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Effects of flower-like ZnO nanowhiskers on the mechanical, thermal and antibacterial properties of waterborne polyurethane
Polymer Degradation and Stability 94 (2009) 1103–1109
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Effects of flower-like ZnO nanowhiskers on the mechanical, thermal and antibacterial properties of waterborne polyurethane Xue-Yong Ma, Wei-De Zhang* Nano Science Research Center, School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 February 2009 Received in revised form 17 March 2009 Accepted 22 March 2009 Available online 7 April 2009
A novel waterborne polyurethane/flower-like ZnO nanowhiskers (WPU/f-ZnO) composite with different f-ZnO content (0–4.0 wt%) was synthesized by an in-situ copolymerization process. The f-ZnO consisting of uniform nanorods was prepared via a simple hydrothermal method. In order to disperse and incorporate f-ZnO into WPU matrix, f-ZnO was modified with g-aminopropyltriethoxysilane. Morphology of f-ZnO in WPU matrix was characterized by scanning electron microscope. The properties of WPU/f-ZnO composites such as mechanical strength, thermal stability as well as water swelling were strongly influenced by the f-ZnO contents. It was demonstrated that appropriate amount of f-ZnO with good dispersion in the WPU matrix significantly improved the performance of the composites. The mechanical property was enhanced with an increase of f-ZnO content up to the optimum content (1 wt%) and then declined. Incorporation of f-ZnO enhanced the water resistance of the composites remarkably. It was amazing to observe that the thermal degradation temperatures of the composites initially decreased significantly and then leveled off with content increase of f-ZnO, which was different from the results of other WPU composite systems reported. Antibacterial activity of WPU/f-ZnO composite films against Escherichia coli and Staphylococcus aureus was also tested. The results revealed that the antibacterial activity enhanced with the increasing f-ZnO content, and the best antibacterial activity was obtained at the loading level of 4.0 wt% f-ZnO. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Waterborne polyurethane Nanocomposites ZnO nanowhiskers Thermal decomposition Antibacterial activity
1. Introduction Nanocomposite is a class of materials with unique physical properties and wide application potential in diverse areas [1]. Dispersion of nanoscaled inorganic fillers into an organic polymer to form polymer nanocomposites has gained increasing interest in recent years. Controlling the nanostructure, composition and morphology of nanocomposites plays an essential role in their applications. Novel properties of nanocomposites can be obtained by successful imparting of the characteristics of parent constituents to a single material [2]. These materials differ from both pure polymers and inorganic fillers in some physical and chemical properties. The combination of polymers and nanoscale inorganic fillers is opening pathways for engineering flexible composites that exhibit attractive mechanical, thermal, optical and electrical properties compared with conventional composites [3,4]. ZnO is an important and attractive semiconductive material. It has drawn enormous attention due to its fantastic characteristics in
* Corresponding author. Tel.: þ86 20 87114099; fax: þ86 20 87112053. E-mail address: [email protected] (W.-D. Zhang). 0141-3910/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2009.03.024
optics, photonics and electronics [5]. Furthermore, ZnO shows a marked antibacterial activity at pH values in the range from 7 to 8 without the presence of light [6,7]. Zinc is also a mineral element essential to human beings. ZnO nanostructures can be obtained by various methods including thermal evaporation, electrochemical deposition, sonochemical method, sol-gel, hydrothermal synthesis and so forth. Various one-dimensional (1D) ZnO nanostructures have been realized, such as nanorods, nanowires, nanobelts, nanosheets, nanotubes, nanonails and so on [8–12]. Among the 1D ZnO nanostructures, nanorods have been widely studied because of their easy preparation and wide applications [8]. In previous studies, ZnO nanostructures were combined with PMMA [13,14], polystyrene [15], polyamide [16], polyacrylonitrile [17], polyacrylate [18], etc. Compared with the original polymers, the yielded composite materials present a lot of excellent performance. Polyurethanes (PU) are probably the most versatile class of polymers due to the great variety of raw materials that can be used for their formation. Waterborne polyurethane (WPU) shows many excellent features compared to conventional organic solvent-based polyurethane with the advantages of non-pollution and nontoxicity and can be used in various fields, such as coating, adhesives for textile, paper, wood or glass fibers, and so forth [19]. Thermal
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stability, insolubility, and mechanical properties of WPU, however, are still lower than those of solvent-based PU and need to be improved. Meanwhile, as a type of polymer, PU is susceptible to be degraded by many types of bacteria [20]. Nanosized additives are used as an effective strategy to alter and enhance the properties of WPU. Various types of filler, like clay [21], CNTs [22], silica [23], hydroxyapatite [24], Au [25], Ag [26] and flax cellulose [27] have been incorporated into WPU to prepare nanocomposites. The results demonstrated that homogeneous dispersion of fillers in WPU matrix significantly improved the performance of the nanocomposites. To our knowledge, no work has been reported about the preparation of WPU modified by ZnO nanowhiskers. Flower-like ZnO nanowhiskers can be widely used as polymer additives to make functional nanocomposites because of their high aspect ratio, together with good comprehensive properties such as semiconductivity, high mechanical strength, wear resistance, vibration insulation, microwave absorption and antibacterial effect. Herein, we report preparation and characterization of waterborne polyurethane/flower-like ZnO nanowhiskers (WPU/f-ZnO) composites. In this study, flower-like ZnO nanowhiskers (f-ZnO) were synthesized by a simple hydrothermal method and modified with g-aminopropyltriethoxysilane (APS) to improve the bonding between f-ZnO and the WPU matrix. Properties of WPU/f-ZnO composites such as mechanical strength, thermal stability, water swelling as well as the antibacterial effect against Escherichia coli and Staphylococcus aureus were all influenced by the content of fZnO. 2. Experiment 2.1. Materials ZnO and NaOH were of analytical grade supplied by Guanghua Chemical Factory Co., Ltd. (Guangdong, China). Silane coupling agent g-aminopropyltriethoxysilane (APS) was purchased from Shuguang Chemical Co., Ltd. (Nanjing, China) and used without further purification. Poly (butyl adipate) diol (PBA; average molecular weight Mn ¼ 1000 g/mol) was obtained from Hodotani Co. (Tokyo, Japan). Dimethylol propionic acid (DMPA), 3-isocyanatemethyl-3,5,5-trimethyl-cyclohexylisocyanate (IPDI), trimethylolpropane (TMP) and dibutyl tin dilaurate (DBTDL) were purchased from First Chemicals of Tianjin (Tianjin, China). IPDI was used after dehydration with 4A molecular sieve. PBA and DMPA were dehydrated at 90 C under vacuum for 24 h before use. Triethyl amine (TEA) and ethylene diamine (EDA) were purchased from Lingfeng Chemicals of Shanghai (Shanghai, China) and used as received. 2.2. Preparation of flower-like ZnO nanowhiskers The flower-like ZnO nanowhiskers were prepared by a simple hydrothermal method. A transparent solution saturated with Zn(OH)2 4 was formed by dissolving commercial ZnO powder in 5 M NaOH solution. Then, the Zn(OH)2 4 saturated solution was loaded into distilled water (the volume ratio of Zn(OH)2 4 and H2O is 2:25) under slow stirring. The diluted Zn(OH)2 4 solution was transferred into a sealed vessel and maintained at 90 C for 10 h in an oven. The precipitate was collected by centrifugation, washed with distilled water until neutral, and then dried at 65 C for 48 h. 2.3. Functionalization of flower-like ZnO nanowhiskers The introduction of reactive groups onto the surface of f-ZnO was achieved through the reaction between APS and the hydroxyl groups on the surface of f-ZnO. Typically, 1 g APS was slowly added
