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Low-k and Recyclable High-Performance POSS/ Polyamide Composites Based on Diels-Alder Reaction Kaiju Luo, Guocheng Song, Yan Wang, Junrong Yu, Jing Zhu, and Zuming Hu ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00215 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Polymer Materials

Low-k and Recyclable High-Performance POSS/Polyamide Composites Based on Diels-Alder Reaction Kaiju Luo, Guocheng Song, Yan Wang*, Junrong Yu, Jing Zhu and Zuming Hu State key laboratory for modification of chemical fibers and polymer materials, college of material science and engineering, donghua university, 201620, shanghai (P. R. China) Corresponding author. *E-mail: [email protected]

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ABSTRACT The rapid development in electronics requires high-performance and low-k materials. Introducing additional recyclability to these materials could further promote their application from environmental and economic aspects. In this paper, we report a reversibly cross-linked composites from a newly designed maleimide functionalized POSS (mPOSS) and aromatic polyamide with pendent furan groups (POF). The Diels-Alder reaction between maleimide and furan groups allowed the facile cross-linking of POF by mPOSS, thus significantly improved the thermal and mechanical properties of mPOSS/POF composites. The reversible feature of Diels-Alder reaction also conferred good recyclability to the mPOSS/POF composites. Tensile test suggested that more than 80% of mechanical properties were retained in reprocessed composites even with multiple reprocessing cycles. Moreover, the porous nature of POSS molecules and the restricted motion of polymer chains by cross-linking significantly reduced the dielectric constant and dielectric loss of composites. These fascinating features of our mPOSS/POF composites make them promising candidate for high-performance, recyclable, and low-k dielectrics. KEYWORDS Polyaimde; POSS; Diels-Alder Reaction; Recycling; Dielectric Constant

1. INTRODUCTION The rapid miniaturization of microelectronics is asserting a claim for smaller dimensions of electronic devices, thus materials of low dielectric constant (k) are urgently needed to decrease the leakage current, capacitance effect, heating phenomenon and cross-talk noise between adjacent conducting wire

1-3.

For example, the k required for the technology nodes that down to

10 nm is as low as 2.3, while the k of traditionally used silica is around 4.0 4. Introducing pores and voids into dielectrics can increase the volume fraction of air (k≈1.0) and scale down the


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dipoles density, which will eventually reduce the k value 5-9. However, this result might be built on the sacrifice of mechanical strength, unless the size and distribution of pores are wellcontrolled under nanoscale

10-12.

Decreasing the polarizability by using chemical bonds of C-C,

C-H, C-Si, and especially C-F groups can also lead to low-k polymeric materials

13-15,

but the

concurrently increased chain flexibility by these groups is harmful to the dimensional stability, solvent-resistance and mechanical properties of resultant materials

16-18.

Although dense cross-

linking among interchains could potentially address these issues, the synthesis process is relatively complex and costly, and the cross-linked dielectrics could not be recycled and reprocessed once they were failure, which results in additional economic and environmental problems. An alternative strategy for robust and low-k materials is the incorporation of porous molecules into high-performance polymers

19-22.

As a representative, polyhedral oligomeric silsesquioxane

(POSS) has attracted numerous attentions in dielectrics recently. POSS consists of cage-like inorganic skeleton with size of 0.53 nm and eight organic arms 23. The introduction of POSS can lower the k value of composites efficiently, and improve the physical properties once the organic peripheries of POSS were delicately designed to provide compatibility or covalent linkage to matrix. The choice of aromatic polymers, including poly(imides) poly(aryl ethers)

29-30,

or polyamide

31-32

24-25,

poly(benzoxazole)

26-28,

as host takes advantages of mechanically strong,

thermally stable, and resistance to heat or solvent. However, the rigid backbone and multiple molecular attractions among polymer chains of these polymers make most of them insoluble and non-meltable that prevent the post-processing

33.

