Organic–Inorganic Nanocomposites via Self-Assembly of an

Article pubs.acs.org/JPCB

Organic−Inorganic Nanocomposites via Self-Assembly of an Amphiphilic Triblock Copolymer Bearing a Poly(butadiene‑g‑POSS) Subchain in Epoxy Thermosets: Morphologies, Surface Hydrophobicity, and Dielectric Properties Wenjun Peng, Sen Xu, Lei Li, Chongyin Zhang, and Sixun Zheng* Department of Polymer Science and Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ABSTRACT: Organic−inorganic nanocomposites composed of polyhedral oligomeric silsesquioxane (POSS) and epoxy resin were prepared via self-assembly of an amphiphilic triblock copolymer bearing a poly(POSS) midblock in epoxy thermosets. First, this organic−inorganic amphiphilic triblock copolymer was synthesized via hydrosilylation of heptaphenylhydro POSS with an existing triblock copolymer containing a short polybutadiene midblock. It was found that this novel amphiphilic block copolymer can self-assemble into nanophases in epoxy thermosets. In the presence of preformed nanophases, the curing reaction was performed, and the organic−inorganic nanocomposites containing poly(POSS) microdomains were thus obtained. Compared with plain epoxy, the as-obtained thermosets exhibited enhanced surface hydrophobicity; the enhanced surface hydrophobicity is attributed to enrichment of the POSS component at the surface of the materials. Owing to the formation of poly(POSS) microdomains, the dielectric constants of the materials significantly reduced, whereas the dielectric loss remained almost unchanged.

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INTRODUCTION

subchains, whereas block copolymers containing inorganic subchains were seldom employed. Epoxy polymers are a class of important thermosets that can be applied as a matrix of composites, electronic encapsulation materials, adhesives, and coatings.2,7,8 Recently, it was found that epoxy thermosets can even display some functional properties by incorporating block copolymers bearing optoelectronic subchains via the formation of nanostructures.9−12 As a class of important inorganic building blocks, polyhedral oligomeric silsesquioxanes (POSS) have been widely used to prepare organic−inorganic epoxy composites.13−15 It has been reported that several reactive POSS, bearing amino, epoxide, and silanol groups, have been introduced into epoxy to obtain nanostructured thermosets.16−26 In these multicomponent systems, chemical linkage between the epoxy networks and POSS is very important for the formation of nanostructures. With chemical linkage, POSS macromers can be dispersed in the epoxy network in the form of single cages and/or POSS microdomains of a couple of nanometers.16−26 While utilizing a functional (or reactive) POSS macromer, we must take into account the competitive kinetics of the intercomponent reactions, macroscopic phase separation, and polymerization. From a viewpoint of a materials process, the physical blending

Incorporating amphiphilic block copolymers into thermosets has been demonstrated to be one of the most effective approaches to accessing nanostructured thermosets. The formation of a nanostructure endows materials with improved properties.1,2 Using an amphiphilic block copolymer, Bates et al.3,4 first reported the formation of a nanostructure in epoxy thermosets via the self-assembly approach. In this approach, the amphiphilic block copolymer can self-organize into nanoobjects in epoxy precursors. With the processing of the curing reaction, the preformed nano-objects can be fixed. Recently, Zheng et al.5,6 reported that nanostructured thermosets can be alternatively obtained via a reaction-induced microphase separation mechanism. In this mechanism, no self-assembly behavior is observed before the curing reaction. The nanostructure will be generated while the curing reaction is carried out at a very high conversion. In the past few years, considerable research has been carried out to understand the correlation of the morphologies with the properties of nanostructured thermosets containing a variety of block copolymers. It has been realized that the architecture, composition, and concentration of amphiphilic block copolymers are critical in affecting the nanostructures of the thermosets.2 Nonetheless, in the previous work, most block copolymers used for modulating the nanostructures of thermosets were assemblies of some organic polymer © 2016 American Chemical Society

Received: August 9, 2016 Revised: October 26, 2016 Published: November 11, 2016 12003

