Effect of comonomer content on the crystallization kinetics and morphology of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

Crystallization kinetics and morphology of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) containing 7, 10 and 18 mol% 3-hydroxyhexanoate (HHx) comonomer were investigated by differential scanning calorimetry, polarized optical microscopy and wide angle X-ray diffraction in detail in this work. The experimental results indicate that overall isothermal crystallization rates of PHBHHx copolymers are reduced with increasing crystallization temperature and HHx content; however, the crystallization mechanism remains unchanged. Moreover, the equilibrium melting point temperatures of PHBHHx copolymers decrease with increasing the HHx content. Banded spherulites morphology and spherulitic growth rates of PHBHHx have been studied in a wide crystallization temperature range. Both band spacing and spherulite growth rate decrease with increasing the HHx comonomer content. All the investigated PHBHHx copolymers exhibit a crystallization regime II to III transition, and the crystallization regime transition temperature shifts to low temperature range with increasing the HHx content. In addition, increasing the HHx content does not modify the crystal structure or crystal cell parameters but decreases the crystallinity of PHBHHx.