A spontaneous approach to prepare damping structural integration

Article pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Spontaneous Approach To Prepare Damping Structural Integration Materials via Gradient Plasticization Mechanism at Nanometer Scale Zhengguang Heng,† Muxuan Li,† Yi Li,‡ Yang Chen,*,† Huawei Zou,*,† and Mei Liang† †

The State Key Lab of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China System Engineering Institute of Sichuan Aerospace, Chengdu 610065, China

‡

ABSTRACT: The gradient phenomenon is commonly seen in nature, and the gradual changes in structure could make materials with unique excellent performances compared with homogeneous materials. In this study, a novel approach to prepare damping structural integration epoxy thermosets has been developed. The damping mechanism was systematically discussed through TEM, SAXS, DSC, and DMA. It is shown that the damping property was obviously improved via the gradient plasticization at nanometer scale. Dynamic mechanical analysis showed that when the content of poly(ε-caprolactone)-b-polystyrene was 40 wt %, the glass transition temperature was decreased from 171.66 to 107.84 °C while the damping temperature range (tan δ > 0.2) of the composites was broadened from 28.02 to 52.95 °C. Meanwhile, the tensile properties of epoxy thermosets were also improved when the concentration of poly(ε-caprolactone)-b-polystyrene diblock copolymer was below 20 wt %. It is expected this method could provide a new idea to fabricate damping structural integration materials in the future.

1. INTRODUCTION Vibrations are undesirable in many systems, because they would cause problems that range from noise to fatigue failure, which not only limit the service duration of machine but also are undesirable for human health. Therefore, damping materials are finding numerous applications in many fields, such as the automobile industry, architecture, naval vessel industry, aerospace industry, electronic mechanical equipment, and daily life.1 Recently, with the development of the science and technology, the requirement of damping materials is transforming to versatile and lightweight, and the demand of composites that possess superior damping ability and high mechanical properties is becoming larger and larger. It is therefore desirable to develop damping structural integration materials with not only high damping but also high strength and modulus. Epoxy (EP) with excellent physical and mechanical performance, chemical resistance, processability, and thermal stability is a kind of attractive thermosets matrices, which is widely applied in aerospace, electronic devices, automobiles, coatings, adhesives, etc. However, because of their high crosslinking density, epoxy thermosets are inherently fragile and their damping property is very poor, which would inevitably limit their application. Various techniques have been studied to improve the damping properties of materials. Among them, additive damping is an effective method. Damping properties can be improved either by inserting damping layers between © XXXX American Chemical Society

composite plies or by adding damping particles into the resin. Generally, incorporating soft rubber in resin is helpful for increasing damping properties while it will significantly sacrifice mechanical properties of the composites.2,3 Several functional organic or inorganic particles were also hybridized in the polymer matrix, such as calcium sulfate whisker,4 carbon fiber,5 silica,6 and ZnO7 nanoparticles. However, compatibility, dispersibility, and interfacial friction between the polymer matrix and the functional hybrid particles play important roles in determining the damping properties of the composite. Nowadays, embedded cocuring and its variant technologies (e.g., embedded perforated cocuring method) has been the most common and successful method to prepare damping structural integration materials, whereas the relatively weak interface adhesion between damping layer and constrained layers limits their wide applications in harsh environment, which prompt researchers to seek new ideas and methods to fabricate damping structural integration materials. Gradient phenomenon is commonly seen in nature, such as bamboo, plant stems, and bone, where the strongest elements are located in regions that experience the highest stresses.8 These gradual changes in structure make them show excellent strength and toughness at the same time. For centuries, Received: Revised: Accepted: Published: A

August 24, 2017 November 3, 2017 December 15, 2017 December 15, 2017 DOI: 10.1021/acs.iecr.7b03509 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Synthesis of PCL−PS Diblock Copolymer

synthesized via the combination of ring-opening polymerization and atom transfer radical polymerization, and then incorporated into epoxy thermosets. By controlling the lengths of each block, spherical nanostructures are constructed in epoxy thermosets. Because of the advantage of nanostructure, gradient coronas structure around the nanostructures and the plasticization of PCL subchains, a novel approach to prepare damping structural integration materials is reported. Besides, the damping mechanism and tensile properties of the damping structural integration epoxy thermosets are systematically investigated. As far as we know, this is the first report on the study of preparing damping structural integration materials via the gradient plasticization mechanism at nanometer scale, it is expected this method could provide a new idea to fabricate damping structural integration materials in future.

scientists have aspired to exploit nature’s gradation principles to create artificial materials, with hierarchical structures and tailored properties, for the fabrication of functional devices. It also has been reported that polymer materials with excellent damping properties can be prepared through the design of gradient structure.9 Meanwhile, the development of nanotechnology has focused the researchers’ attention on microstructure design of materials. Via the suitable design of microstructure in polymer matrix, various high-performance materials with desired properties are obtained.8,10,11 According to the self-stratification mechanism, a new type of continuous gradient polyurethane/epoxy interpenetrating polymer network (IPN) material is prepared by Lv, and the dynamic mechanical analysis tests show that it undergoes more than 117.69 °C with loss tangent larger than 0.3, much better than the traditional IPN with the same component.12 Wang9 prepares a gradient rubber with a progressive change in sulfur modification of the molecular structure along the direction of thickness through in situ chemical modification of rubber during vulcanization. The gradient polymer exhibits a broad damping temperature range spanning over 100 °C. Recently, block copolymer has attracted considerable attention, because amphiphilic block copolymer composed of epoxy miscible blocks and immiscible blocks could selforganize into nanostructures, such as, spherical, wormlike, lamellar, or mixtures of them13,14 when epoxy act as the selective solvent. Among them, spherical nanostructure is the most stable and common morphology. In this case, immiscible blocks would self-organize into spherical dispersed phase, whereas miscible blocks would arrange in the radial direction to form “coronas”15−18 around the spherical nanostructure own to the covalent bond between miscible and immiscible blocks. Besides, It has been reported that epoxy miscible blocks could be demixed from epoxy to some extent with the occurrence of curing reaction in amphiphilic block copolymer and epoxy blends,19,20 the concentration of miscible blocks gradually decreases along the radial direction. So far, the studies of epoxy containing amphiphilic block copolymer with polystyrene as immiscible blocks have been extensively reported.21−25 The synthesis of poly(ε-caprolactone)-b-polystyrene diblock copolymer (PCL−PS) is very mature. Thereby, in this study, PCL−PS diblock copolymer is

