)

Ca (wt%)

Oa (wt%)

Hb (wt%)

Chemical formula

PHMS-0 PHMS-10 PHMS-50 PHMS-100 PHMS-200

42.8 38.8 30.6 24.1 18.3

19.7 26.0 43.2 54.6 63.7

30.5 27.1 18.9 13.7 10.3

7.0 8.1 7.3 7.6 7.7

SiC1.07O1.25H4.6 SiC1.56O1.22H5.85 SiC3.30O1.08H6.7 SiC5.30O1.00H8.83 SiC8.12O1.00H11.8

TMTS-0 TMTS-10 TMTS-50 TMTS-100 TMTS-200

42.8 39.2 29.9 22.6 16.6

18.7 24.5 43.9 56.9 66.1

33.6 31.3 19.5 13.8 9.9

4.9 5.0 6.7 6.7 7.4

SiC1.02O1.37H3.21 SiC1.46O1.40H3.57 SiC3.43O1.14H6.27 SiC5.87O1.06H8.3 SiC9.30O1.04H12.5

Estimated average errors for Si: 2%; C: 1%; O: 3%. H was not analyzed and the amount was estimated by difference.

from the simulation of the spectra are reported in Table 3. In the NMR spectra of the two pure Si precursors (PHMS-0 and TMTS-0) three peaks are present: at 36 ppm due to the starting DH units, but also at 66 ppm (T3 units) and 56 ppm (T2 units) arising from the dehydrocoupling of the Si–H bonds into Si–O bonds. For the PHMS-derived resin, an additional peak is present at 9.9 ppm due to the terminal M units. A clear distinction by 29Si MAS NMR of the DH sites in linear and cyclic siloxanes seems difficult. Indeed, a liquid 29Si NMR investigation of PHMS and TMTS pure precursors showed resonances at 34 ppm for PHMS and at –32 ppm for TMTS. This chemical shift difference is too small compared to the peak linewidth in the solid state spectra to be able to distinguish between cyclic and linear motifs. It is interesting to observe that for the linear-derived precursor (PHMS-0) up to 43% of the initial Si–H bonds have been transformed into Si–O bonds by the dehydrocoupling reaction with ambient moisture and this percentage increases up to 58% for the cyclic-derived sample (TMTS-0) in agreement with chemical analysis. The solid-state 13C CP MAS NMR spectra of the PHMS- and TMTS-derived resins are plotted in Fig. 2. The spectra of the pure siloxanes (PHMS-0 and TMTS-0) show two peaks at +1.5 and 2.3 ppm, which correspond to the CH3 groups in DH and in T units, respectively. The intensity of the peak related to the methyl groups in T units (d = 2.3 ppm) is higher for the TMTS-0 than for the PHMS-0, in good agreement with the 29Si NMR and chemical analysis results previously reported. By increasing the DVB amount for both resins the intensity of the two peaks assigned to DH and T units progressively decreases and a new peak at d  0 ppm emerges. This new component can be assigned to the CH3 groups of D units, C2SiO2, formed through hydrosilylation reactions. Beside the resonances around 0 ppm, for the DVB-containing samples, the 13C NMR spectra show more complexity with various aliphatic and aromatic signals. The samples with a DVB/Si ratio of 10 and 50 wt% present resonances due to aliphatic groups: –CH3 (d = 15 ppm) and –CH2 /–CH– groups (d = 19 and 29 ppm). The same samples show peaks at 127, 144 ppm related to protonated (–CH@) and non-protonated C of the aromatic rings, respectively [21]. The presence of the peaks at 15, 19, 29 ppm substantiates the occurence of a and b hydrosilylations already suggested by the 29Si NMR. Indeed, hydrosilylation of the vinyl group in a position leads to the formation of CH3 and CH groups while in b position produces two CH2 groups (see Scheme 1). For these compositions with 10% and 50% of DVB, hydrosilylation reaction of the vinyl groups is complete since the typical resonance at 114 ppm due to C atoms of C@C groups [22] is absent in the 13C MAS NMR spectra. When the amount of DVB increases to 100 and 200 wt%, additional peaks appear in the 13C NMR spectra. The PHMS and TMTS formulations show differences in both the aliphatic and aromatic ranges. Resonances at 114 ppm and at 138 ppm are assigned, respectively, to CH and CH2 of un-reacted vinyl groups of DVB [22]. These peaks are present for both formulations for the highest amount of DVB (200%) but only for the linear-derived resin when the DVB/Si ratio is 100%. For the cyclic-derived samples with DVB/Si = 100 and 200 wt% another modification is clearly visible in the NMR spectra: the peak at 29 ppm is strongly enhanced compared to the other aliphatic resonances and new peaks appear in the range 40–50 ppm. All these NMR signals can be assigned to – CH2 groups suggesting the self-polymerization of the vinyl groups of DVB with formation of CH2–CH2-chains. In summary, for the linear-derived resin when DVB/Si P 100% residual vinyl groups are present in the structure but there is no clear evidence for self polymerization of DVB. On the other hand for the cyclic-derived samples with DVB/Si = 100% there is an indication for the polymerization of DVB and no residual vinyl can be detected. When the DVB content is increased up to 200% both residual vinyl

