Nanostructured Epoxy Thermosets Containing Poly(vinylidene fluoride

Article pubs.acs.org/IECR

Nanostructured Epoxy Thermosets Containing Poly(vinylidene fluoride): Preparation, Morphologies, and Dielectric Properties Jingang Li, Lei Li, Yixin Xiang, and Sixun Zheng* Department of Polymer Science and Engineering and the State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ABSTRACT: The nanostructured thermosets involving epoxy and poly(methyl methacrylate)-grafted poly(vinylidene fluorideco-trifluoroethylene) copolymer (P(VDF-TrFE)-g-PMMA) were successfully prepared in this work. The P(VDF-TrFE)-gPMMA graft copolymer was synthesized via the atom transfer radical polymerization of methyl methacrylate (MMA) with poly(vinylidene fluoride-co-chlorotrifluoroethylene) copolymer as the macromolecular initiator. It was characterized by means of proton nuclear magnetic resonance spectroscopy, gel permeation chromatography, differential scanning calorimetry, and dynamic laser scattering. Owing to the amphiphilicity, this graft copolymer can be readily incorporated into epoxy to obtain the nanostructured thermosets. Transmission electron microscopy showed that the spherical nanophases with a size of 10−100 nm in diameter were dispersed into the continuous epoxy matrix, depending on the content of P(VDF-TrFE)-g-PMMA. The measurement of dielectric properties showed that the modified epoxy thermosets displayed enhanced dielectric constants and quite low dielectric loss. The surface contact angle measurements showed that the surface hydrophobicity of the nanostructured thermosets was higher than the plain epoxy thermoset.

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INTRODUCTION Nanostructured thermosets have attracted tremendous interest in the past years. It is realized that the generation of the nanostructures in thermosetting polymers can significantly optimize the intercomponent interactions and thus endows the materials with new and improved thermomechanical properties.1−4 In the 1970s, de Gennes5,6 first proposed that ordered mesoscopic structures in thermosets can be obtained through performing the cross-linking reaction of the precursors in liquid crystalline state. In 1990s, Bates et al.7,8 reported the nanostructured epoxy thermosets were prepared with a selfassembly methodology with amphiphilic block copolymers. In their approach, the epoxy precursors were used as the selective solvents of the diblock copolymers, allowing the creation of the self-organized microdomains (e.g., spherical micelles) before the curing reaction. In the presence of the self-assembled nanophases, the curing reaction was carried out to fix the nanostructures in the thermosets. More recently, Zheng et al.9,10 found that with amphiphilic block copolymers the nanostructures in thermosetting polymers can be alternatively formed by following reaction-induced microphase separation (RIMPS) mechanism. Since Bates et al. first reported the formation of nanophases in epoxy thermosets with amphiphilic block copolymers,7,8 the modulation of nanostructures in thermosets has been paid considerable attention in the past years. In most of the previous reports, the nanostructures of thermosets were obtained with well-defined linear block copolymers through the formation of microphase-separated morphologies via self-assembly or RIMPS mechanism. The nanostructures resulting from the microphase separation of amphiphilic block copolymers have motivated utilization of other amphiphiles other than linear block copolymers, which are readily available. Epoxy polymers are a class of important thermosets and have widely been applied as the matrix of high-performance © 2016 American Chemical Society

composites and adhesives owing to their excellent mechanical properties and high chemical resistance. In addition, epoxies are the choice of the electric encapsulation materials for embedded passives such as capacitors, inductors, and resistors. Depending on the assignment of the materials, epoxy thermosets must be modified to exhibit different dielectric properties. For the encapsulation materials of high-speed digital circuit with high frequency, epoxy thermosets need to have low dielectric constants so that the miniaturization and integration of the devices can be achieved with high information transfer rate and reduced crosstalk.9 For the applications of electric charge storage, transportation, and heat dissipation, epoxy thermosets are required to have high dielectric constants.10,11 It is crucial to modulate the dielectric properties of epoxy thermosets to meet various requirements. The dielectric constants of epoxy thermosets can be enhanced by incorporating conductive fillers such as metals under the concentrations that the percolation structure is formed.12,13 Alternatively, epoxy thermosets can be also modified via the preparation of the organic−inorganic nanocomposites by using the inorganic nanoparticles with high dielectric constants as the dielectric properties of other organic polymers were improved.14−20 The dielectric modification of epoxy with organic polymers is attractive owing to the simplicity in processing of materials. It has been reported that the dielectric properties of epoxy thermosets were improved with a few conductive polymers such as polyaniline,21 polythiophene,22 and poly(sodium p-styrenesulfonate).23 In addition, the dielectric constants of epoxy thermosets can be enhanced by incorporating the dielectric with high relative Received: Revised: Accepted: Published: 586

October 27, 2015 December 29, 2015 January 5, 2016 January 12, 2016 DOI: 10.1021/acs.iecr.5b04057 Ind. Eng. Chem. Res. 2016, 55, 586−596

