Elizabeth A. Lofgren

Microstructure of amorphous and crystalline poly(ethylene terephthalate)

The microstructures of amorphous and crystalline poly(ethylene terephthalate) (PET) homopolymers have been determined in terms of their trans and gauche conformational isomer contents by using a combination of infrared and density characterization techniques. The effects of isothermal crystallization (from the glassy state between 105–150°C), as well as the effects of different monomer units in the polymerization process, have been investigated. Results indicate that samples, polymerized from different monomer and catalyst systems, show different microstructures in terms of trans and gauche isomers.These variations result in significant differences in PET optical properties. Further investigations find that these dissimilar behaviors accompany conformational isomer variations in the amorphous phase, suggesting different transformation mechanisms of trans and gauche isomers at early stages of crystallization. These unlike microstructural transformation processes give rise to further changes, which are evident in terms of the intensity of Vv light scattering, haze values, thermal properties, and FTIR spectral results.

The effect of particle size distribution on the dispersion of nanoclays in poly(ethylene terephthalate)/clay nanocomposites

The effects of clay particle size distributions on their dispersion in poly(ethylene terephthalate) (PET)/clay nanocomposites have been studied. A two-step centrifugation method was used to remove large particles from commercial montmorillonite (MMT). Scanning electron microscopy of aqueous dispersions of MMT and centrifuged clay (CMMT) illustrated that average particle size of CMMT in water is much lower than that of MMT in water. PET/clay nanocomposites with MMT and CMMT were prepared by using a modified melt-blending method of mixing PET and a clay–water slurry instead of direct introduction of dried clay. Transmission electron microscopy and X-ray diffraction results showed that CMMT with their smaller particles have better dispersion into the polymer matrix than uncentrifuged MMT. Melt viscosity results revealed that good dispersion of CMMT into the PET matrix reduces the effect of hydrolytic degradation of PET in the presence of water. Differential scanning calorimetry results indicated that the particle size distribution of nanoclays and consequently, the morphology of nanocomposites have a key role on the nucleating effect of clay particles and crystallinity of final samples. Nanoclays with smaller particle size distributions exhibit better dispersion of particles in the polymer matrix and this decreases their nucleating effects and the crystallinity of nanocomposites.

Shrinkage Behavior of Oriented Poly(Ethylene Terephthalate)

The kinetics of thermal shrinkage of poly(ethylene terephthalate) films have been characterized and related to various parameters of the stretching process. Amorphous orientation functions and levels of crystallinity have been found to be of major importance to the shrinkage process. As film extension ratios increase, shrinkage behavior passes through five different regions. Shrinkage first increases with extension ratio, decreases with further extension to reach a minimum, and then increases again as extension is continued to higher levels. A schematic model has been proposed to describe molecular changes in polymer chain structures, within each of the shrinkage regions. Activation energies of shrinkage have been determined in addition to equilibrium shrinkage and shrinkage rate constants.