DOI: 10.1021/acs.jpcb.6b08026 J. Phys. Chem. B 2016, 120, 12003−12014

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The Journal of Physical Chemistry B

methanol, tetrahydrofuran (THF), toluene, and petroleum ether, were purchased from Shanghai Reagent Co., China. Synthesis of POSS-H. First, heptaphenyltricycloheptasiloxane trisodiumsilanolate [Na3O12Si7(C6H5)7] was synthesized according to the reported method.34 To a 500 mL flask, phenyltrimethoxysilane (45.540 g, 250 mol), NaOH (3.959 g, 99.0 mmol), THF (250 mL), and deionized water (5.259 g, 292 mmol) were charged. The mixture was heated and allowed to reflux for 5 h at an elevated temperature. Thereafter, the system was cooled to room temperature, and the reaction was further carried out for 15 h. After removing the solvents, the product (30.080 g) was obtained at a yield of 93.1%. Second, the obtained Na3O12Si7(C6H5)7 (30.080 g) was dispersed in anhydrous THF (400 mL) at 0 °C. Thereafter, trichlorosilane (5.900 g, 35.54 mmol) was injected into the suspension with a syringe. The reaction was performed at 0 °C for 1 h and at room temperature for 24 h. After the insoluble components were filtered out, the solvent in the filtrate was evaporated. The crude product was washed with methanol (100 mL) and recrystallized in a mixture of petroleum ether with THF (1:2 vol). After purification, the resulting product (viz., POSS-H) (11.80 g) was obtained at a yield of 62.7%. 1H NMR (CDCl3, ppm): 7.35−7.85 (m, 35H, −C6H5) and 4.5 (s, 1H, −SiH). Fourier transform infrared spectroscopy (FTIR) (cm−1, KBr window): 2262 (Si−H), 1000−1100 (Si−O−Si). Synthesis of Triblock Copolymer PCL-b-P(B-g-POSS)b-PCL. First, PCL-b-PB-b-PCL was synthesized via a ringopening polymerization (ROP) approach. To a fume-dried flask, HO−PB−OH (0.999 g, 0.285 mmol with respect to the hydroxyl groups) and anhydrous toluene (30 mL) were added; toluene was distilled to remove traces of moisture from HO− PB−OH. Thereafter, CL (7.992 g, 70.04 mmol) was added. The flask was degassed via three pump−freeze−thaw cycles and Sn(Oct)2 (24 μL) was injected into the flask with a syringe. ROP was performed at 120 °C for 48 h. After purification, PCL-b-PB-b-PCL (8.820 g) was obtained with a conversion of CL of 98%. FTIR (cm−1, KBr window): 3440 (O−H), 2710− 3100 (C−H), and 1725 (>CO). GPC: Mn = 1.478 × 104 Da, with Mw/Mn = 1.38. Second, the obtained PCL-b-PB-b-PCL (1.950 g, 7.302 mmol with respect to the CC double bonds in the PB block) and POSS-H (4.650 g, 4.859 mmol with respect to the Si−H bond) were dissolved in anhydrous toluene (45 mL). Thereafter, the system was purged with highly pure nitrogen for 45 min, and then, the Karstedt catalyst (400 μL) was added. The reaction was carried out at 90 °C for 48 h. The reacted mixture was subjected to rotary evaporation and then added to 100 mL petroleum ether to obtain the precipitates. After drying, the block copolymer [i.e., PCL-b-P(B-g-POSS)-b-PCL] (3.65 g) was obtained at a yield of 62.7%. FTIR (cm−1, KBr window): 3440 (O−H), 2710−3100 (C−H), 1725 (>CO), and 880− 1330 (Si−O−Si). GPC: Mn = 8.10 × 104 Da with Mw/Mn = 2.39. Preparation of Epoxy Thermosets. A desired amount of PCL-b-P(B-g-POSS)-b-PCL was mixed with DGEBA. Then, the curing agent MOCA, with a molar ratio of amine groups to epoxide of DGEBA of 1:2, was added to the mixture while stirring for 30 min. The ternary mixture was stirred until it was clear and homogenous. It was then transferred to molds and cured at 150 °C for 3 h and 180 °C for 2 h. Measurement and Characterization. 1H NMR spectroscopy was conducted on a Varian Mercury Plus 400 MHz NMR spectrometer. GPC measurements were conducted on a Waters