Several methodologies, such as grafting side

bulk groups and introducing flexible bonds into rigid backbone, have been proposed to resolve these problems at the sacrifice of physical properties. Subsequent cross-linking could enhance


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the lost properties, but traditional covalent bonds result in permanent network that can not be reprocessed as other cross-linked dielectrics. Therefore, it would be of great important to develop processable and recyclable materials for low-k applications on the text of environmental and economic aspects. Unfortunately, no such material has been reported in literatures to the best of our knowledge. On the contrary, dynamic reactions, such as Diels-Alder (D-A) reaction exchange reaction

38

or transesterification

39,

34-37,

disulfide based

could generate reversibly covalent bonds that

formed or broken under certain stimuli, which provide an attractive approach for reusable materials. However, these reactions have rarely been applied in aromatic polymers due to the fact that it is difficult to synthesize or functionalize an intractable polymer with reactive groups to afford cross-linkable precursors. In a previous work, we have synthesized a furanic polyamide from a newly designed furan-containing diacid 40. The pendent furan groups permitted solution processing in common solvents, and allowed the reversible cross-linking of polyamide with bismaleimide molecules through D-A reaction, which offered great opportunity for fabrication of high-performance and reversibly cross-linked materials. Herein, we report a novel reversibly cross-linked composite on the basis of our previous work. Maleimide functionalized POSS (mPOSS) was designed and synthesized as fillers for furanic polyamide (Scheme 1). The mPOSS played many important roles in these composites. Firstly, the maleimide functionalities of mPOSS could not only provide cross-linking points to furanic polyamide, but also endow these cross-linked networks with good recyclability based on D-A reaction. Secondly, the inorganic cage of mPOSS could afford better mechanical and thermal properties of the crosslinked composites than previously reported pure organic cross-linked networks

40.

Lastly, the

porous cage of mPOSS and restricted chain mobility could effectively reduce the k value of


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composites. The triple roles of mPOSS, that is, the cross-linking agent, the reinforcement agent, and the pore-forming agent, are thus expected to make our composite promising candidate for high-performance, recyclable, and low-k dielectrics. Scheme 1. Synthetic procedure of mPOSS and thermal reversible mPOSS/POF composite.

2. EXPERIMENTAL SECTION 2.1. Materials Methanol, acetone, glacial acetic acid, triethylamine, 5-aminoisopathalic acid, furfural, anhydrous sodium sulfate, acetic anhydride, sodium acetate, sodium borohydride, triphenyl phosphite and pyridine were commercially available and used as received. 4,4-Diaminodiphenyl ether (ODA), 3-aminpropyltriethoxysilane (KH550) and 1-methyl-2-pyrrolidone (NMP) were purchased from J&K Chemical Co. Ltd. (Shanghai). NMP was distilled from CaH2 and stored over 4 Å activated molecular sieves before use. Lithium chloride was dried overnight in muffle under 400 °C for 4 hours.


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2.2. Synthesis of furanic polyamide (POF) The aromatic furanic diacid monomer and polyamide were synthesized according to our previous work

40.

In brief, 5-aminoisopathalic acid and equimolar amount of furfural were dissolved in

methanol to form homogenous solution at room temperature. Glacial acetic acid and anhydrous sodium sulfate were added in turns in the mixture as catalyst and water absorbent. Then fourfold sodium borohydride was added in four times with an interval of 30 min reduce the in-situ formed imine. The obtained diacid monomer (FA) was subsequently reacted with 4,4-diaminodiphenyl ether (ODA) at 60 °C for half an hour and 90 °C for another half an hour and finally 130 °C for four hours to give the finally furanic pendent polyamides, named POF. 2.3. Synthesis of maleimide functionalized POSS (mPOSS) Octa(aminpropyl)silsesquioxane, (NH2CH2CH2CH2)8Si8O12 (POSS-NH2) was synthesized from KH550 according to previously reported literatures