into 80 ml toluene, then 2 g f-ZnO was dispersed in the abovementioned solution under vigorous stirring. After sonication for 20 min, the suspension was refluxed for an additional 24 h with constant stirring. After that, the resultant was separated by centrifugation and then subjected to Soxhlet extraction with boiling ethanol for 12 h to remove excess APS absorbing on the surface of f-ZnO. The final APS functionalized f-ZnO (f-ZnO-NH2) was dried at 65 C for 48 h. 2.4. Synthesis of WPU/f-ZnO composites Before prepolymerization reaction, IPDI, f-ZnO–NH2 and BDTBL (served as catalyst) were added into a conical flask and sealed to form an airtight system under ultrasonic at 60–70 C for 1 h. Under the catalysis of BDTBL, the reaction was initiated between the –NH2 groups available on the surface of the modified f-ZnO and the –NCO groups of IPDI. Uniformly dispersed suspension was obtained after sonication. A 500 ml round-bottom, 3-necked glass flask with a mechanical stirrer and a condenser, was used as the reactor for the preparation of WPU/f-ZnO composites. The reaction was carried out in a constant temperature oil bath. The synthesis procedures of the composites are described briefly as follows. PBA and DMPA were charged into the flask, which was heated at 70 C until they were melted completely. And then, the above IPDI/f-ZnO hybrid suspension was added into the flask while stirring. The mixture was allowed to react in the presence of DBTDL (0.03 phr based on the total solid) at 80 C for 2 h. The NCO-terminated prepolymer was obtained by adding TMP into the mixture to react at 80 C for 3 h. Subsequently, acetone was slowly added to reduce viscosity so as to obtain a homogeneous mixture. After cooling to room temperature, TEA was fed into the reactor and agitated for 30 min to neutralize DMPA unit in PU. An aqueous emulsion of NCO-terminated prepolymer was obtained by adding water to the mixture. EDA dissolved in water was then fed to the emulsion and chain extension was carried out at 50 C for 1 h. The final product was a WPU emulsion with a solid content about 38 wt%. The stoichiometric ratio of IPDI/PBA/DMPA/TMP/EDA/TEA was 2.5/1.0/0.5/0.6/0.1/0.5. The films of the WPU/f-ZnO composites for the measurements were prepared by casting the emulsions onto Teflon plates. 2.5. Characterization and property measurements X-ray diffraction (XRD) was performed by an X-ray diffractometer (Rigaku D/MAX-2500H) at 35 kV and 30 mA with a Cu Ka radiation source (k ¼ 0.15404 nm), at a scan speed of 4 /min. The structure of inorganic powder and composites were identified by a Fourier transfer infrared spectrophotometer (FTIR; Bruke, Tensor 27, Germany). Thermogravimetric analysis (TGA) of composite samples was carried out with a thermogravimetric analyzer (TA Instruments, Q50, USA). The samples of 5–10 mg each in an alumina crucible were used with a heating rate of 10 C/min under nitrogen atmosphere. The morphology of the as-synthesized ZnO sample and the microstructure of the fractured surfaces of the WPU/f-ZnO composite samples were observed by scanning electron microscopy (SEM; JEOL JSM 6700F, Japan). Mechanical properties of the casting films were measured with simple extension on dumbbell specimens about 2 mm thickness using a universal tensile machine (Tinius Olsen, USA) at a crosshead speed of 50 mm/min. For each nanocomposite, five specimens were tested and the average value was reported. Water swelling (the degree of water absorption) value of the composite films was obtained as follows: pre-weighed dry samples (20 mm 20 mm in size) were immersed in distilled water at 25 C. The samples were then blotted with filter paper and weighed.
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Water swelling was expressed as the weight percentage of water in the swollen sample and calculated by the following equation:
Swellingð%Þ ¼ ðWS WD Þ=WD 100%
(1)
where WD is the weight of the original dry sample and WS is the weight of the swollen sample. E. coli ATCC 25922 (E. coli, Gram-negative) and S. aureus ATCC 6538 (S. aureus, Gram positive) were chosen as target microorganisms. All glassware used was sterilized in an autoclave at 120 C for 30 min. Sample films of 50 mm 50 mm were washed with 70 wt% ethanol to kill any bacteria on the surface, and then washed with sterilized water. 0.2 ml bacterial suspension of 2.0–5.0 106 colony forming units per ml (CFU/ml) was pipetted onto the surface of the dried film in a Petri dish and then covered with a PE film (40 mm 40 mm). The films were incubated at a relative humidity (RH) higher than 90 wt% and temperature of 37 C for 24 h. Subsequently, each film was transferred to a new Petri dish and thoroughly washed with a 20 ml of 0.87% NaCl solution containing Tween 80 at pH 7.0 0.2. For determination of the actual number of microorganism colonies, the washing solution from each Petri dish was diluted to series of smaller dilutions with sterile phosphate buffer solution (PBS). Afterwards, 1 ml diluted solution was spread onto the solid growth agar plate (containing 5 g/L beef extract, 10 g/ L peptone, 5 g/L NaCl and 15 g/L agar powder). After incubation of the plates at 37 C for 24 h, the number of viable microorganism colonies was counted manually and the results after multiplication with the dilution factor were expressed as mean CFU after averaging the duplicate counts. The survival ratio was calculated using the following equation:
Survival ratioð%Þ ¼ ðN=N0 Þ 100%
(2)
where, N0 is the mean number of bacteria on the pure WPU film samples (CFU/sample), and N is the mean number of bacteria on the composite film samples (CFU/sample). 3. Results and discussion 3.1. Characterization of the f-ZnO The crystal structure of the synthesized ZnO was characterized by XRD. As shown in Fig. 1, the XRD pattern of the sample is in good accordance with the standard hexagonal phase ZnO (JCPDS Card
Fig. 1. XRD pattern of the as-prepared ZnO nanowhiskers.
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No. 36-1451). Well-crystallized diffraction peaks and no characteristic peaks of impurities of the sample were observed, suggesting that the prepared ZnO sample under the present experimental conditions is well-crystallized and with high purity. Fig. 2 shows the SEM image of the as-synthesized ZnO. Flower-like morphology was achieved and these ZnO flowers were composed of uniform nanorods with smooth surface. The diameter and length of the nanorods were about 400 nm and 3 mm, respectively. 3.2. FTIR analysis The structures of the pristine f-ZnO, modified f-ZnO and WPU/fZnO composite were analyzed by FTIR spectroscopy, as shown in Fig. 3. The FTIR spectrum of the f-ZnO–NH2 (Fig. 3b) reveals some new peaks compared to pristine f-ZnO. As characteristic bands of APS molecules [28], the stretching and bending mode of the –NH2 group were observed at 3355 and 1568 cm1, respectively. The absorption peaks at 1182 and 1025 cm1 indicate the presence of Si–O bonds which were attributed to the APS attachment [29]. In addition, the peak at 886 cm1 can be assigned to the bending mode of Si–OH group [28]. Therefore, the FTIR data indicated that APS providing amino groups was grafted onto the surface of f-ZnO. The result shows that the connection is based on covalent bonds between f-ZnO surface and APS molecules. In order to determine whether the reaction was initiated by the –NH2 group on the surface of f-ZnO–NH2, the interaction product of f-ZnO–NH2 and IPDI was examined by FTIR. The reacted ZnO powder (f-ZnO–NCO) was washed with acetone via centrifugation to completely remove the residual IPDI before FTIR measurement. As indicated in Fig. 3c, the spectrum of f-ZnO–NCO reveals new bands compared to f-ZnO– NH2. A peak at 2262 cm1 appears in the spectrum of f-ZnO–NCO, indicating the presence of –NCO group on the surface of f-ZnO. The bands at 2956 and 2921 cm1 can be ascribed to the symmetrical and asymmetrical stretching vibrations of C–H in –CH3 and –CH2– groups of IPDI, respectively. The newly formed bands at 1639 and 1560 cm1 can be assigned to the stretch vibration of C]O group and the coupling of N–H bending vibration with C–N stretching vibration in –NH–CO–NH–, respectively. On the basis of the above result, we conclude that after being reacted with IPDI, the f-ZnO were attached with –NCO group by the formation of urea bonding. On the other hand, we can only find the major characteristic peaks of PU in the WPU/f-ZnO composite since the content of f-ZnO is very low. It is very hard to find any new peak except for the slight peak at 540 cm1 belonging to ZnO in WPU/f-ZnO composite compared with the pure WPU.
Fig. 2. SEM image of the as-prepared ZnO nanowhiskers.
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important role in improving the mechanical performance of the composite films as discussed later. It is obvious that a large aggregate of nanowhiskers appeared in Fig. 4d. The content of f-ZnO was so high that some of them could not homogeneously disperse in WPU matrix. This would inhibit the reinforcement of the mechanical property of the composites. 3.4. Mechanical properties of the WPU/f-ZnO composites
Fig. 3. FTIR spectra of (a) pristine f-ZnO, (b) f-ZnO–NH2, (c) f-ZnO–NCO and (d) WPU /f-ZnO composite.
