Toughening of bismaleimide resin based on self-assembly of flexible

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Toughening of Bismaleimide Resin Based on the Self-Assembly of Flexible Aliphatic Side Chains Qi Zou, Feng Xiao, Shan Q. Gu, Jun Li, Dai J. Zhang, Yan F. Liu,* and Xiang B. Chen*

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Science and Technology on Advanced Composites Laboratory, Beijing Institute of Aeronautical Materials, Aero Engine Corporation of China, Beijing 100079, China ABSTRACT: The poor toughness of bismaleimide (BMI) resin hampers its application in the aeronautics and space field. Herein, a flexible monohydric aliphatic amine (MAA) was introduced to in-situ toughen BMI resins, in which a novel self-assembled second-phase structure was formed. The micro-nanostructure can be adjusted by changing MAA to perfectly improve the comprehensive performance of BMI. In addition, only a small amount of MAA can achieve substantial increase of toughness of BMI. The morphology of MAAmodified BMI was observed via scanning electron microscopy and atomic force microscopy. Consequently, after adding 1% MAA, the elongation at break of MAA-modified BMI increased by 32.3%. Compared to the neat BMI, the flexural strength of MAA-modified BMI exhibited enhancement by 30.1%. Significantly, the presence of MAA in the BMI resin resulted in a remarkable increase in KIC of 24.3% and GIC of 54.2% without sacrificing the processability and heat resistance.

1. INTRODUCTION In recent years, bismaleimide (BMI) had been widely used in the aeronautics and space field, owing to its excellent thermal resistance, good radiation resistance, and extraordinary dampheat resistance.1−4 Additionally, BMI resins exhibits easy processability similar to epoxy resins so that the cost of manufacturing was low. However, the conventional BMI resins possessed many disadvantages such as, brittleness, poor impact resistance, and crack resistance resulting from high crosslinking density. Thus, it was essential to toughen BMI resins under the premise of ensuring the preponderant inherent properties. Diamine modification as a chemical method had been reported on toughening of BMI resins in previous articles. After using diamine chain extenders, the crosslink density of BMI resins was reduced because the distance between the two functional end-groups was increased. Because of the existence of a great degree of molecular freedom, diamine-modified BMI resins exhibited an enhanced energy-absorbing ability, resulting in improved impact performance.4,5 For example, bis(4aminophenyl)methane was added into 4,4-bismaleimidodiphenyl methane (BDM) and the improvement in fracture toughness (KIC, GIC) was over 100%.6 However, modified BMI resins demonstrated reduction of the glass transition temperature (Tg) and modulus. This result suggested that the method was extremely unfavorable. As another chemical modifying method, copolymerization modification with allyl compounds such as diallyl bisphenol A (DABA) was regarded and was a relatively successful toughening process for BMI resins. The copolymerized BMI resins had notable stability and good solubility. Besides, the copolymerized BMI resins present much better processability and adhesion-ability than aforementioned © XXXX American Chemical Society

diamine-modified BMI resins. Moreover, after curing, the copolymerized BMI resins maintained their original high crosslink density, good thermal resistance, remarkable dampheat resistance, and excellent mechanical properties.7−10 The Cytec 5250 BMI resin was synthetized by using BDM, DABA, and 2,4-bismaleimidotoluene. The obtained BMI resin exhibited good toughness, high thermal resistance (Tg = 293 °C), and low viscosity, which was favorable to application in the resin transfer molding process.11 To further toughen BMI resins, rubber or thermoplastic resin was physically incorporated as the second phase into the BMI resin system. Generally, the blending rubber modification was based on active liquid rubber as the toughening agent. Because BMI resins had poor compatibility with rubber, the rubber separated from the resin matrix and formed a distinct sea-island structure. The rubber phase can greatly improve ductile deformation and toughness of the matrix.12−15 Wang et al. used vinyl-terminated butadiene acrylonitrile (VTBN) as the toughening agent in bis[4-(4-maleimidephen-oxy)phenyl]propane/2,4-bismaleimidotoluene/DABA. The results revealed that an increased KIC of 48.7% can be realized when 6 wt % of VTBN is used.15 However, it had a large negative impact on rigidity and thermal resistance of the matrix. When adding thermoplastic resin, a macroscopically homogeneous and microscopically phase-separated structure was generated, and the structure can effectively induce production of the craze and Received: Revised: Accepted: Published: A