2. EXPERIMENTAL SECTION 2.1. Materials. The monomer of ε-caprolactone (ε-CL), Stannous octanoate [Sn(Oct)2], N,N,N′,N′,N′-pentamethyldiethylenetriamine (PMDETA), methyl 2-bromopropionate (MBP), 2-bromoisobutyryl bromide, and copper(I) bromide (CuBr) were purchased from Aldrich Co. Benzyl alcohol and styrene (St) were purchased from Chemical Reagent Factory of Kelong. 4-Dimethylaminopyridine (DMAP) was purchased from Nanjing Tianhua Reagent Co., China. Prior to use, all of these materials are purified and dried by calcium hydride (CaH2). Diglycidyl ether of bisphenol A-based (DGEBA) epoxy resin E-51, obtained from Jiangsu Wuxi Resin Plant, China,was utilized as the thermoset matrix. Curing agent 3,3′dichloro-4,4′-diamino diphenylmethane (MOCA) was purchased from Changshan beier Co., China. Other solvents were used as received. 2.2. Synthesis of Diblock Copolymer and Homopolymers. The combination of the ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP) were carried out to obtain PCL−PS diblock copolymer. The synthesis of PCL−PS block copolymer was similar to that reported by Meng.21 Generally, hydroxyl-terminated poly(εcaprolactone) was first synthesized via ROP of ε-caprolactone (63.28g, 555.09 mmol) with benzyl alcohol (234 μL, 2.26 mmol) and Sn(Oct)2 (1/1000 (w/w) with respect to ε-CL) as the initiator and catalyst, respectively. The reaction was carried out at 120 °C for 36 h. Then the above PCL homopolymers B

DOI: 10.1021/acs.iecr.7b03509 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Transmission Electron Microscopy (TEM). The nanostructures of epoxy thermosets containing PCL−PS diblock copolymers was observed by a transmission electron microscope (TEM; Tecnai G2 F20, FEI, USA), at an acceleration voltage of 120 kV. The ultrathin sections with a thickness of 100−150 nm were cryogenically microtomed by using an ultramicrotome (EM UC7, LEICA, Germany). Prior to measurement, the ultrathin sections of the epoxy composites was stained with RuO4 to increase the electron density contrast at room temperature for 12 min. Small-Angle X-ray Scattering (SAXS). The SAXS measurements were taken on the Xeuss 2.0 SAXS/WAXS system. Twodimensional diffraction patters were recorded using PILATUS 3R 300 K detector. The experiments were carried out with the radiation of X-ray with the wavelength of λ = 1.54 Å at room temperature (25 °C) operating at 50 kV, 0.6 mA. The intensity profiles were output as the plot of scattering intensity (I) versus scattering vector, q = (4π/λ) sin (θ/2) (θ = scattering angle). Field-Emission Scanning Electron Microcopy (FESEM). The samples were fractured in liquid nitrogen and the fracture surfaces were coated with thin layers of gold to ensure surface conductivity before observation. The microstructures in epoxy thermosets containing PCL and PS homopolymers were evaluated by Field emission scanning electron microscope (Nova NanoSEM 450, FEI, USA) instrument with an acceleration voltage of 5 kV. Dynamic Mechanical Analysis (DMA). Dynamic mechanical experiment was measured using dynamic-mechanical thermal analysis with a Q800 DMA from TA Instruments, USA. A three-point bending mode at 1 Hz was employed using test specimens 20 mm × 10 mm × 4 mm in size. The temperature range used was 30 to 200 °C with a heating rate of 3 °C/min. The temperature at the peak of tan δ was utilized to determine the glass transition temperature of epoxy composites. Differential Scanning Calorimeter (DSC). Samples (about 5−8 mg) were investigated by differential scanning calorimeter on a TA Q200 thermal analysis instruments. After the thermal history was removed, Samples were first equilibrated at 20 °C for 5 min, and then scanned from 20 to 200 °C at a heating rate of 20 °C/min. PCL−PS diblock copolymer was scanned between −20−140 °C. A heating rate of 20 °C/min was used at all cases. Glass transition temperature (Tg) was taken as the midpoint of heat capacity change. The melting temperatures were taken as the temperatures of the maximum of endothermic peak. Mechanical Properties. Tensile properties of epoxy composites containing PCL−PS diblock copolymer were determined on an Instron 5567 universal testing instrument (USA), the size of tensile specimen was shown in Scheme 2. According to GB/T 1040.2−2006, the test was measured at a rate of 10 mm/min. All mechanical values mentioned in following results were the average of at least five samples. The values at the maximum point were used to characterize the tensile properties of the composite.