3.3. Morphology characterization of the WPU/f-ZnO composites Examination of the fractured surfaces of WPU/f-ZnO composites, which were broken at liquid nitrogen temperature, was carried out by SEM. Fig. 4 shows the images of the composites filled with 1.0 wt% and 4.0 wt% f-ZnO, respectively. As compared to the matrix, the morphology of the f-ZnO can be easily identified. The white dots in the images correspond to f-ZnO on the fractured surfaces of the composites. Well-dispersed f-ZnO in the composites can be observed in Fig. 4a, while Fig. 4c shows not only well-dispersed ZnO but also aggregates in the composites with higher filler contents. For details, as shown in Fig. 4b, a homogeneous distribution of the nanorods embedded in the WPU matrix was observed, implying that there exists good adhesion between fillers and matrix. Such an even and uniform distribution of the fillers in the matrix played an
The mechanical properties of the composite films incorporated with different contents of f-ZnO were investigated by tensile testing. From the stress–strain curves in Fig. 5, two characteristic regions of deformation behavior of the samples were observed. The stress increased rapidly with the increase in strain at low strains (<15%), and the stress increased regularly at higher strain with the strain increasing up to the break of the films. The tensile strength and elongation at break were determined from the stress–strain curves and summarized in Fig. 6. The results demonstrated that the f-ZnO content showed an intense effect on the mechanical properties of the composites. It is known that active –NH2 groups on the surface of modified f-ZnO can react with the –NCO groups of the pre-polyurethane. Hence, more chemical interactions occurred between f-ZnO and WPU with increasing the content of f-ZnO, thus resulted in more networks in the composites. It is well known that the network structure of the nanocomposite is favorable for reinforcing mechanical strength [24]. It is worth noting that, with 0.5 wt% f-ZnO filler, the tensile strength of the composite increased to the maximum (12.6 MPa) by about 34%, compared with the neat counterpart (9.4 MPa). When the content of f-ZnO was in excess of 1.0 wt%, the tensile strength of the composites decreased and higher filler content (up to 4.0 wt%) resulted in more serious decrease in the strength. On the other hand, the elongation at break feebly decreased for all of the tested composite films compared to the original WPU film, with a maximum decrease at 1 wt% loading level. This phenomenon can be explained by the fact that the rigid filler network structure, which is responsible for the enforcing effect, was formed perfectly as the f-ZnO content is lower than 1.0 wt%. The decrease of the
Fig. 4. SEM images of the fractured surfaces of the WPU/f-ZnO composites with different f-ZnO contents: (a–b) 1.0 wt%, (c–d) 4.0 wt%.
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Fig. 5. Stress–strain curves of (a) pure WPU and WPU/f-ZnO composites with (b) 0.5 wt%, (c) 1.0 wt%, (d) 1.5 wt%, (e) 2.0 wt% and (f) 4.0 wt% of f-ZnO, respectively.
mechanical strength of the composites with more than 1.0 wt% may be attributed to the aggregation of excess filler in WPU matrix. Furthermore, the aggregation behavior increased with increasing f-ZnO content, indicating an increase in the incompatibility of the WPU/f-ZnO composites with excess f-ZnO content. In combination with the result of SEM, we can summarize that an optimum incorporation amount of f-ZnO exists for an effective enhancement of the mechanical property of the composites. Good dispersion of f-ZnO in composites reduces the stress concentration and enhances the uniformity of stress distribution; as a result, the composites with low f-ZnO contents show higher performance in mechanical properties than those with high f-ZnO loading levels. Good reinforcement was achieved with the homogeneous dispersion of f-ZnO in the composites. The agglomerates of f-ZnO can be the points of stress to damage the structure of the polymeric matrix, which results in mechanical property decrease. 3.5. Thermal properties of the WPU/f-ZnO composites Fig. 7 displays thermal decomposition behavior of pure WPU and WPU/f-ZnO composites. Two stages of decomposition appear at the TGA curves of all samples. As is evident, incorporation of
Fig. 6. Effect of the f-ZnO content on tensile strength and elongation at break of pure WPU and WPU/f-ZnO composites.
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Fig. 7. TGA thermograms of (a) pure WPU and WPU/f-ZnO composites with different fZnO contents: (b) 0.5 wt%, (c) 1.5 wt%, and (d) 4.0 wt%, respectively.
f-ZnO has no effect on the decomposition stage of WPU/f-ZnO composites. However, the decomposition temperature alters remarkably, which implies the thermal stability of the composites changes. Temperature for 50 wt% weight loss of the WPU/f-ZnO composites with 0, 0.5 wt%, 1.5 wt% and 4.0 wt% f-ZnO contents was obtained at 361.3, 331.2, 323.0 and 318.5 C, respectively. Initial degradation temperature of the composites significantly decreased and then leveled off. This result indicates that the composites decompose at lower temperatures than the pure WPU matrix; that is to say, incorporating f-ZnO into the WPU decreases the thermal stability. It is worth noting that our result is different from the results of other WPU composite systems, such as clay [21], CNTs [22], slica [23], SiC [30] and so on. The following reasons suggest a possible explanation for the change of the thermal stability of the composites. On one hand, fZnO network structure in the WPU matrix could confine the motion of polymer chains or act as thermal insulator and mass transport barrier to the volatile products generated during decomposition, thus results in delay of thermal degradation. This is also the main reason why clay and silica improve thermal ability of the composites. On the other hand, as a n-type intrinsic semiconductor, ZnO is able to form free oxygen and oxygen vacancies in the lattice induced by thermal excitation. The oxygen vacancies can trap and bound electrons to form active catalytical sites in ZnO, and free oxygen promotes the formation of peroxy radicals to damage the polymer chains. Thus, formation of free oxygen and oxygen vacancies plays an important role in degrading polymers. Furthermore, due to the one-dimensional nanostructure extended along the [0001] direction, f-ZnO is composed of nanorods with a larger population of (0002) planes [31]. Since the (0002) plane with the highest surface energy is the most unstable plane of ZnO, it is reasonable that the interaction between (0002) plane and polymer chains would also be more active. Therefore, it is logical to infer that the enhancement of thermal degradation of WPU/f-ZnO composites is attributed to the effect of thermal catalysis performance of f-ZnO. These two effects compete with each other and the thermal catalysis effect of f-ZnO dominates in the present system. Consequently, the decomposition temperature at which the weight loss reached 50 wt% was shifted by 30.1 C towards a lower temperature even when the f-ZnO content was as low as 0.5 wt%. However, comparing with the low f-ZnO loading level, excess amount of f-ZnO in the composites caused a smaller temperature shift for the effect of heat-resistance of f-ZnO network structures enhanced.
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3.6. Water resistance property of the WPU/f-ZnO composites Fig. 8 shows water swelling of pure WPU and WPU/f-ZnO composite films as a function of the f-ZnO content. It is clear that incorporation of f-ZnO decreases the value of water swelling significantly. The value of water swelling decreased by approximate 11.9% as the content of f-ZnO is up to 4.0 wt%. The result demonstrates that the presence of impermeable f-ZnO in WPU matrix reduces the water swelling, and enhances the resistance to water. Increase of the mean free path of water molecules to pass through the matrix of WPU/f-ZnO composite due to the reinforcing effect of the f-ZnO network structure, seems to be the cause of reduced water swelling. The improvement of water resistance in other WPU nanocomposite systems containing inorganic nanofillers such as WPU/clay nanocomposites [21] and WPU/silica hybrids [23] has also been reported. 3.7. Antibacterial activity of the WPU/f-ZnO composites Antibacterial activity of the WPU/f-ZnO composite films with different f-ZnO contents was tested using E. coli and S. aureus in comparison with the pure WPU film, as displayed in Fig. 9. The survival ratio of E. coli and S. aureus decreased with increase of f-ZnO content, and the best antibacterial activity was obtained with 4.0 wt% f-ZnO. Concerning the mechanism of the antibacterial activity of ZnO nanomaterials, several mechanisms have been proposed: (1) the release of Zn2þ ions from the powder [32], (2) mechanical destruction of the cell membrane caused by penetration of the nanoparticles [33], (3) active oxygen generated from the powder [34,35] and (4) the generation of hydrogen peroxide (H2O2) from the surface of ZnO [10,36]. It is well known that ZnO is unstable in the solution and the concentration of Zn2þ ions increases as a result of ZnO decomposition. In our work, since the nanorods of f-ZnO were coated with g-aminopropyltrimethoxysilane, the effect of Zn2þ released from ZnO could be restrained. Thus, the release of Zn2þ ions was not a main factor. ZnO nanorods with an average diameter about 400 nm used in this work were less likely to penetrate into the cell wall to damage the bacteria from the interior [37]. Tam et al. examined a large number of cells and found very few cases of internalization of the ZnO nanorod [7]. Therefore, mechanical damage of the cell membrane should not be considered as the mechanism of
Fig. 9. Effects of WPU/f-ZnO composites with different f-ZnO contents on survival ratio of E. coli and S. aureus.