April 3, 2019 August 9, 2019 August 13, 2019 August 13, 2019 DOI: 10.1021/acs.iecr.9b01822 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Scheme 1. Schematic Diagram of the Self-Assembled Phase Structure of a Modified BMI Resin with Flexible Aliphatic Chains

2. EXPERIMENTAL SECTION 2.1. Materials. The BMI resin (main components included BDM and DABA) was synthesized in our laboratory; BDM (99%) was purchased from Honghu City Shuangma Advanced Materials Tech Co., Ltd., China; DABA (96%) was purchased from Laiyu Chemical Co., Ltd., China; and n-dodecylamine (nDDA, 95%) and n-octadecylamine (n-ODA, 97%) were purchased from Aladdin Industrial Corporation, China. 2.2. Sample Preparation. According to the ratio of the formulation, the BMI resin was mixed well with n-DDA and nODA at 90 °C. Then, the mixtures including BMI-0, BMIDDA, and BMI-ODA were degassed in a vacuum oven at 75 °C for 30 min to remove the trapped air bubbles. Afterward, the degassed mixtures were poured into a copper mold precoated with a mold release agent and cured following the stepwise schedule: 150 °C for 2 h, 180 °C for 2 h, 200 °C for 2 h, and 250 °C for 4 h. On the other hand, viscosity at 90 °C of BMI-0, BMI-DDA, and BMI-ODA was tested on a TA Instruments AR2000 rotational rheometer at a frequency of 1 Hz. The detailed formulations for the preparation and viscosity at 90 °C of the samples are shown in Table 1.

the shear band for consuming impact energy. Besides, in the phase-separated structure, the resultant thermoplastic particles can further prevent crack propagation to achieve toughening. Especially, the thermoplastic resin with excellent thermal resistance and rigidness hardly caused a hostile effect on inherent predominant properties of BMI resins.16−20 Wei et al. incorporated polyphenylene oxide particles into a Cytec 5250 BMI resin and the KIC of composite system was increased by 64.7% without altering its Tg. However, the addition of the thermoplastic resin was relatively high (wt % = 15−25%) and the thermoplastic resin possessed a high viscosity, which was exceedingly detrimental on the processability of BMI resins. Thus, physical mixing was not a high-efficiency method to introduce second-phase toughening materials into BMI resins. In contrast, chemical modification to form the second phase can be a competent method. Nevertheless, there were scarcely any corresponding research works until now. In this article, a novel and unique self-assembled secondphase structure was created to in-situ toughen BMI resin by chemically inducing a flexible long-chain monohydric aliphatic amine (MAA) into the crosslinking network of the copolymerized modification of BMI and DABA. In the MAA-modified BMI resins, the aliphatic side chains from MAA had large different solubility parameter with the benzene ring and the strong polar succinimide group from the BMI resin. Thus, the flexible and regular aliphatic side chains easily generated a thermodynamically stable self-assembled phase structure, resulting in in-situ toughening of BMI resins (the schematic diagram is shown in Scheme 1). Interestingly, the size of the self-assembled phase structure can be adjusted by changing the aliphatic side chain. The morphology of MAAmodified BMI resins was observed via scanning electron microscopy (SEM) and atomic force microscopy (AFM), and the thermal stability of MAA-modified BMI resins was characterized by thermomechanical analysis (DMA) and thermogravimetric analysis (TGA). The fracture toughness test revealed that the KIC and GIC of MAA-modified BMI resins increased by 30.1 and 54.2% after adding 1% MAA, respectively. Compared to the neat BMI resin, the elongation at break of MAA-modified BMI resins increased by 32.3%. Besides, the flexural strength of MAA-modified BMI resins exhibited the enhancement by 30.1%. In addition, the amount of MAA was extremely low, and the processability and superior heat resistance of the MAA-modified BMI resins were hardly affected.

Table 1. Formulations (Mass Ratio) and Viscosity at 90 °C of BMI-0, BMI-DDA, and BMI-ODA sample

BMI/g

DDA/g

BMI-0 BMI-DDA BMI-ODA

100 100 100

1

ODA/g

viscosity at 90 °C/Pa·s

1

0.319 0.345 0.336

2.3. Self-Assembled Phase Structure Characterization. The morphology of the samples was measured by the Nanoscope IIIa AFM (Bruker, Germany) in the tapping mode under a frequency of 0.99 MHz. Besides, the morphology of the samples was investigated using an SEM (Hitachi S4800, Japan) with an accelerating voltage of 5 kV. 2.4. Fracture Surface Characterization. The cured samples of BMI-0, BMI-DDA, and BMI-ODA were cryofractured in liquid nitrogen, and the fracture surfaces were observed on an SEM (Quanta 600, America) running at an accelerating voltage of 15 kV. All surfaces were sputtered with a thin gold film before observation. 2.5. Thermomechanical Analyzer. The dynamic mechanical properties of BMI-0, BMI-DDA, and BMI-ODA were B