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Industrial & Engineering Chemistry Research permittivity.24−31 Poly(vinylidene fluoride) (PVDF) is an important thermoplastic fluoropolymer, which has high mechanical properties and good resistance to wear, heat, corrosion, and weather. More importantly, PVDF is one of the best dielectrics with high dielectric constants depending on its crystallinity.29 The excellent properties motivated modification of epoxy thermosets by the use of PVDF or its copolymers. To the best of our knowledge, however, there have been few previous reports in this aspect. One of the main reasons is that PVDF was inherently immiscible with epoxy. The immiscibility is attributable to the high cohesive energy of PVDF, which results from the high electronegativity of fluorine atom, short C−F bond, and high bond energy. It has long been a challenging task to incorporate PVDF or its copolymers into epoxy to obtain the modified materials with fine morphologies. It should be pointed out that there have been a few reports on the preparation of the nanostructured thermosets involving epoxy thermosets and several semifluorinated block copolymers.30,31 In this contribution, we reported a facile preparation of the nanostructured epoxy thermosets with PVDF nanophases; the dielectric properties of the nanostructured thermosets were thus investigated. First, we synthesized a poly(methyl methacrylate) (PMMA)-grafted poly(vinylidene fluoride-cotrifluoroethylene) [denoted P(VDF-TrFE)-g-PMMA] via atom transfer radical polymerization (ATRP). Thereafter, this P(VDF-TrFE)-g-PMMA graft copolymer was used to obtain the modified epoxy thermosets containing P(VDF-TrFE) nanophases. The P(VDF-TrFE)-g-PMMA graft copolymer was employed by knowing that PMMA is miscible with epoxy23 whereas P(VDF-TrFE) is inherently immiscible with epoxy. It is anticipated that the nanostructured epoxy thermosets would be obtained through the self-assembly behavior of the amphiphilic graft copolymer. In this work, P(VDF-TrFE)-g-PMMA graft copolymer was characterized by means of proton nuclear magnetic resonance (1H NMR) spectroscopy, gel permeation chromatography (GPC), dynamic laser scattering (DLS), and differential scanning calorimetry (DSC). The formation of P(VDF-TrFE) microdomains in epoxy thermosets was investigated by means of transmission electron microscopy (TEM) and dynamic mechanical thermal analysis (DMTA). The dielectric properties were measured by means of a broad-band dielectric spectroscopy (BDS).

(NMP) and N,N′-dimethylformamide (DMF) were of chemically pure grade, obtained from Shanghai Reagent Co., China. Synthesis of P(VDF-TrFE)-g-PMMA Graft Copolymer. To a flask, P(VDF-CTrFE) (0.800 g), MMA (8.000 g, 0.08 mol), NMP (8 mL), and Cu(I)Cl (170 mg, 1.7 mmol) were charged with vigorous stirring until a full dissolution was attained. Taking advantage of a Schlenk line, the system was degassed via three pump−freeze−thaw cycles. Thereafter, 2,2′bipyridine (530 mg, 3.39 mmol) dissolved in 1.0 mL of NMP was added with vigorous stirring. After redegassed via three pump−freeze−thaw cycles, this flask was immersed into an oil bath to perform the polymerization at 100 °C for 48 h. Cooled to room temperature, the reacted mixture was passed through a neutral alumina column to remove the catalyst. The solution was concentrated and then dropped into a great amount of cold mixture of methanol with water (10/90 vol) to afford the precipitates. This dissolution−precipitation procedure was repeated three times to purify the polymer. After drying in vacuo at 35 °C for 24 h, the product (1.980 g) was obtained and the conversion of MMA was about 19.4%. 1H NMR (CDCl3, ppm): 3.4−3.7 [3H, −COOCH3], 2.7−3.2 [4H, −CF2CH2CF2CH2−], 2.2−2.4 [4H, CF2CH2CH2CF2], 1.6− 2.0 [2H, −CH2C(CH3)(COO)−], 0.6−1.2 [3H, −CH2C(CH3)(COO)]. Preparation of Epoxy Thermosets. First, the desired amount of P(VDF-TrFE)-g-PMMA dissolved in a small amount of DMF was added to DGEBA at 80 °C with vigorous stirring until the mixture became homogeneous and transparent. In a vacuum oven, the mixture was held at 80 °C for 12 h to remove the majority of the solvent. Thereafter, equimolar 4,4′-methylene-bis(2,6-diethylaniline) with respect to DGEBA was added with vigorous stirring until the curing agent was fully dissolved. The ternary mixture was poured into a Teflon mold to cure at 150 °C for 3 h plus 180 °C for 2 h. In this work, the thermosets were prepared with the content of P(VDF-TrFE)-gPMMA up to 40 wt %. Measurement and Techniques. Proton nuclear magnetic resonance (1H NMR) spectroscopy was carried out on a Varian Mercury Plus 400 MHz NMR spectrometer at 25 °C. The samples were dissolved with deuterium dimethyl sulfoxide [(CD3)2SO] and the solutions were measured with tetramethylsilane (TMS) as an internal reference. Gel permeation chromatography (GPC) was conducted on a Waters 1515 system, which was composed of an isocratic HPLC pump and a RI detector. This apparatus was equipped with three Waters HR columns (HR4, HR3, and HR1). In the measurements, DMF containing 0.01 M LiBr was used as the eluent at a flow rate of 1.0 mL·min−1. The molecular weights were expressed relative to polystyrene standard. Dynamic laser scattering (DLS) was performed on a Malvern Nano ZS90 apparatus equipped with a He−Ne laser operated at the wavelength of λ = 633 nm. The graft copolymer (10.0 mg) was dispersed into 0.5 mL of acetone, and the as-obtained suspension was dropwise added to 20 mL of anhydrous toluene with vigorous stirring. The small amount of acetone was removed via rotary evaporation, and then the resulting suspension was used for the DLS measurement. All the data were acquired at a fixed scattering angle of 90°. Differential scanning calorimetry (DSC) was performed on a PerkinElmer Pyris-1 system in a dry nitrogen atmosphere. Before the measurements, the samples (about 10.0 mg) were rapidly heated up to 150 °C and maintained at this temperature for 3 min, followed by quenching to −70 °C to erase the thermal history. Thereafter,