antibacterial activity of f-ZnO. Our results show that the WPU/f-ZnO composites exhibited antibacterial activity in the absence of light, supporting the fact that antibacterial activity is most likely induced by the H2O2 generated from the surface of f-ZnO. Hydrogen peroxide is a powerful oxidizing agent and more reactive than oxygen molecules, it is harmful to the cells of living organisms [10,36]. The tendency of WPU absorbing moisture facilitates H2O2 generation. It is assumed that H2O2 generated damages the cell membrane of bacteria, produces some type of injury, and inhibits the growth of the cells or even kills them. Therefore, the generation of H2O2 from the surface of f-ZnO is considered as the primary factor of antibacterial activity of WPU/f-ZnO composites. The antibacterial effect of the composites on E. coli is stronger than on S. aureus when the content of f-ZnO was lower than 4.0 wt%. The difference in activity against these two types of bacteria can be attributed to structural and chemical compositional differences of the cell surfaces. Gram-positive bacteria typically have one cytoplasmic membrane and thick wall composed of multilayers of peptidoglycan [38]. However, gram-negative bacteria have more complex cell wall structure, with a layer of peptidoglycan between outer membrane and cytoplasmic membrane [33,38]. In a word, antibacterial effect can be attributed to the damage of cell membranes, which leads to leakage of cell contents and cell death. Therefore, the difference in antibacterial action towards E. coli and S. aureus is assumed to be caused by the different sensitivities towards H2O2 generated by f-ZnO. The mechanisms responsible for antibacterial activity of ZnO nanostructures are still not fully clear, so the exact cause of the membrane damage requires further study. 4. Conclusion
Fig. 8. Variation of water swelling of the WPU/f-ZnO composites by the amount of f-ZnO.
We have successfully prepared flower-like ZnO nanowhiskers (f-ZnO) composed of uniform nanorods via a simple hydrothermal method. WPU/f-ZnO composites were synthesized by in-situ copolymerization process using f-ZnO modified with APS. Composite films with low weight percentages loading of f-ZnO yielded materials with enhanced tensile strengths, water resistance and antibacterial properties compared with neat WPU film. The f-ZnO content showed an intense effect on the mechanical properties of the composites. The tensile strength of composite films increased significantly up to the optimum value (1.0 wt%), and then decreased gradually with excess f-ZnO content. However, the
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elongation at break feebly diminished for all of the tested composite films compared with the neat WPU film. Incorporation of f-ZnO decreased the thermal stability of the composites. The enhancement of thermal degradation of WPU/f-ZnO composites was mainly attributed to the effect of thermal catalysis performance of f-ZnO. Water swelling character declined with the increase of f-ZnO content in WPU/f-ZnO composites due to the reinforcing effect of the f-ZnO network structure. More importantly, the composite films exhibited a strong antibacterial effect against E. coli and S. aureus, and the best antibacterial activity was obtained with 4.0 wt% f-ZnO. From these results, the synthesized WPU/f-ZnO composites are potentially useful in a variety of coating applications because of their combination of enhanced mechanical and antibacterial properties. References [1] Balazs AC, Emrick T, Russell TP. Nanoparticle polymer composites: where two small worlds meet. Science 2006;314:1107–10. [2] Tang EJ, Cheng GX, Ma XL. Preparation of nano-ZnO/PMMA composite particles via grafting of the copolymer onto the surface of zinc oxide nanoparticles. Powder Technol 2006;161:209–14. [3] Lin Y, Boker A, He JB, Sill K, Xiang HQ, Abetz C, et al. Self-directed self-assembly of nanoparticle/copolymer mixtures. Nature 2005;434:55–9. [4] Zhang MQ, Yu G, Zeng HM, Zhang HB, Hou YH. Two-step percolation in polymer blends filled with carbon black. Macromolecules 1998;31:6724–6. [5] Schmidt-Mende L, Macmanus-Driscoll JL. ZnO – nanostructures, defects, and devices. Mater Today 2007;10:40–8. [6] Yamamoto O. Influence of particle size on the antibacterial activity of zinc oxide. Int J Inorg Mater 2001;3:643–6. [7] Tam KH, Djurisic AB, Chan C, Xi YY, Tse CW, Leung YH, et al. Antibacterial activity of ZnO nanorods prepared by a hydrothermal method. Thin Solid Films 2008;516:6167–74. [8] Chen YW, Liu YC, Lu SX, Xu CS, Shao CL, Wang C, et al. Optical properties of ZnO and ZnO: in nanorods assembled by sol-gel method. J Chem Phys 2005;123:134701–5. [9] Zhang Y, Jia HB, Luo XH, Chen XH, Yu DP, Wang RM. Synthesis, microstructure, and growth mechanism of dendrite ZnO nanowires. J Phys Chem B 2003;107:8289–93. [10] Chen SJ, Liu YC, Shao CL, Mu R, Lu YM, Zhang JY, et al. Structural and optical properties of uniform ZnO nanosheets. Adv Mater 2005;17:586–90. [11] Wang CL, Mao BD, Wang EB, Kang ZH, Tian CG. Solution synthesis of ZnO nanotubes via a template-free hydrothermal route. Solid State Commun 2007;141:620–3. [12] Kong XY, Wang ZL. Spontaneous polarization-induced nanohelixes, nanosprings, and nanorings of piezoelectric nanobelts. Nano Lett 2003;3:1625–31. [13] Sun D, Miyatake N, Sue HJ. Transparent PMMA/ZnO nanocomposite films based on colloidal ZnO quantum dots. Nanotechnology 2007;18:215606–11. [14] Demir MM, Memesa M, Castignolles P, Wegner G. PMMA/zinc oxide nanocomposites prepared by in-situ bulk polymerization. Macromol Rapid Commun 2006;27:763–70. [15] Ma C, Chen YJ, Kuan HC. Polystyrene nanocomposite materials: preparation, morphology, and mechanical, electrical, and thermal properties. J Appl Polym Sci 2005;98:2266–73. [16] Wu M, Yang GZ, Wang M, Wang WZ, Zhang WD, Feng JC, et al. Nonisothermal crystallization kinetics of ZnO nanorod filled polyamide 11 composites. Mater Chem Phys 2008;109:547–55.
