Article pubs.acs.org/IECR
Dramatic Toughness Enhancement of Polydicyclopentadiene Composites by Incorporating Low Amounts of Vinyl-Functionalized SiO2 Yaqi Wang, Li Zhang,* Jiacheng Sun, Jin-Biao Bao, Zongbao Wang, and Linbin Ni Ningbo Key Laboratory of Specialty Polymers, Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, P. R. China S Supporting Information *
ABSTRACT: A dramatic improvement of the toughness of polydicyclopentadiene (PDCPD) composite containing low amounts of vinyl-functionalized SiO2 was achieved, and the toughening mechanism was investigated. Considering the nonpolarity of the DCPD monomer and the probability of covalent bonding between SiO2 and DCPD, vinyl-functionalized SiO2 (VSiO2) with a high content of vinyl groups was prepared by a facile, reproducible, one-step, remodeled synthetic sol−gel process. The VSiO2 was characterized with respect to the content of vinyl groups, particle size, and morphology by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), dynamic light scattering (DLS), and field-emission scanning electron microscopy (FE-SEM). PDCPD/V-SiO2 composites were prepared by in situ polymerization. The mechanical properties of the PDCPD/V-SiO2 composites were investigated by universal testing machine (UTM) and dynamic mechanical analysis (DMA). A slight decrease in the yield strength was observed with increasing loading of V-SiO2, whereas the elongation at break increased substantially from 9.0% to 143.4% and the tensile toughness increased by a factor of 14 compared to that of neat PDCPD when just 0.2 wt % V-SiO2 had been added. The dramatic toughness enhancement is attributed to the formation of microvoids and microcracks during the process of stretching, which can absorb a great deal of energy.
1. INTRODUCTION Polydicyclopentadiene (PDCPD) has been extensively studied on account of its excellent mechanical properties and chemical resistance. For these reasons, it is one of the most promising materials for applications in aerospace, aeronautical and ground vehicles, windmill blades, and sport utility vehicles.1 Commercial PDCPD products are usually manufactured using reaction injection molding (RIM) technology because of the low viscosity and suitability for fast processing of PDCPD, with thermal triggering on demand.2−5 However, PDCPD is a comparativey brittle and rigid material, and its low elongation at break (low tensile toughness) particularly limits its applications. Many researchers have attempted to enhance the performance of PDCPD by forming composites using various fillers,6−11 because the very low viscosity of the DCPD monomer can offer fast and efficient wetting by fillers within a short time.5 In addition, some researchers have used copolymerization with other monomers to improve the performance of PDCPD resin.12−15 Vallons et al.5 analyzed several aspects of the mechanical performance and damage behavior of glass-fiber-reinforced PDCPD composites and compared the results with those for an equivalent (brittle) epoxy composite. Yoonessi et al.16 reported the partial exfoliation of organically modified montmorillonite © 2017 American Chemical Society
clay to a highly delaminated state in the low-viscosity nonpolar monomer and prepared montmorillonite clay/PDCPD composites by in situ ring-opening polymerization/curing. Jeong and Kessler17 prepared norbornene-functionalized multiwalled nanotubes (f-MWNTs), achieving a homogeneous dispersion of f-MWNTs in DCPD. Moreover, f-MWNT/PDCPD composites with high tensile toughness were prepared by in situ polymerization. Tensile toughness was enhanced by 925%, and the modulus and yield strength were increased slightly upon the addition of 0.4 wt % functionalized MWNTs. Pan et al.18 successfully grafted norbornene functional groups onto nanoscale SiO2, addressing the issues of dispersion and interfacial interactions between the SiO2 and PDCPD, and studied the mechanical properties and wear performance of SiO2/PDCPD. The addition of nanoscale SiO2 at low loadings had an positive influence on hardness and bending strength compared to the results for neat PDCPD, with the bend strength increasing from 42.2 to 56.8 MPa upon the incorporation of 0.25 wt % grafted silica. Hu et al.19 prepared Received: Revised: Accepted: Published: 4750
January 8, 2017 March 29, 2017 April 4, 2017 April 4, 2017 DOI: 10.1021/acs.iecr.7b00093 Ind. Eng. Chem. Res. 2017, 56, 4750−4757
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Industrial & Engineering Chemistry Research
composites and their industrial production using RIM technology.
