The influence of nanoporous crystalline structure on low molecular weight mass transport in syndiotactic polystyrene

Justin Paul Brandt

Abstract

Solid-state structure, crystalline morphology, crystallization kinetics, thermal, free volume, and gas transport properties of semicrystalline syndiotactic polystyrene (sPS) and ethylene vinyl alcohol copolymers (EVOH) have been investigated. Solid-state structure of sPS after crystallization from the melt and glassy state was examined by differential scanning calorimetry (DSC), density and wide angle x-ray diffraction analysis (WAXD). The measurements confirmed low density of syndiotactic polystyrene crystalline forms, which in the case of α and δe was smaller and in the case of β and γ crystalline forms was slightly larger than the density of the glassy amorphous sPS. Positron annihilation lifetime spectroscopy (PALS) experiments have been carried out to study the free volume properties of these materials. The diffusion characteristics of amorphous, α, and β forms of sPS were successfully measured via a custom made dynamic gas permeation system utilizing a mass-spectrometer. It was shown experimentally that the amorphous and β forms demonstrate similar diffusion. Conversely, experimental results as well as molecular dynamics simulations have shown that the unique "superstructure," packing of triplets forming hexagonally shaped nanochannels along the chain direction, found in the α form of sPS, facilitate high diffusion rates parallel to the chain direction in larger permeants (greater than 2.6Å). This was attributed to a transition from a nearly 3-dimensional behavior for small permeant size, to a 1-dimensional behavior for larger permeant sizes. Larger permeants are confined to the hexagonally shaped nanochannels between the triplets in the α" form, and cannot move between parallel channels. The smaller permeants, however, can move relatively freely between neighboring channels, resulting in a 3-dimensional diffusion behavior. The vapor sorption of several industrial and chemical simulants within semicrystalline domains of syndiotactic polystyrene was studied with a quartz crystal microbalance. To increase the overall sensitivity of the sensing medium a new method for sample preparation was proposed. This method has utilized the ability to prepare high degrees of γ phase crystallinity, through supercritical CO 2 , to generate highly δe crystalline samples via crystalline-crystalline solid state transitions. Sorption characteristics are established as a function of crystalline fraction and chemical properties of the test analytes. It has been demonstrated experimentally that increased crystallinity directly influences the sensitivity of the sensing medium. The use of δ e crystalline phase sPS as a sensing medium in cooperation with QCM to selectively detect the chemical warfare simulant 2-chloroethyl ethyl sulfide was shown. Transport in a broad range of ethylene vinyl alcohol copolymers (EVOH) as it is related to hydrogen bonding interaction through a measure of free volume and cohesive energy characteristics was conducted. Solid state structure was probed utilizing wide angle x-ray diffraction, differential scanning calorimetry, density measurements, and positron annihilation lifetime spectroscopy, to directly study free volume behavior. Characteristic FTIR spectra of EVOH copolymers demonstrate that the width and intensity of the OH stretching vibration increase with increasing vinyl alcohol content. The measure of intermolecular interaction, cohesive energy density (CED), was calculated through group contribution methods and also obtained using molecular dynamics computer simulations. Oxygen transport characteristics of the copolymers, i.e. permeability, diffusivity, and solubility were measured and correlations were made between vinyl alcohol content, free volume, and CED.