were utilized to react with 2-bromoisobutyryl bromide to obtain bromine-terminated poly(ε-caprolactone) macroinitiator (PCL-Br). Finally, PCL-Br macroinitiator (28 g, 0.996 mmol), styrene (13.95 g, 134.136 mmol), PMDETA (418 μL, 1.993 mmol), and CuBr (143 mg, 0.996 mmol) was charged into a Schlenk flask to synthesize poly(ε-caprolactone)-blockpolystyrene diblock copolymer. The procedure of synthesis for PCL−PS diblock copolymer was shown in Scheme 1. The synthetic procedures of poly(ε-caprolactone) (PCL) and polystyrene (PS) homopolymers were similar to that of PCL−PS diblock copolymer. The dosage were expressed as below. For PCL homopolymer, benzyl alcohol (117 μL, 1.13 mmol), ε-CL (31.7 g, 278.07 mmol) and the catalyst Sn(Oct)2 (32 mg). The polymerization was carried out at 120 °C for 36 h. For PS homopolymer, MBP (193 μL, 1.73 mmol), St (24.22g, 232.93 mmol), CuBr (248.3 mg, 1.73 mmol) and PMDETA (724 μL, 3.46 mmol). The polymerization was carried out at 110 °C for 4 h. 2.3. Preparation of Epoxy Composites. Desired amount of PCL−PS diblock copolymer and DGEBA were mixed with continuous vigorous stirring at elevated temperature until the mixtures became homogeneous and transparent. Then molten MOCA was added with continuous stirring. The mixtures were degassed under vacuum, poured into Teflon molds and subjected to the thermal curing at 150 °C for 2 h plus 180 °C for 2 h for post curing. The thermosets containing PCL−PS diblock copolymer up to 40 wt % were obtained. Epoxy thermosets containing PCL and PS homopolymers were also obtained by the above method. The specific additions were shown in the Table 1. Table 1. Composition of Epoxy Composites material weight sample code

epoxy (g)

MOCA (g)

modifier (g)

neat epoxy PCL−PS-10a PCL−PS-20 PCL−PS-40 PCL-10 PCL-20 PCL-40 PCL/PS-10 PCL/PS-20 PCL/PS-40

50 50 50 50 50 50 50 50 50 50

20 20 20 20 20 20 20 20 20 20

0 5 10 20 3.33b 6.67 13.34 3.33/1.67c 6.67/3.33 13.34/6.66

a

PCL−PS-10: The mass ratio of PCL−PS diblock copolymer to epoxy resin is 10%. b3.33: The addition of PCL is 3.33 g. c3.33/1.67: The additions of PCL and PS are 3.33 and 1.67 g, respectively.

2.4. Measurement and Characterization. Nuclear Magnetic Resonance Spectroscopy (NMR). The NMR measurements were carried out on a Bruker DRX-400 400 MHz NMR spectrometer (Germany) at room temperature. PCL, PS and PCL−PS block copolymers were dissolved in CDCl3. Gel Permeation Chromatography (GPC). The molecular weights were measured using a gel permeation chromatography (GPC), which consists of a HLC-8320 chromatograph (Tosoh, Japan) equipped with two columns (TSK gel super HMH 6.0 *150 mm) in serials. Polystyrene (PS) was used as calibration standards. The samples were analyzed at 40 °C with THF as an eluent and the flow rate was 0.6 μL/min.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Diblock Copolymer and Homopolymers. The number molecular weights and the chemical compositions of all the products obtained in this work were investigated by GPC (Figure 1 and Figure 3) and 1H NMR (Figures 2 and 4), respectively. Besides, according to the ratio C


Article

Industrial & Engineering Chemistry Research Scheme 2. Dimensions of the Tensile Specimens

Figure 1. GPC curves of PCL and PCL−PS diblock copolymers. Figure 4. GPC curves of of PCL and PS homopolymers.

the molecular weight of PCL blocks and PS blocks in PCL−PS diblock copolymer could also be calculated. The detail results of polymerization for PCL−PS diblock copolymer were presented in Table 2. According to the results of 1H NMR and GPC, it is known that PCL−PS diblock copolymers, PCL, and PS homopolymers were successfully synthesized. 3.2. Nanostructures in Epoxy Thermosets Containing PCL−PS Diblock Copolymer. As mentioned above, PCL− PS diblock copolymer was incorporated into epoxy to prepare nanostructured damping epoxy thermosets. Before curing, all the mixtures composed of diblock copolymer, DGEBA and MOCA were homogeneous and transparent at both room temperature and elevated temperatures. And the relative cured samples remained homogeneous and transparent, which suggested that no macroscopic phase separation occurred. In order to further investigate the morphology of these epoxy thermosets, the thermosets containing 10, 20, and 40 wt % PCL−PS diblock copolymer were subjected to the observation by means of TEM, which were shown in Figure 5. Prior to the test, the ultrathin sections of the epoxy thermosets were stained with RuO4. According to the dyeing mechanism of ruthenium oxide, both the epoxy matrix and PS microdomains would be stained to some extent, whereas the staining degree of PS microdomains was higher than that of the epoxy matrix. Considering the volume fraction in the epoxy blends and the results of staining, the dark areas in the TEM images should be ascribed to PS domains, whereas the continuous gray areas to epoxy matrix. It was shown that the spherical PS particles were homogeneously imbedded in the continuous epoxy matrix. When the content of PCL−PS diblock copolymer was 10 wt %, the average size was 22 nm in

Figure 2. 1H NMR spectrum of PCL−Br and PCL−PS diblock copolymers.