graphite/PDCPD composites and studied the morphological effects of the fillers on both mechanical and thermal properties. Culver et al.20 used BiTO3 nanocrystals functionalized with 10undecenoic acid to enhance the nanocrystal−polymer interface and mitigate losses in breakdown strength, as well as investigate the dielectric properties of PDCPD/BiTO3 nanocomposites. Peng et al.21 successfully prepared MoS2/PDCPD nanocomposites to improve mechanical and wear resistance properties of PDCPD by means of in situ ring-opening metathesis polymerization (ROMP) using the RIM process. In addition, they successfully obtained a homogeneous dispersion by grafting dialkyldithiophosphate onto the MoS2 surface by an in situ surface grafting method. Although a few of the studies mentioned above tried to investigate the toughness improvement of PDCPD composites, one very important practical question has received relatively little attention: There still remains significant room for improving toughness of PDCPD. Researchers have previously reported that rigid particles can enhance the toughness of composites to a certain extent.22−25 One of the most common, and most important, of these rigid particles is silica. Some studies have reported that silica has been used to improve the toughness of polymers such as polycarbonate,26 poly(lactic acid),27 poly(phenylene sulfide),28 epoxy resin,29 and polycyanurate.30 Nevertheless, the toughness improvements have been limited, and the toughening mechanisms are not well elucidated: They are related to the polymer matrix and fillers used and are rather complicated. Additionally, silica nanoparticles are usually modified, allowing for the dispersion and interfacial interactions of silica in the polymer matrix, which will determine the strengthening and toughening effects. In general, SiO2 nanoparticles are modified with silane coupling agents [e.g., vinyltrimethoxysilane (VTMS), (3-mercaptopropyl)trimethoxysilane (MPS), and aminopropyltriethoxysilane (APTES)] to introduce functional groups such as vinyl, thiol, and amine groups onto the SiO2 nanoparticle surfaces.31 Staub et al.32 reported SBA-15 silica materials functionalized with vinyl (C2), allyl (C3), hexenyl (C6), and octenyl (C8) groups by using suitable alkenyl trichlorosilanes. Staub et al. also confirmed that the functional groups in silica materials could be initiated by the Grubbs I catalyst. In this work, we prepared PDCPD composites by incorporating functionalized SiO2 to study whether the rigid silica particles could reinforce and toughen PDCPD and discuss the corresponding mechanisms. Generally, chemical modifications of SiO2 not only make the preparation procedure multistep, troublesome, and high-cost, but also cannot introduce sufficient active groups, which is unfavorable for industrial RIM production. Here, considering the nonpolarity of the DCPD monomer and the probability of covalent bonding between SiO2 and DCPD, the formation of vinyl-SiO2 (V-SiO2) with a high content of vinyl groups is favorable. Thus, a facile, reproducible, one-step, remodeled synthetic sol−gel process was developed for the preparation of V-SiO 2 . The results showed that the introduction of nonpolar vinyl groups onto the silica surface not only solves the dispersion problem of silica in the DCPD monomer, but also improves the adhesion strength between SiO2 and the PDCPD matrix through covalent bonding. The results also demonstrate that the addition of V-SiO2 effectively enhance the toughness of PDCPD resin, and pave the way for using SiO2 nanoparticles as toughness fillers for SiO2/PDCPD
2. EXPERIMENTAL SECTION 2.1. Materials. Dicyclopentadiene (DCPD, 95%) was purchased from J&K Chemical Co., Ltd. Vinylmethylsilane (VMTS, 98%) was obtained from TCI Co., Ltd. Tetraethylorthosilicate (TEOS, 98%) and dichloromethane (CH2Cl2, 99.9%) were obtained from Aladdin Chemical Co., Ltd. Dichloro(3-methyl-2-butenylidine)bis(tricyclopentyl)phosphine ruthenium (first-generation Grubbs catalyst) was purchased from Sigma-Aldrich Chemcal Co., Ltd. Ammonia (NH4OH) was supplied by Sinopharm Chemical Reagent Co., Ltd. All chemicals and solvents were used without further purification. 2.2. Synthesis of V-SiO2. A one-step procedure was used to synthesis the V-SiO2 by a remodeled sol−gel process. Specifically, 0.5 mL of VMTS was added to 50 mL of distilled water under vigorous stirring for 2 h at 40 °C. After a transparent solution had been obtained, an adequate amount of NH4OH was added to the mixture solution under stirring for 4 h at 50 °C. Subsequently, the cream-white solution was centrifuged and washed several times using distilled water and ethanol. Finally, the product was vacuum-dried at 50 °C for 12 h, giving V-SiO2 (Scheme 1). Scheme 1. Synthesis of V-SiO2