41

and used as starting material for the

preparation of mPOSS. For the synthesis of mPOSS, 6.0 g maleic anhydride was dissolved in 50 ml methanol and slowly added to the solution of freshly prepared POSS-NH2 (2.4 g) at 25 °C. The mixture was stirring overnight and the suspension was filtered to obtain the intermediate POSS-COOH. Then 2.0 g POSS-COOH, 1.5 g sodium acetate, 15 ml acetic anhydride and 2.4 ml triethylamine were added successively into a 250 ml three-necked round-bottomed flask with 100 ml acetone as solvent. The mixture was refluxed at 70 °C for 8h and then dried by rotary evaporator. The white powder was finally washed with water for twice and dried in vacuo to obtain the final mPOSS. 1H NMR (DMSO-d6, 600 MHz), δ (ppm) 0.59-0.66 (m, 16H), 1.51-1.59 (m, 16H), 3.14-3.20 (m, 16H), 6.22-6.27 (m, 5H), 6.38-6.42 (m, 5H), 6.94-6.99 (m, 6H), 9.069.11 (m, 5H), 15.07 (s, 4H).

29Si

NMR (DMSO-d6, 600 MHz), δ (ppm) -66.3, -66.6. FT-IR

(ATR, cm-1) 3280 (O-H stretching vibration), 2937 and 2878 (alkyl -CH2- stretching vibration),


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1700 (C=O stretching vibration), 1566 (C=C stretching vibration), 1256 (Si-C asymmetric stretching vibration), 1196 and 1115 (Si-O-Si asymmetric stretching vibration), 696 (maleimide stretching vibration), 610 (POSS skeletal deformation vibration). 2.4. Preparation of cross-linked mPOSS/POF composites POF (0.21g) and variable content of mPOSS were dissolved in 2 ml NMP and stirred until homogenous. Then the mixture was cast on clean glass plate and dried at 60 °C for 72 h to obtain composites POF-Ima/fur, where Ima/fur represents the molar ratio of maleimide to furan groups (Ima/fur = 0, 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50, respectively). According to the Ima/fur, the contents of mPOSS in composites were listed in Table 1. Moreover, we have performed TGA on mPOSS under air atmosphere and found the inorganic part in mPOSS was about 24.5 wt%, thus, the actual mass fractions of inorganic cages (Si8O12) in composites can be calculated, which were also listed in Table 1. Table 1. Contents of mPOSS and corresponding Si8O12 cage with in composites varied Ima/fur.

Ima/fur

0

0.05

0.10

0.20

0.30

0.40

0.50

mPOSS (wt%)

0

6.1

11.5

20.6

28.0

34.1

39.3

Si8O12 (wt%)

0

1.50

2.82

5.05

6.86

8.35

9.63

2.5. Characterizations NMR spectrometer (Bruker, 600 MHz) was employed to detect the 1H and 29Si spectra at 25°C using DMSO-d6 as solvent. The Attenuated total reflectance fourier transform infrared (ATRFTIR) spectroscopy was performed on samples to record the infrared spectra. Small-angle x-ray scattering (SAXS) measurement of composites were conducted on SAXSess mc2. Dynamic


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thermal mechanical properties of composites were measured using Q800 instrument (TA instruments, Co. Ltd., USA) with temperature ranging from 30 to 200 °C. The heating rate and frequency were set as 10 °C/min and 1Hz, respectively. Electronic universal testing machine (MTS, model C44-104) was used to evaluate the mechanical properties of composites at room temperature and relative humidity of 60 %. Test was carried out on specimen (with length of 40 mm, width of 6 mm and thickness of 0.04-0.10 mm) at a constant rate of 5 mm/min. The surface morphology were observed with a field emission scanning electron microscope (FE-SEM, SU8010, Hitachi, Japan) after sputter-coated with 5-10 nm of gold. A broadband dielectric spectrometer (concept 40, Germany) was employed to investigate the dielectric properties of circular samples with 22 mm in diameter and 0.06-0.15 mm in thickness.

