Oligosilylarylnitrile: The Thermoresistant Thermosetting Resin with

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Oligosilylarylnitrile: the thermoresistant thermosetting resin with high comprehensive properties Mingcun Wang, and Yi Ning ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00238 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

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ACS Applied Materials & Interfaces

Oligosilylarylnitrile: the thermoresistant thermosetting resin with high comprehensive properties Mingcun Wang *†, Yi Ning † †

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, 37 Xueyuan Road, Beijing 100191, P. R. China

ABSTRACT: One of the highest thermoresistant thermosetting resins ever studied so far, oligosilylarylnitrile resin, was investigated firstly in this study. Oligosilylarylnitrile was synthesized by lithium-reduced Wurz-Fittig condensation reaction, and the prepared viscous resin exhibited moderate rheological behaviors while heated purely or together with 20% polysilazane as crosslinking agent. The thermal cure temperature was found by DSC (Differential Scanning Calorimetry) at 268 oC (pure) and 158 oC (with polysilazane crosslinking agent), which is comparably close to that of polysilylarylacetylene resin (normally at 220-250 oC), however much lower than those of polyimide and phthalonitrile resins (normally >300 oC), indicating the admirable material processability of oligosilylnitile. The cured oligosilylarylnitrile resins have extremely high thermal resistance, indicated by the results of TGA (Thermogravimetric Analysis, mass residue at 800 oC is > 90% under N2) and DMA (Dynamic Mechanical Analysis, glass transition temperature is > 420 oC). The mechanical property of oligosilylarylnitrile matrixed silica-cloth reinforced laminate is comparably close to those of polyimide and phthalonitrile, however much higher than that of polysilylarylacetylene, indicating the enviable thermal and mechanical properties of oligosilylnitrile. Thus, among high temperature resins ever studied so far, oligosilylarylnitile resin was found to have the almost best comprehensive characteristics of processability and properties. KEYWORDS: Oligosilylnitrile; Polysilazane; Thermal cure; Thermoresistant; Mechanical property

INTRODUCTION High temperature polymers and their composites have been attracting ceaseless interests in materials research and engineering, and find ever growing applications in aerospace, aviation, automotive, electrical and electronics, and oil and gas markets, etc. 1 High temperature resin with high temperature performance, chemical resistance, toughness, speed of cure and ease of processing is being under carefully focused all over the world. 2 Among the ever-found high temperature thermosetting resins, the most important species popular in scientific research and engineering applications include polyimide, polysilylarylacetylene and phthalonitrile, etc. 3 Nevertheless, in aerospace and aviation areas, bismaleimide, cyanate ester resin and benzoxazine can 1

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be regarded as high temperature resins if the material service temperature is below 300oC. 4,5 Polyimides are well known to have extremely thermal stability and have a glass transition temperature at >300 oC. Polyimides and polyimide composites have been applied in aerospace and electronics industries due to their excellent high temperature mechanical performance, adhesive property, moisture retardance and oxidation endurance. 6,7 Phthalonitrile is another star member of high temperature engineering thermoset, which was started by Keller TM; 8 by now it has evolved to act as an attractive high thermal thermoset with superior mechanical performance, low water absorption and exceptional flame retardancy. 9,10 Polysilylarylacetylene is one kind of high temperature nonpolar resin, which has enhanced heat resistance, excellent mechanical properties, dielectric properties and high temperature ceramic performances. The cured polysilylarylacetylene can be used as high temperature resistant materials, wave transmitting materials, semiconductor materials, and ceramic precursors, etc. 11,12 Another group of acetylene-containing polymers have been reported that phenolic compounds such as Novolac or polyphenols were successfully modified with ethynyl groups for the preparation of high performance addition curable resins. 13 However, it is still an extreme challenge to prepare high temperature thermoset with ideal comprehensive properties of broad processing window, moderate cure condition, high thermal resistance and high mechanical property. 14 For example, for polyimide and phthalonitrile resins, it has admirable high temperature mechanical property, but its processability is really poor (high melting point, extremely high curing temperature, narrow processing window); however, for polysilylarylacetylene resin, it has excellent processability (liquid resin, moderate curing temperature, appropriate processing window) and thermal resistance, but its mechanical property is limited (weak interfacial adhesion of fiber and matrix). Recently the ceramic precursor of polysilazane has been successfully employed as cross-linking agent for benzoxazine, epoxy and diisocyanate resins, 15-17 and the accelerated thermal-cure could proceed at a decreased temperature. The hybridized thermosetting resins showed highly improved thermal stability and interfacial wettability with inorganic fibers. This fact initiated the idea in this paper, we focus on preparation of one brand-new resin which selectively binds the advantages of polyimide and polysilylarylacetylene, however suppresses their deficiencies; i.e., oligosilylarylnitrile resin in this paper will have an excellent processability and high thermal stability like polysilylarylacetylene, meanwhile have a profound mechanical property like polyimide. Such a thermosetting resin will have a promising application in high temperature advanced composite. The synthesis of oligosilylarylnitrile is via Wurtz-Fittig protocol using lithium as reductant in Lewis base type good solvent of tetrahydrofuran. By such synthetic 2


