Bismaleimide Resins with

Article pubs.acs.org/IECR

High Performance Miscible Polyetherimide/Bismaleimide Resins with Simultaneously Improved Integrated Properties Based on a Novel Hyperbranched Polysiloxane Having a High Degree of Branching Bin Sun, Guozheng Liang,* Aijuan Gu,* and Li Yuan 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: Amino-terminated hyperbranched polysiloxane (AHBSi) with high degree of branching (= 0.8) was synthesized by a control hydrolysis of γ-aminopropyl triethoxysilane (APTES) without using any catalyst. Besides, AHBSi was used as the compatibilizer of the miscible polyetherimide (PEI)/bismaleimide (BD) blend, and the influence of the content of AHBSi on the compatibility and integrated performance of the PEI/BD blend was systematically investigated. The corresponding investigation of the APTES/PEI/BD system was also carried out for comparison. Results show that although AHBSi and APTES have similar chemical segments, their different topological structures endow them with completely different effects. AHBSi can remarkably improve the compatibility between PEI and BD resin, and the AHBSi/PEI/BD system not only has obviously improved toughness and decreased brittleness while maintaining good stiffness. It also exhibits decreased dielectric constant and loss. Conversely, the APTES/PEI/BD system has greatly deteriorated compatibility and integrated performance. These interesting results demonstrate that AHBSi is a super multifunctional compatibilizer. those of a traditional linear polymer.9−11 Some hyperbranched polymers were used to improve the compatibility of two thermoplastic polymers. For instance, Wang’s group grafted polyamide 12 onto a hyperbranched polyethyleneimide, which was then used to compatibilize the polyamide 12/polybutylene terephthalate blends.12 Jannerfeldt and his associates used two commercial hyperbranched polymers (Boltorn H30 and Boltorn E2) grafted polypropylene as the compatibilizer of the polypropylene/polyamide 6 blend.13 These research works have shown that hyperbranched polymers may have the potential to be good compatibilizers; however, some key and basic topics have not been studied, and consequently, it is difficult to utilize the advantages of hyperbranched polymers. First, the investigations on the compatibilization effect and its mechanism are still not intensively carried out. Second, the utilization of hyperbranched polymer in compatibilizing HTP/TS blends is still not reported. As we’ve known that the compatibilization effect of a compatibilizer is closely dependent on the rheologic properties, reactivity, and the chain configuration of a macromolecule, these parameters are just the main differences between a hyperbranched polymer and a linear polymer, hence the compatibilization effect and mechanism of a hyperbranched polymer is definitely different from that of a linear polymer. Therefore, it is of great importance and interest for studying these topics. Besides the above theoretic researches, another aim of our research is developing high performance resins for cutting-edge

1. INTRODUCTION High performance thermoplastic polymer (HTP) modified thermosetting (TS) resin, coded as HTP/TS, is an important type of high performance TS resins,1−3 of which the outstanding feature is that toughening can be obtained without declining the remarkable thermal resistance of the original TS resin; moreover, the modified resins often possess additional attractive performance features according to the properties of HTP and TS used.4,5 Therefore, the HTP/TS system has attracted great attention worldwide. Many investigations have demonstrated that the performance of the HTP/TS system is closely related to the morphology of the system. As TS resin and HTP have greatly different polarities, the phase separation is a common phenomenon during the preparation of a HTP/TS resin. For the HTP/TS system with fixed compositions, the dispersion and the domain size of the disperse phase as well as the interaction between HTP and TS are the key factors in determining the performance of the system.6,7 At present, there are two main techniques for the compatibilization of the HTP/TS system, one is introducing reactive groups onto the thermoplastic polymer, and the other is using a reactive compatibilizer. However, very few commercial thermoplastics with reactive functional groups are available, while the synthesis of thermoplastics with reactive functional groups in a lab is also rather complex, so this method is hard to apply. Comparatively, it is a simple and effective method to improve the compatibility between HTP and TS by using a reactive compatibilizer.8 Hyperbranched polymer is a kind of polymer with highly branched chains, presenting an ellipsoidal or spherical structure. This unique structure endows a hyperbranched polymer with special physical and chemical properties that are not similar to © 2013 American Chemical Society

Received: Revised: Accepted: Published: 5054

September 29, 2012 March 11, 2013 March 14, 2013 March 14, 2013 dx.doi.org/10.1021/ie302665m | Ind. Eng. Chem. Res. 2013, 52, 5054−5065