3. RESULTS AND DISCUSSION 3.1. Synthesis and characterizations of mPOSS The synthetic procedure of mPOSS was illustrated in Scheme 1. Traditional method for synthesis of bismaleimide was adopted here for the synthesis of a novel maleimide functionalized POSS from POSS-NH2. NMR was used here to monitor the chemical structure changes of POSS, as shown in Figure 1. After reaction of POSS-NH2 with maleic anhydride, the POSS-COOH exhibited several characteristic peaks as marked in Figure 1a, and the ratio of integral areas (IA: IB: IC: ID: IE: IF: IG =1: 1 : 0.95 : 0.47 : 0.49 : 0.50 : 0.46) was in consistence with its theoretical ratio of H protons in POSS-COOH, indicating the success in converting amine groups to carboxyl groups. The 29Si NMR spectrum of POSS-COOH (Figure 1b) showed that there was only an intense signal located at δ=-66.5 ppm, which was in consistent with the position of signal of silicon atoms in cage-like structure

42-43.

In the 1H NMR spectrum of


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mPOSS, besides the characteristic peaks of POSS-COOH, the appearance of new peak located at 7.00 ppm (marked as b) was ascribed to the H proton of maleimide group and its integral ratio as compared with the stable methylene (marked as a) was approximately 0.37, suggesting the degree of maleimide functionality was about 3 (0.37×8). Therefore, the mPOSS would serve as a tri-functional cross-linker, which is sufficient for cross-linking of linear polymers. The 29Si NMR spectrum of mPOSS evidenced the integration of the silsesquioxane cages after the reaction

44,

and the splitting of the single peak in POSS-COOH into two peaks of mPOSS also demonstrated the partial imidization of mPOSS. In fact, we have increased the degree of functionality to around 4.0 by increasing the reagent, but the solubility of mPOSS was deteriorated. Thus, mPOSS with functionality of three was used for subsequent investigation. Note that the residual carboxyl groups in mPOSS could non-covalently interact with polyamide via hydrogen bonding, which was also benefit to the mechanical and behavior of composites 45-47.

Figure 1. (a) 1HNMR spectra of POSS-COOH and mPOSS. (b)

29Si

NMR spectra of POSS-

COOH and mPOSS.


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3.2. Analysis of Diels-Alder cross-linking between POF and mPOSS The reversible characteristic of D-A reaction between POF and mPOSS was evidenced under the assistance of 1H NMR spectra from molecular level, as shown in Figure 2. Due to the complexity of proton signals in mPOSS, here we employed excess polyamide to provide sufficient furan groups (eightfold to maleimide groups) so as to detect the changes of signals of maleimide signal and D-A adduct. While the peaks attributed to maleimide group around 7.0 ppm and the D-A adduct around 6.2-6.4 ppm (b, c and b', c') were overlapped, the regular appearance and disappearance of peaks of D-A adduct a and a' around 5.2-5.3 ppm as well as the D-A conformation changes around 3.6-4.0 ppm could be distinct proof of the reversible association/dissociation of maleimide and furan groups. Moreover, the regular strengthen and weaken of peaks around 2.6 ppm marked as o, p and o', p', which were also ascribed to the signals of protons in D-A adduct, were contributed another evidence to the existence of reversible D-A reaction. The thermal reversibility of cross-linking between POF and mPOSS were also investigated from macroscopic level by sol-gel transition using POF-0.20 as an example and the digital photos were presented in Figure S1. It is found that the sol-gel transition of mixture of POF and mPOSS could be repeated for at least three times, indicating the potential of mPOSS/POF composites for repeated use.


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Figure 2.The cyclic process of reversible D-A reaction monitored by 1H-NMR spectra. The sample was (1) allowed to react at 60 °C for 24h; (2) exposed to 130 °C for 30 minutes; (3) placed again at 60 °C for 24h; (4) exposed again at 130 °C for 30 minutes. 3.3. Preparation of mPOSS/POF cross-linked composites The obtained composites were investigated by FT-IR spectra and compared with that of neat POF and mPOSS, as shown in Figure S2. It revealed that the characteristic peaks of furan rings in POF were gradually diminished with the increased amount of incorporated mPOSS, and the sharp characteristic peak of maleimide group in mPOSS could not be identified or attenuated to a large extent in all mPOSS/POF composites. These observations demonstrated the occurrence of D-A cross-linking in mPOSS/POF composites.


