Polymorphic and miscibility behavior in crystalline/crystalline blends of poly(pentamethylene terephthalate) with poly(heptamethylene terephthalate)

BACKGROUND: The phase behavior of blends of semicrystalline aryl polyesters with long methylene segments (? (CH₂)_n? with n = 5 or 7) in the repeat units has not been much studied. Thus, crystalline/crystalline blends comprising monomorphic poly(pentamethylene terephthalate) (PPT) and polymorphic poly(heptamethylene terephthalate) (PHepT) were prepared and the crystal growth kinetics, polymorphism behavior and miscibility in this blend system were probed using polarized‐light optical microscopy, differential scanning calorimetry and wide‐angle X‐ray diffraction. RESULTS: The PPT/PHepT blends of all compositions were first proven to be miscible in the melt state or quenched amorphous phase, whose interaction strength was determined (χ₁₂ = ? 0.35), showing favorable interactions and phase homogeneity. Although the spherulites of neat PPT and PHepT could exhibit ring bands at different crystallization temperature (T_c) ranges (100–110 and 50–65 °C, respectively), the spherulites of PPT/PHepT (50/50) blend became ringless in the range 50–110 °C. Growth analysis and polymorphic behavior in the crystalline phases of the blends provided extra evidence for the miscibility between these two crystalline polymers. Spherulitic growth rates of PPT in the PPT/PHepT blends were significantly reduced in comparison with those of neat PPT. In addition, miscible blending of a small fraction of monomorphic PPT (20 wt%) with polymorphic PHepT altered the crystal stability and led to the originally polymorphic PHepT exhibiting only the β‐crystal form when melt‐crystallized at all values of T_c. CONCLUSION: The highly intimate mixing in polymer chains of crystalline PPT and PHepT causes significant disruption in ring‐band patterns and reduction in crystallization rates of PPT as well as alteration in the polymorphic behavior of PHepT. Copyright © 2009 Society of Chemical Industry