2.3. Synthesis of SiO2. Silica was synthesized using the Stoeber synthesis method.33 First, 30 mL of ethanol and 2.4 mL of NH4OH hydroxide were stirred for 5 min in a round flask. Then, 1.2 mL of TEOS was added to the mixed solution under stirring for 6 h at room temperature. The solution was initially clear; after 15−20 min, the solution started to turn cloudy; and the final solution was very turbid and consisted of a suspension of silica. The solution was centrifuged and washed several times using distilled water and ethanol. Finally, the product was vacuum-dried at 50 °C for 12 h. 2.4. Preparation of PDCPD/V-SiO2 Composites. The procedure to fabricate PDCPD/V-SiO2 composites was as follows: The desired quantity of V-SiO2 (0, 0.02, 0.05, 0.2, or 0.3 wt %) was dispersed in DCPD monomer by sonication. Because the crystal size and morphology of the Grubbs catalyst affect its dissolution kinetics in the monomer, the Grubbs catalyst was first dissolved in a small amount of dichoromethene (6.4 × 10−3 g of catalyst/100 μL of CH2Cl2), forming much smaller catalyst crystals (see Supporting Information, Figure S1). Subsequently, the solution containing the dissolved catalyst was injected into the V-SiO2/DCPD solution (6.4 × 10−3 g of catalyst/5 mL of DCPD) using a syringe, forming a homogeneous solution under stirring at 40 °C. Finally, the solution was injected into a hot mold and cured for 2 h at 70 °C, then for 2 h at 110 °C, and finally for 2 h at 150 °C. All mechanical tests were performed 1 day after sample preparation to minimize the effects of surface oxidation. For comparison, we prepared PDCPD/0.2 wt % SiO2 composites by the same methodology. 4751
DOI: 10.1021/acs.iecr.7b00093 Ind. Eng. Chem. Res. 2017, 56, 4750−4757
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Industrial & Engineering Chemistry Research 2.5. Fourier Transform Infrared (FTIR) Spectroscopy. The vinyl groups of V-SiO2 were confirmed by Fourier transform infrared (FTIR) spectroscopy using a Nicolet 6700 FTIR spectrophotometer (Thermo Fisher Scientific, Waltham, MA) within the range of 4000−400 cm−1. The particles were then diluted with KBr powder to roughly 1% by volume and pressed into pellets. 2.6. Thermogravimetric Analysis (TGA). The content of vinyl groups in V-SiO2 was measured by thermogravimetric analysis (TGA) in a TG/DTA 7300 instrument (Seiko, Chiba, Japan) from 25 to 800 °C at a heating rate of 10 °C/min under a constant nitrogen flow. 2.7. Tension Tests. Tension tests of the composites were performed with a universal testing machine at a crosshead speed of 5 mm/min. Testing samples were prepared according to standard method ISO 527-2, and the dimensions were 75 mm × 4 mm × 2 mm. The tension toughness is defined as the area under the tensile curve.10,17 2.8. Dynamic Mechanical Analysis (DMA). DMA was carried out on a DMA+1000 instrument (Metravib, Limonest, France) using rectangular samples (∼35 mm × 4 mm × 2 mm). DMA tests were performed in tensile mode at a frequency of 1 Hz and a ramp rate of 3 °C/min over the temperature range of 30−210 °C. The glass transition temperature (Tg) was recorded as the peak temperature of the tan δ curve. 2.9. Other Characterization Methods. Dynamic light scattering (DLS) was carried out on a Zetasizer ZS90 instrument (Malvern Instruments, Malvern, U.K.) to confirm the particle size and homogeneity of the prepared silica. The compatibility of V-SiO2 in DCPD monomer was measured with an Axio Imager A2 research microscope (Zeiss, Oberkochen, Germany). Field-emission scanning electron microscopy (FESEM) images were observed on an SU 70 FE-SEM instrument (Hitachi, Tokyo, Japan) to observe the internal morphology of the composites.
