New Bismaleimide Resin Toughened by In Situ Ring-Opening

Article pubs.acs.org/IECR

New Bismaleimide Resin Toughened by In Situ Ring-Opening Polymer of Cyclic Butylene Terephthalate Oligomer with Unique Organotin Initiator Yuanzhen Wang, Li Yuan, Guozheng Liang,* and Aijuan Gu* Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Materials Science & Engineering, College of Chemistry, Chemical Engineering, and Materials Science, Soochow University, Suzhou 215123, P. R. China S Supporting Information *

ABSTRACT: A new organotin initiator with amino-terminated hyperbranched polysiloxane (HSiSn) for the ring-opening polymerization of cyclic butylene terephthalate (CBT) oligomer was synthesized. Compared with a traditional initiator, butyltin chloride dihydroxide (BCD), HSiSn has a moderate initiate speed, lower toxicity, and good reactivity. Poly(butylene terephthalate) (PBT) is the ring-opening polymer of CBT, of which the crystalline and molecular weight are almost independent of the initiator used, but the PBT initiated by HSiSn has better thermal stability than that by BCD. On this basis, a series of new toughened bismaleimide (BD) resins (HSiSn/CBT-BD) were prepared through the in situ formation of PBT during the prepolymerization of BD resin with HSiSn. Compared with BCD/CBT-BD and BD resins, HSiSn/CBT-BD resins with small loadings of CBT (≤5 wt %) have remarkably improved integrated performances, including higher impact strengths, improved flexural, and tensile properties, better dielectric properties, and excellent heat resistance. These attractive performances are attributed to the unique cross-linked structure induced by HSiSn.

1. INTRODUCTION Since the first synthetic thermosetting (TS) resin, phenolic resin, was born,1 TS resins have been the key and basic raw material for lots of industries due to their attractive merits such as good processing characteristics, rich varieties, high mechanical strengths, outstanding corrosion resistance, good adhesive properties, and so on.2,3 However, the nature of the cross-linked networks brings significant brittleness for TS resins, and this drawback is more significant with regard to thermally resistant resins such as bismaleimide (BMI),4 cyanate ester (CE),5 etc. In fact, toughening has been almost the most important issue with the development of TS resins, while this issue meets more challenge because many cutting-edge industries urgently require high performance TS resins that not only have high toughness but also should have outstanding thermal resistance, high strength, and excellent dielectric properties.6 There are a lot of methods for toughening TS resins.7−12 Among them, blending or copolymerizing with engineering thermoplastic polymer (ETP) has big potential to improve the toughness while maintaining thermal resistance.13 However, ETP generally has a high melting point poor solubility and lacks reactive groups, so ETP is difficult to homogeneously blend with TS resins, and the structure of the resultant ETP/TS system is not easy to controlled. To overcome these shortcomings, many efforts were conducted. One example is introducing reactive groups onto the molecule of ETP,14,15 and thus making ETP have good dispersion in the TS resin; however, the synthesis of thermoplastics with reactive functional groups is generally complex. Another effort was conducted by synthesizing ETP with low molecular weight. Typically, Zhang and associates put polyaryletherketone with low molecular weight and phenolph© XXXX American Chemical Society

thalein groups (PEK-C) into BMI and found that the PEK-C/ BMI system has a good processing feature. However, with the addition of 20 wt % PEK-C, the glass transition temperature (Tg) decreased from 310 to 238 °C.16 A similar result was also reported in hyperbranched polyaryletherketone modified BMI resin.17 Kinloch’s group reported a better result on modified CE resin by adding 25 wt % low molecular weight polyether sulfone (PES), of which Tg did not remarkably decrease (from 310 to 300 °C) with 220% increased toughness.18 The above research has proven the toughening effect of the ETP with low molecular weight but also suggested that maintaining the outstanding thermal resistance and good processing characteristics of original TS resins (especially the thermally resistant TS resins) is the bottleneck of toughening; in other words, how to solve this problem is still an interesting but challenge project. Cyclic oligomer is one kind of oligomers that can ringopening polymerize, and shows great potential in molecular recognition and preparation of functional materials.19,20 Cyclic butylene terephthalate oligomer (CBT) is such an oligomer that has low melting point (130 to 180 °C) and low melting viscosity (18 MPa·s at 200 °C, only 1/5000 of that of poly(butylene terephthalate) (PBT)); moreover, with suitable initiator, CBT will rapidly polymerize to produce PBT without releasing small molecular materials under 190 °C. Up-to-date, CBT was mainly used to improve the flowing ability of thermoplastics21 but has not been used to modify TS resins. Received: January 8, 2015 Revised: May 2, 2015 Accepted: May 12, 2015