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EXPERIMENTAL SECTION Materials. Diglycidyl ether of bisphenol A (DGEBA) was supplied by Shanghai Resin Co., China; it has a quoted epoxy equivalence of 177 g·mol−1. Methyl methacrylate (MMA), copper(I) chloride (CuCl), 4,4′-methylene-bis(3-chloro-2,6diethylaniline) (MCDEA), and 2,2′-bipyridine were purchased from Sigma-Aldrich Co., Shanghai, China. Before use, MMA was passed through a basic alumina column to remove the inhibitor. Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-CTrFE)) was supplied by Solvay Co., China, and it contained 9 mol % of chlorotrifluoroethylene (CTrFE) structural units. The GPC measurement showed that P(VDFCTrFE) had a molecular weight of Mn = 223 000 Da with Mw/ Mn = 1.16. Prior to use, the P(VDF-co-CTrFE) was dissolved in N-methylpyrrolidone (NMP) at a concentration of 10 wt % and then the solution was dropped into a great amount of methanol to obtain the precipitates. This dissolution−precipitation procedure was repeated three times to remove possible impurities. Organic solvents such as N-methylpyrrolidone 587

DOI: 10.1021/acs.iecr.5b04057 Ind. Eng. Chem. Res. 2016, 55, 586−596

Article

Industrial & Engineering Chemistry Research Scheme 1. Synthesis of P(VDF-TrFE)-g-PMMA Graft Copolymer

the samples were scanned from −70 to 150 °C at a heating rate of 20 °C/min. A Shimadzu XRD-6000 X-ray diffractometer with Cu Kα (λ = 0.154 nm) irradiation at 40 kV and 30 mA and with a Ni filter was used for the wide-angle X-ray diffraction (XRD) measurements. The experiments were carried out from the diffraction angle of 2θ = 5−35° at a scanning rate of 3.0°/ min and a step size of 0.01°, respectively. To investigate the morphologies of the thermosets, the modified thermosets were sliced into the sections with the thickness of ∼70 nm with a Leica UC6 ultrathin microtome machine equipped with a diamond knife and then the sections were stained with RuO4 to increase the contrast and the stained sections were placed in 200 mesh copper grids for observations. The morphological observations were performed on a JEOL JEM-2010 highresolution transmission electron microscope (TEM) at an acceleration voltage of 120 kV. All the thermosets were subjected to dynamic mechanical thermal analysis (DMTA) on a TA Instruments DMA Q800 apparatus. This system was equipped with a liquid nitrogen accessary, and the measurements were carried out from −100 °C until the specimens became too soft to be tested. The measurements were performed in a single cantilever mode, at a frequency of 3.0 Hz and a heating rate of 3.0 °C/min. The thermosets were machined into the cuboid specimens with the dimension of 1.2 × 2 × 0.1 cm3. For the dielectric measurements, the control epoxy and the modified thermosets were machined into cylindrical specimens with a height of ∼1.0 mm and a diameter of 10.0 mm. Their surfaces were carefully polished, and the two surfaces of the cylinders were pasted on aluminum foils to act as the working electrodes. The measurements of dielectric permittivity and loss were performed on an Aglient 4294A impedance analyzer with 16451B dielectric test fixture in the frequency range of 103−107 Hz, applying 0.5 V of alternating current voltage across two sides of the cylinder-shaped specimens.