Figure 3. 1H NMR spectrum of PCL and PS homopolymers.

of integral intensity of the peaks at 4.04−4.07 ppm [OCO(CH2)4CH2] to that at 5.11 ppm [C6H5CH2OCO] and the ratio of integral intensity of the peaks at 6.3−7.2 ppm [CH2CHC6H5] to that at 4.04−4.07 ppm [OCO(CH2)4CH2], D


Article

Industrial & Engineering Chemistry Research Table 2. Characteristics of the Synthesized Polymers diblock copolymer PCL Mn (g/mol)

homopolymer PCL−PS

PCL

PS

GPC

NMR

GPC

NMR

GPC

NMR

GPC

NMR

27920

25080

42120

41512

31640

28840

16160

15080

Figure 6. SAXS profiles of epoxy thermosets containing 5, 10, 20, and 40 wt % of PCL−PS diblock copolymer.

slightly shifted to the high Q values with increasing the content of PCL−PS diblock copolymer, suggesting that the average distance between neighboring domains decreased. This result was in a good agreement with those obtained by means of TEM. 3.3. Formation of Nanostructures in Epoxy Thermosets Containing PCL−PS Diblock Copolymer. It is known that amphiphilic block copolymers could self-organize into nanostructures with various morphologies in epoxy thermosets via “self-assembly”19,20,26−32 and “reaction induced microphase separation”33−40 (RIMPS). Noted that the blends of PS and epoxy displayed an upper critical solution temperature (UCST) behavior, PS blocks were miscible with epoxy before curing, while reaction-induced phase separation would occur during the process of the subsequent curing reaction.40 Own to the intermolecular hydrogen bonding interactions between the carbonyls groups of PCL and the free hydroxyl groups of epoxy matrix, PCL subchains remained compatible with epoxy during the whole curing process. The intermolecular specific interactions between PCL and the epoxy matrix can be evidenced by Fourier transform infrared spectroscopy (FTIR). Figure 7 was the FTIR spectra of epoxy thermosets containing 10 and 20 wt % of PCL in the range of 3000−3700 cm−1. Noted that the band at 3404 cm−1 was ascribed to the hydrogen bonded hydroxyl groups in epoxy matrix. When PCL was incorporated, the associated hydroxyl band was shifted to higher frequencies (3421 cm−1), suggesting intermolecular hydrogen bonding interactions.41 Besides, it can be seen that as the amount of PCL increased the intensity of the associated hydroxyl band increased, indicating that the intermolecular hydrogen bonding interactions between the carbonyls groups of PCL and the free hydroxyl groups of epoxy matrix also increased. Thus, reaction-induced microphase separation occurred during the curing process of epoxy and PCL−PS diblock copolymer blends. According to the results of SAXS and TEM, the formation of nanostructures in the epoxy thermosets

Figure 5. TEM images of the epoxy thermosets containing (a, a′) 10 wt %, (b, b′) 20 wt %, and (c, c′) 40 wt % PCL−PS diblock copolymer.

diameter (Figure 5a′). With increasing the content, the amount of PS nanodomains was increased while the distance between adjacent domains was decreased. Meanwhile, the diameter of spherical PS particles was almost unchanged (∼23 nm, Figure 5b′) when the addition was 20 wt %. When the concentration of diblock copolymer was further increased, the diameter of PS nanostructure was slightly increased to 26 nm (Figure 5c′). The nanostructures were further investigated by means of small-angle X-ray scattering (SAXS). Figure 6 was the SAXS profiles of epoxy thermosets containing PCL−PS diblock copolymer. It is seen that scattering peaks were observed in all cases when diblock copolymer was incroporated, indicating that the diblock copolymer was microphase separated in epoxy matrix. And the intensity of scattering peaks was increased with increasing the content of PCL−PS diblock copolymer, which implied that the order of nanostructures was improved. It was noted that the positions of the first-order scattering peaks E


Article


Figure 7. FTIR spectra of epoxy thermosets containing 10 and 20 wt % PCL homopolymer in the range of 3000−3700 cm−1.

containing PCL−PS diblock copolymer can be depicted in Figure 8. Prior to curing, PCL−PS diblock copolymer was absolutely compatible with the precursors of epoxy (DGEBA +MOCA), and the blends were homogeneous and transparent, suggesting no macroscopic phase separation occurred. During the process of curing reaction, diblock copolymers would be aggregated and arranged in the radial direction to form spherical micelles due to component ratio of the PCL−PS diblock copolymer and the compatibility among each block and epoxy. It is believed that the self-organized micelles were composed of the cores from PS subchains and the coronas of PCL subchains. Then reaction-induced microphase separation of PS subchains occurred, spherical PS nanostructures were fixed in epoxy matrix, whereas PCL subchains remained miscible with epoxy. 3.4. Microstructures in Epoxy Thermosets Containing PCL and PS Homopolymers. Both of PCL and PS homopolymers were added to epoxy to prepare control samples. After curing, the samples of epoxy thermosets containing PCL and PS homopolymers were homogeneous but opaque, suggesting macroscopic phase separation occurred during the curing process. The microstructures of epoxy containing PCL/PS homopolymers were studied by FESEM and the images were shown in Figure 9. When the concentration is 10 wt %, the diameter of spherical particles is larger than 5 μm. And the size of particles is increased with increasing the concentration. Considering the miscibility among PCL, PS and epoxy, the spherical domains are assignable to PS homopolymer whereas the continuous phase to epoxy that is miscible with PCL. The FESEM result indicates that PS homopolymer is macroscopic separated during the curing process, whereas PCL homopolymer remains miscible with epoxy matrix.

