Yang Mengab,
Junfeng Chuab,
Jiajia Xueab,
Chaohao Liuab,
Zhen Wangc and
Liqun Zhang*ab
aState Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, 100029, P. R. China. E-mail: zhanglq@mail.buct.edu.cn; Tel: +86 10 64423312
bKey Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, 100029, P. R. China. E-mail: zhanglq@mail.buct.edu.cn; Tel: +86 10 64423312
cAVIC Beijing Institute of Aeronautical Materials, 100095, P. R. China. E-mail: jenny.wzh@163.com
First published on 2nd July 2014
Although polysiloxane elastomers have many merits, their fast crystallization at low temperature is problematic in some fields. In this study, a novel non-crystallizable, low-Tg epoxidized polysiloxane (ESR) with functional epoxy groups in side chains was designed and synthesized though two steps: (i) the preparation of poly(methylvinylsiloxane) (SR) by anionic ring-opening copolymerization of 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl cyclotetrasiloxane and octamethylcyclotetrasiloxane, and (ii) the subsequent epoxidation of the SR. Reaction kinetic studies demonstrated that the epoxidation of SR was a second-order reaction and more than 90% of the double bonds were converted into epoxy groups during the epoxidation. Despite a slight increase in the Tg of ESRs as the content of epoxy groups increased, the low-temperature performances of ESRs were greatly improved because of the inhibition of the crystallization of polysiloxane chains. Surprisingly, the ESRs also showed higher thermal degradation temperatures than the traditional poly(dimethylsiloxane) did. The excellent low-temperature performance and high degradation temperatures endowed the ESR with great potential as an elastic material in the aerospace industry where materials have to undergo very high and low temperature.
Despite having many excellent properties, polysiloxane elastomers show some weaknesses, which affect their practical use in some fields. One major drawback of polysiloxane elastomers is their fast crystallization at low temperature. Although polysiloxane elastomers have very low glass transition temperature (Tg), most of them will become stiff and cannot be used as elastic materials at temperatures far higher than their Tg.10–12 For instance, poly(dimethylsiloxane) (PDMS), the most extensively studied polysiloxane by far, shows its Tg at about −125 °C but its melting point is as high as about −40 °C.13 Because of the crystallization, polysiloxane elastomers cannot make full use of the advantage of their very low Tg and their applications, especially the aerospace where the materials have to undergo very low temperature, are seriously limited. In addition to the crystallization, the low mechanical properties, weak oil resistance, and poor hydrophobicity are also problematic for most of polysiloxane elastomers.14–20 In addition, some properties of polysiloxane elastomers, such as thermal stability, still need to be improved to meet the applied requirements. Therefore, designing new polysiloxane elastomers with high performances is of great importance.
Chemical modification is a useful method to alter and optimize the properties of polymers. Of all the well-known chemical modifications of polymers, epoxidation is one of the most promising and advantageous methods due to its relatively mild reaction conditions and significant improvement in the properties of the polymers.21–24 Epoxidation of polymers is typically performed with organic peracids (such as m-chloroperbenzoic acid and magnesium monoperoxyphthalate) or a combination of a transition metal catalyst and a co-oxidant (such as H2O2 and t-BuOOH).25,26 Theoretically, all polymers containing unsaturated carbon bonds can be epoxidized under appropriate reaction conditions. The introduction of polar, functional epoxy groups into polymer chains can improve the properties of the final polymer, such as oil resistance, damping property, and gas barrier property.27–30 Owing to the formation of hydrogen bonds or covalent bonds between epoxy groups and inorganic fillers (e.g., silica), the presence of epoxy groups can enhance the interaction between polymers and inorganic fillers, leading to a good dispersion of fillers and improved mechanical properties of polymer composites.31,32 Epoxidation of bacterial polyesters carried out by Park et al.33 showed that epoxidation can also affect the thermal properties, such as raising the glass transition temperature, decreasing the melting temperature, and inhibiting crystallization. Moreover, the epoxy groups can react with other groups, such as amino and carboxyl, and endow polymers with great potential for further chemical modification.34,35
Based on the merits of epoxidized polymers, the introduction of epoxy groups into polysiloxane elastomer chains would be meaningful because the epoxy groups would inhibit the crystallization of polysiloxane elastomers and endow the polysiloxane elastomers with reactive groups for further chemical modification. The polar and reactive epoxy groups would be also beneficial for improving the compatibility of polysiloxane with other polar polymers and enhancing the interaction between polysiloxane and fillers. A few studies on the synthesis of silicone–epoxy resins or silicone–epoxy monomers have been reported.36–39 In these studies, the epoxy groups were mainly introduced into silicones by the hydrosilylation at the presence of catalyst, e.g. platinum. The silicones should have Si–H in side or end of the silicone chains, and the molecular weights of the silicones were usually low, resulting in a low molecular weight of the silicone–epoxy products. Yang et al.40 reported a method of synthesizing silicone–epoxy polymer by the condensation of silicone–epoxy monomer and dimethyldiethoxysilane, but the molecular weight of the product was still very low. Eckberg et al.41 prepared a silicone–epoxy polymer by epoxidation of the silicone containing CC groups; however, the silicone containing C
C groups still needed to be prepared first by the hydrosilylation. In addition, the above reported silicone–epoxy monomers or linear silicone–epoxy polymers are often used for reins or coatings. The use of silicone–epoxy polymers as elastomers, to our knowledge, has not been reported.
