Chinese Chemical Letters  2019, Vol. 30 Issue (3): 710-713   PDF    
Enhanced mechanical and thermal properties of γ-allyloxymethyl 18-crown-6 and polyimide composites through hydrosilylation crosslinking
Chuqi Shi, Shumei Liu*, Yang Li, Jianqing Zhao*, Haohao Huang     
School of Materials Science and Engineering, Key Laboratory of Polymer Processing Engineering of the Ministry of Education, South China University of Technology, Guangzhou 510640, China
Abstract: The complexation of γ-allyloxymethyl 18-crown-6 (AC6) induced inferior mechanical and thermal properties of polyimide (PI) in spite of lowered dielectric constant (k). To solve this puzzle, tetrakis-(dimethylsiloxy)-silane was employed to crosslink the complex of AC6 and PI (AC6-PI) through hydrosilylation reaction. The crosslinked AC6-PI (SiAC-PI) composites possessed excellent mechanical and thermal properties as well as low k. The tensile strength and fracture energy of SiAC-PI were increased by 87% and 716%, and the glass transition temperature and 5% weight loss temperature elevated 14.5℃ and 38.8℃, respectively, compared with those of AC6-PI. The structure of SiAC-PI was characterized by FTIR spectra, crosslinked density and XRD diffraction patterns.
Keywords: Polymeric composites     Dielectrics     Mechanical properties     Thermal properties     Hydrosilylation crosslinking    

Polyimide (PI) has been widely employed as interlayer dielectrics in microelectronics industry for a long time by virtue of low dielectric constant (k) [1-3]. However, today lower k is always demanded for its application in advanced microelectronic devices [4, 5]. Macromolecular chains of PI can form the complex with crown ether molecules via hydrogen bond, to exhibit a decrease in k as a result of shielding effect of low-polarity crown ethers on the strong polar imide groups [6-8].18-Crown-6 induces the k of PI from 3.59 to 2.99 [9], and γ-allyloxymethyl 18-crown-6 (AC6) could even cause the k as low as 2.58 by virtue of an additional evaporation voiding during thermal imidization. The porous structure favors the decrease in the k, but it is harm to mechanical properties, and the tensile strength and fracture energy of the complex of AC6 and PI (AC6-PI) decrease sharply. It becomes a great challenge to improve mechanical properties of the complex on the premise of maintaining low dielectric constant [10, 11]. In addition, the introduction of crown ethers with lower thermal stability resulted in the obvious reduction in the glass transition temperature and 5% weight loss temperature of PI. It was found that polyhedral oligomeric silsesquioxane with outstanding thermal oxidative stability could improve thermal properties of the complex of crown ether and PI to some degree [12]. But less toughness was usually induced by the structural defect originating from agglomeration of nanoparticles [13].

Improvement in both mechanical and thermal properties of PI were achieved by the introduction of crosslinked network structure of stable Si-O-Si skeleton [14, 15]. Vinyl-functionalized polysiloxane-block-polyimide owned high-temperature durability and elevated tensile strength through the crosslinking of poly (hydromethylsiloxane) [16]. Moreover, the thermal stability of AC6 could be ameliorated via hydrosilylation reaction of its reactive γ-allyloxy group with triethoxysilane [17, 18]. It was expected that a Si-O-Si crosslinking approach can balance the lower k with mechanical properties and thermal stability of the AC6-PI complex. In this present work the complex of AC6 and poly(amic acid) (PAA) precursor underwent the crosslinking through hydrosilylation of tetrakis-(dimethylsiloxy)-silane (TDSS) during thermal imidization. The mechanical, thermal properties and dielectric constant of crosslinked AC6-PI (SiAC-PI) composites were investigated.

AC6 was synthesized in our lab according to Ref. [19, 20]. SiAC-PI composites were prepared through a"one-pot"process.The moleratio ofAC6to4, 4'-diaminodiphenyl ether(ODA)inthefeedwas0.4foreach composite. The mole ratios of Si-H of TDSS to CH=CH2 of AC6 were changed into 0.5, 1.0 and 2.0 to obtain three composites with different crosslinked degree, named as SiAC-PI-1, SiAC-PI-2 and SiAC-PI-3. Typically, SiAC-PI-2was preparedas follows.Firstly, 1.03g(3.07mmol) AC6 and 1.57g (7.67mmol) ODA at the mole ratio of 0.4 was dissolved in 24g N-methyl-2-pyrrolidone (NMP) in a three-neck flask. The mixture was stirred for 2 h at room temperature to form AC6-ODA host-guest inclusion complex. Secondly, 1.71 g (7.67 mmol) 1, 2, 4, 5- pyromellitic dianhyride (PMDA) was subsequently added to form thecomplexofAC6andPAAsolution.The mixture was stirred for 6 h at room temperature. Thirdly, 0.26 g (0.77 mmol) TDSS and 0.035 g (0.03 mmol) tetrakis-(triphenylphosphine) palladium (Pt(PPh3)4) were poured into AC6-PAA solution, and the mixture solution was continuously stirred for 2 h. Whereafter, the obtained solution was successively heated at 80 ℃/1 h, 100 ℃/1 h, 200 ℃/1 h, 300 ℃/2 h under vacuum to accomplish thermal imidization progress. The hydrosilylation reaction between AC6-PAA and TDSS occurred during thermal imidization.The content of AC6 and TDSS inSiAC-PI- 2 was 13.87 wt% and 6.82 wt%, respectively, according to 50% inclusion rates of crown ether [9]. AC6-PI complex without crosslinking of TDSS was prepared as a contrast.