approach is simple and attractive. However, simple physical blending is less successful as macroscopic phase separation would occur. It was found that this issue could be solved by using POSS-containing amphiphiles. Zheng et al.27−29 recently reported that POSS nanodomains were successfully formed in epoxy on using POSS-containing polymer telechelics via the self-assembly approach. Nonetheless, it is difficult to change the quantity of POSS microdomains in the thermosets if only POSS-capped polymer telechelics are used toward this end, as the portion of POSS at the ends of chains is too small. It is suggested that POSS microdomains can be generated in the thermosets when amphiphilic block copolymers containing POSS subchains are used. This case closely resembles the formation of nanophases in thermosets containing organic amphiphilic block copolymers.1−6 Recently, Gérard and Zheng30,31 et al. reported on several block copolymers bearing poly(POSS) blocks. Upon introduction of these block copolymers, POSS microdomains would be generated in the thermosets, with sizes of up to 30 nm. Nonetheless, these previous works have mainly focused on the synthesis of block copolymers bearing poly(POSS) subchains. The formation mechanism of POSS microdomains together with the thermomechanical properties of the materials has not been investigated in depth. In the present work, we aimed at investigating the selfassembly behavior of an organic−inorganic block copolymer bearing poly(POSS) subchains in epoxy thermosets. First, we synthesized poly(ε-caprolactone)-block-poly(butadiene-gPOSS)-block-poly(ε-caprolactone) [denoted PCL-b-P(B-gPOSS)-b-PCL], a novel organic−inorganic amphiphilic triblock copolymer, via hydrosilylation of an existing PCL-b-PB-b-PCL triblock copolymer with heptaphenylhydro polyhedral oligomeric silsesquioxane (POSS-H). The design and synthesis of this triblock copolymer are based on the knowledge that the PCL endblock is miscible with epoxy,32,33 whereas the P(B-gPOSS) midblock is immiscible with epoxy. Thereafter, this organic−inorganic amphiphilic triblock copolymer bearing poly(POSS) subchains was incorporated into epoxy to obtain nanostructured thermosets. The correlation of the nanostructures with the thermomechanical properties has been investigated on the basis of morphological observations, measurements of dynamic mechanical properties, surface hydrophobicity, and dielectric properties of the thermosets.

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EXPERIMENTAL SECTION Materials. Phenyltrimethoxysilane was purchased from Zhejiang Institute of Chemical Engineering, Hangzhou, China. ε-Caprolactone (CL) was purchased from Fluka Co., Germany. Diglycidyl ether of bisphenol A (DGEBA) with a quoted epoxide equivalence of 180−210 g mol−1 was supplied by Shanghai Resin Co., China. α,ω-Dihydroxyl-terminated polybutadiene (HO−PB−OH) was supplied by Qilong Chemical Co., Shandong, China; it had a quoted hydroxyl value of 0.77 mmol g−1, from which the molecular weight was calculated to be Mn = 2600 Da. Gel permeation chromatography (GPC) measurements gave a molecular weight of Mn = 4400 Da, with Mw/Mn = 1.52. 1H NMR spectroscopy showed that the contents of 1,2- and 1,4- addition structural units were 80 and 20 mol %, respectively. Trichlorosilane, stannous octanoate [Sn(Oct)2], and the Karstedt catalyst were purchased from Aldrich Co., China. Sodium hydroxide; 4,4′-methylenebis(2-chloroaniline) (MOCA); and organic solvents, such as 12004

DOI: 10.1021/acs.jpcb.6b08026 J. Phys. Chem. B 2016, 120, 12003−12014

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The Journal of Physical Chemistry B Scheme 1. Synthesis of Triblock Copolymer PCL-b-P(B-g-POSS)-b-PCL

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RESULTS AND DISCUSSION Synthesis of Triblock Copolymer PCL-b-P(B-g-POSS)b-PCL. The synthesis of PCL-b-P(B-g-POSS)-b-PCL is depicted in Scheme 1. First, POSS-H was prepared via the silylation reaction of trichlorosilane with heptaphenyltricycloheptasiloxane trisodiumsilanolate [Na3O12Si7(C6H5)7].34 Second, PCL-b-PB-b-PCL was synthesized via ROP of CL, with HO−PB−OH as the macromolecular initiator; the mass fraction of the PB block was controlled to be about 10%. Finally, the hydrosilylation reaction of POSS-H with PCL-bPB-b-PCL was carried out, affording a new triblock copolymer, PCL-b-P(B-g-POSS)-b-PCL. Shown in Figure 1 are the 1H NMR spectra of POSS-H, HO−PB−OH, PCL-b-PB-b-PCL, and PCL-b-P(B-g-POSS)-b-PCL. For POSS-H, the resonance signals at 4.5 and 7.35−7.85 ppm are assignable to the proton of the Si−H bond and the protons of the phenyl groups, respectively. For HO−PB−OH, the resonance peaks at 1.10− 1.60 and 2.04 ppm are assignable to the protons of the ethylene and methine groups, respectively. The proton resonance signals at 4.80−5.80 ppm are attributed to the CC double bonds of the methylene and methine groups. The resonance signals for the hydroxymethyl protons at the ends of HO−PB−OH occurred at 4.10 ppm. In terms of the integral intensities of proton resonance at 4.97 and 5.40 ppm, the percentage of 1,2addition structural units in the PB was calculated to be about 20 mol %. Compared to those for HO−PB−OH, new signals at 1.40, 1.65, 2.31, and 4.06 ppm appeared in the 1H NMR spectrum of PCL-b-PB-b-PCL. These new signals are attributed to the resonance of the protons of the ethylene groups in the backbone of PCL. The minor resonance peak at 3.65 ppm is