In this study, we bring out a strategy to synthesize epoxidized polysiloxane (ESRs) elastomers, in which we firstly synthesized poly(methylvinylsiloxane) (SRs) via common anionic ring-opening polymerization based on 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl cyclotetrasiloxane and octamethylcyclotetrasiloxane, and then conducted an epoxidization reaction. The peracid, m-chloroperbenzoic acid (MCPBA), was used to synthesize ESR because MCPBA has proved to be very effective for the epoxidation of unsaturated polymers.42–44 Chloroform, because of its solubility in both SR and MCPBA, was selected as the reaction medium for the epoxidation. A series of non-crystallizable, low-Tg epoxidized polysiloxane (ESRs) with high molecular weights were successfully prepared. These ESRs have excellent low-temperature performance and high thermal degradation temperatures.
![]() | (1) |
![]() | (2) |
![]() | ||
Fig. 1 NMR spectra of poly(methylvinylsiloxane) (SR-20): (a) 1H NMR spectrum and (b) 13C NMR spectrum. |
Table 2 shows the number-average molecular weights (Mn), weight-average molecular weights (Mw), and molecular weight distributions of SRs with different molar ratios of V4 to D4. The number-average molecular weights are in the range 200000–300
000, which is similar to those of commercially available PDMS. Although anionic ring-opening polymerization method was adopted to synthesize SRs, the molecular weight distributions of SRs are a little broad. As the molar ratio of V4 to D4 increases, the molecular weight distributions of SRs increase from 1.67 to 2.46. The broad molecular weight distributions of SRs could be ascribed to the rearrangement reaction and chain transfer reaction during the polymerization, as shown in Scheme 2.47 In the process of chain growth, the active center can bite back to form different cyclic oligomers (Scheme 2(a)) or attack the other polysiloxane chains to form new active center (Scheme 2(b)), leading to randomization of polysiloxane chains and broad molecular weight distributions.
Samples | D4![]() ![]() |
Mn | Mw | Mw/Mn | Vinyl content (mol%) |
---|---|---|---|---|---|
PDMS | — | 278![]() |
501![]() |
1.80 | 0.15 |
SR-5 | 95![]() ![]() |
269![]() |
449![]() |
1.67 | 5.4 |
SR-10 | 90![]() ![]() |
242![]() |
443![]() |
1.83 | 10.2 |
SR-15 | 85![]() ![]() |
221![]() |
514![]() |
2.46 | 15.6 |
SR-20 | 80![]() ![]() |
228![]() |
533![]() |
2.34 | 20.1 |
![]() | ||
Scheme 2 The (a) rearrangement reaction and (b) chain transfer reaction during the synthesis of SRs. |
Additionally, we measured the vinyl content of SR by the bromine-iodometric technique, considering that the practical vinyl content would be different from the theoretical vinyl content because some cyclic oligomers existed at the end of polymerization. Table 2 shows that the measured vinyl content of a SR is higher than the theoretical value calculated from the molar ratio of D4 to V4, but the difference between the measured vinyl content and the theoretical value is very small.
![]() | ||
Fig. 2 NMR spectra of epoxy-poly(methylvinylsiloxane) (ESR-20): (a) 1H NMR spectrum and (b) 13C NMR spectrum. |
To calculate the epoxidation yield of ESRs with different contents of vinyl groups, two methods—1H NMR and titration—were used. In the 1H NMR method, the unchanged integral of CH3 was selected as the primary integral 1 for all the samples, and the epoxidation yield was calculated from the integrals of the –CHCH2 signals before and after epoxidation (see Fig. S3†). In the titration method, the epoxidation yield was calculated from the ratio of measured epoxy values (EV) to theoretical EV. The epoxidation yields, epoxy contents, and EV of the various ESR samples are shown in Table 3. The epoxidation yields calculated by the 1H NMR method and the titration method are similar, and are all higher than 90%. The molecular weights, molecular weight distributions and gel contents of the ESR samples are also listed in Table 3. Compared with those of SR, the molecular weights and molecular weight distributions of the ESR samples do not change much, indicating that there are not many side reactions, such as degradation, during the epoxidation. The gels contents of the ESR samples are very low, demonstrating that there is little crosslinking between the ESR macromolecular chains.
