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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.51 no.2 Concepción June 2006

http://dx.doi.org/10.4067/S0717-97072006000200015 

 

J. Chil. Chem. Soc., 51, Nº 2 (2006) , pags: 913-917

 

OBTENTION OF POWDERED CERAMIC MATERIAL BY PYROLYSIS OF POLY [(DIMETHYLSILOXANE)-CO-(DIMETHYLSILAZANE)] COPOLYMERS AS PRECURSORS

 

MARIO RODRÍGUEZ-BAEZA* *, MARÍA JOSÉ TRUJICHETT BRITO

Departamento de Polímeros, Facultad de Ciencias Químicas, Universidad de Concepción. Concepción, Chile.


ABSTRACT

A series of new poly(siloxane) and poly(silazane) derivates were synthesized by cationic ring opening mass copolymerization of the cyclic monomers octamethylcyclotetrasiloxane and hexamethylcyclotrisilazane, obtaining poly[(dimethylsiloxane)-co-(dimethylsilazane) ] copolymers with different concentrations of the comonomer units. The molar masses of the soluble fractions of the copolymers and of the macrocycles generated in the copolymerization reaction medium, were determined by gel permeation chromatography. Thermogravimetric analysis shows that poly(dimethylsiloxane) is more stable than the copolymers and the homopolymer poly(dimethylsilazane). Differential scanning calorimetry of poly(dimethylsilazane) presents a melting endotherm, indicating that this homopolymer is crystalline. The copolymers were characterized by FT-IR and 1H and 13C NMR spectroscopy..

The polymers were thermally cured and pyrolysed, obtaining ceramic powder. The morphology of the ceramic powder, studied by scanning electronic microscopy, corresponds to highly uniform spherical particles. The ceramic material was characterized by FT-IR.

It was shown that these cyclic monomers copolymerize and that they constitute pre-polymer precursors of the powdered ceramic material since the products have Si, C, N and O atoms.

Keywords: Poly[(dimethylsiloxane)-co-(dimethylsilazane) ] copolymers, thermal cured, pyrolysis, ceramic powder, morphology.


INTRODUCTION

Ionic via can be used to prepare poly(siloxane)s by either cationic or anionic polymerization of cyclic siloxanes (1). One of the most common monomers is the tetramer octamethylcyclotetrasiloxane (D4) (2), which can be ionically polymerized by ring opening polymerization using diverse cationic initiators: mineral acids, Lewis acids, and organic acids (3,4). Linear poly(siloxane)s can be obtained as well through polycondensation by the reaction of magnesium alkyl chloride and SiCl4 (Grignard), by hydrosilation (5-7), or by live anionic polymerization, as well as by condensation polymerization employing functionalized prepolymers (8). Furthermore, poly(silazane)s (PSZ) are organosilicon polymers containing Si-N- bonds, which are described as polymer precursors for ceramic material (9). These polymers can crosslinked through the labile H atoms bonded to nitrogen atoms, a necessary condition to obtain ceramic materials (10). Poly(silazane) is obtained by ionic polymerization of cyclosilazane monomers (11-14). Given that both 1,3,5,7-octamethylcyclotetrasiloxane (D4) and 2,2,4,4,6,6-hexamethylcyclotrisilazane (SZ) cationically polymerize, they were copolymerized using triflic acid (CF3SO3H) as cationic initiator. These copolymerizations are performed with the objective of obtaining copolymers that are precursors of ceramic materials that contain CSi, Si3N4, SiO2 and C in a form similar to the compounds obtained by other ways (15). Furthermore, compounds that contain Si, N, O and C atoms are described as possible precursors of ceramic materials in fibers (16, 17). Additionally, materials forming silazane [-Si(CH3)2NH-] units are used in refractory material production and as dielectric coverings in microelectronics (18).

