A study of co-reaction mechanism and properties of propenyl-substituted monomer/bismaleimide system

Abstract

A practical modifier, 4,4′-bis[2-(1-propenyl)phenoxy]benzophenone (MT), has been synthesized and copolymerized with 4,4′-bismaleimidodiphenylmethyene (4,4′-BMI) to yield a low-temperature cure bismaleimide resin. Structural information of MT was obtained using Fourier transform infrared spectroscopy and nuclear magnetic resonance (proton and carbon) spectroscopy, elemental, and mass spectrometry analyses. The differential scanning calorimetric curve of the MT/4,4′-BMI system shows two exothermic peaks at 164°C and 210°C, respectively. This value was compared with other polymerization reactions involving diallyl bisphenol A (DP) and diallyl bismaleimide (DBMI), with an exothermic peak at around 257°C. Unlike normal bismaleimide resins with DP postcured at 250°C for about 4–8 h, the MT/4,4′-BMI resin was cured at 230°C for a total of 6 h (i.e. MBMI-230) or at 200°C for 6 h (i.e. MBMI-200). Furthermore, the resins exhibit high thermal resistance, excellent mechanical properties, and exceptionally low dielectric loss. For MBMI-230 and MBMI-200, the dielectric loss values are all in the range of 0.0024–0.0054 from 7 GHz to 18 GHz and feature excellent thermal stabilities (5% weight loss temperature (T _d5) > 415°C under nitrogen atmosphere) and thermo-oxidative stabilities (T _d5 > 425°C in air atmosphere). Additionally, the cured resins also display a high bending modulus (>3.2 GPa) at room temperature and an excellent mechanical stability at high temperature.

Keywords

Bismaleimide lower temperature cure thermal properties dielectric properties

Introduction

One critical use of bismaleimides (BMIs), a group of thermosetting polyimides, involves the replacement of epoxy resins, circumventing the poor hot/wet performance, exhibited by this traditional industry standard in a variety of high-performance applications.^1,2 BMIs feature a high glass transition temperature (T _g), excellent mechanical properties, humidity resistance, and corrosion resistance. Among other applications, BMIs have been widely used in various electronics and advanced technological products.^2,3 The latter applications include radar and microwave structural composites, aerospace, space-ware composites, circuit boards, interconnections, and adhesives for microelectronics.^4
–6

In addition to other highly desirable properties of the cured resins, BMI also feature a low shrinkage without the formation of volatile by-products. The materials can be fabricated using epoxy-like conditions, including resin transfer molding (RTM), vacuum-assisted RTM, and composite prepregs.^7,8 The largest difference in the process characteristics between epoxy resins and BMI resins is the relatively high curing temperature for the latter. Unfortunately, the process of high-temperature curing may result in an exceeding high consumption of energy, and more importantly, the cured resins tend to exhibit higher stress concentrations and internal defects. As a consequence, poor stability features of such materials in service performance can be observed.^9,10 Furthermore, high temperature may be vastly impractical and, in some cases, even unsafe for certain temperature-sensitive components.¹¹ Examples for such parts include, microelectromechanical system buildups or complementary metal-oxide semiconductor memory, which could degenerate or could be destroyed at temperatures exceeding 200°C.¹²

As mentioned above, a high-temperature cure will cause issues to particular application types and production processes, ultimately resulting in poor performance of the cured resins. The advanced development of high-performance BMI resins with a lower cure temperature has been a critical, albeit difficult goal to achieve both scientifically and from an engineering perspective. One challenge in the development of such materials is that the mechanical properties and T _g of resins cured at low temperatures will always be reduced. Because of a lower cross-linking density of resins cured at a lower temperature, the mobility of the chains increases accordingly. For example, the tensile strength of a benzocyclobutene layer, with a complete cure at 250°C, was found to be approximately 110 MPa. However, a low-temperature cure at 210°C leads to a considerably lower tensile strength of approximately 70 MPa.¹²

Much effort was made in the past decade. Many research studies indicated that propenyl-substituted monomer/BMI systems showed lower cure temperatures than allyl-substituted monomer/BMI system. Aijuan Gu⁹ has reported the successful development of a novel performance matrix consisting of BMI and N-propenyl diaminodiphenylether, featuring low cure and postcure temperatures (200°C). John M Barton et al.¹³ reported that the cure peak of propenyl-functionalized cyanate ester (CE) appeared at 180°C, while that of the allyl-funtionalized CE appeared at 260°C, respectively. A propenyl-functionalized Diels–Alder commoner for BMI, commercially available under the trade name Compimide TM123, which has been reported^14,15 draws the attention of the researchers. However, the open systematic report is barely mentioned about TM123/BMI system, especially of fast-cure mechanism and the dielectric properties.