717 system equipped with a Waters 2414 refractive index detector and two Waters RH columns; THF was used as the eluent. The FTIR spectra of the samples were recorded on a Perkin-Elmer Paragon 1000 FTIR spectrometer at room temperature. Differential scanning calorimetry (DSC) was carried out on a TA Q2000 apparatus in a highly pure nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed on a TA Q5000 TGA apparatus in an air or nitrogen atmosphere. Small-angle X-ray scattering (SAXS) experiments were conducted on the BL16B station of the Shanghai Synchrotron Radiation Facility, as detailed elsewhere.9 For the experiments at an elevated temperature, a Linkam hot stage (THMS600) was mounted. Dynamic mechanical thermal analysis (DMTA) was performed on a TA Q800 apparatus. The rectangular specimens, with dimensions of 1.75 × 5 × 25 mm3, were measured in a tensile mode. The morphologies of the samples were observed on a JEOL JEL-2010 microscope operating at 120 kV. The thermosetting samples were sliced into sections with thicknesses of about 70 nm. The samples were observed without any staining. Contact angle measurements were performed on a DSA30 apparatus at room temperature. In the measurements, the drop sizes and amount of probe liquids were controlled to be 1.8 mm and 3.0 μL, respectively. X-ray photoelectron spectroscopy (XPS) was conducted on an AXIS UltraDLD system with Mg and Al anode radiations. The X-ray anode was run at hν = 1253.6 eV and at a voltage of 14.0 kV, with an inclination angle of 5°. Dielectric measurements were performed on a Microtest Precision 6630 impedance analyzer, as detailed elsewhere.9,10 12005

DOI: 10.1021/acs.jpcb.6b08026 J. Phys. Chem. B 2016, 120, 12003−12014

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Figure 1. 1H NMR spectra of POSS-H, HO−PB−OH, PCL-b-PB-b-PCL, and PCL-b-P(B-g-POSS)-b-PCL in CDCl3.

assignable to the hydroxymethyl groups at the ends of the PCL blocks. According to the integral intensities of the resonances at 3.65 and 4.06 ppm, the length, LPCL, of the PCL blocks was estimated to be 5200 Da. With the occurrence of hydrosilylation between POSS-H and PCL-b-PB-b-PCL, the signals assignable to the resonance of the methine protons in phenyl groups appeared in the range of 7.0−8.0 ppm, indicating that POSS cages were grafted onto the PB block of the triblock copolymer. Notably, the signals at 4.96 ppm, assignable to the proton resonance of the 1,2-addition methylene groups, almost disappeared, suggesting that the hydrosilylation dominantly occurred on the vinyl groups of the 1,2-addition structural units. In the meantime, we examined the change in the integral intensity of proton resonance at δ = 5.40 ppm, which was attributable to the double bonds of the methine groups of the main chains, by taking the integral intensity of the methylene of the PCL main chains (e.g., at δ = 4.10 ppm) as the internal reference. It was found that the intensity of resonance at δ =

5.40 ppm was decreased by 40% after hydrosilylation. This result indicates that the hydrosilylation reaction also occurred between POSS-H and the main chain of PB block. Iraqi et al.35 investigated the hydrosilylation behavior of polybutadiene with a few low-molecular organosilanes; they observed that there were high conversions of double bonds pendant from the main chains of polybutadiene, but hydrosilylation also occurred on the double bonds of the backbone of the polymer. Our results are in accordance with those of Iraqi et al.35 Herewith, we estimated the overall fraction of hydrosilylated double bonds to be about 60 wt %, according to the ratio of 1,2- to 1,4-addition structural units in the PB midblock (i.e., 20 mol % of 1,2addition structures) according to 1H NMR spectroscopy. The results from 1H NMR spectroscopy indicate that POSS cages have been successfully grafted on the midblock of PCL-b-PB-bPCL by the hydrosilylation reaction. HO−PB−OH, PCL-b-PB-b-PCL, and PCL-b-P(B-g-POSS)b-PCL were subjected to GPC; the GPC curves are presented 12006

DOI: 10.1021/acs.jpcb.6b08026 J. Phys. Chem. B 2016, 120, 12003−12014

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Figure 2. GPC curves of HO−PB−OH, PCL-b-PB-b-PCL, and PCLb-P(B-g-POSS)-b-PCL in THF.