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[17] Chae DW, Kim BC. Effects of zinc oxide nanoparticles and the physical properties of polyacrylonitrile. J Appl Polym Sci 2006;99:1854–8. [18] Liufu SC, Xiao HN, Li YP. Thermal analysis and degradation mechanism of polyacrylate/ZnO nanocomposites. Polym Degrad Stab 2005;87:103–10. [19] Noble KL. Waterborne polyurethanes. Prog Org Coat 1997;32:131–6. [20] El-Sayed AHMM, Mahmoud WM, Davis EM, Coughlin RW. Biodegradation of polyurethane coatings by hydrocarbon-degrading bacteria. Int Biodeterior Biodegrad 1996;37:69–79. [21] Kim BK, Seo JW, Jeong HM. Morphology and properties of waterborne polyurethane/clay nanocomposites. Eur Polym J 2003;39:85–91. [22] Kuan HC, Ma CC, Chang WP, Yuen SM, Wu HH, Lee TM. Synthesis, thermal, mechanical and rheological properties of multiwall carbon nano tube/waterborne polyurethane nanocomposite. Compos Sci Technol 2005;65: 1703–10. [23] Jeon HT, Jang MK, Kim BK, Kim KH. Synthesis and characterizations of waterborne polyurethane–silica hybrids using sol-gel process. Colloid Surface A 2007;302:559–67. [24] Zhao CX, Zhang WD. Preparation of waterborne polyurethane nanocomposites: polymerization from functionalized hydroxyapatite. Eur Polym J 2008;44:1988–95. [25] Hsu SH, Chou CW, Tseng SM. Enhanced thermal and mechanical properties in polyurethane/Au nanocomposites. Macromol Mater Eng 2004;289: 1096–101. [26] Chou CW, Hsu SH, Chang H, Tseng SM, Lin HR. Enhanced thermal and mechanical properties and biostability of polyurethane containing silver nanoparticles. Polym Degrad Stab 2006;91:1017–24. [27] Cao XD, Dong H, Li CM. New nanocomposite materials reinforced with flax cellulose nanocrystals in waterborne polyurethane. Biomacromolecules 2007;8:899–904. [28] Ogasawara T, Nara A, Okabayashi H, Nishio E, O’Connor CJ. Time-resolved near-infrared and two-dimensional near-infrared correlation spectroscopic studies on polymerization of the silane coupling agent perfluorooctyltrimethoxysilane. Colloid Polym Sci 2000;278:946–53. [29] Compton OC, Mullet CH, Chiang S, Osterloh FE. A building block approach to photochemical water-splitting catalysts based on layered niobate nanosheets. J Phys Chem C 2008;112:6202–8. [30] Guo Z, Kim TY, Lei K, Pereira T, Sugar JG, Hahn HT. Strengthening and thermal stabilization of polyurethane nanocomposites with silicon carbide nanoparticles by a surface-initiated-polymerization approach. Compos Sci Technol 2008;68:164–70. [31] Jang ES, Won JH, Hwang SJ, Choy JH. Fine tuning of the face orientation of ZnO crystals to optimize their photocatalytic activity. Adv Mater 2006;18: 3309–12. [32] Heinlaan M, Ivask A, Blinova I, Dubourguier HC, Kahru A. Toxicity of nanosized and bulk ZnO, CuO and TiO2 to bacteria Vibrio fischeri and crustaceans Daphnia magna and Thamnocephalus platyurus. Chemosphere 2008;71:1308–16. [33] Brayner R, Ferrari-Iliou R, Brivois N, Djediat S, Benedetti MF, Fievet F. Toxicological impact studies based on Escherichia coli bacteria in ultrafine ZnO nanoparticles colloidal medium. Nano Lett 2006;6:866–70. [34] Xu T, Xie CS. Tetrapod-like nano-particle ZnO/acrylic resin composite and its multi-function property. Prog Org Coat 2003;46:297–301. [35] Zhang LL, Jiang YH, Ding YL, Povey M, York D. Investigation into the antibacterial behaviour of suspensions of ZnO nanoparticles (ZnO nanofluids). J Nanopart Res 2007;9:479–89. [36] Sawai J, Shoji S, Igarashi H, Hashimoto A, Kokugan T, Shimizu M, et al. Hydrogen peroxide as an antibacterial factor in zinc oxide powder slurry. J Ferment Bioeng 1998;86:521–2. [37] Sawai J. Quantitative evaluation of antibacterial activities of metallic oxide powders (ZnO, MgO and CaO) by conductimetric assay. J Microbiol Meth 2003;54:177–82. [38] Fu GF, Vary PS, Lin CT. Anatase TiO2 nanocomposites for antimicrobial coatings. J Phys Chem B 2005;109:8889–98.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Effects of flower-like ZnO nanowhiskers on the mechanical, thermal and antibacterial properties of waterborne polyurethane Xue-Yong Ma, Wei-De Zhang* Nano Science Research Center, School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 February 2009 Received in revised form 17 March 2009 Accepted 22 March 2009 Available online 7 April 2009
A novel waterborne polyurethane/flower-like ZnO nanowhiskers (WPU/f-ZnO) composite with different f-ZnO content (0–4.0 wt%) was synthesized by an in-situ copolymerization process. The f-ZnO consisting of uniform nanorods was prepared via a simple hydrothermal method. In order to disperse and incorporate f-ZnO into WPU matrix, f-ZnO was modified with g-aminopropyltriethoxysilane. Morphology of f-ZnO in WPU matrix was characterized by scanning electron microscope. The properties of WPU/f-ZnO composites such as mechanical strength, thermal stability as well as water swelling were strongly influenced by the f-ZnO contents. It was demonstrated that appropriate amount of f-ZnO with good dispersion in the WPU matrix significantly improved the performance of the composites. The mechanical property was enhanced with an increase of f-ZnO content up to the optimum content (1 wt%) and then declined. Incorporation of f-ZnO enhanced the water resistance of the composites remarkably. It was amazing to observe that the thermal degradation temperatures of the composites initially decreased significantly and then leveled off with content increase of f-ZnO, which was different from the results of other WPU composite systems reported. Antibacterial activity of WPU/f-ZnO composite films against Escherichia coli and Staphylococcus aureus was also tested. The results revealed that the antibacterial activity enhanced with the increasing f-ZnO content, and the best antibacterial activity was obtained at the loading level of 4.0 wt% f-ZnO. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Waterborne polyurethane Nanocomposites ZnO nanowhiskers Thermal decomposition Antibacterial activity
1. Introduction Nanocomposite is a class of materials with unique physical properties and wide application potential in diverse areas [1]. Dispersion of nanoscaled inorganic fillers into an organic polymer to form polymer nanocomposites has gained increasing interest in recent years. Controlling the nanostructure, composition and morphology of nanocomposites plays an essential role in their applications. Novel properties of nanocomposites can be obtained by successful imparting of the characteristics of parent constituents to a single material [2]. These materials differ from both pure polymers and inorganic fillers in some physical and chemical properties. The combination of polymers and nanoscale inorganic fillers is opening pathways for engineering flexible composites that exhibit attractive mechanical, thermal, optical and electrical properties compared with conventional composites [3,4]. ZnO is an important and attractive semiconductive material. It has drawn enormous attention due to its fantastic characteristics in
* Corresponding author. Tel.: þ86 20 87114099; fax: þ86 20 87112053. E-mail address: [email protected] (W.-D. Zhang). 0141-3910/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2009.03.024
optics, photonics and electronics [5]. Furthermore, ZnO shows a marked antibacterial activity at pH values in the range from 7 to 8 without the presence of light [6,7]. Zinc is also a mineral element essential to human beings. ZnO nanostructures can be obtained by various methods including thermal evaporation, electrochemical deposition, sonochemical method, sol-gel, hydrothermal synthesis and so forth. Various one-dimensional (1D) ZnO nanostructures have been realized, such as nanorods, nanowires, nanobelts, nanosheets, nanotubes, nanonails and so on [8–12]. Among the 1D ZnO nanostructures, nanorods have been widely studied because of their easy preparation and wide applications [8]. In previous studies, ZnO nanostructures were combined with PMMA [13,14], polystyrene [15], polyamide [16], polyacrylonitrile [17], polyacrylate [18], etc. Compared with the original polymers, the yielded composite materials present a lot of excellent performance. Polyurethanes (PU) are probably the most versatile class of polymers due to the great variety of raw materials that can be used for their formation. Waterborne polyurethane (WPU) shows many excellent features compared to conventional organic solvent-based polyurethane with the advantages of non-pollution and nontoxicity and can be used in various fields, such as coating, adhesives for textile, paper, wood or glass fibers, and so forth [19]. Thermal
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stability, insolubility, and mechanical properties of WPU, however, are still lower than those of solvent-based PU and need to be improved. Meanwhile, as a type of polymer, PU is susceptible to be degraded by many types of bacteria [20]. Nanosized additives are used as an effective strategy to alter and enhance the properties of WPU. Various types of filler, like clay [21], CNTs [22], silica [23], hydroxyapatite [24], Au [25], Ag [26] and flax cellulose [27] have been incorporated into WPU to prepare nanocomposites. The results demonstrated that homogeneous dispersion of fillers in WPU matrix significantly improved the performance of the nanocomposites. To our knowledge, no work has been reported about the preparation of WPU modified by ZnO nanowhiskers. Flower-like ZnO nanowhiskers can be widely used as polymer additives to make functional nanocomposites because of their high aspect ratio, together with good comprehensive properties such as semiconductivity, high mechanical strength, wear resistance, vibration insulation, microwave absorption and antibacterial effect. Herein, we report preparation and characterization of waterborne polyurethane/flower-like ZnO nanowhiskers (WPU/f-ZnO) composites. In this study, flower-like ZnO nanowhiskers (f-ZnO) were synthesized by a simple hydrothermal method and modified with g-aminopropyltriethoxysilane (APS) to improve the bonding between f-ZnO and the WPU matrix. Properties of WPU/f-ZnO composites such as mechanical strength, thermal stability, water swelling as well as the antibacterial effect against Escherichia coli and Staphylococcus aureus were all influenced by the content of fZnO. 2. Experiment 2.1. Materials ZnO and NaOH were of analytical grade supplied by Guanghua Chemical Factory Co., Ltd. (Guangdong, China). Silane coupling agent g-aminopropyltriethoxysilane (APS) was purchased from Shuguang Chemical Co., Ltd. (Nanjing, China) and used without further purification. Poly (butyl adipate) diol (PBA; average molecular weight Mn ¼ 1000 g/mol) was obtained from Hodotani Co. (Tokyo, Japan). Dimethylol propionic acid (DMPA), 3-isocyanatemethyl-3,5,5-trimethyl-cyclohexylisocyanate (IPDI), trimethylolpropane (TMP) and dibutyl tin dilaurate (DBTDL) were purchased from First Chemicals of Tianjin (Tianjin, China). IPDI was used after dehydration with 4A molecular sieve. PBA and DMPA were dehydrated at 90 C under vacuum for 24 h before use. Triethyl amine (TEA) and ethylene diamine (EDA) were purchased from Lingfeng Chemicals of Shanghai (Shanghai, China) and used as received. 