|
Moisture content
(
%
) =
(
W
0
−
W
1
W
0
)
×
100
(1)
2.3.3. Water contact angle measurement
The water contact angle of the samples was measured using an OCA
15EC contact angle analyzer (DataPhysics Instruments GmbH, Filder-
stadt, Germany) with SCA 20 software for data acquisition. Distilled
water (3
μ
L) was dropped on the film surface (3 cm
×
3 cm) prior to
measurement for 200 s.
2.3.4. Fourier transform infrared spectroscopy
The Fourier transform infrared (FTIR) spectra were recorded in an
attenuated total reflectance (ATR) mode using a Bruker Tensor 27 FT-IR
spectrometer (Bruker, Germany) at a wavenumber range from 500 cm
−
1
to 4000 cm
−
1
with 32 accumulated scans and a resolution of 4 cm
−
1
. The
temperature-dependent FTIR spectroscopic studies were carried out at
temperatures ranging from 30
◦
C to 170
◦
C.
2.3.5. X-ray diffraction analysis
X-ray diffraction (XRD) patterns were analyzed at a diffraction angle
(2
θ
) range from 5
◦
to 40
◦
with a scan rate of 0.02
◦
/s, using a JEOL JDX-
3530 X-ray diffractometer (JEOL, Japan).
2.3.6. Tensile test
Tensile properties were tested according to ASTM D882
–
12. To
reduce the effect of shrinkage, the films were first stored at 25
±
2
◦
C, 52
±
2%RH for two weeks, before being cut into rectangular-shaped
specimens with a dimension of 3 cm
×
10 cm. The thickness of each
specimen was measured from five points, one at the center and four at
the perimeter, using an ID-C112BS micrometer (Mitutoyo, Japan) with
an accuracy of
±
0.0001 mm. The specimens were again conditioned at
25
±
2
◦
C, 52
±
2%RH for two days prior to testing. Tensile testing was
performed with an initial grip distance of 100 mm and a crosshead speed
of 50 mm/min using a 5965 universal testing machine (Instron, USA).
Four specimens were tested for each sample. Tensile strength (MPa),
Young's modulus (MPa), and elongation at break (%) were evaluated
from the stress-strain curves as mean
±
SD (
n
=
4).
2.3.7. Impact test
The pendulum impact resistance of the films was evaluated accord-
ing to ASTM D3420. Square-shaped specimens (10 cm
×
10 cm) with a
double-wall film thickness in the range of 150
–
300
μ
m of each sample
were prepared and conditioned at 25
±
2
◦
C, 52
±
2%RH for two days.
The test was performed with an impact head size of 19 mm in diameter
and pendulum energy of 1 J using a FIT-01 Pendulum Impact Tester
(Labthink International, Inc., Medford, USA). Three specimens were
tested for each sample. The energy required to fracture the specimen
(impact energy) is measured and then used to calculate the impact
strength.
2.3.8. Dynamic mechanical thermal analysis
Dynamic mechanical thermal properties were measured in a tension
mode using an EPLEXOR DMA (GABO Qualimeter, Germany). The
measurement was carried out at a temperature range from
−
80
◦
C to
55
◦
C with a heating rate of 3
◦
C/min, a constant frequency of 1.5 Hz, a
static load of 0.5 N, and a dynamic load of 0.15 N. The T
g
of starch was
reported.
2.3.9. Scanning electron microscopy
The morphological characteristics at tensile fracture surfaces of the
samples were observed using an FEI Quanta 450 scanning electron
K.M. Dang and R. Yoksan
International Journal of Biological Macromolecules 188 (2021) 290–299
292
microscope (SEM) (FEI, Oregon, USA) at an accelerating voltage of 12.5
kV. The samples were placed on a stub using a two-sided carbon tape
and coated with a thin layer of gold on the tensile fracture surfaces prior
to SEM observation.
2.3.10. Determination of water vapor transmission rate
Water vapor transmission rate (WVTR) was determined according to
ASTM E96. A circle-shaped film with a diameter of 7.5 cm was first dried
in a hot air oven at 45
◦
C for 12 h and then mounted using a paraffin wax
on the open mouth of a test cup, with an inner diameter of 6.3 cm, filled
with dried desiccant (20 mL). The initial weight of the sample assem-
blies was recorded before they were placed in an incubator at 25
◦
C, 50%
RH. The weight of the sample assemblies was recorded periodically until
it was constant. Three specimens were tested for each sample. WVTR
was determined as a slope of the linear portion of a plot of weight gained
versus time (g/s) divided by the sample permeation area (m
2
). Steady
state over time (slope) yielded a regression coefficient of 0.99 or greater.
The water vapor permeability (WVP, g/
m.s.Pa
) was calculated using Eq.
(2)
.
WVP
=
WVTR
×
L
∆
P
(2)
where L is the mean film thickness (m), and
Δ
P is the partial water vapor
pressure difference between the two sides of the film (Pa). The results
were recorded as mean
±
SD.
2.3.11. Determination of oxygen transmission rate
Oxygen transmission rate (OTR, mol/m
2
.s) was determined under
ASTM D3985 using a model 8000 Oxygen Permeation Analyzer (Illinois
Instruments, Inc., Johnsburg IL, USA). Each sample was cut into a circle
with a diameter of 14 cm. Three specimens were tested for each sample.
Oxygen permeability (OP, mol/
m.s.Pa
) was calculated using Eq.
(3)
.
OP
=
OTR
×
L
∆
P
(3)
where L is the mean thickness (m) and
Δ
P is the oxygen partial pressure
difference between two sides of the film (Pa).
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