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DOI: 10.1021/acs.iecr.5b00103 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

2.3. Ring-Opening Polymerization of CBT. Under the protection of N2 atmosphere, CBT (10 g) was mixed with BCD (0.04 g) or HSiSn (0.08 g) under stirring at 140 °C for 10 min to obtain a mixture, which was then heated to 200 °C and maintained at that temperature for 60 min. The resultant ringopening polymer of CBT was coded as PBT(BCD) or PBT(HSiSn). 2.4. Preparation of Prepolymers. Appropriate amounts of BDM and DBA with a weight ratio of 1:0.73 were thoroughly blended at 140 °C for 30 min to obtain a transparent liquid, which was BD prepolymer. DBA (42.2 g) was blended with CBT and BCD or HSiSn and then heated to 140 °C for 10 min, and BDM (57.8 g) was added to the mixture maintained at that temperature with stirring for 30 min to get a prepolymer, denoted as mBCD/ CBT-BD or mHSiSn/CBT-BD, where m represents the weight loading of CBT in the mBCD/CBT-BD or mHSiSn/CBT-BD system, taking values of 3, 5, 7, 10, and 15. The detailed formulations are listed in Table S1 in the Supporting Information. 2.5. Preparation of Cured Resins. Each prepolymer was put into a preheated mold and thoroughly degassed to remove entrapped air at 140 °C in a vacuum oven, and then the mold was put into an oven for curing and postcuring following the protocol of 150 °C/2 h + 180 °C/2 h + 200 °C/2 h + 220 °C/ 2 h + 240 °C/4 h, successively, to obtain a cured resin. 2.6. Characterizations. Fourier transform infrared (FTIR) spectra were detected on a Nicolet-5700 (U.S.A.) in the region of 4000−400 cm−1 using KBr pellets. 1 H NMR and 13C NMR spectra were recorded using a UNITY INOVA-400 (400-MHz NMR spectrometer, U.S.A.), while the 29Si NMR spectrum was obtained with a Bruker Avance 400 (400-MHz NMR spectrometer, Germany). Tetramethylsilane (TMS) was used as an internal standard in CDCl3 for all NMR measurements. Gel permeation chromatography (GPC) measurements were performed at 35 °C on an Agilent 1100 system (USA). Tetrahydrofuran was used as the eluant (1.0 mL/min) and polystyrene as the standard. Viscosity (η) measurements were carried out on Ubbelohde viscometer (CN60M, China) in phenol/s-tetrachloroethane mixture (50:50 by weight) at 30 ± 0.5 °C. The molecular weight (M̅ n) was calculated using the equation [η] = 4.3 × 10−4M̅ n0.76. Scanning electron microscope (Hitachi S-4700, Japan) was employed to observe the morphologies of the fractured surfaces of samples. Flexural, tensile, and impact property measurements were carried out with a universal tester according to Chinese Standard (GB/T2570-2008). For each property of a system, at least five samples were tested, and the average value was taken as the tested value. Differential scanning calorimetry (DSC) was performed on a TA calorimeter (Q200, TA) from room temperature to 300 °C with a heating rate of 10 °C/min under a nitrogen atmosphere. All samples were heated to eliminate the influence of the thermal history. Dynamic mechanical analysis (DMA) scans were performed with a single-cantilever blending mode using a dynamic mechanical analyzer TA Q800 (USA) from 30 to 350 °C at a heating rate of 5 °C/min and a frequency of 1 Hz. The dimensions of each sample were (35 ± 0.02) mm × (13 ± 0.02) mm × (3 ± 0.02) mm.