In this study, we report a practical thermosetting resin with a low-temperature cure consisting of bismaleimide subunits and a propenyl-endcapped compound. The propenyl-endcapped monomer was synthesized and characterized by Fourier transform infrared (FTIR), proton (¹H) and carbon (¹³C) nuclear magnetic resonance (NMR) spectroscopies, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and elemental analysis. The effects of a low-temperature cure on the thermal, mechanical, and dielectric properties of the BMI resins, MBMI-200 and MBMI-230, have been experimentally investigated and compared the conventional diallyl bisphenol A (DP)/diallyl bismaleimide (DBMI) system.^16,17

Experimental section

Materials

2-Allylphenol was purchased from TCI Co. Ltd (Shanghai, China). 4,4′-Difluorobenzophenone was obtained from Sun Chemical Technology Co. Ltd (Shanghai, China). N,N-4,4′-Bismaleimidodiphenylmethyene (4,4′-BMI) was purchased from Honghu Shuangma Advanced Materials Tech Co. Ltd (Hubei, China). DP was purchased from Laizhou Laiyu Chemical Co. Ltd (Shandong, China). All organic compounds from commercial sources were used without further purification. Potassium carbonate and the solvents o-xylene and N,N-dimethylacetamide (DMAc) were purchased from different vendors (analytical reagent grade) and were also used without further purification.

Monomer synthesis

Synthesis of MT

The 4,4′-bis[2-(1-propenyl)phenoxy]benzophenone (MT) monomer was synthesized via a standard nucleophilic substitution reaction of 2-allylphenol with 4,4′-difluorobenzophenone. In a typical experiment, MT was prepared according to the following procedure: a mixture of 2-allylphenol (29.48 g, 0.22 mol), 4,4′-difluorobenzophenone (21.89 g, 0.1 mol), potassium carbonate (34.50 g, 0.25 mol), o-xylene (50 ml), and DMAc (200 ml) was placed in a 500 ml three-necked flask equipped with a mechanical stirrer, a thermometer, a nitrogen (N₂) gas inlet, and a condenser. The mixture was stirred at room temperature for 10 min until the aromatic phenol and fluorobenzophenone completely dissolved to give a homogeneous solution. The reaction mixture was slowly heated to 160°C and maintained at that temperature for 3 h while stirring. Evolving water was removed simultaneously by azeotropic distillation. After the nucleophilic substitution reaction was completed, the reaction solution was cooled to room temperature and then poured into an excess of water with vigorous stirring in an effort to precipitate the MT monomer. The precipitate was filtered off, thoroughly washed with water, and dried at 100°C for 2 h to remove the majority of the water. Complete dryness was achieved by heating the solid at 160°C under vacuum for 4 h. The product was obtained as a rufous ropy solid (35.7 g/80%); melting point (m.p.) = 19–21°C (by differential scanning calorimetry (DSC)).

¹H NMR (400 MHz, deuterated chloroform (CDCl₃), δ): 6.9–7.8 (m, Ar–H, 16H), 6.52–6.56 (d, J = 16 Hz, Ar–CH=CH–CH₃, 2H), 6.21–6.26 (m, J = 20 Hz, Ar–CH=CH–CH₃, 2H), 1.78–1.80 (d, J = 6.8 Hz, CH₃, 6H). ¹³C NMR (400 MHz, CDCl₃, δ): 193.87 (C¹), 161.64 (C⁵), 151.30 (C⁶), 132.07 (C³), 131.68 (C²), 130.56 (C¹²), 127.96 (C⁸), 127.91 (C¹⁰), 126.66 (C¹¹), 125.09 (C¹³), 124.33 (C⁹), 121.11 (C⁷), 116.04 (C⁴), and 18.69 (C¹⁴). FTIR (potassium bromide (KBr), powder): 3070 cm⁻¹ and 3040 cm⁻¹ (=CH), 1595 cm⁻¹ (C=C), 2965 cm⁻¹ and 2879 cm⁻¹ (CH₃), 1645 cm⁻¹ (C=O), 1230 cm⁻¹ (Ar–O–Ar), 1500–1454 cm⁻¹ (aromatic ring C=C), 875–750 cm⁻¹ (Ar–H). MALDI-time-of-flight (TOF)-MS: m/z: 447 [M+H]⁺. Elemental analysis calculated for C₃₁H₂₆O₃: C, 83.38; H, 5.87. Found: C, 83.29; H, 5.97.

