The primary focus of this work was to evaluate the relationship between free volume distribution, chain motion, crystallinity, and the gas properties of PET upon strain-induced crystallization (SIC) at different stretch ratios. The formation of a three-phase structure containing rigid amorphous phase, mobile amorphous phase, and crystalline phase upon SIC was confirmed by differential scanning calorimetry and positron annihilation lifetime spectroscopy (PALS). Dynamic mechanical analysis and PALS indicated that there was a significant reduction in the fractional free volume upon orientation at an extension ratio of 3 × 3. Sub-T g relaxation studies indicated that the activation energy of mechanical relaxation decreased with increasing the stretch ratio. Gas transport studies revealed that the reduction in permeability coefficient was mainly due to reduction in diffusivity. Permeation studies using gas molecules with different sizes revealed that strain-induced crystallization affects the free volume distribution as well as reducing the average fractional free volume.

SynopsisT h e kinetics of solid state polymerization of poly(ethy1ene terephthalate) (PET) have been investigated a t a variety of conditions. Equations have been developed to describe the relationships of time, temperature, and final molecular weight for PET precursors prepared from specified catalyst and monomer systems. These studies show effects of: time and temperature of solid stating, moisture concentration, oxygen exposure, and nitrogen purge flow rate. Measurements of inherent viscosity, carboxyl end group concentration, melting point, residual acetaldehyde, and acetaldehyde generated during melting are used to monitor molecular weight, purity, and thermal stability of these solid stated resins.

SYNOPSISThe environmental degradation of high-density polyethylene (HDPE ) has been studied, in addition to that of HDPE blends, containing various concentrations of ethylene carbon monoxide copolymer. Extruded sheets of each material were exposed to natural Arizona sunlight for times up to 6 months. Exposed samples were then analyzed with respect to molecular weight, density, thermal behavior, mechanical properties, and infrared absorption. Additional samples were exposed to laboratory weathering conditions, evaluated in terms of property changes, melted, reformed, and then reevaluated without further weathering exposure. Results indicate that sunlight exposure causes decreased elongation to break, increased embrittlement, decreased molecular weight, and increased crystallinity. Environmental oxidative degradation is elucidated by the measurement of specific infrared bands, sensitive to the formation of carbonyl and vinyl end groups. As environmental degradation causes reductions of molecular weight, polymer chain mobility increases, leading to a higher degree of crystallinity. This increased crystallinity, along with the decreased molecular weight, accounts for the loss of ductility, indicated by a sharp decrease in ultimate elongation. The presence of carbon monoxide copolymer in the blended samples accelerates the process of environmental degradation, however, the degradation mechanisms appear to be similar to those observed for nonblended HDPE.

ABSTRACT:The solid-state polymerization (SSP) reaction kinetics of poly(ethylene terephthalate) were investigated in connection with the initial precursor intrinsic viscosity (IV; molecular weight). Evaluations were performed with otherwise equivalent precursors melt-polymerized to IVs of 0.50, 0.56, and 0.64 dL/g. The changes in the molecular weight and other properties were monitored as functions of the reaction times at solid-state temperatures of 160 -230°C. Precursors with higher initial molecular weights exhibited higher rates of SSP than those with lower initial values, as discussed in connection with the levels of crystallinity and the carboxyl and hydroxyl end-group composition. Activation energies decreased at temperatures above 200°C, and this indicated a change in the SSP reaction mechanism. At temperatures of 200 -230°C, similar activation energies were required for the polymerization of all three precursors. Lower temperature polymerizations, from 160 to 200°C, required higher activation energies for all precursors, with the 0.50-IV material requirement almost twice as high as that calculated for the higher IV precursors.

SynopsisThe crystallization behavior of polyethylene terephthalate (PET) was investigated as a function of molecular weight, temperature of crystallization, and polycondensation catalyst system. A detailed analysis of the crystallization c o w has been made utilizing the Avrami expression. The crystallization rate constants and the Avrami exponents were calculated. The results show that the rate constant and the mechanism of crystalhation are dependent on the molecular weight, temperature, and the polycondensation catalyst system. The catalyst system often exhibits a more significant influence than the molecular weight in controlling the rate of crystallization of PET.