Influence of metallic palladium on thermal properties of polysiloxane networks

Polymer Degradation and Stability 109 (2014) 249e260

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Influence of metallic palladium on thermal properties of polysiloxane networks  jcik-Bania, Justyna Olejarka, Teresa Gumuła, Agnieszka Ła˛cz, Monika Wo Magdalena Hasik* w, Poland Faculty of Materials Science and Ceramics, AGH-University of Science and Technology, Al. Mickiewicza 30, 30-059 Krako

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 May 2014 Received in revised form 18 July 2014 Accepted 23 July 2014 Available online 12 August 2014

Polysiloxane networks differing in cross-link densities were prepared by hydrosilylation of linear polysiloxanes containing various amounts of vinyl groups uniformly distributed in their chains. Thus, D2V polymer with a vinyl group at every third Si atom in the macromolecule and 4D3V3 copolymer with vinyl groups located in a block constituting one fifth of its chain were reacted with hydrogensiloxanes of different molecular structures and functionalities in the presence of Karstedt catalyst. The process was conducted at excessive amounts of SieH groups with respect to vinyl ones which ensured that unreacted SieH groups remained in the polysiloxane networks formed. Such systems were treated with palladium(II) acetate solution in THF. As established by X-ray diffraction and FTIR spectroscopic studies, the redox reaction between Pd2þ ions from the solution and SieH groups of the networks occurred which resulted in the appearance of metallic Pd particles in the systems. Systematic investigations conducted using thermogravimetry coupled with mass spectrometry allowed to conclude that the presence of Pd modifies thermal properties of polysiloxane networks, influencing mainly redistribution of Si bonds taking place during thermolysis of the systems. It was found that higher cross-link densities of the D2V polymer-derived networks than those of the 4D3V3 copolymer-based ones are beneficial for thermal stability of the systems with incorporated Pd. The type of the cross-linking agent, in turn, decides on residual mass remaining after pyrolysis of the Pd-containing samples conducted at 1000  C in Ar flow. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Polysiloxanes Polysiloxane networks Palladium Thermal properties

1. Introduction Because of their unique properties, polysiloxanes are among the most important synthetic polymers. These inorganic-organic macromolecular compounds have extremely flexible chains resulting from freedom of rotation around SieO bonds, exhibit high permeability to gases and are chemically stable [1]. They are also nontoxic, physiologically inert materials that find numerous biomedical applications [2]. Furthermore, cross-linked polysiloxanes can be used as precursors for SiCO ceramics [3]. Polysiloxanes are known for their outstanding thermal properties [4]. In particular, they show very good stability at high temperatures. The onset of their degradation is usually close to or above 300  C; their thermal stability depends on substituents attached to Si atoms, kind of polymer end-groups as well as conditions of heat treatment [5e9]. Composition and atmosphere of heating affect

* Corresponding author. Tel.: þ48 12 6173788; fax: þ48 12 6337161. E-mail address: [email protected] (M. Hasik). http://dx.doi.org/10.1016/j.polymdegradstab.2014.07.025 0141-3910/© 2014 Elsevier Ltd. All rights reserved.

also the type of products formed during thermal degradation of polysiloxanes and, consequently, the char yield obtained after this process. For example, poly(dimethylsiloxane) (PDMS), i.e. the simplest and the most widely applied polymer of this group, in an inert atmosphere gives no residue because its decomposition is accompanied by the evolution of exclusively volatile compounds (cyclic siloxanes) [5,6]. After decomposition under aerobic conditions, ca. 30%e60% of the material remains as a solid residue (SiO2), whereas CO2 and H2O are released as volatile products [5,6]. These differences have been attributed to cross-linking of PDMS occurring in the presence of oxygen at elevated temperatures which prevents polymer chains from scissions necessary for the formation of cyclosiloxanes [5,6]. Thermal properties of polysiloxanes can be modified by incorporation of additives and fillers which also influence flame retardancy of these polymers [10]. Among additives, the use of platinum has been described in several papers. Thus, platinum from platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex, i.e. Karstedt catalyst, added to silicone rubber (lightly cross-linked PDMS filled with SiO2) has been found to improve flame