Curing procedure

BMI blended with MT and DP with a molar ratio of 0.75:1 was placed in beakers, and the designations, MBMI and DBMI, were assigned. The prepolymers were obtained through a melting process at 130°C with subsequent deaeration at 125°C for 30 min using a vacuum oven. The material was then poured into a preheated mold releasing the agent. The curing procedure included two different routes: The first route involved 130°C/45 min, 180°C/2 h, 200°C/2 h, and 230°C/6 h. The second route involved 130°C/45 min, 160°C/2 h, 180°C/2 h, and 200°C/6 h. The cured resins were assigned as MBMI-230, MBMI-200, DBMI-230, respectively. After completion of the curing procedure, the sample was allowed to slowly cool down to room temperature to prevent cracking. The prepared sample was then subjected to characterization and property evaluation.

Characterization techniques

FTIR spectra were recorded on a Nicolet 6700 spectrometer (Madison, Wisconsin, USA) with KBr pellets, and spectra averages were obtained from at least 128 scans at a standard wave number range of 600–4000 cm⁻¹. ¹H NMR and ¹³C NMR spectra were obtained on a Bruker Avance III 400 spectrometer (Billerica, Massachusetts, USA) at frequencies of 400 MHz using CDCl₃ as the solvent. Mass spectra analyses were obtained on a Bruker Microflex MALDI-TOF spectrometer. Elemental analysis was performed on a Vario MICRO (Elementar, German) cube elemental analyzer. Rheological measurements have been carried out on a Gemini 200 strain-controlled rotational rheometer using the test specimen disks with a diameter of 25 mm and thickness of 1.0 mm. The top plate was oscillated at 1.0 Hz and 10 Pa stress at a heating rate of 4°C min⁻¹, whereas the lower plate was attached to a transducer used to record the resultant torque. DSC analyses were performed on a DSC 6220 with sample masses of approximately 5–10 mg under nitrogen atmosphere at a heating rate of 10°C min⁻¹. Thermogravimetric analyses (TGAs) were performed on a Perkin–Elmer TG-DTA 6300 (Waltham, Massachusetts, USA) at a heating rate of 10°C min⁻¹ under nitrogen atmosphere and in air atmosphere at temperatures ranging from 100°C to 800°C. Dynamic mechanical analysis (DMA) experiments were carried out on a DMS 6100 (SEIKO, Japan) calorimeter. The storage modulus E′ and tan δ were measured at a heating rate of 5°C min⁻¹ from ambient to 350°C at a frequency of 1 Hz and an amplitude of 10 µm in a three-point bending mode. Dielectric constant and loss were performed with an Agilent Technologies N5230A measurement system (Santa Clara, California, USA) at 25°C by a two-parallel–plate mode over a wide frequency from 7 GHz to 18 GHz. The dimensions of each sample were Ø = 50 mm and thickness <2 mm, and all samples were dried under vacuum at 105°C for 1 h before tests. Three-point bend test of the cured resins were performed on Instron 4467 universal testing machine (Norwood, Massachusetts, USA) per ASTM D 790-10 standard.¹⁸ The specimens ((15 ± 0.2) mm width and (4.0 ± 0.2) mm thickness) were tested at a loading speed of 2.0 mm min⁻¹, with a span 60 mm.

Results and discussion

Synthesis and characterization of monomer

The monomeric unit, containing propenyl and benzophenone groups, was prepared by reaction of 2-allylphenol and 4,4′-difluorobenzophenone as shown in Figure 1. The mechanism of the reaction involves the base-supported nucleophilic attack of an oxygen anion to replace a fluorine atom as part of a standard, nucleophilic substitution reaction. The next step involves the isomerization of the 2-propenyl (allyl) group using potassium carbonate to form a pair of geometric isomers of the propenyl-substituted phenoxy compound, see discussion below. The solubility of MT was evaluated in various solvents, and the results are listed in Table 1. The monomer shows good solubility in common strong polar and nonpolar organic solvents, for example, DMAc, N-methyl pyrrolidone, toluene, chloroform (CHCl₃), and so on.