Figure 9. FESEM images of the epoxy thermosets containing (a) 10 wt %, (b) 20 wt %, and (c) 40 wt % PCL and PS homopolymers.

3.5. Dynamic Mechanical Properties. These above three systems (PCL/epoxy, PCL/PS/epoxy, and PCL−PS/epoxy) were investigated by means of dynamic mechanical analysis. The temperature at the peak of tan δ was utilized to determine the glass transition temperature of epoxy composites. Figure 10 was the dynamic mechanical curves of epoxy thermosets containing PCL and PCL/PS homopolymers. It is shown that the glass transition temperature and damping temperature range (tan δ > 0.2) in these two systems have the same tendency when the homopolymers were added. With increasing content, the glass transition temperature was

Figure 8. Formation of nanostructures in the epoxy thermosets containing PCL−PS diblock copolymer. F


Article


Figure 10. Dynamic mechanical curves of epoxy thermosets containing PCL and PCL/PS homopolymers.

decreased, whereas the damping temperature range (Tan δ > 0.2) was broadened. Besides, damping property of epoxy thermosets containing PCL homopolymer was better than containing PCL and PS homopolymers. This phenomenon could be ascribed to the intermolecular specific interactions between the carbonyls groups of PCL and the free hydroxyl groups of epoxy matrix, which has been evidenced in Figure 8. Because of the appearance of the intermolecular hydrogen bonding interactions between PCL and epoxy, PCL homopolymer can play a role of plasticizer in the systems. Besides, the amount of intermolecular hydrogen bond has a direct proportion of the concentration of PCL, leading to better damping in the PCL/epoxy system. The plots of tan δ versus temperature of epoxy containing PCL−PS diblock copolymer were shown in Figure 11. It is

was shown in Figure 12. Even through the added amount was exactly the same, the damping property of epoxy containing PCL and PS homopolymers were worse than that of epoxy containing PCL−PS diblock copolymer. The detail results were shown in Table 3. Thus, it reasonable to believe that the excellent damping performance of epoxy containing PCL−PS diblock copolymer was attributed to the appearance of nanostructures. 3.6. Damping Mechanism of Epoxy Thermosets Containing PCL−PS Diblock Copolymer. To further study the damping mechanism, PCL−PS diblock copolymer, epoxy containing PCL−PS diblock copolymer, epoxy containing PCL homopolymer, and epoxy containing PCL and PS homopolymers were measured by means of DSC. As for the neat PCL−PS diblock copolymer (Figure 13), DSC curve displays a sharp endothermic peak at 55 °C, which is attributable to the melting transition of the PCL subchain. Meanwhile, the glass transition of PS was also observed at 61 °C, which is consistent with the literature.21 The results show that PCL−PS diblock copolymers were microphase-separated. The DSC curves of epoxy thermosets containing PCL homopolymer was presented in Figure 14. It is shown that every sample displayed only one glass transition, and the glass transition temperature of epoxy composites decreased with increasing PCL homopolymer, suggesting the PCL homopolymer is matrix homogeneously dispersed and not crystalline in the resin matrix. Figure 15 was the DSC results of epoxy thermosets containing PCL and PS homopolymers. When PCL and PS homopolymers were incorporated, the DSC curves exhibited two glass transitions at 90−130 °C and 60−64 °C. The former glass transition was attributed to epoxy that mixed with PCL homopolymer, whereas the latter was assignable to PS microdomains. The results suggested PS homopolymer was macroscopically separated in epoxy matrix, which was in good agreement with the FESEM results. For epoxy thermosets containing 10 and 20 wt % diblock copolymer, a transition appears at 66−69 °C besides the glass transitions assignable to epoxy-rich regions (Figure 16). It has been reported that epoxy miscible blocks could be demixed from epoxy to some extent with the occurrence of curing reaction in amphiphilic block copolymer and epoxy blends.19,20 Meanwhile, the formation of PS nanostructures also happened during the curing process. This is to say, the formation of PS nanostructures and demixing of epoxy miscible blocks

Figure 11. Dynamic mechanical curves of epoxy thermosets containing PCL−PS diblock copolymer.

interesting to find that the damping property of epoxy containing PCL−PS diblock copolymer is even much better than that containing same amount of PCL homopolymer. With increasing the content of PCL−PS, the glass transition temperature was decreased from 171.66 to 107.84 °C, whereas the damping temperature range (tan δ > 0.2) of the composites was broadened from 28.02 to 52.95 °C. The comparison of glass transition temperature and damping temperature range (tan δ > 0.2) among PCL/EP, PCL/PS/EP and PCL−PS/EP G


Article


Figure 12. Comparison of glass transition temperature and damping temperature range (tan δ > 0.2) among PCL/EP, PCL/PS/EP, and PCL−PS/ EP.

Table 3. Results of Dynamic Mechanical Analysis of Epoxy Composites sample code

glass transition temperature Tg (°C)

damping temperature in range tan δ > 0.2 (°C)

EP PCL−PS10 PCL−PS20 PCL−PS40 PCL-10 PCL-20 PCL-40 PCL/PS10 PCL/PS20 PCL/PS40

171.66 159.49

156.50−184.52 138.80−175.46

145.52

121.52−164.72

107.84

78.82−131.77

165.60 142.92 126.31 165.08

147.86−179.69 120.33−159.99 102.64−146.35 146.31−179.10

150.56

128.40−168.51

129.93

105.71−150.10

Figure 14. DSC curves of epoxy thermosets containing PCL homopolymer.