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retardant properties of the material by facilitating further polymer cross-linking which results in the increased char yield [11]. Similar effect, observed in the case of SiO2-filled uncross-linked PDMS, has been explained by synergistic role of platinum and silica in the PDMS cross-linking process [12]. Platinum ions originating from H2PtCl6 incorporated into the cross-linked poly(methylphenylsiloxane) have been demonstrated, however, to lower thermal stability and char yield of the initial material [13]. Another platinum group metal, palladium, has been introduced into polysiloxanes in order to obtain catalytically active systems [14e20]. In these studies, Pd ions have been incorporated into appropriately functionalized polymers [14e16] and Pd metal particles - into poly(methylhydrosiloxane) [17e20]. Deteriorative influence of Pd ions on thermal stability of the polymer matrix has been reported; the effect, however, has not been studied in detail [14]. The present investigations have been aimed at evaluating the influence of palladium on thermal stability and char yield of regular polysiloxane networks. The networks have been obtained by crosslinking of two linear polysiloxanes containing vinyl groups regularly distributed at Si atoms in their chains: D2V polymer and 4D3V3 diblock copolymer using the - well-known in organosilicon chemistry [21] - hydrosilylation reaction. Linear (HMMH), branched (QMH4) and cyclic (DH 4 ) hydrogensiloxanes have been applied as cross-linking agents. In this way, polysiloxane networks of various cross-link densities have been obtained. Metallic Pd has been introduced into these systems from palladium(II) acetate solution in THF employing redox properties of the SieH groups present in them. It should be noted that polysiloxane networks-Pd systems prepared and investigated in the work can be potentially applied as heterogeneous catalysts and as flame retarders. In both applications, thermal properties are of crucial importance. 2. Experimental 2.1. Materials The monomers: hexamethylcyclotrisiloxane (D3) and 1,3,5trivinyl-1,3,5-trimethylcyclotrisiloxane (V3) were purchased from SIGMA-Aldrich and ABCR, respectively, whereas 1,3,3,5,5pentamethyl-1-vinylcyclotrisiloxane (D2V) was synthesized according to the procedure presented in Ref. [22]. They were dried over CaH2 and vacuum-distilled before use in the polymerizations. The initiator, n-buthyllithium (n-BuLi) and the chain terminating agent, chlorotrimethylsilane (CTMS) were supplied by SIGMAALDRICH and ABCR, respectively and applied in the syntheses without further purification. Triethylamine (Et3N) was purchased from SIGMA-ALDRICH, dried over P2O5 and distilled under atmospheric pressure before use. Cross-linking agents: 1,1,3,3-tetramethyldisiloxane (HMMH), 2,4,6,8-tetramethylcyclotetrasiloxane (DH 4 ), tetrakis(dimethylsiloxy)silane (Q(MH)4) were provided by ABCR and applied as received. Karstedt catalyst, i.e. platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex, applied in the cross-linking processes, was supplied by Aldrich (Poland) as the solution in xylene (2% of Pt) and used in the experiments as received. Palladium(II) acetate was purchased from ACROS. It was applied in the experiments without any preliminary purification procedure. Solvents: tetrahydrofurane (THF) and toluene were bought from POCh (Poland). Before use, THF was dried using benzophenone and sodium, and then distilled under Ar, whereas toluene, after preliminary drying over CaCl2, was distilled from the suspension with P2O5.

2.2. Synthetic methods The polymers, further on in this work referred to as D2V polymer and 4D3V3 copolymer, were obtained by kinetically-controlled anionic ring-opening polymerization of D2V and sequential, kinetically-controlled anionic ring-opening copolymerization of D3 and V3 at their molar ratio equal to 4:1, respectively. The reactions were carried out under inert atmosphere (Ar). In the polymerization of D2V, 0.186 mol of D2V, 1.57$103 mol of n-BuLi, 7.89$103 mol of CTMS and 7.17$103 mol of Et3N were used. The process was carried out at 16  C. Details of the procedure can be found in Ref. [23]. In the D3 and V3 copolymerization, 0.091 mol of D3, 9.42$104 mol of n-BuLi, 0.02 mol of V3, 3.94$103 mol of CTMS and 3.58$103 mol of Et3N were applied. The procedure was as follows: D3 and THF were first vacuum-distilled into a Schlenk reactor, then under flowing Ar e n-BuLi was added into the obtained solution. The polymerization was carried out at room temperature until 80% conversion degree of the monomer was reached (controlled by gas chromatography). After that, freshly vacuum-distilled V3 was added to the reactor, which was placed in a cryostat at 0  C. The polymerization was continued to 80% conversion degree of V3. Then the reaction was stopped by addition of CTMS and Et3N. The synthesized 4D3V3 copolymer was purified by dissolution in methylene chloride and precipitation in methanol. Finally, all the volatile compounds were removed from it during heating at 60  C on a vacuum line. Average molecular weight of D2V polymer was equal to 13 000 g/mol, whereas that of 4D3V3 copolymer - 19 800 g/mol, (determined by GPC, polystyrene standards, eluent: methylene chloride). Both polymers had similar molecular weight distribution, Mw/Mn ¼ 1.2. 29 Si NMR spectra of the polymers (not shown) contained the signals at 21 and 35 ppm assigned to Si atoms in Si(CH3)2 and Si(CH3) (CH]CH2) units [24], respectively. D2V polymer regularity calculated based on its spectrum according to the method proposed in Ref. [22] was equal to 84.1%. This means that the composition of this polymer was highly regular, with a vinyl group attached to every third Si atom in most units of the macromolecule. The ratio of the units originating from D3 to those of V3 in 4D3V3 copolymer was equal to 1: 0.25, i.e. precisely as expected. The polymers were cross-linked by hydrosilylation, in which H various hydrogensiloxanes: HMMH, DH 4 , Q(M )4 and Karstedt catalyst were used. Molar ratio of SieH groups from the cross-linker to eCH]CH2 groups from the polymer amounted to 1.5:1. Crosslinking processes were carried out without any solvent (D2V polymer) or in toluene (4D3V3 copolymer, 1 ml of toluene per 0.5 g of the copolymer), under inert atmosphere (Ar), at the temperature of 60  C for 48 h. The reactions were catalyzed by 0.2$106 mol of Pt introduced into the system from Karstedt catalyst per 0.5 g of the polymer subjected to cross-linking. Palladium was introduced into the prepared cross-linked polymers from the solution of palladium(II) acetate in THF (4.7$103 mol dm3). In the experiments, the cross-linked polymers were treated with such amounts of palladium(II) acetate solution to get 1 wt% of Pd in the system. The reactions were carried out at room temperature for 48 h. After the reaction, the Pd-containing material was separated from the solution, washed with THF and dried on a vacuum line. It should be mentioned at this point that - out of two possible ways of Pd incorporation into the polysiloxane networks studied, i.e. during or after cross-linking of the polymers - the method adopted in the present work which used previously crosslinked polymers seems to be more advantageous. This is because the reaction between Pd2þ ions from palladium(II) acetate solution and the cross-linked polymer is the only chemical process that can take place in the system. Moreover, the previously obtained polymer