Industrial & Engineering Chemistry Research

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fields. Therefore, the selection of the composition of the blend is very important. In this study reported herein, polyetherimide (PEI)/allyl bisphenol A modified bismaleimide (BD) system is chosen as the object owing to its importance and wide applications.14,15 Actually, this system has been investigated,16,17 some researchers tried to improve the compatibility of the system by adding a third component, but the results were far from satisfactory.18,19 In this paper, amino-terminated hyperbranched polysiloxane (AHBSi) with high degree of branching (DB) was synthesized through the control hydrolysis of γ-aminopropyl triethoxy silane (APTES). As AHBSi combines the advantages of both hyperbranched polymer and polysiloxane, which was added into the immiscible PEI/BD blend to develop a new system. Note that AHBSi is originated from APTES, so AHBSi and APTES have the same structure units; besides, APTES has low viscosity as AHBSi. More interestingly, APTES has been often used as a compatibilizer in polymer blends,20,21 so the influence of APTES on the compatibility of the PEI/BD system was also investigated for comparison. The morphologies and macroscopic performances (mechanical, thermal, and dielectric properties) of AHBSi/PEI/BD and APTES/PEI/BD systems with different contents of AHBSi and APTES were systematically investigated, and the compatibilization and properties of the corresponding systems are discussed.

mAPTES/PEI/BD, where m represents the weight content of APTES in BD resin, taking the values of 0.5, 1.0, 1.5, and 2.0. DBA was added into the clear PEI/CH2Cl2 solution at 25 ± 2 °C with stirring to get a clear liquid. When the liquid was heated to 150 °C, BDM was added into the liquid with stirring to obtain a brown-red transparent liquid, which was coded as PEI/BD. Appropriate quantities of BDM and DBA were put into a reactor, which was then heated to 150 °C and maintained at that temperature with stirring until a clear and brown liquid was obtained. The liquid was maintained at that temperature for additional 0.5 h to obtain a transparent liquid, which was BD prepolymer. 2.4. Preparation of Cured Resins. Each prepolymer was put into a preheated mold followed by degassing at 140 °C for 2 h in a vacuum oven. After that, the mold was put into an oven for curing and postcuring per the procedures of 160 °C/2 h + 185 °C/2 h + 230 °C/2 h, and 240 °C/4 h, successively. 2.5. Preparation of AHBSi/PEI and APTES/PEI Blends. Appropriate amounts of AHBSi were added into the PEI/ CH2Cl2 solution with stirring for 10 min to get a homogeneous mixture, and then, the mixture was put into a vacuum oven at 50 °C to remove CH2Cl2. The residual was washed with a large amount of ethanol to eliminate unreacted AHBSi and then was dried. The product was coded as xAHBSi/PEI, where x was the weight percent of AHBSi per 100 g PEI, taking the values of 5, 10, 15, and 20. The blend based on APTES and PEI was also prepared using above procedure for xAHBSi/PEI except that AHBSi was replaced by APTES. The resultant product was coded as yAPTES/PEI, where y was the weight percent of APTES per 100 g PEI, taking the values of 5, 10, 15, and 20. 2.6. Measurements of the Content of NH2 Groups and Grafting Yield. The content of NH2 groups in AHBSi was determined by a titration method,22 the detail procedure is described as follows. A 0.1 g portion of AHBSi was blended with an overdosed amount of 0.1 mol/L HCl solution to give a tested liquid, which was then titrated by using a standard NaOH solution (0.05 mol/L) until the pH value of the solution was 5.0. The content of NH2 groups of AHBSi (QNH2) can be calculated according to eq 1:

2. EXPERIMENTAL SECTION 2.1. Materials. 4,4′-Bismaleimidodiphenylmethane (BDM) was bought from Xibei Institute of Chemical Engineering (Xi′an, China). o,o′-Diallylbisphenol A (DBA) was purchased from Laiyu Chemical Factory (Shandong, China). PEI (Ultem1000) with a number-average molar mass of 20 kg/ mol and a density of 1.27 g/cm3 was made in General Electric, Ltd. (Shanghai, China), which was dried at 150 °C for 4 h in a vacuum oven before use. APTES was bought from Jingzhou Jianghan Fine Chemical Ltd. (Jingzhou, China). Methylene chloride (CH2Cl2) and ethanol (C2H5OH) were commercial reagents with analysis grades, which were used as received. 2.2. Preparation of AHBSi. Appropriate amounts of APTES, distilled water, and ethanol were put into a threenecked flask equipped with a thermometer and condenser to form a solution. After being maintained at room temperature for 15 min, the solution was heated to 55 °C and maintained at that temperature with stirring for 5 h, and then the resultant product was put into a vacuum oven at 60 °C to give off ethanol. Finally, a transparent and viscous liquid was obtained, which was AHBSi. The synthesis route was shown in Figure S1 in the Supporting Information. 2.3. Preparation of Prepolymers. The formulation of each resin was shown in Table S1 in the Supporting Information. At 25 ± 2 °C, appropriate PEI was dissolved in CH2Cl2 to form a clear solution (PEI/CH2Cl2); and then preweighted AHBSi and DBA were added into the solution with stirring until a homogeneous liquid was obtained. When the liquid was heated to 150 °C, BDM was added into the liquid, and the mixture was maintained at 150 °C with stirring until a transparent brown-red liquid was formed. The product was coded as nAHBSi/PEI/BD, where n represents the weight content of AHBSi in BD resin, taking the values of 0.5, 1.0, 1.5, and 2.0. Similarly, the prepolymer based on APTES, PEI, and BD was also prepared following above procedure except that AHBSi was replaced by APTES, and the prepolymer was coded as

