ORIGINAL PAPER
Biodegradable Poly(butylene succinate) and Poly(butyleneadipate-co-terephthalate) Blends: Reactive Extrusionand Performance Evaluation
Rajendran Muthuraj • Manjusri Misra •
Amar Kumar Mohanty
� Springer Science+Business Media New York 2014
Abstract Two biodegradable polyesters, poly(butylene
adipate-co-terephthalate) (PBAT) and poly(butylene suc-
cinate) (PBS) were melt-compounded in a twin screw
extruder to fabricate a novel PBS/PBAT blend. The com-
patibility of the blend was attributed to the transesterifi-
cation reaction that was confirmed by Fourier transform
infrared spectroscopy. The Gibbs free energy equation was
applied to explain the miscibility of the resulting blend.
Dynamic mechanical analysis of the blends exhibits an
intermediate tand peak compared to the individual com-
ponents which suggests that the blend achieved compati-
bility. One of the key findings is that the tensile strength of
the optimized blend is higher than each of the blended
partner. Rheological properties revealed a strong shear-
thinning tendency of the blend by the addition of PBAT
into PBS. The phase morphology of the blends was
observed through scanning electron microscopy, which
revealed that phase separation occurred in the blends. The
spherulite growth in the blends was highly influenced by
the crystallization temperature and composition. In addi-
tion, the presence of a dispersed amorphous phase was
found to be a hindrance to the spherulite growth, which was
confirmed by polarizing optical microscopy. Furthermore,
the increased crystallization ability of PBAT in the blend
systems gives the blend a balanced thermal resistance
property.
Keywords Biodegradable polyester � Transesterification �Tensile strength � Morphology
Introduction
The development of biodegradable material as a potential
substitute for non-biodegradable material is an emerging
field of research and development. In recent years, different
types of biodegradable polymers have received an immense
amount of attention for developing various new materials
and to reduce environmental concerns [1]. Some biode-
gradable polymers are commercially available in the market,
such as poly(butylene adipate-co-terephthalate) (PBAT),
polyhydroxyalkanoates (PHAs), polycaprolactone (PCL),
poly(propylene carbonate) (PPC), poly(butylene succinate)
(PBS), poly(lactic acid) (PLA), and thermoplastic starch [2–
5]. Biodegradable polymers are not currently widely used
due to some limitations such as their cost, mechanical
properties, and thermal stability.
Researchers have been trying to address these issues by
utilizing blending techniques to obtain biodegradable blends
with tailored properties. Compared to other methods, melt
blending is a cost effective and less time consuming process
for the development of new materials with balanced prop-
erties [6, 7]. During melt blending, dipole interactions,
hydrogen bonding, or a combination of these occur naturally,
and such interactions can enhance the overall performance of
the resulting products [8, 9]. The modification can also be
made during processing to further improve the strength of the
material. Well established literature is available for modifi-
cation studies of polymer blends using techniques such as
R. Muthuraj � M. Misra � A. K. Mohanty
School of Engineering, Thornbrough Building, University of
Guelph, 50 Stone Road East, Guelph, ON N1G 2W1, Canada
R. Muthuraj � M. Misra (&) � A. K. Mohanty (&)
Department of Plant Agriculture, Bioproducts Discovery and
Development Centre, Crop Science Building, University of
Guelph, 50 Stone Road East, Guelph, ON N1G 2W1, Canada
e-mail: [email protected]
A. K. Mohanty
e-mail: [email protected]
123
J Polym Environ
DOI 10.1007/s10924-013-0636-5
in situ compatibilization [10], graft copolymerization [11],
copolymerization [12], and transreactions [8, 13]. Transre-
actions include an alcoholysis, acidolysis, and ester inter-
change reaction. These three reactions are generally referred
to as transesterification. The transesterfication reaction is an
exchange mechanism which can help to form a new type of
ester linkage between the components in the blends [14]. The
resulting transesterification products very often play
important roles in the miscibility, compatibility, crystallin-
ity, and mechanical properties of the blends. During the past
several years of research, many studies have been done on
the transesterification of polyester blends such as PBS/PCL
[6], poly(ethylene terephthalate)/poly(ether imide) [15],
poly(triethylene terephthalte)/polycarbonate [16], PHB/
PLA [17], and polycarbonate/poly(trimethylene terephthal-
ate) [18].
Among the biodegradable polyesters, both PBS and
PBAT have been intensively studied due to their inherent
biodegradability and commercial availability [1, 19]. PBS
is an aliphatic polyester which is synthesized from the
polycondensation reaction of petroleum based aliphatic
dicarboxylic acid (succinic acid) and 1,4-butane diol [20–
22]. The biodegradability of PBS is similar to cellulose and
bacterial polyesters like poly(hydroxybutyrate-co-valerate)
(PHBV) [23]. Currently, PBS is synthesized from renew-
able resource based succinic acid, which reduces its carbon
footprint while preserving its total performance [24]. PBS
is a good candidate for making biodegradable products as
well as having some unique physical properties such as
semicrystalline nature, thermal stability, good processing
properties, good gas barrier properties, and a lower melting
point [25–28].
Poly(butylene adipate-co-terephthalate) (PBAT) is com-
mercially synthesized from petroleum based adipic acid, 1,4-
butane diol, and terephthalic acid, which is a good biode-
gradable polymer in the presence of naturally occurring
microorganisms [29–31]. Furthermore, it has excellent
toughness and is mostly used for film extrusion and coatings
[32]. PBAT is a promising material to improve the toughness
of polymer blends which contain brittle polymers like
poly(lactic acid) [33], polycarbonate [34], and poly(-
hydroxybutyrate-co-valerate) [35]. As noted above, PBS and
PBAT are the most promising candidates for future biode-
gradable materials in various potential applications. Many
studies have reported the blending of either PBS or PBAT
with other biodegradable polymers. For instance, PBS has
been incorporated with many polymers such as poly(trieth-
ylene succinate) [21], poly(ethylene oxide) [36], poly(pro-
pylene carbonate) [37], poly(butylene terephthalate) [28],
copolyesters [30, 38], polyhydroxybutyrate [27], and
polycaprolactone (PCL) [6]. Although many studies have
reported stiffness-toughness balanced biodegradable binary
blends, to the best of our knowledge no literature is available
for PBS/PBAT binary blends. As two typical thermoplastic
biodegradable polyesters, the blend of PBS and PBAT are of
great interest due to their unique properties, which can
extend their applications in diversified areas. Hence, the
present work focused on the fabrication of a novel high
performance PBS/PBAT blend. The binary blend was pre-
pared by an extrusion followed by the injection molding
technique. The binary blends were characterized by differ-
ential scanning calorimetry (DSC), dynamic mechanical
analysis (DMA), polarizing optical microscopy (POM),
tensile properties, Fourier transform infrared spectroscopy
(FTIR), and scanning electron microscopy (SEM).