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cross-linking level is not affected by this process. Upon incorporation of Pd during cross-linking of the polymer, in turn, formation of the polymer network, reactions of the cross-linking agent as well as the network formed with Pd2þ ions occur simultaneously. Hence, the system is complex and both, incorporation of Pd and polymer cross-linking processes, are difficult to control. In the following parts of the paper the symbols: D2V(HMMH), H H H H H D2V(DH 4 ), D2V(Q(M )4), 4D3V3( MM ), 4D3V3(D4 ), 4D3V3(Q(M )4) will be used to denote D2V polymer and 4D3V3 copolymer crosslinked with the appropriate hydrogensiloxane. The respective systems containing Pd, in turn, will be referred to as: D2V(HMMH)Pd, D2V(DH D2V(Q(MH)4)Pd, 4D3V3(HMMH)Pd, 4D3V3(DH 4 )Pd, 4 )Pd, 4D3V3(Q(MH)4)Pd. 2.3. Analytical methods The obtained systems were characterized by the following experimental methods: - Equilibrium swelling measurements e the initial polymer networks were subjected to swelling in THF at room temperature. In the experiments, into the weighed amount of the studied cross-linked polymer sample an excessive amount of the solvent was added to ensure complete swelling of the network. After a certain period of time (usually 12 h), excess of the solvent was separated from the swollen polymer, which was then weighed. After that, a new portion of the solvent was added to the polymer. In order to attain equilibrium, this procedure was repeated until no further change in the weight of the swollen sample was observed (72 h). Equilibrium swelling degree was expressed as the ratio: (mm0/m0)  100%; where m, m0  weight of the swollen and the initial sample, respectively. - FTIR spectroscopy - the spectra were collected using the transmission method, on a Bruker 70 V spectrometer. The range of the measurements was 400e4000 cm1, their resolution was equal to 4 cm1. Standard KBr pellet technique was used. Correction of the spectra baseline was performed in the WinIR™ program. Calculations of SieH/SieCH3 groups ratios were performed using Digilab Win-IR Pro program, by comparing the areas of the changing SieH band at 2140-2150 cm1 and the constant SieCH3 band at 1260 cm1 in the obtained, baselinecorrected spectra; - Powder X-ray diffraction (XRD) e was conducted on a Philips X'Pert diffractometer using Ni-filtered CuKa radiation (l ¼ 1.54 Å). Range of 2q angles: 3 e 90 . - Thermal Analysis e thermogravimetric (TG) measurements were carried out on SDT 2960 TA INSTRUMENTS apparatus. The samples of mass around 11 mg were placed in the standard platinum crucibles and heated at a rate of 10  C min1. The measurements were carried out in helium atmosphere (purity 99,999%) under dynamic conditions (the flow of 100 cm3 min1). To analyze the volatile products evolved during thermal treatment of the samples the SDT 2960 system was connected on-line with the quadrupole mass spectrometer (THERMOSTAR QMD 300 BALZERS) by a quartz capillary heated up to 200  C. The mass spectrometry measurements were performed in the scan mode for m/z (where m is mass of molecule and z is a charge of the molecule in electron charge units) range from 10 to 100.

temperatures: 250  C, 400  C, 530  C, 700  C and 1000  C. The experiments were conducted in a quartz furnace in the flow of Ar at the heating rate of 5 /min. In a typical experiment, ca. 0.1 g of the sample studied was placed in the furnace, heated to the required temperature at which it was maintained for 15 min. Then it was slowly cooled down in the furnace to room temperature. 3. Results and discussion 3.1. Swelling of the initial polysiloxane networks As mentioned in the “Introduction”, D2V and 4D3V3 polymers applied in the work for the preparation of polysiloxane networks showed regular chain compositions. D2V polymer contained a vinyl group attached mainly to every third Si atom in its chain, whereas 4D3V3 copolymer - vinyl groups occurring in a block constituting one fifth of its chain. Such compositions of both compounds were confirmed by their 29Si NMR spectra (Section 2.2). Polysiloxane networks investigated in the work have been obtained by hydrosilylative cross-linking of D2V and 4D3V3 polymers with hydrogensiloxanes of various functionalities and molecular structures (Section 2.2). Results of equilibrium swelling experiments, presented in Table 1, show that the content and distribution of vinyl groups in the starting polymer chain as well as the type of the cross-linking agent applied strongly affect cross-linking densities of the systems prepared. Thus, swelling degrees of 4D3V3copolymer derived networks are significantly higher and, hence, cross-linking densities lower than those of the networks prepared from the D2V polymer. This is the result of lower content of vinyl groups as well as their location in a block of the former macromolecular chains. When the influence of hydrogensiloxanes is analyzed, it is seen that for both polymers hydrosilylated with linear, difunctional HMMH, swelling degrees are the highest, i.e. cross-linking densities the lowest. Incorporation of moieties origiH nating from four-functional, cyclic DH 4 and linear, branched Q(M )4 into the networks results in lower swelling degrees, i.e. higher cross-linking densities than those recorded for the samples obtained with HMMH. Interestingly, for D2V polymer molecular structure of the four-functional hydrogensiloxane does not seem to H matter significantly: cross-linking with DH 4 and Q(M )4 leads to the materials of close swelling degrees. In contrast, value of this parameter is ca. 1.5 times higher for 4D3V3(DH 4 ) than for 4D3V3(Q(MH)4) sample. Hence, swelling measurements clearly demonstrate that the investigated networks have shown various cross-linking densities. 3.2. XRD and IR characterization of the prepared materials Upon incorporation of palladium, spectacular change in the appearance of all samples takes place. Colorless, transparent starting cross-linked polymers turn black after soaking in palladium(II) acetate solution. This suggests that metallic palladium is introduced into the investigated networks. Since during cross-

