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3.7. Mechanical properties
The mechanical properties of TPS blown films were tested in both
tension and impact modes.
Fig. 4
a-c shows that TPS films plasticized
with glycerol had significantly lower tensile strength and Young's
modulus, but higher elongation at break than the films plasticized with
mixed plasticizers of glycerol/xylitol (
Fig. 4
d-f) and glycerol/sorbitol
(
Fig. 4
g-h). The result indicates that the molecular size of the plasticizer
affects the tensile properties of TPS film. Glycerol, a small-molecular-
sized plasticizer, more easily diffuses into starch polymer chains to
disrupt their hydrogen bond interaction
[26]
, while lower penetration of
larger-sized plasticizers such as xylitol and sorbitol causes less hydrogen
bond destruction between starch molecules
[16]
. Similar behaviors have
been reported by Muscat, Adhikari, Adhikari and Chaudhary
[9]
and
Talja, Hel
´
en, Roos and Jouppila
[7]
. In addition, Talja, Hel
´
en, Roos and
Jouppila
[7]
reported that xylitol and sorbitol could crystallize at 54%
RH, appearing as white spots on the surfaces of solution-cast xylitol- and
sorbitol-plasticized starch films. This caused a decrease in the number of
polyols in the film matrices and thus changed their mechanical prop-
erties. Following the above explanation, in this study, the films plasti-
cized with glycerol/sorbitol mixture were stronger, stiffer, and less
extensible (
Fig. 4
g-h) than the ones plasticized with a glycerol/xylitol
mixture (
Fig. 4
e-f).
By considering elongation at break, as presented in
Fig. 4
C, it was
unexpectedly found that among G, i.e. G38, G40, and G42 (
Fig. 4
Ca-c),
and TPS with the highest plasticizer content of 42 phs, i.e. G42, GX42,
and GS42 (
Fig. 4
Cc,f,h), the elongation at break did not differ. This
might be a result of high water uptake and high moisture content (20 wt
%) in those TPS films at equilibrium during conditioning; the role of
water was thus more pronounced than the role of polyol plasticizers in
the cases of TPS films with a high level of plasticization. During high
water absorption, the number of hydrogen bonds formed between
starch-polyol and starch-starch molecules decreased, but the hydrogen
bond formation between starch-water and polyol-water molecules
became more evident
[7]
. Consequently, the mobility of starch chains
was maximized and greater free volume was created
[27]
. It should be
noted that plasticizer type hardly affected the impact strength of the TPS
films, as shown in
Fig. 4
D.
After increasing plasticizer content, the tensile strength and Young's
modulus of TPS film tended to decrease, while elongation at break and
impact strength tended to increase, suggesting that the films became
more extensible and tougher with lower tensile strength and stiffness.
This is explained by the reduction of intermolecular interaction of
starch-starch molecules resulting in increased free volume and starch
chain mobility.
3.8. Morphological characteristics
The morphological characteristics at tensile fracture surfaces of TPS
films were observed by SEM, as illustrated in
Fig. 5
. All films showed
uniform morphology without obvious remaining starch granules, sug-
gesting that all plasticizer types and contents used, along with the
application of heat and shear force in the extrusion processes, were
effective in destroying starch granules. This evidence is consistent with
the absence of XRD diffraction peaks belonging to native starch crystals
(
Fig. 3
a). It was seen that G exhibited rough tensile fracture surfaces at
all concentrations (
Fig. 5
a-c), while GX and GS exhibited smoother
(
Fig. 5
d-f) and the smoothest surfaces (
Fig. 5
g-h), respectively. The re-
sults suggest that G is the most flexible and GS possesses the highest
brittleness, which corresponds with the tensile property result. It is
Fig. 3.
XRD patterns of (a) native cassava starch and (b)-(i) different TPS blown
films as mentioned in
Section 2.2
: (b) G38, (c) G40, (d) G42, (e) GX38, (f)
GX40, (g) GX42, (h) GS40, and (i) GS42.
K.M. Dang and R. Yoksan
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