ARTICLE IN PRESS
Polymer Degradation and Stability xxx (2010) 1e6
Contents lists available at ScienceDirect
Polymer Degradation and Stability
journal homepage: www.elsevier.com/locate/polydegstab
Silver sulfide/poly(3-hydroxybutyrate) nanocomposites: Thermal stability
and kinetic analysis of thermal degradation
S.Y. Yeo, W.L. Tan, M. Abu Bakar*, J. Ismail
Nanoscience Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 November 2009
Received in revised form
19 February 2010
Accepted 19 February 2010
Available online xxx
The non-isothermal degradation of poly(3-hydroxybutyrate) (PHB) and silver sulfide/poly(3-hydroxybutyrate) (Ag2S/PHB) nanocomposites was investigated using thermogravimetric (TG) analysis. In the
composite materials, Ag2S caused the degradation of PHB at a lower temperature as opposed to that of
neat PHB. Moreover, an increase Ag2S loading in the PHB decreased the onset temperature (Tonset) of
thermal degradation, whereas it was raised upon augmenting the heating rate. From Kissinger plots, the
observed trend of the degradation activation energy, Ed, was attributed to polymereparticle surface
interactions and the agglomeration of Ag2S. The thermal degradation rate constant, k, was linearly
related to the Ag2S loading in PHB. Thus, the Ag2S nanoparticles effectively catalyzed the thermal
degradation of PHB in the Ag2S/PHB nanocomposites. Differential scanning calorimetry (DSC) data also
supported the catalytic property of Ag2S.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
PHB
Silver sulfide
Nanocomposite
Thermal degradation
Kissinger
1. Introduction
Poly(3-hydroxybutyrate), PHB is a biodegradable polymer that
has been widely studied. However, PHB possesses unfavorable
properties such as high crystallinity, which lead to brittleness, and
low resistance to thermal degradation, which cause the material to
be easily degradable. These properties have limited the application
range of PHB [1e7], and consequently, new approaches are sought
after in order to alleviate the weaknesses of the material. Examples
include blending of PHB with other polymers [1,6,8e11] or filling
PHB with nanofillers, such as montmorillonite [12] or carbon
nanotubes [13], in order to obtain enhanced mechanical properties
and improved thermal stability of the resultant composites. Among
the investigated methods, polymer blending has gained increasing
attention and interest of researchers as it represents a good route
for producing materials with a decent biocompatibility. Blending
PHB with other polymers may thereby improve the mechanical and
thermal properties as well as retain the biodegradability of the
initial polymer.
PHB chains have been found to degrade at temperatures slightly
above their melting point, thus producing shorter chain fragments
with carboxylic terminal groups according to random chain scission
mechanisms with crotonic acid as one of the signature by-products
* Corresponding author. Tel.: þ60 604 6533888; fax: þ60 604 6574854.
E-mail address: [email protected] (M. Abu Bakar).
[14e16]. The carboxylic terminal groups of the degraded PHB react
with other compatible functionalized polymers at sufficiently high
temperatures. In fact, our group has previously reported upon melt
reactions in blends of PHB and a modified natural rubber, namely
50% epoxidized natural rubber, ENR-50 [17]. In this case, oxirane
ring opening reactions were induced by carboxylic terminal groups
of the degraded PHB upon heating the PHB/ENR-50 blend to slightly
above the melting temperature of PHB. The initially immiscible
blend of PHB/ENR-50 demonstrated a progressive change towards
miscibility that consequently afforded a single glass transition peak
as an effect of the melt reaction upon annealing at 190 C. Although
the annealing time and temperature constitute main considerations
for the melt reaction during blending of these polymers, the presence of a catalyst for the thermal degradation of PHB may lower the
temperature and reduce the time, hence increasing the rate of the
melt reaction. Obviously, this could have a large impact on industries dealing with PHB or other polymer blending processes.
Dispersed metal nanoparticles in a polymer matrix provide
strong possibilities of fabricating functional materials with unique
properties for catalytic, electrical and sensing applications. It is
anticipated that metal nanoparticles can potentially be employed
as catalysts in melt processing of polymer blends. It is known that
metal nanoparticles may enhance or suppress the thermal stability
depending on the type of polymer, as reported by Lee et al. [18,19].
Thus, Pd nanoparticles have been seen to enhance the thermal
stability of polystyrene, polypropylene and polymethacrylate but to
suppress that of polyamide 6 and poly(ethylene terephthalate).
0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymdegradstab.2010.02.025
Please cite this article in press as: Yeo SY, et al., Silver sulfide/poly(3-hydroxybutyrate) nanocomposites: Thermal stability and kinetic..., Polym
Degrad Stab (2010), doi:10.1016/j.polymdegradstab.2010.02.025
ARTICLE IN PRESS
2
S.Y. Yeo et al. / Polymer Degradation and Stability xxx (2010) 1e6
There also exists a report on similar effects of Ag nanoparticles on
sago starch in Ag/sago starch nanocomposites [20].