Figure 4. TGA curves of HO−PB−OH, PCL-b-PB-b-PCL, and PCLb-P(B-g-POSS)-b-PCL; the heating rate is 20 °C min−1.

the distribution of the molecular weight of PCL-b-P(B-gPOSS)-b-PCL became fairly broad in comparison with that of HO−PB−OH and PCL-b-PB-b-PCL. This result was proposed to be associated with the strong interaction between the P(B-gPOSS) block and the chromatography columns, which caused prolonged retention of the organic−inorganic block copolymer in the columns and thus broadened GPC profiles. Both PCL-b-PB-b-PCL and PCL-b-P(B-g-POSS)-b-PCL were subjected to DSC measurements, and the DSC curves are shown in Figure 3. In all cases, endothermic and exothermic peaks appeared in the heating and cooling DSC curves, which are attributable to the melting and crystallization transitions of the PCL blocks in these two-block copolymers, respectively. Under identical thermal treatment, the melting temperature (Tm = 56 °C) of the PCL block in PCL-b-PB-b-PCL was significantly higher than that (Tm = 50 °C) of the PCL block in PCL-b-P(B-g-POSS)-b-PCL. The depressed melting point is accounted for by the increased confinement of P(B-g-POSS) microdomains on crystallization of PCL. The confinement resulted from the increase in the mass fraction of the midblock [viz., P(B-g-POSS)] due to the occurrence of the hydrosilylation reaction. For PCL-b-PB-b-PCL, the mass fraction of PCL was calculated to be f PCL = 0.8 and the PCL microdomains in the triblock copolymer remained continuous. Therefore, no confinement was exerted on crystallization of PCL. With the occurrence of hydrosilylation, the mass fraction of the midblock (viz., PB-g-POSS) was increased up to 0.87, which was much higher than that of the PCL block (viz., 0.13). In the latter case, PCL became the isolated microdomains. It is known that the crystallization behavior of crystalline subchains is closely related to the mechanical states, intermicrodomain connectivity, and Tg of the matrix.36−39 In this work, the nonconnectivity of PCL microdomains in the PCL-b-P(B-gPOSS)-b-PCL triblock constituted the environment of confinement on crystallization of PCL. The confinement can be further evidenced by the fact that the PCL blocks in PCL-b-P(B-g-

Figure 3. DSC curves of PCL-b-PB-b-PCL and PCL-b-P(B-g-POSS)b-PCL, the heating and cooling rates were 20 and 10 °C min−1, respectively.

in Figure 2. Notably, all samples exhibited a unimodal distribution of molecular weights. The molecular weight of HO−PB−OH was measured to be Mn = 4.400 × 103 Da, with Mw/Mn = 1.52. After the ROP of CL, with HO−PB−OH as the macromolecular initiator, the molecular weight was increased to Mn = 1.480 × 104 Da, with Mw/Mn = 1.38. For PCL-b-P(B-gPOSS)-b-PCL, the molecular weight was further increased to Mn = 8.10 × 104 Da, with Mw/Mn = 2.39. It is noteworthy that 12007

DOI: 10.1021/acs.jpcb.6b08026 J. Phys. Chem. B 2016, 120, 12003−12014

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The Journal of Physical Chemistry B

Figure 5. TEM images of the organic−inorganic nanocomposites containing (A) 10, (B) 20, (C) 30, and (D) 40 wt % of PCL-b-P(B-g-POSS)-bPCL.

Self-Assembly Behavior of PCL-b-P(B-g-POSS)-b-PCL in Epoxy Thermosets. The triblock copolymer PCL-b-P(B-gPOSS)-b-PCL was incorporated into epoxy to obtain organic− inorganic nanocomposites. All nanocomposites were homogenous and transparent, suggesting that macroscopic phase separation did not occur in the composite system. The morphologies of the nanocomposites were observed by TEM. Shown in Figure 5 are the TEM images of the materials containing various contents of PCL-b-P(B-g-POSS)-b-PCL. All composites displayed microphase-separated morphologies. Notably, some nonspherical microdomains, with sizes of 20− 50 nm, were dispersed into the continuous matrix; the quantity of the microdomains increased with the content of PCL-b-P(Bg-POSS)-b-PCL. According to the difference in electron density between the organosilicon domains and epoxy matrix, it is judged that the dark features are assignable to the P(B-g-POSS) microdomains, via POSS−POSS interactions,23,24 whereas the light features are assignable to the epoxy matrices. Notably, the nonspherical P(B-g-POSS) microdomains were in contrast to the spherical microdomains generally formed in epoxy thermosets containing organic block copolymers. The morphological features could result from the different viscoelastic properties of the poly(POSS) subchains of the organic polymers during the process of microdomain formation. The above microphase-separated morphologies were further evidenced by the results from SAXS. The SAXS profiles of the