Figure 1. (a) FTIR spectra and (b) TGA curves of pure SiO2 and vinyl-SiO2.
the amount of hydroxyl groups in V-SiO2 is very limited. The FTIR spectroscopy and TGA results thus demonstrate that vinyl silica was successfully synthesized and that the vinyl group content was about 4.33 wt %, much higher than the amounts obtained in vinyl silica prepared by chemical modifications (generally less than 2 wt %).34 To obtain small-sized and uniformly distributed silicon spheres, we investigated the preparation parameters (reaction temperature, reaction time, and amount of ammonia) in detail. Dynamic light scattering (DLS) was used to measure the variation trends in the particle size and polydispersity index (PDI) of the prepared silicon spheres. All data are listed in Table 1. We found that the particle size and PDI gradually decreased with increasing amount of ammonia. However, the particle size tended to become stable when the amount of ammonia increased to a certain degree. With increasing reaction temperature, the particle size of the silicon spheres
3. RESULTS AND DISCUSSION 3.1. Characterization and Dispersion of V-SiO2. V-SiO2 was characterized by FTIR spectroscopy. Figure 1a shows FTIR spectra of pure SiO2 and V-SiO2. In the spectrum of pure SiO2, the absorption peaks at 1138 and 3452 cm−1 are attributable to the stretching vibrations of SiO and hydroxyl groups, respectively, whereas in the spectrum of V-SiO2, the absorption peak for the stretching vibrations of hydroxyl groups is obviously weaker. Meanwhile, the absorption peaks at 3061 and 2958 cm−1 in the spectrum of V-SiO2 are assigned to C H stretching vibrations, and the peaks at 1409 and 1050 cm−1 correspond to CC stretching vibrations and SiC stretching vibrations, respectively. V-SiO2 was further characterized by thermogravimetric analysis (TGA). Figure 1b shows TGA curves for pure SiO2 and synthesized V-SiO2. For pure SiO2, the weight loss between 30 and 200 °C is mainly due to the dehydration of adhered water. In the temperature range of 200−700 °C, dehydration condensation reactions of the silicon alcohol groups (SiOH) within the SiO2 occur, and the weight loss is about 5.35%. For synthesized V-SiO2, the minimal weight loss in the 30−200 °C range indicates the presence of less adhered water than for pure SiO2. Significant weight loss occurs in the range of 400−600 °C that can be attributed to the decomposition of the vinyl groups, with a weight loss of 4.33 wt %. Aside from the weight loss due to vinyl groups in the 400−600 °C range, there is no other obvious weight loss in the range of 200−700 °C, indicating that
Table 1. Mean Diameters and PDIs of Silica Spheres Obtained under Different Reaction Conditions
4752
temperature (°C)
NH4OH (mL)
time (h)
mean diameter (nm)
PDI
40 40 40 40 40 50 60
0.5 1 2 4 2 2 2
4 4 4 4 2 4 4
685.3 ± 32.2 644.8 ± 17.5 530.3 ± 22.0 565.3 ± 14.2 485.1 ± 28.6 339.9 ± 5.8 336.6 ± 4.1
0.211 0.088 0.075 0.061 0.242 0.028 0.140
DOI: 10.1021/acs.iecr.7b00093 Ind. Eng. Chem. Res. 2017, 56, 4750−4757
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Industrial & Engineering Chemistry Research gradually decreased, whereas the homogeneity increased, which might be the result of forming more nucleation sites because of the increase in the reaction rate at higher temperatures. Interestingly, the particle size remained almost unchanged after the reaction temperature had been increased to a certain value. In addition, reaction time is also an important parameter, and we found that silicon spheres with a larger size and better homogeneity could be obtained by prolonging the reaction time. Considering the size and uniformity of the silicon spheres, we determined the optimal preparation parameters to be as follows: amount of ammonia, 2 mL of ammonia/50 mL of H2O; reaction temperature, 50 °C; reaction time, 4 h (highlighted in bold in Table 1). To further test the uniformity and size of the silica particles, we obtained SEM images of the silica particles (see Figure 2b), which indicated that the prepared V-SiO2 had a mean diameter of about 400 nm and exhibited excellent uniformity.
