Crystallization kinetics and miscibility of blends of polyhydroxybutyrate (PHB) with ethylene vinyl acetate copolymers (EVA)

A series of ethylene vinyl acetate random copolymer EVA, with vinyl acetate (VA) varied from 9 to 91m%, was investigated by differential scanning calorimetry DSC and polarized optical microscopy. Biodegradable polymer blends of polyhydroxybutyrate PHB and EVA, having VA in EVA in range from 40m% till 91m%, were prepared by film casting from a chloroform solution. The miscibility and crystallization behavior of these blends were investigated. The isothermal crystallization behaviors of PHB and PHB/EVA blends are discussed in terms of the half time of crystallization t(1/2). Experimental results indicated that blends of PHB/EVA91 are completely miscible blend in the entire (0 to 100 m%) compositional ranges. Blends PHB/EVA, for VA varied from 40 till 70 m% are immiscible as evidenced by the existence of unchanged composition independent glass transition temperatures (T-g), crystallization and melting behavior. The isothermal crystallization of PHB blends was investigated from room temperature till 130 degrees C. 80 degrees C was found to be the best temperature for comparison of different blends. At 80 C t(1/2) strongly depends on the content of VA in PHB/EVA blend, mainly due to differences in miscibility as well as due to differences in segmental mobility as identified by differences in glass transition temperature. Since both components in PHB/EVA80, pure PHB and pure EVA80, have glass transition temperatures close to 0 degrees C, it is difficult to decide its miscibility from Tg. However from the strong dependence of the value of crystallization half time t(1/2) of PHB/EVA80 on blend composition, it was possible to reasonably infer that PHB/EVA80 is partially miscible.