TMDSC and atomic force microscopy studies of morphology and recrystallization in polyesters including oriented films

The thermal and crystal morphological properties of poly[ethylene teraphtha late] (PET) and poly(ethylene-2,6-naphthalenedicarboxylate) (PEN) biaxially oriented films were compared to amorphous and other isotropic semi-crystal line samples. Crystal melting as a function of temperature was characterize d by temperature modulated DSC (TMDSC) and found to begin just above the gl ass transition for both oriented films. About 75 degrees C above the glass transitions, substantial exothermic recrystallization begins and continues through the final melting region in oriented films. The maximum in the non- reversing TMDSC signal for the oriented films signifies the maximum recryst allization exothermic activity with peaks at 248 degrees C and 258 degrees C for PET and PEN, respectively. The final melting endotherm detected was 2 60 degrees C and 270 degrees C for PET and PEN, and is shown by the TMDSC d ata and by independent rapid heating rate melting point determinations to b e due to the melting of species recrystallized during the heating scan. The results are compared with TMDSC data for initially amorphous and melt crys tallized samples. The volume fraction of rigid species (F-rigid=total cryst al fraction plus 'rigid amorphous or non-crystalline species') were measure d by TMDSC glass transition data, and contrasted with the area fraction of rigid species at the oriented film surface characterized with very high res olution atomic force microscopy (AFM) phase data. The data suggest that the 11 nm wide hard domains in PET, and 21 nm wide domains in PEN film detecte d by AFM consist of both crystal and high stiffness interphase species.