Table 1.

Solubility of MT in various solvents.

Toluene	Cyclohexane	CHCl₃	DMAc	DMSO	NMP	H₂O	Ethanol	Dioxane
++	++	++	++	+	++	−	+	++

MT: 4,4′-bis[2-(1-propenyl)phenoxy]benzophenone; ++: easily soluble; +: slightly soluble; –: Insoluble; DMAc: N,N-dimethylacetamide; DMSO: dimethyl sulfoxide; NMP: N-methyl-2-pyrrolidone; H₂O: water.

Figure 2 shows the FTIR spectrum of the monomer. The broad band at 3070 cm⁻¹ to 3040 cm⁻¹ corresponds to the stretching vibration of the =C–H bond. The peaks in the range of 2965–2879 cm⁻¹ corresponds to the CH₃ stretching vibrations, whereas the CH₃ bending deformation vibrations for the propenyl group appeared at 1376 cm⁻¹. The peaks at 1595 cm⁻¹ can be assigned to the stretching vibrations of C=C–CH₃. This provides strong evidence for a successful isomerization reaction of the allyl group to form a propenyl group under alkaline conditions. The strong bands at about 1500–1454 cm⁻¹ and 875–750 cm⁻¹ represent the stretching vibrations of the benzene ring C=C and the out-of-plane bending vibration of Ar–H, respectively. The stretching vibration of the C=O groups is detected from the band at 1645 cm⁻¹, exhibiting a significant shift toward a lower wave number, which may be caused by the π–π conjugative effect. The C–O stretching of Ar–O–Ar groups is detected from the band at 1230 cm⁻¹. This is an indication for the aromatic ether group being produced by the nucleophilic substitution between 2-allylphenol and 4,4′-difluorobenzophenone at an elevated temperature.

The representative NMR spectra of the monomer are shown in Figures 3 and 4 and provide further evidence for the structures proposed in Figure 1. ¹H NMR spectroscopy (Figure 3) of MT clearly shows signals for CH₃ groups appearing at around 1.8 ppm, whereas =CH groups attached to methyl groups (=CH–CH₃ ) and phenyl groups (Ar–CH=) resonated at around 6.3 ppm and 6.6 ppm, respectively. The disappearance of the signals (at δ = 3.4 ppm) assigned to the methylene of the allyl group confirms that a successful isomerization reaction has been achieved. The doublet of the methyl group (in the propenyl group) is selected as this resonates at δ 1.78 ppm (cis) and δ 1.80 ppm (trans) respectively. The ¹H NMR results are in good agreement with the infrared spectrum of the MT compound, providing further evidence for a successful isomerization reaction of the allyl groups to form propenyl groups. The characteristic peaks of aromatic ring at 6.9–7.8 ppm are observed. The ratios between the areas for the aromatic protons and the areas for the aliphatic protons are close to the theoretical values.

The ¹³C NMR spectrum of the monomer in Figure 4 shows three types of carbon signals. It shows the appearance of methyl carbons at around 18 ppm. This further evidences the occurrence of isomerization reaction of the allyl groups to form propenyl groups. The peaks at around 110–162 ppm correspond to the double bond carbon in the aromatic ring and propenyl carbon signals of MT. Carbonyl carbons of MT can be detected from the peaks observed in the range of 193–194 ppm. The obtained spectrum is in good agreement with the proposed molecular structures, and the results obtained through elemental analysis correspond to the calculated values.

To characterize the structures of the monomer, the compound is analyzed by the corresponding mass observed from a high-resolution TOF mass spectrum using electron impact ionization (cf. Figure 5). The expected quasimolecular ion [M+H]⁺ (m/z = 447) appears with high intensity in the MALDI mass spectrum and the [M+H]⁺ ion corresponds to the target molecule, accompanied only by minor signals. The peaks at 485 (m/z) and 237 (m/z) correspond to [M+K]⁺ and a fragment of MT, respectively.

