Thermal and morphological study of fractionated poly(3-hydroxybutyric acid-co-3-hydroxypropionic acid)

The thermal and morphological characteristics of fractionated poly(3‐hydroxybutyric acid‐co3‐hydroxypropionic acid)s (P(3HB‐CO‐3HP)) with narrow comonomer compositional distribution, as well as the natural poly(3‐hydroxybutyric acid) (P(3HB)) and chemosynthetic poly(β‐propiolactone) (PPL) were studied by means of differential scanning calorimeter (DSC) and infrared spectroscopy (IR). It is seen that melting point (Tm), heat of fusion (ΔH) and glass transition temperature (Tg) decrease with the increase of the content of 3HP units up to 60.5 mol‐%. Employing 13C NMR in the solution, the sequence distributions of the fractionated copolyesters were revealed not to be in accordance with the results of the random statistical model. The carbonyl IR absorption bands were further resolved by curve fitting into several components stemming from the bond stretchings in the different molecular environments known as the amorphous, crystalline and intermediate regions. It is demonstrated that the contributions which belong to the crystalline regions show intrinsic absorption peaks at 1724.0, 1723.8 and 1729.1 cm−1 for P(3HB), P(3HB‐CO‐3HP)s and PPL, respectively. In the case of P(3HB‐CO‐3HP)s, the relative intensities of contributions originating from the crystalline regions decrease with the increase of 3HP units up to 60.5 mol‐%. However, only the P(3HB) type of lattice structure is formed despite the variation of the 3HP unit content. Moreover, the effect of the substrates (silicon wafer and KBr) used for preparation of the IR samples on the crystallization behavior was also investigated.