The effects of transesterification on structure development in polycarbonate - poly(butylene terephthalate) blends.
A series of polycarbonate-poly(butylene terephthalate) blends has been formed via
melt blending in a torque rheometer. Initially polycarbonate and poly(butylene
terephthalate) were blended alone to determine the extent to which the two
homopolymers were able to cross-react (transesterify), and hence, how the
morphology of the blends developed as a result of transesterification.
Subsequently the degree of transesterification between the two homopolymers was
controlled by the incorporation of an alkyl titanium catalyst, and the effect on
morphological development determined. Resultant uncatalysed and catalysed
materials were characterised by DSC, DMA, FTIR, microscopy, solubility studies
and X-ray scattering.
No evidence for direct inter-chain transesterification was seen for the uncatalysed
blends. No new thermal transitions or absorptions were noted from DSC, DMA or
FTIR, nor was there any evidence for phase refinement from microscopy, or
variation in solubility behaviour. However, there was evidence for some degree of
phase mixing from DMA damping behaviour, indicated by convolution of the PC
and PBT transition peaks. Due to the lack of transesterification relatively coarse
morphologies, indicative of an immiscible two phase blend, were apparent from
microscopical analysis. Thermal behaviour of the uncatalysed blends showed
evidence of thermal degradation above 270°C, promoting transesterification via
As the degree of transesterification was increased (catalysed materials) the
composition of the blends became increasingly complex, comprising mixtures of
the homopolymers and various AB-type block copolymers of polycarbonate and
poly(butylene terephthalate), with concomitant changes in their thermal behaviour.
DSC and FTIR proved useful in analysing blends containing greater than 150 ppm
of added catalyst, whilst DMA highlighted the subtle differences in the blends with
less than 200 ppm additional titanium transesterification catalyst. Microscopical
analysis provided visual evidence for the transformation in the materials
morphology through progressive transesterification: the relatively coarse structure
characteristic of two phase blends developing into a more refined sub-micron
structure exhibited by blends containing a significant volume of interphase material.
This morphological change was due to the formation of increasing concentrations
of random block copolymers. At increasing degrees of transesterification this
change in morphology was accompanied by variations in the solubility of the
blends. With increasing amounts of additional catalyst up to 150 ppm, the blends
became increasingly resistant to solvent (dichloromethane). Whereas above this
level the blends became increasingly soluble, as copolymers of a more random
nature (with increased solubility) were formed.
X-ray scattering showed the pure PBT to exhibit highly reproducible crystallisation
behaviour. In contrast a 50/50 blend showed a progressive reduction in the degree
of crystallinity, melting and crystallisation temperatures with increasing