POSS)-b-PCL displayed a much lower temperature of crystallization (Tc) than that in PCL-b-PB-b-PCL in the cooling scans of DSC (see Figure 3). HO−PB−OH, PCL-b-PB-b-PCL, and PCL-b-P(B-g-POSS)b-PCL were subjected to TGA measurements in an air atmosphere; TGA curves of the samples are shown in Figure 4. For HO−PB−OH, the initial temperature of degradation was measured to be Td = 385 °C; this polymer was completely decomposed at ca. 630 °C. For PCL-b-PB-b-PCL, a two-step degradation was observed, at 300 and 480 °C. The first step of degradation is responsible for the endblocks of the triblock copolymer (viz., PCL), whereas the second step of degradation is responsible for the midblock (i.e., PB). The PCL blocks had a thermal stability lower than that of the PB blocks. It is noted that the degradation temperature of the first step was significantly increased to ca. 380 °C after the POSS cages were grafted onto the PB blocks, although the PCL endblocks remained unchanged. It is proposed that the mass loss due to the formation of gaseous fragments due to segmental decomposition could be significantly suppressed by the inorganic P(B-g-POSS) microdomains, which was just like the case in which inorganic layered silicates were well dispersed in organic matrices.40,41 The formation of the P(B-g-POSS) midblock resulted in the yield of the degradation residue increasing up to 24.7 wt %, as the midblock [viz., P(B-gPOSS)] is transformed into silica at an elevated temperature. 12008

DOI: 10.1021/acs.jpcb.6b08026 J. Phys. Chem. B 2016, 120, 12003−12014

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Figure 8. DMTA curves of the organic−inorganic nanocomposites; the heating rate and frequency were 3 °C min−1 and 1.0 Hz, respectively. Figure 6. SAXS curves of the organic−inorganic nanocomposites.

Figure 9. TGA curves of the organic−inorganic nanocomposites.

Notably, the position of the scattering peaks increasingly shifted to high q values with an increase in the content of PCL-b-P(Bg-POSS)-b-PCL, suggesting that the average distance between adjacent P(B-g-POSS) microdomains decreased. The SAXS experiments confirmed the results of TEM. In terms of the miscibility of PCL and P(B-g-POSS) with epoxy, it is judged that the formation of P(B-g-POSS) microdomains would follow a self-assembly mechanism. To confirm this mechanism, the ternary mixtures [viz., DGEBA, MOCA, and PCL-b-P(B-g-POSS)-b-PCL] were subjected to SAXS measurements at room and curing temperatures.

Figure 7. SAXS profiles of the epoxy thermosets containing 40% PCLb-P(B-g-POSS)-b-PCL triblock copolymers: (A) at 25 °C, (B) at 150 °C, and (C) after curing.

nanocomposites are shown in Figure 6. Each nanocomposite displayed a single broad scattering peak, the intensity of which increased with an increase in the content of the triblock copolymer. The broad peaks resulted from the form factor scattering of these nonspherical P(B-g-POSS) microdomains. 12009

DOI: 10.1021/acs.jpcb.6b08026 J. Phys. Chem. B 2016, 120, 12003−12014

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Figure 10. Dielectric constants and dielectric losses of the organic− inorganic nanocomposites. Figure 12. Static contact angles of the organic−inorganic nanocomposites.

