3.3
Material Properties of PHA
A variety of optically pure PHA has been biosynthesized using microorganisms. P
(3HB) is the first type of PHA to be discovered and to be characterized. Following
the earlier characterization of P(3HB) (Holmes et al. 1984), researchers were interested to compare its properties to polypropylene (PP). Interestingly, P(3HB) has a
melting temperature (T m ) of about 177
C and glass transition temperature (T g )
around 4
C (Taguchi et al. 2012) which are nearly similar to PP. However, owing to
the stereoregular nature of P(3HB) obtained from microbes, it is highly crystalline,
unlike PP which is relatively flexible. Highly crystalline property has made it be a
very stiff and brittle polymer. Moreover, as for the hot-drawing process of P(3HB)
materials, one of the major problems is the decrease in molecular weight of polymer
due to rapid thermal degradation near its melting temperature. All these cumulative
drawbacks of P(3HB) have hindered it from being developed as an industrial
material targeting a wide range of applications. Therefore, 3HB monomer unit has
to be copolymerized with other monomers to circumvent the aforementioned
limitations. Table 3.3 summarizes chemical structures of some of the biosynthesized
3HB-based copolymers for better material properties. 3HB-based copolymers with
even small fraction of other monomers will help to reduce the T m of the polymer in
overall. It was shown that with just about 3.5 mol% of 3HHx fractions, the T m of the
copolymer was reduced noticeably compared to P(3HB) homopolymer (Tsuge et al.
2004b). These copolymers melt during the drawing of polymer material at a low
temperature while avoiding thermal degradation. Various attempts have been made
to characterize the bacterially produced optically active 3HB-based copolymers for
improved material properties. Some of such copolymers’ material properties are
summarized in Table 3.4 along with material properties of P(3HB) and PP and LDPE
for a comparison. The higher molecular weight gives P(3HB) a higher mechanical
strength, and unlike lower molecular-weight P(3HB) polymers, ultrahigh-molecular
weight-P(3HB) [UHMW-P(3HB)] can be processed into strong films or fibers by hot
or cold drawing (Kusaka et al. 1999). It is expected that the improved material
properties of UHMW-P(3HB) will, therefore, expand the number of applications for
this polymer. It was found that the mechanical properties of the stretched P(3HB)
films were markedly improved in comparison with those of the unstretched films
where the elongation at break and tensile strength of the stretched films increased to
35% and 1320 MPa (Kusaka et al. 1999, Iwata 2005), respectively. As mentioned
earlier, another way to improve the physical properties of P(3HB) is by the
incorporation of different HA units into the P(3HB) sequence to form PHA
copolymers. P(3HB-co-3HV) is one of the very first commonly biosynthesized
copolymer, and it even was produced at a bulk amount under the trademark of
Biopol™. However, isodimorphous is one of the biggest impediments in P(3HB-co3HV) where 3HB and 3HV units are isodimorphous and get incorporated into the P
(3HB) crystal lattice. Incorporation of 8 mol% of 3HV units (Taguchi et al. 2012)
has reduced the T m to 165
C and T g to 1
C. However, the elongation at break
increased to 35%. On the other hand, incorporation of 20 mol% 3HV units, T m and
T g of the copolymer decrease to 145
C and Àl
C, respectively, and its elongation to
3 Development of Polyhydroxyalkanoate (PHA) and Its Copolymers as a Possible. . .
73
Material Properties of PHA
A variety of optically pure PHA has been biosynthesized using microorganisms. P
(3HB) is the first type of PHA to be discovered and to be characterized. Following
the earlier characterization of P(3HB) (Holmes et al. 1984), researchers were interested to compare its properties to polypropylene (PP). Interestingly, P(3HB) has a
melting temperature (T m ) of about 177
C and glass transition temperature (T g )
around 4
C (Taguchi et al. 2012) which are nearly similar to PP. However, owing to
the stereoregular nature of P(3HB) obtained from microbes, it is highly crystalline,
unlike PP which is relatively flexible. Highly crystalline property has made it be a
very stiff and brittle polymer. Moreover, as for the hot-drawing process of P(3HB)
materials, one of the major problems is the decrease in molecular weight of polymer
due to rapid thermal degradation near its melting temperature. All these cumulative
drawbacks of P(3HB) have hindered it from being developed as an industrial
material targeting a wide range of applications. Therefore, 3HB monomer unit has
to be copolymerized with other monomers to circumvent the aforementioned
limitations. Table 3.3 summarizes chemical structures of some of the biosynthesized
3HB-based copolymers for better material properties. 3HB-based copolymers with
even small fraction of other monomers will help to reduce the T m of the polymer in
overall. It was shown that with just about 3.5 mol% of 3HHx fractions, the T m of the
copolymer was reduced noticeably compared to P(3HB) homopolymer (Tsuge et al.
2004b). These copolymers melt during the drawing of polymer material at a low
temperature while avoiding thermal degradation. Various attempts have been made
to characterize the bacterially produced optically active 3HB-based copolymers for
improved material properties. Some of such copolymers’ material properties are
summarized in Table 3.4 along with material properties of P(3HB) and PP and LDPE
for a comparison. The higher molecular weight gives P(3HB) a higher mechanical
strength, and unlike lower molecular-weight P(3HB) polymers, ultrahigh-molecular
weight-P(3HB) [UHMW-P(3HB)] can be processed into strong films or fibers by hot
or cold drawing (Kusaka et al. 1999). It is expected that the improved material
properties of UHMW-P(3HB) will, therefore, expand the number of applications for
this polymer. It was found that the mechanical properties of the stretched P(3HB)
films were markedly improved in comparison with those of the unstretched films
where the elongation at break and tensile strength of the stretched films increased to
35% and 1320 MPa (Kusaka et al. 1999, Iwata 2005), respectively. As mentioned
earlier, another way to improve the physical properties of P(3HB) is by the
incorporation of different HA units into the P(3HB) sequence to form PHA
copolymers. P(3HB-co-3HV) is one of the very first commonly biosynthesized
copolymer, and it even was produced at a bulk amount under the trademark of
Biopol™. However, isodimorphous is one of the biggest impediments in P(3HB-co3HV) where 3HB and 3HV units are isodimorphous and get incorporated into the P
(3HB) crystal lattice. Incorporation of 8 mol% of 3HV units (Taguchi et al. 2012)
has reduced the T m to 165
C and T g to 1
C. However, the elongation at break
increased to 35%. On the other hand, incorporation of 20 mol% 3HV units, T m and
T g of the copolymer decrease to 145
C and Àl
C, respectively, and its elongation to
3 Development of Polyhydroxyalkanoate (PHA) and Its Copolymers as a Possible. . .
73