Mechanical Properties and Higher Order Structures of the Bacterial Polyester Highly Oriented Films

Abstract

IntroductionPoly(3-hydroxybutyrate)(PHB) is produced by a wide variety of microorganisms as intracellular carbon and energy storage material [1], and has been extensively studied as a biodegradable and biocompatible thermoplastic [2][3][4]. However, it is well known that the processing of PHB and its copolymers into fibers, films, and other products is very hard. These polyesters have a glass transition point below room temperature, and the crystallization rate is quite low [5]. Because of these properties, the melt cools down to the room temperature without crystallization and the spherulitic crystallization occurs after processing resulting in the brittle character. Further the defect in the spherulites formed during the secondary crystallization deteriorates the mechanical properties [6,7].Several groups have attempted to improve the mechanical properties of PHB solution cast films. Holmes tried to obtain uniaxially and biaxially oriented films of PHB to improve the mechanical properties [4]. However, these films were too brittle to draw either uniaxially or biaxially to high ratio. De Koning et al. have reported that the stiffness and brittleness of PHB film was avoided by annealing at elevated temperatures [8]. This procedure seems to eliminate the defect in the spherulites. Barham and Keller [9] reported that the ductility of PHB films can be significantly improved by the application of cold drawing. The elongation at break measured at room temperature increases up to 300 % after 300 % of elongation by rolling, while as compression molded PHB film shows the elongation at break as low as 7 %. One can apply similar process to obtain highly oriented PHBH films with good mechanical properties.In this study, the roll drawing was applied to the compression molded poly ( 3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) films. While brittle spherulite structure tends to break easily by extension, it is possible to make a crystal orientation to some extent, without breaking in roll drawing. Roll drawn PHBH films are ductile and can be stretched to high extension ratio without breaking. Films obtained were further annealed at an elevated temperature. The objective of this study is to investigate the changes in mechanical properties and higher order structures of PHBH films during these processes.Abstract : Bacterial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)(PHBH) highly oriented films were prepared by the combination of roll and uniaxial drawing processes. Compression molded films, which is rather brittle, turned to be ductile after roll drawing up to x 2, and the films were further uniaxially drawn to a draw ratio as high as 1000 %. The PHBH film with the tensile strength of 178 MPa and the modulus of 1.4 GPa were obtained. The roll drawn PHB film showed a slight crystalline orientation of α-form. The orientation advanced with uniaxial draw ratio and the formation of β-form crystal was observed. Relations among the processing conditions, higher order structures and mechanical properties were investigated using...

Mechanical properties and higher-order structures of biaxially drawn bacterial poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanote] films

“…Irrespective of the roll drawing condition, the highest biaxial draw ratio was achieved at 80 o C. Roll draw ratio also affected the biaxial draw ratio achieved similarly to the results obtained for the uniaxial drawing process. 8 The highest biaxial draw ratio was achieved to the film roll drawn up to ×1.4, although the optimum roll draw ratio for the uniaxial drawing was ×2.0. The films roll drawn up to the ratio higher than ×1.4 tended to break easily in the transverse direction.…”

“…This tendency in the mechanical property has been obtained for the uniaxially drawn PHBH films. 8 In uniaxial drawn PHBH films, the tensile strength and modulus increased monotonically and the elongation at break decreased with increasing uniaxial draw ratio. Crystal deformation and orientation to only MD occurred in the uniaxial drawing process.…”

“…5 Because of these properties, the melt cools down to the room temperature without crystallization and the spherulitic crystallization occurs after processing resulting in the brittle character. 6,7 We have reported that poly(3-hydroxybutyrate-co-hydroxyhexanoate) (PHBH) films with superior mechanical properties and high ductility can be obtained by a uniaxial drawing after applying a roll drawing process 8 . The tensile strength and modulus of 178 MPa and 1.4 GPa, respectively were achieved.…”

Fiber structure formation in melt spinning of bio-based aliphatic co-polyesters

“…al. [3]. When the extrusion temperature was high, crystallization rate of PHBH was low and molten polymer in the spinning line reached to the take-up bobbin without crystallization, i.e.…”

“…Yamane et al reported that the PHBH fibers can be obtained at the extrusion temperature of 160 °C [3]. They indicated that the crystallization rate was faster if the extrusion temperature was lower than the melting temperature of pure PHB (176°C) and discussed that a certain amount of nucleus was remaining in the polymer melt at this temperature.…”