Isomerization effect on the heat capacities and phase behavior of oligophenyls isomers series

•Isomerization effect on the heat capacity of the polyphenyls isomers series.•Isomerization effect on solid–liquid equilibrium, enthalpies and entropies of fusion.•Why higher melting temperature for the para than for the ortho/meta series?•Thermal Behavior of the polyphenyls isomers series.

In this work an analysis of the effect of ortho, meta and para isomerization and the number of phenyl rings on the heat capacity and phase behavior of oligophenyls, n(Ph), from 3 up to 5 is presented. The phase behavior from 298.15 K to the melting temperature was analysed by differential scanning calorimetry (DSC) and the temperature, enthalpies and entropies of fusion along the oligophenyls series were determined. With the exception of terphenyl isomers, the meta oligophenyls series show lower melting temperatures and higher entropies of fusion. Heat capacities, at T = 298.15 K, were determined for the ortho and meta quaterphenyl and p-quinquephenyl oligomers by means of high precision drop calorimetry. The heat capacities of the ortho and meta isomers are systematically lower than those of the corresponding para isomers. It was found that the subtle isomerization effect on the heat capacity is partially related to the ring rotation restrictions in their supramolecular structure and increases with the increase of the number of phenyl rings.