Measuring Thermal Crystallinity in PET
The mechanical properties of poly (ethylene terephthalate) (PET) depend on the degree of crystallinity. PET crystallizes from both thermal and strain-induced process paths. Strain-induced crystallization occurs under uniaxial or biaxial stretching, the latter is common in stretch blow molding. Thermal crystallization takes place when cooling from the melt or under sustained elevated temperature which are both common during injection and blow molding. The current work focuses on comparing methods used to quantify the percent thermal crystallinity in injection molded water bottle preforms. An injection molded preform with a gradient of induced thermal crystallinity was examined using Differential Scanning Calorimetry (DSC), Micro X-ray Diffraction (μXRD), and Raman Spectroscopy. Correlating these three techniques for percent thermal crystallinity, helps with the development of nondestructive crystallinity measurement methods to aid with process optimization and part quality.
Structure of terephthalate polymers I—Infra-red spectra and molecular structure of poly(ethylene terephthalate)
The ‘rigid’ crystalline model for poly(ethylene terephthalate) is not wholly consistent with the inra-red spectra of PET and model compounds, including polarized spectra in the far infra-red (600-20 cm−1). The results indicate that the extent of rotation of the glycol fragment about the terephthalate framework in the PET crystal appears to be greater than in the above model. The space group P1 is, however, conserved. There appears to be more rotational hindrance in the crystal than in the amorphous polymer about the CH2O and CO bonds and the existence of the discrete trans and gauche glycol configurations has been confirmed. The degree of crystallinity of PET is correlated with the trans glycol residue and a planar terephthalate framework. Conversely the amorphous content is correlated with the gauche form of the glycol residue and an accompanying distortion of the terephthalate framework from Vh to C2v symmetry. Assignments for the vibrational modes of the terephthalate framework have been made on the basis of Vh and C2v symmetries for the trans and gauche isomers in PET, respectively, using information from the related poly(cyclohexane-1,4-dimethylene terephthalate). The assignments of the vibrational modes for the ethylene glycol fragment on the basis of C2h symmetry (trans isomer) and C2 symmetry (gauche isomer) are also given together with assignments for the principal stretching, deformation and torsional modes of the backbone chain for each configuration. These are found to be in reasonable agreement with calculated values.
HRTEM study of orthorhombic zirconia in MgO-PSZ
High-resolution transmission electron microscopy (HRTEM) was used to study the phase of orthorhombic ZrO2 formed in magnesia partially stabilized zirconia (MgO-PSZ) during HRTEM specimen preparation. Based on the three reported crystal structures of orthorhombic ZrO2, with the space groups Pbcm, Pbc21 and Pbca, here it is shown that orthorhombic ZrO2 formed in MgO-PSZ has the Pbcm structure.