Curing analysis of MT/BMI system by DSC

The cure behavior of the MT/BMI system was investigated by DSC (cf. Figure 6) and compared with that of DP/BMI and the pure BMI system. The MT/BMI system shows a main peak at low temperature of approximately 164°C and another main exothermic peak at around 210°C. The latter can be ascribed to Diels–Alder reaction and homopolymerization of BMI, respectively. The exothermic peak of the pure BMI also appears at approximately 210°C. H Stenzenberger¹⁴ reported that the reaction mechanism of propenyl-substituted monomer/BMI system was different from the allyl-substituted monomer/BMI. It is illustrated that the MT/BMI system occurred due to the Diels–Alder reaction and then went through ene reaction to form a more thermal stable structure. This two-step reaction mechanism is illustrated in Figure 7. The remarkably lower cure temperature can be compared with that of the DP/BMI system (peak temperature (T _p) = 257°C). Generally accepted in the scientific community is the notion that the rate of Diels–Alder reactions is mainly affected by steric interactions as well as electronic properties. However, in the MT/BMI system, the Diels–Alder reaction between the double bonds of BMI and the conjugated double bonds of the propenyl groups with high electron density from methyl (−CH₃) can proceed directly and with low steric hindrance. Therefore, the cure reaction of MT/BMI system occurred at a very low temperature (T _p = 164°C). Since the cure reaction mechanism of the DP/BMI system is found to be considerably different, the main peak of Diels–Alder reaction for DP/BMI system appeared at 257°C. The difference of the two systems is illustrated in Figure 7. From inspection of the cure reaction mechanism, it becomes obvious that the MBMI resins feature a stronger link, a better symmetry, and almost no polar groups. Presumably, this will have a significant effect on the thermal, dielectric, and mechanical properties compared to the DBMI resins, as discussed below.

Curing analysis of the MT/BMI system by IR spectroscopy

The chemical changes of MT/BMI are investigated by FTIR changes throughout the cure process as shown in Figure 8. The absorption peak in the range of 3030–3068 cm⁻¹ and 823 cm⁻¹ can be attributed to the stretching vibration of the =C–H bond in the propenyl group and the C=C maleimide band, respectively. The absorption of propenyl group decreased as the curing temperature was increased to 160°C and disappeared after the cure reached a temperature of 180°C. The decrease in the relative absorbance of the =C–H bond can be mainly attributed to the Diels–Alder reaction with the double bond in BMI. This finding indicates that the reaction can proceed completely at 180°C, which is in good agreement with the DSC curve of the MT/BMI system. The absorption of C=C maleimide band disappeared until reaching a temperature of 230°C. Given the fact that the MT/BMI system contains an excess of BMI and, combined with the DSC curves of pure BMI, it can be determined that the homopolymerization of BMI reacts independently from 180°C to 230°C after copolymerization of the MT/BMI system is completed. This finding provides further data supporting the proposed reaction mechanism of the MT/BMI system.

Process properties of MT/BMI

In order to further fully understand the processing properties of the MT/BMI system, measurements of the melt viscosity during the cure procedure at varying temperatures were conducted as shown in Figure 9. The isothermal rheological experiment indicates that the resin retains a low viscosity level (below 1 Pa·s) above 120°C and can be injected efficiently at classical RTM processing temperatures. According to viscosity data, the processing time under 1 Pa·s is found to be approximately 20, 30, and 60 min for isothermal cures at temperatures of 140, 130, and 120°C, respectively. The key processing properties of a prepolymer for a given RTM technique are viscosity at an injection temperature, pot life, and reactivity. Generally, the minimum pot life for an RTM resin is 30–45 min under 1 Pa·s.¹⁹ Therefore, the MBMI system does meet the requirements of an RTM process under 120°C. Furthermore, it can be determined that the MBMI system features a high reactivity above 130°C. The latter finding may be an important feature for the MBMI system to find use in low-temperature cure.