Experimental Section
Materials
Poly(butylene succinate) (PBS) pellets with a molecular
weight (Mw) of 1.4 9 105 g mol-1 and PDI of 1.82, com-
mercially named Bionolle 1020, manufactured by Showa
Highpolymer Co. Ltd, Japan, were used. The commercially
available PBAT (Biocosafe 2003F) was purchased from
Xinfu Pharmaceutical Co., Ltd, China. The molecular
structures of the neat polymers are shown in Scheme 1.
Blend Preparation
Prior to blending, polymer pellets were dried at 80 �C for
at least 12 h in a vacuum oven. Samples with different
compositions of PBS/PBAT were prepared in a micro
compounder (DSM Xplore, Netherlands). The micro
extruder was equipped with a co-rotating twin screw and
had a barrel volume of 15 cm3. A twin screw length of
150 mm and aspect ratio of 18 was used in the melt
blending process. The process temperature, cycle time, and
screw speed were kept constant at 140 �C, 2 min, and
100 rpm, respectively for different compositions of the
PBS/PBAT blend. The molten polymer was collected and
injected into the mold at 30 �C using a 12 cm3 micro-
Scheme 1 Molecular structure of PBAT and PBS
J Polym Environ
123
injection molder (DSM Xplore) kept at 140 �C. The mol-
ded test specimens were used for further characterization.
Fourier Transform Infrared Spectroscopy (FTIR)
FTIR analysis was performed by using a Thermo Scientific
Nicolet 6700 ATR-FTIR at ambient temperature. All the
scans were performed from 400 to 4,000 cm-1 with a
resolution of 4 cm-1. For each sample of the spectrum 32
accumulated scans were produced and the absorbance was
recorded as a function of wavenumbers.
Mechanical Properties
Tensile strength and percent elongation of the neat poly-
mers and their blends were obtained by Instron 3382, using
a constant strain rate of 50 mm min-1 at room tempera-
ture. The tensile testing was performed according to the
ASTM D638 testing method using dumbbell shaped sam-
ples. Data was collected by blue hill software. All the
reported values are an average of at least five samples for
each formulation.
Melt Flow Index (MFI)
Melt flow index of the neat polymers and their blends was
measured according to the ASTM D1238 standard using a
Melt Flow Indexer (Qualitest model 2000A) at 190 �C with
a standard weight of 2.16 kg. For each measurement, 6 g of
the material was loaded in the instrument. The presented
results are an average of at least five replicates of each
formulation.
Differential Scanning Calorimetry (DSC)
DSC analysis was carried out in a thermal analysis (TA)
instrument Q 200, and the analysis was performed under
the nitrogen atmosphere. Each sample, weighing about
5–10 mg, was placed in an aluminum pan and placed in the
instrument. The sample was first scanned from room tem-
perature to 150 �C with a heating rate of 10 �C min-1 and
subsequently cooled down from 150 to -50 �C at a cooling
rate of 5 �C min-1. A second heating scan of the samples
was performed from -50 to 150 �C at a rate of 10 �C
min-1. The first heating cycle was used for the removal of
thermal history and the reported results are from the second
heating scan. The data were analyzed using TA Instrument
Universal Analysis software.
Dynamic Mechanical Analysis (DMA)
The storage modulus and tan d of the neat polymers and
their blends were measured as a function of temperature by
a DMA Q800 from TA Instruments. The analysis was
performed between -50 and 100 �C at a heating rate of
3 �C min-1. The experiment was carried out in a dual
cantilever clamp with 1 Hz frequency and 15 lm oscil-
lating amplitude.
Heat Deflection Temperature (HDT)
HDT measurement was performed based on the ASTM
D648 standard at a constant load 0.455 MPa in the same
DMA instrument. The analysis was performed at a heating
rate of 2 �C min-1 from ambient temperature to 100 �C in
a three point bending mode.
Thermogravimetric Analysis (TGA)
Thermal stability of the neat polymers and their blends was
measured using a TA Q500 Instrument. Analysis was
performed under the nitrogen atmosphere at a flow rate of
60 ml min-1 from room temperature to 600 �C with a
heating rate of 20 �C min-1. The maximum rate of deg-
radation was observed from the derivative thermogram
(DTG).
Rheological Studies
A strain-controlled rheometer (Anton Paar Modular Com-
pact Rheometer MCR– 302) was used to observe the rhe-
ological properties of neat polymers and their blends.
Injection molded samples were placed between the paral-
lel-plates (diameter of the parallel plate is 25 mm), and the
experiment was performed at 140 �C using a gap width of
1 mm. Dynamic properties were determined by a dynamic
frequency sweep test. During the test, the range of fre-
quency was 500 to 0.01 rad s-1 and the strain was kept
constant at 3 % in the LVE region of neat polymers and
blends, respectively. These limits were fixed based on the
polymer torque sensitivity and their thermal stability.
Polarizing Optical Microscopy (POM)
Polarizing optical microscopy was performed on a Nikon,
Universal Design Microscope. The microscope was
equipped with a Linkam LTS 420 hot stage, which is used
to control the temperature. A DS-2Mv (with DS-U2) color
video camera with the capture NIS element imaging soft-
ware was used for POM observations. Samples were
sandwiched between two microscope glass slides and
heated to 150 �C for 5 min to remove the thermal history.
Subsequently, samples were annealed at the crystallization
temperature with a heating rate of 10 �C min-1. The
spherulities growth was observed at two different crystal-
lization temperatures of 80 and 90 �C.
J Polym Environ
123
Scanning Electron Microscopy (SEM)
Scanning electron microscopy images of the polymer
blends were captured by an Inspect S50—FEI Company.