Figure 15. DSC curves of epoxy thermosets containing PCL and PS homopolymers. Figure 13. DSC curves of PCL−PS diblock copolymer.

suggested the existence of gradient plasticization structure along the radial direction of PS nanostructures. When the concentration of PCL−PS diblock copolymer was 40 wt %, the glass transition temperature of epoxy-rich regions would further decreased, meanwhile, the amount of demixed PCL subchains was further increased. In terms of this fact, these two glass transitions appeared in epoxy thermosets containing 10 and 20 wt % PCL−PS diblock copolymer would merged into

occurred at the same time. Because of the covalent bond between PCL and PS subchains, the demixed PCL subchains had to enrich at the surfaces of PS nanodomains. In this case, the glass transition temperature in these regions would much lower than that of epoxy-rich regions. Thus, it was judged that the transition appears at 66−69 °C was attributable to the PCL-rich epoxy regions surrounding PS nanostructures, which H


Article


epoxy composites. Thus, the damping mechanism of epoxy thermosets containing PCL−PS diblock copolymer could be summarized as a gradient plasticizing effect at the nanometer scale. 3.7. Mechanical Properties. Figures 18 and 19 displayed the tensile strength of epoxy thermosets containing PCL−PS

Figure 16. DSC curves of epoxy thermosets containing PCL−PS diblock copolymer.

one. Thus, a board peak was detected when the concentration was 40 wt %. It was shown that own to the covalent bond between PCL and PS subchains, macroscopic phase separation of PS was avoided and gradient plasticization structure existed along the radial direction of PS nanostructures. Thereby, according to the formation process of nanostructures and results of TEM, SAXS, DMA, and DSC, the damping mechanism of epoxy thermosets containing PCL−PS diblock copolymer was a gradient plasticization mechanism at the nanometer scale. After curing, PS subchains self-organized into nanostrucutres, whereas PCL subchains remained miscible with epoxy and the concentration was gradually decreased along the radial direction. The schematic diagram of the gradual nanostructure is shown in Figure 17, and the depth of

Figure 18. Tensile properties of epoxy thermosets containing PCL− PS block copolymer.

Figure 19. Stress−strain curves of the epoxy thermosets containing PCL−PS diblock copolymer.

block copolymer and the corresponding stress−strain plots, respectively. Compared with the neat epoxy, the incorporation of PCL−PS diblock copolymer facilitated the improvement of tensile properties. When the addition was lower than 20 wt %, the tensile strength of epoxy thermosets was increased with increasing the content of block copolymer. The maximum of the tensile strength 73.2 MPa was obtained when the concentration was 20 wt %, 27% improved compared with neat epoxy. With further increasing the content, the tensile strength of epoxy thermosets showed a tendency to decrease, whereas the elongation at break was cautiously increased. Because of the presence of epoxy miscible PCL blocks in PCL−PS diblock copolymer, macrophase separation of PS was avoided. PCL blocks remained miscible with the epoxy network, whereas PS blocks would be generated together to form spherical nanodomains and embedded in epoxy after curing (Figure 5). Because of the large specific surface area of the spherical nanodomains, the interface adhesion between the

Figure 17. Schematic diagram of gradual nanostructure.

color represented the concentration of PCL subchains. Because of the presence of intermolecular hydrogen bonding interactions between PCL and epoxy, gradient plasticization of PCL subchains led to the gradual increasing of glass transition temperature along the radial direction (Tg1 < Tg2 < ... < Tgn). When the epoxy thermosets containing PCL−PS diblock copolymer suffered from the outer vibrations, the segmental motion of composites was not uniform, showing a better damping property compared with PCL/epoxy and PCL/PS/ I


