Polymer Degradation and Stability 94 (2009) 1103 1109
Contents lists available at ScienceDirect
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:
A novel waterborne polyurethane/flower-like ZnO nanowhiskers (WPU/f-ZnO) composite with different
Received 24 February 2009
f-ZnO content (0 4.0 wt%) was synthesized by an in-situ copolymerization process. The f-ZnO consisting
Received in revised form
of uniform nanorods was prepared via a simple hydrothermal method. In order to disperse and incor-
17 March 2009
porate f-ZnO into WPU matrix, f-ZnO was modified with g-aminopropyltriethoxysilane. Morphology of
Accepted 22 March 2009
f-ZnO in WPU matrix was characterized by scanning electron microscope. The properties of WPU/f-ZnO
Available online 7 April 2009
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
Keywords:
dispersion in the WPU matrix significantly improved the performance of the composites. The mechanical
Waterborne polyurethane
property was enhanced with an increase of f-ZnO content up to the optimum content (1 wt%) and then
Nanocomposites
declined. Incorporation of f-ZnO enhanced the water resistance of the composites remarkably. It was
ZnO nanowhiskers
Thermal decomposition amazing to observe that the thermal degradation temperatures of the composites initially decreased
Antibacterial activity
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.
1. Introduction optics, photonics and electronics [5]. Furthermore, ZnO shows
a marked antibacterial activity at pH values in the range from 7 to 8
Nanocomposite is a class of materials with unique physical without the presence of light [6,7]. Zinc is also a mineral element
properties and wide application potential in diverse areas [1]. essential to human beings. ZnO nanostructures can be obtained by
Dispersion of nanoscaled inorganic fillers into an organic polymer various methods including thermal evaporation, electrochemical
to form polymer nanocomposites has gained increasing interest in deposition, sonochemical method, sol-gel, hydrothermal synthesis
recent years. Controlling the nanostructure, composition and and so forth. Various one-dimensional (1D) ZnO nanostructures
morphology of nanocomposites plays an essential role in their have been realized, such as nanorods, nanowires, nanobelts,
applications. Novel properties of nanocomposites can be obtained nanosheets, nanotubes, nanonails and so on [8 12]. Among the 1D
by successful imparting of the characteristics of parent constituents ZnO nanostructures, nanorods have been widely studied because of
to a single material [2]. These materials differ from both pure their easy preparation and wide applications [8]. In previous
polymers and inorganic fillers in some physical and chemical studies, ZnO nanostructures were combined with PMMA [13,14],
properties. The combination of polymers and nanoscale inorganic polystyrene [15], polyamide [16], polyacrylonitrile [17], poly-
fillers is opening pathways for engineering flexible composites that acrylate [18], etc. Compared with the original polymers, the yielded
exhibit attractive mechanical, thermal, optical and electrical prop- composite materials present a lot of excellent performance.
erties compared with conventional composites [3,4]. Polyurethanes (PU) are probably the most versatile class of
ZnO is an important and attractive semiconductive material. It polymers due to the great variety of raw materials that can be used
has drawn enormous attention due to its fantastic characteristics in for their formation. Waterborne polyurethane (WPU) shows many
excellent features compared to conventional organic solvent-based
polyurethane with the advantages of non-pollution and non-
toxicity and can be used in various fields, such as coating, adhesives
* Corresponding author. Tel.:þ86 20 87114099; fax:þ86 20 87112053.
E-mail address: [e-mail] (W.-D. Zhang). for textile, paper, wood or glass fibers, and so forth [19]. Thermal
0141-3910/$  see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymdegradstab.2009.03.024
1104 X.-Y. Ma, W.-D. Zhang / Polymer Degradation and Stability 94 (2009) 1103 1109
stability, insolubility, and mechanical properties of WPU, however, into 80 ml toluene, then 2 g f-ZnO was dispersed in the above-
are still lower than those of solvent-based PU and need to be mentioned solution under vigorous stirring. After sonication for
improved. Meanwhile, as a type of polymer, PU is susceptible to be 20 min, the suspension was refluxed for an additional 24 h with
degraded by many types of bacteria [20]. Nanosized additives are constant stirring. After that, the resultant was separated by
used as an effective strategy to alter and enhance the properties of centrifugation and then subjected to Soxhlet extraction with
WPU. Various types of filler, like clay [21], CNTs [22], silica [23], boiling ethanol for 12 h to remove excess APS absorbing on the
hydroxyapatite [24], Au [25], Ag [26] and flax cellulose [27] have surface of f-ZnO. The final APS functionalized f-ZnO (f-ZnO-NH2)
been incorporated into WPU to prepare nanocomposites. The was dried at 65 C for 48 h.
