The effect of flexible chain length on thermal and mechanical properties of poly(m-methylene 2,6-naphthalate)s

The effect of flexible chain length on the thermal and mechanical properties such as melting temperature, glass transition temperature, dynamic mechanical relaxation behavior, and uniaxial tensile deformation for melt-quenched poly(m-methylene 2,6-naphthalate) (PmN) films was investigated using differential scanning calorimeter (DSC), dynamic mechanical thermal analyzer, and universal tensile machine. It was found from DSC thermograms that PmNs with even number of methylene group have higher melting temperatures and faster crystallization rates than PmNs with odd number of methylene group, showing an odd–even fluctuation. The plots of tan δ versus temperature show that all PmN samples have three relaxation processes (β, β∗, and α) regardless of the number of methylene group in their backbone. It was found that both β∗- and α-relaxations are cooperative processes and that the activation energies of both relaxations as well as the glass transition temperature associated with the α-relaxation show odd–even fluctuations as a function of the number of methylene group. The initial tensile modulus at the low drawing rate of 0.15 cm/min also shows an odd–even fluctuation. In summary, the macroscopic thermal and mechanical properties of PmN such as melting temperature, glass transition temperature, crystallization rate, activation energies of α- and β∗-relaxations, and initial modulus measured under a slow drawing rate exhibit odd–even fluctuations as the number of methylene group in PmN increases.