Article


fiber-reinforced epoxy composites. Compos. Sci. Technol. 2004, 64, 2517. (6) Trakulsujaritchok, T.; Hourston, D. Damping characteristics and mechanical properties of silica filled PUR/PEMA simultaneous interpenetrating polymer networks. Eur. Polym. J. 2006, 42, 2968. (7) Zheng, J.; Ozisik, R.; Siegel, R. W. Disruption of self-assembly and altered mechanical behavior in polyurethane/zinc oxide nanocomposites. Polymer 2005, 46, 10873. (8) Suresh, S. Graded materials for resistance to contact deformation and damage. Science 2001, 292, 2447. (9) Wang, Y. Q.; Wang, Y.; Zhang, H. F.; Zhang, L. Q. A Novel Approach to Prepare a Gradient Polymer with a Wide Damping Temperature Range by In-Situ Chemical Modification of Rubber During Vulcanization. Macromol. Rapid Commun. 2006, 27, 1162. (10) Hesse, S. A.; Werner, J. r. G.; Wiesner, U. One-Pot Synthesis of Hierarchically Macro-and Mesoporous Carbon Materials with Graded Porosity. ACS Macro Lett. 2015, 4, 477. (11) Garate, H.; Bianchi, M.; Pietrasanta, L. I.; Goyanes, S.; D’Accorso, N. B. High-Energy Dissipation Performance in Epoxy Coatings by Synergistic Effect of Carbon Nanotube/Block Copolymer Conjugates. ACS Appl. Mater. Interfaces 2017, 9, 930. (12) Lv, X.; Huang, Z.; Huang, C.; Shi, M.; Gao, G.; Gao, Q. Damping properties and the morphology analysis of the polyurethane/epoxy continuous gradient IPN materials. Composites, Part B 2016, 88, 139. (13) Cong, H.; Li, L.; Zheng, S. Formation of nanophases in epoxy thermosets containing ABC and ACB triblock copolymers: A comparative investigation. Polymer 2015, 80, 146. (14) Cong, H.; Li, L.; Zheng, S. Formation of nanostructures in thermosets containing block copolymers: From self-assembly to reaction-induced microphase separation mechanism. Polymer 2014, 55, 1190. (15) Redline, E. M.; Declet-Perez, C.; Bates, F. S.; Francis, L. F. Effect of block copolymer concentration and core composition on toughening epoxies. Polymer 2014, 55, 4172. (16) Zhu, L.; Zhang, C.; Han, J.; Zheng, S.; Li, X. Formation of nanophases in epoxy thermosets containing an organic−inorganic macrocyclic molecular brush with poly (ε-caprolactone)-blockpolystyrene side chains. Soft Matter 2012, 8, 7062. (17) Mai, Y.; Eisenberg, A. Selective localization of preformed nanoparticles in morphologically controllable block copolymer aggregates in solution. Acc. Chem. Res. 2012, 45, 1657. (18) Li, D.; Li, C.; Wan, G.; Hou, W. Self-assembled vesicles of amphiphilic poly (dimethylsiloxane)-b-poly (ethylene glycol) copolymers as nanotanks for hydrophobic drugs. Colloids Surf., A 2010, 372, 1. (19) Hillmyer, M. A.; Lipic, P. M.; Hajduk, D. A.; Almdal, K.; Bates, F. S. Self-assembly and polymerization of epoxy resin-amphiphilic block copolymer nanocomposites. J. Am. Chem. Soc. 1997, 119, 2749. (20) Lipic, P. M.; Bates, F. S.; Hillmyer, M. A. Nanostructured thermosets from self-assembled amphiphilic block copolymer/epoxy resin mixtures. J. Am. Chem. Soc. 1998, 120, 8963. (21) Meng, F.; Xu, Z.; Zheng, S. Microphase Separation in Thermosetting Blends of Epoxy Resin and Poly(caprolactone)block-Polystyrene Block copolymers. Macromolecules 2008, 41, 1411. (22) Bates, F. S.; Fredrickson, G. H. Block CopolymersDesigner Soft Materials. Phys. Today 1999, 52, 32. (23) Yu, R.; Zheng, S.; Li, X.; Wang, J. Reaction-Induced Microphase Separation in Epoxy Thermosets Containing Block Copolymers Composed of Polystyrene and Poly(ε-caprolactone): Influence of Copolymer Architectures on Formation of Nanophases. Macromolecules 2012, 45, 9155. (24) Meng, F.; Zheng, S.; Li, H.; Liang, Qi; Liu, T. Formation of Ordered Nanostructures in Epoxy Thermosets: A Mechanism of Reaction-Induced Microphase Separation. Macromolecules 2006, 39, 5072. (25) Fan, W.; Zheng, S. Reaction-induced microphase separation in thermosetting blends of epoxy resin with poly(methyl methacrylate)-

nanodomains and epoxy matrix was greatly improved, resulting in the increase in tensile strength. Besides, PCL blocks connected with PS nanostructures through covalent bond were remain fixed in the epoxy network after curing, which was further optimized the interactions between the matrix and the modifier. Thus, the tensile strength of epoxy thermosets was improved. However, when the content of the PCL−PS diblock copolymer was further increased, the decrease in tensile strength of epoxy thermosets was attributed to the increased nanostructure size.

4. CONCLUSION In this work, a novel approach to prepare damping structural integration materials was provided. Spherical nanostructures were constructed in epoxy thermosets through controlling the lengths of each block in the synthesis of PCL−PS diblock copolymer. Together with PCL/EP and PCL/PS/EP composites, the damping mechanism of epoxy thermosets containing PCL−PS diblock copolymer were systematically discussed through DSC and DMA. Because of the gradient coronas structure around the nanostructures and the plasticization of PCL subchains, gradient plasticizing effect at nanometer scale was formed. Dynamic mechanical analysis shown that when the content of PCL−PS was 40 wt %, the glass transition temperature were decreased from 171.66 to 107.84 °C, whereas the damping temperature range (tan δ > 0.2) of the composites were broaden from 28.02 to 52.95 °C. Meanwhile, the tensile properties of epoxy thermosets were also improved when PCL−PS diblock copolymer was incorporated.

■

AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-28-85408288. Fax: +86-28-85402465. E-mail: [email protected] (Y.C.). *Tel: +86-28-85408288; Fax: +86-28-85402465. E-mail: [email protected] (H.Z.). ORCID

Yang Chen: 0000-0002-1228-3250 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (51703137), the Fundamental Research Funds for the Central Universities (2012017yjsy186) and the Fundamental Research Funds for the Central Universities of China (2015SCU11008) for financial support.