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method, the coupling condensates from aromatic halide was successfully prepared, 18 and a series of polysilanes was also prepared from chlorosilanes. 19,20 Based on the experienced synthesis, oligosilylarylnitrile in this paper was readily synthesized using methyldichlorosilane, 4-bromophthalonitrile and 2,5-dichlorobenzonitrile as starting chemicals.

EXPERIMENTAL Oligosilylnitrile synthesis and thermal cure. Oligosilylnitrile resin was prepared by Wurtz-Fittig reduction condensation as the similar procedure in reported literature. 19,20 In a three-necked flask equipped with flux condenser and addition funnel, the reductant of lithium (5.6 g, 0.8 mol) was firstly added in 100mL tetrahydrofuran, the air in the flask was blown out by nitrogen flow. After the reaction temperature reached 50oC, the mixture of methyldichlorosilane (0.2 mol, 23 g), 4-bromophthalonitrile (0.2 mol, 36.4 g) and 2,5-dichlorobenzonitrile (0.1 mol, 17.2 g) in 100 mL THF was added dropwise in about 120 min while the reaction temperature was maintained below 60 oC. After addition, the reaction turned yellowish and plenty of white salt precipitated. The reaction was run at 60 oC until lithium disappeared (normally in 3 hours). The reaction was cooled down naturally after the removal of heat resource. The salts were separated by vacuum filtration, the resin solution was evaporated on rotary evaporator, and the resin was purified by rinsing with 50 mL de-ionized water (at least 5 times until the water phase was negative to AgNO3). Finally, the resin was recovered after water removal by rotary evaporation at a yield of 90%. Thermal cure was conducted in convention electric oven. For pure oligosilylarylnitrile, the resin was pre-heated at 160 oC till gelation, then heated at 200 o C for 2 h, finally post-cured at 250 oC for 2 h. For oligosilylarylnitrile with 20% polysilazane (made in our own lab by ammonolysis of chlorosilanes), yellowish liquid, viscosity at 25 oC is 30 mPa·s), the resin was pre-heated at 100 oC till gelation, then heated at 150 oC for 2 h, finally post-cured at 200 oC for 2h. The cured resin exhibited as dark-red dense solid.

Preparation and evaluation of oligosilylnitrile matrixed silica cloth laminate. Oligosilylnitrile matrixed silica cloth laminate was fabricated by the method as reported in literature. 21 Silica cloth (plain, 1K) was impregnated in the oligosilylarylnitrile acetone solution at 50 wt% concentration, and the prepregs were dried under ambient conditions for about 24h. The resin content of the prepregs was about 40 wt%. The prepregs were stacked and put between two steel plates in a hydraulic press pre-heated at 160 oC. After resin flow ceased, the plies were cured under 4 MPa at 200 oC for 2 h, then was post-cured at 250 oC for 2 h. Laminate composites using oligosilylarylnitrile-polysilazane as matrix were also 3



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prepared. The press was started at 100 oC, the thermal cure was under 4 MPa at 150 o C for 2 h and post-cure was at 200 oC for 2 h.