results demonstrated that homogeneous dispersion of fillers in
WPU matrix significantly improved the performance of the 2.4. Synthesis of WPU/f-ZnO composites
nanocomposites.
To our knowledge, no work has been reported about the prep- Before prepolymerization reaction, IPDI, f-ZnO NH2 and BDTBL
aration of WPU modified by ZnO nanowhiskers. Flower-like ZnO (served as catalyst) were added into a conical flask and sealed to
nanowhiskers can be widely used as polymer additives to make form an airtight system under ultrasonic at 60 70 C for 1 h. Under
functional nanocomposites because of their high aspect ratio, the catalysis of BDTBL, the reaction was initiated between the  NH2
together with good comprehensive properties such as semi- groups available on the surface of the modified f-ZnO and the  NCO
conductivity, high mechanical strength, wear resistance, vibration groups of IPDI. Uniformly dispersed suspension was obtained after
insulation, microwave absorption and antibacterial effect. Herein, sonication.
we report preparation and characterization of waterborne poly- A 500 ml round-bottom, 3-necked glass flask with a mechanical
urethane/flower-like ZnO nanowhiskers (WPU/f-ZnO) composites. stirrer and a condenser, was used as the reactor for the preparation
In this study, flower-like ZnO nanowhiskers (f-ZnO) were synthe- of WPU/f-ZnO composites. The reaction was carried out in
sized by a simple hydrothermal method and modified with a constant temperature oil bath. The synthesis procedures of the
g-aminopropyltriethoxysilane (APS) to improve the bonding composites are described briefly as follows. PBA and DMPA were
between f-ZnO and the WPU matrix. Properties of WPU/f-ZnO charged into the flask, which was heated at 70 C until they were
composites such as mechanical strength, thermal stability, water melted completely. And then, the above IPDI/f-ZnO hybrid
swelling as well as the antibacterial effect against Escherichia coli suspension was added into the flask while stirring. The mixture was
and Staphylococcus aureus were all influenced by the content of f- allowed to react in the presence of DBTDL (0.03 phr based on the
ZnO. 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.
2. Experiment Subsequently, acetone was slowly added to reduce viscosity so as to
obtain a homogeneous mixture. After cooling to room temperature,
2.1. Materials TEA was fed into the reactor and agitated for 30 min to neutralize
DMPA unit in PU. An aqueous emulsion of NCO-terminated pre-
ZnO and NaOH were of analytical grade supplied by Guanghua polymer was obtained by adding water to the mixture. EDA dis-
Chemical Factory Co., Ltd. (Guangdong, China). Silane coupling agent solved in water was then fed to the emulsion and chain extension
g-aminopropyltriethoxysilane (APS) was purchased from Shuguang was carried out at 50 C for 1 h. The final product was a WPU
Chemical Co., Ltd. (Nanjing, China) and used without further puri- emulsion with a solid content about 38 wt%. The stoichiometric
fication. Poly (butyl adipate) diol (PBA; average molecular weight ratio of IPDI/PBA/DMPA/TMP/EDA/TEA was 2.5/1.0/0.5/0.6/0.1/0.5.