Our group has observed a similar catalytic effect for Ag nanoparticles on the thermal degradation of PHB [21]. Thus, the metal
nanoparticles such as Pd and Ag act as catalysts for the thermal
degradation of polymers. In addition to metal nanoparticles, other
types of inorganic nanoparticle catalysts such as metal oxides or
metal chalcogenides may also be equally effective. Amongst the metal
chalcogenides, Ag2S has been incorporated in numerous polymers
such as polyvinylpyrrolidone, poly(vinyl alcohol), polyurethane,
polystyrene [22e25] and even into sago starch [20]. Numerous
techniques have been employed to synthesize Ag2S nanoparticles,
including hydrothermal methods, precipitation, etc. [26e28]. Thioacetamide [29], thiourea [26,30], hydrogen sulfide [28,31] and carbon
disulfide [22,23] often constitute the sulfur source.
The possibility of utilizing various catalysts for the thermal
degradation of polymers, thereby giving rise to a new approach for
blending polymers, is of great interest to us. The present fundamental study has thus been undertaken in order to eventually
extend the work on the melt reaction of PHB/ENR-50 blends. To the
best of our knowledge, no investigation of metal sulfide/PHB
nanocomposites has ever been published. The present article thus
describes the synthesis and kinetic study of the thermal degradation of Ag2S/PHB nanocomposites.
2. Experimental section
2.1. Materials
Silver nitrate, AgNO3 (Fisher Chemical, UK), sodium hydroxide,
NaOH (Malinckrodt, Malaysia), chloroform (Systerm, Malaysia),
thiourea, NH2CSNH2 (Riedel-de Haen, Germany) and cetyl-trimethyl
ammonium bromide, CTAB (Merck, Germany) were all used without
further purification. Poly(3-hydroxybutyrate), PHB was supplied by
BIOCYCLE, Brazil, and purified according to a previous report [32].
The PHB used in this work was a homopolymer with an average
molecular weight of 370,000 g/mol and a polydispersity of 2.12.
2.2. Method
The Ag2S/PHB nanocomposites were obtained via a two-step
aqueous to organic phase transfer method [33] where the Ag2S
particles were first formed in the aqueous phase followed by the
transfer of the Ag2S particles into an organic solution of PHB in
chloroform. In a typical procedure, two sets of aqueous solutions
were prepared, where one was comprised of 1 104 mol CTAB and
1 104 mol silver nitrate, and the other consisted of 4 104 mol
NaOH and 3 104 mol thiourea. These two solutions were stirred
separately for 30 min, after which they were mixed to give a mole
ratio of Agþ:CTAB:thiourea:NaOH equaling 1:1:3:4 in a total
volume of 200 ml solution. This final mixture was stirred vigorously
and refluxed at w120 C (bath temperature) for 12 h.
Upon cooling the Ag2S hydrosol, a solution of PHB in chloroform was added under vigorous stirring to allow the Ag2S
nanoparticles to transfer into the organic phase. The aqueous and
the organic phases were then separated. The solvent of the
organic phase was removed via rotary evaporation and the
obtained solid Ag2S/PHB nanocomposite sample was dried in
a vacuum oven at w45 C for 24 h. Various Ag2S/PHB nanocomposites were prepared in a similar manner by varying the
amount of PHB. Thus, several Ag2S loadings (viz. 0, 0.05, 0.12,
0.26, 0.50, 1.28 and 2.60 wt%) in PHB were obtained and denoted
PHB, Ag2S/PHB2000, Ag2S/PHB1000, Ag2S/PHB500, Ag2S/PHB250,
Ag2S/PHB100, and Ag2S/PHB50, respectively, as listed in Table 1.
2.3. Characterization
The determination of the metal content in the Ag2S/PHB nanocomposites (expressed as wt% of Ag2S) was performed using
a PerkineElmer Analyst 100 atomic absorption spectrometer (AAS).
The x-ray diffraction (XRD) patterns of the samples were obtained
using the SIEMENS D5000 X-ray diffractometer. The XRD sample of
the Ag2S particles in the aqueous phase was prepared by precipitating the particles by addition of ethanol. The particles were then
collected via centrifugation. In contrast, the XRD samples of the
Ag2S/PHB nanocomposites were obtained by evaporating the
organic phase, which gave rise to a thin solid film.