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a

b

TMTS-200

PHMS-200

TMTS-100

PHMS-100

PHMS-50

TMTS-50

PHMS-10

TMTS-10

T3 DH

DH T2

M

TMTS-0

PHMS-0 40

20

T3

T2

0

-20

-40

-60

-80

40

20

0

-20

δ (ppm) Fig. 1.

-40

-80

-60

δ (ppm)

29

Si MAS NMR spectra of (a) PHMS and (b) TMTS compositions.

Table 3 29 Si MAS NMR analysis of the PHMS- and TMTS-derived resins. Sample

M (%) d (ppm)

D (%) d (ppm)

DH (%) d (ppm)

T2 (%) d (ppm)

T3 (%) d (ppm)

4 (9.9)

–

–

53 (35.5)

6 (55.9)

37 (65.5)

PHMS-10

4 (9.7)

4 (19.7)

5 (23.6)

40 (35.5)

14 (55.8)

33 (65.6)

PHMS-50

4 (9.1)

15 (20.3)

27 (24.1)

44 (35.8)

6 (55.5)

4 (65.6)

PHMS-100

3 (7.7)

30 (21.5)

30 (25.4)

38 (36.5)

–

–

PHMS-200

4 (7.9)

46 (22.0)

36 (25.9)

14 (36.6)

–

–

TMTS-0

–

–

–

42 (36.4)

9 (56.3)

49 (66.0)

TMTS-10

–

4 (20.1)

3 (23.6)

43 (35.7)

10 (56.6)

40 (65.8)

TMTS-50

–

25 (18.6)

20 (22.2)

43 (33.7)

6 (54.6)

6 (64.6)

TMTS-100

–

54 (19.9)

28 (23.9)

18 (34.7)

–

–

TMTS-200

–

65 (20.5)

35 (24.8)

–

–

–

PHMS-0

groups and CH2–CH2 chains seem to be present in the TMTS-derived structure. 3.1.3. FT-IR FT-IR spectra are reported in Fig. 3. For both precursors are evident peaks due to: C–H bonds at 2960–2830 cm1 (stretching); Si– H bonds at 2160 cm1 (stretching) and 830 cm1 (bending), Si–CH3 groups at 1265 cm1 (stretching) and at 760 cm1 (rocking), C–C bonds of vinyl groups at 1625 cm1 (stretching) and C–C bonds

of phenyl at 1600 and 1590 cm1 (stretching). Si–O bonds of the siloxane network give rise to absorption at 1150 and 1000 cm1 (stretching). The band at 460 cm1 is associated with Si–O–Si bending of cage-type siloxane network [23] like the one present in silsesquioxanes, R-SiO1,5. Thus, the absorption at 460 cm1 indicates the existence, in the siloxane network, of T cages, which are formed from the starting DH sites, through dehydrocoupling reactions. These units disappear for composition with DVB/ Si P 50%. The evolution of the FT-IR spectra with the amount of

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a

b

PHMS-200

TMTS-200

TMTS-100

PHMS-100

* ** *

** **

*

*

*

*

TMTS-50 PHMS-50

TMTS-10 PHMS-10

TMTS-0

PHMS-0 200

150

50

100

0

200

δ (ppm) Fig. 2.

R

R

- product H CH3 RH= *

CH3

O Si O Si O Si O H

H

O n

*

0

C CPMAS NMR spectra of (a) PHMS and (b) TMTS compositions. *Spinning side bands.

+

CH3

50

13

+

H3C

100

δ (ppm)

H3C

R-H

150

or

H

H

Si

Si O Si

CH3 H

O H3C O Si H3C H

Scheme 1. Schematic representation of the hydrosilylation reaction.

