Miscibility with thermal and mechanical properties for poly(pentamethylene 2,6-naphthalate) and poly(heptamethylene 2,6-naphthalate) blends

We have prepared the blends of poly(pentamethylene 2,6-naphthalate) (PPN) with poly(heptamethylene 2,6-naphthalate) (PHepN) by solution blending method and investigated their glass transition behaviour, melting behaviour, and tensile properties. It was observed that the blends of PPN/PHepN(9/1) and PPN/PHepN(1/9) have a single glass transition, reflecting a homogeneous phase, whereas those of PPN/PHepN(7/3), PPN/PHepN(5/5), and PPN/PHepN(3/7) exhibit double glass transitions, representing the existence of two phases. The PPN homopolymer annealed below 90 °C shows triple melting peaks (Tm1, Tm2, and Tm3). It was proved that Tm1 is attributed to melting of thin lamellar formed during secondary crystallization process, Tm2 to melting of thick lamellar created during primary crystallization, and Tm3 to melting of crystals recrystallized after melting the primary crystals at Tm2. For the annealed PHepN homopolymer, double melting endotherms (Tm1 and Tm2) were observed, caused by dual lamellar population with different thickness, i.e. Tm1 corresponding to the melting of secondary crystal and Tm2 to primary one. The Hoffman-Weeks plots, applied to the melting of primary crystals (Tm2s), indicate that the equilibrium melting temperatures of PPN homopolymer, PPN/PHepN(9/1), and PPN/PHepN(7/3) blends are same to be 147 °C, and those of PHepN homopolymer, PPN/PHepN(1/9), and PPN/PHepN(3/7) blends to be 145 °C. Both the glass transition and melting behaviours demonstrate that the PPN/PHepN blend system is partially miscible. In addition, both the modulus and strength for the blends almost follow additive rule against blend composition, indicating that the PPN/PHepN blends are mechanically compatible over all blend compositions.