In fact, CBT cannot be directly used to modify thermally resistant TS resins because its low molecular weight tends to destroy the heat resistance of TS resins. Therefore, it is necessary to in situ produce PBT through ring-opening polymerization of CBT during the process of preparing modified TS resins, and then it is possible to get high toughness without sacrificing the processing feature and thermal resistance. The occurrence of the ring-opening polymerization of CBT needs initiators. The commonly used ones are organic tin compounds (such as dihydroxy butyl tin chloride and tin naphthenic),22 but they cannot be directly used in toughening thermally resistant TS resins due to following three main problems. First, PBT obtained under isothermal polymerization with the aid of organotin compounds has a relatively large brittleness.23 Second, the performance of the modified resin system is closely related to the phase structure,24,25 and the chemical interaction between thermally resistant TS and ETP is beneficial to get better performance;26,27 however, the PBT initiated by organotin compounds has no active groups. Third, the toxicity of organotin compounds has been confirmed,28 so the synthesis of organic tin with low toxicity becomes a common concern to researchers worldwide. Due to above problems, the design and synthesis of a new initiator for ringopening polymerization of CBT undoubtedly becomes the key point. Owing to the importance and wide applications in many cutting-edge fields,29,30 BMI resin is selected as the base for developing high performance resins in our research reported herein. A new type of organotin initiator (HSiSn) with high efficiency is synthesized and fully characterized, and then a series of modified CBT-BMI resins with HSiSn were prepared. The structure and properties of modified resins were systematically studied and compared with those of CBT-BMI resins using the traditional initiator, butyltin chloride dihydroxide (C4H9Sn(OH)2Cl, coded as BCD).

2. EXPERIMENTAL SECTION 2.1. Materials. The BMI used was 4,4′-bismaleimidodiphenylmethane (BDM), bought from Xibei Institute of Chemical Engineering (Xi′an, China). o,o′-Diallylbisphenol A (DBA) was purchased from Laiyu Chemical Factory (Shandong, China). CBT was purchased from Cyclics Co, U.S.A., and was dried at 60 °C for 4 h in a vacuum oven before use. BCD and triethylamine (TEA) were purchased from Shanghai Chemical Reagents (China). γ-Aminopropyl triethoxysilane (APTES) was bought from Jingzhou Jianghan Fine Chemical Ltd. (Jingzhou, China). Phenol (C6H6O), s-tetrachloroethane (C2H2Cl4), isopropanol (C3H8O), acetone (C3H6O), and ethanol (C2H5OH) were commercial reagents with analysis grades, which were used as received. Amino-terminated hyperbranched polysiloxane (AHBSi) was synthesized in our lab.4 2.2. Preparation of HSiSn. BCD (0.7 g, 2.85 mmol) and TEA (0.58 g, 5.7 mmol) were dissolved in isopropanol to form a mixture, into which an excessive amount of AHBSi dissolved in isopropanol was added slowly. The solution was heated to 55 °C under a nitrogen atmosphere and maintained at that temperature with stirring for 6 h. White precipitate was obtained and collected through filtration, followed by drying in vacuo, and then 1.82 g of product was obtained, which was HSiSn. B


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Figure 1. Synthesis mechanism of HSiSn.

Thermogravimetric (TG) analyses were performed on a TGA SDTQ600 (TA Instruments, U.S.A.) from 25 to 800 °C with a flow rate of 100 mL/min and a heating rate of 10 °C/ min. The temperature at which the weight loss of a sample reaches 5 wt % is regarded as the Tdi, and the char yield at 800 °C is regarded as Yc. The dielectric constant and loss were measured on a broad band dielectric spectrometer (Novocontrol Concept 80, Germany). The dimensions of each sample were (2.5 ± 0.1) mm × (25 ± 1) mm × (25 ± 1) mm. Positron annihilation lifetime spectra (PALS) were recorded using a positron lifetime spectrometer with a fast−slow coincidence system (U.S.A.). The radioactive source was22Na (20 μCi), the spectrometer resolution was 300 ps, and cumulative counts for each spectrum were 1 × 106. All PALS measurements were performed at 20 ± 0.5 °C under an air atmosphere. The dimensions of each sample were (10 ± 0.02) mm × (10 ± 0.02) mm × (1 ± 0.02) mm.