Samples | Mn | Mw | Mw/Mn | Gel (%) | Epoxidation yield by NMR (%) | Epoxidation yield by titration (%) | Epoxy contenta (mol%) | Epoxy valuea (mol 100 g−1) |
---|---|---|---|---|---|---|---|---|
a Epoxy content and epoxy value were calculated by titration. | ||||||||
ESR-5 | 254![]() |
444![]() |
1.75 | 0 | 91.4 | 92.2 | 4.98 | 0.066 |
ESR-10 | 243![]() |
459![]() |
1.89 | 0.2 | 93.3 | 90.3 | 9.21 | 0.120 |
ESR-15 | 210![]() |
567![]() |
2.69 | 0.1 | 94.1 | 91.1 | 14.21 | 0.191 |
ESR-20 | 201![]() |
480![]() |
2.35 | 0.3 | 93.0 | 90.5 | 18.20 | 0.238 |
Some studies using MCPBA to prepare epoxides showed that the epoxidation reaction was initially second-order.22,48 Assuming that the epoxidation reaction is second order, we can express the epoxidation rate for SR by eqn (3):
![]() | (3) |
![]() | (4) |
![]() | (5) |
Eqn (5) can be further expressed in terms of the double bond conversion w:
![]() | (6) |
Fig. 5 shows the results of Fig. 4 plotted in accordance with eqn (6). For all the SR samples, the plots are linear, illustrating that the epoxidation of SR using MCPBA follows a second-order mechanism. In addition, the SR samples with different contents of vinyl groups show different apparent second-order rate constants (k), and the rate constant decreases with increasing the content of vinyl groups. The difference in k values is probably due to the different chemical structures of the ESRs. The commonly accepted mechanism for epoxy formation is the “butterfly” mechanism, which involves a cyclic polar process where a proton is transferred intramolecularly to the carbonyl oxygen with a concerted attack on the alkene by the hydrogen-bonded peracid.49 The reaction rate depends significantly on the chemical structures of alkene and peracid; the electron-donating groups on the alkene and the electron-withdrawing groups on the peracid can accelerate the epoxidation rate.50 Steric hindrance also plays an important role in the epoxidation rate.51 Because the groups adjacent to the carbon double bonds are similar, the difference in k values should not be ascribed to the electron-donating ability of the adjacent groups but to the steric hindrance. For the SR samples with high contents of vinyl groups, the adjacent vinyl groups and the generated epoxy groups would hinder the epoxidation, resulting in a decrease in epoxidation rate.
![]() | ||
Fig. 6 DSC curves of PDMS and SRs with different contents of vinyl groups: (a) DSC cooling curves and (b) DSC heating curves. |
When the vinyl groups in SR are converted into epoxy groups, the thermal properties such as glass transition temperatures (Tg) and crystallization show great changes. As shown in Fig. 7(a), when the content of epoxy groups reach 4.98 mol% (ESR-5), no exothermic peak of crystallization can be seen in the DSC cooling curves, indicating that the crystallization of ESR is inhibited when the sample is cooled to −150 °C at 10 °C min−1. However, in the DSC heating curve of ESR-5 (Fig. 7(b)), an exothermic peak corresponding to cold crystallization appears in the range of −105 to 85 °C when the sample is heated from −150 °C to 20 °C, implying that the ESR-5 is still crystallizable. In addition, ESR-5 shows a lower ΔHm (21.2 J g−1) than SR-5 does (see Table S4†), suggesting that epoxy groups have a greater impact on the crystallization of PDMS than vinyl groups do, probably because of the larger steric hindrance of epoxy groups than that of vinyl groups. In the ESR samples with contents of epoxy groups higher than 9.21 mol% (e.g., ESR-10), neither an endothermic peak nor an exothermic peak can be seen in the DSC curves. The results from the DSC curve of ESR-10 further confirm that epoxy groups have a greater impact on the crystallization of polysiloxane than vinyl groups do in that an endothermic peak and an exothermic peak can be still seen in the DSC heating curve of SR-10 (Fig. 6(b)).
