2017 Volume 58 Issue 5 Pages 842-844
Graphene oxide/polyurethane/epoxy resin nanocomposites containing various contents of graphene oxide were prepared by a sequential physical and polymeric technique. For the nanocomposites with a 0.3 mass% loading of graphene oxide, great improvement in tensile properties such as the elongation at break and toughness have been achieved by 45% and 87%, respectively. Meanwhile, the damping property of the nanocomposites is superior to that of the polyurethane/epoxy composite, with a 1℃ loss for the glass transition temperature. Combined with the morphology analysis, it has been proved that the polyurethane prepolymer terminated with hydroxyl groups and graphene oxide exhibit synergistic effect on improving mechanical properties of neat epoxy resin.
Epoxy resin (EP) is classified as one group of the most important thermosetting polymers characterized by high temperature resistance1), electrical insulation2), chemical resistance3), and anti-corrosion performance4). However, EP is generally brittle polymers with poor toughness and damping property, which restricts its application. Polyurethane (PU) is a flexible and elastic polymer, which has been widely used to improve the mechanical properties of EP. Generally the modification process of EP with PU is conducted by reacting polyol and isocyanate first to produce PU prepolymer terminated with isocyanate groups, then the PU prepolymer product is reacted with EP5,6). With the rise of nano science and technology, an increasing number of composites of EP and PU modified by graphene compounds have been reported, which are featured by enhanced mechanical property and excellent thermal stability7–11). Nevertheless, seldom study focused on the comprehensive influence of PU prepolymer and graphene compounds on EP based composites. In this research, different with the process in general, the PU prepolymer terminated with hydroxyl groups was utilized to modify graphene oxide (GO)/EP dispersion matrix which was obtained through a modified pressurized oxidation method followed by a facile two-phase extraction method. Then the GO/PU/EP nanocomposites were prepared via in situ thermal polymerization. Furthermore, the synergistic effect of GO and PU prepolymer terminated with hydroxyl groups on properties and morphologies of these nanocomposites was investigated.
Graphite flakes (100 mesh) was bought from Tianjin Dingshengxin Chemical Co. (China). Toluene-2,4-diisocyanate (TDI) and dibutyltin dilaurate (DBTDL) were purchased from Sigma-Aldrich Ltd. Potassium permanganate (KMnO4), hydrochloric acid (HCl), sulfuric acid (H2SO4), hydrogen peroxide (H2O2, 30% aq.), and polytetramethylene-oxide glycol (PTMG) were provided by Nanjing Chemical Reagent Co. (China). Diglycidyl ether of biphenol A (DGEBA) epoxy resin E-51 (with an epoxide equivalent weight of 196 g/mol), Methyl tetrahydrophthalic anhydride (MTHPA), and 2,4,6-tri(dimethylaminomethyl) phenol (DMP-30) were made in Tianjin Chemical Co. (China).
2.2 Process 2.2.1 Preparation of the graphite oxideGraphite oxide was synthesized via a modified pressurized oxidation method. More details about the synthesis of graphite oxide can be found in our earlier work12).
2.2.2 Preparation of the GO/PU/EP compositeFirstly, 14.96 g PTMG was dehydrated in vacuum oven, and then mixed with 1.04 g TDI (OH:NCO = 1:0.8, molar ratio) and 1～2 drops DBTDL. Afterward, the mixture was sealed and heated to 60℃ for 1 h to obtain the polyurethane prepolymer terminated with hydroxyl groups. Secondly, calculated amount of graphite oxide was exfoliated in 30 mL deionized water using ultrasonication with a power of 70 W for 0.5 h at room temperature, and poured to 80 g E-51 epoxy resin. The mixtures were vigorously stirred for 4 h in an oil bath (～50℃), and then allowed to stand for several hours to separate into two layers. After completely removing the upper layer of water in an oil bath (～70℃), the GO/epoxy resin dispersion was obtained. Thirdly, the GO/epoxy resin dispersion was mixed with the as prepared polyurethane preploymer, 60 g MTHPA, and 0.8 g DMP-30. After that, the mixtures were vigorously stirred for 0.5 h at room temperature and vacuumed to remove air bubbles. The as prepared mixtures were poured into a steel mold, and then vacuumed twice within 1 h at 80℃. Finally, the samples were pre-cured at 120℃ for 3 h and post-cured at 140℃ for another 3 h. Thereafter, the GO/PU/EP composites were obtained via in situ thermal polymerization and cut into specific dimensions for characterization. The GO/PU/EP composites with different GO content (0.1%, 0.3%, by weight, based on the amount of E-51 epoxy resin and MTHPA) were marked as GO/PU/EP1 and GO/PU/EP2 respectively. In control experiments, the neat epoxy resin and PU/EP composite samples were manufactured in a similar way.
2.3 MeasurementsThe tensile properties of samples were determined using an Instron universal testing machine (model 4502) according to ASTM D638-2014. 5 type I specimens with a thickness of 4 mm for each sample were tested at a tensile rate of 5 mm/min. Specimens were conditioned 48 h at 23℃ and 50% relative humidity, and tested at 23℃ and 50% relative humidity. Dynamic mechanical analysis (DMA) of the neat PU and PU/GO nanocomposite films was performed using a Netzsch instrument (DMA 242C) with a three points bending mode of 1 Hz and a heating rate of 4℃/min. A scanning electron microscopy (SEM, JEOL JSM-6480) was employed to investigate the fracture morphologies of the post-tensile testing samples.
