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International Journal of Biological Macromolecules 188 (2021) 290–299
296
worth noting that the tensile fracture surfaces of all TPS films became
more homogeneous with increased plasticizer content, confirming the
greater plasticizing effect.
The thickness of all TPS films obtained after blown film extrusion
was rather consistent; however, upon storage, the thickness of G films
significantly increased and became greater than that of GX and GS films.
In addition, the thickness of TPS films was inversely proportional to the
incorporated plasticizer content. These results suggest that the greater
the plasticizing effect, the thicker the film after storage, most likely due
to the larger swelling resulting from higher water/moisture absorption
[28]
, enhanced molecular mobility
[29]
, and greater relaxation to in-
crease the entropy of starch polymer chains. SEM images of the samples
after tensile testing and storage under the same conditions of 25
◦
C and
52%RH also support the above results.
Fig. 5
shows that the thickness of
TPS film tended to increase with increasing plasticizer content and with
decreasing the molecular size of the plasticizer. A similar result was
reported by Al-Hassan and Norziah
[30]
, who found that sago starch/
fish gelatin films (with a ratio of 2:1) plasticized with glycerol were
softer and more easily shrank than the films plasticized with sorbitol
when stored in the lab environment, even for short time.
3.9. Water vapor and oxygen barrier properties
Fig. 6
shows the water vapor permeability (WVP) and oxygen
permeability (OP) of TPS films. Those plasticized with glycerol had
higher WVP (3.88
–
5.59
×
10
−
12
g/m.s.Pa) than the films plasticized
with mixed plasticizers of glycerol/xylitol (1.10
–
2.02
×
10
−
12
g/m.s.Pa)
and glycerol/sorbitol (1.14
–
1.17
×
10
−
12
g/m.s.Pa) (
Fig. 6
A). The
poorer water vapor barrier property of G than that of GX and GS was
most likely due to the greater hydrophilic nature of glycerol, leading to
increased adsorption of water molecules throughout the film network
[31]
. In addition, the water diffusion rate through the film matrices of G
was also higher than that of the other two TPS groups as T
g
of the cor-
responding G was significantly lower (
Table 2
). Moreover, if the same
mass loading of plasticizer is considered, films containing lower-
molecular-weight plasticizer possess a greater molar content of such
plasticizer
[32]
. Consequently, many OH groups of the small-molecular-
sized glycerol readily formed hydrogen bonds with water molecules,
resulting in increased water holding capacity of the film matrix. On the
other hand, partially replacing glycerol with xylitol or sorbitol signifi-
cantly reduced WVP (
Fig. 6
Ad-h), or in other words, improved the water
vapor barrier property of the TPS film, corresponding to the lower
moisture content (less than 10%) and the increased T
g
(
Table 2
).
Although glycerol was present in the GX and GS films, xylitol and sor-
bitol had a greater influence on film structure and performance.
WVP of the TPS films tended to increase with increasing plasticizer
content, particularly at a high loading of 42 phs. The poorer water vapor
barrier property of the TPS films as a function of plasticizer content was
associated with the looser film matrix, or enhanced mobility of the
polymer chains, due to the plasticizing effect, resulting in increased
water absorption and diffusion. In addition, at a very high concentration
of plasticizer, the plasticizer-plasticizer interactions became more
outstanding than the plasticizer-starch interactions
[9]
; as a result, the
number of free hydroxyl groups of starch available for interacting with
water increased. Water molecules can also act as a plasticizer to increase
free volume, allowing water vapor to pass through the films. This effect
was predominant for the small-molecular-sized plasticizer.
The effect of plasticizer on OP of TPS films is presented in
Fig. 6
B. G
Fig. 4.
(A) Tensile strength, (B) Young's modulus, (C) elongation at break, and (D) impact strength of different TPS blown films as mentioned in
Section 2.2
: (a) G38,
(b) G40, (c) G42, (d) GX38, (e) GX40, (f) GX42, (g) GS40, and (h) GS42.
K.M. Dang and R. Yoksan
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