merization of methacrylates with PVDF homopolymer as the macroinitiator.39 Zhang et al.40 reported the grafting polymerization of polystyrene and poly(tert-butyl acrylate) with poly(vinylidene fluoride-co-chlorotrifluoroethylene) [P(VDFco-CTrFE)] copolymer with the macroinitiator. The catalysts used were the complexes of copper(I) chloride and 2,2′bipyridine. To the best of our knowledge, there have been few reports on the synthesis of P(VDF-TrFE)-g-PMMA graft copolymers via ATRP. It should be pointed out that Xu et al.41 recently carried out the graft copolymerization of methyl methacrylate (MMA) onto PVDF with Cu(0)/2,6-bis(imino)pyridines as the catalyst via a single-electron transfer-living radical polymerization approach. In this work, the grafting polymerization of MMA onto PVDF was performed via ATRP approach with a P(VDFCTrFE) copolymer (with 9 mol % of CTrEF copolymerization units) as the macromolecular initiator and the complex of copper(I) chloride (CuCl) with 2,2′-bipyridine was used as the catalyst (Scheme 1). With this ATRP approach, the main chain of the PVDF-CTrFE copolymer would be transformed into poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrEF)], since the PMMA chains were grown from the chlorine atoms. Shown in Figure 1 are the 1H NMR spectra of the P(VDFCTrFE) and P(VDF-TrFE)-g-PMMA. For P(VDF-CTrFE), two groups of resonance signals were exhibited at 2.3 and 2.9 ppm, respectively. The former is assignable to the protons of methylene in the head-to-head structure, whereas the latter is assignable to those in the head-to-tail structure. For P(VDFTrFE)-g-PMMA, there appeared several new peaks besides those assignable to P(VDF-CTrFE). The peaks of resonance at 0.5−1.3 and 3.6 ppm are assignable to the protons of methyl groups connected to the main chains of PMMA and oxygen atoms, respectively. The proton resonance of methylene groups in the main chains of PMMA was discernible at 1.6−2.0 ppm. 1 H NMR spectroscopy indicates that the polymerized product combined the structural features from PVDF and PMMA. According to the ratio of the integral intensity of the protons of all the methylene groups of P(VDF-TrFE) to that of the methyl protons at 3.6 ppm, the mass ratio of P(VDF-TrFE) to PMMA in the resulting product was calculated to be 1:1.42, and thus the molecular weight of the graft copolymer was estimated to be Mn = 539 700 Da. The above polymerized product was subjected to gel permeation chromatography (GPC), and the GPC curves are shown in Figure 2. For the P(VDF-CTrFE), a unimodal distribution of molecular weight was exhibited with Mn = 223 000 Da with Mw/Mn = 1.16. Notably, the P(VDF-CTrFE) gave a negative peak with this RI detector. This phenomenon resulted from the refractive index of the copolymer lower than that of the eluent (viz., DMF). Upon grafting PMMA chains onto this copolymer, there appeared a positive peak at the

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RESULTS AND DISCUSSION Synthesis of P(VDF-TrFE)-g-PMMA Graft Copolymer. It has been reported that the graft copolymers of PVDF can be synthesized via conventional radical polymerization approach.32−39 The free radicals on the backbone of PVDF chains were produced by the treatments with ionizing radiation32−34 and a free-radical initiator.35,36 In addition, free radicals were also created via thermal decomposition of peroxide groups which were introduced onto PVDF backbone via ozone treatment.37,38 One of the disadvantages for the conventional free copolymerization is that the homopolymer of comonomer was produced.39 Recently, there have been a few reports that PVDF graft copolymers were obtained from commercial PVDF homopolymer or copolymers via ATRP.39−42 Hester et al. reported the direct graft copoly588


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Industrial & Engineering Chemistry Research

Figure 3. Photos of (A) PMMA, (B) P(VDF-CTrFE), and (C) P(VDF-TrFE)-g-PMMA dispersed in toluene.

they were dispersed in toluene. It is seen that PMMA was readily dissolved in toluene (Figure 3A) whereas P(VDFCTrFE) was insoluble in this solvent (Figure 3B). Notably, the polymerized product can be fully dispersed in toluene (Figure 3C), indicating that all the P(VDF-CTrFE) chains were grafted with PMMA. A suspension of P(VDF-TrFE)-g-PMMA dispersed in toluene was further subjected to dynamic laser scattering (DLS) measurement, and a plot of hydrodynamic radius distribution as a function of hydrodynamic radius (Rh) is shown in Figure 4. Notably, the intensity-averaged hydro-

Figure 1. 1H NMR spectra of P(VDF-CTrFE) and P(VDF-TrFE)-gPMMA.

Figure 4. Plots of the distribution of hydrodynamic radius for the P(VDF-TrFE)-g-PMMA copolymer in toluene.

Figure 2. GPC curves of P(VDF-CTrFE) and P(VDF-TrFE)-gPMMA.

dynamic radius displayed unimodal distribution with about Rh = 90 nm. The DLS result shows that P(VDF-TrFE)-g-PMMA graft copolymer was self-organized into the nanoobjects in toluene. The self-assembly behavior is a sign of the amphiphilicity behavior of the graft copolymer, suggesting that PMMA has been successfully grafted onto P(VDF-TrFE) chains. The results of the 1H NMR spectroscopy, solubility tests, and DLS indicate that the P(VDF-TrFE)-g-PMMA graft copolymer has been successfully obtained.

longer retention time side, whereas the negative peak was still preserved and slightly shifted to the lower elution time side. The appearance of the positive peak indicates that the PMMA chains were produced with the ATRP process. Similar results have also been reported in other PVDF-containing block or graft copolymers.43−45 In this work, we explored using the solubility test indirectly to demonstrate the formation of the graft copolymers. Shown in Figure 3 are the photos of plain P(VDF-CTrFE), PMMA, and P(VDF-TrFE)-g-PMMA while 589