The cryofractured samples were used to observe the phase
morphology of the blends. A selective dissolution of
polyester in tetrahydrofuran (THF) was used to distinguish
the polymer phases. All the samples were dried and sput-
tered with gold prior to imaging in order to make them
conductive.
Results and Discussions
Fourier Transform Infrared Spectroscopy (FTIR)
The FTIR analysis was performed to identify the physical
and chemical interaction between the neat polymers during
melt blending. The FTIR spectra of neat polymers and their
blends are shown in Fig. 1. The carbonyl group frequency
of neat polymers and their blends was observed at 1,712
and 1,716 cm-1, respectively. The peak of the carbonyl
group was shifted towards higher wavenumbers for the
PBS/PBAT blends compared to each of the neat polymers,
which clearly indicates that a strong chemical interaction
occurred during the melt process at 140 �C. Kwei [39] has
reported that the shift (from 1,722 to about 1,705 cm-1) of
the carbonyl group peak in the blends occurred as a result
of chemical interactions between the parent polymers. John
et al. [6] also observed a similar type of transesterification
reaction in the PBS/PCL and PCL/EASTAR blends. These
results can be explained by the formation of copolyester of
PBS and PBAT by an ester–ester interchange reaction. The
resulting copolyester which is compatible with the
homopolymer of the unreacted PBS and PBAT may play
the role of a compatibilizer in the blend system. In the
present study, no external transesterification catalyst was
added into the blends. Even though a transesterification
reaction was observed during melt blending, the amount of
the reaction gradually decreased with increasing PBAT
content. This resultant ester exchange reaction was due to
the presence of residual catalysts existing in the homo-
polymer synthesis [40–42]. The transesterification product
can further enhance the mechanical performance of the
resultant blends. Scheme 2 shows the expected chemical
structure of the transesterification product in the PBS/
PBAT blends.
Mechanical Properties
Figure 2 shows the stress–strain curves of PBS, PBAT, and
their blends. Neat PBS showed a higher tensile yield
strength but lower elongation compared to neat PBAT. For
PBS, no apparent strain hardening was observed during the
tensile test. On the other hand, PBAT showed excellent
elongation and obvious strain hardening regions in the
stress–strain curves, while its’ tensile and yield strength
was poor. As for the blends, all the samples presented three
clear regions such as elastic, plastic deformation, and strain
hardening. The first region showed linear stretching with
recoverable deformation, followed by the second region
which revealed plastic deformation which is a non recov-
erable deformation of the samples. The second region
indicated cold drawing behavior after the neck forming
occurred in the samples. The third region showed strain
hardening, the tensile stress gradually increasing until the
samples broke. Crystalline slippage was also observed.
Interestingly, after the strain increased to 150 %, each
composition of the blend showed clear evidence of cold
drawing which affected the polymer chain alignment and
resulted in strain hardening. Generally, amorphous and
semi crystalline polymer chain entanglement can lead to
strain hardening. Strain hardening behavior is of great
importance in polymer processing such as film blowing,
thermoforming, and blow molding due to its good resis-
tance against stretching of polymer segments. Also, strain
hardening behavior can make the process easier and lead to
higher quality products [43]. Figure 3 shows tensile
strength and percentage elongation data. A significant
improvement in the tensile strength and elongation was
observed for the blends.
The tensile strength of the blend was higher than that of
the neat polymers. The tensile strength of the PBS/PBAT
(70/30 wt %) blend increased by 30 and 148 % over the neat
PBS and PBAT, respectively. The percent elongation of
PBS/PBAT (70/30 wt%) blend was 150 % higher than neat
PBS. Tensile strength improvement is directly related to the
Fig. 1 Evaluation of the normalized FTIR spectra of the carbonyl
region (1,800–1,600 cm-1) of PBS, PBAT and their blends
J Polym Environ
123
intermolecular forces, crystallinity, miscibility, compatibil-
ity, and molecular orientation of the polymers in the blend
[6]. In the present study, tensile strength improvement was
directly related to the amount of PBS present in the blends.
Tensile strength decreased with increasing PBAT content in
the blend system, which reveals that the blend transesterifi-
cation ability was reduced. Furthermore, the increased
PBAT content in the blend system may cause phase sepa-
ration due to the decreased compatibility of the blend which
can lead to the reduction of tensile strength. These results
were also observed from morphological analysis of the
blends. John et al. [6] observed that a similar phase separa-
tion occurs when increasing one component in a PBS/
EASTAR and PCL/EASTAR blend leading to reduced ten-
sile strength in the blends. Tensile properties of the PBS/
PBAT blends were sharply increased compare to their parent
polymers. Apparently, the PBS/PBAT blends mechanical
properties are comparable with literature polyethylene
mechanical properties [44]. So, we believe that the PBS/
PBAT blends can be potential substitute for non-biode-
gradable polymers in the packaging applications.
Melt Flow Index
MFI measurement is a common technique for studying the
flow behavior of the polymers [45]. Table 1 shows the melt
flow index values of neat polymers and their blends. The MFI
value of PBAT and PBS was 9 and 25 g/10 min, respec-
tively. After blending both polymers, the MFI of all the
blends increased compared to the neat polymers. The
reduction of molecular weight and changing thermal prop-
erties during the melt blending may be responsible for this
[3]. PBS/PBAT (70/30 wt%) blend had the highest melt flow
rate compared to neat polymers and other PBS/PBAT blends.
The increased MFI of the blend can be related to a reduction
in molecular weight of PBS and PBAT through thermal
degradation because these polyesters are thermally sensitive.
Possible degradation can be occurring by chain scission of
the polymers, depolymerization, oxidative degradation, and
transesterification reactions. In addition, reactive end
groups, residual catalyst, unreacted starting monomers in the
Scheme 2 Expected
transesterification product of
PBS/PBAT blend
Fig. 2 Tensile stress–strain curves of PBS, PBAT, and their blends
Fig. 3 Tensile strength and elongation at break of PBS, PBAT, and
their blends: A PBS, B PBS/PBAT (70/30 wt%), C PBS/PBAT (60/
40 wt%), D PBS/PBAT (50/50 wt%), E PBAT
J Polym Environ
123
polymers, and other impurities can accelerate the thermal
degradation of the polymers [46]. Increased MFI of the
blends indicate better flow behavior compared to neat
polymers. Therefore, this blend system is suitable to use as a
new matrix system for polymer composites.
