Abstract
The first polyhydroxyalkanoate (PHA) block copolymer poly(2-hydroxybutyrate-b-3-hydroxybutyrate) [P(2HB-b-3HB)] was previously synthesized using engineered Escherichia coli expressing a chimeric PHA synthase PhaCAR with monomer sequence-regulating capacity. In the present study, the physical properties of the block copolymer and its relevant random copolymer P(2HB-ran-3HB) were evaluated. Stress–strain tests on the P(88 mol% 2HB-b-3HB) film showed an increasing stress value during elongation up to 393%. In addition, the block copolymer film exhibited slow contraction behavior after elongation, indicating that P(2HB-b-3HB) is an elastomer-like material. In contrast, the P(92 mol% 2HB-ran-3HB) film, which was stretched up to 692% with nearly constant stress, was stretchable but not elastic. The differential scanning calorimetry and wide-angle X-ray diffraction analyses indicated that the P(2HB-b-3HB) contained the amorphous P(2HB) phase and the crystalline P(3HB) phase, whereas P(2HB-ran-3HB) was wholly amorphous. Therefore, the elasticity of P(2HB-b-3HB) can be attributed to the presence of the crystalline P(3HB) phase and a noncovalent crosslinked structure by the crystals. These results show the potential of block PHAs as elastic materials.
Highlights
Bacterial polyhydroxyalkanoates (PHAs) are biobased polyesters that can be used as commodity plastics[1] and have attracted considerable interest because of their superior biodegradability in a range of different natural environments, including the s ea[2]
PHA synthases play a central role in the biosynthesis of 2HA-containing PHAs
We previously reported the class I PHA synthase PhaCAR that can efficiently incorporate 2HB u nits19. PhaCAR is an engineered chimeric enzyme, composed of N- and C-terminal regions of Aeromonas caviae and Ralstonia eutropha (Cupriavidus necator) PHA synthases, respectively20. PhaCAR produces a copolymer of 2HB and 3HB in Escherichia coli
Summary
Bacterial polyhydroxyalkanoates (PHAs) are biobased polyesters that can be used as commodity plastics[1] and have attracted considerable interest because of their superior biodegradability in a range of different natural environments, including the s ea[2]. Block copolymers are generally known to possess characteristic and useful properties, and used in the broad range of applications, such as e lastomers[21], drug delivery[22] and lithography[23,24]. For this reason the biosynthesis of block PHAs has attracted great research interest[25]. A typical attempt to generate a block copolymer is to switch the monomer precursors in the medium during PHA p roduction[26,27]. It was demonstrated that P(2HB-b-3HB) can be processed into an elastomer-like material
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