They stated that a small quantity of plasticizer
could be easily inserted between polymer chains, producing a
“cross-linker” effect that would decrease the free volume and the
segmental mobility of the polymer, decreasing the mechanical
strength of the films and enhancing their extensibility. Sorbitol has
been extensively used in other biopolymer films, but it appeared
to have no significant effect (p > 0.05) on plasticizing kefiran film,
at least at the concentrations tested. As described earlier, this was
due to the differences in molecular size and structure, the number
of active hydroxyl groups, polarity, and some other properties
that distinguish sorbitol from glycerol. The TS values of kefiranplasticized
films were 5.04–16.18 MPa, which were lower than
those of synthetic polymers such as LDPE (9–17 MPa), polystyrene
(35–55 MPa), and cellophane (114 MPa) (Smith, 1986). However,
kefiran film plasticized with glycerol showed extremely high elongation
values – for example, they were higher than cellophane
(20%) and polystyrene (1%) – but could not be stretched nearly LDPE