Optical anisotropy, molecular orientations, and internal stresses in thin sulfonated poly(ether ether ketone) films

The thickness, the refractive index, and the optical anisotropy of thin sulfonated poly(ether ether ketone) films, prepared by spin-coating or solvent deposition, have been investigated with spectroscopic ellipsometry. For not too high polymer concentrations (B5 wt%) and not too low spin speeds (C2000 rpm), the thicknesses of the films agree well with the scaling predicted by the model of Meyerhofer, when methanol or ethanol are used as solvent. The films exhibit uniaxial optical anisotropy with a higher in-plane refractive index, indicating a preferred orientation of the polymer chains in this in-plane direction. The radial shear forces that occur during the spin-coating process do not affect the refractive index and the extent of anisotropy. The anisotropy is due to internal stresses within the thin confined polymer film that are associated with the preferred orientations of the polymer chains. The internal stresses are reduced in the presence of a plasticizer, such as water or an organic solvent, and increase to their original value upon removal of such a plasticizer

The effects of water on the morphology and the swelling behavior of sulfonated poly(ether ether ketone) films

Thin sulfonated poly(ether ether ketone) films swell excessively in water. The extent of water-induced swelling is shown to be correlated with the optical anisotropy of the films, due to two distinct phenomena. Firstly, the optical anisotropy is directly related to the amount of water taken up from the surrounding ambient atmosphere, and thus to amount of water present in the material just prior to swelling. Secondly, the optical anisotropy corresponds to internal stresses in the film that affect the free energy of the film, and thus the potential of the film to swell. The anisotropy vanishes upon sorption of liquid water and returns when the water is desorbed. When the water is completely removed, the film changes from more or less colorless to an intense yellow color that can be attributed to molecular assembly of the aromatic rings in the polymer backbone. The color change is reversible and occurs immediately upon exposure to low humidity. For films prepared in the absence of water, the lack of hydration of the sulfonic acid groups affects the microphase separation behavior of the polymer. This is manifested by an apparent lower propensity to water-induced swelling. The possibility to affect the properties of sulfonated polymer films by varying the hydration state of the polymer during preparation can have important implications for applications of such films

Thin sulfonated poly(ether ether ketone) films for the dehydration of compressed carbon dioxide

In this thesis, the properties of thin films from highly sulfonated poly(ether ether ketone) (SPEEK) have been investigated within the context of their application as membranes for the dehydration of compressed carbon dioxide. Spectroscopic ellipsometry has been used as the predominant measurement technique. The relationships between the presence of water during film formation and final film properties have been investigated, as well as the water-induced dynamic swelling. Furthermore, the effects on thermal stability of thin SPEEK films when the protons in the sulfonic acid groups are exchanged with sodium ions has been evaluated. Additionally, the feasibility of using composite hollow fibers in the dehydration of compressed carbon dioxide has been assessed with simulations in Aspen Plus®. \ud \ud Thin SPEEK films swell excessively in water, and the extent of water-induced swelling is shown to be correlated with the inherent optical anisotropy of the films. The inherent anisotropy in SPEEK films is indicative for the presence of molecular orientations and internal stresses in the material. Additionally, the hydration state of SPEEK during film formation impacts the final morphology of the films, which can significantly affect the swelling behaviour. Films formed without the presence of water, swell significantly less than their counterparts, prepared in identical way but in a humid atmosphere. Swelling of SPEEK films can be also suppressed by grafting of polyols molecules on sulfonic acid groups, predominantly reducing the affinity for water. Sodium counter-ions strongly enhances thermal stability of SPEEK. No thermally-induced changes have been observed for SPEEK-sodium films when these are heated up to 220 °C for 15 hours. In contrast, SPEEK-proton films that are heated up to only 160 °C for 15 hours exhibit significant decomposition. For dehydration application via membranes, a method is required to remove water from the permeate side such that the high driving force over the membrane is maintained for water permeation. In this study, the usage of a sweep gas in a low pressure, also carbon dioxide, has been discussed