Figure 3. (a,b) Optical photographs of (i) SiO2 in DCPD, (ii) V-SiO2 in DCPD, and (iii) DCPD monomer: (a) immediately after sonication and (b) 72 h after sonication. (c,d) Microscopy images of (c) SiO2 in DCPD and (d) V-SiO2 in DCPD.
the double bonds of the norbornene rings are opened, driven by ring-strain relief owing to the higher ring-strain energy of the norbornene rings compared with that of the cyclopentene rings,35 forming a linear chain. The remaining double bonds of the cyclopentene rings can be polymerized in a similar way to form a cross-linked PDCPD network.36 As Staub et al.32 reported, Grubbs catalyst can initiate double bonding in V-SiO2, and covalent bonds can be formed between PDCPD and V-SiO2. However, there are two possible types of polymerization mechanisms between V-SiO2 and DCPD, as illustrated in Scheme 2. In one mechanism, covalent bonds can Scheme 2. Polymerization of DCPD and V-SiO2: (I) Cyclopentene and (II) Norbornene Sites
Figure 2. (a) Size distribution histogram measured by DLS. (b) SEM image of V-SiO2 spheres.
In this work, to improve the dispersion and interfacial interactions of SiO2 in the DCPD monomer, we synthesized silica with nonpolar vinyl groups (V-SiO2). The V-SiO2 exhibited an obvious enhancement in dispersion and compatibility in DCPD, as confirmed by visual inspection. Figure 3 shows that the pure SiO2 started to aggregate within a short time after sonication, whereas V-SiO2 formed stable suspensions without visible aggregation even after 72 h. The microscope image of the V-SiO2/DCPD monomer suspension shown in Figure 3d provides further evidence of their compatibility. Micrometer-scale aggregates of SiO2 were observed in SiO2/DCPD suspensions (Figure 3c), whereas the V-SiO2 particles remained well-dispersed and homogeneously stable in V-SiO2/DCPD suspensions without obvious aggregation (Figure 3d). 3.2. Reaction Mechanism of DCPD and V-SiO2. The DCPD monomer contains two functional groups: norbornene and cyclopentene. When Grubbs catalyst is added to DCPD,
be formed between the vinyl groups of V-SiO2 and the double bonds of the cyclopentene rings of DCPD (Scheme 2I). In the other mechanism, the norbornene rings of DCPD encounter the vinyl groups on the silica surface and form covalent bonds (Scheme 2II). 3.3. Tensile Properties of PDCPD/V-SiO2 Composites. PDCPD/V-SiO2 composites were fabricated in this study to verify the effectiveness of V-SiO2 in improving the mechanical performance of PDCPD. Representative stress−strain curves are shown in Figure 4a. All specimens exhibited similar tensile behaviors. First, the stress increased linearly with strain, and the sample became uniformly stretched. When the elongation at break reached about 5%, yield occurred, and a necking zone 4753
DOI: 10.1021/acs.iecr.7b00093 Ind. Eng. Chem. Res. 2017, 56, 4750−4757
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elongation at break and toughness of the PDCPD composites improved. Neat PDCPD was found to have an εf of 9% and a toughness of 2.58 MPa. Surprisingly, the composites showed a dramatic increase in εf from 9.0% to 143.4% when just 0.2 wt % V-SiO2 was added, resulting in an increase in toughness from 2.58 to 40.94 MPa; that is, the tensile toughness of the composite with 0.2 wt % V-SiO2 increased by a factor of 14. In addition, compared with the sample containing 0.2 wt % VSiO2, that containing 0.3 wt % V-SiO2 exhibited an obvious decrease in elongation at break from 143.0% to 73.4%; thus, the toughness was reduced from 40.94 to 19.18 MPa, which might have been the result of a detrimental stress concentration caused by the partial agglomeration of V-SiO2, as discussed in the next section. In addition, the tensile samples exhibited the obvious phenomenon of stress whitening (inset of Figure 4b), possibly caused by the formation of subcritical microvoids and microcracks during stretching.37,38 For comparison, we prepared DCPD/0.2 wt % SiO2 and measured its mechanical properties. As shown in Figure S2 and Table S1 (Supporting Information), the enhancement of the PDCPD composites was similar to that observed using V-SiO2. However, compared with PDCPD/0.2 wt % V-SiO2, PDCPD/ 0.2 wt % SiO2 exhibited a marked decrease in elongation at break from 143.0% to 52.5%, which might have resulted from not only a considerable decrease of adhesion strength but also the agglomeration of SiO2 due to the incompatibility between the polar SiO2 particles and nonpolar DCPD monomers. 3.4. Toughening Mechanisms of PDCPD/V-SiO 2 Composites. In general, the fracture morphology of a composite is important because it correlates with the mechanical properties and many other physical properties.39 To investigate the dispersibility of V-SiO2 in PDCPD, the freeze-fractured surfaces of PDCPD/V-SiO2 composites were observed, and the results are shown in Figure 5. On the whole,
Figure 4. (a) Representative stress−strain curves of PDCPD composites. (b) Changes in tensile toughness with respect to VSiO2 content, with morphological changes in the lengths of elongated samples in the inset.