DVB shows a progressive decrease of the Si–H related peaks at 2160 and 830 cm1 and a progressive increase of the phenyl and vinyl-related absorption at 1600 and 1650 cm1. Residual Si–H bonds (absorption at 2160 and 830 cm1) are present in the PHMS-200 composition and absent in the TMTS-200 sample. The peak at 1700 cm1 present in the FT-IR spectrum of the TMTS200 sample could be associated to the formation of few C@O bonds due to the very exothermic character of DVB polymerization.

3.2. Characterization of the SiCO glasses TG analysis in Fig. 4 reveals that for both series of samples, the polymer-to-ceramic transformation occurs with a main decomposition step between 500 °C and 800 °C. The pyrolysis is complete at 1000 °C. Above 1000 °C the ceramics are stable up to 1500 °C without showing any carbothermal reduction reactivity at high temperature, even for the samples containing the highest amount of DVB. The ceramic yield is similar for the two series of samples and

ranges from 90% for the PHMS-10 and TMTS-10 to 60–65% for the 200% samples. The SiCO samples are still X-ray amorphous at 1200 °C. The synchrotron patterns recorded on samples pyrolyzed at 1400 °C are reported in Fig. 5. Usually, the increase of the pyrolysis temperature above 1200 °C results in a phase separation of the SiCO amorphous network into SiC4 and SiO2 rich domains, with formation of nanocrystalline b-SiC [6]. PHMS derived SiCO glasses follow this known evolution and the diffraction patterns are characterized by reflections assigned to b-SiC and by the component assigned to amorphous silica. In contrast, the TMTS-derived SiCOs show, together with the b-SiC peaks, a sharp component at 2h = 6.4° superimposed to the amorphous silica halo which indicates the partial crystallization of the silica phase into cristobalite. A peak around 2h = 12° is also present in all the diffraction patterns. This peak, which is weak at low amount of DVB and grows with the DVB content, is related to graphite and indicates the presence of the free carbon phase. 4. Discussion The objective of this study was the preparation of siloxane resins, precursors for SiCO glasses, having the same chemical composition but different molecular organization. The final aim being the investigation of the role of the polymer architecture on the structure and properties of the related SiCO glasses. For this reason a great effort has been put in the chemical and structural characterization of the cross-linked polysiloxanes. Two preceramic polymer systems have been synthesized starting from a linear and a cyclic siloxane. The precursors have been crosslinked via hydrosilylation reactions with divinylbenzene

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a

b TMTS-200

PHMS-200

PHMS-100

TMTS-100

PHMS-50

TMTS-50

C=C

C=C

PHMS-10

TMTS-10

PHMS-0

TMTS-0

Si-CH3 C-H

Si-CH3

Si-H

C-H

Si-O-Si

Si-H

Si-O 3000

2500

2000

1500

1000

3000

500

Si-O-Si

Si-O

2500

2000

1500

1000

500

-1

-1

Wavenumbers (cm )

Wavenumbers (cm )

Fig. 3. FT-IR spectra of (a) PHMS and (b) TMTS compositions.

a

b

100

100

PHMS-10

90

80

PHMS-50

80

70

PHMS-100

60

PHMS-200

Weight (%)

Weight (%)

TMTS-10

90

TMTS-50 TMTS-200

70 TMTS-100

60 50

50

40

40 0

200

400

600

800

1000

1200

1400

0

200

400

600

800

1000

1200

1400

Temperature (°C)

Temperature (°C) Fig. 4. TGA spectra of (a) PHMS and (b) TMTS compositions.

according to the reaction reported in Scheme 1. However, when the amount of DVB is low (samples with DVB/Si 6 50) dehydrocoupling reaction with ambient moisture can also take place with the formation of trifunctional, T, CSiO3 units. Indeed, this mechanism is the only one responsible for the crosslinking of the pure siloxanes when DVB is not used. The evidence for the formation of CSiO3 units have been obtained from the 29Si and 13C MAS NMR spectra, from the FT-IR investigation with the band at 460 cm1 and from the chemical analysis data by the Si/O ratio.

4.1. Chemical characterization of the crosslinked resins Elemental analysis allowed comparing the chemical composition of the two series of samples. Accordingly, it has been found that, as expected, the composition of the samples obtained with the same amount of DVB is very close to each other as can be observed from the data in Table 2 and visually from the ternary Si–C– O diagram of Fig. 6. This way of representing the chemical composition of siloxane resins and their derived SiCO glasses is common in the PDC field [24,25].

