English

• 1. INTRODUCTION

  Polyurethane (PU), prepared through the addition reaction between disocyanate and polyol, is a special group of segmented copolymers and is used in the field of coatings^[1], adhesives^[2], rubbers and foams^{[3, 4]}. Through the modification of polyol polymer chain and optimizing synthesis routes, PU can be prepared to different final products with various mechanical properties including polyurethane foams^[5], cast polyurethane elastomers^[6], thermoplastic polyurethanes^[7], and so on. In the market of elastomers, cellular elastomers, a casted elastomer comprising isocyanate, polyol, chain extenders, and additive, domains up to about 40% of the elastomers. Because of the better resistance to compaction during wearing than ethylene vinyl acetate (EVA), cellular elastomers are used in the field of footwear as midsoles^[8]. Nevertheless, the poor antibacterial for PU is still uncapable to fulfill the requirement proposed by footwear industry. As a commonly used wearable material in daily life, a better antibacterial activity and better mechanical properties were demanded.

  To solve this problem above, various antibacterial agents were developed^[9-12]. Among them, silver nanoparticles (AgNPs) were most widely used due to its wide-spectrum antibiotic, high efficiency, and long-term durability^[13]. Additionally, thanks to the nano effects, the addition of AgNPs usually leads to an enhancement in the mechanical properties of polymer matrix^[14]. However, high specific surface area and high surface energy of AgNPs together make it easy to agglomerate while being dispersed into polymer matrix, which deteriorates both antibacterial activity and the enhancement of mechanical properties^[15]. Vinay et al.^[16] introduced AgNPs into auxetic PUFs, and the as-prepared materials showed an improvement in both compression strength and antibacterial properties. Zhao et al.^[17] introduced AgNPs into waterborne PUFs and found that the antibacterial property of as-prepared nanocomposites were prominently improved along with the tensile properties. Nevertheless, they also found that the tensile properties were decreased with a further increase of AgNPs loading because of the agglomeration of nano-particles. Wattanodorn et al.^[18] introduce AgNPs into PUFs by in situ reduction method and tried to disperse the antibacterial agent uniformly in the polymer matrix. Result showed that as-prepared antibacterial foams exhibit an obvious increase in mechanical properties. Considering the literatures mentioned above, how to disperse AgNPs uniformly in the polymer matrix is what researchers are concerning about.

  With the rapid development of nanotechnology, carbon materials on nano-dimensional materials have attracted a wide concern. Recently, carbon-based nanomaterials including carbon nanotube, carbon nanofiber, and graphene play important roles in various fields including aerospace^[19], biomedical^[20], automotive^[21], electronic^[22], etc. Due to its favorable introduced chemical and physical properties, carbon nanomaterials, which are treated as a reinforcement phase, can be combined into a polymer matrix to prepare nanocomposites. The uniformly dispersed carbon nanocomposites in the polymer matrix would enhance the mechanical properties^{[23, 24]}, electrical properties^[25], thermal properties^[26], wave absorbing^[27], and electromagnetic shielding^[28]. Among various carbon nanomaterials, graphene nanomaterials with a planar structure have attracted great interests whether in academia or industry^[29]. Polyurethane/graphene nanocomposite also has aroused research interests in the field of polyurethane foam. Coated with graphene nanoparticles and their derivates, oil absorbent property^[30], hydrophobic property^[31], sound damping property^[32], and electronic property^[33] can be also greatly improved.

  In recent years, researchers found that AgNPs can be uniformly anchored on the graphene oxide (GO) nanosheet^[34]. Besides, the anchored Ag nanoparticles will construct a codispersing GO–Ag nanosystem, in which the AgNPs and GO sheet can support each other hindering their individual aggregation^[35]. With the combination of the properties of GO and AgNPs, silver-coated graphene nanocomposite (Ag/GO) would exhibit a better stability in polymer matrix. Bao et al.^[36] took hydroquinone as a reductant and prepared GO-AgNPs successfully. As-prepared AgNPs exhibited excellent antibacterial activity for E. coli. and S. aureus. Besides that, Ag/GO nanoparticle has been reported to show a great effect in anticancer^[37] and antiviral^[38]. Moreover, compared with AgNPs or GO nanosheets only, Ag/GO is easier to be dispersed in the polymer matrix^[35]. Though Ag/GO nanoparticles have great potential in the enhancement and functionalization of the polymer matrix, few reports about their application in polyurethane foams (PUFs) have been reported. Therefore, in this paper, we developed an efficient and non-toxic method to prepare the Ag/GO nanoparticles. Accordingly, a series of PUFs containing Ag/GO was prepared. Corresponding mechanical properties and antibacterial properties were also investigated in detail.

  2. EXPERIMENT

  2.1 Reagents and materials

  The AgNPs (30 ± 5 nm) were obtained from InnoShines Technology Co. Ltd. The polyester polyol and methylene diphenyl diisocyanate were kindly provided by BASF China; the hydroxyl value of the polyester polyol was measured at 56 mg KOH/g. The graphite was supplied by Shanghai Macklin Biochemical Co., Ltd. Trimethylene diamine, H₂SO₄ (98%), KNO₃, KMnO₄, H₂O₂, pepsin from the porcine stomach, and AgNO₃ were commercial products with analytical purity and used without further purification. Distilled water was produced in our laboratory.