Q NH = 2

C1V1 − C2V2 m

(1)

where C1 and C2 are the concentrations of standard HCl and NaOH solution, respectively; V1 and V2 are the consumed amounts of the standard HCl and NaOH solution, respectively; m is the consumed mass of AHBSi. xAHBSi/PEI was ground into powders and then blended with an overdosed standard HCl solution to form a homogeneous dispersion. The dispersion was titrated using a standard NaOH solution (0.05 mol/L) until the pH value of the solution was 5.0. According to the method, both unreacted amino groups of AHBSi and the amide groups formed during the blending of AHBSi and PEI can be titrated; however, the amount of the amide groups is far less than that of unreacted amino groups, hence the former can be negligible. As a result, the mass of AHBSi grafted onto PEI and the grafting yield can be calculated using eqs 2 and 3, respectively. 5055

dx.doi.org/10.1021/ie302665m | Ind. Eng. Chem. Res. 2013, 52, 5054−5065


mA =

C1V1′ − C2V2′ Q NH 2

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good compatibilizers. First, hyperbranched polymers have fine rheologic properties that are good for compatibilization because a compatibilzer with a lower viscosity is easy to diffuse and locate at the interface of the phases, and then, the effect of the compatibilizer could be embodied.23 Second, the PEI/BD blend investigated herein should be prepared using a solution method, so the candidate compatibilizer should have a favorable solubility in common solvents; while the good solubility is one typical merit of hyperbranched polymers.24 Third, it is known that more free volumes are favorable to get a higher toughness of the material,25,26 while hyperbranched polymers usually have big free volumes, especially those with high DB values. According to above analyses, a hyperbranched polysiloxane with high DB is designed. Hyperbranched polysiloxane can be synthesized by two methods. One is grafting linear polysiloxane onto a presynthesized core or grafting one polysiloxane onto another.27,28 Obviously, the resultant hyperbranched polysiloxane has a high content of linear polysiloxane linkages, so this method is not suitable to synthesize hyperbranched polysiloxane with high DB. Another method is the control hydrolysis of trialkoxysilane with the acid or base catalyst.22,29 Note that in the process with an acid catalyst, the rate of hydrolysis is significantly faster than that of the condensation, providing a favorable condition for preparing linear polymers, so the DB value of the corresponding hyperbranched polysiloxane is usually low (about 0.4). Meanwhile, in the process with the base catalyst, the condensation rate is faster than the hydrolysis rate, hence it is easy to produce highly branched polymeric structures. However, the DB of the polymer should be controlled; otherwise, a gelation tends to appear, and this may be the reason why few literature works reported the hydrolysis and condensation of trialkoxysilane with a base catalyst. In the case of APTES, no corresponding hyperbranched polysiloxane can be found in the literatures because amino group will react with an acid to produce an ammonium salt, so the acid condition is not suitable for the hydrolysis of APTES. Keeping in mind that we designed a hyperbranched polysiloxane with a high DB value, hence the control hydrolysis of APTES should proceede under a base condition and the hydrolysis and condensation rates need to be controlled to avoid the gelation. Specifically, we took two measures, one is employing the alkalescence of APTES,30 that is, no additional base is incorporated, and the other is adding an appropriate amount of ethanol for reducing the rate of the positive reaction. Figure 1 shows the FTIR spectra of AHBSi and APTES. There are three sharp peaks attributing to Si−O−C groups from 1000 to 1200 cm−1 in the spectrum of APTES; while AHBSi exhibits a broad and overlapped peak in this range, indicating the existence of polysiloxane because the siloxane linkages become longer with the progress of polycondensation, and thus, the characteristic peak attributing to Si−O−Si linkages becomes broader and more complex (showing two or more overlapping bands).31 Figure 2 shows the 1H NMR spectrum of AHBSi, the integration ratios of the hydrogen on different groups are equal to the theoretical results, indicating the formation of AHBSi. The 13C NMR spectrum of AHBSi as shown in Figure S2 in the Supporting Information suggests the occurrence of the hydrolysis and condensation. The above statement can be further proved by the 29Si NMR spectrum of AHBSi as shown in Figure 3. There is a distinct