Table 1 Swelling degrees of the polymer networks studied in the work. D2V polymer-derived networks

4D3V3 copolymer-derived networks

Sample

Swelling degree [wt.%]

Sample

Swelling degree [wt.%]

D2V(HMMH) D2V(DH 4) D2V(Q(MH)4)

109 54 46

4D3V3(HMMH) 4D3V3(DH 4) 4D3V3(Q(MH)4)

469 398 265

2.4. Pyrolysis of the systems Samples of the cross-linked D2V polymer before and after incorporation of Pd were subjected to pyrolysis at various

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linking of the polymers excessive amounts of SieH with respect to vinyl groups have been applied (Section 2.2), unreacted SieH groups, which exhibit reducing properties [21], have remained in the systems. Therefore the black color of the samples containing palladium indicates that the redox reaction between Pd2þ ions from the solution and SieH groups present in the cross-linked polymers occurs. To verify this we have performed XRD and FTIR studies of the systems. XRD patterns of D2V polymer and 4D3V3 copolymer containing palladium are shown in Fig. 1 and Fig. 2, respectively. In all of them broad reflections at 2q angles lower than 30 are present. Similar reflections appear in the XRD patterns of the starting cross-linked polymers (not shown in the figures). Therefore they can be ascribed to the polymer phase and prove that some ordering exists in the networks studied. Additionally, characteristic reflections at 2q angles equal to 40 , 46 , 68 , 82 are of low intensity, but well resolved in the XRD traces of 4D3V3 copolymer-based systems (Fig. 1). They correspond to interplanar distances d ¼ 2.23, 1.93, 1.37 and 1.17 Å and are assigned respectively to (111), (200), (220) and (311) planes in fcc crystals of metallic palladium [25]. XRD patterns recorded for D2V polymer with incorporated palladium (Fig. 2) also comprise the reflections due to metallic Pd. They are, however, less intense than in the diffractograms of 4D3V3 copolymer-based systems. Their

intensity and shape depend on the cross-linking agent applied for the preparation of the network. In the XRD pattern of D2V(HMMH) Pd sample the reflection at 2q ¼ 40 and raised background at ~46 are seen, in that corresponding to D2V(DH 4 )Pd system - less intensive peak at 2q ¼ 40 is observed, whereas the pattern of D2V(Q(MH)4)Pd sample contains only a raised background around 2q angle value equal to 40 (Fig. 2). Thus, XRD investigations confirm that metallic Pd has been present in all the systems. Differences in intensities of its reflections in the patterns may be due to differences in metal dispersion in various matrices. Since, as has already been mentioned, formation of metallic Pd in the systems studied should have occurred with the participation of SieH groups present in the networks, lowering in their amounts after reactions in palladium(II) acetate solution has been expected. FTIR spectroscopy is a perfect tool to examine changes in the amounts of SieH bonds in polysiloxanes [23,26,27]. Therefore in the present work FTIR spectra of all initial networks as well as those of the networks containing Pd have been measured. Their quantitative analysis, based on the ratios of the bands at 2140 - 2150 cm1 (due to SieH bonds [28]) and at ~1261 cm1 (assigned to CeH bonds in the unchanged SieCH3 groups [28]) has been also performed (Section 2.3). Changes in the spectra are illustrated by showing only those recorded for the D2V polymer-derived systems in Fig. 3. Results of all FTIR spectra quantitative analyses are collected in Table 2. Decrease in intensities of the bands due to SieH bonds at ~910 cm1 (bending vibrations [28]) and at 2140 e 2150 cm1 (stretching vibrations [28]) in the spectra of the samples containing palladium with respect to those of the initial ones is evident (Fig. 3). Quantitative spectra analysis shows that the decrease is the highest for the 4D3V3(HMMH) network in which after incorporation of Pd no SieH groups remained (Table 2). Closer inspection of the data presented in Table 2 reveals that in the initial networks the highest amounts of SieH groups are preserved when DH 4 is applied as the cross-linking agent for their preparation. This fully corroborates our previous findings concerning low reactivity of this hydrogensiloxane towards D2V polymer [23] and indicates that the same effect occurs in siloxane block copolymers containing vinyl moieties. Similarly, after incorporation of Pd the highest amounts of SieH groups remain in the samples obtained with DH 4 . It is also important to note that for both polymer-based networks proportions of the SieH bonds consumed in the reductions decrease in the same order: the polymers cross-linked with HMMH > Q(MH)4 > DH 4 . This is understood since the fraction of SieH groups needed for the reduction of a given amount of Pd2þ ions is related to their content in the initial network. Therefore in the polymers cross-linked with HMMH, of the lowest contents of SieH groups, their highest proportions have been used in the process, whereas in the systems obtained with DH 4 e the lowest, because initially they have contained the most SieH groups. Thus, XRD and FTIR investigations clearly show that metallic palladium has been present in the networks obtained from D2V and 4D3V3 polymers and that it has resulted from the redox reaction between Pd2þ ions from the solution and SieH groups of the crosslinked macromolecules. 3.3. Thermal properties and pyrolysis of the systems

Fig. 1. XRD patterns of 4D3V3 copolymer-derived systems containing Pd.