DOI: 10.1021/acs.iecr.9b01822 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research measured by DMA (TA Instruments Q800, USA) in the double-cantilever mode from 25 to 350 °C at a heating rate of 5 °C min−1 and under a frequency of 1 Hz. The dimensions of the rectangular samples were 60 mm × 10 mm × 2 mm. Tg‑onset was determined as the onset in the storage modulus (E′) decrease during the glass transition drop, and Tg‑tanδ defined by the tan δ peaks was obtained from the DMA spectrum. 2.6. Thermogravimetric Analysis. The thermal stability of BMI-0, BMI-DDA, and BMI-ODA was measured by TGA (NETZSCH STA 499F3, Germany) under a nitrogen atmosphere (50 mL min−1) from 25 to 800 °C at a heating rate of 10 °C min−1. 2.7. Mechanical Performance Test. The tensile test was carried out at room temperature according to ASTM D638-14 and the crosshead speed was set at 5 mm min−1; the bending test was carried out at room temperature according to ASTM D790 and the crosshead speed was set at 1.36 mm min−1; and fracture toughness (KIC, GIC) was carried out at room temperature according to ASTM D5045 and the crosshead speed was set at 10 mm min−1.

Figure 2. SEM images of three BMI systems. BMI-0 [(a), 20 000×], BMI-DDA [(b), 20 000×], and BMI-ODA [(c), 5000×, (c′) 10 000×].

3. RESULTS AND DISCUSSION 3.1. Self-Assembled Phase Structure Characterization. Figure 1 shows AFM phase images and 3D height images

self-assembly of the aliphatic side chains. The second-phase structure was produced from the aliphatic side chains of MAA because they had a large different solubility parameter with the benzene ring and a strong polar succinimide group from the BMI resin. In addition, the difference in length of the aliphatic side chains caused the variation in the size of the selfassembled phase structure. As the length of the aliphatic side chains increased, larger-sized self-assembled phase structures were generated. For BMI-DDA (see Figures 1b and 2b), a large number of nanosize self-assembled phase structures were displayed and the size ranged from 100 to 500 nm. The peak height of the self-assembled phase structure was not over 61.6 nm. A multilevel self-assembled phase structure was found in BMI-ODA (see Figures 1c and 2c), including nanosize structures (300−700 nm) and micron-size structures (1.0− 1.7 μm). The largest peak height of the self-assembled phase structure of BMI-ODA reached up to 123.7 nm because the length of the aliphatic side chains of BMI-ODA was longer than that in BMI-DDA. However, compared to the micron-size phase structure, the nanosize phase structure had relatively larger specific surface area and it could absorb more damage energy, and thus, BMI-DDA with a large number of nanosize phase structures had huge potential in toughening the BMI resin. 3.2. Fracture Surface Characterization. Figure 3 showed SEM micrographs of cryo-fractured surfaces of BMI-0, BMIDDA, and BMI-ODA cured samples. BMI-0 exhibited brittleness due to its high crosslink density and strong rigidity. Figure 2a,a′ shows that the fracture surface of BMI-0 was relatively smooth, which suggested a representative brittle fracture morphology. This result indicated that the toughness of BMI-0 was poor, and crack propagation was basically unimpeded during fracture. However, the micro−nano selfassembled phase structure formed in BMI-DDA and BMIODA can hinder crack propagation and toughen the matrix through changing and increasing the diffusion path of the crack during the fracture process to absorb more damage energy. As shown in Figure 3b,c, a large amount of river lines were seen on the fracture surface of BMI-DDA and BMI-ODA, which implied that BMI-DDA and BMI-ODA possessed a ductile fracture morphology. After zooming in the river lines of BMIDDA and BMI-ODA (see Figure 3b′,c′), a kind of “fish scale”

Figure 1. AFM images of three BMI systems. Top. BMI-0 [(a), phase; (a′), 3D height]; middle. BMI-DDA [(b), phase; (b′), 3D height]; bottom. BMI-ODA [(c), phase; (c′), 3D height].

