Thermal analysis and Raman spectroscopic studies of crystallization in poly(ethylene 2,6-naphthalate)

The combination of Fourier transform Raman spectroscopy and thermal analysis has been proved to be adequate for the study of the quantitative structural changes which take place in amorphous poly(ethylene 2,6-naphthalate) on annealing. Different conformer contents were found in the annealed samples depending on annealing conditions. In general, annealing of the amorphous poly(ethylene 2,6-naphthalate) from the glassy state induces a conformational transition of gauche to trans. The structure obtained during crystallization is characterized by a three-phase conformational model, including an amorphous phase, a rigid amorphous phase and a crystalline phase. The crystallization is further characterized by a three-zone process, firstly a primary crystallization process, secondly a variation of the rigid amorphous phase with a constant value of the crystalline phase and thirdly a secondary crystallization process. The bandwidth at half intensity at 1721 cm−1 in the Raman spectrum varied between 32 cm−1 for the complete amorphous phase and 7 cm−1 for the total rigid phase, the sum of the rigid amorphous and crystalline phase. The bandwidth at half intensity at 1721 cm−1 was directly related to the amount of the total rigid phase and confirmed by the variation of the heat capacity increase at the glass transition temperature. Two complementary bands in the Raman spectrum, at 1107 and 1098 cm−1, were found to be related to the trans and gauche isomers. A difference was measured between the total trans content and the amount of rigid phase due to the presence of some trans conformations in the amorphous phase. The extrapolation of the bandwidth at half intensity at 1721 cm−1 to the value of zero, corresponding to the complete crystalline phase, gave a melting enthalpy of 196 J/g and the corresponding density of the crystalline phase was 1.4390 g/cm3. A complete rigid phase structure was obtained by a melting enthalpy of 144 J/g and a density of 1.4070 g/cm3.