assembly mechanism. It is noteworthy that upon heating the mixture to the curing temperature the scattering peak shifted to a lower q position (qm = 0.19 nm−1). This observation suggests that the average distance between adjacent P(B-g-POSS) microdomains increased when the mixture was heated to an elevated temperature. The increased intermicrodomain distance could reflect a decrease in the size of the microdomains due to the extraction of the solvent (viz., DGEBA + MOCA) from the swollen microdomains at room temperature. Notably, the SAXS profile of the mixture at the beginning of the curing reaction was quite close to that of the mixture cured at 150 °C for 5 h, indicating that the curing reaction did not alter the preformed microphase-separated morphologies. It is judged that in the organic−inorganic nanocomposites, P(B-g-POSS) blocks were segregated from the epoxy network to form dispersed microdomains, whereas PCL blocks remained miscible in the epoxy matrix. This microphase behavior can be evidenced by DMTA. The DMTA spectra of plain epoxy and the nanocomposites containing P(B-g-POSS) microdomains are shown in Figure 8. For plain epoxy, the sharp α peak was exhibited at 159 °C, and it is assignable to the Tg of the cross-linked network. In addition, plain epoxy displayed secondary transitions at 70 and −56 °C. They are attributable to the motions of diphenyl and hydroxyether structural units, respectively, in the aromatic amine-crosslinked bisphenol-Atype epoxy.42,43 Upon incorporating the triblock copolymer into epoxy, the α transitions shifted to a lower temperature; the Tg’s of the epoxy matrices decreased, as the content of PCL-bP(B-g-POSS)-b-PCL increased. The decreased Tg’s are attributable to the intimate mixing of the PCL blocks with the epoxy network. It has been recognized that the miscibility of PCL with the epoxy network resulted from intercomponent hydrogen-bonding interactions.32,33 It should be pointed out that over the range of temperatures the Tg’s of the P(B-g-

Figure 11. Dielectric constants and dielectric losses of the organic− inorganic nanocomposites.

Representatively shown in Figure 7 are the SAXS curves of the mixture containing 30 wt % PCL-b-P(B-g-POSS)-b-PCL. At room temperature, a scattering peak appeared at qm = 0.23 nm−1, indicating the formation of a nanostructure. Upon increasing the temperature to 150 °C, notably, the scattering peak still existed, suggesting that the as-formed nano-objects survived at an elevated temperature. This observation indicates that the curing reaction of the thermosetting system occurred in the presence of the preformed P(B-g-POSS) microdomains. Therefore, it is concluded that the formation of P(B-g-POSS) microdomains in the thermosetting matrix followed the self12010

DOI: 10.1021/acs.jpcb.6b08026 J. Phys. Chem. B 2016, 120, 12003−12014

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The Journal of Physical Chemistry B Table 1. Contact Angles and Surface Free Energy of Organic−Inorganic Nanocompositesa,b surface free energy (mN m−1)

static contact angle (deg) θH2O

PCL-b-P(B-g-POSS)-b-PCL (wt %) 0 10 20 30 40 a

85.2 95.8 97.9 98.6 99.4

± 0.7 ± 0.6 ± 0.8 ± 1.1 ±0.5

θethylene glycol

γds

γps

γs

± ± ± ± ±

25.5 21.93 22.25 22.95 20.67

4.9 2.31 1.72 1.45 1.67

30.5 24.23 23.97 24.4 22.34

57.9 71.1 72.9 73.1 75.4

1.0 0.4 1.0 0.6 0.5

H2O: γL = 72.80 mN m−1, γdL = 21.80 mN m−1, γpL = 51.00 mN m−1. bEthylene glycol: γL = 48.3 mN m−1, γdL = 29.3 mN m−1, γpL = 19.0 mN m−1.

that of plain epoxy. Upon incorporating PCL-b-P(B-g-POSS)b-PCL into epoxy, the initial degradation temperatures (Td’s) remained almost unchanged or decreased slightly. However, the yields of the degradation residues were significantly enhanced, increasing with an increase in the percentage of PCL-b-P(B-gPOSS)-b-PCL. In terms of the compositions of the nanocomposites, it is judged that the degradation residues mainly contained char and ceramics, resulting from the oxidation of the silsesquioxane cages. Compared to that of plain epoxy, the improved thermal stability is attributable to the formation of P(B-g-POSS) microdomains. It is proposed that the mass loss from gaseous fragments due to segmental decomposition was suppressed with fine dispersion of the P(B-g-POSS) microdomains in the epoxy matrix. Similar cases were also reported in the organic−inorganic nanocomposites, in which inorganic layered silicates were fully exfoliated.40,41 Dielectric Properties. The nanocomposites were subjected to dielectric measurements to investigate the effect of the P(Bg-POSS) microdomains on the dielectric properties of the epoxy thermosets. The dielectric properties of the nanocomposites are determined from the following components: (1) the PCL subchains, (2) the P(B-g-POSS) microdomains, and (3) the epoxy networks. Among them, PCL remained mixed with the epoxy network and constituted the matrix, whereas the P(B-g-POSS) subchains formed microdomains in the epoxy matrix. To isolate the effect of the POSS microdomains, we measured the dielectric properties of the thermosets composed of epoxy and PCL-b-PB-b-PCL. Shown in Figure 10 are plots of the dielectric constants and dielectric losses of the thermosets in the alternating current (AC) frequency range of 103−106 Hz. The dielectric constants of plain epoxy were detected to be 6.1 at an AC frequency of 103 Hz and 5.9 at 106 Hz. As expected, the dielectric constant of the epoxy decreased with an increase in the AC frequency. This phenomenon can be explained by the fact that establishment of effective polar polarization did not catch up with the increase in the frequency, which is typical of dipolar polarization behavior. In other words, dipolar polarization is dominant for plain epoxy. When PCL-b-PB-b-PCL was incorporated into the epoxy, the dielectric constants and losses remained unchanged or decreased slightly. This phenomenon suggests that the