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Figure 3. SEM images of surface morphology of (a) POF, (b) POF-0.05, (c) POF-0.10, (d) POF0.20, (e) POF-0.30, (f) POF-0.40, and (g) POF-0.50. (h) Energy dispersive spectroscopy of POF0.20. The homogenous dispersion of nanofiller in polymer matrix plays a crucial role in composite materials. Thus SEM was employed to analyze the dispersion state of mPOSS in polymer matrix, as shown in Figure 3a-g. It was found that mPOSS exhibited good compatibility with matrix at lower loadings (Figure 3b-d) without any visible particles as that of pure POF (Figure 3a). EDS mapping of POF-0.20 (Figure 3h) also demonstrated the homogeneous distribution of element Si across the fracture surface, corresponding to the evenly dispersed mPOSS in POF-0.20. However, some small agglomerations appeared in POF-0.30 (Figure 3e), and such agglomerations were more obvious in POF-0.40 (Figure 3f) and POF-0.50 (Figure 3g), suggesting some aggregations of mPOSS might form at higher loadings. It is previously reported that phase separation could happen in POSS/polymer composites with high content of POSS particles 44. Therefore, we have carried out SAXS characterization to evaluate the microphase of our mPOSS/POF composites, as shown in Figure S3. The SAXS spectra of all POF and mPOSS/POF were featureless with no sign of phase separation, suggesting the mPOSS aggregates did not form long-range ordered crystallites.


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3.3. Thermal and mechanical properties of mPOSS/POF composites. One of the well known merits of polyamide lies in its outstanding thermal stability. Thus it is essential to investigate the thermal properties of composites by TGA and the results were shown in Figure S4. It is found that all POF and mPOSS/POF composites were thermally stable without any loss below 240 °C, and the maximum decomposition temperature (Td) of composites elevated monotonously with the increasing contents of mPOSS. The Td of POF-0.05, POF-0.10, POF-0.20, POF-0.30, POF-0.40, and POF-0.50 were 462.1, 478.5, 482.2, 491.1, 496.4, and 514.6 °C, respectively, in comparison to that of 458.6 °C of POF, suggesting the cross-linking was helpful to delay the pyrolysis of polymer chains, and the inorganic Si8O12 cages could hinder the release of volatile degradation products and heat transfer during decomposition process, which was also beneficial to the delay of decomposition rate of the whole composites 48-50. The dynamic thermo-mechanical properties of composites with varied Ima/fur were were measured and the results were shown in Figure 4 and Table S1. The storage modulus (G’) of composites increased gradually with the increasing content of mPOSS because of the increased cross-linking degree and inorganic portion of composites, and achieved an increment of 74.1% from 1275 MPa of pure POF to 2220 MPa of POF-0.40 at 50 °C. Further addition of mPOSS resulted in decreased G’ of POF-0.50 to 2015 MPa, which was anticipated to be caused by the aggregation of mPOSS that decreased the cross-linking density inversely. The Tg was reflected by the peak value of tan δ and increased greatly from 130.6 °C of POF to 154.8 °C of POF-0.20, proving the restriction of polymer chains by cross-linking. However, the Tg decreased upon further addition of mPOSS despite the degree of cross-linking was theoretically higher in POF-0.30, POF-0.40 and POF-0.50. It is tentatively attributed this contradictive phenomenon to the alkyl chains attached on mPOSS, more mPOSS addition means more flexible alkyl chains that could lower


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the overall Tg of composites. Interestingly, there were small steps in the curves of storage modulus in all composites at around 95 °C, especially at higher Ima/fur (0.30-0.50), which was anticipated to be caused by the de-cross-linking of some D-A linkages because pure POF did not show this step in G’ curve.

