Xibing Zhana,
Xiqing Caib and
Junying Zhang*b
aCollege of Chemical and Material Engineering, Quzhou University, Zhejiang 324000, China
bLab of Adhesives and In-situ Polymerization Technology, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: zjybuct@gmail.com; Fax: +86 10 64425439; Tel: +86 10 64425439
First published on 3rd April 2018
A novel cross-linker polymethyl(ketoxime)siloxane (PMKS) with dense pendant reactive groups based on polymethylhydrosiloxane (PMHS) was synthesized via dehydrocoupling reaction. The novel PMKS cross-linker was applied to a hydroxyl-terminated polydimethylsiloxane (HPDMS) matrix to prepare a series of novel RTV silicone rubbers. The chemical structure of PMKS and curing reaction between HPDMS and PMKS by hydrolytic condensation were verified by IR spectroscopy and 1H NMR. Thermal stability and mechanical properties of these novel RTV silicone rubbers have been studied by means of thermal gravimetric analysis (TGA) and universal tensile testing machine, respectively. The results displayed that a pronounced enhancement effect of the novel cross-linker PMKS on thermal stabilities and mechanical properties of RTV silicone rubbers as compared with the traditional cross-linking agent of methyltris(methylethylketoximino)silane (MTKS). Subsequently, the degradation residues were also characterized by FT-IR and X-ray photoelectron spectrometer (XPS). It was found that the striking enhancements in thermal properties and improvements on mechanical properties could be the synergistic effect of the T-type branched structure of PMKS cross-linker, in situ formation of dense PMKS phase in the chain network by self-crosslinking and the uniform distribution of PMKS cross-linker in the HPDMS matrix.
As the most common member of the polysiloxane family, polydimethylsiloxane (PDMS) has been shown to be thermally unstable above 300 °C under vacuum.14 At elevated temperatures, the polysiloxane materials will undergo a dramatic chemical change such as the rearrangement of molecular bonds.15 So far, a variety of techniques and processes have been reported on the improvement in thermal resistance of polysiloxane materials at elevated temperatures. One of the most popular techniques is the composition or hybridization with some heat-resistant groups and elements, including phenyl,16,17 fluorene and adamantine,18,19 polysilsesquioxane (POSS),20–26 and boron elements in siloxane chains,27,28 or inorganic additive blending (such as silica, Al2O3 and montmorillonite clay, etc.) with polysiloxane compounds.29–32 The incorporation of methyl-phenyl siloxane or diphenyl siloxane as a copolymer with PDMS has been shown to increase the onset temperature of degradation to nearly 400 °C.16 Most of the methods mentioned above focused on the incorporation of heat-resistant and rigid groups or inorganic additives. Nevertheless, these polysiloxanes with heat-resistant organic groups generated a few complex degradation products which were difficult to identify and analyze, and the degradation mechanisms were very various and complicated at high temperature.33,34 The inorganic additives are not compatible or hard to disperse in polysiloxane polymer. Some investigation showed that the use of concentrative crosslinking might be one of the effective methods to improve the mechanical strength of siloxane elastomer.35 However, very little attention had been paid to improvement of thermal resistance by enhancement of crosslinking network structure. In the past few years, our research team has been carried out some studies on polysiloxane crosslinking agents with many pendant alkoxy groups (PMOS) instead of traditional small crosslinking reagents (such as tetraethoxysilane).11,36 This new cross-linkers can be cured with atmospheric moisture and form 3D networks that had much more thermal resistance (6% mass loss at 600 °C for product of PMOS self-crosslinking) compared to the polysiloxane incorporated with phenyl groups. But the tack free time of this system including PMOS crosslinker and hydroxyl-terminated polydimethylsiloxane (HPDMS) by dealcoholization was much longer than traditional curing systems of deacidification and deketoximization. Herein, it is necessary to explore a new macromolecular crosslinking agent with plenty of pendant active groups combined with good thermal stability and high curing reactivity on the basis of PMOS compounds.