Figure 2. FTIR spectra of AHBSi, BCD, and HSiSn.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of HSiSn. As mentioned above, BCD is the traditional and effective initiator of CBT but also has three major problems. To overcome these drawbacks, hyperbranched polysiloxane (AHBSi) was designed to react with BCD to synthesize a new organic tin initiator as shown in Figure 1. The introduction of hyperbranched polysiloxane has three advantages. First, previous study showed that the ring-opening polymer using traditional initiator has significant brittleness,31 and this is not beneficial to be used for toughening TS resins. Therefore, AHBSi was designed to be introduced in the molecule of initiator, and then the PBT obtained will have reduced crystalline ability and improved toughness. Second, silicone has good biocompatibility and thus reduces the toxicity of tin.32 Third, the resultant PBT has active groups, which will react with the BD resin, tending to obtain good improvement. Figure 2 shows FTIR spectra of AHBSi, BCD, and HSiSn. The spectrum of HSiSn looks like a combination of those of AHBSi and BCD, indicating that HSiSn simultaneously contains polysiloxane and organic tin units. However, in the spectrum of HSiSn, the absorption peaks at 926 cm−1 (Sn− OH) and 554 cm−1 (Sn−Cl) appearing in the spectrum of BCD are not observed; instead, a new absorption peak at 770 cm−1 (Si−O−Sn) appears, clearly manifesting the occurrence of the condensation between BCD and AHBSi. This statement is further confirmed by the 29Si NMR spectra of AHBSi and HSiSn. As shown in Figure 3, the spectrum of HSiSn shows the

Figure 3. 29Si NMR spectra of AHBSi and HSiSn.

chemical shifts representing dendritic unit (D), linear unit (L), and terminal unit (T),33 suggesting that HSiSn has hyperbranched structure. Especially, compared with the T shift (−53.60 ppm) in the spectrum of AHBSi, that of HSiSn appears at −48.71 ppm due to the condensation between BCD and AHBSi. On the basis of Figure 3, the degree of branch (DB) of AHBSi was calculated to be 0.80. 3.2. Ring-Opening Polymerization of CBT in BD Resin. As stated above that CBT should in situ ring-opening polymerize during the curing of BD resin to get good toughening effect; therefore, it is necessary to study the ringopening polymerization of CBT in BD resin and evaluate the efficiency of both commercial initiator (BCD) and the new one prepared herein (HSiSn). Figure 4 gives the nonisothermal C


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Figure 4. DSC curves of BCD/CBT and HSiSn/CBT at different heating rates.

Figure 6. DSC curves of original and modified BD prepolymers.