The FTIR spectra of PI films were recorded on a Bruker Vector-22 Fourier transform infrared spectroscopy (FTIR). The fracture surface morphology of PI films was observed on a Nova NanoSEM430 scanning electron microscopy (SEM). According to ASTM D882-12, mechanical properties of the films were analyzed with a universal material testing machine. The glass transition temperature (Tg) and the storage modulus (E') of PI films were obtained from a Netzsch 242C dynamic mechanical analyser (DMA) at a heating rate of 5 ℃/min. Thermal stability of the samples was conducted under nitrogen with a Netzsch 209F1 thermogravimetric analyzer (TGA) at a heating rate of 20 ℃/min. The morphological structure of the samples was performed using a Bruker D8ADVANCE wide-angle X-ray diffractometer (XRD) and Cu Kα (l = 1.542 Å) was chosen as the radiation source. Capacitance of PI films was measured by an Agilent 4284ALCR meter at a frequency of 1 MHz. The film density was measured at 23 ± 0.5 ℃ via liquid pyknometer method according to ISO 1183-1:2012. The dielectric constant (k) of PI films was calculated according to the following equation: k = Cd/ε0S, where C is capacitance (F), d is thickness of the films (m), S is the area of the electrodes (m2), ε0 is the permittivity of vacuum which equals to 8.854 -10-12 F/m.

AC6 is a kind of crown ethers involving reactive γ-allyloxy group. Like 18-crown-6, it can form the interlocking complex with the macromolecular chain of PI to decrease the k. It is more difficult for AC6 to evaporate from the matrix than 18-crown-6. The escape of small amounts of unlocked AC6 at nearly 300 ℃ leaves pores in the matrix after imidization of AC6-PAA precursor. The density of the obtained AC6-PI is only 1.250 g/cm3, which is 13% lower than the density of PI (1.441 g/cm3). The presence of pores can be confirmed by the comparison of cross-sectional SEM micrographs of PI (Fig. 1a) and AC6-PI (Fig. 1b). In fact, the porous structure favors the decrease in k, which drops to 2.58 from 2.99 of the complex of 18-crown-6 and PI. But it is detrimental to mechanical properties of PI. The tensile strength and fracture energy of AC6-PI sharply decrease to 67.5 MPa and 3.1 MJ/m3 from 119.3 MPa and 20.9 MJ/m3 of PI, respectively. It is necessary to overcome the undesirable results of evaporation voiding.

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Fig. 1. Cross-sectional SEM images of (a) pristine PI, (b) AC6-PI, (c) SiAC-PI-1, (d) SiAC-PI-2, and (e) SiAC-PI-3.

Here TDSS containing four Si-H bonds was applied for hydrosilylation with γ-allyloxy group of AC6 during thermal imidization of AC6-PAA. For one thing, TDSS can fasten unlocked AC6 to alleviate evaporation voiding. For another, it acts as a crosslinking agent for macromolecular chains of AC6-PI. The preparation process of AC6-PI complex and its crosslinked composites (SiAC-PI) was shown in Scheme 1a. The structure of SiAC-PI composites was diagramed in Scheme 1b.

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Scheme 1. (a) The preparation process of AC6-PI complex and SiAC-PI, (b) the structure diagram of SiAC-PI composites.

FTIR spectra and section spectra of AC6-PI, SiAC-PI-2 and TDSS are shown in Fig. 2. The structure of SiAC-PI composites can be proved to some degree from their FTIR spectrum variation (Fig. 2a). The absorption bands of C=C at 3100–3050 cm-1 (Fig. 2b) are characteristics of γ-allyloxy group, and the absorption bands of SiH at 2250-2090 cm-1 cannot be observed in the spectrum of SiACPI-2, indicating the reaction occurrence between AC6 and TDSS. Besides, C–H stretching absorption of -CH3 from TDSS at 2960 cm-1 (Fig. 2b) becomes quite evident.