The present work is mainly concerned with thermal stability and mechanical properties of novel room temperature vulcanized (RTV) silicone materials. Thus, a new kind of cross-linking agent polymethyl(dimethylketoxime)siloxane (PMKS) with dense pendant ketoxime groups was synthesized by means of dehydrocoupling reaction, and cured with HPDMS compounds in the moisture environment via hydrolysis–condensation to form three-dimensional cross-linking networks. We also investigated the mechanical properties and thermal stabilities (including thermal degradation process and degradation residues by TGA, infrared spectroscopy and XPS) of the novel silicone rubber. It is found that PMKS cross-linker was favorable to enhancement in the thermal stabilities and mechanical properties of the novel RTV silicone rubbers in comparison with traditional crosslinking agent methyltris(methylethylketoximino)silane (MTKS).
The catalyst of 0.3 wt% dibutyltin dilaurate was added to the mixture of PMKS (or MTKS) and hydroxyl-terminated PDMS and stirred by mechanical agitator, and then was cured in the following curing equipment (Fig. 1) for 36 h at 25 °C. The curing equipment is a glass container in which the mixture of PMKS (or MTKS)/HPDMS/catalyst is poured into a Teflon mould, a bottle of water about 10 mm depth is added to keep at 100% relative humidity and nitrogen was purged at intervals through inlet and outlet to remove the small molecule from the condensation reaction between HPDMS and PMKS (MTKS).
The samples S10 as reference materials were designed to compare the difference between traditional cross-linker such as methyltris(methylethylketoximino)silane (MTKS) and PMKS, in which the mole content of methylethylketoximino is the same as that of acetone oxime in sample of S5.
Elemental composition of the degradation residues were got by X-ray photoelectron spectrometer (XPS) (ESCA Lab 250, UK) using a standard Al Kα source in the analysis chamber (operating conditions: 15 kV voltage, 20 mA current and 2 × 107 Pa pressure) during the experiments.
Thermogravimetric curves (TGA) were obtained using a Thermogravimeter (NETZSCH STA 449C, Germany) ranging from 50 °C to 800 °C with a heating rate of 10 °C min−1 in N2.
Five specimens with dumb-bell shape were fabricated according to GB/T 528-2009 standard. Tensile tests were conducted using a universal testing machine (CMT4104, China) at speed of 10 mm min−1. The Young modulus was determined from the initial slope of the stress–strain curve (1–5% strain range of stress–strain curve).
The FT-IR spectra of PMHS and PMKS are shown in Fig. 2(a). From spectrum of PMHS to spectrum PMKS, the signal of Si–H group (2250 cm−1) disappears while that of Si–O–NC(CH3)2 group (1650 cm−1) emerges. These observations seem to reflect the occurrence of dehydrocoupling reaction between PMHS and acetoxime. As we all known, the chemical shift of the Si–H proton of PMHS was at around 4.6 ppm. In Fig. 3, this signal disappeared and a new signal at 1.86 ppm appeared which belonged to the proton of Si–ON
C(CH3)2 units of PMKS. Therefore, these data from both FT-IR and 1HNMR spectra can indicate that the dehydrogenation between PMHS and acetoximes occurs and the molecular structure of PMKS is the same as expected.
