Effects of isothermal crystallization on the mechanical properties of a elastomeric medium chain length polyhydroxyalkanoate

In the present study, the relationship between molecular structure and mechanical properties for a medium chain length polyhydroxyalkanoate (mcl-PHA) composed of 3-hydroxyoctanoate and 3-hydroxyhexanoate was elucidated. The mcl-PHA was crystallized from the melt at four different temperatures between its glass transition and melting point (37, 21, 3 and −21 °C) and its molecular structure was analysed by means of differential scanning calorimetry (DSC) and wide-angle X-ray diffractometry (WAXD). The mechanical properties, which were analysed via tensile-tests and dynamic mechanical analysis (DMA), were clearly affected by the selected crystallization temperature and corresponding molecular structure of the polymer. In this sense, samples crystallized at 37, 21 and 3 °C displayed higher secant moduli calculated at 2% (E_2% ∼ 20 MPa) than the sample crystallized at −21 °C (E_2% ∼ 7 MPa) due to their higher crystallinity. Even if samples crystallized at 37, 21 and 3 °C had very similar degree of crystallinity, their secant moduli calculated at 50, 100 and 200% (E_50%, E_100% and E_200%) and yield strength (σ_y) were clearly affected by the selected crystallization temperature, showing a positive correlation (i.e., higher crystallization temperatures and corresponding more ordered crystalline domains with narrower crystal distributions resulted in higher E_50%, E_100% and E_200% values).