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a

b #

# SiO2

#

2 ° C (SP )

* β-SiC * °

*

*

* PHMS-200

°

*

*

TMTS-200

PHMS-100 TMTS-100

PHMS-50 TMTS-50

PHMS-10

0

10

20

30

TMTS-10

40

Synchrotron Radiation 2θ (°)

0

10

20

30

40

Synchrotron Radiation 2θ (°)

Fig. 5. Synchrotron radiation of: (a) PHMS, and (b) TMTS samples at 1400 °C.

DVB-based spacer or via direct Si–O–Si bridges. Residual Si–H bonds are present and the C@C bonds are totally consumed in both the formulations. The PHMS-derived resins are ladder type (with the steps of the ladder formed by the DVB units instead of the usual Si–O–Si bridges) while the TMTS-derived ones are based on siloxane rings.

Carbon

E D SiC

4.3. Structural characterization of samples with high amount of DVB (DVB/Si P 100%)

C

B A

Silicon

SiO2

Oxygen

Fig. 6. Composition (mol%) of the different formulations. (j) PHMS and (H) TMTS. A: DVB/Si = 0; B: DVB/Si = 10%; C: DVB/Si = 50%; D: DVB/Si = 100% and E: DVB/ Si = 200%.

4.2. Structural characterization of samples with low amount of DVB (DVB/Si 6 50%) For the samples with a low DVB/Si ratio (DVB/Si 6 50%) the amount of vinyl groups available for the hydrosilylation is understoichiometric (see Table 1) and dehydrocoupling reaction occurs easily. The amount of T units is revealed by the quantitative analysis of the 29Si MAS NMR spectra and it is higher for the cyclic precursor compared to the linear one (see Table 3). The polymer architecture of the two series of samples is reported in Scheme 2. In both cases the cross-linking of the siloxanes occurs either via

For compositions with a DVB/Si ratio P100% the C@C/Si–H molar ratio is P0.9 and the crosslinking of the siloxanes occurs exclusively via hydrosilylation reactions (29MAS NMR spectra do not show anymore the presence of the peak at 55 and 65 ppm due to CSiO3 units). Both compositions with DVB/Si = 100% show residual Si–H groups while for DVB/Si = 200% Si–H moieties are present only in the linear-derived sample (PHMS-200). 13 C MAS NMR (Fig. 2) reveals that un-reacted vinyl groups are present in the PHMS-100 and PHMS-200 samples while for the cyclic siloxane residual C@C bonds are present only in the 200% samples (TMTS-200). The simultaneous presence of Si–H and vinyl bonds for the PHMS-100 sample suggests that for the linear polysiloxane the development of the network leads to a rapid decrease of the molecular mobility with entrapment of residual functional groups. On the other hand, for the corresponding TMTS-100 sample vinyl groups are totally consumed even through self polymerization with formation of CH2–CH2, as is clearly shown by the enhancement of the resonance at 29 ppm in the 13C NMR spectrum. For the two compositions with high amount of DVB (DVB/ Si P 100%) the polymer architecture which emerges from this study can be schematically described as follows (see Scheme 3): for the PHMS-derived samples the bridges between the PHMS chains contain one aromatic ring. This assumption is based on the evidence that, for this precursor, the self polymerization of

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a

CH3

O

CH3

O Si O Si O Si O Si O O CH3 CH3 CH3 H H O Si O Si O Si O Si O CH3 CH H

CH3 O

Si CH3 CH3

O

H3C

CH3

O Si O Si O Si O Si O CH3 CH

*

O

O

Si O

Si H3C Si CH3 O H O Si H3C CH3 Si O O Si O CH3 O H3C Si H O Si CH3

3

CH3

O CH3

b

CH3

CH3 H H Si O Si O Si O Si O CH3 CH3 O CH

O

*

H O Si

CH3

O O Si O HC Si O Si O 3 O CH3 Si O H3C H3C O

CH3O

Si H3C

CH3

Si

H3C Si

H n

3

3

CH3 Si O

CH3

CH3 Si O CH 3 Si

O O Si O H3C

Scheme 2. Schematic representation of the microstructure of the resins with low amount of DVB (DVB/Si 6 50%), (a) PHMS-derived and (b) TMTS-derived.

a

b CH3

CH3

CH3

CH3

CH3

O Si O Si O Si O Si O SiO H H C CH H H 3

CH3

CH3

CH3

Si O Si O Si O H H HC CH 3

H3C

CH3 O Si O Si H Si O Si O CH HC 3

H3C

3

H3C CH

HC CH 3

CH3 O Si

Si O Si

CH3

O H3C O Si H3C

CH3 H3C O

H O Si O Si O Si O Si O Si O Si O Si O CH3 H CH3 CH CH3 CH3 CH3 3

H3C

Si O

Si CH3 Si O H3C O Si H3C

CH3

H3C CH3 O Si Si O H3C O Si CH3 Si O H3C CH3

Scheme 3. Schematic representation of the microstructure of the gels with high amount of DVB, (a) PHMS-derived and (b) TMTS-derived.

