Biomedical Engineering Reference
In-Depth Information
the permeation is assumed not to occur in and through a crystal (Tsujita, 2003) involv-
ing the improvement of the barrier properties of the materials. The barrier properties
are based on tortuosity concept which is one of the most important parameters that
increases molecule migration pathways and consequently limits permeability. Com-
pared to polymers with fossil origin, the barrier properties are increased due primarily
to tortuous concept linked to presence of crystalline domains.
Concerning water permeation measurements, the presence of ester groups in back-
bone structure induced water affinity leading to an increase of permeability coefficient,
as observed with PLA. The PHA are thermoplastics with high degree of crystallinity.
The crystalline structure affected the transport properties of PHA films directly. It
provides valuable informations for interpreting the water vapor permeation properties.
The water vapor permeability is thus related to the chemical structure and also to the
morphology of the PHA films. The higher hydrophilic nature of polymer is enhanced,
the higher the water resistance is reduced. The water affinity of PHBV polymers helps
the diffusion of water molecules within the film due to water plasticization effect. But
compared to bio-based polyester such as PLA, the permeabilities are lower.
With moderate hydrophilic character such as PHBV copolymers, the water barrier
properties are maintained close to values corresponding to medium barrier polymer for
water. In particular, the permeation properties of PHBV copolymers reveal that they
could serve as useful packaging material. The PHB copolymers showed the medium
barrier properties with polar molecules and the highest with polar ones.
We can observe that the same behavior is observed with the three diffusing gas
molecules and the PHB films are more permeable to CO
2
than O
2
and N
2
gas. This
result is usually obtained whatever the polymer studied (Tsujita, 2003). The general
behavior in permeation is maintained with bacterial based polymers.
In conclusion, the barrier effects towards gas and liquid molecules are mainly at-
tributed to the tortuosity pathway through the polymer (due to the crystalline phase)
independently to the hydrophilic character of polymer backbone structure. However,
comparisons between different bio-based materials are complicated and sometimes
not possible due to the use of different types of equipments and dissimilar conditions
for the measurements.
CoNClusioN
In this work, the relationship between structure, physical and transport properties of
bio-resources polymers obtained from bacteria was analyzed, showing the high po-
tential presented by these eco-friendly polyesters. Currently, only three prominent
polymers in PHA family [PHB, poly(3-hydroxybutyrate-
co
-3-hydroxyvalerate) and
poly(3-hydroxyhexanoate-
co
-3-hydroxyoctanoate)] were produced to a relatively
high concentration with high productivity. Because of its renewable character, its bio-
degradability, its low water affinity which is arguably the main advantages of PHA
family, PHBV polymers can be considered in many applications by replacement of
conventional polymers. In fact, PHBV copolymers exhibited properties much closer to
those of polyethylene LDPE or polypropylene but their availability and price still can
represent a hindrance for this family to be considered as serious competitors against
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