Mnź1000 g/mol) was obtained from Hodotani Co. (Tokyo, Japan). The films of the WPU/f-ZnO composites for the measurements
Dimethylol propionic acid (DMPA), 3-isocyanatemethyl-3,5,5-tri- were prepared by casting the emulsions onto Teflon plates.
methyl-cyclohexylisocyanate (IPDI), trimethylolpropane (TMP) and
dibutyl tin dilaurate (DBTDL) were purchased from First Chemicals 2.5. Characterization and property measurements
of Tianjin (Tianjin, China). IPDI was used after dehydration with 4A
molecular sieve. PBA and DMPA were dehydrated at 90 C under X-ray diffraction (XRD) was performed by an X-ray diffractom-
vacuum for 24 h before use. Triethyl amine (TEA) and ethylene eter (Rigaku D/MAX-2500H) at 35 kV and 30 mA with a Cu Ka
diamine (EDA) were purchased from Lingfeng Chemicals of radiation source (kź0.15404 nm), at a scan speed of 4 /min. The
Shanghai (Shanghai, China) and used as received. structure of inorganic powder and composites were identified by
a Fourier transfer infrared spectrophotometer (FTIR; Bruke, Tensor
2.2. Preparation of flower-like ZnO nanowhiskers 27, Germany). Thermogravimetric analysis (TGA) of composite
samples was carried out with a thermogravimetric analyzer (TA
The flower-like ZnO nanowhiskers were prepared by a simple Instruments, Q50, USA). The samples of 5 10 mg each in an alumina
hydrothermal method. A transparent solution saturated with crucible were used with a heating rate of 10 C/min under nitrogen
Zn(OH)2 was formed by dissolving commercial ZnO powder in 5 M atmosphere. The morphology of the as-synthesized ZnO sample
4
NaOH solution. Then, the Zn(OH)2 saturated solution was loaded and the microstructure of the fractured surfaces of the WPU/f-ZnO
4
into distilled water (the volume ratio of Zn(OH)2 and H2O is 2:25) composite samples were observed by scanning electron micros-
4
under slow stirring. The diluted Zn(OH)2 solution was transferred copy (SEM; JEOL JSM 6700F, Japan).
4
into a sealed vessel and maintained at 90 C for 10 h in an oven. The Mechanical properties of the casting films were measured with
precipitate was collected by centrifugation, washed with distilled simple extension on dumbbell specimens about 2 mm thickness
water until neutral, and then dried at 65 C for 48 h. using a universal tensile machine (Tinius Olsen, USA) at a crosshead
speed of 50 mm/min. For each nanocomposite, five specimens were
2.3. Functionalization of flower-like ZnO nanowhiskers tested and the average value was reported.
Water swelling (the degree of water absorption) value of the
The introduction of reactive groups onto the surface of f-ZnO composite films was obtained as follows: pre-weighed dry samples
was achieved through the reaction between APS and the hydroxyl (20 mm 20 mm in size) were immersed in distilled water at 25 C.
groups on the surface of f-ZnO. Typically, 1 g APS was slowly added The samples were then blotted with filter paper and weighed.
X.-Y. Ma, W.-D. Zhang / Polymer Degradation and Stability 94 (2009) 1103 1109 1105
Water swelling was expressed as the weight percentage of water in No. 36-1451). Well-crystallized diffraction peaks and no charac-
the swollen sample and calculated by the following equation: teristic peaks of impurities of the sample were observed, suggesting
that the prepared ZnO sample under the present experimental
Swellingð%ÞźðWS WDÞ=WD 100% (1)
conditions is well-crystallized and with high purity. Fig. 2 shows
the SEM image of the as-synthesized ZnO. Flower-like morphology
where WD is the weight of the original dry sample and WS is the
was achieved and these ZnO flowers were composed of uniform
weight of the swollen sample.
nanorods with smooth surface. The diameter and length of the
E. coli ATCC 25922 (E. coli, Gram-negative) and S. aureus ATCC
nanorods were about 400 nm and 3 mm, respectively.