A Leo Supra 50VP field emission scanning electron microscope
(FE-SEM) equipped with energy-dispersive X-ray spectroscopy (EDS)
was used to identify the elemental composition of the Ag2S/PHB
nanocomposites. The morphology of the Ag2S particles was investigated using a Philips CM12 transmission electron microscope (TEM)
operating at 80 kV. The samples were prepared by casting a few drops
of the respective aqueous or organic phase onto the 400 mesh
carbon-coated grid, followed by air drying under ambient conditions.
The average particle sizes and size distributions were analyzed
based on at least 300 particles from the TEM micrographs using
the image analysis computer software “AnalySIS Docu” Version 3.2
(Soft Imaging System GmbH, Munster, Germany).
The thermal behavior and degradation of the Ag2S/PHB nanocomposites were investigated with differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis, respectively, on
w10 mg samples. The DSC measurements were performed using
a PerkineElmer Pyris DSC instrument. Samples were first heated
from 30 C to 190 C at a heating rate of 20 C min1 and held for
1 min. The samples were then quenched to 40 C at a rate of
100 C min1 and held for another 3 min. Finally, the samples
were subjected to the second heating run where they were again
heated from 40 C to 190 C at a rate of 20 C min1. The first
heating run was conducted to remove the thermal history of the
sample, and thermal data was acquired based on the second
heating run. Thermal degradation of the samples was determined
using a PerkineElmer TGA-7 at various heating rates of 2, 10, 20,
30 and 40 C min1, under nitrogen atmosphere. The onset
Table 1
Kinetic parameters of thermal degradation and DSC data (from 2nd heating) of neat PHB and the various Ag2S/PHB nanocomposites.
Sample
Ag2S loading (wt%)
Kinetic parameters
r,a Linear regression
Purified PHB
Ag2S/PHB2000
Ag2S/PHB1000
Ag2S/PHB500
Ag2S/PHB250
Ag2S/PHB100
Ag2S/PHB50
a
0.00
0.05
0.12
0.26
0.50
1.28
2.60
0.9980
0.9945
0.9940
0.9950
0.9975
0.9980
0.9995
DSC data
Ea (kJ/mol)
136.8
110.2
95.1
87.5
91.8
94.5
99.1
A (s1)
k (s1)
12
2.2 10
2.0 1010
7.8 108
1.1 108
5.3 108
2.1 109
9.5 109
2
1.37 10
7.14 102
10.34 102
8.74 102
15.42 102
33.70 102
48.58 102
Tg ( C)
Tc ( C)
Tm ( C)
5.9
2.2
2.0
2.4
6.9
9.0
13.4
55
56
51
56
40
44
27
e
147
139
133
144
138
132
172
163
157
153
150
148
143
DHm (J/g)
cc (%)
88.4
80.9
86.6
83.7
82.6
72.3
60.3
60.5
55.4
59.3
57.3
56.5
49.5
41.3
Refers to the plots of Fig. 6.
Please cite this article in press as: Yeo SY, et al., Silver sulfide/poly(3-hydroxybutyrate) nanocomposites: Thermal stability and kinetic..., Polym
Degrad Stab (2010), doi:10.1016/j.polymdegradstab.2010.02.025
ARTICLE IN PRESS
S.Y. Yeo et al. / Polymer Degradation and Stability xxx (2010) 1e6
3
temperature of thermal degradation, Tonset, defined by the intersection point between the linear extrapolation of the baseline and
linear fitted maximum slope of the TG curve [34], was acquired
from the computer program STARe (version 9.20) software.
2.4. Kinetic analysis
The thermal degradation of PHB in neat and nanocomposite
form were evaluated using non-isothermal TG analyses. The Kissinger equation [19,35,36], equation (1), was employed in the
kinetic analysis of the thermal degradation.
ln
q
E
AR
¼ d ln
RTp
Ed
Tp2
(1)
Here, q is the heating rate (K min1), Tp is the maximum degradation
rate temperature (K), Ed is the degradation activation energy, R is the
gas constant and A is a pre-exponential factor. The plot of ln q/(Tp)2
against 1/Tp gives a straight line. The Ed and A could thus be obtained
from the slope and the Y-intersection of the plot respectively. The
degradation rate constant, k was calculated at a selected temperature (T ¼ 230 C) using the Arrhenius equation (2) shown below.
k ¼ AeEd =RT
(2)
3. Results and discussion
3.1. Morphology
Typical XRD patterns of the Ag2S nanoparticles (obtained from the
aqueous phase) and the Ag2S/PHB nanocomposite are shown in Fig. 1.
In both cases, the Ag2S particles adopted the stable monoclinic form
(JCPDS 14-0072) [26,30]. The broad diffraction peaks of Ag2S in both
samples indicate that the synthesized Ag2S particles were in the
nanosize regime. Fig. 1(b), depicting the XRD pattern of the Ag2S/PHB
nanocomposite, also displays peaks attributed to the crystalline PHB.