(2)

where mA represents the mass of AHBSi grafted onto PEI; V1′ or V′2 is the consumed amount of the standard HCl or NaOH solution, respectively. Then mA is substituted into eq 3: mA G= × 100% mAHBSi/PEI − mA (3) where G represents the grafting yield, and mAHBSi/PEI is the mass of xAHBSi/PEI. 2.7. Characterizations. Fourier transform infrared (FTIR) spectra were detected on a Nicolet-5700 (USA) in the region of 4000−400 cm−1 using KBr pellets. 1 H NMR and 13C NMR spectra were recorded using tetramethylsilane (TMS) as an internal standard in CDCl3 on a UNITY INOVA-400 (400-MHz NMR spectrometer, USA). The 29Si NMR spectrum was recorded using TMS as an internal standard in CDCl3 on a Bruker Avance 400 (400-MHz NMR spectrometer, Germany). Gel permeation chromatography (GPC) measurements were performed at 35 °C using tetrahydrofuran as the eluant (1.0 mL min−1) and polystyrene as the standard with an Agilent 1100 system (USA). Scanning electron microscope (Hitachi S-4700, Japan) was employed to observe the morphologies of the fractured surfaces of samples. UV−vis transmittance spectra from 200 to 800 nm were recorded on a Shimadzu RF540 spectrophotometer (Kyoto, Japan). Flexural, tensile, and impact tests were done using 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 with a TA calorimeter (2910 MDSC, TA) from room temperature to 300 °C with a heating rate of 10 °C/min under a nitrogen atmosphere. 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. 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 spectroscopy (PALS) was recorded using a positron lifetime spectrometer with a fast− slow coincidence system (USA). The radioactive source selected was 22Na (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.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of AHBSi. Compared with linear polymers, hyperbranched polymers have some unique characteristics that are beneficial to act as 5056



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(T) as shown in Figure S1 in the Supporting Information. The DB of the hyperbranched polymer can be calculated using eq 4.34 2D DB = (4) 2D + L The DB of AHBSi was calculated to be 0.80. In addition, the molecular weight (Mw) of AHBSi is 8000, and the molecular weight distribution is 1.14, measured by GPC. On the basis of the above analyses, a kind of aminoterminated hyperbranched polysiloxane with high DB was successfully synthesized. 3.2. Influences of AHBSi and APTES on the Structure of the PEI/BD Blend. 3.2.1. Curing Behavior and Chemical Structure. The reaction between amino groups and BMI has been certified by some researchers.35,36 Bonnaud’s group37 and Li’s group19 proved that amines can also react with PEI through the mechanism shown in Figure S3 in the Supporting Information. However, it is still necessary to evaluate the possibility of the reaction between PEI and AHBSi because the reactivity of an active group in a hyperbranched polymer may be different from that in a compound.38 The reactivity between APTES and PEI is also discussed for comparison. Figure 4 is the FTIR spectra of PEI, 5AHBSi/PEI, and 20APTES/PEI. There are a tiny peak (1660 cm−1) assigning to

Figure 1. FTIR spectra of APTES and AHBSi.

Figure 2. 1H NMR spectrum of AHBSi.

Figure 4. FTIR spectra of PEI, 5AHBSi/PEI, and 20APTES/PEI blends.

the carbonyl peak of −NH−CO−39 and a wide absorption band (1000−1200 cm−1) belonging to the characteristic peak of Si−O−Si linkage31 in the FTIR spectrum of 5AHBSi/PEI, indicating that AHBSi is grafted onto PEI. Comparatively, no peak for the Si−O−C bond can be observed in the FTIR spectrum of the 20APTES/PEI, although the content of APTES is as large as 20 wt %, demonstrating that APTES does not react with PEI. The grafting rate of AHBSi onto PEI was determined by the titration method, the grafting rates of 5AHBSi/PEI, 10AHBSi/ PEI, and 20AHBSi/PEI are 4.28, 8.12, and 13.44, respectively, reflecting that a larger content of AHBSi is beneficial to get a higher grafting rate. Generally, a hyperbranched polymer has a big steric effect, so it is not possible for all active groups of the hyperbranched polymer to be reacted with other groups; however, with the increase of the content of AHBSi, the alkalinity enhances, making the reaction between PEI and

Figure 3. 29Si NMR spectrum of AHBSi.

chemical shift at −68.79 ppm, which represents the dendritic unites O1.5Si(CH2)3NH2,32 while the nearby smaller peaks arise from the incompletely condensed oligomers.33 The overlaid peaks from −59.55 to −61.02 ppm are attributable to linear units including NH2(CH2)3SiOC2H5O and NH2(CH2)3SiOHO; while the peaks around −53.89 ppm are assigned to the terminal groups including NH2(CH2)3Si(OH)2O0.5, NH2(CH2)3SiOHOC2H5O0.5, and NH2(CH2)3Si(OC2H5)2O0.5. A hyperbranched polymer prepared from AB2 monomer contains dendritic unit (D), linear unit (L), and terminal unit 5057