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Industrial & Engineering Chemistry Research The P(VDF-TrFE)-g-PMMA graft copolymer was subjected to differential scanning calorimetry (DSC), and the DSC curves of P(VDF-CTrFE) and P(VDF-TrFE)-g-PMMA are shown in Figure 5. For P(VDF-CTrFE), two endothermic peaks were

Figure 6. XRD curves of P(VDF-CTrFE) and P(VDF-TrFE)-gPMMA copolymer.

diffraction peaks were significantly decreased whereas the amorphous halo increased. The XRD result was in good agreement with that of DSC. Nanostructured Epoxy Thermosets Containing P(VDF-TrFE). The P(VDF-TrFE)-g-PMMA graft copolymer was employed to prepare the modified thermosets. It was observed that the graft copolymer was readily dispersed in the epoxy precursors (viz., DGEBA + MCDEA) to form stable and homogeneous mixtures. All the mixtures of the epoxy precursors (i.e., DGEBA + MCDEA) with P(VDF-TrFE)-gPMMA were transparent at room and elevated temperature, indicating that the P(VDF-TrFE)-g-PMMA can be well dispersed into the epoxy monomers. This observation was in sharp contrast to the cases of P(VDF-CTrFE) and PVDF. In view of the miscibility of PMMA and P(VDF-CTrFE) with the epoxy precursors, it is proposed that the P(VDF-TrFE)-gPMMA graft copolymer was self-organized into the nanoobjects. If the mixtures were cured at elevated temperature, the nanostructured epoxy thermosets containing PVDF nanophases would be obtained. The formation of the nanostructures in epoxy is readily investigated by transmission electron microscopy (TEM). All the cured thermosets containing P(VDF-TrFE)-g-PMMA graft copolymer were homogeneous and transparent, suggesting that macroscopic phase separation did not occur in the process of curing reaction. Nonetheless, the transparency did not exclude the possibility that the thermosets were microphaseseparated, since P(VDF-TrFE) was inherently immiscible with epoxy. To investigate the microphase separation behavior, the thermosets were subjected to the morphological observation by means of transmission electron microscopy (TEM). Shown in Figure 7 are the TEM images of the epoxy thermosets containing 10, 20, 30, and 40 wt % of P(VDF-TrFE)-g-PMMA. To increase the contrast between epoxy matrix and P(VDFTrFE) microdomains, the ultrathin sections of the samples were stained with RuO4. In this case, the epoxy matrix was stained whereas the P(VDF-TrFE) microdomains remained almost unaffected. For all the epoxy thermosets containing

Figure 5. DSC curves of P(VDF-CTrFE) and P(VDF-TrFE)-gPMMA copolymer.

displayed at 140 and 170 °C, respectively; they are ascribed to the melting transitions of α and β phases, respectively.46 The glass transition of P(VDF-CTrFE) was indiscernible under the present quenching condition owing to its high degree of crystallinity. Upon grafting PMMA chains onto P(VDF-TrFE) chain, the intensity of the melting peaks was greatly decreased; a feeble melting transition was discernible at 154 °C. In addition, there appeared one weak cold crystallization peak at 108 °C and one glass transition at 29 °C. Notably, the value of the melting enthalpy (ΔHm) was quite close to that of the crystallization enthalpy (ΔHc), implying that the amorphous P(VDF-TrFE)-g-PMMA was obtained while this graft copolymer was quenched from its melt to −70 °C. The thermal transition behavior indicates that the crystallization of P(VDFCTrFE) became fairly slow while it was grafted by PMMA chains, which was in sharp contrast to plain P(VDF-CTrFE). The decreased crystallization rate for the P(VDF-TrFE)-gPMMA graft copolymer resulted from the miscibility of PVDF with PMMA.47−51 The incorporation of the miscible and higher-Tg component (viz. PMMA) resulted in the decreases in the rate of crystallization of P(VDF-TrFE), the degree of crystallinity, and the melting temperature. Both P(VDFCTrFE) and P(VDF-TrFE)-g-PMMA were subjected to wideangle X-ray diffraction (XRD), and the X-ray diffraction curves are presented in Figure 6. For P(VDF-CTrFE), the strong diffraction peaks were exhibited at 17.4° 18.4°, 19.9°, and 26.7°. These diffraction peaks are assignable to the reflections of α phases at (100), (020), (110), and (021) planes, since the cocrystallization occurred between PVDF and P(CTrFE).52,53 In marked contrast to P(VDF-CTrFE), the intensities of these 590


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and the DSC curves are presented in Figure 8. Plain epoxy displayed a glass transition at 166 °C. Upon addition of

Figure 7. TEM images of the nanostructured thermosets containing (A) 10, (B) 20, (C) 30, and (D) 40 wt % of P(VDF-TrFE)-g-PMMA copolymer. Figure 8. DSC curves of the nanostructured thermosets containing P(VDF-TrFE)-g-PMMA copolymer.