From the XRD patterns of Fig. 1, the presence of impurities, if any,
cannot be clearly seen. In view of this, EDS analyses (not shown) were
carried out on a selection of samples. There were no detectable
impurities such as Na or N in any of the samples. Nevertheless, traces
of Cl and Br were observed in the Ag2S/PHB50 but not in other
nanocomposites. It was thus reasonable to believe that the nanocomposites were composed solely of PHB and Ag2S.
TEM images of the Ag2S particles and the various Ag2S/PHB
nanocomposites are given in Fig. 2. As can be seen in Fig. 2(a), the
Ag2S particles are dispersed and sphere-like, with an average
particle size of 18.0 3.1 nm. On the other hand, the particles in all
the nanocomposite samples are aggregated. The morphology of the
Ag2S particles in the nanocomposite is dependent on the PHB
concentration. At low concentrations of PHB (i.e., for Ag2S/PHB50
and Ag2S/PHB500), the particles are hexagonal in shape. However,
the Ag2S nanoparticles became spherical upon increasing the
concentration of PHB (i.e., for Ag2S/PHB1000 and Ag2S/PHB2000).
The Ag2S particles in the Ag2S/PHB500 sample show the largest
average particle size of 23.9 4.0 nm while the corresponding
values are 15.5 4.3 nm for Ag2S/PHB50, 14.8 5.5 nm for
Ag2S/PHB1000 and 13.8 5.5 nm for Ag2S/PHB2000.
3.2. Thermal stability and degradation
The effect of the concentration of Ag2S in PHB was studied in order
to obtain a better understanding on the influence of the nanoparticles
on the thermal degradation of the polymer. Fig. 3 shows typical
TG thermograms for neat PHB and the various Ag2S/PHB
Fig. 1. Typical X-ray diffraction patterns of the (a) Ag2S particles and (b) Ag2S/PHB
nanocomposite [# is due to PHB].
nanocomposites obtained at a heating rate of 20 C min1. As can be
observed, the Tonset decreased with an increased Ag2S loading in the
PHB. For example, the Tonset values of the Ag2S/PHB2000 and
Ag2S/PHB50 samples are 277 and 232 C, respectively, while the Tonset
of neat PHB is 296 C. To further confirm this trend, the Tonset of all
samples at all studied heating rates i.e., 2, 10, 20, 30 and 40 C min1,
were plotted against the Ag2S concentration as shown in Fig. 4.
The Tonset decreased with an increase Ag2S loading at all heating rates.
The Ag2S/PHB50 nanocomposite with the highest particle loading
(2.6 wt%) displayed the lowest Tonset while neat PHB (0 wt%) showed
the highest Tonset for all heating rates. These results thus demonstrate
that the thermal degradation of PHB in the nanocomposite is
significantly influenced by the amount of Ag2S loading as well as by
the heating rate.
In the Ag2S/PHB degradation process, the presence of Ag2S
nanoparticles promoted the degradation of PHB. This caused the
Tonset to shift to lower temperatures. Furthermore, it is believed that
the decrease in thermal stability may have been caused by the low
molecular weight polymer chains resulting from the degradation of
PHB catalyzed by the Ag2S nanoparticles. This phenomenon
becomes more obvious upon increasing the Ag2S concentration in
PHB. The Tonset shifts to higher temperatures upon increasing the
heating rate as shown in Fig. 4. This is attributed to the heat transfer
lag or slow heat diffusion at higher heating rates, which causes
a slow equilibrium between the sample and its environment.
All samples in this work seem to follow a similar mechanism since
the degradation occurred as a one-stage weight loss. Furthermore,
Please cite this article in press as: Yeo SY, et al., Silver sulfide/poly(3-hydroxybutyrate) nanocomposites: Thermal stability and kinetic..., Polym
Degrad Stab (2010), doi:10.1016/j.polymdegradstab.2010.02.025
ARTICLE IN PRESS
4
S.Y. Yeo et al. / Polymer Degradation and Stability xxx (2010) 1e6
Fig. 3. TG thermograms of (a) neat PHB, (b) Ag2S/PHB2000, (c) Ag2S/PHB1000, (d) Ag2S/
PHB500, (e) Ag2S/PHB250, (f) Ag2S/PHB100 and (g) Ag2S/PHB50 at a heating rate of
20 C min1.
Fig. 2. TEM images of (a) Ag2S particles and the various nanocomposites of (b) Ag2S/
PHB2000, (c) Ag2S/PHB1000, (d) Ag2S/PHB500 and (e) Ag2S/PHB50.
TG-FTIR analyses (not shown) revealed the presence of crotonic acid
from the neat PHB and Ag2S/PHB nanocomposites samples when
degraded at a heating rate of 20 C min1 [21]. Therefore, it can be
suggested that the thermal degradation of PHB in the presence of
Ag2S nanoparticles occurs according to the typical random chain
scission mechanism.