EXPERIMENTAL PART

Materials and polymerization procedures

Two series of poly[(dimethylsiloxane)- co-(dimethylsilazane)] copolymers were synthesized by cationic ring opening copolymerization of the cyclic monomers 1,3,5,7-octamethylcyclotetrasiloxane (D4) and 2,2,4,4,6,6-hexamethylcyclotrisilazane (SZ), varying the monomer concentrations in the initial reaction mixtures. Triflic acid (CF3SO3H) was used as cationic initiator, and was added in CH2Cl2 solution to the polymerization flasks, with a syringe through silicon rubber stoppers. The cyclic monomer D4 ( Fluka) was purified by refluxing distillation over molecular sieves for 48 h in nitrogen atmosphere and then was distilled in vacuum directly into the polymerization flasks and was kept in nitrogen atmosphere. SZ (UCT, United Chemical Technologies, Inc.) was purified by vacuum distillation over CaH2. CH2Cl2 was dried by refluxing distillation on AlLiH4 for 72 h. The copolymerizations were performed in nitrogen atmosphere to eliminate traces of water.

The reactions were performed at 30 ºC in mass (without solvent) with a CF3SO3H concentration of 2.5x10-3 mol/L during 8 days (series 1), and also in mass at 68 ºC with a CF3SO3H concentration of 5x10-3 mol/L during 12 days (series 2). The initial stoichiometric quantities of the comonomers were varied to obtain the copolymer series. The experimental work was carried out as previously described for other polymerizations using similar cyclic monomers (19, 20). The copolymerizations were stopped by adding a small amount of a 1% Et3N/CH3OH solution. The copolymers were dried in vacuum at room temperature for 8 h. The comonomer unit concentrations, dimethylsiloxane (-OSi(CH3)2-) and dimethylsilazane (-NHSi(CH3)2-), in the copolymers depends on the monomer concentration in the initial reaction mixtures. The composition was determined by elemental analysis, expressed as a molar fraction of the D4 units in the copolymers, XD4, (Tables 1 and 2).



Measurements

Fourier-transform infrared spectra (FT-IR) were obtained in a Nicolet Magna-550 instrument using KBr pellets. The nuclear magnetic resonance analysis of 1H and 13C (Bruker AC-250-P) were measured on the soluble fractions of the samples in acetone with TMS as internal standard. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on the samples obtained without previous thermal treatment. A Polymer Laboratories STA 625 thermal analyzer was used. The samples (3-6 mg) were placed in aluminum pans and heated under nitrogen flow (50 cm3 min-1) at a heating rate of 10 ºC min-1 between 25 and 550 ºC. The thermal curing treatment was performed at 200 ºC for 2 h. The pyrolysis was performed in a Thermolyne 59300 high temperature tube furnace in air at 970 ºC. The molecular weights were determined by gel permeation chromatography (GPC) using a Perkin-Elmer-Serie 200 with THF as solvent and using a calibration curve constructed from poly(styrene) standards. The partially soluble copolymer solutions were filtered, and then their molecular weights were determined. The polarized light optical microscope analysis was performed in a Leitz Ortholux II-Pol-BK. The electron micrographie were taken with an ETEC-Siemens Autoscan U-1 scanning electronic microscope (SEM) in samples previously metalized with Au (metalizer S 150 Sputter Coater).

RESULTS AND DISCUSSION

The copolymers poly[(dimethylsiloxane)-co-(dimethylsilazane) ]s were synthesized according to the following scheme:

The copolymer chains grow by addition of the cyclic monomers to the cationically active chain ends and simultaneously by polycondensation reactions as we previously informed for similar copolymer systems (21,22). These mechanisms are reported as well to explain the formation of poly(dimethylsiloxane) (PDMS) obtained by cationic ring opening polymerization of hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane monomers (23-25). Additionally, these mechanisms explain the formation of the siloxane-silazane bonds in the chains of poly[(dimethylsiloxane)-co-(dimethylsilazane) ] copolymers synthesized in this study.

Besides, the homopolymerization of SZ with triflic acid as initiator has be reported to produce higher molar mass polymer due to ring opening polymerization reaction and polycondensation through the trisilation of the nitrogen atoms (26,27).

Furthermore, in the homopolymerization of SZ and cyclodisilazanes, it is assumed that cyclic quaternary "silazanammonium" salts are the active centers, and propagation, cycle formation, and back-biting reactions are proposed as mechanisms (28).