DMA of MBMI

Figure 10 shows the E′ and the tan δ data of the cross-linked BMI resins (MBMI-230, MBMI-200, and DBMI-230, respectively). At room temperature, the value of E′ of the cured MBMI resins is approximately 5.3 GPa, indicating high rigidity. The MBMI-230 and MBMI-200 resins can keep their modulus stability below 255°C and 217°C, respectively. However, with an increase of the cure temperature, the T _g (measured by DMA) of the MBMI resins shifted to higher temperature in the following order: MBMI-200 (232°C) and MBMI-230 (268°C and 293°C). Presumably, this latter finding is due to the change in reaction characteristics from reaction-controlled to diffusion-controlled processes beyond the gelation point. If cured at a low temperature, the degree of cure increased slowly, leading to a lower T _g. Furthermore, compared with DBMI (T _g = 254°C measured by DMA), the MBMI-230 resin features two T _gs up to 293°C. According to the reaction mechanism (cf. Figure 7), two types of network structures in the cured MBMI resin can be observed. The two T _gs of the MBMI-230 resin found are 254°C (Diels–Alder reaction) and 293°C (homopolymerization of BMI). From the DSC curve of the pure BMI resin, it can be determined that the homopolymerization of BMI cannot result in the formation of the low-temperature cure MT/BMI system. Therefore, the cured MBMI-200 resin shows only one T _g. For the DP/BMI system, the Diels–Alder reaction and the homopolymerization of BMI take place synchronously (cf. Figure 6). The latter finding may be due to the homogeneous structure with one T _g. Via DMA, it has been shown before that the T _g of the classical XU292 (DBMI), cured at 200°C for 10 h, is 218°C. Taken in concert, the MBMI-200 resin shows a relatively high storage modulus and T _g with low temperature processability.

Thermal properties of MBMI

The thermal stabilities and thermo-oxidative stabilities of the MBMI resins have been investigated by TGA and compared with that of the DBMI resins (cf. Figure 11). The onset temperatures, corresponding to 5% weight loss (T _d5), were found to be above 415°C under nitrogen atmosphere and above 425°C in air atmosphere for both the MBMI-230 and the MBMI-200 resins. Similarly to the degradation patterns of other BMI resins, the thermal degradation of the MBMI-230 and MBMI-200 resins experiences a one-stage weight loss pattern under nitrogen atmosphere and a two-stage pattern in air atmosphere. We guess that the thermal degradation of the MBMI resin is divided into two steps in air atmosphere. At first step, MBMI resin degraded into carbon at low temperature and then the produced carbon is oxidized to carbon dioxide at high temperature. However, for the MBMI resin under nitrogen atmosphere, thermal degradation is simply just with the first step. With this data in hand, it becomes obvious that the decomposition rate of MBMI is significantly slower than that of DBMI. Furthermore, the char yields (Y _c) of MBMI-230, MBMI-200, and DBMI-230 at 800°C obtained were 39%, 37%, and 27%, respectively. This demonstrates that the MBMI resins exhibit a better thermal stability and thermo-oxidative stability than the DBMI resin.

In order to further evaluate the thermal properties of the MBMI resins. The thermo-oxidative aging properties of the MBMI-230 and MBMI-200 resins were measured and compared with the properties of the DBMI-230 resin. Figure 12 shows the weight loss ratios of the MBMI-230, MBMI-200, and DBMI-230 resins at different time points during the aging process at 230°C. The data display an average of the values obtained from five test sequences. From inspection of the data, it becomes clear that the weight loss of the BMI-200 resin is lower than that of the MBMI-230 resin at the same aging time point. This finding may be caused by a microcracking zone. It has been reported previously that the weight losses, particularly from the cut edges, are affected by the additional surface created by the formation and the growth of microcracking sections in the resin body.²⁰ Furthermore, the presence of residual stress left from the curing process increases the affinity of the material to form microcracking.²¹ Since the MBMI-200 resin cured at a lower temperature with a lower residual stress, compared to the MBMI-230 resin, we forecast that a reduced amount of microcracking would occur. It can therefore be concluded that the MBMI-200 resin shows a better thermo-oxidative stability. Furthermore, beyond 300 h for all the three systems, a relatively rapid increase in the weight loss rate can be observed. Interestingly, the weight loss ratio reached a low value for both MBMI and DBMI resins. It is believed that the networks of the BMI resins are being oxidized, leading to a weight gain at this point. The weight loss ratio of the DBMI resins, which is found to be lower than the ratio of the MBMI resins at the very start, dramatically increased beyond the weight loss ratio of the MBMI resins for an extended period of aging time. This finding can be explained by the characteristics of the molecular structures (cf. Figure 1) and the copolymerization mechanisms (cf. Figure 7): the two groups in the DBMI structure, that is, –CH₂– and –C(CH₃)₂–, do not exist in the MBMI resins, therefore reducing the thermal stability of DBMI. It can therefore be predicted that the weight loss ratio of the MBMI resin does decrease slower than that of the DBMI resin with a longer aging time. Based on the above analysis, for the MBMI-230 and MBMI-200 systems, low-temperature curing does not impede the thermal properties.