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amines easier to be proceeded,40 and consequently, the grafting rate of AHBSi on PEI increases. In order to investigate the influence of AHBSi and APTES on the curing behavior of the PEI/BD blend, DSC curves of AHBSi/PEI/BD and APTES/PEI/BD prepolymers were recorded and shown in Figure 5. The characteristic data from

be calculated by a semiempirical equation based on DMA analyses41,42 as shown in eq 5. log10 E′ = 7 + 293Xdensity

(5)

where E′ is the storage modulus (Figure 6) of the cured sample in the rubbery plateau region above the glass transition

Figure 6. Overlay storage modulus−temperature plots for cured PEI/ BD and AHBSi/PEI/BD resins.

temperature (Tg). Herein, E′ is chosen as the modulus at that temperature which is 20 °C higher than Tg, and the corresponding Xdensity values of cured PEI/BD and AHBSi/ PEI/BD resins are depicted in Figure 7. Note that the APTES/

Figure 5. DSC curves of BD, PEI/BD, APTES/PEI/BD, and AHBSi/ PEI/BD prepolymers.

these curves were summarized in Table S2 in the Supporting Information. It can be seen that each prepolymer has a single exothermic peak, which appears at the similar temperature range, reflecting that the prepolymer with AHBSi or APTES can still be cured with the same procedure as the PEI/BD blend. However, either AHBSi/PEI/BD or APTES/PEI/BD system has a higher curing enthalpy than the PEI/BD blend, meaning that the former has more reactions than the latter. For the PEI/BD blend, its curing reactions include the copolymerization between BDM and BDA as well as the homopolymerization of BDM. Because PEI does not take part in these reactions, PEI is thought to act as a “dilutent” that reduces the contact possibility among active groups. With regard to the AHBSi/PEI/BD system, some additional reactions induced by the presence of AHBSi will take place, they are the coreaction between AHBSi and PEI, the reaction between AHBSi and BDM, and the homopolymerization of AHBSi. For the APTES/PEI/BD system, the additional reaction is the coreaction between APTES and BDM. However, APTES is a small molecule, which is easier to react with BDM than AHBSi. The APTES/PEI/BD system also exhibits high curing enthalpy. 3.2.2. Cross-Linking Density. Cross-linking density (Xdensity) is an important index of a cured thermosetting resin, which can

Figure 7. Cross-linking densities of cured PEI/BD and AHBSi/PEI/ BD resins.

PEI/BD system will decompose at high temperature (>230 °C), so it is not possible to get a usable DMA curve and the Xdensity value of the APTES/PEI/BD system can not be obtained. It can be observed from Figure 7 that all AHBSi/PEI/BD resins have higher cross-linking densities than the PEI/BD blend. In the case of the AHBSi/PEI/BD system, as the content of AHBSi increases, the AHBSi/PEI/BD resin almost remains a similar cross-linking density until the content of AHBSi is larger than 1.5 wt %, and then increases. This can be explained from the dependence of the curing behavior on the content of 5058



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initially increase to a slight degree until the content of AHBSi is 1.5 wt %; continuously increase the content of AHBSi, the free volume fraction remarkably decreases, and the free cavity volume significantly increases. These results can be attributed to the multiple influences of the addition of AHBSi on the structure of the PEI/BD blend. Specifically, the presence of AHBSi improves the compatibility between PEI and BD resin and increases the cross-linking density of the network; these influences tend to decrease the free cavity volume and free volume fraction. However, generally, a hyperbranched polymer has cavities, so the addition of AHBSi into the PEI/BD blend increases the fraction of free volume and enlarges the free cavity volume. As shown in Figure 8, the free cavity volume increases as the content of AHBSi increases, suggesting that the size of the cavity in AHBSi is much larger than the free cavity volume of the PEI/BD blend. With regard to the APTES/PEI/BD system, it has similar free cavity volume as the PEI/BD resin, because APTES does not improve the compatibility between PEI and BD resin. However, the APTES/PEI/BD system has a smaller free volume fraction than the PEI/BD blend, and this trend is intensified as the content of APTES increases. This maybe results from the fact that the presence of APTES reduces the viscosity of the PEI/BD blend, and makes PEI have a somewhat better dispersion in BD resin, leading to a smaller free volume fraction while maintaining the free cavity volume. 3.2.4. Morphology. Figure 9 gives the digital photos of cured BD, PEI/BD, AHBSi/PEI/BD, and APTES/PEI/BD resins. It can be seen that cured BD resin is uniform and transparent reddish brown, while the PEI/BD resin is uniform but completely opaque owing to the poor compatibility between PEI and BD resin. It is interesting to note that the addition of AHBSi and that of APTES into the PEI/BD blend brings different effects on the appearance of the resultant system. Specifically, with the addition and the increasing of the content of AHBSi, the transparency of cured AHBSi/PEI/BD resin improves, meaning an improvement of the compatibility. Conversely, with the addition and the increasing of the content of APTES, all APTES/PEI/BD resins are opaque; moreover, 1.5APTES/PEI/BD and 2.0APTES/PEI/BD resins have many aggregates, demonstrating that the resins have serious phase separation. The influence of AHBSi and that of APTES on the morphologies of the PEI/BD blend can be further evaluated by the positions and shapes of the glass transition temperature (Tg) peaks in the loss modulus−temperature curves from DMA tests. It is known that an absolutely miscible blend presents only one Tg peak; while the peak of each phase of an immiscible blend will be observed, and the peak of each phase will shift toward each other with the improvement of the compatibility.44 Figure 10 presents the loss modulus−temperature curves from DMA tests of cured PEI/BD, APTES/PEI/BD, and AHBSi/PEI/BD resins, the corresponding Tg values from these curves are shown in Table S3 in the Supporting Information. The curve of the PEI/BD resin has a weak peak and a strong peak at 195 and 300 °C, representing the PEI phase and BD resin phase, respectively. With the addition of AHBSi, the two peaks shift toward each other, indicating the improvement in the compatibility between the two phases; note that a third peak can be observed in the curve of the 2.0AHBSi/PEI/BD resin, meaning that the phase resulting from the homopolymerization of AHBSi can not be neglected. However, these phenomena can not be observed in the curves of APTES/PEI/