Figure 4. (a) Storage modulus and (b) Tan δ of POF and mPOSS/POF composites. The tensile properties of composite films were also investigated and the results were presented in Figure 5a. The average Young’s modulus (E), tensile strength (σ) and elongation at break (ε) were compared in Figure 5b and listed in Table S1. It can be seen that the mechanical properties of cross-linked composites exhibited a strong dependence on the content of mPOSS and thus could be controlled by the varied Ima/fur. The E of POF gradually increased with an increasing


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amount of mPOSS from 1134 MPa to 1904 MPa in composite of POF-0.40, corresponding to a 67.9 % increase, but the E of POF-0.50 slightly decreased to 1839 MPa, which was in agreement with the variation tendency of G’ of composites. The σ improved sharply from 41.3 MPa of POF to 72.7 MPa of POF-0.10 and the highest value appeared in POF-0.20 of 73.3 MPa, corresponding to an increment of 77.5 %, after which it started to decrease with the rising amount of Ima/fur. The ε of POF was also improved from 12.4 % to 22.5 % of POF-0.20 and then started to decline, which was consistent with the variation of σ. These results can be explained by several reasons. At lower loadings, the mPOSS could be homogenously dispersed in matrix, resulting in somewhat loosely cross-linked network, which could effectively transfer external loading among polymer chains and dissipate external energies, thus greatly reinforce the whole mechanical properties of composites. Further increasing the amount of mPOSS would lead to higher cross-linking degree of network and higher fraction of inorganic components in composites. These factors resulted in hardening of composites and further improved the modulus, but reduced the ductility of composites, and thus decreased the strength because of the less elastic deformation. While too much mPOSS would inevitably result in aggregation that served as stress concentration pot, and the steric hindrance effect of POSS cages could prevent the access of maleimide groups to furan groups, thus more mPOSS would provide excess maleimide groups in cross-linked network, these factors co-contributed to the decreased mechanical properties in composite with high loading of mPOSS (POF-0.50). Nevertheless, the above mechanical experiments made it evident that the overall tensile properties of POF could be simultaneously improved by mPOSS through the cross-linking at proper loadings.


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Figure 5. (a) Typical stress-strain curves of POF and mPOSS/POF composites. (b) Comparisons of modulus, tensile strength and elongation at break of mPOSS/POF composites. 3.5. Recycling of mPOSS/POF composites Thanks to its thermal reversible character of D-A reaction, the cross-linked composites exhibited not only improved mechanical performances at ambient condition, but also ability of recycling under heat stimulation. To test the recyclability of composites, we cut the composites into small pieces, and the fractured composites were then dissolved in NMP at 130 °C for 30 min to form homogenous solution, uniform films were obtained again by a cast-drying process and the reprocessed films were characterized by tensile test, as shown in Figure 6a, the recovery efficiency of σ and ε were compared in Figure 6b. In general, all the reprocessed samples showed a slight decrease in mechanical properties, including the pure POF. The slightly higher recovery efficiency of σ in POF-0.05 (92%) was anticipated to be result of the low cross-linking density, which endowed the sample with good recyclability. The lower recovery efficiency of σ in composites with higher mPOSS contents were considered to be the presence of some mPOSS aggregation and non-redispersable portion of cross-linked network, or the less cross-linking efficiency during the cyclic procedure, which was commonly seen in D-A reaction cross-linked composites. The recovery efficiency of ε maintained around 80% in POF and composites except for the POF-0.50, which exhibited a recovery efficiency of 94.8%, this is because mPOSS was


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already presented as agglomeration in original POF-0.5, reprocessing did not alter the dispersion state of mPOSS in this composites significantly. Nevertheless, the reprocessed samples could regain about 80 % of their original strength, which was comparable with most composites crosslinked by D-A reaction in literatures 34, 51-54. The mechanical properties of our composites after multiple recycling process were also evaluated by using POF-0.20 as model. The stress-strain curves of the corresponding recycled composites were shown in Figure 6c, and the recovery efficiency of σ and ε were compared in Figure 6d. As can been seen, the recovery efficiency of σ was slightly decrease with the increasing of recycle number, while the recovery efficiency of ε was in reverse tendency. This variation were anticipated to be caused by the insufficient dissociation and re-association of D-A linkages after multiple drying and dissolving process, which resulted in a loss of σ and increase of ε. Anyway, the recovery efficiency about 80 % after three recycle was meaningful and proposed a bright potential of our composites to be used as recyclable materials.