The DTG thermograms of Fig. 5 reveal that the Tp are shifted to
lower temperatures with increasing Ag2S content. This is in accordance with the results of the TG analyses discussed above. Moreover, the obtained DTG thermograms comprised sharp peaks, with
the exception of the thermograms for Ag2S/PHB500 and Ag2S/PHB250
which exhibit broader thermal decomposition ranges. This is
believed to be caused by a thermal effect occurring in the polymer,
influenced by the various particle morphologies or particle distributions within the PHB during sample preparation. Such a thermal
effect would result in inefficient heat diffusion or the trapping of the
degraded PHB chains within the sample, causing a time lapse during
which the volatiles could escape from the sample.
For a further explanation of the thermal degradation of PHB and
Ag2S/PHB nanocomposites, Kissinger plots were constructed. The
plots are shown in Fig. 6 and a summary of the values of the extracted
kinetic parameters of thermal decomposition are listed in Table 1. As
a note, the Ed for PHB obtained in this work is comparable to other
previously reported values [12,16]. The values of Ed for the Ag2S/PHB
nanocomposites are lower than the Ed for neat PHB. Moreover,
a trend was found for Ed upon increasing the Ag2S content in the
nanocomposite. The results of Ed for PHB and Ag2S/PHB2000 are 136.8
and 110.2 kJ mol1, respectively, and the Ed continued to decrease to
87.5 kJ mol1 for Ag2S/PHB500. However, the Ed became slightly
increased once the Ag2S loading in PHB exceeded 0.26 wt%. The
respective Ed of Ag2S/PHB250, Ag2S/PHB100 and Ag2S/PHB50 nanocomposites are 91.8, 94.5 and 99.1 kJ mol1. This trend (for Ed) is
consistent, regardless of the number of times the experiment was
repeated. The initial decrease in Ed was probably due to surface
interactions between the particles and the polymer matrix, providing
an effective heat transfer between the particles and the polymer and
thereby promoting degradation. This is supported by the study
conducted by Gorghiu and co-workers [37] on the effect of metals on
the thermal degradation of a polymer. They reported that the thermal
degradation of polyethylene was enhanced due to the surface interaction between metal particles and the polymer. The later increment
of Ed was believed to be due to the agglomeration of Ag2S aggregates
during heating [38]. The pre-exponential factor, A, followed a similar
trend to that of Ed. On the other hand, for all the Ag2S/PHB nanocomposites, k exhibited a linear relationship with the Ag2S loading
and was apparently not affected by the trends with regard to Ed or A.
Fig. 4. Tonset ( C) versus Ag2S loading in PHB (wt%) at various heating rates ( C min1);
(A) 2, (,) 10, (:) 20, () 30 and (B) 40.
Please cite this article in press as: Yeo SY, et al., Silver sulfide/poly(3-hydroxybutyrate) nanocomposites: Thermal stability and kinetic..., Polym
Degrad Stab (2010), doi:10.1016/j.polymdegradstab.2010.02.025
ARTICLE IN PRESS
S.Y. Yeo et al. / Polymer Degradation and Stability xxx (2010) 1e6
Fig. 5. The DTG thermograms of (a) neat PHB, (b) Ag2S/PHB2000, (c) Ag2S/PHB1000, (d)
Ag2S/PHB500, (e) Ag2S/PHB250, (f) Ag2S/PHB100 and (g) Ag2S/PHB50 at a heating rate of
20 C min1.
Consequently, based on the obtained Ed, the Ag2S nanoparticles
could be said to act as effective catalysts for the degradation of PHB
in Ag2S/PHB nanocomposites. In this study, an Ag2S loading of
0.26 wt% apparently provided the highest catalytic effect. Beyond
this concentration, the catalytic efficiency of Ag2S for the thermal
degradation of PHB decreased. Bozanic et al. [20] have reported on
the catalytic effect of Ag2S nanoparticles on sago starch where
a reduction of w32% in Ed was achieved with 8.3 wt% Ag2S. In our
case, it is worth noting that as little as 0.05 wt% Ag2S in the PHB
gave rise to a reduction in Ed of w20%. At higher Ag2S loadings (viz.
0.12e1.28 wt%), a further decrease in Ed of up to 30% was observed.
Fig. 7 shows the DSC thermograms of neat PHB and the Ag2S/
PHB nanocomposites. The corresponding thermal data, including
the glass transition temperature (Tg), crystallization temperature
(Tc), melting temperature (Tm), the enthalpy of melting (DHm) as
well as the crystallinity (cc), is listed in Table 1. The DSC results
pointed at a generally decreasing trend for Tg, Tc, Tm, DHm and cc
with an increasing loading of Ag2S in the PHB.