Characterizations. DSC profile was recorded on Mettler-Toledo DSC1Q2000, nitrogen flow at 50mL/min, temperature range of 50-350 oC, heating rate at 10 oC/min. FT-IR (Fourier transform infrared spectrum) was measured on Nicolet iS50, wave number range of 4000-400 cm-1, resolution 4cm-1, scanning times 32, KBr tablet. GPC (Gel permeation chromatography) analysis was recorded on Waters 1525, using THF as eluent at 1mL/min, and standard mono-disperse polystyrenes as references. Rheological analysis was measured on Gemini 200 Advanced Rheometer at 1Hz and heating rate of 5 oC/min ranging from RT to 250 oC. TGA was plotted on NETZSCH STA 409pc at a temperature ramp of 20oC/min, N2 flow at 50mL/min, and temperature range of 28-900 oC. DMA was detected on NETZSCH DMA 242E at 1Hz at a temperature ramp of 5 oC/min, N2 flow at 50mL/min, and temperature range of 50-500 oC. The mechanical properties of the laminate composites were evaluated as per the standards of ASTM D7264/D7264 M-2007 for the flexural properties and ASTM D3846-08 for the shear strength.

RESULTS AND DISCUSSIONS Synthesis and processing

capability

of

oligoarylnirile.

Oligosilylarylnitrile with low softening temperature was readily prepared according to the procedure as shown in Scheme 1. The formation Si-Ph bond by Lithium reduction of methyldichlorosilane, 4-bromophthalonitrile and 2,5-dichlorobenzonitrile proceeds preferably in tetrahydrofuran to in toluene, because the polarity and Lewis basicity of tetrahydrofuran accelerate the metallization of arylhalide. The polycondensation degree or molecular weight can be tailored feasibly by adjusting the relative ratio of mono-halide and di-halide starting materials. In the viewpoint of ideal processability required for high temperature resins, the resin was chemically formulated as comparably low molecular weight polymer, i.e., oligomer. It should be pointed out that the synthetic yield was as high as 90% and the recover method was feasible, thus the preparation of oligosilylarylnitrile resin is easily scaled up.

4


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N

N

8mol

CH3 N

Br

+

Cl

Cl

+ Cl

Si

Lithium in THF

Cl drop at 50oC & reaction at 60oC

H

2mol

1mol

2mol N

N

N CH3

CH3 N

Si

Si

H

H

N

Oligosilylarylnitrile

reaction route N

N

N

methyldichlorosilane

lithium N

Br

N

N

Li

SiHMeCl

N

N N N

methyldichlorosilane ClMeHSi Cl

Li

SiHMeCl

Cl

N

N

lithium

lithium

N

N

methyldichlorosilane Cl

SiHMe

N

N

N

Cl

Li

N

N

SiHMeCl

( SiHMe

N

reaction mechanism

)n SiHMe

N

Oligosilylarylnitrile species

Scheme 1. Synthesis of oligosilylarylnitrile resin

A simple method in which gas (hydrogen) bubbles escape due to hydrolysis of Si-H can be applied to ascertain the existence of Si-H in oligosilylarylnitrile molecule (Part 1, Supporting Information). Elemental analysis of oligosilylarylnitrile is another effective technique to indicate structure components (Part 2, Supporting Information): By Flash EA1112 analyzer, nitrogen content in oligosilylarylnitrile was found at 16.2%, while nitrogen content in oligosilylarylnitrile-20%polysilazane was found at 17.3% (the combination of nitrogen amount in oligosilylarylnitrile and polysilazane). By EDS (Energy Disperse Spectroscopy) attached to SEM (Scanning Electron Microscope), for the cured resins, silicon, nitrogen and carbon distribution mappings were qualitatively provided in Figure S1 & S2 in Part 2 of Supporting Information: all the elements of silicon, nitrogen and carbon were found uniformly distributed in oligosilylarylnitrile and oligosilylarylnitrile-20%polysilazane (meaning the molecule-level miscibility of oligosilylarylnitrile and polysilazane); silicon content in oligosilylarylnitrile was 12.9% by weight, while silicon content in oligosilylarylnitrile-20%polysilazane was 19.2% by weight (the combination of silicon amount in oligosilylarylnitrile and polysilazane). FT-IR spectrum of oligosilylarylnitrile was showed in Figure 1, in which the molecular structure characteristics of oligosilyarylnitrile was revealed and ascertained: 5



aromatic C-H at 3050 cm-1, aliphatic C-H at 2900 cm-1, nitrile C≡N at 2250 cm-1, Si-H at 2140 cm-1, and Si-C at 1270 cm-1. After thermal cure into three dimensional crosslinking network, the inert groups still displayed their absorption peaks (aromatic C-H at 3050 cm-1, aliphatic C-H at 2900 cm-1, and Si-C at 1270 cm-1), while the curable groups disappeared (nitrile C≡N at 2250 cm-1, Si-H at 2140 cm-1). The new broad peak at 3300 cm-1 was owing to the oxidation / hydrolysis of Si-H by oxygen or moisture during elevated curing temperature under air condition. 22,23 The molecular structure was confirmed by comparison of IR spectra of the parent and cured resins. The molecular characteristics revealed by FT-IR were in agreement with 1HNMR spectrum of oligosilylarylnirile (Figure S3, Part 3 of Supporting Information): in 1 HNMR spectrum, aromatic hydrogen was at ppm, hydrogen in Si-H was at 4.3-4.8 ppm, hydrogen in Si-Me was at 0-0.8 ppm, aromatic hydrogen was at 6.7-7.7 ppm. Oligosilylarylnitrile Oligosilylarylnitrile cured 100

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

60

3050 2900 Ph-H C-H 40

2250 C≡N

20

2140 Si-H

1270 Si-C

0 3500

3000

2500

2000

1500

1000

Wave number (cm-1) Figure 1. FT-IR spectra of oligosilylarylnitrile (before & after thermal cure)

The purposed application of oligosilylarylnitrile in this paper is to be as composite matrix, so the resin processability is critical property to be evaluated. Gel permeation chromatography showed that the number-average molecular weight is 490, the weight-average molecular weight is 730, and polydispersity index is 1.49, showing the typical characteristic of polycondensation oligomer. If the dominent individual peak was excluded and only higher molecular polycondensates were under consideration, the Mn is 890, the Mw is 1750, and index of polydispersity is 1.97 , which indicated the polycondensate was dominantly composed of small molecular compound, together with some chain-elongated species(Figure S4 in part 4 of Supporting Information). The low average molecular weight and wide polydispersity guarantee oligosilylarylnitrile with ideal process capability. Shown in Figure 2 by dynamic rheological profile, with temperature increase, the pure oligosilylarylnitrile quickly reached its minimum viscosity at about 70 oC (its 6


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softening point at around 60 oC), shaped a horizontal basin bottom in the temperature range of 60-240 oC, and climbed fast after 240 oC owing to initiated cross-linking cure reaction; the results revealed the pure oligosilylarylnitrile has a quite good processing capability (wide processing window over 150 oC, low melting point, moderate gelation temperature). The dependence of static viscosity on temperature can be found in Table S1 in Supporting Information. Oligosilylarylnitrile and oligosilylarylnitrile-20%polysilazane resins possessed stable viscosity with rising temperature till 120oC, and the results showed their processing capability for environment-friendly melt processing with no use of solvent in composite preparation. In order to further optimize the rheology of oligosilylarylnitrile, oligosilylarylnitrile was homogeneously blended with 20 wt% polysilazane as curing agent, the addition amount of 20 wt% was chosen because at this point oligosilylarylnitrile-polysilazane turned from gel-like soft solid to viscous state. Even more important thing was that polysilazane highly accelerated thermal cure reaction of oligosilylarylnitrile, revealed by the fact that the gelation temperature (at which viscosity increased rapidly) shifted dramatically to lower temperature from 240 oC to 155 oC. Surely oligosilyarylnitrile-20%polysilazane has much better processing capability (liquid state at room temperature, wide processing window over 120 oC, low gelation temperature). The acceleration effect of polysilazane on thermal cure of oligosilylarylnitrile might result from different reaction mechanism, the details will be discussed in the section of DSC analysis. 3000

oligosilylarylnitrile (pure)

2500

oligosilylarylnitrile (20% polysilazane as crosslinking agent)

2000

Viscosity (Pa·s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1500

1000

500

0 50

100

150

200

250

o

Temperature ( C)

Figure 2. Rheological profile of oligosilylarylnitrile (pure resin & polysilazane as crosslinking agent)

The thermal cure behaviors were displayed in Figure 3. Compared the DSC behaviors of the pure and blended (with 20 % polysilazane as curing agent) oligosilylarylnitrile, some results were revealed as follows: 7



In the DSC profile of pure oligosilylarylnitrile: (1) the exothermic peak of thermal cure reaction occurred in the range of 200-300 oC, peaking at 268 oC; (2) the thermal curing peak was quite similar with that of polysilylarylacetylene (normally in the range of 190-280 oC and with a peak at around 240 oC); 11 (3) however quite lower than those of polyimide and phthalonitrile (normally in the range of 250-400 oC and with a peak at over 300 oC). 7,9 In the DSC profile of oligosilylarylnitrile with 20 % polysilazane cross-linking agent: (1) the exothermic peak of thermal cure shifted to lower temperature range of 130-200 oC and peaked at 158 oC; (2) the thermal curing peak was even lower than that of polysilylarylacetylene, and dramatically lower than those of polyimide and phthalonitrile; (3) the accelerated cure process led the liquid resin to solid readily at moderate temperature upon heating, attributing oligosilylarylnitrile-polysilazane resin to be suitable to be conveniently processed into varying shapes. 158oC

Heat flow (W/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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oligosilylarylnitrile (20% polysilazane) 268oC

oligosilylarylnitrile (pure) 50

100

150

200

250

Temperature (oC)

300

350

400

Figure 3 DSC profiles of oligosilylarylnitrile (pure resin & polysilazane as crosslinking agent)

Due to the complexity of multiple cure reactions in thermal cure of oligosilylarylnitrile resins and still inadequate characterization techniques for solidification process of thermosetting resins, thermal cure mechanism is not fully understood yet. Figure 4 is the hypothesized mechanism of thermal cure of pure and polysilazane accelerated oligosilyarylnitriles. Generally, thermal cure reactions in pure oligosilylarylnitrile involved hydrosilylation (between C≡N and Si-H), 24 cyclotrimerization of nitrile to triazine, 25 cyclization of phthalonitrile to phthalocyanine rings, 26 etc. Although such reactions normally require a certain catalyst to proceed at mild conditions, heating at high temperature is an effective method to initiate these reactions in oligomer or polymer, in view of the purity demand for polymer material. For the plain resin, gelatinated resin (at gel-like solid state) and totally cured resin, the functional groups evolution were shown in Figure 5. For oligosilylarylnitrile, upon 8


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heating in thermal cure process, the groups of Si-H and C≡N were gradually weakening, and the appearance of peaks at 1010-1020 cm-1 implied the generation of triazine, but the phthalocyanine at 1510-1530 cm-1 was not evidently visible. For oligosilylarylnitrile-20%silazane, the functional groups showed an accelerated evolution: the faster appearance of triazine at 1010-1020 cm-1 was observed; there was new peak at about 1510 cm-1, but it was hard to assign it to phthalocyanine. 27 Revealed by rheology in Figure 2 and DSC in Figure 3 the thermal cure of oligosilylarylnitrile can be highly accelerated by polysilazane. The nucleophilic addition reaction between Si-N and C≡N 28 as well as Michael reaction between N-H and C≡N

29

might be responsible for the shape change of differential scanning calorimetric profile, the above reactions occurred at much lower temperatures, thus the exothermic peak appeared at lower temperature range than that of pure oligosilyarylnitrile. The molecular structure of cross-linked resin might be different due to the different cure mechanism, but in each circumstance the network is consist of thermally stable bonds, so the high temperature resistance of pure and polysilazane accelerated oligosilylarylnitrile resins are reasonably expected. Compared with conventional catalytic groups such as amino and hydroxyl, 30,31 the advantages of hydrosilylation and nitrosilylation lie in the following three points: (1) the crosslinking network of oligosilylarylnitrile-20%polysilazane is composed of silicon-participated hybrid chains, so the thermal stability is guaranteed and the thermo-oxidation resistance is highly improved in air atmosphere; (2) the crosslinking reaction proceeds smoothly at lower temperature to form silicon-containing hetero-chains, so the thermally curing property is ideal for moderate cure of material fabrication; (3) polysilazane has low viscosity (ca. 30 mPa·s at 25 oC) and good miscibility with oligosilylarylnitrile, so the rheological property of oligosilylarylnitrile is ideal for melt processing of diverse materials.

9



N

N

N CH3

CH3 N

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Si

Si

H

H

N

N

Hydrosilylation addition to Schiff base structure

HMeSi

N SiMe

SiMe

HMeSi

SiMeH

Cyclotrimerization to triazine

SiMeH N

N SiMeH N HMeSi

SiMeH

SiMeH

Cyclization to phthalocyanine rings N N

HMeSi

N

N N N

N N SiMeH

SiMeH

Thermal cure of pure oligosilylarylnitrile N

N

N

N

CH3

CH3

Si

Si

H

H

H

+

N

Si

NH

CH3

N

Hydrosilylation addition to Schiff base structure

HMeSi

N SiMe

SiMe

CH3 Si

NH

Nitrosilylation addition to heterochain network

N

HMeSi

H CH3

N

CH3

Si H

Si NH

N

H

SiMe

CH3 Si N

H NH

Nucleophilic addition of N-H to nitrile group

HMeSi

N

CH3

CH3 Si H

N

Si H NH

SiMe

Crosslinking agent polysilazane to accelerate oligosilylarylnitrile curing

Figure 4. Thermal cure mechanisms of pure and polysilazane-crosslinked oligosilylarylnitrile

10


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2100-2250

1510-1530 1010-1020

100

Transmittance (%)

A 80

60

C

40

20

B

A: oligosilyarylnitrile B: gelatinated resin C: cured resin

0 3500

3000

2500

2000

1500

1000

Wave number (cm-1) 2100-2250 1510-1530 1010-1020

100

80

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

D F E

40

20

D: oligosilylarylnitrile-20%polysilazane E: gelatinated resin F: Cured resin

0 3500

3000

2500

2000

1500

1000

Wave number (cm-1)

Figure 5. FT-IR spectra of plain, gelatinated and cured resins

Thermal resistance characteristics. Thermal stability can be revealed by TGA and DMA techniques. Figure 6 and 7 displayed the thermo-decomposition behaviors of the cured oligosilylarylnitrile under nitrogen and air atmosphere respectively, while Figure 7 displayed the dynamic mechanical property of the cured oligosilylarylnitrile under nitrogen. The information revealed by TGA in nitrogen flow (Figure 6) include: (1) both of pure and polysilazane-crosslinked oligosiylarylnitrile resins can tolerate high temperature with little mass loss before 550 oC (the intersection point temperature of tangent lines is about 550oC, which is badly close to that of polysilylarylacetylene and phthalonitrile); (2) only beyond 550 oC the apparent decomposition start at a quite slow descent of mass loss, implying the organic network might rearrange to form carbon rather than collapse to release a large amount of volatiles; the final carbonaceous residue was found at 92.5 % for pure oligosilylarylnitrile and at 90 % for oligosilylarylnitrile-20%polysilazane respectively; the ultrahigh residue after pyrolysis is comparably a bit higher than that of polysilylarylacetylene (the ever reported thermoset with the highest carbon residue of 85-88 %),11,12 and much higher than those of polyimide and phthalonitrile (normally 60 % and 75 % respectively). 7-9 Under nitrogen atmosphere the thermal stability of oligosilylarylnitrile is slightly 11



better than that of oligosilylarylnitrile-20%polysilazane. This might be due to the fact that polysilazane contains easily decomposed groups of –NH- and -CH3, the mass residue of polysilylarylnitrile-20%polysilazane is a bit inferior to that of pure oligosilylarylnitrile, hence in the circumstance of inert atmosphere the higher silicon content helps not too much to improve thermal stability of such an aromatic backbone polymer. 32 100

Mass residue (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90

Oligosilylarylnitrile-20%polysilazane onset: 540oC; Td5: 650oC; 800oC residue: 90% Oligosilylarylnitrile onset: 537oC; Td5: 660oC; 800oC residue: 92.5%

80

70 100

200

300

400

500

600

700

800

Temperature (oC) Figure 6. TGA profiles of oligosilylarylnitrile (pure resin & polysilazane as crosslinking agent) under nitrogen atmosphere

However under air atmosphere (oxidative condition), the thermal stability of pure and polysilazane-crosslinked oligosilylarylnitrile (shown in Figure 7) showed quite different behaviors: (1) actually the pure oligosilyarylnitrile possesses good thermo-oxidative stability, ascertained by its Td5 of 460 oC and residue of 55 % at 800 o C; (2) polysilazane crosslinked oligosilyarylnitrile has highly improved oxidation resistance, i.e., its Td5 of 580 oC and residue of 74 % at 800 oC; (3) the thermo-oxidative stability of oligosilylarylnitrile is higher than that of polyimide and phthalonitrile (normally mass residue of 450 C ; onset of E: 440 C

0.2

Oligosilylarylnitrile-20%polysilazane o o Tg: >450 C ; onset of E: 425 C

0.1

200 100

0.0 100

200

300

400

Temperature (oC)

Figure 8. DMA profiles of oligosilylarylnitrile (pure resin & polysilazane as crosslinking agent) under nitrogen atmosphere

Silica cloth laminate property. Ascertained by the results of rheology (in Figure 2) and thermal cure (in Figure 3) behaviors, oligosilylarylnitrile has excellent processing capability. Furthermore, high thermal stability is also confirmed by TGA in 13



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Figure 6&7 and DMA in Figure 8. The mechanical property is highly relevant to interfacial bonding. In view of the ideal viscosity and wide processing window, there is sufficient time for oligosilylarylnitrile to wet and impregnate silica cloth during laminate fabrication. The high mechanical property is highly possible because the strong adhesion between the polar chemical groups in oligosilylarylnitrile and the surface polar groups of silica fibers. High temperature mechanical property was listed in Table 1. As indicated by the data, oligosilylarylnitrile laminate exhibited so high mechanical properties, and oligosilylarylnitrile crosslinked by 20 % polysilazane had little negative influence on mechanical properties. The high retention rate of flexural strength and interlaminar shear strength at 400 oC implied the excellent high temperature mechanical performance of oligosilylarylnitrile composite, the profound flexural and ILSS strength may be the result of rigid aromatic crosslinked network, furthermore silicon-containing hetero-chain network can improve thermo-oxidative stability and help maintain the mechanical property under air atmosphere at elevated temperature. Compared with polyimide and phthalonitrile, oligosilylarylnitrile composite has closely similar high temperature mechanical properties. 34-36 Table 1. Flexural property of silica cloth laminate Items Laminate matrix Oligosilylarylnitrile Oligosilylarylnitrile -20%polysilazane

Measurement temperature RT o

400 C 30 min RT o

400 C 30 min

Flexural strength (MPa)

Flexural modulus (GPa)

Interlaminar shear strength (MPa)

820 492

32 29

56 32

(conservation rate 60%)



730 409

30 26

60 33




The profound processability, thermal stability and mechanical property of oligosilylarylnitrile made it become the almost best high-temperature resistant thermosetting resin for advanced engineering composites. Summarized in Table 2, compared with polyimide and phthalonitrile, oligosilylarylnitrile has superior processing ability (liquid viscous resin, moderate thermally curing temperature), while has extremely close thermo-stability and mechanical property. 37-40 Compared with polysilylarylacetylene, oligosilylarylnitrile has superior high temperature mechanical and thermo-resistant properties, while has similar processing capability. 12, 41 Thus, oligosilylarylnitrile is one of high temperature resins with almost highest comprehensive material properties.

14


Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Comparison of oligosilylarylnitrile, polysilyarylacetylene, phthalonitrile and polyimide Resin & property

oligosilylarylnitrile

polysilyarylacetylene

phthalonitrile

polyimide

Thermal curing temperature (oC)

200-250

200-300

>300

>300

Glass transition temperature (oC)

>400

>400

>400

300-400

Residue at 800oC

>90%

>85%

≥70%

≥60%

Shear strength of laminate (MPa)

ca. 60

ca. 20

ca. 60

ca. 70

Refs: for phthalonitrile, [9, 24, 31, 37, 38]; for polyimide, [39, 40]; for polysilyarylacetylene, [12, 41].

CONCLUSIONS In summary, based on the above results and discussion, the following conclusions were drawn: (1) Oligosilylarylnitrile resin was prepared by lithium-reduced Wurz-Fittig condensation at a yield of 90 %, and the viscous resin exhibited ideal processing window (for pure resin, processing window is over 160 oC from 70 to 240 o C; for resin with 20% polysilazane as crosslinking agent, processing window is over 120 oC from 30 to 155 oC). (2) The thermal curing temperature was found at 268 oC (pure) and 158 oC (with 20 % polysilazane), which is comparably close to that of polysilylarylacetylene resin (normally at 220-250 oC), however much lower than those of polyimide and phthalonitrile resin (normally >300 oC). Liquid oligosilylnitile possesses admirable material processability. (3) The dynamic mechanical property revealed by DMA technique is excellent for both pure and polysilazane-crosslinked oligosilyarylnitrile resins, Tg indicated by Tgδ peak was higher than 450 oC. The pure and polysilazane-crosslinked oligosilylarylnitrile both have high thermal decomposition resistance, while polysilazane-crosslinked oligosilylarylnitrile has extremely enhanced thermo-oxidative resistance due to the ceramic-like pyrolytic product which acts as oxidation barrier. (4) The mechanical property of oligosilylarylnitrile matrixed silica-cloth reinforced laminate is comparably close to those of polyimide and phthalonitrile, however much higher than that of polysilylarylacetylene. Oligosilylarylnitrile matrixed composite has enviable thermal and mechanical properties. (5) Among high temperature resins ever studied so far, oligosilylarylnitile resin has the best comprehensive characteristics of processability and properties. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 15



