Miscibility and morphology of blends of poly(3-hydroxybutyrate) and poly(vinyl butyral)

The miscibility and morphology of blends of biodegradable poly(3-hydroxybut yrate) (PHB) and poly(vinyl butyral) (PVB) were studied by differential sca nning calorimetry (DSC) and scanning electron microscopy (SEM). PHB is a se mi-crystalline polymer while PVB can be regarded as a random copolymer cont aining vinyl butyral (VB) and vinyl alcohol (VA) units whose composition ra tio can be systematically changed over a wide range. DSC measurements show two T(g)s in the amorphous phase for all PVB and PHB blends, indicating pha se immiscibility. However, partial miscibility was observed in 50/50 w/w bl ends containing PVBs with 25-36 wt% VA content, demonstrated by the inward shift of the T(g)s of the two phases relative to those of the neat componen ts. This shift exhibits a maximum value around 31 wt% of VA in PVB. The seg mental interaction parameters chi (VA,HB), chi (VB,HB) and chi (VA,VB) were determined and the copolymer composition dependence of the overall interac tion parameter between PVB and PHB, chi (12), was then calculated. This par ameter shows a minimum around 31 wt% VA content, consistent with the experi mental data. The presence of a minimum can be interpreted in terms of a cop olymer effect in which the unfavorable interaction between PVB and PHB is m inimized at a certain copolymer compositions due to the repulsive intramacr omolecular interaction between VA and VB units within the PVB chains. The c omposition manifesting minimum immiscibility was also consistent with the m elting temperature (T-m) depression and a minimum in the fractional crystal linity of the PHB phase in the blends around this VA content. A co-continuo us morphology was observed in the blends containing 25-33 wt% VA content PV Bs, which might well result from more favorable interactions between the tw o component polymers at this copolymer composition. (C) 2001 Published by E lsevier Science Ltd.