■

REFERENCES

(1) Finegan, I. C.; Gibson, R. F. Recent research on enhancement of damping in polymer composites. Compos. Struct. 1999, 44, 89. (2) Kishi, H.; Nagao, A.; Kobayashi, Y.; Matsuda, S.; Asami, T.; Murakami, A. Carboxyl-terminated butadiene acrylonitrile rubber/ epoxy polymer alloys as damping adhesives and energy absorbable resins. J. Appl. Polym. Sci. 2007, 105, 1817. (3) Heng, Z.; Zeng, Z.; Chen, Y.; Zou, H.; Liang, M. Silicone modified epoxy resins with good toughness, damping properties and high thermal residual weight. J. Polym. Res. 2015, 22, 1. (4) Chen, S.; Wang, Q.; Wang, T. Mechanical, damping, and thermal properties of calcium sulfate whisker-filled castor oil-based polyurethane/epoxy IPN composites. J. Reinf. Plast. Comp. 2011, 30, 509. (5) Kishi, H.; Kuwata, M.; Matsuda, S.; Asami, T.; Murakami, A. Damping properties of thermoplastic-elastomer interleaved carbon J


Article

Industrial & Engineering Chemistry Research block-polystyrene block copolymers: Effect of topologies of block copolymers on morphological structures. Polymer 2008, 49, 3157. (26) Heng, Z.; Li, R.; Chen, Y.; Zou, H.; Liang, M. Preparation of damping structural integration materials via the formation of nanostructure in triblock copolymer modified epoxy resins. J. Polym. Res. 2016, 23, 1. (27) Blanco, M.; López, M.; Kortaberria, G.; Mondragon, I. Nanostructured thermosets from self-assembled amphiphilic block copolymer/epoxy resin mixtures: effect of copolymer content on nanostructures. Polym. Int. 2010, 59, 523. (28) Grubbs, R. B.; Dean, J. M.; Bates, F. S. Methacrylic block copolymers through metal-mediated living free radical polymerization for modification of thermosetting epoxy. Macromolecules 2001, 34, 8593. (29) Mijovic, J.; Shen, M.; Sy, J. W.; Mondragon, I. Dynamics and morphology in nanostructured thermoset network/block copolymer blends during network formation. Macromolecules 2000, 33, 5235. (30) Guo, Q.; Thomann, R.; Gronski, W.; Thurn-Albrecht, T. Phase Behavior, Crystallization, and Hierarchical Nanostructures in SelfOrganized Thermoset Blends of Epoxy Resin and Amphiphilic Poly (ethylene oxide)-b lock-poly (propylene oxide)-b lock-poly (ethylene oxide) Triblock Copolymers. Macromolecules 2002, 35, 3133. (31) Kosonen, H.; Ruokolainen, J.; Nyholm, P.; Ikkala, O. Selforganized cross-linked phenolic thermosets: thermal and dynamic mechanical properties of novolac/block copolymer blends. Polymer 2001, 42, 9481. (32) Kosonen, H.; Ruokolainen, J.; Nyholm, P.; Ikkala, O. SelfOrganized Thermosets: Blends of Hexamethyltetramine Cured Novolac with Poly (2-vinylpyridine)-b lock-poly (isoprene). Macromolecules 2001, 34, 3046. (33) Ocando, C.; Serrano, E.; Tercjak, A.; Pena, C.; Kortaberria, G.; Calberg, C.; Grignard, B.; Jerome, R.; Carrasco, P. M.; Mecerreyes, D.; Mondragon, I. Structure and properties of a semifluorinated diblock copolymer modified epoxy blend. Macromolecules 2007, 40, 4068. (34) Fan, W.; Wang, L.; Zheng, S. Double Reaction-induced Microphase Separation in Epoxy Resin Containing Polystyrene-blockpoly (ε-caprolactone)-block-poly (n-butyl acrylate) ABC Triblock Copolymer. Macromolecules 2010, 43, 10600. (35) Heng, Z.; Chen, Y.; Zou, H.; Liang, M. Simultaneously enhanced tensile strength and fracture toughness of epoxy resins by a poly (ethylene oxide)-block-carboxyl terminated butadiene-acrylonitrile rubber dilock copolymer. RSC Adv. 2015, 5, 42362. (36) Meng, F.; Zheng, S.; Zhang, W.; Li, H.; Liang, Q. Nanostructured Thermosetting Blends of Epoxy Resin and Amphiphilic Poly (ε-caprolactone)-block-polybutadiene-block-poly(ε-caprolactone) Triblock Copolymer. Macromolecules 2006, 39, 711. (37) Meng, F.; Zheng, S.; Li, H.; Liang, Q.; Liu, T. Formation of ordered nanostructures in epoxy thermosets: a mechanism of reaction-induced microphase separation. Macromolecules 2006, 39, 5072. (38) Xu, Z.; Zheng, S. Reaction-induced microphase separation in epoxy thermosets containing poly (ε-caprolactone)-block-poly (nbutyl acrylate) diblock copolymer. Macromolecules 2007, 40, 2548. (39) Yu, R.; Zheng, S.; Li, X.; Wang, J. Reaction-induced microphase separation in epoxy thermosets containing block copolymers composed of polystyrene and poly (ε-caprolactone): Influence of copolymer architectures on formation of nanophases. Macromolecules 2012, 45, 9155. (40) Hu, D.; Zheng, S. Reaction-induced microphase separation in epoxy resin containing polystyrene-block-poly (ethylene oxide) alternating multiblock copolymer. Eur. Polym. J. 2009, 45, 3326. (41) Parameswaranpillai, J.; Sidhardhan, S. K.; Jose, S.; Hameed, N.; Salim, N. V.; Siengchin, S.; Pionteck, J. r.; Magueresse, A.; Grohens, Y. Miscibility, Phase Morphology, Thermomechanical, Viscoelastic and Surface Properties of Poly (ε-caprolactone) Modified Epoxy Systems: Effect of Curing Agents. Ind. Eng. Chem. Res. 2016, 55, 10055.

K


A spontaneous approach to prepare damping structural integration

Recommend Documents