Thermal properties of the initial networks as well as those of the networks with incorporated Pd have been studied by thermogravimetric analysis (TG), based on which derivative (DTG) curves have been constructed. Volatile products released during thermal decomposition of the materials have been investigated by mass spectrometry (MS).

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Fig. 2. XRD patterns of D2V copolymer-derived systems containing Pd.

TG and DTG curves corresponding to the systems prepared are presented in Fig. 4. Values of residual mass remaining at 1000  C and temperatures of maximum mass losses determined using these curves are collected in Table 3. As can be seen, all the initial networks undergo a two or three-step thermal degradation marked in their DTG curves as minima of various intensities; evidently this process is modified by the presence of Pd (Fig. 4). In the light of the previous studies concerning polysiloxane networks, it can be supposed that the two-stage thermal decomposition of D2V polymer and 4D3V3 copolymer-based samples involves Si-containing bonds (SieO/SieC, SieH/SieO, SieO/SieO) redistribution reactions first [29e35]. They usually occur in the temperature range of 300e600  C and are accompanied by the evolution of volatile Si compounds. The second degradation step, in turn, must be due to the so-called mineralization (ceramization) process which in most polysiloxane systems takes place between 600 and 900  C and leads to the formation of the final ceramic product as well as H2 and gaseous hydrocarbons resulting from the cleavage of SieC and CeH bonds [29e35]. In the case of the D2V polymer and 4D3V3 copolymer-derived samples which undergo a three-step thermal degradation, the second and the third transformations can be ascribed to Si bonds redistributions and ceramization of the material, respectively. The origin of the first DTG minimum, occurring at the lowest temperature is, however, not clear. An additional

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Fig. 3. FTIR spectra of the D2V polymer-derived systems before and after incorporation of Pd.

degradation step at lower temperatures has been observed for the cross-linked polysiloxanes containing reactive groups, such aseOH, eOC2H5 [30e35], but these moieties have not been present in the systems studied in this work. Nevertheless, intensities of the DTG minima suggest that redistributions of Si bonds are the major way of thermal degradation of all the samples obtained from D2V polymer, i.e. the initial ones and those containing Pd, as well as the 4D3V3(HMMH) and 4D3V3(HMMH)Pd systems, whereas cleavage of

Table 2 SieH/SiCH3 band ratios in the FTIR spectra of D2V and 4D3V3 polymer-based networks before and after incorporation of palladium. Sample

D2V(HMMH) D2V(DH 4) D2V(Q(MH)4) 4D3V3(HMMH) 4D3V3(DH 4) 4D3V3(Q(MH)4)

SieH/SiCH3 band ratio Initial sample

Sample containing palladiuma

0.213 0.362 0.217 0.054 0.137 0.069

0.008 0.226 0.026 0.000 0.080 0.024

(96.2) (37.6) (88.0) (100.0) (41.6) (65.2)

a In parentheses: fractions of the SieH groups consumed (in %), calculated as the difference: 100%-(SieH/SiCH3 band ratio in the spectrum of the sample containing Pd/ SieH/SiCH3 band ratio in the spectrum of the initial sample x100%).

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Fig. 4. TG and DTG curves of the systems studied in the work.

SieC and CeH bonds is the main process responsible for thermal H H decomposition of 4D3V3(DH 4 ), 4D3V3(D4 )Pd and 4D3V3(Q(M )4), H 4D3V3(Q(M )4)Pd samples (Fig. 4). Consequently, the temperatures of their maximum mass loss are the highest (Table 3). TG and DTG curves show that incorporation of Pd influences thermal stability, residual mass remaining in the systems after treatment in Ar atmosphere at 1000  C as well as the mechanism of thermal degradation of the cross-linked polymers studied (Fig. 4,

Table 3). The effects of Pd presence are more pronounced for the D2V polymer than the 4D3V3 copolymer-based systems. In particular, Pd increases thermal stability of the D2V(HMMH) and D2V(Q(MH)4) networks: degradation of D2V(HMMH)Pd and D2V(Q(MH)4)Pd samples starts at higher temperatures than that of the respective ones not containing Pd. However, drop in the residual mass remaining at 1000  C is observed; the difference is especially significant (20.7 wt.%, Table 3) for the D2V(HMMH) and

jcik-Bania et al. / Polymer Degradation and Stability 109 (2014) 249e260 M. Wo Table 3 Residual mass at 1000  C and temperatures of maximum mass losses determined by TG and DTG investigations of the initial polysiloxane networks and the networks containing palladium. Sample

D2V(HMMH) D2V(DH 4) D2V(Q(MH)4) 4D3V3(HMMH) 4D3V3(DH 4) 4D3V3(Q(MH)4)

Residual mass at 1000  C [wt.%]

Temperature of maximum  mass loss [ C]