Bending properties of MBMI

Figure 13 shows the bending properties of MBMI resins at different test temperatures. The bending strength of the MBMI resins increased with elevated curing temperatures, yet the bending modulus slightly decreased. The two types of BMI resins, cured at different temperature, both feature excellent mechanical properties. Although the bending strength and modulus of the MBMI-230 and MBMI-200 resins weaken at 177°C, the bending strength and strength retention still maintain at 73 MPa (64%) and 67 MPa (77%), respectively. A similar phenomenon can be observed for the DBMI resins. The bending strength and modulus of the classical XU292 BMI resin (DBMI), cured at 250°C for 6 h, are 166 MPa and 4.0 GPa. This finding can be attributed to the high-temperature cure and hydrogen bond interaction in the network structure from DP. Even though the MBMI-200 resin cured at 200°C, the cured system also exhibits a good balance between the mechanical properties (i.e. 85.6 MPa and 3.69 GPa, respectively) and the thermal properties, similar to the MBMI-230 resin. This finding provides further support for the MBMI systems to be potentially useful in applications involving a low-temperature cure.

Dielectric properties of MBMI

The dielectric constant and loss of the cured MBMI resins and the DBMI resins as a function of the frequency at room temperature are shown in Figure 14. The two BMI resins exhibit high dielectric constant stabilities and loss in a wide frequency range from 7 GHz to 18 GHz. Surprisingly, the MBMI resins at different curing temperatures all show the same low dielectric loss (below 0.004) compared to the DBMI resin (above 0.012). Most notably, the dielectric loss depends on the polarity of the polymer molecules and the polar group density. The greater the polarity and the polar group density, the higher the dielectric loss.²² Compared to traditional DP, the chemical structure of MT is highly symmetrical and features almost no polar groups (cf. Figure 1). These features may contribute to the reduction of the dielectric loss for the cured BMI resins. On the other hand, the high density of the rigid benzene ring on MT improves the rigidity and stability of the cured MBMI resins and further reduces the dielectric loss. Furthermore, the curing temperature can also influence the dielectric properties. With the same chemical structure for a thermosetting resin, the dielectric properties also depend on the cross-linking density. The dielectric properties of the polymers generally depend on the orientation and relaxation of dipoles, accompanied by the movement of polymer chain segments.²³ Compared to the MBMI-200 resin, the MBMI-230 resin cured at 230°C features a larger cross-linking density. The latter feature provides a large restriction influence on the orientation and relaxation of dipoles in the applied electric field, thus resulting in a decreased dielectric loss. However, the dielectric constant is reduced by an increase in the free volume of the molecules or by a decrease in polarization.^24
–26 Therefore, the MBMI-230 resin exhibits a higher dielectric constant, owing to the higher cross-linking density resulting in a lower free volume.

Superior dielectric properties, particularly a low dielectric loss, represent the most critical features of resins in applications involving microeletronics.²⁷ Currently, CE, with a low dielectric loss between 0.002 and 0.008, is the most attractive thermosetting resin in the field of microelectronics.²² MBMI resins exhibit additional advantages over CE with respect to combinatory properties and cost efficiency while maintaining a low dielectric loss in the range of 0.0024–0.0045. Taken in concert, the MBMI resins are believed to represent a potential replacement for CE in order to meet the strict requirements set by, for example, the microelectronics sectors or aerospace industry for applications in high-frequency and high-speed printed circuit boards or radome enclosures.

Conclusions

A practical modifier for bismaleimide featuring a low-temperature cure has been developed. The curing peak of the MT/BMI system is found to be approximately 50°C lower than that of the normal DP/BMI system. The cure mechanism between allyl and propenyl has been verified by DSC and FTIR analyses. The cured MBMI-230 and MBMI-200 resins, without the need of postcuring procedures, exhibit excellent thermal, mechanical, and dielectric properties. These properties, combined with the superior processability (low melting point, low viscosity, etc.) and cost-effectiveness, render the MBMI resins attractive materials to find use as high-performance matrices for advanced composites and adhesives, particularly in future applications involving microelectronics.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Youth Science Foundation of Heilongjiang Province, China (grant no. QC2014C008) and Science Foundation of Heilongjiang Academy of Sciences (no. 2014-YQ-01, no. 2015-YQ-08, and no. 2015-YQ-01).

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