Gas bubble escape in basic solvent to show Si-H existence in oligosilyarylnitrile; Element analysis of Silicon and Nitrogen; 1H-NMR spectrum of oligosilylarylnitrile; GPC profile of oligosilylarylnitrile; Viscosity of oligosilylarylnitrile and oligosilyarylnitrile-20%polysilazane at different temperatures. AUTHOR INFORMATION Corresponding author Email: [email protected] (Mingcun Wang) ORCID Mingcun Wang: 0000-0002-6254-1633 Notes The authors declare there is no conflict of interest. ACKNOWLEDGMENTS The research in this paper is financially supported by the Research Institute for Special Structures of Aeronautical Composite, Aviation Industry Corporation of China (Contract number 527859-01). REFERENCES (1) Mittal, K. L. Polyimides and other high temperature polymers: synthesis, characterization and applications (vol 5). CRC Press: Leiden, 2009. (2) Hamerton, I. Chemistry and technology of cyanate ester resins. Blackie Academic & Professional: Glasgow, 1994. (3) Tandon, G. P.; Pochiraju, K. V.; Schoeppner, G. A. Thermo-Oxidative Behavior of High-Temperature PMR-15 Resin and Composites. Mat. Sci. Eng.: A 2008, 498(1), 150-161. (4) Ramirez, M. L.; Walters, R.; Lyon, R. E.; Savitski, E. P. Thermal Decomposition of Cyanate Ester Resins. Polym. Degrad. Stab. 2002, 78(1), 73-82. (5) Ren, Z.; Cheng, Y.; Kong, L.; Qi, T.; Xiao F. High Glass Transition Temperature Bismaleimide- Triazine Resins Based on Soluble Amorphous Bismaleimide Monomer. J. Appl. Polym. Sci. 2016, 133(3), 42882-42888. (6) Karra S, Rajagopal K R. Modeling the Non-Linear Viscoelastic Response of High Temperature Polyimides. Mech. Mater. 2011, 43(1), 54-61. (7) Droske, J. P.; Stille, J. K.; Alston, W. B. Biphenylene End-capped Polyquinoline and Polyimide Prepolymers as Matrix Resins for High-use-temperature Composites. Macromolecules 2002, 17(1), 14-18. (8) Keller, T. M. Phenolic-cured phthalonitrile resins. US Patent 4,410,676 A. 1983-10-18. (9) Keller, T. M. Phthalonitrile- Based High Temperature Resin. J. Polym. Sci., Part A: Polym. Chem. 1988, 26(12):3199-3212. (10) Laskoski, M.; Clarke, J. S.; Neal, A.; Harvey, B. G.; Ricks-Laskoski, H. L.; Hervey, W. J.; Daftary, M. N.; Shepherd, A. R.; Keller T. M. Sustainable High Temperature Phthalonitrile Resins Derived from Resveratrol and Dihydroresveratrol. Chem. Select 2016, 1(13), 3423-3427. 16


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TOC graphic (Graphic Abstract)

N CH3

N

N CH3

Si

Si

H

H

N

90

Oligosilylarylnitrile-20%polysilazane onset: 540 oC; Td5: 650o C; 800oC residue: 90% Oligosilylarylnitrile onset: 537 oC; Td5: 660 oC; 800oC residue: 92.5%

80

N2

Oligosilylarylnitrile

70 100

200

30 0

400

500

600

700

80 0

Temperature (oC)

Thermogravimetric analysis under nitrogen 0.5

8 00 7 00

0.4

N2

6 00 0.3

5 00

tg d

N

Mass residue (%)

100

Storage modulus (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Oligosilylarylnitrile o

o

4 00

Tg: >450 C ; onset of E: 440 C

3 00

Oligosilylarylnitrile-20%polysilazane o

2 00

0.2

o

Tg: >450 C ; onset of E: 425 C

0.1

0.0

1 00 100

2 00

300

400

o

Temperature ( C)

Dynamic thermomechanical analysis under nitrogen

19


Oligosilylarylnitrile: The Thermoresistant Thermosetting Resin with

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