![]() | ||
Fig. 7 DSC curves of PDMS and ESRs with different epoxy values: (a) DSC cooling curves and (b) DSC heating curves. |
The introduction of epoxy groups into PDMS side chains also has a great impact on the Tg of PDMS. As shown in Fig. 7(b), the Tg increases substantially with increasing content of epoxy groups. Similarly, epoxidation was also reported to increase the Tg s of natural rubber, styrene butadiene rubber, and other polymers such as polyesters.33,39,57 The increase in Tg of ESR is attributed to the polar epoxy groups, which can increase the intramolecular interactions. Fig. 8 shows the plot of Tg versus epoxy content of ESR. The plot is linear and shows that the Tg increases by 0.69 °C as the content of epoxy groups increases 1 mol%. The increase rate of Tg with the content of epoxy groups is close to the that of other polymers reported in the literature.33,57
Dynamic mechanical analysis (DMA) can measure the phase transitions, such as melting, crystallization, alpha transition (glass), and beta transition of a material by vibrating the material sinusoidally at a constant frequency and small amplitude. For the determination of the phase transitions of materials, DMA is more sensitive than other measurements such as differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (mDSC).53 In this study, DMA was used to further investigate the phase transitions and low-temperature resistance of the crosslinked ESR composites with silica. Fig. 9(a) shows the elastic modulus (E′) of ESR composites with different epoxy contents versus temperature. The E′ of PDMS composite decreases as the temperature increases from −120 °C to −100 °C, and then decreases rapidly as the temperature increases from −50 °C to −40 °C. The changes of E′ in the range of −120 to 100 °C is ascribed to the glass transition, and that in the range of −50 to 40 °C owns to the crystal melting. The change of E′ in the range of −120 to 100 °C is far smaller than that in the range of −50 to 40 °C, implying that PDMS has a very high degree of crystallinity. In addition, E′ can reflect the mechanical properties of PDMS composite; the sharp increase of E′ in the range of −50 to 40 °C means that the PDMS composite becomes very stiff and cannot be used as an elastic material below −50 °C. For ESR-5 composite, a sharp decrease of E′ because of crystal melting can be still seen in the range of −60 to 45 °C. However, the change of E′ is lower than that for PDMS composite, implying a decrease in the degree of crystallinity. For ESR-10 composite, E′ begins to decrease significantly at −115 °C because of glass transition, but then increases at approximately −95 °C owing to cold crystallization. As the temperature reaches −80 °C, the E′ of ESR-10 composite begins to decrease again because of crystal melting. In the DSC curve of ESR-10 (Fig. 7), however, neither an endothermic peak nor an exothermic peak can be seen. Because DMA test was carried out at a constant frequency and a small amplitude, the cold crystallization of ESR-10 during the DMA test was probably induced by stress, which has been reported to accelerate the crystallization of polymers.54–56 At a very low cooling rate, it was found that there is still an exothermic peak in the DSC cooling curve of ESR-10, but the ΔHc is very low, indicating a low degree of crystallinity of ESR-10 (see Fig. S5†). For ESR-15 composite, only a sharp decrease of E′ corresponding to glass transition can be seen in Fig. 9(a), demonstrating an amorphous structure for ESR-15. The amorphous structure of ESR-15 can be further confirmed by the DSC cooling curve, in which none of exothermic peaks can be seen even at a very low cooling rate (see Fig. S5†). At epoxy contents higher than 14.21 mol%, the temperature corresponding to the sharp decrease of E′ increases because of the increase of Tg of ESR. Generally, the introduction of epoxy groups into the PDMS side chains can improve the low-temperature resistance of PDMS. As the epoxy content increases, the temperature corresponding to crystal melting decreases and the low-temperature resistance increases. Among these ESRs, however, ESR-15 should show the best low-temperature resistance because it is amorphous and has a lower Tg than ESR-20 does.
![]() | ||
Fig. 9 Dynamic viscoelastic curves of PDMS composites and ESR composites: (a) storage modulus versus temperature; (b) tan![]() |
A material's tanδ designates the material's ratio of viscous to elastic components and is sometimes called the damping ability of the material.57 Fig. 9(b) shows the tan
δ of ESR composites with different epoxy contents versus temperature. Consistent with DSC results, the Tg of ESR composites obtained from the plot of tan
δ vs. T increases with increasing epoxy content of ESR. The tan
δ of ESR composites in the range −120 to 80 °C increases significantly as the epoxy content of ESR increases. One possible reason for the increase of the tan
δ is that the introduction of epoxy groups inhibits the crystallization of PDMS and the PDMS chains constrained by the crystals are able to move to lose energy. Another possible reason is that the epoxy groups on the side chains increase the friction between the macromolecular chains, resulting in an increase of energy loss during the segmental motions in the glass transition region.