Differential Scanning Calorimetry
The thermal behaviors of the neat PBS, PBAT, and their
blends were measured through non-isothermal differential
scanning calorimetry analysis. Non-isothermal DSC results
of PBS, PBAT, and their blends are given in Fig. 4. The
neat PBS and PBAT showed melting points at 114 and
116 �C respectively. Interestingly, the second heating cycle
showed a double melting peak for neat PBS and their
blends due to the melt re-crystallization of the polymers.
During the second heating cycle as shown in Fig. 4, the
less perfect crystals melt at lower temperatures but the
more structurally perfect crystals melt at higher tempera-
ture [3]. Another possible reason may be that the molecular
weight distribution could also affect the melt of the poly-
mers [47]. The PBS/PBAT blend showed a similar melting
behavior to that of PBS. The enthalpy of fusion for neat
PBS and PBAT was found to be 32 and 10 J g-1, respec-
tively. The blends showed a high enthalpy of fusion com-
pared to the neat polymers. The PBAT phase may act as a
nucleating agent for the PBS phase, which will improve the
crystallization of PBS in the blend. Another reason is a
change in the regular structure of the interchange reaction
product, which may lead to the production of thicker
lamellar crystals that melt with a higher enthalpy of fusion
[6]. In all the blends, a single glass transition temperature
(Tg) was observed because the Tg values of both the neat
polymers are very close to each other. Therefore, the values
may be overlapping. The Tg value of the blends shifted to
lower temperatures compared with that of the neat poly-
mer. A similar observation was found through DMA ana-
lysis. This variation in Tg could be the cause of an
interchange reaction which occurs during the melt blending
process and it is also evidence for compatibility of the
polymer in the blends. John et al. [6] have observed similar
synergistic effects in PBS/PCL blends. Miscibility of the
two polymers can be predicted from Gordon-Taylor (G-T)
equation (Eq. 1) [48, 49]
Tg ¼W1Tg1 þ kW2Tg2
W1 þ kW2
ð1Þ
where Tg1 and W1 are the glass transition temperature and
weight fraction of PBAT, respectively. The Tg2 and W2 are
the glass transition temperature and weight fraction of PBS,
respectively, and the parameter k is the fitting constant. The
Tg values are observed from the DSC analysis. If k = 1,
the Gordon and Taylor theory represents a good interaction
between two blended components. If the k value is lower or
higher than 1, it indicates poor interaction between the
components in the blend. Figure 5 shows the Tg value
obtained from the G-T equation. From the DSC, the Tg
values observed for all the blends were -35 �C, close to
the G-T curve. This indicates good agreement with the G-T
equation. The k value of the present work was found to be
0.98; this semi-quantitatively measured value can further
support the interaction which occurred between the poly-
mers in the blends. The k value of the diglycidyl ether of
bisphenol-A/poly(ethylene terephthalate) blend was 0.10;
the small value of k suggests that only weak interactions
exist between the components in the blend [8, 50]. Richard
et al. [50] observed a k value of 0.18 for the PLA/PHBV
blend indicating poor miscibility, which was confirmed
through SEM analysis. These results are supportive of our
present work: that the components have a good compati-
bility in the blend system.
Miscibility and compatibility of the blends can be
explained by Gibbs free energy. Thermodynamically
compatible PBS/PBAT blends were analyzed according to
the Gibbs free energy equation (Eq. 2) [51]:
Table 1 Melt flow index (MFI) of the neat polymers and their blends
Samples MFI (g/10 min)
Neat PBS 25.3 ± 2.4
PBS/PBAT (70/30 wt%) 41.5 ± 3.2
PBS/PBAT (60/40 wt%) 33.3 ± 2.8
PBS/PBAT (50/50 wt%) 33.6 ± 1.6
Neat PBAT 9.4 ± 1.8
Fig. 4 Second heating DSC thermograms of PBS, PBAT, and their
blends after cooling at 5 �C/min
J Polym Environ
123
DGm ¼ DHm - T DScm þ DSe
m
� �ð2Þ
where DGm is Gibb’s free energy, T is absolute tempera-
ture, DHm is heat of mixing, DSem is mixing of combina-
torial entropy and DSem is the mixing of excess entropy. The
molar volume of the components inversely depends on the
combinatorial entropy. Hence, molecular weight of the
polymers is directly related to combinatorial entropy. If the
polymer has a higher molecular weight, the DScm becomes
zero. Therefore, the system is spontaneous; it could lead to
a DGm which is less than zero while DHm is less than zero.
In practical fields this is rarely possible and so can be
ignored. The DHm is calculated using expression Eq. (3)
[51]:
DHm ¼ ðd1�d2Þ2u1u2 ð3Þ
where d1, u1, d2, u2 are the solubility parameter values and
the volume fraction of PBS and PBAT. The solubility
parameter (d) of the PBS and PBAT was calculated as
follows (Eq. 4) [51]:
d ¼ qX
G� �
=M ð4Þ
where q, G and M are the density of the polymer, group
molar attraction constant of the polymer, and molecular
weight of the monomer unit, respectively. The group molar
attraction constant was calculated from Mark [52]. The
Gibbs free energy and solubility parameter values for PBS
and PBAT were calculated by Eqs. (3) and (4), and the
values are given in Table 2. Gibbs free energy values for
PBS/PBAT blends are low and very close to each other,
indicating that some extent of compatibility was achieved
in the blend system. Previous studies have reported similar
observations for some of the biopolymer blends such as
PLA/PCL, PLA/PHBV, and PHBV/PCL blends, and they
too have reported slight miscibility was achieved in their
melt blend process [7, 51, 53].
