X-RAY-SCATTERING AND THERMAL-ANALYSIS STUDY OF THE EFFECTS OF MOLECULAR-WEIGHT ON PHASE-STRUCTURE IN BLENDS OF POLY(BUTYLENE TEREPHTHALATE)WITH POLYCARBONATE

Blends of Poly(butylene terephthalate), PET, with Polycarbonate, PC, w ere studied for a range of molecular weights and blend compositions. B lends were available in PBT/PC compositions 80/20 and 40/60, and with M(w) designated by H (high) or L (low). Samples were prepared by melt crystallization, or by cold crystallization following a rapid quench f rom the melt. Addition of PC reduces the crystallization kinetics of P ET so that the resulting crystals are more perfect than those which fo rm in the homopolymer. Degree of crystallinity of the blends followed the rank ordering: L/L > L/H > H/L = H/H. The glass transition behavio r was investigated using dynamic mechanical analysis (DMA) and modulat ed differential scanning calorimetry (MDSC). All blends exhibited two glass transitions at intermediate temperatures between the T(g)s of th e homopolymers, indicating existence of a PET-rich phase and a PC-rich phase. Blends L/L were most, and H/H the least, miscible. Small-angle X-ray scattering was performed at room temperature on cold crystalliz ed blends, or at elevated temperature during melt crystallization. The long period was consistently larger, and the linear stack crystallini ty was consistently smaller, in blends L/L or H/L. These results indic ate that in blends containing low M(w) PC, there is more PC located wi thin the PET-rich phase. The long period was consistently smaller in c old crystallized samples, while the linear stack crystallinity was nea rly the same, regardless of melt or cold crystallization treatment. Re duction of the average long period in cold crystallized samples could result from crystallization of PET within the PC-rich phase. This is c onsistent with thermal analysis results, which indicate that cold crys tallized samples have greater overall crystallinity than melt crystall ized samples. A hypothetical liquid phase diagram is presented to expl ain the differences between melt and cold crystallized blends. (C) 199 6 John Wiley & Sons, Inc.