centrifugation, washing, and drying, the ring-opening polymer of CBT was obtained. According to the initiator used, the resultant polymer was named as L-PBT(BCD) or L-PBT(HSiSn), of which the DSC curves are shown in Figure 5. From these curves, no melting point of CBT appears at 130−160 °C; instead, the melting peak representing the melting of PBT was observed at about 220 °C,34 indicating that the ring-opening polymerization occurs in the perpolymerization stage of BD. With comparison (Table S2 in the Supporting Information), it is found that, for the same initiator, the ring-opening polymer of CBT and that with the presence of BD resin have a difference of 10 °C in the melting points; moreover, the LPBT(BCD) has slightly decreased molecular weight and Tg compared with those of PBT(BCD), and similar phenomenon also appears in the pair of L-PBT(HSiSn) and PBT(HSiSn). This means that the existence of BD slightly affects the ringopening polymerization of CBT due to the steric effect. 3.3. Curing Behavior and Chemical Structure of CBT/ BD Resin with Initiators. The cross-linked structure of TS resins is closely related to the curing behavior, so it is necessary to investigate the effect of CBT and initiator on the curing behavior of BD resin. The chemical reactions in the HSiSn/ CBT-BD system including the ring-opening polymerization of CBT (Figure 7a),35 the coreaction of imide ring in BDM with DBA (Figure 7b), the homopolymerization of BDM (Figure 7b),36 and the coreaction of imide ring in BDM with −NH2 group in HSiSn (Figure 7c).4 These reactions also exist in the BCD/CBT-BD except for the coreaction between BDM and CBT. It is known that the curing of TS resin is initially governed by the chemical action and then determined by the molecular diffusion,37 so the presence of PBT definitely plays a steric effect on the cross-linking of BD resin, which is why the curing peak gradually moves to the direction of high temperature along with the increase of the content of CBT in the BCD/ CBT-BD resin as shown in Figure 8. Note that L-PBT(HSiSn) has −NH2 groups, which have high reactivity with imide ring in BDM; the chemical interaction offsets above steric effect in some extent, so the shift of the curing peak is moderate somewhat in the HSiSn/CBT-BD prepolymers. As we have known that the properties of a polymer are not only dependent on the chemical structure but also determined by the aggregation state structures, the latter is often characterized by the cross-linking density (Xdensity) and free volume.38 Using a semiempirical equation (eq 1)39 based on the curve of storage modulus−temperature plots from DMA tests (Figure

DSC curves of CBT/BCD and CBT/HSiSn at different heating rates. It can be seen that BCD can initiate the ring-opening polymerization of CBT under different heating rates; however, HSiSn can only initiate the ring-opening polymerization under relatively low heating rates (2.5 °C/min and 5 °C/min). These results suggest that HSiSn is such an initiator that has a relatively slower speed for the ring-opening polymerization of CBT; this is an attractive feature for producing a homogeneous cross-linked network. Figure 5 shows the DSC curves of CBT, PBT(BCD), and PBT(HSiSn). The two ring-opening polymers have the same

Figure 5. DSC curves of CBT and ring-opening polymers with different initiators.

melting points, indicating that the crystalline ability of PBT is independent of the initiator. However, PBT(HSiSn) has lower crystalline and molecular weight than PBT(BCD) (Table S2 in the Supporting Information); this is because hyperbranched structure weakens the crystalline ability, and moreover, the Sn− O−Si structure in HSiSn hinders the break from double bond to a single bond for the carbonyl groups in CBT. Figure 6 shows the DSC curves of BD, CBT-BD, BCD/ CBT-BD, and HSiSn/CBT-BD prepolymers. Without using any initiator, the CBT-BD prepolymer exhibits an obvious melting point at 130 °C, but other prepolymers with initiator do not show the melting point, demonstrating that the presence of initiator makes the ring-opening polymerization of CBT take place in the stage of the BD prepolymerization. To further confirm this statement, each prepolymer was put into acetone, and then insoluble material was obtained. After D


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Figure 7. Chemical reactions in the HSiSn/CBT-BD system (a: ring-opening polymerization of CBT; b: self-polymerization of BDM and copolymerization between BDM and DBA; c: copolymerization between BDM and L-PBT(HSiSn)).

Figure 9. Storage moduli of original and modified BD resins. Figure 8. DSC curves of original and modified BD prepolymers.

with suitable loadings of CBT (10 wt %), the contribution for the reduced flexibility induced by the chemical reaction between HSiSn and BD resin increases. Besides the impact strength, the tensile elongation at break is also closely related to the toughness of a material. As shown in Figure 14, BD resin has low tensile elongation due to its

Figure 14. Tensile elongations at break of original and modified BD resins.

Figure 15. Flexural (a) and tensile (b) moduli of original and modified BD resins. G


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Figure 16. Tensile (a) and flexural (b) strengths of original and modified BD resins.