6538 (S. aureus, Gram positive) were chosen as target microor-
ganisms. All glassware used was sterilized in an autoclave at 120 C
for 30 min. Sample films of 50 mm 50 mm were washed with 3.2. FTIR analysis
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 The structures of the pristine f-ZnO, modified f-ZnO and WPU/f-
colony forming units per ml (CFU/ml) was pipetted onto the surface ZnO composite were analyzed by FTIR spectroscopy, as shown in
of the dried film in a Petri dish and then covered with a PE film Fig. 3. The FTIR spectrum of the f-ZnO NH2 (Fig. 3b) reveals some
(40 mm 40 mm). The films were incubated at a relative humidity new peaks compared to pristine f-ZnO. As characteristic bands of
(RH) higher than 90 wt% and temperature of 37 C for 24 h. APS molecules [28], the stretching and bending mode of the  NH2
Subsequently, each film was transferred to a new Petri dish and group were observed at 3355 and 1568 cm 1, respectively. The
thoroughly washed with a 20 ml of 0.87% NaCl solution containing absorption peaks at 1182 and 1025 cm 1 indicate the presence of
Tween 80 at pH 7.0 0.2. For determination of the actual number of Si O bonds which were attributed to the APS attachment [29]. In
microorganism colonies, the washing solution from each Petri dish addition, the peak at 886 cm 1 can be assigned to the bending
was diluted to series of smaller dilutions with sterile phosphate mode of Si OH group [28]. Therefore, the FTIR data indicated that
buffer solution (PBS). Afterwards, 1 ml diluted solution was spread APS providing amino groups was grafted onto the surface of f-ZnO.
onto the solid growth agar plate (containing 5 g/L beef extract, 10 g/ The result shows that the connection is based on covalent bonds
L peptone, 5 g/L NaCl and 15 g/L agar powder). After incubation of between f-ZnO surface and APS molecules. In order to determine
the plates at 37 C for 24 h, the number of viable microorganism whether the reaction was initiated by the  NH2 group on the
colonies was counted manually and the results after multiplication surface of f-ZnO NH2, the interaction product of f-ZnO NH2 and
with the dilution factor were expressed as mean CFU after aver- IPDI was examined by FTIR. The reacted ZnO powder (f-ZnO NCO)
aging the duplicate counts. The survival ratio was calculated using was washed with acetone via centrifugation to completely remove
the following equation: the residual IPDI before FTIR measurement. As indicated in Fig. 3c,
the spectrum of f-ZnO NCO reveals new bands compared to f-ZnO
Survival ratioð%ÞźðN=N0Þ 100% (2)
NH2. A peak at 2262 cm 1 appears in the spectrum of f-ZnO NCO,
indicating the presence of  NCO group on the surface of f-ZnO. The
where, N0 is the mean number of bacteria on the pure WPU film
bands at 2956 and 2921 cm 1 can be ascribed to the symmetrical
samples (CFU/sample), and N is the mean number of bacteria on the
and asymmetrical stretching vibrations of C H in  CH3 and  CH2
composite film samples (CFU/sample).
groups of IPDI, respectively. The newly formed bands at 1639 and
1560 cm 1 can be assigned to the stretch vibration of C]O group
3. Results and discussion
and the coupling of N H bending vibration with C N stretching
vibration in  NH CO NH , respectively. On the basis of the above
3.1. Characterization of the f-ZnO
result, we conclude that after being reacted with IPDI, the f-ZnO
were attached with  NCO group by the formation of urea bonding.
The crystal structure of the synthesized ZnO was characterized
On the other hand, we can only find the major characteristic peaks
by XRD. As shown in Fig. 1, the XRD pattern of the sample is in good
of PU in the WPU/f-ZnO composite since the content of f-ZnO is
accordance with the standard hexagonal phase ZnO (JCPDS Card
very low. It is very hard to find any new peak except for the slight
peak at 540 cm 1 belonging to ZnO in WPU/f-ZnO composite
compared with the pure WPU.
Fig. 1. XRD pattern of the as-prepared ZnO nanowhiskers. Fig. 2. SEM image of the as-prepared ZnO nanowhiskers.
1106 X.-Y. Ma, W.-D. Zhang / Polymer Degradation and Stability 94 (2009) 1103 1109
important role in improving the mechanical performance of the
composite films as discussed later. It is obvious that a large aggre-
gate 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
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
Fig. 3. FTIR spectra of (a) pristine f-ZnO, (b) f-ZnO NH2, (c) f-ZnO NCO and (d) WPU
/f-ZnO composite.