Base on the DSC and DMA results, ESRs, especially ESR-15 and ESR-20, show excellent low-temperature performance and can be used as low-temperature-resistant materials. In practical applications, the cold resistance factor under compression (Kc) is often used to evaluate the low-temperature performance of a material. In this study, ESR composites with silica were prepared and the cold-resistance of ESR composites was investigated by using a cold-resistance-factor tester. The results, which are used to evaluate the low-temperature performance of ESR composites, are shown in Fig. 10. The cold resistance factor under compression is a measurement of the elasticity of a material, and the higher the Kc the higher the elasticity of the material. The Kc is influenced significantly by the Tg and crystallinity of the material. If a material has a Tg higher than the test temperature or easily crystallizes at the test temperature, the segmental motions will be inhibited and the material will have a low Kc. As shown in Fig. 10, all the composites have high Kc (>0.9) at room temperature and the difference in Kc between these composites is small, indicating that these composites have similar and very high elasticity. For the PDMS composite, the Kc is still high (Kc = 0.67) at −50 °C, but begins to decrease significantly as the temperature decreases from −50 °C. At −60 °C, the Kc of the PDMS composite decreases to 0.31 because of the crystallization of PDMS. When the temperature decreases to −70 °C, the Kc of PDMS composite is almost 0, demonstrating that the PDMS composite has lost almost all its elasticity as a result of crystallization and cannot be used as an elastic material at temperatures lower than −70 °C. ESR-5 composite shows a better low-temperature performance than PDMS composite does and still remains highly elastic at −60 °C. However, the Kc of ESR-5 composite decreases to only 0.11 at −70 °C, suggesting that the ESR-5 composite has also lost its elasticity at this temperature because of crystallization. The ESR composites with contents of epoxy groups higher than 9.21 mol% show high Kc at −50 °C, −60 °C, and −70 °C. Among these ESR composites (ESR-10, ESR-15, and ESR-20), ESR-10 composite shows the best low-temperature performance according to the Kc value, probably because it has the lowest Tg. As indicated by DMA results, however, ESR-15 composite and ESR-20 composite would show better low-temperature performance than ESR-10 composite does at temperatures lower than −70 °C due to the crystallization of ESR-10 in the temperature range −95 to 80 °C. Because the cold-resistance-factor tester is cooled by dry ice, Kc values at a lower temperature than −70 °C cannot be obtained. Nevertheless, according to DSC and DMA results, we can confirm that these ESR composites, especially ESR-15 composite and ESR-20 composite, can be used as low-temperature-resistant materials at temperatures lower than −70 °C.
In this study, we also studied the thermal stability and high-temperature applicability of ESR. Fig. 11 shows the TGA curves of uncured PDMS and uncured ESRs in inert atmosphere. It can be seen from these TGA curves that the thermal stability of ESR increases with increasing the content of epoxy groups. The temperature of 10% weight loss increases by 52 °C when 14.21 mol% of epoxy groups is introduced into the side chains of PDMS.
The thermal degradation of polysiloxane elastomer end-blocked with (CH3)3Si-groups in inert atmosphere can be explained by a well-known depolymerisation mechanism, as shown in Scheme 3.6 When polysiloxane is heated in an inert atmosphere, silicone d-orbital participation is postulated with siloxane bond rearrangement, leading to the elimination of cyclic oligomers and the shortening of the residual chain length. The trimer cyclic oligomer is the most abundant product, with irregularly decreasing amounts of the tetramer, pentamer, and higher oligomers.58 The transition state can be formed at any point of the silicone rubber chain, and the degradation process can take place continuously until the residual chain is too short to form cyclic oligomers.59,60 Camino et al. suggested that the formation of cyclic transition is the rate-determining step during the thermal degradation.60 In other words, the more easily the cyclic transition state forms, the weaker the thermal stability of silicone rubber is. Obviously, the flexibility of silicone rubber chains is the dominant factor for the thermal degradation. As confirmed by the increase of Tg, the flexibility of PDMS will decrease when polar epoxy groups are introduced into the side chains of PDMS. Therefore, the presence of epoxy groups will inhibit the formation of cyclic transition and increase the thermal degradation temperature. In addition, the steric hindrance of epoxy groups also inhibits the formation of cyclic transition.
Footnote |
† Electronic supplementary information (ESI) available: The photographs of SR and ESR; 1H NMR spectra of SRs and ESRs; thermal properties of PDMS, SRs and ESRs; and the DSC cooling curves of ESRs (cooling rate: 1 °C min−1). See DOI: 10.1039/c4ra02293a |
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