Micrographs were obtained of sheets previously
immersed in chloroform for 24h in order to solubilize the
PLA and allowed observation of PLA regions in the TPS
matrix. Images were also taken of sheets not immersed in
chloroform.
Fig. 1(a)
shows the fracture micrograph of the
TPS25/PLA sheet (section
2.2
) and
Fig.1(b)
shows the sheet
previouslyimmersed in chloroform. In
Fig.1(a)
it is possible
to observe the PLA domains (lighter) distributed in the TPS
matrix (darker).
Fig. 1(b)
shows the voids previously occu-
pied by PLA domains, characterizing the immiscibility
between the PLA and TPS. This behavior is associated with
the difference between the hygroscopicity of starch
(hydrophilic) and PLA (hydrophobic). These
fi
ndings verify
the observations of a previous study of Müller et al.
[8]
.
Fig. 2
shows the surface micrographs of the TPS25/PLA
sheet uncoated (
Fig. 2
a), and coated by spraying (
Fig. 2
b)
or by immersion (
Fig. 2
c) with cross-linked chitosan.
Regardless of the coating process, the micrographs indi-
cate a continuous covering of chitosan on the sheet
surface.
Coating the sheets with chitosan led to a higher surface
roughness compared with the uncoated sheets, indepen-
dently of the method used for coating. This feature may be
associated with the reaction of glutaraldehyde with
repeating units of the chitosan polymer chain due to the
cross-linking process. Furthermore, the sheets coated by
immersion showed a more irregular surface than the spray-
coated samples, probably because the immersion favored
water diffusion into the starch matrix which modi
fi
es the
surface appearance of the sheet.
Gierszewska-Dru
_
zy
nska and Ostrowska-Czubenko
[16]
studied the ionic cross-linking of chitosan membranes
with sodium tripolyphosphate and observed the surface of
the
fi
lms by SEM. They reported that the surface of the
chitosan
fi
lms was smooth, while on the surface of the
cross-linked chitosan membrane there were numerous
visible lumps. These differences in the surface morphology
were con
fi
rmed by AFM, with calculated values for surface
roughness of 3.56 and 23.71 nm for the uncross-linked and
cross-linked chitosan
fi
lms, respectively.