Crystallization and Equilibrium Melting Behavior of PBT/PETG Blends D. NABI SAHEB, J. P. JOG Polymer Science and Engineering Group, Chemical Engineering Division, National Chemical Laboratory, Pune 411008, India Received 24 November 1998; revised 12 April 1999; accepted 10 May 1999 ABSTRACT: The results of studies of equilibrium melting point and crystallization behavior of PBT/PETG blends are reported for the first time. A single composition- dependent glass-transition temperature is observed in the DSC studies. The isothermal crystallization studies of the blends indicate retardation in crystallization rate as evidenced by the increase in crystallization half time. The retardation in crystallization rate has been attributed to the miscibility in the molten state and the hindrance to the diffusion of crystallizable units. This assumption is further supported by the composi- tion dependence of the crystallization half time. A composition-dependent melting point depression has been observed which has been attributed to the possible thermodynamic and morphological effects. The interaction parameter calculated by analyzing equilib- rium melting point depression shows composition-dependent negative values confirm- ing the miscibility of the systems. These results are in good agreement with our earlier results on mechanical and dynamic mechanical properties of PBT/PETG blends. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2439 –2444, 1999 Keywords: PBT; PETG; crystallization; isothermal; equilibrium melting point INTRODUCTION The crystallization of polymer blends is a very complex process as the crystallization of compo- nent polymers is modified and consequently, a variety of behaviors can be envisaged. 1–2 Depend- ing upon the interaction between the polymers, interesting effects, such as depression of melting point/equilibrium melting point, increase/de- crease of crystallinity and the rate of crystalliza- tion, and changes in morphological structures can be observed. In semicrystalline and noncrystal- line blends, such changes are attributed to misci- bility and/or to the transport of the noncrystalliz- able units (amorphous) across the crystallizable units/crystals at the interface in the melt and diffusion process. Polybutylene terephthalate (PBT) is one of the engineering polyesters with a high crystallization rate. It has been blended with other engineering polymers such as polyesters, polyarylate, and polycarbonate. 3–10 These blends are of special in- terest as they exhibit miscibility and other fea- tures corresponding to the observed miscibility such as melting point depression, reduced crys- tallization rate, etc. In our earlier communication we have noted that the PBT/PETG (an amor- phous copolyester) blends exhibit miscibility as evidenced by the single composition-dependent glass-transition temperature and enhanced prop- erties due to interaction between the polymers. 11 In addition, the DSC scans (melting and crystal- lization) show significant differences in the melt- ing and crystallization temperatures for blended samples. However, the changes observed were un- der quasi-equilibrium conditions. It was therefore interesting to study the equilibrium melting be- havior of PBT in PBT/PETG blends and the effect Correspondence to: J. P. Jog (E-mail: jyoti@che.ncl.res.in) Journal of Polymer Science: Part B: Polymer Physics, Vol. 37, 2439 –2444 (1999) © 1999 John Wiley & Sons, Inc. CCC 0887-6266/99/172439-06 2439