In this work, to assure that the reactions reached the expected thermodynamic ceiling-equilibrium, the polymerization times were set at 8 and 12 days.

The molecular weights of the soluble fractions of the synthesized copolymers were determined by GPC, and the majority of these presented two bands. In series 1, one of the bands corresponded to copolymers with molecular weights that fluctuated between 1075 and 860 g/mol; while in series 2, they ranged between 2940 and 740 g/mol. The other band corresponds to molecular weights of macrocycles of 5 units, probably of silazane (-NHSi(CH3)2-) of 390 g/mol, formed in the polymerization reactions medium through "back-biting" reactions (29); besides, the polydispersity index Q=1.03, corresponding to a low molar mass compound. The molecular weight of PDMS obtained in series 1 is 61590 g/mol, with a polydispersity index of Q=3.3, which signals a wide molecular weight distribution. Another band present in GPC of PDMS corresponding to a molar mass of 1700 g/mol, is assigned to a macrocycle of 23 units of OSi(CH3)2- as reported in the literature (3). The PDMS obtained in series 2 has a molecular weight of 216500 g/mol. PSZ presented a band corresponding to a molecular weight of 1004 (series 1) and 906 g/mol (series 2) and a band of a cycle of 5 units of silazane (-NHSi(CH3)2-) of 390 g/mol.

The molecular weights were determined only from the soluble fractions of the copolymers in THF corresponding to oligomers as well as macrocycles formed in the copolymerization reactions, which explains the low molar mass values obtained.

The thermogravimetric analysis (TGA) of the polymers obtained in series 1 (Fig. 1) and 2 shows that PDMS is thermally more stable than the copolymers and PSZ. Copolymer 2 (series 1) presents clearly two decomposition processes suggests that its structure corresponds to block copolymer formed by units of silazane (-NHSi(CH3)2-) and siloxane (-OSi(CH3)2-) (30). Tables 1 and 2 summarize the TGA analysis data.


Figure 1. TG curves of poly[(dimethylsiloxane)-co-(dimethylsilazane) ] copolymers (curves 2,3,5) and of the homopolymers PDMS and PSZ, recorded at a heating rate of 10 ºC min-1. (series 1).


Figure 2. DSC/TG curves of poly(dimethylsilazane) (PSZ). Heating rate of 10 ºC min-1 obtained in Series 1.

Differential scanning calorimetry (DSC) of PSZ presents a melting endotherm at 96.9 ºC and a heat of melting of 66.1 J/Kg. The decomposition heat is 127.7 J/Kg. The melting process indicates that PSZ is crystalline and that it is formed by similar sized crystallites, as is deduced from the narrow melting endotherm (Fig 2). The melting processes of other crystalline PSZ have also been described (31).

The crystallinity of the PSZ obtained in this work was corroborated with the polarized light optical microscope photographs (Fig. 3), which show that the molecules are oriented (arranged), presenting birefringence. The texture is fibrilar and the coloration is due to light dispersion.


Figure 3. Polarized light optical micrograph of poly(dimethylsilazane) (PSZ).

FT-IR analysis of the copolymers (Fig. 4) present typical absorption bands of the respective homopolymers, PDMS and PSZ, such as: d N-H at 3382 cm-1; d C-H (CH3) at 2961, 2904 and 1408 cm-1 ; d Si-C at 1259 cm-1 ; d N-Si at 1180 and 937 cm-1 ; and an intense absorption band of the bond Si-O-Si at 1080 cm-1.


Figure 4. FT-IR spectra of poly[(dimethylsiloxane)-co-(dimethylsilazane) ] copolymer. Sample 5, series 1.

The copolymers were also characterized by NMR of 1H and 13C. Fig. 5, typical for 1H- NMR of these copolymers, presents the following signals according to the structure:


Figure 5. 1H-NMR spectra of poly[(dimethylsiloxane)-co-(dimethylsilazane) ] copolymer. Sample 5, series 2.