of the self-assembled morphologies of BMI-0, BMI-DDA, and BMI-ODA cured samples, and Figure 2 shows their SEM images. As seen from Figures 1 and 2, the morphology of BMI0 was homogeneous and featureless, and its peak height did not exceed 7.5 nm, as expected for a monophase material. By contrast, it can be clearly observed that BMI-DDA and BMIODA had plenty of bump structures, namely, self-assembled phase structures mentioned above, which were ascribed to the C

DOI: 10.1021/acs.iecr.9b01822 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. SEM micromorphology of the fracture surfaces of three BMI resin systems. Left. BMI-0 (a, 150×; a′, 2000×); middle. BMI-DDA (b, 200×; b′, 2000×); and right. BMI-ODA (c, 200×, c′, 2000×).

obtained crosslinking densities and the dynamic mechanical property parameters are listed in Table 2. It can be observed that the E′−temperature curves of BMI-0, BMI-DDA, and BMI-ODA were similar and their corresponding values of crosslinking density were also close, resulting from the very small amount of monoamine added (1 wt %) to BMI-DDA and BMI-ODA. From the inset in Figure 4, Tg‑onset and Tg‑tanδ can be obtained, which were used as an index to evaluate the thermal resistance of the material. The Tg‑onset of BMI-0, BMI-DDA, and BMI-ODA exceeded 290 °C, and the Tg‑tanδ outstripped 300 °C, as listed in Table 2, indicating that all three systems had excellent thermal resistance. The thermogravimetry (TG) and derivative thermogravimetry (DTG) curves of BMI-0, BMI-DDA, and BMI-ODA in N2 atmosphere are shown in Figure 5. The TG curves of BMI-0, BMI-DDA, and BMI-ODA were substantially coincident until 380 °C, and the weight loss of the three systems was less than 5 wt % at this moment. Generally, the temperature at 5 wt % weight loss can be used as an index to evaluate the thermal stability of the material. When the weight loss of BMI-0, BMIDDA, and BMI-ODA was 5 wt %, the corresponding temperature of BMI-DDA and BMI-ODA was slightly lower than that of BMI-0, which can be because the aliphatic side chains began decomposing over 380 °C. However, the temperature at 5 wt % weight loss of BMI-DDA and BMIODA exceeded 400 °C, which indicated that modified BMI resins maintained excellent thermal stability. As shown in Table 2, the other thermal stability parameters of BMI-0, BMIDDA, and BMI-ODA were basically similar. Because of the low amount of monoamine added, BMI-DDA and BMI-ODA possessed thermal stability similar to that of BMI-0. 3.4. Mechanical Performance. Figure 6 shows the mechanical performance column of BMI-0, BMI-DDA, BMIODA, and the corresponding stress−strain curves are shown in Figure 7. Fracture toughness of BMI resin modified by MAAs exhibited a remarkable increase. Combined with the results of

pattern of BMI-DDA and linear pattern of BMI-ODA were observed. Compared to BMI-DDA, the crack propagation’s paths of BMI-DDA were bended and transferred because the nanosize phase structure had a relatively larger specific surface area, which could be more likely to give rise to crack deflection and absorb more damage energy. Based on these results of SEM and AFM, it can be predicted that BMI-DDA and BMIODA possessed excellent toughness and mechanical properties, especially BMI-DDA. 3.3. DMA and TGA. Figure 4 showed the variation of storage modulus of BMI-0, BMI-DDA, and BMI-ODA with an

Figure 4. DMA curves of BMI-0, BMI-DDA, and BMI-ODA.

elevating temperature. By using the rubber elasticity model,21 the effective crosslinking density of these three systems could be calculated using the following eq 1 υe =

Er 6RT

(1)

where υe is the crosslinking density; Er is the storage modulus at Tg + 40 °C that is the rubbery stage modulus; R is the gas constant; and T is the absolute temperature at Tg + 40 °C. The

Table 2. Dynamic Mechanical Performance Parameters and Thermal Stability Parameters for BMI-0, BMI-DDA, and BMIODA samples BMI-0 BMI-DDA BMI-ODA

Tg‑onset/°C Tg‑tan/°C υe/(mol cm−3) 5 wt % weight loss temperature/°C 800 °C carbon residue rate/% DTG curve peak temperature/°C 292.6 292.0 295.3

306.5 308.8 316.0

2.66 × 10−3 2.54 × 10−3 2.26 × 10−3

412.4 404.7 402.4

30.32 30.02 27.68 D

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Figure 5. TG and DTG curves of BMI-0, BMI-DDA, and BMI-ODA.