Dynamic Mechanical Analysis
Figure 6 shows the storage moduli of neat PBS, PBAT, and
their blends. The storage modulus value of the PBS and
PBAT at room temperature was found to be 0.6 and
0.1 GPa, respectively. PBS had a higher storage modulus
compared to PBAT at all temperatures, and their blends
had values in between the PBS and PBAT. A similar trend
was observed in the tensile modulus of the PBS/PBAT
blends. Reduction in modulus with increasing temperature
Table 2 Solubility parameter
values for polymersSample Group molar
attraction constant G
(J1/2 cm3/2 mol-1)
Solubility
parameter
d
(J1/2 cm3/2)
Gibbs free
energy DGm
(J g-1 m-3)
PBS 2,990 20.93 –
PBAT 6,154 22.28 –
PBS/PBAT (70/30 wt%) – – 0.382
PBS/PBAT (60/40 wt%) – – 0.437
PBS/PBAT (50/50 wt%) – – 0.455
Fig. 6 Storage moduli of PBS, PBAT, and their blends
Fig. 5 Theoretical and experimental values of Tg for PBS/PBAT
blends
J Polym Environ
123
is attributed to increasing polymer chain mobility. Gener-
ally, above the alpha transition temperature, molecular
motion increases and polymer segments move from glassy
to a rubbery state, which is accompanied by an increase in
the molecular relaxation in the polymers [54].
Figure 7 depicts the tan d curves of PBS, PBAT, and
their blends. It shows the primary and secondary transition
peaks in neat PBAT at -20 and 62 �C, respectively. The
primary transition peak corresponds to the poly(butylene
adipate) segment mobility, and the secondary transition
peak corresponds to the terephthalate unit mobility [55].
The Tg values of the neat polymers and their blends were
calculated from the maximum height of the tan d peak.
Generally, an incompatible blend shows two transition
peaks which correspond to the glass transition temperature
of individual components in the system [56]. For a highly
compatible and partially compatible blend, a single tran-
sition peak can be seen lying between the transition
temperature of individual components with an increased
broadness in the transition peak [9, 57]. In our present
study, all the blends were observed to have a single
transition peak. The Tg values of the PBS/PBAT blends
were shifted towards lower temperatures compared to the
neat PBS, which is a dilution effect with the addition of
PBAT into PBS. Another possible reason is that for par-
tially or completely compatible blends, the Tg shifts
towards lower or higher temperatures as a function of
composition and the broadness of the tan d peak [56].
Moreover, the small variation in the Tg value shows fur-
ther evidence of an interchange reaction occurring
between the neat homopolymers. The Tg shift was
observed by the influence of a transesterification reaction
when polycarbonate was incorporated into the poly(tri-
methylene terephthalate) [56].
Heat Deflection Temperature
Heat deflection temperature represents the maximum
working temperature of materials and is defined as the
temperature at which a material will be deformed by
250 lm under a constant load of 0.455 MPa [58]. The
HDT value of the neat polymers and their blends is shown
in Table 3. The HDT value of the neat PBS and PBAT is
88 and 46 �C, respectively. In general, the HDT of amor-
phous polymers is low, and around their glass transition
temperature. In the crystalline polymers, the HDT is close
to its melting point [7, 59]. In the present study, a balanced
HDT value of PBS/PBAT blends was observed due to the
PBAT having a lower crystallinity and thermal resistance
compared to PBS.
Thermogravimetric Analysis
Thermogravimetric analysis is the most accepted method
for studying the thermal stability of polymeric materials.
Figure 8 shows the thermal stability of PBS, PBAT, and
their blends as a function of temperature. PBS undergoes a
cyclic degradation mechanism and some of the predomi-
nant byproducts are anhydrides, olefins, carbon dioxide,
and esters [60]. PBAT degradation takes place by the
breakdown of the ester groups and chain scission of C–O
and C–C bonds on the polymer backbone. The onset deg-
radation temperature (Tonset) of PBAT was 377 �C and
PBS was 372 �C. This suggests that PBAT has slightly
more thermal stability compared to PBS. TGA results
reveal that PBS and PBAT present a relatively good ther-
mal stability up to 300 �C. These data are in good agree-
ment with the literature results [55, 61]. The maximum
degradation temperature (Tmax) was observed at 405, 413,
408, 410, and 413 �C for extruded PBS, PBAT, PBS/PBAT
(70:30 wt%), PBS/PBAT (60:40 wt%), and PBS/PBAT
(50:50 wt%), respectively. The Tonset and Tmax of the
blends were quite similar to those estimated for PBS and
PBAT homopolymer. All the blends showed single step
degradation because the neat polymer degradation tem-
peratures were close to each other, which was made clear
through a derivative thermogram (Fig. 9). According to
Fig. 7 Tan d curves of PBS, PBAT, and their blends
Table 3 Heat deflection temperatures of the neat polymers and their
blends
Samples HDT (�C)
Neat PBS 88.06 ± 0.4
PBS/PBAT (70/30 wt%) 73.66 ± 1.2
PBS/PBAT (60/40 wt%) 70.27 ± 2.1
PBS/PBAT (50/50 wt%) 65.88 ± 2.3
Neat PBAT 46.12 ± 1.5
J Polym Environ
123
melt flow rate results, all the blends showed a molecular
weight reduction, but the thermal stability of the blends
was gradually increased compared to neat PBS. This may
be due to improved compatibility between the polymer
phases. While increasing the PBAT content, thermal sta-
bility increased because PBAT is more thermally stable
compared to PBS. The results indicate that a compatible
blend was achieved and supports the DSC results.
Rheological Properties
Rheological properties were investigated to identify the
interaction between polymer phases in the blends. Fig-
ure 10 shows the complex viscosity (g*) of the neat
polymers and their blends as a function of frequency at
140 �C. A higher complex viscosity was observed in the
lower frequency range compared to its higher frequency
region. Furthermore, the neat PBS and PBAT exhibited
almost Newtonian behavior at below 1 rad s-1 frequency,
and a strong shear thinning behavior was observed beyond
1 rad s-1 frequency range.