2.2. Preparation of flower-like ZnO nanowhiskers The flower-like ZnO nanowhiskers were prepared by a simple hydrothermal method. A transparent solution saturated with Zn(OH)2 4 was formed by dissolving commercial ZnO powder in 5 M NaOH solution. Then, the Zn(OH)2 4 saturated solution was loaded into distilled water (the volume ratio of Zn(OH)2 4 and H2O is 2:25) under slow stirring. The diluted Zn(OH)2 4 solution was transferred into a sealed vessel and maintained at 90 C for 10 h in an oven. The precipitate was collected by centrifugation, washed with distilled water until neutral, and then dried at 65 C for 48 h. 2.3. Functionalization of flower-like ZnO nanowhiskers The introduction of reactive groups onto the surface of f-ZnO was achieved through the reaction between APS and the hydroxyl groups on the surface of f-ZnO. Typically, 1 g APS was slowly added
into 80 ml toluene, then 2 g f-ZnO was dispersed in the abovementioned solution under vigorous stirring. After sonication for 20 min, the suspension was refluxed for an additional 24 h with constant stirring. After that, the resultant was separated by centrifugation and then subjected to Soxhlet extraction with boiling ethanol for 12 h to remove excess APS absorbing on the surface of f-ZnO. The final APS functionalized f-ZnO (f-ZnO-NH2) was dried at 65 C for 48 h. 2.4. Synthesis of WPU/f-ZnO composites Before prepolymerization reaction, IPDI, f-ZnO–NH2 and BDTBL (served as catalyst) were added into a conical flask and sealed to form an airtight system under ultrasonic at 60–70 C for 1 h. Under the catalysis of BDTBL, the reaction was initiated between the –NH2 groups available on the surface of the modified f-ZnO and the –NCO groups of IPDI. Uniformly dispersed suspension was obtained after sonication. A 500 ml round-bottom, 3-necked glass flask with a mechanical stirrer and a condenser, was used as the reactor for the preparation of WPU/f-ZnO composites. The reaction was carried out in a constant temperature oil bath. The synthesis procedures of the composites are described briefly as follows. PBA and DMPA were charged into the flask, which was heated at 70 C until they were melted completely. And then, the above IPDI/f-ZnO hybrid suspension was added into the flask while stirring. The mixture was allowed to react in the presence of DBTDL (0.03 phr based on the total solid) at 80 C for 2 h. The NCO-terminated prepolymer was obtained by adding TMP into the mixture to react at 80 C for 3 h. Subsequently, acetone was slowly added to reduce viscosity so as to obtain a homogeneous mixture. After cooling to room temperature, TEA was fed into the reactor and agitated for 30 min to neutralize DMPA unit in PU. An aqueous emulsion of NCO-terminated prepolymer was obtained by adding water to the mixture. EDA dissolved in water was then fed to the emulsion and chain extension was carried out at 50 C for 1 h. The final product was a WPU emulsion with a solid content about 38 wt%. The stoichiometric ratio of IPDI/PBA/DMPA/TMP/EDA/TEA was 2.5/1.0/0.5/0.6/0.1/0.5. The films of the WPU/f-ZnO composites for the measurements were prepared by casting the emulsions onto Teflon plates. 2.5. Characterization and property measurements X-ray diffraction (XRD) was performed by an X-ray diffractometer (Rigaku D/MAX-2500H) at 35 kV and 30 mA with a Cu Ka radiation source (k ¼ 0.15404 nm), at a scan speed of 4 /min. The structure of inorganic powder and composites were identified by a Fourier transfer infrared spectrophotometer (FTIR; Bruke, Tensor 27, Germany). Thermogravimetric analysis (TGA) of composite samples was carried out with a thermogravimetric analyzer (TA Instruments, Q50, USA). The samples of 5–10 mg each in an alumina crucible were used with a heating rate of 10 C/min under nitrogen atmosphere. The morphology of the as-synthesized ZnO sample and the microstructure of the fractured surfaces of the WPU/f-ZnO composite samples were observed by scanning electron microscopy (SEM; JEOL JSM 6700F, Japan). Mechanical properties of the casting films were measured with simple extension on dumbbell specimens about 2 mm thickness using a universal tensile machine (Tinius Olsen, USA) at a crosshead speed of 50 mm/min. For each nanocomposite, five specimens were tested and the average value was reported. Water swelling (the degree of water absorption) value of the composite films was obtained as follows: pre-weighed dry samples (20 mm 20 mm in size) were immersed in distilled water at 25 C. The samples were then blotted with filter paper and weighed.
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Water swelling was expressed as the weight percentage of water in the swollen sample and calculated by the following equation:
Swellingð%Þ ¼ ðWS WD Þ=WD 100%
(1)
where WD is the weight of the original dry sample and WS is the weight of the swollen sample. E. coli ATCC 25922 (E. coli, Gram-negative) and S. aureus ATCC 6538 (S. aureus, Gram positive) were chosen as target microorganisms. All glassware used was sterilized in an autoclave at 120 C for 30 min. Sample films of 50 mm 50 mm were washed with 70 wt% ethanol to kill any bacteria on the surface, and then washed with sterilized water. 0.2 ml bacterial suspension of 2.0–5.0 106 colony forming units per ml (CFU/ml) was pipetted onto the surface of the dried film in a Petri dish and then covered with a PE film (40 mm 40 mm). The films were incubated at a relative humidity (RH) higher than 90 wt% and temperature of 37 C for 24 h. Subsequently, each film was transferred to a new Petri dish and thoroughly washed with a 20 ml of 0.87% NaCl solution containing Tween 80 at pH 7.0 0.2. For determination of the actual number of microorganism colonies, the washing solution from each Petri dish was diluted to series of smaller dilutions with sterile phosphate buffer solution (PBS). Afterwards, 1 ml diluted solution was spread onto the solid growth agar plate (containing 5 g/L beef extract, 10 g/ L peptone, 5 g/L NaCl and 15 g/L agar powder). After incubation of the plates at 37 C for 24 h, the number of viable microorganism colonies was counted manually and the results after multiplication with the dilution factor were expressed as mean CFU after averaging the duplicate counts. The survival ratio was calculated using the following equation:
Survival ratioð%Þ ¼ ðN=N0 Þ 100%
(2)
where, N0 is the mean number of bacteria on the pure WPU film samples (CFU/sample), and N is the mean number of bacteria on the composite film samples (CFU/sample). 3. Results and discussion 3.1. Characterization of the f-ZnO The crystal structure of the synthesized ZnO was characterized by XRD. As shown in Fig. 1, the XRD pattern of the sample is in good accordance with the standard hexagonal phase ZnO (JCPDS Card
Fig. 1. XRD pattern of the as-prepared ZnO nanowhiskers.
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No. 36-1451). Well-crystallized diffraction peaks and no characteristic peaks of impurities of the sample were observed, suggesting that the prepared ZnO sample under the present experimental conditions is well-crystallized and with high purity. Fig. 2 shows the SEM image of the as-synthesized ZnO. Flower-like morphology was achieved and these ZnO flowers were composed of uniform nanorods with smooth surface. The diameter and length of the nanorods were about 400 nm and 3 mm, respectively. 3.2. FTIR analysis The structures of the pristine f-ZnO, modified f-ZnO and WPU/fZnO composite were analyzed by FTIR spectroscopy, as shown in Fig. 3. The FTIR spectrum of the f-ZnO–NH2 (Fig. 3b) reveals some new peaks compared to pristine f-ZnO. As characteristic bands of APS molecules [28], the stretching and bending mode of the –NH2 group were observed at 3355 and 1568 cm1, respectively. The absorption peaks at 1182 and 1025 cm1 indicate the presence of Si–O bonds which were attributed to the APS attachment [29]. In addition, the peak at 886 cm1 can be assigned to the bending mode of Si–OH group [28]. Therefore, the FTIR data indicated that APS providing amino groups was grafted onto the surface of f-ZnO. The result shows that the connection is based on covalent bonds between f-ZnO surface and APS molecules. In order to determine whether the reaction was initiated by the –NH2 group on the surface of f-ZnO–NH2, the interaction product of f-ZnO–NH2 and IPDI was examined by FTIR. The reacted ZnO powder (f-ZnO–NCO) was washed with acetone via centrifugation to completely remove the residual IPDI before FTIR measurement. As indicated in Fig. 3c, the spectrum of f-ZnO–NCO reveals new bands compared to f-ZnO– NH2. A peak at 2262 cm1 appears in the spectrum of f-ZnO–NCO, indicating the presence of –NCO group on the surface of f-ZnO. The bands at 2956 and 2921 cm1 can be ascribed to the symmetrical and asymmetrical stretching vibrations of C–H in –CH3 and –CH2– groups of IPDI, respectively. The newly formed bands at 1639 and 1560 cm1 can be assigned to the stretch vibration of C]O group and the coupling of N–H bending vibration with C–N stretching vibration in –NH–CO–NH–, respectively. On the basis of the above result, we conclude that after being reacted with IPDI, the f-ZnO were attached with –NCO group by the formation of urea bonding. On the other hand, we can only find the major characteristic peaks of PU in the WPU/f-ZnO composite since the content of f-ZnO is very low. It is very hard to find any new peak except for the slight peak at 540 cm1 belonging to ZnO in WPU/f-ZnO composite compared with the pure WPU.