P(VDF-TrFE)-g-PMMA, notably, the microphase-separated morphologies were exhibited. For the thermoset containing 10 wt % of P(VDF-TrFE)-g-PMMA, the spherical microdomains with a size of about 10 nm in diameter were dispersed into the continuous matrix; the size and number of the P(VDFTrFE) microdomains increased as the content of P(VDFTrFE)-g-PMMA increased. While the content of P(VDFTrFE)-g-PMMA was 40 wt %, the size of the P(VDF-TrFE) microdomains was increased to about 100 nm. According to the miscibility of P(VDF-TrFE), PMMA with epoxy, it is judged that the spherical microdomains are attributable to P(VDFTrFE) whereas the continuous matrix is attributable to epoxy that was miscible with PMMA. The TEM results showed that P(VDF-TrFE) copolymer has been successfully incorporated into epoxy thermosets in the form of spherical microdomains. It is proposed that the formation of the P(VDF-TrFE) microdomains in the thermosets followed the self-assembly mechanism. First, the P(VDF-TrFE)-g-PMMA graft copolymer was self-assembled into the spherical P(VDF-TrFE) microdomains in its mixtures with the epoxy precursors. The microdomains were then fixed via the subsequent curing reaction at elevated temperature. Notably, the sizes of P(VDFTrFE) microdomains were about 100 nm while the content of P(VDF-TrFE)-g-PMMA was 30 wt % or higher, which was significantly larger than those of the self-assembly of block copolymers in selective solvents (viz. 20−50 nm).54 The increased sizes of the microdomains could result from the additional interactions within the P(VDF-TrFE) microdomains. The DSC results (see infra) showed that the microdomains were crystalline, and thus the lattice energy contributed to the increase in the size of self-assembly in addition to the van der Waals force. Similar cases have been reported by Williams et al. in the nanostructured epoxy thermosets containing polyethylene-block-poly(ethylene oxide) (PE-b-PEO) diblock copolymer.55,56 The crystallinity of the P(VDF-TrFE) microdomains in the epoxy thermosets can be examined with DSC,

P(VDF-TrFE)-g-PMMA, the glass transition shifted to the lower temperature sides. The decreased glass transition temperatures reflected the miscibility of the PMMA side chains with the epoxy network. After the glass transitions, all the nanostructured thermosets containing P(VDF-TrFE)-g-PMMA displayed the endothermic peaks at ∼154 °C. The temperatures of these peaks remained unchanged, irrespective of the content of P(VDF-TrFE)-g-PMMA, but their intensity increased with the content of P(VDF-TrFE)-g-PMMA. The endothermic peaks are attributable to the melting transitions of the P(VDF-TrFE) microdomains in the nanostructured thermosets. The detection of the melting peaks indicates that the crystalline microdomains were formed in the nanostructured thermosets. It is worth pointing out that the intensities of the P(VDF-TrFE) melting peaks in the nanostructured thermosets were even higher than that of plain P(VDF-TrFE)-g-PMMA graft copolymer (see Figure 5). This observation suggests that the PMMA chains have been extracted from the P(VDF-TrFE) microdomains, since in the nanostructured thermosets PMMA chains remained mixed with epoxy matrix. The plain epoxy and its nanostructured thermosets containing P(VDF-TrFE)-g-PMMA were subjected to dynamic mechanical thermal analysis (DMTA). The DMTA curves are shown in Figure 9. For the control epoxy, the major transition appeared at 174 °C, whereas one secondary transition appeared at −63 °C. The former is attributable to the glass transition of this cross-linked polymer, whereas the latter is attributable to the motion of hydroxy ether structural unit [−O−CH2− CH(OH)−CH2−O−] in amino-cross-linked epoxy.57−59 With incorporation of P(VDF-TrFE)-g-PMMA, the glass transitions were observed to shift to the lower temperatures, indicating that the epoxy was mixed with PMMA side chains at the segmental level. In the DMTA spectra of the nanostructured 591


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Figure 9. DMTA curves of control epoxy and the binary blends of epoxy resin containing P(VDF-TrFE)-g-PMMA. Figure 10. Dielectric constants and dielectric loss tangent of the epoxy thermosets containing PMMA at 25 °C as functions of ac frequency. The content of PMMA is 0, 6, 12, 18, and 24 wt %, respectively.