The crosslinking reaction of RTV silicone rubber can be monitored by attenuated total reflection infrared (ATR-IR) spectroscopy on the silicone elastomeric surface. The FT-IR spectra of cured PDMS/HPDMS system were dramatically changed in comparison with that of PMKS, and the difference were easily discerned in Fig. 2(b). It is reported that a broad peak at 3448 cm−1 and a weak peak at 1630 cm−1 of HPDMS spectrum are attributed to stretching and deformation vibration of silanol (Si–OH), respectively.20 Two peaks mentioned above completely diminished in the spectrum of cured product. Additionally, the signal at 1650 cm−1 belonging to Si–O–NC(CH3)2 group of PMKS utterly disappeared after HPDMS was cured and the liquid mixture of PMKS/PDMS becomes elastic solid, which suggested that the silanol groups had reacted completely with cross-linker and new cross-linking network of Si–O–Si had formed in the matrix of PDMS. Moreover, the tack free time (tt) which referred to the surface cure time, namely the surface tackiness disappeared when touching the surface with fingers, at a certain temperature and humidity according to GB/T 13477.5-2002, and full cure time (tf) was 120 min, 36 h for HPDMS/PMKS system, 300 min, 48 h for HPDMS/PMOS system and 110 min, 34 h for HPDMS/MTKS in the presence of same amount of catalyst and reactive groups of cross-linking agent, respectively. It implies that the PMKS has much higher reactivity than PMOS, and is similar reactivity to MTKS.
The thermogravimetric curves of HPDMS/PMKS system with different amounts of PMKS (from S1 to S5) and MTKS/HPDMS in N2 were shown in Fig. 4 and the residual masses at different temperature were summarized in Table 2. The characteristic temperature of 10% weight loss for novel RTV silicone rubber was ranged from 455.6 °C (S1) to 492.4 °C (S5), which was much higher than that of HPDMS/MTKS (470.7 °C for S10) except for S1 because the content of PMKS crosslinker in S1 was far less than that of S10. The residual mass at 400 °C showed little difference, but the residual mass at 500 °C was very different and the value was 64.8%, 77.2%, 86.8%, 87.3% and 35.2% corresponding to S1, S3, S4, S5 and S10, respectively. The TG curves exhibited that the residual mass for HPDMS/PMKS system rose at 500 °C with an increase in the content of PMKS. Similarly, the residual mass at 600 °C and 800 °C exhibited the same trends, and all of PMKS/HPDMS (except for S1) silicone rubbers were of higher decomposition temperature and more residual mass than MTKS/HPDMS (S10) system. From the above discussion, it is clearly found that PMKS cross-linker has a significant enhancement on thermal stability of the PDMS polymer system relative to conventional cross-linkers.
![]() | ||
Fig. 4 Thermogravimetric curves of crosslinked siloxanes with different weight percent of PMKS and MTKS. |
Sample | Residual mass at different temperature (%) | |||
---|---|---|---|---|
400 °C | 500 °C | 600 °C | 800 °C | |
S1 | 99.6 | 35.2 | 6.5 | 6 |
S3 | 98.9 | 77.2 | 13.3 | 12.9 |
S4 | 99.5 | 86.8 | 19.1 | 18.9 |
S5 | 98.8 | 87.3 | 23.9 | 21.8 |
S10 | 99.2 | 64.8 | 7.3 | 7.2 |
It's well known that polysiloxane undergoes stepwise degradation of the backbone and oxidation of the methyl groups above 300 °C (Fig. 5).41,42 Although the Si–C bond is thermodynamically less than the Si–O bond, thermal degradation of polysiloxane occurs by depolymerization through the Si–O bonds rearrangement, leading to the production of cyclic oligomers.14,43 PMKS is a branched molecule with rich pendant acetoxime groups, which maybe has the ability to destroy the helical coiling structure of polysiloxane and form T structure units to make the network compact.11 The dense network can constrain the motion of Si–O–Si chain segment. Therefore, the branched and T-type structure of PMKS can prevent the rearrangement of Si–O bonds in polysiloxane and the cyclic oligomers can be blocked. Moreover, the Si–CH3 bond scission at higher temperature will form some radical and these macro-radicals may also cross-link by coupling each other.42 Steric hindrance and the cross-linked networks of the three-dimensional macro-radicals decrease the flexibility of the PDMS chain, prevent splitting of cyclic oligomers still further and retard further degradation of the PDMS chain.44 As a result, PMKS/HPMDS (S3, S4 and S5) have better thermal resistance than MTKS/HPDMS (S10) and the thermal resistance increases with the PMKS contents.