The decrease in Tm is likely due to two factors, of which the first
is the presence of defects in the PHB crystals or lamellar structures
caused by the Ag2S particles in the Ag2S/PHB nanocomposites. The
second factor involves the lower average molecular weight of PHB
as a result of the occurrence of chain scission reactions during the
isothermal treatment of the first heating run. This is enhanced by
the presence of Ag2S. The Ag2S/PHB samples exhibit multiple
melting peaks and increasing the Ag2S content in the nanocomposites caused a broad shoulder to appear, especially in the
Fig. 6. Determination of activation energy by Kissinger method; (A) neat PHB, (þ)
Ag2S/PHB2000, (C) Ag2S/PHB1000, ( ) Ag2S/PHB500, () Ag2S/PHB250, (:) Ag2S/PHB100
and (-) Ag2S/PHB50.
5
Fig. 7. DSC thermograms (2nd heating) of (a) neat PHB, (b) Ag2S/PHB2000, (c) Ag2S/
PHB1000, (d) Ag2S/PHB500, (e) Ag2S/PHB250, (f) Ag2S/PHB100 and (g) Ag2S/PHB50 at
a heating rate of 20 C min1.
Ag2S/PHB250, Ag2S/PHB100 and Ag2S/PHB50 samples. The presence
of a shoulder and the multiple melting peaks was attributed to the
occurrence of recrystallization in the imperfectly crystallized
crystals [7]. Moreover, multiple melting peaks in the DSC thermograms for samples with higher Ag2S loadings suggest that the
process of partial melting, recrystallization and remelting (mrr) has
taken place. This is a result of the PHB with higher Ag2S loading
producing more imperfect crystals that melted over a range of low
temperatures. Furthermore, the Tg was significantly decreased from
5.9 C for the neat PHB to 13.4 C for the Ag2S/PHB50 nanocomposite. As indicated previously, the Ag2S nanoparticles acted as
a degradation catalyst which reduced the molecular weight of PHB.
This consequently decreased the Tg and allowed the polymer to
achieve the rubbery state at a lower temperature. Also the crystallinity decreased due to the low molecular weight of PHB. The
DHm and cc values listed in Table 1 support this statement, and
calculations based on the DSC data show that the lowest crystallinity for PHB is 41.3% in the Ag2S/PHB50 nanocomposite.
From the DSC results, it can be said that the presence of Ag2S
nanoparticles affects the glass transition, crystallization and
melting temperatures of PHB in the Ag2S/PHB nanocomposites. The
Ag2S nanoparticles thus influenced the amorphous and crystalline
structures of PHB in the composite material. Consequently, the DSC
data evidently supported the TG results regarding the lowering of
Tonset of PHB in the nanocomposites.
4. Conclusions
Ag2S particles were prepared by refluxing an aqueous solution
of silver nitrate and thiourea. Several Ag2S/PHB nanocomposites
were then obtained via a two-step phase transfer of Ag2S nanoparticles in the aqueous phase into chloroform containing various
amounts of PHB. Spherically and hexagonally shaped Ag2S nanoparticles with small particle sizes (<24 nm) were obtained. The TG
analyses of non-isothermal degradation of PHB and Ag2S/PHB
nanocomposites showed that the thermal degradations of neat PHB
and PHB in Ag2S/PHB nanocomposites occurred as a one-stage
weight loss. The Tonset of neat PHB and PHB in Ag2S/PHB nanocomposites was shifted to higher temperatures upon increasing the
heating rate. This was attributed to the slow heat diffusion at higher
heating rate. However, increasing the Ag2S loading in the PHB gave
rise to a lowering of Tonset. This can be explained by the shorter PHB
chains that arise due to the degradation. The Ed obtained from the
Kissinger plots showed a decreasing trend upon addition of Ag2S
into PHB up to 0.26 wt%. Further addition of Ag2S led to an increase
Please cite this article in press as: Yeo SY, et al., Silver sulfide/poly(3-hydroxybutyrate) nanocomposites: Thermal stability and kinetic..., Polym
Degrad Stab (2010), doi:10.1016/j.polymdegradstab.2010.02.025
ARTICLE IN PRESS
6
S.Y. Yeo et al. / Polymer Degradation and Stability xxx (2010) 1e6
in Ed. The former was believed to be due to interactions between
the polymer and the particle surface whilst the later was the result
of agglomeration of the Ag2S aggregates. Nevertheless, the presence of Ag2S in the nanocomposites caused the PHB Tonset of
thermal degradation to occur at a lower temperature as opposed to
in neat PHB. DSC analyses showed that the Tg, Tc, Tm, DHm and cc
values for PHB in the nanocomposites generally decreased upon
increasing the Ag2S loading. This was attributed to the formation of
imperfect crystals as well as to the reduced molecular weight of
PHB caused by the presence of Ag2S nanoparticles in the nanocomposites. Moreover, k showed a linear relationship with the Ag2S
loading in all the Ag2S/PHB nanocomposites. It can thus be
concluded that the Ag2S nanoparticles enhanced the degradation
rate and acted as effective catalysts for the degradation of PHB in
Ag2S/PHB nanocomposites.