Figure 6. Mechanical performance column of BMI-0, BMI-DDA, and BMI-ODA.

relatively smaller contact surface with the matrix because a large-size self-assembled phase structure was in-suit created in BMI-ODA. In a word, BMI-DDA demonstrated the best comprehensive performance. The tensile strength and bending strength of BMI-DDA reached 112 and 190 MPa, respectively. Compared with BMI-0, the tensile strength and bending strength of BMIDDA increased by 14.1 and 30.1%, respectively. Moreover, the BMI-DDA displayed tensile elongation of 4.09% at break, which was higher than that of BMI-0 (3.03%). The increase of 32.3% can be achieved for tensile elongation of BMI-DDA. Additionally, the KIC and GIC of BMI-DDA were up to 1.89 MPa m1/2 and 792 J m−2, respectively. The improvement of KIC and GIC of BMI-DDA attained 24.3 and 54.2% compared to those for BMI-0, respectively.

Figure 7. Typical stress−strain curves of BMI-0, BMI-DDA, and BMI-ODA.

AFM and SEM, the micro−nano self-assembled phase structures generated in BMI-DDA and BMI-ODA gave rise to good toughening effect. Significantly, the self-assembled phase structures in situ grew and uniformly dispersed in the matrix (illustrated in Scheme 1), which achieved perfect toughening without damage to the matrix itself. Owing to similar crosslinking density and rigidness, tensile and flexural modulus of BMI-0, BMI-DDA, and BMI-ODA were basically the same. However, compared to BMI-0, tensile and flexural strength of BMI-DDA and BMI-ODA exhibited a significant enhancement because micro−nano self-assembled phase structures in BMI-DDA and BMI-ODA resins can improve their toughness. Moreover, the mechanical properties and fracture toughness of BMI-DDA were higher than those of BMI-ODA because the larger the contact surface area between the toughening phase and the matrix, more is the damage energy absorption. Although the amount of monoamine added in BMI-DDA and BMI-ODA was equal, the self-assembled phase structure of BMI-ODA had a relatively smaller specific surface area and a

4. CONCLUSIONS Two modified BMI resin networks (BMI-DDA and BMIODA) with different flexible aliphatic side chains were designed and prepared. The aliphatic side chains can in-suit form self-assembled phase structures with size ranging from nanometer to micrometer, which were able to effectively toughen and improve the mechanical performance of the BMI resin. Consequently, BMI-DDA and BMI-ODA maintained the excellent thermal resistance and thermal stability of the original BMI resin. Compared to BMI-0, the elongation at break of BMI-DDA increased by 32.3%, and the tensile strength and bending strength of BMI-DDA increased by 14.1 and 30.1%, respectively. Appreciably, in the premise of possessing good processability, BMI-DDA exhibited a remarkable increased KIC of 24.3% and GIC of 54.2%. In a word, the method of chemical modification to form the second phase can be promising for toughening BMI resins. E

DOI: 10.1021/acs.iecr.9b01822 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(17) Bonneau, M. R.; Boyd, J. D.; Emmerson, G. T.; Lucas, S. D.; Howard, S. J.; Jacobs, S. D. Particle-Toughened Fiber-Reinforced Polymer Composites. U.S. Patent 8,313,830B2, 2012. (18) Sun, S.; Guo, M.; Yi, X.; Zhang, Z. Preparation and characterization of a naphthalene-modified poly(aryl ether ketone) and its phase separation morphology with bismaleimide resin. Polym. Bull. 2017, 74, 1519. (19) Wang, Y.; Yuan, L.; Liang, G.; Gu, A. New Bismaleimide Resin Toughened by In Situ Ring-Opening Polymer of Cyclic Butylene Terephthalate Oligomer with Unique Organotin Initiator. Ind. Eng. Chem. Res. 2015, 54, 5948. (20) Wei, G.; Sue, H.-J. Fracture mechanisms in preformed polyphenylene oxide particle-modified bismaleimide resins. J. Appl. Polym. Sci. 1999, 74, 2539. (21) Hagen, R.; Salmén, L.; Stenberg, B. Effects of the type of crosslink on viscoelastic properties of natural rubber. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 1997.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 10 62497436. Fax: +86 10 62497436 (Y.F.L.). *E-mail: [email protected] (X.B.C.). ORCID

Qi Zou: 0000-0002-4581-656X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China under grant no. 51803201.

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REFERENCES

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DOI: 10.1021/acs.iecr.9b01822 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX