Morphological studies of polycarbonate‐poly(butylene terephthalate) blends by transmission electron microscopy

Abstract

The morphology of polycarbonate (PC)‐poly(butylene terephthalate) (PBT) blends prepared by melt‐processing in an extruder or a Brabender Plastograph was studied by Transmission Electron Microscopy. Transesterification during blending was avoided by the use of di‐n‐octadecyl phosphite, an efficient transesterification inhibitor. Ruthenium tetraoxide was used to selectively stain the PC fraction. Observation of ultrathin sections revealed the biphasic nature of the blends and the partial compatibility of the polymers in the molten state. The most striking consequence of this Is the presence, of PBT crystalline lamellae growing inside the PC‐rich phase for samples annealed in the proper temperature range. A preliminary study of the morphology as a function of composition and thermal annealing was undertaken. The RuO4 staining technique was also applied to the commercial blend Xenoy CL 100 from General Electric, which was shown to contain an MBS impact modifier (a methylmethacrylate butadiene‐styrene copolymer) exclusively located In the PC‐rich phase.

Polymer blends: the PC–PBT case

“…Delimoy et al, 44 Birley and Chen, 42 Bertilsson et al, 43 Hamilton et al, 40 Hobbs et al, 45 Kim and Burns, 29 Kong…”

“…24,31 As a consequence of this, copolymers act as compatibilisers in these blends, making it difficult to characterise the inherent miscibility of PC and PBT in melt blends. Delimoy et al 44 reported that IR spectroscopy and NMR gave a very coherent picture of the PC-PBT copolyester structure that resulted from an exchange reaction of the aliphatic ester sequences and the aromatic carbonate sequences. Some researchers believe that the transesterification reaction in PC-PBT blends occurs only at temperatures that are greater than the T m of PBT, approximately 220 uC.…”

“…has shown, nevertheless, that transesterification also occurs after ''thermal treatment'' at 200 and 210 uC (lower than the T m of PBT). The transesterification reaction is catalysed by Ti residues that are present in the PBT, 12,23,25,44 which can, furthermore, lead to the development of a yellow colour in the blend. 40 Consistent performance of PC-PBT blends can be achieved only through control of melt transesterification.…”

“…If this reaction is not controlled, the properties of the blend will change with each thermal history. The transesterification process can be suppressed by added stabilisers, 25,44 but complete suppression cannot be proven. 12,31 The problem is that the analytical detection of a very small, in situ formed copolymer content is limited by the sensitivity of methods such as NMR and FTIR, 12,31,35 which is reported to be restricted to copolymer concentrations above 1 mol%.…”

“…44 Even very low contents of block copolymers are known to influence the morphology of immiscible blends. 31,44 Furthermore, a correct estimation of the copolymer content is impossible, because the transesterification is influenced by many parameters. Additionally these parameters have an interaction.…”