The PBS/PBAT blends had a higher complex viscosity
compared to the neat polymers at lower oscillation fre-
quencies. At higher frequencies, the complex viscosity of
the blends was between that of the neat polymers. This
increased viscosity may have occurred because transeste-
rification can form pseudo structures [62] that can with-
stand shear forces. The transesterification reaction also
plays a predominant role in viscosity improvement of the
blends. At lower frequency, the transesterification product
acts as a solid-like particle in all the PBS/PBAT blends and
leads to a higher viscosity compared to the parent poly-
mers. However, the FTIR results showed transesterification
product gradually decreased with increasing PBAT content
from 30 to 40 and 50 wt%. The higher content of PBAT
reduces the transesterification reaction in the blend system
and thus reduces the viscosity compared to PBS/PBAT
70/30 wt% blend. Li et al. [63] have reported similar
behavior for PLA/PBAT blends at lower frequencies and
also that the interaction between the polymers can increase
the melt viscosity. Also, this is due to phase morphology of
the blends and compatibility between the phases. The
higher compatibility between the two phases leads to good
dispersion of the discrete phase in the blend system. As we
can see in the SEM image, when PBAT content increases
from 30 to 50 wt% in the blend, the discrete phase (PBAT)
morphology is changed form spherical droplet to co-con-
tinuous morphology. This indicates that, the PBS/PBAT
70/30 wt% blend was more compatible than PBS/PBAT
60/40 and 50/50 wt% blends. Consequently, the PBS/
Fig. 8 TGA curves of PBS, PBAT, and their blends
Fig. 9 DTG curves of PBS, PBAT, and their blends
Fig. 10 Complex viscosity of PBS, PBAT, and their blends with
different weight fractions of PBAT at 140 �C
J Polym Environ
123
PBAT 70/30 wt% blend had higher melt viscosity than
PBS/PBAT 60/40 and 50/50 wt% blend. A similar behav-
ior of the PLA/PBS and PLA/PBAT blends was reported in
literature [63, 64].
Figures 11 and 12 show the dynamic loss modulus and
the storage modulus of PBS, PBAT, and PBS/PBAT
blends. Generally, dynamic loss modulus and storage
modulus represent the amount of energy dissipated in the
viscous portion and the ability of a material to store energy
during deformation, respectively. Figure 11 shows that the
storage modulus (G0) of each sample increased with
increase in frequency. It was also observed that with the
blending of PBAT into PBS, there were no changes in the
storage modulus (G0) at higher frequencies. All the blends
showed a higher storage modulus at lower frequencies
compared to the neat polymers. The 70:30 wt% of PBS/
PBAT blend had a higher loss modulus (G00) than other
blends (Fig. 12). When the PBAT phase was finely dis-
persed in the blend, the fine dispersal could be the reason
for interaction existing between the two phases. Stronger
interactions were observed in the PBS/PBAT (70:30 wt%)
blend, which were exhibited as a higher loss modulus. In
addition, the increased storage modulus is attributed to the
PBAT molecular chain entanglement with PBS molecular
chain mediated by the transesterification product acting as
a compatibilizer. The higher entanglement density of the
blends would store more recoverable energy. Generally,
the entanglement density of the blend depends on the
existing compatibility between the two phases. The PBS/
PBAT 70/30 wt% blend had more compatibility than PBS/
PBAT 60/40 and 50/50 wt% blends as shown in SEM. The
higher entanglement density of the PBS/PBAT 70/30 wt%
blend leads to higher storage modulus. However, the
storage modulus gradually decreased at lower frequency
with increasing PBAT content in the blends. A similar
trend was observed in the complex viscosity. This is con-
sistent with a higher amount of trasesterification product
present in 70/30 wt% blend. The reduced storage modulus
of the blends is due to the morphology changes [63] and
entanglement density due to lower transesterification
product present in the blend.
The Cole–Cole plot was used to explain the phase
structure of PBS/PBAT blends and the plot was performed
between the real and imaginary viscosity components of
the blends. If the blend gives a single arc curve, it can
suggest phase homogeneity at the melt stage [65]. Fur-
thermore, if any deviations from a single arc are observed,
they are evidence for inhomogeneous morphology and
phase separation occurring in the blends due to a second
relaxation mechanism occurring in the samples. The Cole–
Cole plot for PBS/PBAT blend at 140 �C is depicted in
Fig. 13. A second circular arc was observed on the right-
hand side of the curve, and it is clear evidence for a second
Fig. 11 Loss modulus versus frequency for PBS, PBAT, and their
blends with different weight fractions of PBAT at 140 �CFig. 12 Storage modulus versus frequency for PBS, PBAT, and their
blends with different weight fractions of PBAT at 140 �C
Fig. 13 Cole–Cole plot of the PBS/PBAT blends at 140 �C
J Polym Environ
123
Fig. 14 (a) Photograph of the
film annealed at 80 �C: (i) PBS;
(ii) PBS/PBAT (70/30 wt%);
(iii) PBS/PBAT (60/40 wt%)
and (iv) PBS/PBAT (50/
50 wt%). (b) Photograph of the
film annealed at 90 �C: (i) PBS;
(ii) PBS/PBAT (70/30 wt%);
(iii) PBS/PBAT (60/40 wt%)
and (iv) PBS/PBAT (50/
50 wt%)
J Polym Environ
123
relaxation mechanism happening for all PBS/PBAT blends.
Nevertheless, when the PBAT reaches 40 and 50 wt% in
the blends, the blends showed a tail on the right hand side
of the plot. This is probably due to the phase inversion
occurring in the blends. This result shows that co-existing
phase morphology was formed in the entire blend system
Fig. 15 (a) SEM images of PBS and PBAT blends (left hand side):
(i) PBS/PBAT (70/30 wt%); (ii) PBS/PBAT (60/40 wt%) and (iii)
PBS/PBAT (50/50 wt%). (b) SEM images of PBS and PBAT blends
surface after etching with THF (right hand side): (i) PBS/PBAT (70/
30 wt%); (ii) PBS/PBAT (60/40 wt%) and (iii) PBS/PBAT (50/
50 wt%)
J Polym Environ
123
and also that the formed morphology may be the droplet-
matrix or co-continuous phase morphology. Consequently,
the Cole–Cole plot shows an inhomogeneous morphology
formed in PBS/PBAT blend systems. There have been
reports of PLA/PBAT blend systems with similar obser-
vations of phase behavior when the PBAT concentration is
more than 30 wt% in the matrix [63]. The Cole–Cole plot
clearly shows that the PBS/PBAT 70/30 wt% blend is more
heterogeneous compared to the 50/50 wt% blend.