 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
3.3. Morphology characterization of the WPU/f-ZnO composites
content of f-ZnO, thus resulted in more networks in the composites.
It is well known that the network structure of the nanocomposite is
Examination of the fractured surfaces of WPU/f-ZnO compos-
favorable for reinforcing mechanical strength [24]. It is worth
ites, which were broken at liquid nitrogen temperature, was carried
noting that, with 0.5 wt% f-ZnO filler, the tensile strength of the
out by SEM. Fig. 4 shows the images of the composites filled with
composite increased to the maximum (12.6 MPa) by about 34%,
1.0 wt% and 4.0 wt% f-ZnO, respectively. As compared to the matrix,
compared with the neat counterpart (9.4 MPa). When the content
the morphology of the f-ZnO can be easily identified. The white
of f-ZnO was in excess of 1.0 wt%, the tensile strength of the
dots in the images correspond to f-ZnO on the fractured surfaces of
composites decreased and higher filler content (up to 4.0 wt%)
the composites. Well-dispersed f-ZnO in the composites can be
resulted in more serious decrease in the strength. On the other
observed in Fig. 4a, while Fig. 4c shows not only well-dispersed ZnO
hand, the elongation at break feebly decreased for all of the tested
but also aggregates in the composites with higher filler contents.
composite films compared to the original WPU film, with
For details, as shown in Fig. 4b, a homogeneous distribution of the
a maximum decrease at 1 wt% loading level. This phenomenon can
nanorods embedded in the WPU matrix was observed, implying
be explained by the fact that the rigid filler network structure,
that there exists good adhesion between fillers and matrix. Such an
which is responsible for the enforcing effect, was formed perfectly
even and uniform distribution of the fillers in the matrix played an
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%.
X.-Y. Ma, W.-D. Zhang / Polymer Degradation and Stability 94 (2009) 1103 1109 1107
Fig. 7. TGA thermograms of (a) pure WPU and WPU/f-ZnO composites with different f-
Fig. 5. Stress strain curves of (a) pure WPU and WPU/f-ZnO composites with (b)
ZnO contents: (b) 0.5 wt%, (c) 1.5 wt%, and (d) 4.0 wt%, respectively.
0.5 wt%, (c) 1.0 wt%, (d) 1.5 wt%, (e) 2.0 wt% and (f) 4.0 wt% of f-ZnO, respectively.
f-ZnO has no effect on the decomposition stage of WPU/f-ZnO
mechanical strength of the composites with more than 1.0 wt% may
composites. However, the decomposition temperature alters
be attributed to the aggregation of excess filler in WPU matrix.
remarkably, which implies the thermal stability of the composites
Furthermore, the aggregation behavior increased with increasing
changes. Temperature for 50 wt% weight loss of the WPU/f-ZnO
f-ZnO content, indicating an increase in the incompatibility of the
composites with 0, 0.5 wt%, 1.5 wt% and 4.0 wt% f-ZnO contents was
WPU/f-ZnO composites with excess f-ZnO content.
obtained at 361.3, 331.2, 323.0 and 318.5 C, respectively. Initial
In combination with the result of SEM, we can summarize that
degradation temperature of the composites significantly decreased
an optimum incorporation amount of f-ZnO exists for an effective
and then leveled off. This result indicates that the composites
enhancement of the mechanical property of the composites. Good
decompose at lower temperatures than the pure WPU matrix; that
dispersion of f-ZnO in composites reduces the stress concentration
is to say, incorporating f-ZnO into the WPU decreases the thermal
and enhances the uniformity of stress distribution; as a result, the
stability. It is worth noting that our result is different from the
composites with low f-ZnO contents show higher performance in
results of other WPU composite systems, such as clay [21], CNTs
mechanical properties than those with high f-ZnO loading levels.
[22], slica [23], SiC [30] and so on.
