The effect of aging on the mechanical properties was investigated for a series of bacterial copolyester
poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HH)] with 3HH molar fraction ranging from 5 to
18 mol %. The mechanism behind the aging phenomenon was studied by using gel permeation chromatography
(GPC), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray
scattering (SAXS) in parallel to the tensile testing. It was found that the high initial crystallinity and the narrower
amorphous region in the P(3HB-co-3HH) films are the key factors that cause the secondary crystallization in the
aging process to embrittle the films of copolymers with 3HH fraction lower than 10 mol %. For the copolymers
with 3HH fractions higher than 10 mol %, because of the intrinsically lower crystallinity and thicker amorphous
region, the secondary crystallization cannot affect the mechanical properties significantly.
A biodegradable, immiscible poly(butylenes adipate‐co‐butylenes terephthalate) [P(BA‐co‐BT)]/poly(ethylene oxide) (PEO) polymer blend film with compositional gradient in the film‐thickness direction has been successfully prepared in the presence of a low‐molecular‐weight compound 4,4′‐thiodiphenal (TDP), which is used as a miscibility‐enhancing agent. The miscibilities of the P(BA‐co‐BT)/PEO/TDP ternary blend films and the P(BA‐co‐BT)/PEO/TDP gradient film were investigated by differential scanning calorimetry (DSC). The compositional gradient structure of the P(BA‐co‐BT)/PEO/TDP (46/46/8 w/w/w) film has been confirmed by microscopic mapping measurement of Fourier‐transform infrared spectra and dynamic mechanical thermal analysis. We have developed a new strategy for generating gradient‐phase structures in immiscible polymer‐blend systems by homogenization, i.e., adding a third agent that can enhance the miscibility of the two immiscible polymers through simultaneous formation of hydrogen bonds with two component polymers.
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