AHBSi as discussed above. Briefly, there are two opposite factors, on one hand, the additional reaction between AHBSi and PEI as well as the homopolymerization of AHBSi increase the number of cross-linked points and thus the cross-linked density; on the other hand, the coreaction between AHBSi and BDM reduces the amount of the homopolymer of BDM, leading to decreased cross-linking density because the product of the former has longer distance among cross-linked points than that from the latter. The data in Figure 7 suggest that when the content of AHBSi is not very large, the effects from above two opposite factors are almost equal, so the resultant cross-linking density nearly remains the similar value; while when the content of AHBSi is large (>1.5 wt %), the equilibrium is broken, resulting in a increased cross-linking density. This is consistent with the remarkably increased grafting rate of AHBSi onto PEI as discussed above. 3.2.3. Free Volume. Free volume reflects the packing of the molecule of a matter,43 which can be reflected by free cavity volume and free volume fraction. The former directly reflects the average size of free volume, and the latter relates to the number of free volume sites. Figure 8 presents free cavity volumes and free volume fractions of cured PEI/BD, AHBSi/PEI/BD, and APTES/PEI/

Figure 8. Free cavity volumes and free volume fractions of cured PEI/ BD, AHBSi/PEI/BD, and APTES/PEI/BD resins.

BD resins. Compared with the PEI/BD resin, with the addition of AHBSi and the increase of the content of AHBSi, both free volume fraction and free cavity volume increase; however, when the content of AHBSi is 2.0 wt %, the free volume fraction remarkably decreases while the free cavity volume continuously increases. The free volume fraction and free cavity volume 5059



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Figure 10. Overlay loss modulus−temperature plots of cured PEI/BD, AHBSi/PEI/BD, and APTES/PEI/BD resins. Figure 9. Digital photos of cured BD, PEI/BD, AHBSi/PEI/BD, and APTES/PEI/BD resins.

far from spherical; however, the size of PEI becomes smaller, and the dispersion of PEI is more homogeneous. The fracture surface of either 1.0AHBSi/PEI/BD or 1.5AHBSi/PEI/BD resin exhibits a single and homogeneous phase. Similar phenomenon also appear in most regions of the fracture surface of the 2.0AHBSi/PEI/BD resin, but some big PEI particles can be found in the BD resin, reflecting that the 2.0AHBSi/PEI/BD resin does not truly have a homogeneous morphology. 3.3. Influences of AHBSi and APTES on the Properties of the PEI/BD Blend. 3.3.1. Impact Strength and Tensile Elongation. The impact resistance of a material reflects its ability to absorb the energy of a rapidly applied load, and the ability to withstand this sudden impact is related to the toughness of the material.45 Figure 12 presents the impact strength of cured BD, PEI/BD, AHBSi/PEI/BD, and APTES/ PEI/BD resins. All AHBSi/PEI/BD resins have obviously higher impact strengths than BD and PEI/BD resins. In detail, as the content of AHBSi increases, the impact strength almost linearly increases and reaches the maximum value (43.9 kJ/m2) at 1.5 wt % of AHBSi, about 1.9 and 2.6 times the impact strengths of the PEI/BD blend and BD resin, respectively. Conversely, all APTES/PEI/BD resins have much lower impact strengths than both BD and PEI/BD resins, and a large content of APTES leads to reduced impact strength.