Polarizing Optical Microscopy
The crystallization behavior of PBS and PBS/PBAT blends
was investigated by optical microscopy. The dark and light
regions represent the amorphous and crystalline phases,
respectively. Figure 14a shows that PBS/PBAT blends were
annealed at a crystallization temperature of 80 �C for
30 min. When the PBAT content was 50 wt% in the blend,
an increased number of PBS spherulites were observed with
decreasing spherulite size. This decrease in size suggests that
the PBS chain mobility was disrupted and PBAT acted as a
nucleating site to promote the formation of crystalline nuclei.
A PBS/PVDF blend exhibited a similar phenomenon when
PVDF was the predominant species in the blend compared to
the PBS [66]. The PBS chain mobility was slowed in the
presence of highly viscous PBAT, thereby decreasing
spherulite growth. With increasing PBAT content in the
blends (from 30 to 50 wt%), the texture of PBS spherulites
became coarse. Figure 14b shows the PBS/PBAT blends
after being annealed at a crystallization temperature of 90 �C
for 30 min. The increasing PBAT composition in the blends
caused a reduced number of spherulites because PBAT has
less crystallinity and the PBS chain mobility increases at the
crystallization temperature of 90 �C compared to 80 �C.
With increasing PBAT content in the blends, the spherulites
became rougher. For 30 wt% PBAT content in the blend, the
PBS spherulites were uniformly distributed with uniform
dimensions after being annealed for a given time. Conse-
quently, our present results conclude that the spherulite size
plays a vital role in the control of morphology and
mechanical properties of the blends.
Scanning Electron Microscopy
For the polymer blends, phase morphology formed in the
blend plays a vital role in determining the properties of the
resulting blend, such as mechanical and thermal properties
as well as permeability. Phase morphology of the blends
depends on the second components, processing parameters,
molecular weight of the virgin polymers, and compatibility
between the polymers. If the blending components have a
similar melt viscosity, the resulting morphology will be
very fine and both polymers will be uniformly distributed
throughout the blend whether it is the minor or major
phase. The same is true if the blend consists of similar melt
viscosity components. If the minor phase has a lower or
higher viscosity compared to the major phase, it leads to
the spherical domains of finely or coarsely dispersed
morphology in the matrix. As shown in Fig. 15, phase
morphology in the blend was identified by the solvent
etching method. Being a good solvent for PBAT while
unable to dissolve PBS, THF was used as the etching
solvent. The observed morphology of PBAT phase selec-
tively removed from the blends without disturbing the PBS
matrix is shown in Fig. 15b. The PBAT phase was com-
pletely extracted under these conditions. The correspond-
ing unextracted samples are shown in Fig. 15a. This
indicates that the holes are the extracted PBAT phase by
THF. The surface morphology of the blend reveals that
spherical PBAT particles were uniformly distributed
throughout the matrix. Finer dispersions were observed in
lower PBAT composition in the blend. When the PBAT
composition was increased in the blend, the domain shape
and size gradually changed due to the coalescence phe-
nomenon. This may also act to decrease the tensile strength
of the blends.
Conclusions
We have succeeded in fabricating a high performance and
biodegradable PBS/PBAT blend through the melt blending
technique. There is a significant improvement in tensile
strength and elongation at break by the incorporation of
PBAT into PBS, indicating that a good level of compati-
bility is achieved between the polymers. The observed
compatibility is caused by the formation of copolyester due
to transesterification between the neat polymers, which was
confirmed by FTIR analysis. DSC and DMA analysis
suggested that the blends show compatibility between the
PBS and PBAT. The rheological properties of blends such
as the complex viscosity, storage modulus, and loss mod-
ulus were increased with the addition of PBAT in the
matrix. As the PBAT composition was increased, phase
morphology changes occurred in the blends, leading to
decreased values of complex viscosity, storage modulus,
and loss modulus. The phase morphology of the PBS/
PBAT blends shows a two phase structure in which PBAT
is the minor phase. Furthermore, polarizing optical
microscopy analysis revealed that the PBAT has disturbed
the spherulite growth of the matrix.
Acknowledgments The authors acknowledge the Ontario Ministry
of Agriculture and Food (OMAF) and Ministry of Rural Affairs
(MRA)—University of Guelph Bioeconomy-industrial uses research,
for their sponsorships. They also gratefully acknowledge the Ontario
Research Fund, Research Excellence, round-4 (ORF RE04) from
J Polym Environ
123
Ontario Ministry of Economic Development and Innovation (MEDI),
Natural Sciences and Engineering Research Council (NSERC), Net-
works of Centers of Excellence (NCE) and AUTO21 project for their
financial support to carry out this research work.