In the multiplete near 0.1 ppm, the following signals can be distinguished: 0.097 ppm signal of the Ha; at 0.11 ppm signal of the Hb and at 0.12 ppm signal of the Hc. The quantity of methyl protons that deutered acetone contains appears at 2.04 ppm. The proton bonded to N appears at 3.46 ppm, and varies its position according to the type of hydrogen bond, temperature and solvent. The RMN-13C analysis of this copolymer (Fig. 6) presents the following signals: at 3.0 ppm corresponds to methyl C; at 30.0 ppm corresponds to C coupled to deuterium of the deutered acetone of multiplicity 7. The signal at 206.5 ppm is also from deutered acetone.


Figure 6. 13C-NMR spectra poly[(dimethylsiloxane)-co-(dimethylsilazane) ] copolymer. Sample 5, series 2.

The thermal curing process of the copolymers was carried out at 200 ºC during 2 h, so that the chains crosslinked through the labile H atoms bonded to the N atom that the silazane co-units possess. For this purpose,

the samples are placed in crucible in an muffle furnace. Schematically, the structure of the partially crosslinked copolymer is the following:

Afterwards, the cured material was pyrolysed at a temperature of 970 ºC in air for 4 h, and then the ceramic powder was maintained in a vacuum oven. The ceramic powder obtained presumably has the following composition: SiC, Si3N4, SiO2 and C (15). The morphology of the ceramic powder, studied by scanning electronic microscope (SEM), corresponds to very well-defined, regular spherical particles with an average diameter in two samples of 1.92 mm (sample 5, series 1) (Fig.7) and 0.82 mm ( (sample 7, series 2).


Figure 7. Scanning electron micrograph of powder ceramic obtained by pyrolysis in air at 970 ºC from sample 5, series 1. (10000X increase).

These characteristics are adequate for the subsequent fabrication of ceramic pieces due to structure uniformity, which provides the material with better mechanical and thermal properties since it inhibits ceramic material segregation during the finished elaboration process (32).

The ceramic powder obtained from the precursor copolymers was characterized by FT-IR (Fig. 8). The principal bands that characterize these ceramic materials are: a) tension band d N-H a 3437 cm-1 typical of the ceramic polysilazane precursor; b) wide band between 900 and 1400 cm-1, in which the following bands are overlapping: i) the band between 940 and 1000 cm-1 corresponds to the stretching vibration of the Si-N-Si bond; ii) near 1160 cm-1 corresponds to the stretching vibration of the N-H bond of Si-NH; iii) at 1080 cm-1 is found the stretching band of the Si-O bond. The stretching vibration band at 830 cm-1 corresponds to d Si-C of the tersiloxane groups. The band at 792 cm-1 is also observed and is assigned to the Si-CH3 bond of the tersiloxane groups (33), which confirms the existence of crosslinked structures.


Figure 8. FT-IR spectra of powder ceramic obtained by pyrolysis in air at 970 ºC from sample 5, series 1.

The FT-IR analysis indicates that a mixture of ceramic powders of SiC, Si3N4 and SiO2 or SixNyCzOp was obtained. These FT-IR spectra present a classic example of residual material obtained from pre-ceramic polymers (34,35).

CONCLUSIONS

It was demonstrated that D4 and SZ copolymerize, obtaining poly [(dimethylsiloxane)-co-(dimethylsilazane)]. These copolymers have co-units of (OSi(CH3)2)- and -NHSi(CH3)2)- with atoms of Si, C, N and O, constituting precursor polymers of ceramic materials (16,17). The curing processes allow production of polymers that are partially crosslinked through labile H atoms bonded to N atoms, and subsequent pyrolysis processes produced powdered ceramic material with the general composition of SiC, Si3N4 and SiO2, or compounds of the SixNyCzOp type (4). The powdered ceramic material presented a good morphology corresponding to very regular spherical ceramic particles with a narrow size distribution. These characteristics are very appropriate for the fabrication of finished ceramic pieces (32). The thermograms obtained by TGA show that some copolymers have characteristics of block copolymers, presenting two decomposition processes. Additionally, PDMS was found to be more stable than the copolymers and PSZ, where PSZ in the synthesized conditions is crystalline.

ACKNOWLEDGEMENTS

This research was supported by the Dirección de Investigación of the Universidad de Concepción (Grant No. 203.024.024-1.0).

 

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e-mail: mrodrigo@udec.cl

 

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