The derivative curves of TGA of PMKS/HPMDS (S1, S3, S4 and S5) are presented in Fig. 6. It can be found that there are two degradation peaks, including a shoulder and a maximum degradation peak, for PMKS/HPMDS from Fig. 6, which indicated the existence of different degradation mechanisms. With increment of PMKS content, the intensity of maximum peaks at 500 °C gradually weakened (S1) to be a shoulder (S3, S4 and S5), and the original shoulder peak at 550 °C gradually strengthened (S1) to be a maximum peak (S3, S4 and S5). Thus, the first step degradation maybe corresponds to the decomposition of structures like PDMS networks, while the second step degradation may be related to the PMKS content.11 The reaction order and decomposition activation energy can be calculated to evaluate the rate of the new degradation (the second step) behavior of HPDMS with different PMKS content.
The pyrolysis reaction order (n) can be calculated by Kissinger method for the non-isothermal degradation:
![]() | (1) |
A dimensionless parameter (α) can be defined as follows:
![]() | (2) |
In eqn (2), M, M0 and Mf represent the sample weight at different temperature, the initial sample weight at 25 °C and the steady-state weight at 800 °C, respectively.
Additionally, a typically kinetic equation can be expressed as:
![]() | (3) |
Eqn (4) is an integral form of eqn (3) with the initial condition of α = 0 at T = T0 and can be expressed as follows
![]() | (4) |
For the temperature (T) ranging from 0.9 TM to 1.1 TM (TM obtained from the peak of DTG curve), the following approximation is made by Van Krevelen method
![]() | (5) |
![]() | (6) |
A slope can be obtained from the plot of ln[g(α)] against lnT, and then the activation energy can be calculated.
Reaction parameters for the second degradation of S3, S4 and S5 are calculated and summarized in Table 3 according to Kissinger's method eqn (1) and Van Krevelen eqn (6). The decomposition activation energy increase from 98.3 kJ mol−1 to 135.5 kJ mol−1 with the PMKS contents increasing at a heating rate of 10 °C min−1 maybe because the mobility of the molecular chain is restrained. Camino47 have made a research about the correlation between activation energy and heating rate for polysiloxane and found that the activation energy was inversely proportional to heating rate and ranged from 54 up to 250 kJ mol−1 in the degradation process. The activation energy measured between 98 and 135 kJ mol−1 for PMKS cross-linked HPDMS may be reasonable compared with those values mentioned above.
Sample | Shape index, s | TM/(°C) | Reaction order, n | ΔE (kJ mol−1) |
---|---|---|---|---|
S3 | 0.107 | 559.5 | 0.462 | 98.3 |
S4 | 0.106 | 563.4 | 0.432 | 121.7 |
S5 | 0.099 | 571.3 | 0.459 | 135.5 |
All of the IR spectra of solid degradation residues exhibited a similar trend, and the strongest bands in the spectra are assigned to the asymmetric Si–O–Si vibration between 1086 and 1012 cm−1. Although the characteristic stretching vibration peak of C–H of Si–CH3 in the vicinity of 2964 cm−1 in Fig. 2(b) is indiscernible in Fig. 7, the characteristic bending vibration peak of Si–C at 798 cm−1 is discernible. It indicates that the degradation residues in air contain the same compositions. The black degradation residues in N2 are mainly siliconoxycarbide,44,48 but the degradation residues in air are the grayish white powder which consists of mainly white silica and small amount of black siliconoxycarbide.48 Furthermore, the Si–O–Si peaks remain but change from double peaks to one broad single peak at 1120 cm−1, which indicates that the Si–O long polymer chains have cleaved (Fig. 7). The bands at 788 cm−1 assigned to Si–CH3 in S4 and S5, however, are much higher than that in spectra of S10. It may be concluded that PMKS can reduce the oxidation degradation of carbon element in polydimethylsiloxane. The observed improvement effect of different contents of PMKS on thermal stability of silicone rubber is ascribed to the T-type branched structure of PMKS. In other words, PMKS has more than 16 active sites in the side chain as compared with the traditional tri-functional cross-linkers (MTKS), which facilitates formation of 3D networks during the curing process. The three-dimensional network of PDMS polymers decrease the flexibility of the PDMS chain, retard motion of polymer chain and block the formation of cyclic oligomers, and hence elevate the decomposition temperature of the PDMS polymer.