thermosets, notably, there were the shoulder peaks at about 100 °C in the neighbor of the α transitions. The shoulder peaks are assignable to the PMMA chains, which could to some extent be demixed out of epoxy matrix during the curing reaction. Such a demixing behavior has been found in some nanostructured thermosetting systems containing block copolymers.7,8,60−62 Bates et al.7,8 have reported that the epoxy-miscible block (viz. poly(ethylene oxide)) of the poly(ethylene oxide)-blockpoly(ethylethylene) was demixed out of epoxy matrix in the process of curing reaction. It is proposed that this demixing behavior resulted from the transition from equilibrium morphology to a chemically pinned metastable state while the curing reaction progressed through the gel point.7,8 Returning to Figure 9, it is seen that the storage moduli of the nanostructured thermosets in the glass state were significantly higher than that of the plain epoxy. The increased moduli for the nanostructured thermosets is attributable to the nanoreinforcement of P(VDF-TrFE) microcrystals on the epoxy matrices. Dielectric Properties. It is of interest to examine the effect of P(VDF-TrFE) microdomain on the dielectric properties of the epoxy thermosets. In this multicomponent system, it is necessary to isolate the effect of PMMA side chains on the dielectric properties. For this purpose, we prepared the binary thermosetting blends of epoxy with PMMA homopolymer (Mn = 52 000 Da with Mw/Mn = 1.32) and measured the dielectric properties of the thermosetting blends. Shown in Figure 10 are the plots of dielectric constant and dielectric loss at 25 °C as functions of ac frequency in the range of 103−107 Hz. It is seen that the dielectric constants of control epoxy and the blends slightly decreased as the ac frequency increased. In the meantime, the dielectric loss did not increase with ac frequency. The decrease in dielectric constants with ac frequency is readily interpreted on the basis of the dipolar motions in the materials that fail to catch up with the external electric fields. Notably, the dielectric constants of the thermosetting blends were significantly lower than control epoxy; the dielectric constants decreased with the content of PMMA. Similarly, the dielectric loss of the thermosetting blends was also lower than the plain

epoxy. This observation suggests that miscible PMMA could behave as the diluent of the dipoles in the epoxy thermosets. As a result, the dielectric constants and loss were also lower than the control epoxy. In addition, the decreased dielectric loss implies that the motion of dipoles in the blends did not significantly increase with the incorporation of PMMA. For the nanostructured thermosets, the plots of dielectric constants as functions of ac frequency are presented in Figure 11. Notably, all the nanostructured thermosets displayed the dielectric constants significantly higher than the control epoxy; the dielectric constants of the nanostructured thermosets were also higher than the P(VDF-TrFE)-g-PMMA graft copolymer while the content of P(VDF-TrFE)-g-PMMA was 20 wt % or higher. The dielectric constants of the nanostructured thermosets increased with the content of P(VDF-TrFE)-gPMMA. For instance, the dielectric constants of the nanostructured thermoset containing 40 wt % of P(VDF-TrFE)-gPMMA at 103 and 107 Hz were measured to be 10.1 and 8.7, respectively, which were much higher than those of the control epoxy (i.e., 7.7 and 6.7, respectively) and P(VDF-TrFE)-gPMMA (i.e., 9.4 and 8.0, respectively). The enhanced dielectric constants were exclusively attributable to the formation of P(VDF-TrFE) microdomains in the epoxy matrix, since the incorporation of PMMA resulted in the decrease in dielectric constants. It is proposed that the increase in the dielectric constant for the nanostructured thermosets is responsible for (i) the incorporation of a great number of dipoles with the inclusion of P(VDF-TrFE) and (ii) the formation of the interfacial polarization around the P(VDF-TrFE) microdomains. The C−F bonds in P(VDF-TrFE) are typical polar covalent bonds owing to the big difference in electronegativity between carbon and fluorine elements. Compared to the control epoxy, the quantity of the dipoles in the nanostructured thermosets was significantly enhanced owing to the inclusion of P(VDF-TrFE) microdomains; the number of the dipoles increased with the content of P(VDF-TrFE)-g-PMMA. It is 592


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this work, the surface hydrophobicity of the nanostructured thermosets was investigated in terms of static contact angle measurements. The flat and free surfaces of control epoxy and the nanostructured thermosets were prepared for the measurement of the static contact angles with water and ethylene glycol as the probe liquids. The results of the static contact angles together with the surface energy are shown in Figure 12 and

Figure 11. Dielectric constants and dielectric loss tangent of the epoxy thermosets containing P(VDF-TrFE)-g-PMMA at 25 °C as functions of ac frequency. The content of copolymer is 0, 10, 20, 30, 40, and 100 wt %, respectively.

the dipoles from the P(VDF-TrFE) that contribute the increase in dielectric constants. On the other hand, a great amount of interface between epoxy matrix and P(VDF-TrFE) microdomains was created owing to the formation of P(VDF-TrFE) microdomains. At the interfaces, a great number of charges were accumulated under an external electrical field according to Maxwell−Wagner polarization mechanism.63 Therefore, the interfacial polarization significantly contributed to the enhancement of dielectric constants. Also shown in Figure 11 are the plots of dielectric loss as functions of ac frequency for control epoxy and the nanostructured thermosets. It is seen that in the range of the lower (103−104 Hz) and higher (106−107 Hz) frequencies of the ac electric field, the values of the dielectric loss of the nanostructured thermosets were slightly higher than that of the control epoxy. On the one hand, the slightly increased dielectric loss is responsible for the increase in the number of dipoles with the incorporation of P(VDF-TrFE)-g-PMMA. On the other hand, the increased dielectric loss could be associated with the relaxation of the polarization at the interface between epoxy matrix and P(VDF-TrFE) microdomains.63,64 In addition, P(VDF-TrFE) possessed a glass transition temperature of −35 °C,65 which was much lower than those of epoxy. At high frequency of ac electric field, the P(VDF-TrFE) chains in amorphous phase would exhibit the relatively high mobility. It is plausible to propose that those chains in the amorphous region of P(VDF-TrFE) microdomains would also contribute to the slightly high dielectric loss, although the crystallinity of P(VDF-TrFE) microdomains was very high. Nonetheless, the overall dielectric constants of the epoxy thermosets modified with P(VDF-TrFE)-g-PMMA graft copolymer were lower than 0.04, which was still at the quite low level. Surface Hydrophobicity. As a fluorocarbon polymer, P(VDF-TrFE) inherently had low surface energy.65,66 It is expected that the epoxy thermosets modified with P(VDFTrFE) would display the improved surface hydrophobicity. In