DVB does not occur to a considerable extent. The network comprises residual Si–H and C@C vinyl groups. The resulting siloxane structure is a ladder-type as the one reported in Scheme 3. On the other hand, the network of the TMTS-derived samples (TMTS-200) is based on tetrasiloxane cycles and contains a low amount (TMTS-100) or even no (TMTS-200) residual Si–H moieties. The bridges between two Si atoms can be formed by more than one benzene ring since, as was inferred from the 13C NMR study, self polymerization of DVB seems to occur for this system.

in the silicon oxycarbide ceramics show in general exceptional stability against devitrification. Moreover, this different behavior for the two systems has to be certainly ascribed to the different polymer architectures since the elemental composition is the same for the two studied formulations. The explanation for these experimental results is however not straightforward. At this stage it can be postulated that the well defined silica rings present in the starting TMTS-derived precursors are retained in the resulting SiCO ceramics and may act as nuclei for the formation of crystalline SiO2.

4.4. Comparison of the high temperature behavior of the various SiCO glasses

5. Conclusion

The SiCO glasses obtained by pyrolysis at 1400 °C of the crosslinked siloxanes have been characterized by XRD analysis and this investigation shows that the structure of the SiCO glasses has memory of the molecular organization of the starting precursor. Indeed, the linear-derived SiCO shows, at high temperature, the typical behavior observed for the majority of SiCO glasses with the crystallization of nanosized b-SiC and the stability of the amorphous SiO2 phase. On the other hand, for the cyclic-derived SiCOs the diffraction spectra clearly display, together with the usual formation of nanocrystalline silicon carbide, the partial crystallization of the silica phase into crystobalite. The reduced stability of the amorphous SiO2 in the TMTS-derived SiCOs compared to the liner-derived glasses is certainly a remarkable effect since, as it has been already recalled, the amorphous silica clusters present

Two series of cross-linked polysiloxanes based on the same repeating units, but with different molecular organization have been obtained starting from a linear and a cyclic siloxanes using the hydrosilylation technique. The two silicon compounds were: polyhydridomethylsiloxane (PHMS) and 1,3,5,7-tetramethyl1,3,5,7-tetracyclotetrasiloxane (TMTS) which have been crosslinked with increasing amounts of divinylbenzene, DVB. Experimental results obtained by 29Si, 13C MAS NMR and FT-IR showed that the PHMS-derived samples have a ladder-type structure with DVB bridges between the siloxane chains while the TMTS-derived network is based on tetrasiloxane rings and the bridges between the rings may contain more than one aromatic ring. Additional differences between the two series include the higher reactivity of the cyclic molecule compared to the linear

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one which leads to a more efficient hydrosilylation reaction and a higher degree of consumption of the Si–H bonds. The influence of the polymer architecture on the structure of the resulting SiCO glasses has been studied and SiCO glasses have been obtained by pyrolysis at 1400 °C. The X-ray diffraction analysis showed that the stability of the silica phase in the two series of SiCO glasses is different and is related to the structure of the precursor resin. For the linear-derived SiCOs the amorphous SiO2 is stable like for the majority of reported silicon oxycarbide glasses while for the cyclic-derived SiCOs a partial crystallization of crystobalite has been observed. Thus, the presence of well defined siloxane rings in the starting polymer may lead to silica-rich clusters in the corresponding SiCO glasses which act as nuclei for the silica devitrification. Acknowledgement Research supported by the European Community, through a Marie Curie Research and Training Network ‘PolyCerNet’ (http:// www.ing.unitn.it/~soraru//), MRTN-CT-019601. References [1] G.M. Renlund, S. Prochazka, R.H. Doremus, J. Mater. Res. 6 (12) (1991) 2716. [2] F. Babonneau, G.D. Soraru, G. D’Andrea, S. Dirè, L. Bois, Mater. Res. Soc. Symp. Proc. 271 (1992) 789. [3] C.G. Pantano, A.K. Singh, J. Sol–Gel Sci. Technol. 14 (1999) 7.

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