Sample without palladium

Sample containing palladium

Sample without palladium

Sample containing palladium

54.4 57.9 63.5 37.4 65.0 51.3

33.7 73.7 62.8 13.2 63.0 51.8

532 525 512 562 656 656

534 569 545 536 651 654

D2V(HMMH)Pd samples. The onset of thermal degradation of the D2V(DH 4 ) system is not changed upon introduction of Pd, but the H D2V(DH 4 )Pd sample decomposes more slowly than the D2V(D4 ) one which results in spectacular, 15.8 wt. % growth in the residual mass at 1000  C as compared to the initial system (Fig. 4, Table 3). Additionally, the presence of Pd suppresses (D2V(HMMH) vs D2V(HMMH)Pd samples) or eliminates (D2V(Q(MH)4) vs D2V(Q(MH)4)Pd samples) the first step of thermal degradation (Fig. 4). Moreover, the major decomposition temperatures of all the D2V polymer-derived systems containing Pd are shifted to higher values with respect to those without Pd (Table 3). When thermal behavior of 4D3V3 copolymer-based systems is analyzed (Fig. 4), it is seen that introduction of Pd either decreases thermal stability of the cross-linked polymer (4D3V3(HMMH) and 4D3V3(Q(MH)4) samples) or does not influence it at the beginning (4D3V3(DH 4 ) sample). Residual mass remaining after treatment of the materials at 1000  C is lower (4D3V3(HMMH) and 4D3V3(DH 4) systems) or comparable (4D3V3(Q(MH)4) system) with respect to the samples not containing Pd (Table 3). Similarly to the D2V polymer-based samples, the highest decrease (24.5 wt. %, Table 3) in residual mass upon introduction of Pd is observed for the 4D3V3 copolymer cross-linked with HMMH. It is also worth noting that the course of thermal decomposition of 4D3V3(HMMH)Pd sample, with three distinct DTG minima, is changed as compared to the 4D3V3(HMMH) one (Fig. 3). Values of the temperature of maximum mass losses of the 4D3V3 copolymer-derived samples with introduced Pd are lower than those of the systems without metal particles (Table 3). Thus, the results of TG and DTG investigations demonstrate that Pd exhibits more advantageous influence on thermal properties of the cross-linked D2V polymer than those of the 4D3V3 copolymer. This suggests that the effect of incorporation of Pd into polysiloxane networks may be related to their cross-link density. Higher crosslink densities, as revealed for the systems obtained from D2V polymer in swelling experiments (Section 3.1), are beneficial for thermal stability or the mass remaining at high temperatures of the networks containing Pd. MS analysis confirms that incorporation of Pd has a stronger effect on thermal stability of the D2V polymer-derived systems than of those obtained from the 4D3V3 copolymer. In Fig. 5 and Fig. 6 results of MS investigations are presented. For the sake of clarity, only the ion current (IC) curves corresponding to the most intensive m/z lines of individual species evolved from the systems upon their thermal decomposition are shown in the figures. As can be seen, all mass spectra recorded contain the signals of m/z equal to 16, 27, 45, 59 and 73. In those of D2V(HMMH) and D2V(HMMH)Pd systems an additional line at m/z 85 is observed. The lines at m/z ¼ 16, 27 can be ascribed to methane (m/z ¼ 16, 15, 14, 13, 12) and ethene (m/z ¼ 28, 27, 26, 25, 24) or ethane (m/

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z ¼ 28, 27, 26, 30, 29) [35], respectively. They are, therefore, related to mineralization of the materials studied which proceeds, as has been already mentioned, with the release of gaseous hydrocarbons and H2. The latter, however, has not been detected in our experimental conditions (Section 2.3). It should be noted that formation of methane is typical of thermal transformations of polysiloxane networks containing CH3 substituents at Si atoms [30e35]. It results from radical processes involving SieC and CeH or SieH bonds [32,34,35]. Appearance of ethane or ethene among the decomposition products is due to the cleavage of SieC bonds in the SieCH2eCH2eSi moieties present in the cross-linked polymers and/or decomposition of vinyl groups whose complete consumption, especially during hydrosilylation of 4D3V3 copolymer e owing to their close location in a block of chain e has been very unlikely. Indeed, IR spectra of the cross-linked 4D3V3 copolymer (not shown here) have confirmed the existence of vinyl groups in the systems. Other lines visible in the mass spectra of the samples studied prove that redistributions of Si bonds occur in the systems. Thus, the line at m/z ¼ 45 may originate from CH3SiH3 (m/z ¼ 44, 45, 43, 42, 41, 40), that at m/z ¼ 59 e from (CH3)2SiH2 (m/z ¼ 59, 58, 44, 45, 43, 42) and the one at m/z ¼ 73 e from Si(CH3)4 (m/z ¼ 73, 74, 75 [35]), whereas the line at m/z ¼ 85 may be due to (CH3)3Si(CH] CH2) (the main m/z lines at 85, 73 [35]). Such lines have been also observed in mass spectra recorded during thermal decompositions of other polysiloxane networks [29e31,33e36]. Their formation can be explained by thermal conversions of siloxane units containing two CH3 groups (i.e. the most abundant units in both polymer chains), CH3 groups and H atoms (i.e. moieties originating from cross-linkers) or three CH3 groups (i.e. ends of both polymer chains) in accordance with the following reactions [33,35]:

SiðCH3 Þ2 O  þðCH3 Þ3 SiO  /SiðO  Þ4 þ SiðCH3 Þ4 [

(1)

SiðCH3 Þ2 O  þðCH3 Þ3 SiO  /CH3 SiðO  Þ3 þ SiðCH3 Þ4 [

(2)



