The crystal structures and possible transformations in poly(ethylene naphthalate) (PEN) were examined using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The crystal cells (α- and β-form) were dependent on several factors related to their thermal histories. For PEN subjected to Tmax = 280 °C (where T max is the maximum melt temperature at which the samples were treated before crystallization), only the α-form developed regardless of the temperature of crystallization or cooling rate. However, for PEN subjected to Tmax = 300 °C, PEN developed mixed dual crystals, with greater fractions of the α-form at low temperatures of crystallization (220 °C or lower) or higher cooling rates, but higher β-form fractions at higher crystallization temperatures (>230 °C). The difference in the relative stability of these two crystals was utilized to prepare PEN containing only the α-form or only the β-form. The equilibrium melting temperatures of these two crystal forms in PEN were then measured separately. For PEN containing only the β-form, Tm0 (289.3 °C) was found by using the Hoffman-Weeks technique, while for PEN containing only the α-form, Tm0 was determined using the same technique but was found to be lower (282.6 °C).
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