BD resins; instead, the two peaks move away from each other to a larger degree as the content of APTES increases, reflecting that APTES can not act as a compatibilizer of the PEI/BD blend. In order to get more direct evidence, SEM micrographs of PEI/BD, AHBSi/PEI/BD, and APTES/PEI/BD resins were taken and shown in Figure 11; the corresponding micrographs of the etched samples in which PEI was completely removed are also provided for better observation. For the PEI/BD blend, its phase separation mechanism is thought to be the spinodal decomposition.16 The prepolymer of the blend is homogeneous; however, as the curing proceeds, the copolymerization between BDM and DBA produces a three-dimensional network with infinite molecular weight, leading to the phase separation in a large scale. Therefore, the cured PEI/BD resin presents bicontinuous morphology and round PEI particles dispersed in the BD resin have clear and sharp boundaries. With the addition of 0.5 wt % AHBSi, the phase boundaries become rather obscure, and interestingly, many small droplike holes surround each big hole (Figure 11b), indicating that the compatibility between PEI and BD resin improves; however, the reduction of the interfacial tension is not enough. Further increasing the content of AHBSi, the shape of the PEI phase is 5060



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Figure 11. SEM micrographs of the fracture surfaces for PEI/BD and AHBSi/PEI/BD resins (left) and etched samples (right). 5061



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Figure 12. Impact strengths of cured resins.

Figure 13. Tensile elongations at break of cured BD, PEI/BD, and AHBSi/PEI/BD resins.

According to Griffith’s Microcrack theory,46 the destruction of a material comes from the expansion of the internal microcracks. In a multiphase system, the existence of tough dispersion phase will hinder the expansion of microcracks through absorbing or transferring the cracking energy if there is a good interfacial adhesion between continuous and disperse phases; otherwise, the crack will expand from the weak interface, resulting in low strength, that is, the failure of toughening effect. Therefore, the above significantly different impact strengths between the AHBSi/PEI/BD and APTES/ PEI/BD systems are attributed to the different influences of AHBSi and APTES on the microstructure of the resins, although AHBSi and APTES have similar chemical compositions. For the PEI/BD blend, under the impact loading, the microcracks can be observed at the interface between PEI and BD resin (Figure 11) owing to the poor interfacial adhesion and poor compatibility. Such problems are greatly overcome with the addition of AHBSi, so nearly all cracks are stopped in the PEI phases (Figure 11), leading to obviously improved impact strength. The effect of the content of AHBSi on the impact strength is in good agreement with that on the morphology of the system. On the other hand, the addition of APTES intensifies the phase separation to such a great extent that the phase separation can even be observed macroscopically, resulting in remarkably decreased impact strengths. Besides the impact strength, the tensile elongation at break is also closely related to the toughness (or brittleness) of the materials. The brittleness of the materials can be calculated according to eq 6.47 B = 1/(εbE′)

Figure 14. Brittleness values of cured BD, PEI/BD, and AHBSi/PEI/ BD resins.

values than BD resin. Similar trend can be also observed in both flexural and tensile moduli of the AHBSi/PEI/BD resins with different contents of AHBSi as shown in Figure 15.

(6)

where B is the brittleness, εb is elongation at break (Figure 13), and E′ is the storage modulus at 1 Hz and the temperature of interest (30 °C) by DMA. The calculated B values of cured BD, PEI/BD, and AHBSi/PEI/BD resins are provided in Figure 14. The influence of the content of AHBSi on the B value is opposite to that on the impact strength; this is reasonable because toughness and brittleness describe the property of a material from opposite angles. 3.3.2. Storage, Flexural, and Tensile Moduli. Storage, flexural, and tensile moduli are usually used to reflect the stiffness of a material.48,49 From Figure 6 it can be seen that as the content of AHBSi increases, the E′ value in the glassy state of AHBSi/PEI/BD resin increases; when the content of AHBSi is larger than 1.0 wt %, AHBSi/PEI/BD resins have higher E′

Figure 15. Flexural and tensile moduli of cured BD, PEI/BD, and AHBSi/PEI/BD resins.

As we’ve known that the stiffness of a material is determined by the secondary valence forces, hence it is dependent on the packing density or concentration of the chain segments in the glassy state.50 PEI and BD resin do not have good compatibility, so the PEI/BD blend has low packing density. This condition is changed with the addition of AHBSi; specifically, the aromatic rings of BD resin can drive the 5062



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aromatic rings of PEI to move cooperatively and disrupt the microsegregation of phenyl groups into stacks owing to the improved compatibility and the low viscosity of AHBSi. Figure 16 shows the UV−visible spectra of cured BD, PEI/BD, and

Figure 17. Flexural strengths of cured resins.

