Poly-4-hydroxybutyrate mechanical properties

Lannace, S., Ambrosio, L., Huang, S.J. and Nicolais, L. 1994. Poly(3- hydroxybutyrate)-co-(3-hydroxyvalerate)/poly-l-lactide blends thermal and mechanical properties. Journal of Applied Polymer Science 54 1525-35. 

Another interesting example of lactones are the p-hydroxyalkanoates, whose ROP affords poly(p-hydroxyalkanoate)s (PHAs), a class of aliphatic polyesters naturally produced by bacteria (Fig. 3) [12, 13]. Poly(3-(R)-hydroxybutyrate) (PHB) is a typical example. PHB is a stiff thermoplastic material with relatively poor impact strength, but the incorporation of other monomers can improve the mechanical properties. 

The copolymer poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-co-PHV) produced by A eutrophus has generated more interest than poly-(R)-3-hydroxybutyrate (PHB) homopolymer. Since these bacterial polyesters are biodegradable thermoplastics, their mechanical and physical properties have received much attention. PHB is a relatively stiff and brittle material because of its high crystallinity. However, the physiochemi-cal and mechanical properties of [P(HB-HV)] vary widely and depend on the molar percentage of 3-hydroxyvalerate (HV) in the copolymer (4,5) as shown inTable 1. Propionic acid is converted by a synthetase to propionyl-CoA, and the biosynthetic P-ketothiolase catalyzes the condensation of propionyl-CoA with acetyl-CoA to 3-ketovaleryl-CoA by the acetoacetyl-CoA reductase. The hydroxyvaleryl moiety is finally covalently linked to the polyester by the PHA synthase (6). 

The blending of different polymers is a frequently used technique in industrial polymer production to optimize the material s properties. The biodegradable polymer poly(3-hydroxybutyrate) (PHB) [45, 46], for example, which can be produced by bacteria from renewable resources, has the disadvantage of being stiff and brittle. The mechanical properties of PHB, however, can be readily enhanced by blending with another biopolymer, poly(lactic acid) (PLA) [47]. In order to prepare the optimum blend, it must be noted that the miscibility of different polymers depends on their concentration, the temperature, and their structural characteristics [48]. 

At room temperature, the mechanical properties of PLA are close to the one of PS but smaller than the one of PET (Table 8.5). Polyolefins present reduced stress at yield compared to PLA but the strain at break of LDPE and HOPE are much higher than the one of PLA. Compared to another biobased polymer, poly(hydroxybutyrate) (PHB), PLA shows better mechanical properties with higher modulus of elasticity and stress at yield. 

Poly(4-hydroxybutyrate) [P(4HB)] is a highly ductile, flexible polymer withstanding an extension of around 1,(XX)% before breaking, compared to P(3HB), which can only extend to less than 10% before breaking. Combining these different monomers to form copolymers, as in P(3HB-co-4HB), been described as one of the most useful PHAs by Sudesh et al. [5], produces a family of materials with mechanical properties that can be tailored to specific needs. P(3HB-co-4HB) has been found to have desirable mechanical properties for applications in the medical and pharmaceutical field [11]. The biocompatibility and bioabsorbable nature of P(3HB-co-4HB) makes it the most valuable type of biopolymer among the vast number of different PHAs synthesized by microorganisms. To date, five wild-type bacteria, which can produce P(3HB-co HB), i.e. R. eutropha... 

TAN 07] Tanaka T., Yabe T., Termachi S., et al, Mechanical properties and enzymatic degradation of poly[(R)-3-hydroxybutyrate] fibers stretched after isothermal crystallization neaxJg , Polymer Degradation and Stability, vol. 92, no. 6, 1016-1024, 2007. 

Luo S, Grubb DT, Netravah AN (2002) The effect of molecular weight on the lamellar structure, thermal and mechanical properties of poly(hydroxybutyrate-co-hydroxyvalerates). Polymer 43 4159 166... 

