Comparative studies of structural, thermal, mechanical, rheological and dynamic mechanical response of melt mixed PHB/bio-PBS and PHBV/bio-PBS blends

PHB/bio-PBS and PHBV/bio-PBS blends have been comparatively evaluated for their thermo-mechanical, morphological and rheological properties. Differential scanning calorimetry indicated crystallisation of PHB and PHBV to be largely restricted, whereas the thermal stability of PHB and PHBV blends marginally increased upon blending with bio-PBS. The immiscibility of the two components of two blends influenced the morphological changes from domain-dispersed to co-continuous type where the domain sizes of the dispersed phase bio-PBS increased in tune with compositions. Dynamic mechanical analysis showed distinct glass transition relaxation peaks of two components whereas WAXD studies confirmed a sharper decline in crystallisation of the PHB/bio-PBS based blends than in PHBV/bio-PBS systems, though the lamellar sizes estimated from Scherrer’s equation. At higher frequencies, the non-Newtonian melt flow dynamics of blends showed a sharp drop in their complex viscosity attributed to alignment of droplets along the flow direction (shear) due to phase separated morphology. Constitutive modelling of complex viscosity indicate the polymer melts to follow Carreau-Yasuda model, where the zero-shear viscosity decreased with bio-PBS content, irrespective of the nature of the matrix. Application of Cox-Merz rule has lead to estimations that PHB and PHBV matrices could give rise to co-continuous morphologies at a 35 wt. % and 41 wt. % of bio-PBS contents respectively.