The elemental compositions of HPDMS/PMKS bulk containing different PMKS contents (S3, S4 and S5) after 800 °C degradation were recorded by XPS which can detect the chemical element with the order of 1–5nm, to verify the conjecture that PMKS could depress the oxidation degradation of carbon portion in polysiloxane. The XPS spectra of S3, S4 and S5 for C1s, O1s and Si2p regions were illustrated in Fig. 8, and each of the atomic ratio (C/O, C/Si and Si/O) from XPS analysis was summerized in Table 4. In general, the polydimethylsiloxane materials could undergo a dramatic chemical change after 800 °C degradation in air and carbon element could disappear mainly leaving SiO2. However, the carbon element didn't totally disappear but still remained in degradation residues, and the C/Si ratio value for S3, S4 and S5 was 0.72, 0.74 and 0.82, respectively. The value of C/Si ratio gradually increased with the PMKS contents rising which indicated that the PMKS could reduce the oxidation degradation of carbon portion. This result was fairly consistent with data from IR analysis.
Sample | C | O | Si | Si/O ratio | C/Si ratio | C/O ratio |
---|---|---|---|---|---|---|
S3 | 18.92 | 55.01 | 26.08 | 0.47 | 0.72 | 0.34 |
S4 | 20.82 | 51.18 | 28.00 | 0.54 | 0.74 | 0.41 |
S5 | 23.52 | 48.12 | 28.48 | 0.59 | 0.82 | 0.49 |
Sample | Tensile strength (MPa) | Elongation at break (%) | Density (g cm−3) | Modulus (MPa) | Crosslink density (10−3 mol kg−1) |
---|---|---|---|---|---|
S1 | 0.38 | 222 | 0.89 | 0.28 | 0.79 |
S3 | 0.86 | 276 | 0.95 | 0.34 | 1.35 |
S4 | 0.47 | 134 | 1.01 | 0.51 | 1.45 |
S5 | 0.46 | 116 | 1.01 | 0.56 | 1.63 |
S10 | 0.27 | 89 | 0.93 | 0.43 | — |
In order to confirm that PMKS can form dense crosslinked network phases in polysiloxanes, the average crosslinked densities of PMKS crosslinked polysiloxanes were studied and can be calculated according to eqn (7) for uniaxial extension.
![]() | (7) |
![]() | (8) |
The average crosslink density can be calculated according to eqn (7) and depicted in Fig. 9 and Table 5. The average crosslink density increases as the loading of PMKS goes up, demonstrating that more dense PMKS phases are formed in the network. In the course of crosslinking reaction, excessive PMKS chains will form high crosslink density phased by the self-hydrolytic condensation of their rich pendant active groups. The hydroxyl-terminated PDMS chains turns to long chain networks because of the hydrolytic condensation between the hydroxyl group of HPDMS and the acetoxime groups of PMKS. As the molecular weight of PMKS (103 g mol−1) is much lower than that of HPDMS (104 g mol−1) and both of them are linear oligomers, HPDMS chain will form a loose crosslinking network with PMKS, whereas the residual PMKS turn to a dense phase by self-condensation and disperse into the continuous phase of HPDMS/PMKS. Therefore, PMKS dense phase might hinder the formation of cyclic oligomers and depress the oxidation degradation of carbon portion in polysiloxanes.
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