with a local decrease in width within the gauge region appeared. Subsequently, the stress remained unchanged as the necking zone was expanded along the gauge region until complete fracture or complete thinning occurred (see the inset of Figure 4b). Finally, the specimens with complete thinning were uniformly stretched again, and the stress increased with the increase in the strain until the specimens failed. The yield strengths (εy), elongations at break (εf), and toughness values (defined as the area under the tensile curve) of the composites are summarized in Table 2. As shown in Table 2, with increasing incorporation of VSiO2, the yield strength decreased slightly, whereas the Table 2. Tensile Properties of PDCPD Composites with VSiO2 V-SiO2 loading (wt %) 0 0.02 0.05 0.15 0.2 0.25 0.3
εy (MPa)
εf (%)
toughness (MPa)
± ± ± ± ± ± ±
9.00 ± 0.90 40.90 ± 3.20 130.69 ± 6.87 135.82 ± 6.13 142.97 ± 7.93 119.88 ± 4.64 73.44 ± 3.19
2.58 ± 0.25 11.67 ± 1.10 37.13 ± 3.50 39.23 ± 1.81 40.94 ± 3.56 32.19 ± 1.12 19.18 ± 1.70
37.50 36.46 34.32 34.17 35.22 33.22 33.74
1.40 4.26 3.38 2.73 3.58 0.65 3.16
change in toughness (%) 352.32 1339.15 1381.78 1468.82 1147.67 643.41
Figure 5. SEM images of the freeze-fractured surfaces of PDCPD/VSiO2 composites: (a) PDCPD/0.2 wt % V-SiO2, (b) PDCPD/0.3 wt % V-SiO2, and (c) magnified image of PDCPD/0.3 wt % V-SiO2. 4754
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particles [Al(OH) 3 , SiO 2 ] by imitating the cavitation mechanism of rubber-particle-toughened plastics. They pointed out that rigid particles could toughen resins by debonding and forming submicrometer voids at the interface between the rigid particles and the matrix and described toughening mechanisms including stress concentration, debonding, and shear yield. The increase in strain led to the appearance of microcracks originated from the coalescence of the voids (Figure 6c). Moreover, the SiO2 particles favored the formation of microcracks with some different directions in the sample. The microcracks tended to merge with each other to form macrocracks with a continuous increase in strain, and the interface was subjected to a wide range of plastic deformation. Finally, the growth of macrocracks led to the fracture of the sample. 3.5. Swelling Behaviors of PDCPD/V-SiO2 Composites. To investigate the change in cross-linking degree after the incorporation of V-SiO2, the swelling ratios of PDCPD/V-SiO2 composites were measured. According to Flory−Rehner theory, the greater the cross-link density of a composite, the lower the swelling ratio (defined as the weight ratio of the absorbed solvent to the specimen) exhibited by the composite.41 The PDCPD/V-SiO2 composites were soaked in toluene for 72 h, and then the amount of absorbed solvent was measured. As shown in Figure 7, the swelling ratio exhibited an upward trend
V-SiO2 was uniformly dispersed in all of the prepared samples (Figure 5a,b). However, it can be seen from Figure 5c and the black circles in Figure 5b that partial SiO2 agglomeration occurred in the PDCPD/0.3 wt % V-SiO2 composite. Aggregation of V-SiO 2 could severely deteriorate the mechanical performance of final products through the undesirable localized concentration of stress, resulting in much lower values of both the yield strength and the tensile toughness compared with those of PDCPD/0.2 wt % V-SiO2. To study the toughness enhancement of the PDCPD/V-SiO2 composites during stretching, the freeze-fractured surfaces of the PDCPD/V-SiO2 composites were prepared along a plane parallel to the tensile direction from the midthickness regions, as shown in Figure 6. When the stress reached a certain value,
Figure 7. Swelling ratios of PDCPD/V-SiO2 composites. Figure 6. SEM images of the freeze-fractured surfaces PDCPD/0.2 wt % V-SiO2 composite taken along a plane parallel to the tensile direction from the midthickness regions: (a) 40% elongation, (b) magnified image of 40% elongation, and (c) 80% elongation.