3  Si CH3 H O  /2CH3 SiðO  Þ3 þ CH3 SiH3 [

(3)



2  Si CH3 H O  /SiðO  Þ4 þ ðCH3 Þ2 SiH2 [

(4)


2ðCH3 Þ2 Si H O  /  SiðCH3 Þ2 O  þðCH3 Þ2 SiH2 [

(5)


3ðCH3 Þ2 Si H O  /  SiðCH3 Þ2 O  þðCH3 Þ3 SiO  CH3 SiH3 [ (6) However, formation of higher molecular weight Si compounds upon thermolysis of the systems derived from D2V polymer and 4D3V3 copolymer, particularly siloxane and silane oligomers, cannot be excluded. In the conditions of our measurements their detection would not be possible owing to condensation in the capillary related to low volatility of these substances. Therefore the m/z values recorded have been limited to 100 (Section 2.3). IC profiles obtained for the initial networks prepared from D2V polymer (Fig. 5) show e in agreement with TG studies e that D2V(HMMH) and D2V(Q(MH)4) are the least thermally stable systems. Their decomposition starts at ~200  C (D2V(Q(MH)4) sample) and ~250  C (D2V(HMMH) sample) with the evolution of Si compounds which in both cases finishes at ~700  C. Hence, these systems are characterized by a wide temperature range of Si bonds redistributions. D2V(DH 4 ) sample exhibits significantly higher thermal stability: Si compounds are released from it at temperatures between 400 and 700  C. Independently of the cross-linking agent applied, the maximum rate of ethene/ethane formation is at

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Fig. 5. IC curves of the D2V polymer-derived systems before and after incorporation of Pd.

~580  C and that of methane at ~700  C. There are, however, additional stages of methane evolution: at ~580  C (visible for all the samples) and at ~480  C (D2V(HMMH) and D2V(Q(MH)4) samples). Introduction of Pd into the cross-linked D2V polymer

improves thermal stability of the systems: the evolution of Si compounds starts at higher temperatures and is suppressed with respect to the samples not containing Pd. Obviously, low amount of volatile Si compounds formed is the reason for the high residual

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Fig. 6. IC curves of the 4D3V3 copolymer-derived systems before and after incorporation of Pd.

mass remaining after treatment of the D2V(DH 4 )Pd sample at 1000  C (Table 3). On the other hand, low residual mass recorded for the D2V(HMMH)Pd system (Table 3) suggests that in this case mainly higher molecular weight oligomers, not detected in our measurements, are formed.

According to IC curves corresponding to the networks prepared from 4D3V3 copolymer (Fig. 6), the lowest thermal stability is shown by the 4D3V3(HMMH) system whose decomposition starts at ~300  C with the release of CH3SiH3 and (CH3)2SiH2. Degradation of the other two cross-linked 4D3V3 copolymer

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samples begins at 400  C and results in the formation of ethene/ ethane and methane. In all the cases, hydrocarbons are released in two stages with maximum rate for ethene/ethane at ~580  C and for methane at ~720  C. Mass spectra confirm the conclusion drawn from the DTG curves (vide supra) that the major way of thermal decomposition of 4D3V3(Q(MH)4) and 4D3V3(DH 4 ) samples involves cleavage of SieC and CeH bonds during the mineralization step. Exceptionally narrow temperature range (600e700  C) of Si bonds redistributions observed for 4D3V3(DH 4) system is worth noting. Incorporation of Pd suppresses the release of Si compounds in the case of 4D3V3(HMMH) and 4D3V3(DH 4 ) samples, but it does not have a significant effect on the 4D3V3(Q(MH)4) one. Similarly to the D2V(HMMH)Pd system, low residual mass at 1000  C remaining from the 4D3V3(HMMH)Pd sample (Table 3) and low amount of Si compounds detected by MS indicate that higher molecular weight Si compounds are released in this case. Nevertheless, MS results suggest that independently of the starting polymer, Pd incorporated into the investigated polysiloxane networks influences mainly the Si bonds redistribution processes. It seemed interesting to complement MS studies by the analysis of residues formed after pyrolysis of the samples conducted at various temperatures. Therefore, D2V polymer e derived samples (initial and those containing Pd) have been subjected to heat treatment at selected temperatures in Ar flow (Section 2.4). Then FTIR spectra of the solid products obtained have been measured. D2V polymer has been chosen for these studies since, as has been shown, incorporation of Pd exerts the most significant influence on its thermal properties. In all the measured FTIR spectra similar trends, with only slight differences, are visible. Therefore results of these investigations are illustrated in Fig. 7 and Fig. 8 by presenting exclusively the spectra recorded for the pyrolyzed D2V(Q(MH)4) and D2V(Q(MH)4)Pd samples, respectively. Conclusions that can be drawn from the spectra are as follows:  Thermal transformations of the systems not containing Pd begin with disintegration of SieCH2eCH2eSi bridges. This is manifested by the disappearance of the band at 1137 cm1 assigned to CH wagging vibrations [28] from the spectra recorded for all the initial samples pyrolyzed at 250  C. Incorporation of Pd leads to increased stability of these groups. The band at 1137 cm1 is distinguished in the spectra of the systems subjected to 400  C;  SieH groups present in the samples also show low thermal stability. The decrease in the intensities of the bands due to vibrations of SieH bonds at 2145 cm1 and 907 cm1 as pyrolysis temperature grows can be observed. The spectra prove, however, that Pd retards decomposition of these groups. In those of  the D2V(Q(MH)4) (Fig. 7) and D2V(DH 4 ) samples treated at 400 C no bands corresponding to SieH groups are visible, whereas in the spectra corresponding to D2V(Q(MH)4)Pd (Fig. 8) and D2V(DH 4 )Pd systems pyrolyzed at this temperature the band at 907 cm-1 of low intensity is still seen. The exception is the D2V(HMMH)Pd sample in which low amounts of SieH groups remained (Table 2). For this reason, the bands originating from SieH groups in its spectrum (not shown here) have been of low intensity already at the beginning and completely disappeared from that of the sample pyrolyzed at 250  C;  Incorporation of Pd into the cross-linked D2V polymer inhibits formation of SieCH2eSi bridges resulting from the reactions between SieH and SieCH3 or SieCH3 groups. The band at 1359 cm1, ascribed to SieCH2eSi linkages [37,38], is seen in the spectra of all the initial samples pyrolyzed at 400  C (Fig. 7) and in those of the Pd-containing samples e only after treatment at 530  C (Fig. 8);