Fig. 2. SEM image of the as-prepared ZnO nanowhiskers.
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important role in improving the mechanical performance of the composite films as discussed later. It is obvious that a large aggregate of nanowhiskers appeared in Fig. 4d. The content of f-ZnO was so high that some of them could not homogeneously disperse in WPU matrix. This would inhibit the reinforcement of the mechanical property of the composites. 3.4. Mechanical properties of the WPU/f-ZnO composites
Fig. 3. FTIR spectra of (a) pristine f-ZnO, (b) f-ZnO–NH2, (c) f-ZnO–NCO and (d) WPU /f-ZnO composite.
3.3. Morphology characterization of the WPU/f-ZnO composites Examination of the fractured surfaces of WPU/f-ZnO composites, which were broken at liquid nitrogen temperature, was carried out by SEM. Fig. 4 shows the images of the composites filled with 1.0 wt% and 4.0 wt% f-ZnO, respectively. As compared to the matrix, the morphology of the f-ZnO can be easily identified. The white dots in the images correspond to f-ZnO on the fractured surfaces of the composites. Well-dispersed f-ZnO in the composites can be observed in Fig. 4a, while Fig. 4c shows not only well-dispersed ZnO but also aggregates in the composites with higher filler contents. For details, as shown in Fig. 4b, a homogeneous distribution of the nanorods embedded in the WPU matrix was observed, implying that there exists good adhesion between fillers and matrix. Such an even and uniform distribution of the fillers in the matrix played an
The mechanical properties of the composite films incorporated with different contents of f-ZnO were investigated by tensile testing. From the stress–strain curves in Fig. 5, two characteristic regions of deformation behavior of the samples were observed. The stress increased rapidly with the increase in strain at low strains (<15%), and the stress increased regularly at higher strain with the strain increasing up to the break of the films. The tensile strength and elongation at break were determined from the stress–strain curves and summarized in Fig. 6. The results demonstrated that the f-ZnO content showed an intense effect on the mechanical properties of the composites. It is known that active –NH2 groups on the surface of modified f-ZnO can react with the –NCO groups of the pre-polyurethane. Hence, more chemical interactions occurred between f-ZnO and WPU with increasing the content of f-ZnO, thus resulted in more networks in the composites. It is well known that the network structure of the nanocomposite is favorable for reinforcing mechanical strength [24]. It is worth noting that, with 0.5 wt% f-ZnO filler, the tensile strength of the composite increased to the maximum (12.6 MPa) by about 34%, compared with the neat counterpart (9.4 MPa). When the content of f-ZnO was in excess of 1.0 wt%, the tensile strength of the composites decreased and higher filler content (up to 4.0 wt%) resulted in more serious decrease in the strength. On the other hand, the elongation at break feebly decreased for all of the tested composite films compared to the original WPU film, with a maximum decrease at 1 wt% loading level. This phenomenon can be explained by the fact that the rigid filler network structure, which is responsible for the enforcing effect, was formed perfectly as the f-ZnO content is lower than 1.0 wt%. The decrease of the
Fig. 4. SEM images of the fractured surfaces of the WPU/f-ZnO composites with different f-ZnO contents: (a–b) 1.0 wt%, (c–d) 4.0 wt%.
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Fig. 5. Stress–strain curves of (a) pure WPU and WPU/f-ZnO composites with (b) 0.5 wt%, (c) 1.0 wt%, (d) 1.5 wt%, (e) 2.0 wt% and (f) 4.0 wt% of f-ZnO, respectively.
mechanical strength of the composites with more than 1.0 wt% may be attributed to the aggregation of excess filler in WPU matrix. Furthermore, the aggregation behavior increased with increasing f-ZnO content, indicating an increase in the incompatibility of the WPU/f-ZnO composites with excess f-ZnO content. In combination with the result of SEM, we can summarize that an optimum incorporation amount of f-ZnO exists for an effective enhancement of the mechanical property of the composites. Good dispersion of f-ZnO in composites reduces the stress concentration and enhances the uniformity of stress distribution; as a result, the composites with low f-ZnO contents show higher performance in mechanical properties than those with high f-ZnO loading levels. Good reinforcement was achieved with the homogeneous dispersion of f-ZnO in the composites. The agglomerates of f-ZnO can be the points of stress to damage the structure of the polymeric matrix, which results in mechanical property decrease. 3.5. Thermal properties of the WPU/f-ZnO composites Fig. 7 displays thermal decomposition behavior of pure WPU and WPU/f-ZnO composites. Two stages of decomposition appear at the TGA curves of all samples. As is evident, incorporation of
Fig. 6. Effect of the f-ZnO content on tensile strength and elongation at break of pure WPU and WPU/f-ZnO composites.
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Fig. 7. TGA thermograms of (a) pure WPU and WPU/f-ZnO composites with different fZnO contents: (b) 0.5 wt%, (c) 1.5 wt%, and (d) 4.0 wt%, respectively.
f-ZnO has no effect on the decomposition stage of WPU/f-ZnO composites. However, the decomposition temperature alters remarkably, which implies the thermal stability of the composites changes. Temperature for 50 wt% weight loss of the WPU/f-ZnO composites with 0, 0.5 wt%, 1.5 wt% and 4.0 wt% f-ZnO contents was obtained at 361.3, 331.2, 323.0 and 318.5 C, respectively. Initial degradation temperature of the composites significantly decreased and then leveled off. This result indicates that the composites decompose at lower temperatures than the pure WPU matrix; that is to say, incorporating f-ZnO into the WPU decreases the thermal stability. It is worth noting that our result is different from the results of other WPU composite systems, such as clay [21], CNTs [22], slica [23], SiC [30] and so on. The following reasons suggest a possible explanation for the change of the thermal stability of the composites. On one hand, fZnO network structure in the WPU matrix could confine the motion of polymer chains or act as thermal insulator and mass transport barrier to the volatile products generated during decomposition, thus results in delay of thermal degradation. This is also the main reason why clay and silica improve thermal ability of the composites. On the other hand, as a n-type intrinsic semiconductor, ZnO is able to form free oxygen and oxygen vacancies in the lattice induced by thermal excitation. The oxygen vacancies can trap and bound electrons to form active catalytical sites in ZnO, and free oxygen promotes the formation of peroxy radicals to damage the polymer chains. Thus, formation of free oxygen and oxygen vacancies plays an important role in degrading polymers. Furthermore, due to the one-dimensional nanostructure extended along the [0001] direction, f-ZnO is composed of nanorods with a larger population of (0002) planes [31]. Since the (0002) plane with the highest surface energy is the most unstable plane of ZnO, it is reasonable that the interaction between (0002) plane and polymer chains would also be more active. Therefore, it is logical to infer that the enhancement of thermal degradation of WPU/f-ZnO composites is attributed to the effect of thermal catalysis performance of f-ZnO. These two effects compete with each other and the thermal catalysis effect of f-ZnO dominates in the present system. Consequently, the decomposition temperature at which the weight loss reached 50 wt% was shifted by 30.1 C towards a lower temperature even when the f-ZnO content was as low as 0.5 wt%. However, comparing with the low f-ZnO loading level, excess amount of f-ZnO in the composites caused a smaller temperature shift for the effect of heat-resistance of f-ZnO network structures enhanced.