Figure 12. Plot of surface water contact angle as a function of the content of P(VDF-TrFE)-g-PMMA copolymer for the thermosets.

summarized in Table 1. For the plain epoxy, the water contact angle was measured to be 80.1°, suggesting that the epoxy was quite hydrophilic. After P(VDF-TrFE)-g-PMMA was incorporated into epoxy, the water contact angles were significantly enhanced. For the nanostructured thermoset containing 40 wt % of P(VDF-TrFE)-g-PMMA, the water contact angle was increased to 94.6°; i.e., the surface of the epoxy thermosets became increasingly hydrophobic. The enhanced water contact angle resulted from the migration and enrichment of the hydrophobic component [viz. P(VDF-TrFE)] onto the surfaces of the thermosets. Nonetheless, the contact angle did not linearly increase with the content of P(VDF-TrFE)-g-PMMA. While the content of P(VDF-TrFE)-g-PMMA was 10 wt % or higher, the increase of the contact angle as a function of the content of P(VDF-TrFE)-g-PMMA was slow, suggesting that P(VDF-TrFE) has been enriched onto the surfaces to saturation. In this work, the surface free energies were calculated according to the geometric mean model:67,68 2 cos θ = [(γLdγsd)1/2 + (γLpγsp)1/2 ] − 1 γL (1) γs = γsd + γsp

(2)

where θ is contact angle and γL is the liquid surface tension; γds and γsp are the dispersive and polar components of γL, respectively. According to the contact angle values with water 593


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Table 1. Static Contact Angles and Surface Free Energy of Epoxy Thermosets Containing P(VDF-TrFE)-g-PMMA Copolymer surface free energy (mN·m−1)

static contact angle (deg) P(VDF-TrFE)-g-PMMA (wt %) 0 10 20 30 40

Θwater 80.1 91.3 92.1 93.1 94.6

± ± ± ± ±

0.7 0.6 0.8 1.1 0.5

θethylene glycol

γds

γps

γs

± ± ± ± ±

23.21 22.75 22.90 22.46 22.28

7.97 3.47 3.17 2.97 2.57

31.18 26.22 26.07 25.43 24.85

53.9 66.1 66.8 68.1 69.7

and ethylene glycol probe liquids, the surface free energy was calculated and summarized in Table 1. Notably, the total surface free energy of the copolymers was reduced from 31.2 to 24.9 mN/m with an increment of the content of P(VDFTrFE)-g-PMMA from 0 to 40 wt %. It is seen that the polar components of the surface tension (viz. γps ) were quite sensitive to the concentration of P(VDF-TrFE)-g-PMMA, implying that P(VDF-TrFE) components have been enriched onto the surfaces of the materials, i.e., P(VDF-TrFE) at the surface of the nanostructured thermosets behaved as the screening agent to promote the surface hydrophobicity.

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CONCLUSIONS

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AUTHOR INFORMATION

REFERENCES

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In summary, poly(methyl methacrylate)-grafted poly(vinylidene fluoride-co-trifluoroethylene) copolymer was successfully synthesized via ATRP approach with poly(vinylidene fluoride-co-chlorotrifluoroethylene) as the macromolecular initiator. It was found that the graft copolymer was readily incorporated into epoxy to obtain the nanostructured thermosets containing P(VDF-TrFE) nanophases. The formation of the nanostructures in epoxy thermosets was indicated by means of transmission electron microscopy. It was found that in the epoxy thermosets the spherical nanophases of P(VDF-TrFE) with a size of 10−100 nm in diameter were dispersed into the continuous epoxy matrix. The results of dynamic mechanical thermal analysis showed that in the nanostructured thermosets the grafted PMMA chains remained mixed with epoxy matrix whereas P(VDF-TrFE) was segregated out of the epoxy matrix. Compared to control epoxy, the nanostructured epoxy thermosets containing P(VDF-TrFE) microdomains displayed the enhanced dielectric constants. The surface contact angle measurements showed that the surface hydrophobicity of the nanostructured thermosets was higher than the plain epoxy thermosets.

Corresponding Author

*E-mail: [email protected]. Tel: 86-21-54743278. Fax: 86-2154741297. Notes

The authors declare no competing financial interest.

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0.2 0.4 1.0 0.6 0.5

ACKNOWLEDGMENTS

The financial support from Natural Science Foundation of China (Grants 51133003, 21274091, and 21304058) are gratefully acknowledged. The authors thank the Shanghai Synchrotron Radiation Facility for support under Project 13SRBL16B14042. 594


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Nanostructured Epoxy Thermosets Containing Poly(vinylidene fluoride

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