chains of polysaccharide is increasing and making the structure
more fragile. This involved a decrease in tensile strength. Similar
results have been previously reported for hydroxypropyl starch
films (Arvanitoyannis and Biliaderis, 1998) and for potato and barley
starch films (Koskinen et al., 1996).
Elongation of the samples was determined to estimate the
stretchability of the films. The elongation of the starch films was
affected by all the process variables except the interactive effect of
starch and span 80 content an also the quadratic effect of the starch
content (Table 5). The addition of agar would destroy the intermolecular
hydrogen between polysaccharide chains and enhance
their mobility and then increased the elongation. The interaction
of polymers and continuity of the polysaccharide network structure
was reduced due to the addition of surfactants (span 80). But
the low molecular hydrophilic plasticizer such as glycerol can disperse
the network. So the continuous network structure was not
disrupted significantly. As a result, the elongation was increased
(Fig. 4) due to the increase in mobility of polymer chains by the
presence of glycerol (Chang et al., 2006). During the processing of
starch film, the granular and crystal structure of starch was mostly
destroyed by high temperature and shear force. This was favorable
to the pervasion of plasticizer into the matrix. The permeated
glycerol molecules could further decrease the interactions among
the starch macromolecules, because of the formation of hydrogen
bonds between the hydroxyl groups of starch macromolecules and
glycerol small molecules. These all are favorable to the movement
and rearrangements of the macromolecule chains of the starch,
which caused the increase in the flexibility of the starch films.