The mechanical properties of individual PHAs depend on the
composition of the monomer units. P(3PHB) is highly crystalline
because it is a completely stereoregular polyester, with all asymmetric
carbon atoms in the R-configuration. The high crystallinity
(typically 55e80%) makes it relatively stiff and brittle. The glass
transition temperature (Tg) of P(3HB) lies between 5 and 9 C, and
the melting point (Tm) ranges from 173 to 180 C. P(3HB-co-3HV) is
another well-studied PHA, composed of both 3HB and 3HV
monomers. Its mechanical properties are considered more desirable
than P(3HB) monomers because its melting point is much
lower, and it is less crystalline, easier to mold and tougher than
pure P(3HB) (Luzier, 1992). These thermo mechanical properties
can be widely varied because the composition of P(3HB-co-3HV)
can range from 0 to 30 mol% 3HV (Braunegg et al., 1998).