Straightforward determination of equilibrium heat of fusion and interaction parameter for a Polymer-Diluent system

Melting point depression approach is a common method to determine equilibrium heat of fusion (& UDelta;H0m) and interaction parameter (& chi;) for a polymer-diluent system. Using melting temperatures of dried crystals with similar thickness prepared from a polymer with different degrees of dilution (dry melting temperature, DMT) rather than that for the undiluted polymer (normal melting temperature, NMT) substantially reduces the error arising from using non-equilibrium instead of equilibrium melting temperature (T0m). However, DMT measurement is a challenging process, because it needs selection of a diluent with particular features and doubles the number of experiments and material consumption. The results of this work for poly(3-hexylthiophene) (P3HT)-3-hexylthiophene (3HT) pair revealed NMT and DMT difference depended on the cooling rate (CR) of the crystallization process and showed a non-symmetrical bell-shape behavior. Nearly acceptable values of & UDelta;H0m and & chi; were obtained via performing crystallization at a low CR (1 degrees C min-1), regardless of using either DMT or NMT. A critical CR was introduced in which DMT equalled NMT. Performing crystallization at this CR guarantees obtaining correct values of & UDelta;H0m and & chi; with no need to use T0m s or DMT data. Finally, & chi; was predicted theoretically using Flory-Huggins and compressible regular solution models.