Figure 13. XPS spectrum of the epoxy thermosets containing 10 wt % PCL-b-P(B-g-POSS)-b-PCL.

POSS) microdomains were not detectable, possibly due to the feeble transition or very high Tg value. Thermomechanical Properties of the Nanostructured Thermosets. Thermal Properties. The above nanocomposites were subjected to TGA to investigate their thermal stabilities (Figure 9). The degradation profiles of the nanocomposites were similar to those of plain epoxy, implying that inclusion of the triblock copolymer did not significantly change the thermal degradation mechanism. For plain epoxy, initial degradation occurred at ca. 397 °C (viz., Td = 397 °C) and the yield of char was measured to be 19.8% at 800 °C. For PCL-b-P(B-g-POSS)b-PCL, initial degradation occurred at Td = 382 °C. Nonetheless, PCL-b-P(B-g-POSS)-b-PCL displayed a yield of degradation of 48% at 800 °C, which was much higher than

Table 2. Elemental Compositions of the Surfaces of Organic−Inorganic Nanocomposites Determined with XPS experimental composition (mol %)

theoretical composition (mol %)

PCL-b-P(B-g-POSS)-b-PCL (wt %)

C

O

Si

Cl

N

C

O

Si

Cl

N

0 10 20 30 40

78.00 69.28 68.62 68.85 66.37

18.14 17.05 16.58 17.02 18.43

0.00 12.27 13.72 13.35 13.74

1.56 0.63 0.47 0.32 0.70

2.30 0.77 0.60 0.47 0.76

83.50 81.71 79.83 77.85 75.72

10.86 11.74 12.67 13.64 14.69

0.00 0.84 1.73 2.67 3.67

2.82 2.61 2.39 2.15 1.90

2.82 2.61 2.39 2.15 1.90

12011

DOI: 10.1021/acs.jpcb.6b08026 J. Phys. Chem. B 2016, 120, 12003−12014

Article

The Journal of Physical Chemistry B concentration of the dipoles remained almost unchanged or decreased slightly on inclusion of PCL-b-PB-b-PCL. In contrast to the thermosets containing PCL-b-PB-b-PCL, the organic−inorganic thermosets containing P(B-g-POSS) microdomains had significantly lower dielectric constants, as shown in Figure 11. The dielectric constants of the nanocomposites decreased with an increase in the percentage of triblock copolymer PCL-b-P(B-g-POSS)-b-PCL. At 40 wt % PCL-b-P(B-g-POSS)-b-PCL, the dielectric constants were decreased to 5.2 and 4.5 at 103 and 106 Hz, respectively. It is proposed that the decreased dielectric constants are attributed to the following factors: (1) inclusion of a component with a low dielectric constant (viz., POSS) and (2) restriction of the motion of the dipoles in the thermosets by P(B-g-POSS) microdomains. The POSS components displayed low dielectric constants due to the low polarities of the silsesquioxane moieties and their cage-like hollow structures. It is known that the P(B-g-POSS) blocks existed in the form of microdomains. As the nanoreinforcement agents, these P(B-g-POSS) microdomains could restrict the motion of the dipoles and thus impose a restriction on their polarization. The above factors can result in decreased dielectric constants and dielectric losses. Decreased dielectric constants are important for the application of epoxy thermosets as electronic encapsulation materials, such as inductors, capacitors, and resistors. Low dielectric constants and dielectric losses are also important for integration and miniaturization of devices, high information transfer rates, and crosstalk reduction.44,45 Notably, the dielectric loss of the nanocomposites increased slightly in the AC frequency range of 103−104 Hz; thereafter, the dielectric loss remained almost unchanged, irrespective of the mass fraction of PCL-b-P(B-gPOSS)-b-PCL. The increased dielectric loss in the AC frequency range of 103−104 Hz could have resulted from the increased mobility of the dipoles in the epoxy thermosets owing to the plasticization of the PCL chains. Nonetheless, the overall dielectric loss of the organic−inorganic nanocomposites remained at a considerably low level (