Figure 16. UV−visible spectra of cured BD, PEI/BD, and AHBSi/ PEI/BD resins.

AHBSi/PEI/BD resins. Compared with cured BD and PEI/BD resins, AHBSi/PEI/BD resins (especially 2.0AHBSi/PEI/BD resin) have higher intensities of the peak assigning to the benzene ring (257 nm), indicating that the presence of AHBSi is beneficial to induce the occurrence of the cooperative motion between benzene rings of both PEI and BD resin and conspicuously increase the packing density of the polymer. Similar phenomena were also reported in other heterogeneous systems.51,52 Meanwhile, the packing density is also related to the free volume fraction of the network. With the addition of AHBSi and the increase of the content of AHBSi, the free volume fraction initially increases and then remarkably decreases when the content of AHBSi is 2.0 wt %. These data mean that when the content of AHBSi is not larger than 1.5 wt %, the increased free volume fraction tends to decrease the packing density of modified resins. On the basis of above discussions, it can be seen that when the content of AHBSi is not larger than 1.5 wt %, the above two aspects induced by the presence of AHBSi play opposites effects on the packing density. When the content of AHBSi is larger than 1.5 wt %, the two aspects are beneficial to increase the packing density and, thus, improve the stiffness. 3.3.3. Flexural Strength. Flexural strength is usually used to evaluate the integrated mechanical properties of a material because the flexural loading contains multitype loadings such as tensile, shearing, and/or compressing loadings.53 Generally, the increased flexural strength of materials primarily arises from the combination of the significant improvement in stiffness and toughness, so those factors that are beneficial to improve the stiffness; the toughness can improve the flexural strength. From corresponding discussions above, it can be seen that the improvement of toughness and stiffness induced by the addition of AHBSi is dependent on the content of AHBSi, so it is reasonable to observe the results shown in Figure 17. 3.4. Dielectric Properties. Figure 18 presents the dependence of dielectric constant and loss on frequency of cured BD, PEI/BD, and AHBSi/PEI/BD resins. Compared with the PEI/BD blend, all AHBSi/PEI/BD resins have lower dielectric constants, which are closely related to the content of AHBSi. As the content of AHBSi increases, the dielectric

Figure 18. Dependence of dielectric constant and loss on frequency of cured BD, PEI/BD, and AHBSi/PEI/BD resins.

constant initially decreases and reaches the lowest value at 1.5 wt %. The similar phenomenon also appears in the dielectric loss−frequency plots. It is known that the dielectric properties of polymers depend on the orientation and relaxation of dipoles in the applied electric field, and the process of dipole polarization accompanies the movement of polymer chain segments.54 Compared with the PEI/BD blend, the AHBSi/PEI/BD system has greatly improved compatibility between PEI and BD resin through the chemical reactions induced by the presence of 5063



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AHBSi, so it is difficult for the dipoles to orient and relax in the applied electric field. Because the effect of the compatibilization is related to the content of AHBSi, the dielectric property of the AHBSi/PEI/BD system is also related to the content of AHBSi.

4. CONCLUSIONS Using a controlled hydrolysis of APTES without using any additional acid or base catalysts, an amino-terminated hyperbranched polysiloxane (AHBSi) with high DB (0.8) was synthesized. The addition of AHBSi can greatly improve the compatibility between PEI and BD resin, and the 1.5AHBSi/ PEI/BD resin exhibits a single and homogeneous phase. Besides the role of the compatibilization, AHBSi can also toughen the PEI/BD blend while maintaining good stiffness and decrease the dielectric constant and loss. Comparatively, the presence of APTES intensifies the immiscibility of the PEI/ BD blend, and the corresponding APTES/PEI/BD resins have greatly deteriorated properties. These attractive results demonstrate that AHBSi is a multifunctional modifier of the PEI/BD blend, resulting from the unique structure and property of hyperbranched polysiloxane.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1: formulations of the BD, PEI/BD, APTES/PEI/BD, and AHBSi/PEI/BD resins. Table S2: typical data from DSC curves of the prepolymers. Table S3: glass transition temperatures from the loss modulus−temperature curves in DMA tests of PEI/BD and AHBSi/PEI/BD resins. Figure S1: synthesis route and the structure of AHBSi. Figure S2: 13C NMR spectrum of AHBSi. Figure S3: reaction between PEI and amines. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 512 61875156. Fax: +86 512 65880089. E-mail address: [email protected] (A. Gu) or [email protected] (G. Liang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Natural Science Foundation of China (21274104, 51173123), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Major Program of Natural Science Fundamental Research Project of Jiangsu Colleges and Universities (11KJA430001), and Suzhou Applied Basic Research Program (SYG201141) for financially supporting this project.

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REFERENCES

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