  2.2 Synthesis of GO and Ag/GO

  A modified Hummers method^[39-41] was involved to prepare GO. In a typical synthesis, 3 g of graphite, 3.6 g of KNO₃, and 200 mL of H₂SO₄ were together added into a three-necked bottle with vigorous stirring in an ice bath. The mixture was reacted at 35 ℃ for 6 h. Then 250 mL distilled water was slowly added to the mixture, and kept the reaction under 5 ℃ for 10 h. After that, the mixture was poured into 80 mL H₂O₂ and further diluted with 600 mL distilled water to stop the reaction. Finally, a yellow powder was obtained after repeatedly washing with distilled water and completely drying by lyophilization, and named as GO.

  The Ag/GO nanocomposite was prepared by pepsin as a reductant. Typically, 100 mg as prepared GO powder was dispersed in 80 mL distilled water and sonicated for 30 minutes to form a homogeneous solution. Then, 100 mg pepsin was fully dissolved into the GO solution. Afterwards, 20 mL AgNO₃ aqueous (1 mM) was rapidly poured into the mixture, then kept stirring for 6 h under dark at room temperature. Finally, the samples were collected by centrifugation at 10, 000 rpm for 8 min with distilled water for several times and dried at 60 ℃ in a vacuum for 3 h.

  2.3 Preparation of nanoparticles reinforced polyurethane foams

  In this study, PUFs were prepared by a two-step method. Appropriate amounts of as-prepared nanoparticles (AgNPs, Ag/GO), polyester polyol, catalysts (triethylene diamine), and distilled water were fully mixed. Followed by that, the isocyanate prepolymer was added into the mixture with vigorous stirring and poured into a steel mold for foaming and then kept post-curing for 12 h at 60 ℃.

  2.4 Characterization

  Transmission electron microscopy (TEM) was performed on a JEM-2010 (JEOL, Japan) at 200 kV accelerating voltage. The as-prepared GO and Ag/GO was prepared by mounting a drop of the micelle solution (0.05 mL) on a copper EM grid covered with a thin film of formvar. Scanning electron microscope (SEM) (HITACHI, SU8010/EDX, JAPAN) was employed to observe the morphologies of the nanoparticles reinforced PUF.

  The apparent density of as-prepared PUF was measured according to ISO 845. The PUFs were cut into small cubes of 1 cm³. The mass of the cubes was measured by electronic analytical balance (Mettler Toledo, MS104TS/02, China).

  The resilience of the particle reinforced PUFs was measured by ball rebound according to ISO 8307. The diameter of the iron ball was 16 mm and the dropping height was 460 mm.

  The tensile property test was performed according to ASTM D638 with a constant speed of 5 mm·min^-1 using a load cell of 1 kN. The results were averaged from five specimens.

  2.5 Antibacterial test

  The turbidimetric method and inhibition zone method were used to test the antibacterial properties of AgNPs or Ag/GO reinforced PUFs. The S. aureus (ATCC 25923) was involved in these two tests. The turbidimetric method was conducted according to ASTM D6756, the bacterial concentration was monitored by measuring the OD at 595 nm on a microplate reader (iMark, Bio-Rad Laboratories, Inc); the inhibition zone method was conducted according to ISO 20645. All the tested samples were cut into small cubes with a size of 1 cm³ and then sterilized for 30 min by UV light.

  3. RESULTS AND DISCUSSION

  3.1 Synthesis and characterization

  As schematically illustrated in Fig. 1a, an efficient and non-toxic method was proposed to prepare Ag/GO nanoparticles. In first step, modified Hummers method was involved to prepare the GO. Since pepsin can be used as an eco-friendly reducing and stabilizing agent to prepare metal nanoparticles^[42], here it was involved to coat silver nanoparticles onto the GO nano-sheets. Then the uniformly coated Ag particles, as a dissolution antibacterial agent, partially dissociated Ag⁺ cation, which can be absorbed on the bacteria's membrane and denature it, finally ruptured the bacterial^[43].

  Figure 1

  

  Figure 1.  (a) Schematic illustration of the preparation of Ag/GO. (b) TEM image of the as-prepared GO (left) and Ag/GO (right)

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  Captured TEM pictures (Fig. 1b) show that the few-layer graphene with high transparency was prepared after oxidation and intercalation by Hummer's method. While after being treated by pepsin, the AgNPs was uniformly coated on the GO nano-sheets, and the diameter of the coated AgNPs was measured around 20~50 nm.

  3.2 Mechanical properties

  The mechanical property plays an important role in materials application. To study the effect on mechanical properties of the PUFs brought by the content of reinforced particles, PUFs with the different ratios (0.2%, 0.4%, 0.6%, 0.8%, and 1.0%) of AgNPs and Ag/GO were prepared.