Good reinforcement was achieved with the homogeneous disper-
The following reasons suggest a possible explanation for the
sion of f-ZnO in the composites. The agglomerates of f-ZnO can be
change of the thermal stability of the composites. On one hand, f-
the points of stress to damage the structure of the polymeric
ZnO network structure in the WPU matrix could confine the motion
matrix, which results in mechanical property decrease.
of polymer chains or act as thermal insulator and mass transport
barrier to the volatile products generated during decomposition,
3.5. Thermal properties of the WPU/f-ZnO composites
thus results in delay of thermal degradation. This is also the main
reason why clay and silica improve thermal ability of the
Fig. 7 displays thermal decomposition behavior of pure WPU
composites. On the other hand, as a n-type intrinsic semiconductor,
and WPU/f-ZnO composites. Two stages of decomposition appear
ZnO is able to form free oxygen and oxygen vacancies in the lattice
at the TGA curves of all samples. As is evident, incorporation of
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. Further-
more, 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 compos-
ites 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. Conse-
quently, 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
Fig. 6. Effect of the f-ZnO content on tensile strength and elongation at break of pure
WPU and WPU/f-ZnO composites. effect of heat-resistance of f-ZnO network structures enhanced.
1108 X.-Y. Ma, W.-D. Zhang / Polymer Degradation and Stability 94 (2009) 1103 1109
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 demon-
strates 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
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 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
antibacterial activity of f-ZnO. Our results show that the WPU/f-ZnO
survival ratio of E. coli and S. aureus decreased with increase of
composites exhibited antibacterial activity in the absence of light,
f-ZnO content, and the best antibacterial activity was obtained with
supporting the fact that antibacterial activity is most likely induced
4.0 wt% f-ZnO.
by the H2O2 generated from the surface of f-ZnO. Hydrogen peroxide
Concerning the mechanism of the antibacterial activity of ZnO
is a powerful oxidizing agent and more reactive than oxygen
nanomaterials, several mechanisms have been proposed: (1) the
molecules, it is harmful to the cells of living organisms [10,36]. The
release of Zn2þ ions from the powder [32], (2) mechanical
tendency of WPU absorbing moisture facilitates H2O2 generation. It
destruction of the cell membrane caused by penetration of the
is assumed that H2O2 generated damages the cell membrane of
nanoparticles [33], (3) active oxygen generated from the powder
bacteria, produces some type of injury, and inhibits the growth of the
[34,35] and (4) the generation of hydrogen peroxide (H2O2) from
cells or even kills them. Therefore, the generation of H2O2 from the
the surface of ZnO [10,36].
surface of f-ZnO is considered as the primary factor of antibacterial
It is well known that ZnO is unstable in the solution and the
activity of WPU/f-ZnO composites.
concentration of Zn2þions increases as a result of ZnO decomposi-
The antibacterial effect of the composites on E. coli is stronger
tion. In our work, since the nanorods of f-ZnO were coated with
than on S. aureus when the content of f-ZnO was lower than
g-aminopropyltrimethoxysilane, the effect of Zn2þ released from
4.0 wt%. The difference in activity against these two types of
ZnO could be restrained. Thus, the release of Zn2þ ions was not
bacteria can be attributed to structural and chemical compositional
a main factor. ZnO nanorods with an average diameter about 400 nm
differences of the cell surfaces. Gram-positive bacteria typically
used in this work were less likely to penetrate into the cell wall to
have one cytoplasmic membrane and thick wall composed of
damage the bacteria from the interior [37]. Tam et al. examined
multilayers of peptidoglycan [38]. However, gram-negative bacteria
a large number of cells and found very few cases of internalization of
have more complex cell wall structure, with a layer of peptido-
the ZnO nanorod [7]. Therefore, mechanical damage of the cell
glycan between outer membrane and cytoplasmic membrane
membrane should not be considered as the mechanism of
[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 nano-
structures are still not fully clear, so the exact cause of the
membrane damage requires further study.
4. Conclusion
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
Fig. 8. Variation of water swelling of the WPU/f-ZnO composites by the amount of
f-ZnO. then decreased gradually with excess f-ZnO content. However, the
X.-Y. Ma, W.-D. Zhang / Polymer Degradation and Stability 94 (2009) 1103 1109 1109
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