with increasing content of V-SiO2, which reveals that V-SiO2 impeded the formation of cross-links in the composites. The decrease in cross-linking degree can be attributed to the fact that, once a covalent bond was formed between a vinyl group of V-SiO2 and a double bond of DCPD, the active sites disappeared and chain propagation terminated. For highly cross-linked PDCPD, according to our toughness results, a decrease in cross-linking degree is important for toughening. 3.6. Dynamic Mechanical Behavior of PDCPD/V-SiO2 Composites. The dynamic mechanical properties of PDCPD composites were characterized by DMA in tensile mode, which provides the storage modulus (E′) and damping factor (tan δ). Figure 8 shows representative curves of the storage modulus and tan δ as functions of temperature, and the numerical results for E′ and Tg are listed in Table 3. The storage moduli in the glassy state and in the rubbery region increased slightly with increasing content of V-SiO2 because of the stiffening effect of the silica. In addition, the glass transition temperature (Tg, defined as the peak temperature of the tan δ curve) shifted toward higher temperatures. This increase in Tg is mainly attributable to the incorporation of the rigid silica particles, which reduce the mobility and hinder the movement of the
the difficulty of deforming rigid SiO2 led to interfacial debonding between the rigid silica particles and the PDCPD matrix, giving rise to substantial microvoids around the particles in the direction parallel to that of the applied stress (Figure 6a). In addition, compared with V-SiO2 in Figure 2a, V-SiO2 in PDCPD/V-SiO2 (Figure 6a,b) showed an increase in mean diameter from about 400 nm to about 550 nm and exhibited a relatively rougher interface, indicating that covalent bonds were probably formed between the vinyl groups and the PDCPD matrix, which would be important for enhancing toughness. Because of the formation of microvoids and the continuous growth of the microvoids themselves, the load state transformed from a plane strain state to a plane stress state, forming weak shear bands in the matrix ligaments between particles, and then shear yielding occurred. These processes absorb a great deal of energy, resulting in an improvement in toughness. Similar results were reported by Kim et al.,40 who proposed a toughening mechanism model of resin toughened with rigid 4755
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the glassy state and the rubbery region increased upon the addition of V-SiO2.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00093. SEM images of pristine Grubbs catalyst and recrystallized Grubbs catalyst. Tensile properties of PDCPD/0.2 wt % V-SiO2 and PDCPD/0.2 wt % SiO2, including yield strength, elongation at break, tensile toughness, and representative stress−strain curves (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-0574-87609986. Fax: +86-0574-87609986. E-mail:
[email protected]. ORCID
Li Zhang: 0000-0003-2684-0414 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51403110), the Scientific Research Fund of Zhejiang Provincial Education Department (No. Y201430774), the Natural Science Foundation of Ningbo City (No. 2015A610047), and the K.C. Wong Magna Fund in Ningbo University.
Figure 8. DMA curves of PDCPD composites: (a) storage modulus (E′) and (b) tan δ.
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Table 3. DMA Results of PDCPD Composites with V-SiO2 V-SiO2 loading (wt %)
E′ at 30 °C (MPa)
E′ at (Tg + 30 °C) (MPa)
Tg (°C)
0 0.02 0.05 0.2 0.3
1270 1290 1300 1330 1360
21.0 21.4 22.8 24.6 27.4
135.6 137.2 138.9 143.7 145.1
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PDCPD molecular chains, leading to a reduction of the free volume of the polymer.
4. CONCLUSIONS We successfully prepared high-toughness PDCPD composites through the incorporation of low amounts of vinyl-functionalized SiO2 (V-SiO2). The V-SiO2 was synthesized by a facile, reproducible, one-step, remodeled synthetic sol−gel process and characterized with respect to the content of vinyl groups, particle size, and morphology by FTIR spectroscopy, TGA, SEM, and DLS. The V-SiO2 was found to have a mean diameter of 340 nm, a vinyl group content of 4.33 wt %, and excellent uniformity. Compared with the properties of neat PDCPD, the elongation at break of the PDCPD/0.2 wt % VSiO2 composite increased markedly from 9.0% to 143.4%. Correspondingly, the tensile toughness was enhanced by a factor of 14 from 2.58 to 40.94 MPa, which can be attributed to the formation of microvoids and the appearance of microcracks during stretching. Our DMA results showed that Tg shifted toward higher temperatures and that the storage moduli in both 4756
DOI: 10.1021/acs.iecr.7b00093 Ind. Eng. Chem. Res. 2017, 56, 4750−4757
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