  As shown in Fig. 2a, with the addition of both AgNPs and Ag/GO, the density increases, and the rate of increase accelerated with the loading of two nanoparticles increased. Among them, compared with the foams reinforced with Ag/GO, the density of PUFs reinforced with AgNPs is higher at each content.

  Figure 2

  

  Figure 2.  Mechanical properties of PUFs with different rates of AgNPs and Ag/GO density (a), resilience (b), tensile tests (c). (d) morphology of pure PUFs (left), 0.4% AgNPs reinforced PUFs (middle), and Ag/GO reinforced PUFs (right)

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  Fig. 2b shows the relation between nanoparticle content and PUFs' resilience. From the picture, it can be concluded that the resilience of PUFs show a trend of rising at initial stage and then decreasing. Opposite with the density, the resilience of the foams reinforced with Ag/GO is always higher than that of AgNPs. The resilience of two nanoparticles reinforced PUFs reaches the maximum value, 40.2% (Ag/GO) and 39.5% (AgNPs), respectively.

  The tensile properties of different PUFs show a similar trend of resilience (Fig. 2c). With the addition of nanoparticles, both the tensile strength and elongation at break are increased firstly when the content of the reinforced particles is lower than 0.4%, and then decreases. As the same as the resilience properties, the PUF reinforced by Ag/GO is always stronger than that of AgNPs at each content. The maximum value of tensile strength and corresponding elongation at the break of AgGO and AgNPs reinforced nanocomposites are 40.25 MPa/778.1% and 39.52 MPa/730.5%, respectively.

  To further study the mechanism of the mechanical property improvement brought by the AgNPs and Ag/GO, the SEM images of pure PUF, PUFs containing 0.4% AgNPs and Ag/GO were captured and presented in Fig. 2d. For pure PUF, the cellular structure is larger compared with nanoparticles reinforced PUFs. For the AgNPs reinforced PUF, the cellular is smaller but more concentrated, and the size is uneven. For the PUFs containing 0.4% Ag/GO, the cellular structure is uniform, and each pore is connected. The added nanoparticles provide active sites when the polyurethane is reacting and forming^[44]. Such active sites make PUFs easier to foam, leading to a smaller cellular generation in PUFs. However, due to the agglomeration of AgNPs, the foams of PUFs nanocomposites are concentrated, while the Ag/GO is easier to disperse, resulting in a more uniform structure along with a significant improvement of mechanical properties.

  3.3 Antibacterial test

  To evaluate the antibacterial properties of two nanoparticles reinforced PUFs, the turbidimetric method and inhibition zone method were involved here.

  The PUFs reinforced with two different weight ratios of nanoparticles was tested by turbidimetric method, and corresponding results are shown in Fig. 3a. With the increase of antibacterial nanoparticles, the bacteriostasis rate is constantly increasing, but the increasing speed is slowing down. Thanks to the well-dispersed Ag/GO, PUFs containing Ag/GO exhibit a better antibacterial ability, no matter what the weight ratio of Ag/GO is.

  Figure 3

  

  Figure 3.  (a) Bacteriostasis rate of PUFs with different rates of AgNPs and Ag/GO tested by the turbidimetric method. (b) Bacteriostasis rate of PUFs with AgNPs (left) and Ag/GO (right) at a content of 0.4% by the inhibition zone method

  DownLoad: Full-Size Img PowerPoint

  The inhibition zone method can display the antibacterial ability more intuitively. Here, the PUFs contents 0.4% AgNPs and Ag/GO were involved in this test (Fig. 3b). From the picture, it can be seen that the foam containing Ag/GO has a larger inhibition zone compared with that with AgNPs, which demonstrated that Ag/GO is a more effective antibacterial agent for PUFs than AgNPs.

  4. CONCLUSION

  In summary, a facile and non-toxic method to prepared Ag/GO nanoparticles was presented in this paper. Compared with traditional antibacterial nanoparticles, Ag/GO nanoparticles can be dispersed more homogeneously in PUFs. By comparison with AgNPs, the as-prepared Ag/GO nanoparticles modified PUFs have a 1.85% improvement in resilience, 7.9% improvement in tensile strength, 6.52% improvement in tensile elongation at break, and 8.74% in bacteriostats rate, at a concentration of 0.4%. These promising results provide a facile and environmentally friendly way to prepare functionalized polyurethane foams, which have high academic and application value.

  
  
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  Figure 1  (a) Schematic illustration of the preparation of Ag/GO. (b) TEM image of the as-prepared GO (left) and Ag/GO (right)

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  Figure 2  Mechanical properties of PUFs with different rates of AgNPs and Ag/GO density (a), resilience (b), tensile tests (c). (d) morphology of pure PUFs (left), 0.4% AgNPs reinforced PUFs (middle), and Ag/GO reinforced PUFs (right)

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  Figure 3  (a) Bacteriostasis rate of PUFs with different rates of AgNPs and Ag/GO tested by the turbidimetric method. (b) Bacteriostasis rate of PUFs with AgNPs (left) and Ag/GO (right) at a content of 0.4% by the inhibition zone method

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