Yu F, Dong T, Zhu B, Tajima K, Yazawa K, Inoue Y (2007) Mechanical properties of comono-mer-compositionally fractionated poly[(3-hydroxybutyrate)-co-(3-mercaptopropionate)] with low 3-mercaptopropionate unit content. Macromol Biosci 7 810-819 Yu J (2001) Production of PH A from starchy wastewater via oigtuiic adds. J Biotechnol... 

Poly[3-hydroxybutyrate] was the first PHA to be produced on an industrial scale, but its brittle nature, its poor mechanical properties and its high production cost limited its application potential. In the early 1990s, Imperial Chemical Industries [ICI] started the production of poly[3-hydroxybutyrate-co-3-hydroxyvalerate] [P3HB3HV] under the trade name Biopol . This material showed lower degrees of crystallinity and superior mechanical properties. Later on, the production of Biopol was continued by Monsanto and subsequently followed up by Metabolix. PHAs were originally intended as bio-based alternatives for polyolefins used in plastic containers, films and bottles. Despite the large interest in PHAs, their application remains, however, limited due to their narrow processing window [84, 85]. 

Multilayer co-extrusion is another technique used in the preparation of starch/ synthetic sheets or films [164, 263-266], in which TPS is laminated with appropriate biodegradable polymers to improve the mechanical, water-resistance and gas-barrier properties of final products. These products have shown potential for applications such as food packaging and disposable product manufacture. Three-layer co-extrusion is most often practiced, in which a co-extrusion line consists of two single-screw extruders (one for the inner starch layer and the other for the outer polymer layers) a feedblock a coat-hanger-type sheet die and a three-roll calendering system [164]. Biodegradable polyesters such as PCL [164, 264], PLA [164, 263], and polyesteramide, PBSA and poly(hydroxybutyrate-co-valerate) [164] are often used for the outer layers. These new blends and composites are extending the utilization of starch-based materials into new value-added products. 

Avella, M., Rota, G., Martuscelli, E., Raimo, M., Sadocco, P. Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and wheat straw fibre composites thermal, mechanical properties and biodegradation behaviour. J. Mater. Sci. 35, 829-836 (2000)... 

Poly[(/ )-3-hydroxybutyrate], P(3HB), accumulated in various bacteria, is extensively studied as a biodegradable and biocompatible thermoplastic with a melting point of 180°C (Alper et al. 1963 Doi 1990 Anderson and Dawes 1990). However, it is well known that the mechanical properties of P(3HB) materials markedly deteriorate by a process of secondary crystallization, since the glass-transition temperature (T) is about 4°C (Holmes 1988 De Koning and Lemstra 1993 Scandola et al. 1989). Accordingly, P(3HB) is considered as a polymer that is difficult to use in industrial applications because of its stiffness and brittleness. 

As an example of P(3HB) copolymers, high-strength commercial-P(3HB-co-3HV) and poly[(/ )-3-hydroxybutyrate-co-(/ )-3-hydroxyhexanoate] [P(3HB-co-3HH)] fibers were produced by same method of stretching after isothermal crystallization (Tanaka et al. 2006, 2007a). The maximum draw ratio of P(3HB-co-3HV) fibers with isothermal crystallization time of 24 h was about ten times in the initial length of the sample. The mechanical properties of one-step-drawn P(3HB-co-3HV) fibers without and after isothermal crystallization are summarized in Table 4. The... 

Aoyagi Y, Doi Y, Iwata T (2003) Mechanical properties and highly ordered structure of ultra-high-molecular-weight poly [(R)-3-hydroxybutyrate] films effects of annealing and two-step drawing. Polym Degrad Stab 79 209-216... 

Yamamoto T, Kimizu M, Kikutani T, FirruhasM Y, Cakmak M (1997) The effect of drawing and annealing conditions on the structure and properties of bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) fibers. Int Polym Process XII 29-37 Yamrme H, Terao K, Hiki S, Kimirra Y (2001) Mechanical properties and higher order structure of bacterial homo poly(3-hydroxybutyrate) melt spun fibers. Polymer 42 3241-3248... 

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