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Fig. 2. (a) FTIR spectra and (b) section spectra from 3200 cm-1 to 2900 cm-1 of AC6-PI, SiAC-PI-2 and TDSS.

The crosslinked density (ve) of SiAC-PI composites can be estimated from the storage modulus (E') of DMA test in accordance with Mooney-Rivlin equation veE'/3RT [21, 22]. Here, T is Tg + 40 ℃, and E' is storage modulus at the temperature of Tg+40 ℃ in E'-temperature curves (Fig. 3a). ve of SiAC-PI-1 rises to 2.25 mmol/cm3 from 1.34 mmol/cm3 of AC6-PI (Table 1), evidencing the increase of crosslinking degree with the introduction of TDSS. v e of SiAC-PI-3 rises slightly to 2.36 mmol/cm3 from 2.33 mmol/cm3 of SiAC-PI-2, indicating the occurrence of full crosslinking among AC6-PAA chains at equal mole ratio of Si—H to -CH=CH2.

Table 1
Properties of SiAC-PI composites.

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Fig. 3. (a) E'-temperature curves, (b) stress-strain curves, (c) tan δ-temperature curves, and (d) weight-temperature curves of five PI films.

The mechanical strength of AC6-PI enhances remarkably via crosslinking. Young's modulus, tensile strength and fracture energy of PI, AC6-PI and three SiAC-PI composites are listed in Table 1, and stress-strain curves are shown in Fig. 3b. It is found that several mechanical properties of SiAC-PI-1 evidently excel over AC6-PI. The properties of the composites become higher with the increase in crosslinking degree. Compared with AC6-PI, Young's modulus and tensile strength of SiAC-PI-2 are increased by 12% and 87%. It is more significant that fracture energy of SiACPI-2 rises to 25.3 MJ/m3 from 3.1 MJ/m3 of AC6-PI, which overmatches pristine PI. This should be attributed to the evident abatement of evaporation voiding (Figs. 1c–e). Less number and smaller size of pores in the matrix can be observed from crosssectional SEM micrographs of SiAC-PI-2 (Fig. 1d). The density of SiAC-PI-2 goes up to 1.361 g/cm3. This modulus and strength variation of three SiAC-PI composites can be obtained well explanation from ve. It is worth noting that two properties of SiAC-PI-3 are slightly superior to those of SiAC-PI-2.

As the crosslinked structure restricts the movement of PI chain segments, the glass transition temperature (Tg) of SiAC-PI-2 goes up to 341.9 ℃ from 327.4 ℃ of AC6-PI, quite close to 352.4 ℃ of pristine PI (Fig. 3c). The 5% weight loss temperature (T5%) of SiACPI-2 increases from 414.3 ℃ to 453.1 ℃ (Fig. 3d). It can be seen from the figures that the crosslinking can actually elevate thermal stability of AC6-PI.

The dielectric constant is an important parameter for the application of PI in microelectronic materials. Less number and smaller size of pores in the matrix usually induce the increase of k value, while the introduction of Si-O-Si skeleton broadens the interchain distance of PI, which is beneficial to the reduction of k. The interchain distance (d) of PI can be calculated from XRD diffraction patterns (Fig. 4) according to Bragg equation 2dsinθ = nλ [23, 24]. The results indicate that d of AC6-PI (4.81 Å) is evidently larger than that of pristine PI (4.23 Å), and d of SiAC-PI composites further increases, and d of SiAC-PI-2 is 4.85 Å. It is found that the k of SiAC-PI-1, SiAC-PI-2 and SiAC-PI-3 is 2.59, 2.61 and 2.63 at 1 MHz, near to 2.58 of AC6-PI, indicating the primary effect of Si-O-Si skeleton.

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Fig. 4. XRD diffraction patterns of five PI films.

The SiAC-PI composites were prepared through hydrosilylation crosslinking of AC6-PAA precursor with TDSS during thermal imidization. Mechanical and thermal properties of AC6-PI were simultaneously enhanced with the formation of crosslinked structure. Especially, the fracture energy was elevated from 3.1 MJ/m3 to 25.3 MJ/m3 owing to the evident abatement of evaporation voiding. Young's modulus, tensile strength and fracture energy of the composites overmatched pristine PI, with the k as low as 2.6. An approach is presented to decrease the dielectric constant as well as upgrade mechanical properties of PI. It shows great application prospects in microelectronic materials.

Acknowledgments

This research is supported by the National Natural Science Foundation of China (No. 51573054) and the Project for Science and Technology of Guangzhou (No. 201604016080).

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