Free volume studies of various polymeric systems using positron annihilation and PVT-EOS analyses
The glass transition phenomenon and free volume behavior below and above the glass transition temperature of various polymeric systems have been investigated. Several novel polymeric systems were considered for this study. Two generations of hyperbranched polyols, H40 and H20, were selected due to large number of hydroxyl groups on the periphery and within the bulk. The effect of hydrogen bonds and molecular weight was related with the glass transition and free volume behavior for the whole range of experimental temperature. The free volume behavior was experimentally studied using PVT and PALS to determine occupied volume, fractional free volume and number density of holes. Molecular dynamic simulation was performed to compare atmospheric pressure V-T data and visualize hydrogen bond structures. Linear as well as crosslinked isomeric polymers were selected for the study of isomerism on glass transition and free volume. Isomers were selected based on para and meta substitution on phenylene ring in the polymer repeat unit. In this way the polymer chemical composition was kept the same and only architecture was varied. Two linear polymers based on isomeric repeat unit, polyethylene terephthalate (para) and polyethylene isophthalate (meta), and five sets of epoxy networks prepared using isomeric diamine crosslinkers, 3,3'-DDS and 4,4'-DDS were used. The crosslinked networks followed the same trend of glass transitions and free volume properties as in linear polymers. The glass transition temperatures of para isomer based linear polymers and epoxy-amine networks were higher. It was observed for linear polymers as well as for all networks that para isomer generates structure with higher amount of free volume in the glassy state; however, in the rubbery state they are the same. Free volume studies were further extended for solvent uptake in epoxy-amine networks to correlate hole free volume and van der Waals volume of solvent. The higher frozen in free volume in all para isomers leads to higher solvent uptake as compared to meta isomers. Pressure-volume-temperature (PVT) data were used to calculate occupied volume and fractional free volume using Simha-Somcynsky (S-S) Equation of State (EOS). PALS was utilized to evaluate average hole free volume for a wide range of temperature. Both PALS and PVT were utilized to evaluate occupied volume and hole number density. The PVT fractional free volume was also correlated with positron annihilation lifetime spectroscopy (PALS) hole free volume, v 3 , and ortho-positronium formation intensity, I 3 , to calculate the correlation coefficient, C.