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Figure 6. (a) Typical Stress-Strain curves of reprocessed composites.(b) Recovery efficiency of strength and elongation at break of reprocessed composites. (c) Typical stress-strain curves of POF-0.20 with multiple reprocessing cycles and (d) the comparison of recovery efficiency in strength and elongation at break. 3.6. Dielectric properties of mPOSS/POF composites It is well known that POSS has the function to alter the dielectric properties of polymers, thus the dependence of dielectric constant and dielectric loss of composites as a function of mPOSS content were investigated. The results were shown in Figure 7, and the dielectric constant values were listed in Table 2. It can be seen that the dielectric constant conspicuously decrease with the increase of mPOSS (from 4.25 of POF to 2.25 of POF-0.20 at 106 Hz), confirming the positive effect of mPOSS in reducing the dielectric constant. In the meantime, the dielectric loss, which determined the energy dissipation by molecular motion in the presence of an electric field, is also


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reduced from 0.029 of POF to 0.018 of POF-0.20 at 106 Hz, such low dielectric constant and dielectric loss of POF-0.20 could well satisfy the demands of electronic devices. In general, the ultimate dielectric constants of composites are determined by the intrinsic value of matrix and the porosity of the nanofillers. To understand the mechanism of reduced dielectric constant, we have measured the density of mPOSS/POF composites and compared to their corresponding theoretical density, as presented in Table 2. The continuous decrease in density of composites revealed that the porous POSS was indeed useful to increase the porosity in composites. Additionally, the measured density of composites were always lower than their corresponding theoretical density, suggesting the cross-linking of POF by mPOSS has created additional porosity, that is, free volume, in composites, which was also helpful for the decreased dielectric constant 55. Furthermore, the mPOSS was homogenously dispersed in polyamide matrix at lower loadings and could lead to radical location of electronic cloud at the hetero-structure junction, thus reduce the dielectric constant of the composite and this phenomenon was called "dielectric confinement effect" 56. Moreover, the mPOSS was covalently bonded to polyamide chains by DA reaction and the cross-linked structure could effectively restrict the mobility of molecular chains and prevented the electron cloud of polyamide from being polarized. All these factors were thought to contribute to the significantly reduced dielectric constant. However, the dielectric constant increased inversely in POF-0.30, POF-0.40 and POF-0.50, which were 2.43, 3.09, and 3.32, respectively, and the dielectric loss of POF-0.30, POF-0.40 and POF-0.50 were 0.042, 0.037, and 0.023, respectively, all of which were higher than that of POF-0.20. These phenomena were attributed to the increased amount of dangling alkyl chains in mPOSS that increased the polarizability and chain mobility in composites with high loading of mPOSS,


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which surpassed the positive effect of porosity and cross-linking induced by POSS cages, and resulted in these irregular variations in dielectric constant and dielectric loss. To further confirm the point that our mPOSS/POF composites could be used repeatedly, we have evaluated the dielectric constant and dielectric loss of the reprocessed composites, as shown in Figure 7c and 7d. It can be seen that the dielectric constant and dielectric loss of reprocessed samples were all similar to their corresponding original ones with negligible changes. For example, the dielectric constant of POF-0.20 changed from 2.25 to 2.32, while the dielectric loss changed from 0.018 to 0.017 at 106 Hz, demonstrating the successful in recycling of these composites.

Figure 7. Dielectric constant (a, c) and dielectric loss (b, d) of original and reprocessed mPOSS/POF composites.


