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

Article history:
Received 24 February 2009
Received in revised form
17 March 2009
Accepted 22 March 2009
Available online 7 April 2009

Keywords:
Waterborne polyurethane
Nanocomposites
ZnO nanowhiskers
Thermal decomposition
Antibacterial activity

a b s t r a c t

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 incor-
porate 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.

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 prop-
erties 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

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]

, poly-

acrylate

[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 non-
toxicity and can be used in various fields, such as coating, adhesives
for textile, paper, wood or glass fibers, and so forth

[19]

. Thermal

*

Corresponding author. Tel.: þ86 20 87114099; fax: þ86 20 87112053.
E-mail address:

[e-mail]

(W.-D. Zhang).

Contents lists available at

ScienceDirect

Polymer Degradation and Stability

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / p o l y d e g s t a b

0141-3910/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymdegradstab.2009.03.024

Polymer Degradation and Stability 94 (2009) 1103–1109

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 prep-

aration 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 semi-
conductivity, high mechanical strength, wear resistance, vibration
insulation, microwave absorption and antibacterial effect. Herein,
we report preparation and characterization of waterborne poly-
urethane/flower-like ZnO nanowhiskers (WPU/f-ZnO) composites.
In this study, flower-like ZnO nanowhiskers (f-ZnO) were synthe-
sized 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 f-
ZnO.

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 puri-
fication. Poly (butyl adipate) diol (PBA; average molecular weight
M

n

¼ 1000 g/mol) was obtained from Hodotani Co. (Tokyo, Japan).

Dimethylol propionic acid (DMPA), 3-isocyanatemethyl-3,5,5-tri-
methyl-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)

4

2

was formed by dissolving commercial ZnO powder in 5 M

NaOH solution. Then, the Zn(OH)

4

2

saturated solution was loaded

into distilled water (the volume ratio of Zn(OH)

4

2

and H

2

O is 2:25)

under slow stirring. The diluted Zn(OH)

4

2

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 above-
mentioned 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-NH

2

)

was dried at 65



C for 48 h.

2.4. Synthesis of WPU/f-ZnO composites

Before prepolymerization reaction, IPDI, f-ZnO–NH

2

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 –NH

2

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 pre-
polymer was obtained by adding water to the mixture. EDA dis-
solved 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 diffractom-

eter (Rigaku D/MAX-2500H) at 35 kV and 30 mA with a Cu K

a

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 micros-
copy (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.

X.-Y. Ma, W.-D. Zhang / Polymer Degradation and Stability 94 (2009) 1103–1109

1104

Water swelling was expressed as the weight percentage of water in
the swollen sample and calculated by the following equation:

Swellingð%Þ ¼ ðW

S

 W

D

Þ=W

D

 100%

(1)

where W

D

is the weight of the original dry sample and W

S

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 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
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  10

6

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 aver-
aging the duplicate counts. The survival ratio was calculated using
the following equation:

Survival ratioð%Þ ¼ ðN=N

0

Þ  100%

(2)

where, N

0

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

No. 36-1451). Well-crystallized diffraction peaks and no charac-
teristic 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

m

m, respectively.

3.2. FTIR analysis

The structures of the pristine f-ZnO, modified f-ZnO and WPU/f-

ZnO composite were analyzed by FTIR spectroscopy, as shown in

Fig. 3

. The FTIR spectrum of the f-ZnO–NH

2

(

Fig. 3

b) reveals some

new peaks compared to pristine f-ZnO. As characteristic bands of
APS molecules

[28]

, the stretching and bending mode of the –NH

2

group were observed at 3355 and 1568 cm

1

, respectively. The

absorption peaks at 1182 and 1025 cm

1

indicate the presence of

Si–O bonds which were attributed to the APS attachment

[29]

. In

addition, the peak at 886 cm

1

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 –NH

2

group on the

surface of f-ZnO–NH

2

, the interaction product of f-ZnO–NH

2

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. 3

c,

the spectrum of f-ZnO–NCO reveals new bands compared to f-ZnO–
NH

2

. 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
bands at 2956 and 2921 cm

1

can be ascribed to the symmetrical

and asymmetrical stretching vibrations of C–H in –CH

3

and –CH

2

–

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

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 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.

X.-Y. Ma, W.-D. Zhang / Polymer Degradation and Stability 94 (2009) 1103–1109

1105

3.3. Morphology characterization of the WPU/f-ZnO composites

Examination of the fractured surfaces of WPU/f-ZnO compos-

ites, 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. 4

a, while

Fig. 4

c shows not only well-dispersed ZnO

but also aggregates in the composites with higher filler contents.
For details, as shown in

Fig. 4

b, 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

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. 4

d. 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
–NH

2

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. 3. FTIR spectra of (a) pristine f-ZnO, (b) f-ZnO–NH

2

, (c) f-ZnO–NCO and (d) WPU

/f-ZnO composite.

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

1106

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 disper-
sion 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

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, f-
ZnO 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. 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
effect of heat-resistance of f-ZnO network structures enhanced.

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.

Fig. 6. Effect of the f-ZnO content on tensile strength and elongation at break of pure
WPU and WPU/f-ZnO composites.

Fig. 7. TGA thermograms of (a) pure WPU and WPU/f-ZnO composites with different f-
ZnO contents: (b) 0.5 wt%, (c) 1.5 wt%, and (d) 4.0 wt%, respectively.

X.-Y. Ma, W.-D. Zhang / Polymer Degradation and Stability 94 (2009) 1103–1109

1107

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

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 Zn

2þ

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 (H

2

O

2

) from

the surface of ZnO

[10,36]

.

It is well known that ZnO is unstable in the solution and the

concentration of Zn

2þ

ions increases as a result of ZnO decomposi-

tion. In our work, since the nanorods of f-ZnO were coated with

g

-aminopropyltrimethoxysilane, the effect of Zn

2þ

released from

ZnO could be restrained. Thus, the release of Zn

2þ

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

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 H

2

O

2

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 H

2

O

2

generation. It

is assumed that H

2

O

2

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 H

2

O

2

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 peptido-
glycan 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 H

2

O

2

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
then decreased gradually with excess f-ZnO content. However, the

Fig. 9. Effects of WPU/f-ZnO composites with different f-ZnO contents on survival ratio
of E. coli and S. aureus.

Fig. 8. Variation of water swelling of the WPU/f-ZnO composites by the amount of
f-ZnO.

X.-Y. Ma, W.-D. Zhang / Polymer Degradation and Stability 94 (2009) 1103–1109

1108

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 perfor-
mance 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 impor-
tantly, 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.

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