Properties of PHA
Freeze-dried cells (upper right) and purified PHA (lower right)
Physical properties of poly[(R)-3-hydroxybutyrate]
P(3HB) isolated from bacteria reveals 55–80% crystallinity , while the molecules within the bacteria are amorphous [23, 24, 25] and exist as water insoluble inclusions. The weight-average molecular weight (Mw) of P(3HB) produced from wild-type bacteria is usually in the range of 1 x 104 to 3 x 106 g/mol with polydispersity of around 2 . The glass transition temperature of P(3HB) is around 4°C while the melting temperature is around 180°C, as measured by calorimetric analysis. The densities of crystalline and amorphous P(3HB) are 1.26 and 1.18 g/cm-3, respectively. The mechanical properties of Young’s modulus (3.5 GPa) and the tensile strength (43 MPa) of P(3HB) material are close to those of isotactic polypropylene. The extension to break (5%) for P(3HB) is however markedly lower than that of polypropylene (400 %). Therefore, P(3HB) appears as a stiffer and more brittle plastic material when compared with polypropylene.
Physical properties of PHA copolymers
Random copolymers containing (R)-3HB as a constituent along with other HA units of chain lengths ranging from 3 to 14 carbon atoms have been produced from various carbon substrates by a variety of bacteria. PHA composition produced by bacteria is dependent on the substrate specificities of enzymes in the PHA biosynthesis pathway. The physical and thermal properties of PHA copolymers can be regulated by varying their molecular structure and copolymer compositions. The PHA family of polyesters offers a wide variety of polymeric materials exhibiting various properties, from hard crystalline plastic to elastic rubber. The PHA materials behave the thermoplastics with melting temperatures of 50–180°C.
For solution-cast film of P(3HB-co-3HHx) copolymer, the melting temperature decreased from 177 to 52°C as the (R)-3HHx fraction was increased from 0 to 25 mol%. The glass-transition temperature decreased from 4 to -4°C. The tensile strength of the films decreased from 43 to 20 MPa as the (R)-3HHx fraction was increased from 0 to 17 mol%. In contrast, the elongation break increased from 6 to 850%. Thus, the P(3HB-co-3HHx) films become soft and flexible with an increase in the (R)-3HHx fraction .
The melting temperature of P(3HB-co-4HB) solution-cast film decreased from 178 to 130°C with the 4HB fraction. The glass-transition temperature decreased from 4 to -48°C as the 4HB fraction was increased from 0 to 100 mol%. The tensile strength of P(3HB-co-4HB) films with compositions of 0–16 mol% 4HB decreased from 43 to 26 MPa with an increase in the 4HB fraction, while the elongation break increased from 5 to 444%. The tensile strength of the films with compositions of 64–100 mol% 4HB increased from 17 to 104 MPa with increasing the 4HB fraction. The true tensile strength of P(4HB) homopolymer was calculated to be as large as 1 GPa if the cross-section was corrected. Thus, P(3HB-co-4HB) copolymers exhibit a wide range of material properties [28, 29].
Recently, it was found that Pseudomonas sp. 61-3 accumulated a novel random copolymer of 3HB and medium-chain-length 3-hydroxyalkanoates of carbon numbers ranging from 6 to 12 atoms [P(3HB-co-3HA)], from glucose and alkanoates . More recently, it was found that genetically engineered Pseudomonas sp. 61-3 can produce a P(3HB-co-3HA) copolymer with high (R)-3HB compositions (up to 94 mol% (R)-3HB) from glucose . In the case of P(3HB-co-6 mol% 3HA) film, the elongation break reached 680% by the introduction of only 6 mol% of MCL 3HA units. The resulting tensile strength was determined to be 17 MPa, indicating that the copolymerization of (R)-3HB units with MCL 3HA units is effective in improving the brittleness of P(3HB) film . The mechanical properties of P(3HB-co-6 mol% 3HA) were also found to be very similar to those of low-density polyethylene.
Poly[(R)-3-hydroxybutyrate] based polymer blends
To regulate the physical properties of P(3HB), the polymer blends of P(3HB) with biodegradable polymers have been investigated to a great extent. Polymer blends are physical mixtures of structurally different polymers, and the mixture of two polymers forms either homogeneous or heterogeneous phase in amorphous region on a microscopic scale at equilibrium. When a mixture of two polymers in the amorphous phase exists as a single phase, the blend is considered to be miscible in the thermodynamic sense. In contrast, a mixture of two polymers separates into two distinct phases consisting primarily of the individual components, the blend is considered to be immiscible in the thermodynamic sense. The physical properties of a mixture are strongly dependent on the phase structures. Therefore, the miscibilities of P(3HB)-based polymer blends have been evaluated extensively.