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.