Polymorphism behavior of poly(ethylene naphthalate)/clay nanocomposites: Role of clay surface modification

The influence of clay surface modification on the polymorphism behavior of poly(ethylene naphthalate) (PEN)/clay nanocomposites was investigated via in situ Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction. The results show that untreated clay has a heterogeneous nucleating effect on PEN and favors the β-crystal form, while the surfactant 1-hexadecyl-2,3- dimethylimidazolium (IMC16) has a plasticization effect and tends to enhance the kinetically favored α-phase instead. In contrast, the nanocomposite (PEN/IMC16-MMT) formed from IMC16-treated clay (IMC16-MMT) exhibits a strong temperature-dependent polymorphic behavior, with the β-phase being more favored at 200 °C, but the α-phase being preferred instead at 180 °C. In situ FTIR spectroscopy of PEN/IMC16-MMT reveals an abrupt change in the concentration of α- and α-"crystalline conformers" between the two temperatures during the induction period of crystallization. This is attributed to the hindered formation of stable nuclei at the organoclay surface. In addition, surfactant degradation gives rise to a highly plasticized polymer/organoclay interface. The combination of the hindered heterogeneous nucleation and plasticization effects gives rise to the unique temperature-dependent polymorphism behavior in PEN/IMC 16-MMT.