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3.6. Water resistance property of the WPU/f-ZnO composites Fig. 8 shows water swelling of pure WPU and WPU/f-ZnO composite films as a function of the f-ZnO content. It is clear that incorporation of f-ZnO decreases the value of water swelling significantly. The value of water swelling decreased by approximate 11.9% as the content of f-ZnO is up to 4.0 wt%. The result demonstrates that the presence of impermeable f-ZnO in WPU matrix reduces the water swelling, and enhances the resistance to water. Increase of the mean free path of water molecules to pass through the matrix of WPU/f-ZnO composite due to the reinforcing effect of the f-ZnO network structure, seems to be the cause of reduced water swelling. The improvement of water resistance in other WPU nanocomposite systems containing inorganic nanofillers such as WPU/clay nanocomposites [21] and WPU/silica hybrids [23] has also been reported. 3.7. Antibacterial activity of the WPU/f-ZnO composites Antibacterial activity of the WPU/f-ZnO composite films with different f-ZnO contents was tested using E. coli and S. aureus in comparison with the pure WPU film, as displayed in Fig. 9. The survival ratio of E. coli and S. aureus decreased with increase of f-ZnO content, and the best antibacterial activity was obtained with 4.0 wt% f-ZnO. Concerning the mechanism of the antibacterial activity of ZnO nanomaterials, several mechanisms have been proposed: (1) the release of Zn2þ ions from the powder [32], (2) mechanical destruction of the cell membrane caused by penetration of the nanoparticles [33], (3) active oxygen generated from the powder [34,35] and (4) the generation of hydrogen peroxide (H2O2) from the surface of ZnO [10,36]. It is well known that ZnO is unstable in the solution and the concentration of Zn2þ ions increases as a result of ZnO decomposition. In our work, since the nanorods of f-ZnO were coated with g-aminopropyltrimethoxysilane, the effect of Zn2þ released from ZnO could be restrained. Thus, the release of Zn2þ ions was not a main factor. ZnO nanorods with an average diameter about 400 nm used in this work were less likely to penetrate into the cell wall to damage the bacteria from the interior [37]. Tam et al. examined a large number of cells and found very few cases of internalization of the ZnO nanorod [7]. Therefore, mechanical damage of the cell membrane should not be considered as the mechanism of
Fig. 9. Effects of WPU/f-ZnO composites with different f-ZnO contents on survival ratio of E. coli and S. aureus.
antibacterial activity of f-ZnO. Our results show that the WPU/f-ZnO composites exhibited antibacterial activity in the absence of light, supporting the fact that antibacterial activity is most likely induced by the H2O2 generated from the surface of f-ZnO. Hydrogen peroxide is a powerful oxidizing agent and more reactive than oxygen molecules, it is harmful to the cells of living organisms [10,36]. The tendency of WPU absorbing moisture facilitates H2O2 generation. It is assumed that H2O2 generated damages the cell membrane of bacteria, produces some type of injury, and inhibits the growth of the cells or even kills them. Therefore, the generation of H2O2 from the surface of f-ZnO is considered as the primary factor of antibacterial activity of WPU/f-ZnO composites. The antibacterial effect of the composites on E. coli is stronger than on S. aureus when the content of f-ZnO was lower than 4.0 wt%. The difference in activity against these two types of bacteria can be attributed to structural and chemical compositional differences of the cell surfaces. Gram-positive bacteria typically have one cytoplasmic membrane and thick wall composed of multilayers of peptidoglycan [38]. However, gram-negative bacteria have more complex cell wall structure, with a layer of peptidoglycan between outer membrane and cytoplasmic membrane [33,38]. In a word, antibacterial effect can be attributed to the damage of cell membranes, which leads to leakage of cell contents and cell death. Therefore, the difference in antibacterial action towards E. coli and S. aureus is assumed to be caused by the different sensitivities towards H2O2 generated by f-ZnO. The mechanisms responsible for antibacterial activity of ZnO nanostructures are still not fully clear, so the exact cause of the membrane damage requires further study. 4. Conclusion
Fig. 8. Variation of water swelling of the WPU/f-ZnO composites by the amount of f-ZnO.
We have successfully prepared flower-like ZnO nanowhiskers (f-ZnO) composed of uniform nanorods via a simple hydrothermal method. WPU/f-ZnO composites were synthesized by in-situ copolymerization process using f-ZnO modified with APS. Composite films with low weight percentages loading of f-ZnO yielded materials with enhanced tensile strengths, water resistance and antibacterial properties compared with neat WPU film. The f-ZnO content showed an intense effect on the mechanical properties of the composites. The tensile strength of composite films increased significantly up to the optimum value (1.0 wt%), and then decreased gradually with excess f-ZnO content. However, the
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elongation at break feebly diminished for all of the tested composite films compared with the neat WPU film. Incorporation of f-ZnO decreased the thermal stability of the composites. The enhancement of thermal degradation of WPU/f-ZnO composites was mainly attributed to the effect of thermal catalysis performance of f-ZnO. Water swelling character declined with the increase of f-ZnO content in WPU/f-ZnO composites due to the reinforcing effect of the f-ZnO network structure. More importantly, the composite films exhibited a strong antibacterial effect against E. coli and S. aureus, and the best antibacterial activity was obtained with 4.0 wt% f-ZnO. From these results, the synthesized WPU/f-ZnO composites are potentially useful in a variety of coating applications because of their combination of enhanced mechanical and antibacterial properties. References [1] Balazs AC, Emrick T, Russell TP. Nanoparticle polymer composites: where two small worlds meet. Science 2006;314:1107–10. [2] Tang EJ, Cheng GX, Ma XL. Preparation of nano-ZnO/PMMA composite particles via grafting of the copolymer onto the surface of zinc oxide nanoparticles. Powder Technol 2006;161:209–14. [3] Lin Y, Boker A, He JB, Sill K, Xiang HQ, Abetz C, et al. Self-directed self-assembly of nanoparticle/copolymer mixtures. Nature 2005;434:55–9. [4] Zhang MQ, Yu G, Zeng HM, Zhang HB, Hou YH. Two-step percolation in polymer blends filled with carbon black. Macromolecules 1998;31:6724–6. [5] Schmidt-Mende L, Macmanus-Driscoll JL. ZnO – nanostructures, defects, and devices. Mater Today 2007;10:40–8. [6] Yamamoto O. Influence of particle size on the antibacterial activity of zinc oxide. Int J Inorg Mater 2001;3:643–6. [7] Tam KH, Djurisic AB, Chan C, Xi YY, Tse CW, Leung YH, et al. Antibacterial activity of ZnO nanorods prepared by a hydrothermal method. Thin Solid Films 2008;516:6167–74. [8] Chen YW, Liu YC, Lu SX, Xu CS, Shao CL, Wang C, et al. Optical properties of ZnO and ZnO: in nanorods assembled by sol-gel method. J Chem Phys 2005;123:134701–5. [9] Zhang Y, Jia HB, Luo XH, Chen XH, Yu DP, Wang RM. Synthesis, microstructure, and growth mechanism of dendrite ZnO nanowires. J Phys Chem B 2003;107:8289–93. [10] Chen SJ, Liu YC, Shao CL, Mu R, Lu YM, Zhang JY, et al. Structural and optical properties of uniform ZnO nanosheets. Adv Mater 2005;17:586–90. [11] Wang CL, Mao BD, Wang EB, Kang ZH, Tian CG. Solution synthesis of ZnO nanotubes via a template-free hydrothermal route. Solid State Commun 2007;141:620–3. [12] Kong XY, Wang ZL. Spontaneous polarization-induced nanohelixes, nanosprings, and nanorings of piezoelectric nanobelts. Nano Lett 2003;3:1625–31. [13] Sun D, Miyatake N, Sue HJ. Transparent PMMA/ZnO nanocomposite films based on colloidal ZnO quantum dots. Nanotechnology 2007;18:215606–11. [14] Demir MM, Memesa M, Castignolles P, Wegner G. PMMA/zinc oxide nanocomposites prepared by in-situ bulk polymerization. Macromol Rapid Commun 2006;27:763–70. [15] Ma C, Chen YJ, Kuan HC. Polystyrene nanocomposite materials: preparation, morphology, and mechanical, electrical, and thermal properties. J Appl Polym Sci 2005;98:2266–73. [16] Wu M, Yang GZ, Wang M, Wang WZ, Zhang WD, Feng JC, et al. Nonisothermal crystallization kinetics of ZnO nanorod filled polyamide 11 composites. Mater Chem Phys 2008;109:547–55.
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