Acknowledgement
The authors wish to thank the Universiti Sains Malaysia for
financial support in the form of the following grants: FRGS 203/
PKIMIA/671029 and USM-RU-PRGS 1001/PKIMIA/831012.
References
[1] Sudesh K, Abe H, Doi Y. Synthesis, structure and properties of polyhydroxyalkanoates: biological polyesters. Prog Polym Sci 2000;25:1503e55.
[2] Chen C, Fei B, Peng SW, Zhuang YG, Dong LS, Feng ZL. The kinetic of the thermal
decomposition of poly(3-hydroxybutyrate) and maleated poly(3-hydroxybutyrate). J Appl Polym Sci 2002;84:1789e96.
[3] El-Hadi A, Schnabel R, Straube E, Müller G, Henning S. Correlation between
degree of crystallinity, morphology, glass temperature, mechanical properties
and biodegradation of poly(3-hydroxyalkanoate) PHAs and their blends.
Polym Test 2002;21:665e74.
[4] Bordes P, Hablot E, Pollet E, Avérous L. Effect of clay organomodifiers on
degradation of polyhydroxyalkanoates. Polym Degrad Stab 2009;94:789e96.
[5] Oliveira LM, Araújo ES, Guedes SML. Gamma irradiation effects on poly
(hydroxybutyrate). Polym Degrad Stab 2006;91:2157e62.
[6] Erceg M, Kova
ci
c T, Klari
c I. Dynamic thermogravimetric degradation of poly
(3-hydroxybutyrate)/aliphatic-aromatic copolyester blends. Polym Degrad
Stab 2005;90:86e94.
[7] Gunaratne LMWK, Shanks RA, Amarasinghe G. Thermal history effects on
crystallisation and melting of poly(3-hydroxybutyrate). Thermochim Acta
2004;423:127e35.
[8] Li SD, He JD, Yu PH, Cheung MK. Thermal degradation of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) as studied by
TG, TG-FTIR and Py-GC/MS. J Appl Polym Sci 2003;89:1530e6.
[9] Al-Salah HA. Crystallization and morphology of poly(ethylene succinate) and
poly(ß-hydroxybutyrate) blends. Polym Bull 1998;41:593e600.
[10] You JW, Chiu HJ, Don TM. Spherulitic morphology and crystallization kinetics
of melt-miscible blends of poly(3-hydroxybutyrate) with low molecular
weight poly(ethylene oxide). Polymer 2003;44:4355e62.
[11] Gassner F, Owen AJ. Some properties of poly(3-hydroxybutyrate)-poly(3hydroxyvalerate) blends. Polym Int 1996;39:215e9.
[12] Erceg M, Kova
ci
c T, Perinovi
c S. Kinetic analysis of the non-isothermal
degradation of poly(3-hydroxybutyrate) nanocomposites. Thermochim Acta
2008;476:44e50.
[13] Yun SI, Gadd GE, Latella BA, Lo V, Russell RA, Holden PJ. Mechanical properties
of biodegradable polyhydroxyalkanoates/single wall carbon nanotube nanocomposite films. Polym Bull 2008;61:267e75.
[14] Grassie N, Murray EJ. The thermal degradation of poly(-(D)-b-hydroxybutyric
acid): part 3 e the reaction mechanism. Polym Degrad Stab 1984;6:127e34.
[15] Gonzalez A, Irusta L, Fernández-Berridi MJ, Iriarte M, Iruin JJ. Application of
pyrolysis/gas chromatography/fourier transform infrared spectroscopy and
TGA techniques in the study of thermal degradation of poly(3-hydroxybutyrate). Polym Degrad Stab 2005;87:347e54.
[16] Ariffin H, Nishida H, Shirai Y, Hassan MA. Determination of multiple thermal
degradation mechanisms of poly(3-hydroxybutyrate). Polym Degrad Stab
2008;93:1433e9.
[17] Lee HK, Ismail J, Kammer HW, Bakar MA. Melt reaction in blends of poly(3hydroxybutyrate)(PHB) and epoxidized natural rubber (ENR-50). J Appl Polym Sci
2005;95:113e29.
[18] Lee JY, Liao Y, Nagahata R, Horiuchi S. Effect of metal nanoparticles on thermal
stabilization of polymer/metal nanocomposites prepared by a one-step dry
process. Polymer 2006;47:7970e9.