References
1. Wu D, Yuan L, Laredo E, Zhang M, Zhou W (2012) Ind Eng
Chem Res 51:2290–2298
2. Tokiwa Y, Calabia BP (2007) J Polym Environ 15:259–267
3. Zhang K, Mohanty AK, Misra M (2012) ACS Appl Mater
Interfaces 4:3091–3101
4. Ouyang W, Huang Y, Luo H, Wang D (2012) J Polym Environ
20:1–9
5. Yu T, Luo F, Zhao Y, Wang D, Wang F (2011) J Appl Polym Sci
120:692–700
6. John J, Mani R, Bhattacharya M (2002) J Polym Sci A Polym
Chem 40:2003–2014
7. Nanda MR, Misra M, Mohanty AK (2011) Macromol Mater Eng
296:719–728
8. Huang P, Zhong Z, Zheng S, Zhu W, Guo Q (1999) J Appl Polym
Sci 73:639–647
9. Varughese KT, Nando GB, De PP, De SK (1988) J Mater Sci
23:3894–3902
10. Nesarikar AR, Carr SH, Khait K, Mirabella FM (1997) J Appl
Polym Sci 63:1179–1187
11. Singh D, Malhotra VP, Vats JL (1999) J Appl Polym Sci
71:1959–1968
12. Tang W, Murthy NS, Mares F, McDonnell ME, Curran SA
(1999) J Appl Polym Sci 74:1858–1867
13. Kotliar AM (1981) J Polym Sci Macromol Rev 16:367–395
14. Jayakannan M, Anilkumar P (2004) J Polym Sci A Polym Chem
42:3996–4008
15. Chen HL (1995) Macromolecules 28:2845–2851
16. Aravind I, Ahn KH, Ranganathaiah C, Thomas S (2009) Ind Eng
Chem Res 48:9942–9951
17. Focarete ML, Scandola M, Dobrzynski P, Kowalczuk M (2002)
Macromolecules 35:8472–8477
18. Aravind I, Eichhorn KJ, Komber H, Jehnichen D, Zafeiropoulos
NE, Ahn KH, Grohens Y, Stamm M, Thomas S (2009) J Phys
Chem B 113:1569–1578
19. Liu B, Bhaladhare S, Zhan P, Jiang L, Zhang J, Liu L, Hotchkiss
AT (2011) Ind Eng Chem Res 50:13859–13865
20. Fujimaki T (1998) Polym Degrad Stab 59:209–214
21. Soccio M, Lotti N, Gigli M, Finelli L, Gazzano M, Munari A
(2012) Polym Int 61:1163–1169
22. Huang CL, Jiao L, Zhang JJ, Zeng JB, Yang KK, Wang YZ
(2012) Polym Chem 3:800–808
23. Kim SW, Lim JC, Kim DJ, Seo KH (2004) J Appl Polym Sci
92:3266–3274
24. Myriant Technologies websites. http://www.myriant.com/succi
nicpage.htm. Accessed on Febraury 2013
25. Yoo ES, Im SS (1999) J Polym Sci B Polym Phys 37:1357–1366
26. Wang J, Zheng L, Li C, Zhu W, Zhang D, Xiao Y, Guan G (2012)
Polym Test 31:39–45
27. Qiu Z, Ikehara T, Nishi T (2003) Polymer 44:2503–2508
28. Kim YJ, Park OO (1999) J Polym Environ 7:53–66
29. Gu SY, Zhang K, Ren J, Zhan H (2008) Carbohydr Polym
74:79–85
30. Gan Z, Abe H, Kurokawa H, Doi Y (2001) Biomacromolecules
2:605–6013
31. Sykacek E, Hrabalova M, Frech H, Mundigler N (2009) Compos
Part A Appl Sci Manuf 40:1272–1282
32. Javadi A, Kramschuster AJ, Pilla S, Lee J, Gong S, Turng LS
(2010) Polym Eng Sci 50:1440–1448
33. Jiang L, Liu B, Zhang J (2009) Ind Eng Chem Res 48:7594–7602
34. Jang MO, Kim SB, Nam B-U (2012) Polym Bull 68:287–298
35. Javadi A, Srithep Y, Lee J, Pilla S, Clemons C, Gong S, Turng LS
(2010) Compos Part A Appl Sci Manuf 41:982–990
36. Qiu Z, Ikehara T, Nishi T (2003) Polymer 44:3095–3099
37. Li Y, Shimizu H (2009) ACS Appl Mater Interfaces 1:1650–1655
38. Huang X, Li C, Zheng L, Zhang D, Guan G, Xiao Y (2009)
Polym Int 58:893–899
39. Kwei TK (1984) J Polym Sci B 22:307–313
40. Wang LH, Huang Z, Hong T, Porter RS (1990) J Macromol Sci
Phys B 29:155–169
41. Takiyama E, Fujimaki T, Seki S, Hokari T, Hatano Y (1994) US
patent no. 5310782
42. Takiyama E, Hatano Y, Fujimaki T, Seki S, Hokari T, Hosogane
T, Harigai N (1995) US patent no. 5436056
43. Mittal V (2012) Functional polymer blends: synthesis, properties,
and performance. CRC Press, New York, p 235
44. Joseph K, Thomas S, Pavithran C (1996) Polymer 37:5139–5149
45. Dobkowski Z (1986) Rheol Acta 25:195–198
46. Carrasco F, Pages P, Gamez-Perez J, Santana O-O, Maspoch ML
(2010) Polym Degrad Stab 95:116–125
47. Corre YM, Bruzaud S, Audic JL, Grohens Y (2012) Polym Test
31:226–235
48. Siciliano A, Seves A, Maro TD, Cimmino S, Martuscelli E,
Silvestre C (1995) Macromolecules 28:8065–8072
49. Liu AS, Liau WB, Chiu WY (1998) Macromolecules
31:6593–6599
50. Richards E, Rizvi R, Chow A, Naguib H (2008) J Polym Environ
16:258–266
51. Parulekar Y, Mohanty AK (2007) Macromol Mater Eng
292:1218–1228
52. Mark JE (2006) Physical properties of polymers handbook, 2nd
edn. Spring Science, New york 293
53. Jain S, Redy MM, Mohanty AK, Misra M, Ghosh AK (2010)
Macromol Mater Eng 295:750–762
54. Menard KP (1999) Dynamic mechanical analysis: a practical
introduction, 2nd edn. CRC Press, New York, p 85
55. Mohanty S, Nayak SK (2012) J Polym Environ 20:195–207
56. Aravind I, Boumod A, Grohens Y, Thomas S (2010) Ind Eng
Chem Res 49:3873–3882
57. Thomas S, Gupta BR, De SK (1987) J Vinyl Technol 9:71–85
58. ASTM Standard D648 (2007) Standard test method for deflection
temperature of plastics under flexural load in edgewise position.ASTM International, West Conshohocken, PA. www.astm.org.
59. Kawamoto N, Saki A, Horikoshi T, Urushihara T, Tobita E
(2007) J Appl Polym Sci 103:244–250
60. Lu SF, Chen M, Chen CH (2012) J Appl Polym Sci
123:3610–3619
61. Chrissafis K, Paraskevopoulos KM, Bikiaris DN (2005) Ther-
mochim Acta 435:142–150
62. Di Y, Iannace S, Maio ED, Nicolais L (2005) Macromol Mater
Eng 290:1083–1090
63. Bhatia A, Gupta RK, Bhattacharya SN, Choi HJ (2007) Korea
Aust Rheol J 19:125–131
64. Li K, Peng J, Turng LS, Huang HX (2011) Adv Polym Technol
30:150–157
65. Wang L, Jing X, Cheng H, Hu X, Yang L, Huang Y (2012) Ind
Eng Chem Res 51:10088–10099
66. Wang T, Li H, Wang F, Schultz JM, Yan S (2011) Polym Chem
2:1688
J Polym Environ
123