Barrier Properties of Biodegradable Bacterial Polyester Films - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 67

permeability was demonstrated. By virtue of limited water solubility in such poly-
mers, water diffusion in moderately hydrophobic polymers is a structurally sensitive
process. The authors estimated that the crystalline structure of PHB led to the decrease
of water diffusivity.
Vapor Transmission Rate (Vpt) through Polymer Film
Some authors measured the vapor permeability by using a gravimetric cell (Miguel,
1997) which is basically a small container partially filled with diffusing liquid mol-
ecules, and with a polymer film sealing its top. The permeation process is reflected as
a reduction in the overall weight of the cell. When the diffusing liquid molecules come
in contact with the film (the gravimetric cell is placed downward) liquid permeation
can be measured. This technique can be used when transport properties against vapors
of diffusing molecules that are liquids at normal pressures and temperatures are tested.
For example, with PHB film obtained by 3% chloroform casting method, Miguel
and co-workers (Miguel, 1999) have followed the weight change by using a comput-
er-connected Sartorius analytical balance with a sensitivity of 10 -5 g. Two different
permeability coefficients have been calculated depending on the units in which the
concentration gradient of the diffusing molecules is expressed. In both cases, the cal-
culated values took into account the exposed area and the partial water vapor pressure
difference across the two sides of the film. The first coefficient that is the permeability,
expressed in Barrer, is calculated from the steady-state slope of the permeation mea-
surement. The second coefficient corresponds to vapor transmission rate coefficient
(VTR, expressed in g cm/cm 2 s) related the driving force of the permeation process to
diffusing molecules activity. The VTR coefficient is proportional to the actual flux of
the diffusing molecules passing through the film.
PhysiCal ProPerties
The purpose of this section is to compare the properties of PHA polymers with those
of polyesters commercially available. In fact, the thermal properties of materials are
important for processing and also during the use of the products derived from these
materials as well as the physical and mechanical properties of these polymers in order
to design novel eco-friendly films. The properties of some biodegradable polymers are
gathered in Table 1 and data available for two most common polymers (LDPE, PP) are
added for comparison.
Due to its high degree of crystallinity of 60-80%, linear polyester PHB is a stiff
material of high tensile strength and especially pronounced brittleness, although excel-
lent mechanical strength and tensile modulus are present. It can be seen that the prop-
erties of PHB are rather close to those of polypropylene, outperforming polyethylene
in some parameters. The primary negative importance of PHB is represented by its
very low deformation at break related to its brittleness and low toughness. These seri-
ous drawbacks have hindered potential applications of PHB. The reason for the PHB
brittleness comes from large volume-filled crystals in the form of spherulites from few
nuclei due to their high purity. This stiffness is reduced by introducing hydroxyvaler-
ate units. As observed in Table 1, the mechanical properties of PHBV were enhanced,
whatever the hydroxyvalerate units used. In addition, these PHB copolymers showed