[19] Lee JY, Horiuchi S, Choi HK. Effect of palladium nanoparticles on the thermal
degradation kinetics of a crystalline syndiotactic polystyrene. J Ind Eng Chem
2006;12(6):862e7.
[20] Bo
zani
c DK, Djokovi
c V, Blanusa J, Nair PS, Georges MK, Radhakrishnan T.
Preparation and properties of nano-sized Ag and Ag2S particles in biopolymer
matrix. Eur Phys J E 2007;22:51e9.
[21] Yeo SY, Tan WL, Abu Bakar M, Ismail J. Non-isothermal degradation of silverbased poly(3-hydroxybutyrate) nanocomposites. In: Book of Abstracts 13th
Asian Chemical Congress, Shanghai China; 2009 (MP-PP79).
[22] Qian XF, Yin J, Feng S, Liu SH, Zhu ZK. Preparation and characterization of
polyvinylpyrrolidone films containing silver sulfide nanoparticles. J Mater
Chem 2001;11:2504e6.
[23] Qian XF, Yin J, Huang JC, Yang YF, Guo XX, Zhu ZK. The preparation and
characterization of PVA/Ag2S nanocomposite. Mater Chem Phys 2001;
68:95e7.
[24] Liu SH, Qian XF, Yin J, Hong L, Wang XL, Zhu ZK. Synthesis and characterization
of Ag2S nanocrystals in hyperbranched polyurethane at room temperature.
J Solid State Chem 2002;168:259e62.
[25] Carotenuto G, Martorana B, Perlo P, Nicolais L. A universal method for the
synthesis of metal and metal sulfide clusters embedded in polymer matrices.
J Mater Chem 2003;13:2927e30.
[26] Dong L, Chu Y, Liu Y, Li L. Synthesis of faceted and cubic Ag2S nanocrystals in
aqueous solutions. J Colloid Interface Sci 2008;317:485e92.
[27] Martínez-Castañón GA, Sánchez-Loredo MG, Dorantes HJ, MartínezMendoza JR, Ortega-Zarzosa G, Ruiz F. Characterization of silver sulfide
nanoparticles synthesized by a simple precipitation method. Mater Lett
2005;59:529e34.
[28] Shukla S, Seal S. Synthesis and characterization of silver sulfide nanoparticles
containing sol-gel derived HPC-silica film for ion-selective electrode application. J SoleGel Sci Technol 2002;23:151e64.
[29] Wu MD, Pan X, Qian X, Yin J, Zhu Z. Solution-phase synthesis of Ag2S hollow
and concave nanocubes. Inorg Chem Commun 2004;7:359e62.
[30] Lu Q, Gao F, Zhao D. Creation of a unique self-supported pattern of radially
aligned semiconductor Ag2S nanorods. Angew Chem Int Ed 2002;41(11):
1932e4.
[31] Xu C, Zhang Z, Ye Q. A novel facile method to metal sulfide (metal ¼ Cd, Ag, Hg)
nano-crystallite. Mater Lett 2004;58:1671e6.
[32] See GL. Polymer alloys with poly(3-hydroxybutyrate), Ph.D. thesis, Universiti
Sains Malaysia, Penang; 2006.
[33] Abu Bakar M, Tan WL, Azizi NJ, Abu Bakar NHH. Synthesis of modified rubberstabilised silver organosol via liquid-to-liquid transfer techniques. J Rubb Res
2006;9(4):193e203.
[34] Muhammad A, Abdul Mutalib MI, Wilfred CD, Murugesan T, Shafeeq A.
Viscosity, refractive index, surface tension, and thermal decomposition of
aqueous N-methyldiethanolamine solutions from (298.15 to 338.15) K. J Chem
Eng Data 2008;53:2226e9.
[35] Kissinger HE. Reaction kinetics in differential thermal analysis. Anal Chem
1957;29(11):1702e6.
[36] Pramoda KP, Liu T, Liu Z, He C, Sue HJ. Thermal degradation behavior of
polyamide 6/clay nanocomposites. Polym Degrad Stab 2003;81:47e56.
[37] Gorghiu LM, Jipa S, Zaharescu T, Setnescu R, Mihalcea I. The effect of
metals on thermal degradation of polyethylenes. Poly Degrad Stab 2004;
84:7e11.
[38] Rong M, Zhang M, Liu H, Zeng H. Synthesis of silver nanoparticles and their
self-organization behaviour in epoxy resin. Polymer 1999;40:6169e78.
Please cite this article in press as: Yeo SY, et al., Silver sulfide/poly(3-hydroxybutyrate) nanocomposites: Thermal stability and kinetic..., Polym
Degrad Stab (2010), doi:10.1016/j.polymdegradstab.2010.02.025