The typical tensile stress-strain curves for neat EP, PU/EP composite, and GO/PU/EP composites are shown in Fig. 1. Table 1 summarizes the tensile mechanical properties of neat EP, PU/EP composite, and GO/PU/EP composites. In Table 1, the tensile strength, elongation at break, and toughness (obtained from the area under the corresponding tensile stress–strain curves) of neat EP were effectively improved by the introduction of PU. Moreover, the tensile properties are further increased significantly with the addition of the GO. For instance, compared with PU/EP composite, with the incorporation of 0.3 mass% of GO (GO/PU/EP2), the tensile strength is enhanced by 22%, the elongation at break is increased by 45%, and the toughness is improved by 87%, respectively. The evolution of tensile properties indicates that the polyurethane prepolymer terminated with hydroxyl groups and GO have the synergistic effect on improving tensile properties of neat EP. On one hand, the introduction of polyurethane prepolymer terminated with hydroxyl groups provides additional free volume, leading to more facilitated segmental movements13). On the other hand, the wrinkled topology at the nanoscale of GO results in an enhanced mechanical interlocking with the polymer chains, while the abundant oxygen-containing groups on GO's basal plane and edges may form covalent bonds with polymer matrix14). The cooperation of additional free volume and the strong interfacial interactions between GO and polymer matrix substantially strengthen and toughen the host polymer.
Tensile strain-stress curves for different samples.
| Tensile properties |
Neat EP | PU/EP | GO/PU/EP1 | GO/PU/EP2 |
|---|---|---|---|---|
| Tensile strength, σb/MPa |
30.29 ± 1.16 | 54.39 ± 2.44 | 65.41 ± 3.08 | 66.38 ± 3.28 |
| Elongation at break, εt/% |
2.64 ± 0.12 | 7.54 ± 0.36 | 9.90 ± 0.41 | 10.96 ± 0.50 |
| Toughness Acurve/MPa |
42.93 | 221.96 | 370.30 | 414.10 |
*5 specimens were tested for each sample, ± means standard deviation
The DMA measurements for neat EP, PU/EP composite, and GO/PU/EP composites are shown in Fig. 2. Figure 2(a) demonstrates temperature dependence of storage modulus for different samples. Clearly, in the glassy state, the storage modulus decreases significantly with the incorporation of the polyurethane prepolymer terminated with hydroxyl groups, which indicates that the introduced additional free volume soften the epoxy resin substantially. Nevertheless, the storage modulus increases from about 42 MPa for PU/EP composite to around 60 MPa for GO/PU/EP2. This can be ascribed to the GO encapsulated in the polymer matrix, which hinders the macromolecular chain segmental movements, and results in relatively harder composites compared with the polymer matrix. The loss factor (tan delta) peaks to maximum in Fig. 2(b) present the glass transition temperature (Tg), as well as the beginning temperature for energy dissipation which occurs during the transition of the glassy state to a rubber state. The single peaks in the loss tangent curves indicate that no phase separation occurred for the polymer base composites. Due to the additional free volume provided by the polyurethane prepolymer terminated with hydroxyl groups, the Tg shifts lower from 134℃ for neat EP to 128℃ for PU/EP composite. However, the Tg of GO/PU/EP composites barely changes with the introduction of GO (128℃ for GO/PU/EP1, and 127℃ for GO/PU/EP2.). Compared with neat EP and PU/EP composite, the segments of all nanocomposites relax in a wider temperature range. Especially for GO/PU/EP2, the absolute value of the loss tangent curve maximum is larger than that of neat EP and PU/EP composite15). The DMA analysis indicates that the polyurethane prepolymer terminated with hydroxyl groups and GO have the synergistic effect on improving damping property of neat EP and PU/EP composite with a little loss for Tg.
(a) Storage modulus versus temperature for different samples, (b) Loss factor versus temperature for different samples.
Figure 3 presents the morphologies of fractured surfaces of neat EP, PU/EP composite, and GO/PU/EP composites after tensile testing. The fracture surface of the neat EP is relatively smooth with straight ravines and gullies, and exhibits stiffness. The straight cracks in different planes without any deformation account for its poor toughness. While the fracture surface of the PU/EP composite become rough with a scaly morphology, and exhibit toughness. The rough fracture surface showed the deviation of cracks from their original plane, and the increasing area of the cracks, indicating that more energy is required for the propagation of the cracks on the fracture surfaces of PU/EP composite. The fracture surface of the GO/PU/EP composites exhibits a river like morphology with a large number of cracks. Because of the tremendous difference of modulus between GO and matrix polymer, GO functions as stress concentration point, and induces microcracks around it, which consumes additional fracture energy. Especially for GO/PU/EP2(Fig. 3(h)), the fracture surface showed tough dimples due to shear yielding. These results prove that the polyurethane prepolymer terminated with hydroxyl groups and GO have the synergistic effect on improving mechanical properties of neat EP.
SEM images of (a) and (b) Neat EP, (c) and (d) PU/EP composite, (e) and (f) GO/PU/EP1 composite, (g) and (h) GO/PU/EP2 composite.
It is concluded from this study that the polyurethane prepolymer terminated with hydroxyl groups and GO exhibit synergistic effect on improving mechanical properties of neat EP. With the incorporation of 0.3 mass% of GO to the PU/EP composite, the tensile strength is enhanced by 22%, the elongation at break is increased by 45%, and the toughness is improved by 87%, respectively. The damping property of EP/PU composite is improved with a 1℃ loss for Tg simultaneously. The evolution of the fracture-surface images coincide well with the improvement of mechanical properties.
This work was supported by the Prospective Joint Research Project of Jiangsu Province (Grant No. BY2016073-02), and the General Project of Natural Science Research of Higher Education Institutions of Jiangsu Province (Grant No. 15KJB430010).