Fig. 7. FTIR spectra of the D2V(Q(MH)4) sample: initial and pyrolyzed at various temperatures.

 In the case of both types of systems thermal transformations of hydrocarbon groups proceed gradually and are completed at the temperatures above 700  C. This can be judged by changes in intensities of the bands at 2964, 2882, 1415, 1359 and 1261 cm1. The spectra of the materials obtained at 1000  C are typical of SiCO ceramics [37e40]. This means that the final pyrolysis products of the systems containing Pd are composites in which metallic particles are dispersed in SiCO matrix. Thus, FTIR spectra of the cross-linked D2V polymer pyrolyzed at various temperatures confirm beneficial influence of incorporated Pd on thermal stability of the systems. In particular, the increased decomposition temperature of SieH groups in the presence of Pd should be noted. Since these groups are involved in the formation of most volatile Si compounds revealed in MS studies (reactions 3e6 shown above), their presence retards release of these species from the Pd-containing systems. The delayed appearance of SieCH2eSi bridges in Pd-containing samples, observed in FTIR investigations can be, in turn, the reason for lowering in residual mass at 1000  C detected for several systems after incorporation of metal particles. These bridges constitute additional cross-linking sites which are especially important when difunctional hydrogensiloxane, i.e. HMMH, is applied as the cross-linking agent. In this case, disintegration of one SieC bond in

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played by the molecular structure of the cross-linking agent applied for the preparation of polysiloxane networks in thermal properties of these networks with incorporated Pd. 4. Conclusions Based on the investigations conducted in the work it can be concluded that: 1. It is possible to prepare metallic Pd particles dispersed in regular polysiloxane networks by a redox process between SieH groups present in the polymer matrix and Pd2þ ions from the solution. 2. Pd incorporated into the networks modifies thermal properties of the systems, influencing mainly redistributions of Si bonds occurring in Ar atmosphere in the temperature range of 200e700  C (the initial systems) and 400e700  C (the systems containing Pd). 3. The retarded emission of volatile Si compounds upon thermolysis observed for most samples with introduced Pd is due to more difficult decomposition of SieH and SieCH3 groups as well as SieCH2eCH2eSi linkages than in the samples not containing Pd. 4. The influence of Pd on thermal properties of polysiloxane networks is related to their cross-link density. Higher cross-link densities of the starting networks are advantageous for thermal stability of the systems with incorporated Pd. Molecular structure of the cross-linking agent applied also matters. Pyrolysis of the highly cross-linked system containing thermally stable cyclic siloxane moieties with introduced Pd particles results in higher residual mass remaining at 1000  C than that recorded for the process to which the starting sample is subjected. 5. Pyrolysis of polysiloxane networks with introduced Pd particles conducted at 1000  C in Ar flow leads to composite materials comprising metallic Pd particles dispersed in SiCO ceramic matrix. Fig. 8. FTIR spectra of the D2V(Q(MH)4)Pd sample: initial and pyrolyzed at various temperatures.

References

SieCH2CH2eSi moiety results in the decrease in cross-linking degree of the polymer. This allows oligomeric siloxane compounds to be formed upon thermal decomposition of the systems which accounts for the dramatic drop in the residual mass of D2V(HMMH)Pd sample vs D2V(HMMH) one (Fig. 4, Table 3). Most probably, similar phenomena are responsible for the difference in residual mass remaining at 1000  C from 4D3V3(HMMH)Pd and 4D3V3(HMMH) systems (Fig. 4, Table 3). It can be proposed that more difficult thermal decomposition of the cross-linked D2V polymer with incorporated Pd than the initial network is related to the location of metal particles between its chains which prevents the groups (SieH, SieCH3) present in the macromolecule from interactions. Stronger influence of Pd on thermal properties of D2V polymer-derived networks than the 4D3V3 copolymer-based ones may be due to the difference in their cross-link densities. Higher cross-link density of the former causes that separation of the reactive groups by Pd particles has a more pronounced effect on their interactions than in the case of the latter one where distances of reactive groups are large even before introduction of Pd. Additionally, significant increase in residual mass preserved after decomposition at 1000  C of the D2V(DH 4 )Pd sample as compared to the D2V(DH 4 )Pd one (Fig. 4, Table 3) may be related to high thermal stability of the cyclic structure of the crosslinker [4] contained in this system. This points to the important role

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