MODULATED DIFFERENTIAL SCANNING CALORIMETRY STUDY OF BLENDS OF POLY(BUTYLENE TEREPHTHALATE) WITH POLYCARBONATE

Modulated differential scanning calorimetry (MDSC) was used to study t he glass-transition relaxation behavior in blends of poly(butylene ter ephthalate) (PET), a semicrystalline polymer, with polycarbonate (PC), an amorphous polymer. Using a temperature-modulated differential scan ning calorimeter (TM-DSC), a sinusoidal temperature oscillation was su perimposed upon the underlying linear temperature ramp. The reversing, total, and non-reversing heat flow curves were then analyzed We exami ned the efficacy of modulated differential scanning calorimeter (MDSC) to extract glass transitions (T-g) when these were covered over by ra pid cold crystallization occurring in the same temperature range. Blen ds were available in PET/PC compositions of 80/20 and 40/60, and with high and low molecular weight, M-w, designated 'H' or 'L', respectivel y. Samples of very low initial crystallinity were prepared by rapid qu enching from the melt. These samples crystallized immediately during t he MDSC scan producing complex exothermic peaks when the scanning temp erature increased over T-g. All blends exhibited a lower glass transit ion assigned to the PET-rich phase. The upper glass transition, assign ed to the PC-rich phase, was never observed in 80H/20L, 80L/20L, or 40 L/60L. This suggests that PC-L has better miscibility with amorphous P ET, while producing a very broad, indistinct grass transition in the P C-rich phase. The Fox equation was used to determine the mass fraction composition of the two phases, and confirms that better miscibility i s achieved when low molecular weight components are blended. Higher cr ystalline blends were prepared by melt crystallization. The size of th e glass-transition step was greatly reduced in the melt crystallized b lends compared to the quenched blends. Nonetheless, MDSC was used succ essfully to observe dual glass transitions at intermediate temperature s between the T(g)s of the homopolymers for all melt crystallized blen ds, except 80L/20L. Analysis of the lower T-g indicates better amorpho us phase miscibility in blends with PBT-L and PC-L; analysis of the up per T-g indicates better amorphous phase miscibility in blends with PB T-L. (C) 1997 Elsevier Science B.V.