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Table 2. Dielectric constant and density of mPOSS/POF composites.

a

Theoretical densitya (g/cm3)

Measured density (g/cm3)

Dielectric constantb

Dielectric constantc

POF

1.332

1.332±0.010

4.25

4.16

POF-0.05

1.328

1.326±0.008

4.18

4.08

POF-0.10

1.325

1.290±0.011

3.17

3.14

POF-0.20

1.319

1.280±0.007

2.25

2.32

POF-0.30

1.315

1.288±0.011

2.43

2.52

POF-0.40

1.311

1.278±0.005

3.09

3.18

POF-0.50

1.308

1.261±0.007

3.32

3.34

Calculated from the weight percentage of Si8O12 cages in composites and the density of Si8O12

and POF. (1.120 and 1.332 g/cm3) b

Dielectric constant of original composites at 106 Hz.

c

Dielectric constant of recycled composites at 106 Hz.

4. CONCLUSIONS Reversibly cross-linked mPOSS/POF composites based on D-A reaction were fabricated from maleimide functionalized POSS and aromatic polyamide with pendent furan groups. Due to the cross-linking of POF and the reinforcement effect of inorganic cages of mPOSS, the thermal and mechanical properties of POF showed significant improvement with the incorporation of mPOSS. The reversible D-A reaction between mPOSS and POF also permitted recycling and reprocessing of the cross-linked composites. The recovery efficiency of σ and ε were about 80 % even after three cycles of reprocessing. In addition, the porous nature of mPOSS and the restricted chain mobility by cross-linking made a great contribution to the decreased dielectric


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constant and dielectric loss of composites, the low k value and dielectric loss of 2.25 and 0.018 of POF-0.20 at 106 Hz could satisfy the use of our composites in low-k dielectrics. The low dielectric constant and dielectric loss were still retained in recycled samples, demonstrating the good recyclability of mPOSS/POF composites. This work was anticipated to put forward a new idea to design high-performance and recyclable composites for low-k applications.

ACKNOWLEDGMENT The authors thank the Natural Science Foundation of Shanghai (Grant No. 17ZR1401100), and National Natural Science Foundation of China (Grant No. 51473031) for the support.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website: [The sol-gel transition test of mPOSS/POF mixed solution; FT-IR spectra of POF, mPOSS, and mPOSS/POF composites; SAXS spectra of POF and mPOSS/POF composites; TGA and DTG curves of POF and mPOSS/POF composites; summary of physical properties of mPOSS/POF composites]. Notes The authors declare no competing financial interest.

REFERENCES (1) Volksen, W.; Miller, R. D.; Dubois, G. Low Dielectric Constant Materials. Chem. Rev. 2010, 110, 56-110.


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(2) Maier, G. Low Dielectric Constant Polymers for Microelectronics. Prog. Polym. Sci. 2001, 26, 3-56. (3) Qiao, Y.; Yin, X.; Zhu, T.; Li, H.; Tang, C. Dielectric Polymers with Novel Chemistry, Compositions and Architectures. Prog. Polym. Sci. 2018, 80, 153-162. (4) Si, L.; Guo, D.; Xie, G.; Luo, J. Mechanical Properties and Interface Characteristics of Nanoporous Low-k Materials. ACS Appl. Mater. Interfaces 2014, 6, 13850-13858. (5) Huang, Y.; Liu, L.; Zhang, S.; Yu, H.; Yang, J. Polycarbosilane-Modified Styrene-Based Polymers with Ultra-Low Dielectric Constant, Greatly Enhanced Mechanical Strength and Thermal Stability. Eur. Polym. J. 2018, 98, 347-353. (6) Fukumaru, T.; Fujigaya, T.; Nakashima, N. Design and Preparation of Porous Polybenzoxazole Films Using the Tert-Butoxycarbonyl Group As a Pore Generator and Their Application for Patternable Low-k Materials. Polym. Chem. 2012, 3, 369-376. (7) Rathore, J. S.; Interrante, L. V.; Dubois, G. Ultra Low-k Films Derived from Hyperbranched Polycarbosilanes (HBPCS). Adv. Funct. Mater. 2008, 18, 4022-4028. (8) Yoon, S. J.; Pak, K.; Nam, T.; Yoon, A.; Kim, H.; Im, S. G.; Cho, B. J. Surface-Localized Sealing of Porous Ultralow-k Dielectric Films with Ultrathin (

Polyamide

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