3.5. Thermal Resistance of CBT/BD Resins. Tg is the highest service temperature for TS resins,52 while TG behavior reflects the thermal stability with temperature,53,54 so the two parameters are often used to evaluate the thermal resistance of TS resins. Figure 17 shows the tan delta−temperature curves of

Figure 18. TG and DTG curves of original and modified BD resins.

low Tdi of PBT. As Tdi is generally used to evaluate the thermal stability,60,61 so it is reasonable to state that HSiSn/CBT-BD resins have excellent thermal stability as BD resin. This result is unique because most thermoplastic polymer modified TS resins have declined thermal stability.62−64 3.6. Dielectric Properties. Electric and insulation fields are two main applications of BMI resins owing to their good dielectric properties,65,66 so the investigation on the dielectric properties of new modified BMI resin is meaningful and necessary. Figure 19 presents the dependence of dielectric constant and loss on frequency of original and modified BD resins. All modified resins have lower dielectric constant than BD resin. For BCD/CBT-BD resins, the dielectric constant decreases as the loading of CBT increases, while the 3HSiSn/CBT-BD resin has the lowest dielectric constant in the HSiSn/CBT-BD system, so besides the fact that PBT has lower dielectric constant (3.1−3.3) than BD resin,67 other factors including the Si−O−Si chains, the reduced free volume, and the reaction between L-PBT(HSiSn) and BDM provide the difficulty for the orientation and relaxing of dipoles in an applied electric field, leading to reduced dielectric constant. With regard to dielectric loss, all modified resins have similar values as BD resin, suggesting that all resins have similar polarity.

Figure 17. Tan delta−temperature curves of original and modified BD resins.

BCD/CBT-BD and HSiSn/CBT-BD resins, and the peak temperature is regarded as Tg.55,56 BCD/CBT-BD resins have lower Tg values (277.5−316.5 °C) than BD resin (319.3 °C), and this is because PBT(BCD) not only has low Tg (55.3 °C) but also blocks the cross-linking of the BD resin. This phenomenon is weakened in the HSiSn/CBT-BD resins owing to the interaction between HSiSn and BDM. Especially, the Tg of 5HSiSn/CBT-BD resin is still as high as 310.3 °C, which is superior over most thermal resistant resins.57−59 Figure 18 shows the TG and DTG curves of BCD/CBT-BD and HSiSn/CBT-BD resins, and the typical data, such as the initial decomposition temperature at which the weight loss of a sample reaches 5 wt % (Tdi), the temperature with the maximum decomposition rate (Tmax), and the char yield (Yc) at 800 °C, are summarized in Table S3 in the Supporting Information. It can be seen that BCD/CBT-BD resins have lower Tdi values than BD resin, and this is expected because PBT has far lower Tdi (352.9 °C), while attractively, HSiSn/ CBT-BD resins have similar Tdi as BD resin; moreover, their experimental Yc values are higher than the theoretical ones, and these results are thought to originate from outstanding thermal stability of Si−O−Si and chemical action between L-PBT (HSiSn) and BD resin, and they offset the negative influence of

4. CONCLUSION Terminal amino groups and hyperbranched polysiloxane make HSiSn show unique nature in the ring-opening polymerization of CBT oligomer and thus the polymer (PBT) and modified BD resin. Besides the similar crystalline and molecular weight as those of PBT(BCD), PBT(HSiSn) has better thermal H



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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21274104) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for financially supporting this project.

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Figure 19. Dependence of dielectric constant and loss on frequency of cured BD, BCD/CBT-BD, and HSiSn/CBT-BD resins.

stability and will react with BD resin, and these endow HSiSn/ CBT-BD resins with attractively better integrated performances than BD and BCD/CBT-BD resins. This investigation provides a new approach to synthesize the efficient and green initiator of CBT oligomer and corresponding high performance tough resins.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 gives formulations of BD, BCD/CBT-BD, and HSiSn/CBT-BD resins. Table S2 shows typical data from DSC curves of ring-opening polymers based on different initiators and the presence of BD or not. Table S3 summarizes the typical parameters representing thermal resistance. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b00103.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 512 65880967. Fax: +86 512 65880089. E-mail: [email protected] (Guozheng Liang). *Tel.: +86 512 65880967. Fax: +86 512 65880089. E-mail: [email protected] (Aijuan Gu). Notes

The authors declare no competing financial interest. I


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