The Role of Additive Interactions in Polymer Nanocomposite Membranes

Abstract

Membranes, one of the most energy efficient separation methodologies, can be further enhanced by the addition of nanoparticle fillers, making them exciting candidates for industrial separations. Despite membranes’ potential for exceptional performance, these advanced materials are not being utilised in industry to improve separation technologies. Many high performance polymers and mixed matrix membranes (MMM) suffer from mechanical weakness and are prone to physical aging, making the films too brittle to withstand industrial operating conditions and exhibit lower performance over time. In recent years, there has been a growing awareness that these properties are key to the realisation of MMM applications, and that polymer-additive interactions have considerable influence over bulk membrane properties. In this dissertation, the relationship between polymers and additives was explored to develop methods that control their interactions and allow the properties of the resulting nanocomposite materials to be tuned. Polymer-Additive interactions were examined against key performance indicators relevant to the application of membranes within industry; specifically focusing on gas permeability and selectivity, mechanical stability, physical aging, and the compatibility of nanocomposites’ constituents. As a central theme, the dissertation builds on the discovery of titanium transmetallated UiO-66 (Ti5UiO-66) and its unusual success as an additive in PIM-1 based membranes. By comparing the structure and polymer interaction of TixUiO-66 against UiO-66 and a number of other additives; an understanding of how additive interactions influence the properties of composite membranes was developed. At just 5 wt. %, Ti5UiO-66 in PIM-1 increased permeability by 274 %, a 153 % increase over the unmodified UiO-66 MOF. The interaction between Ti5UiO-66 and PIM-1 also reinforced the polymer and reduced its age-related permeability loss. In each of the other MMM studied, additive enhancement in one property usually severely compromised the materials’ performance in another. In a study linking the polymer-additive interactions to mechanical performance and physical aging, both strong and weak interactions were found to have useful and adverse effects on the properties of mixed matrix membranes. Based on this work, the facile assumption that MMM properties were solely determined by additive ‘compatibility’ was dispelled, suggesting instead the need to tailor specific interactions in order to control nanocomposite properties. From a number of novel approaches to control interaction, it was found that the composition and distribution of organic linkers in the UiO-66 framework could be predictably controlled through Post Synthetic Exchange (PSE); a promising method of tuning UiO-66’s interaction with polymer matrices. Finally, the findings presented were applied to the development of nanocomposite materials in a commercial setting, finding that MOFs with surfaces targeting specific interactions could improve the properties and processing of an industrial polymer. Overall, this dissertation provides a foundation for the development of the next generation of mixed matrix membranes by revealing how polymer-additive interactions can be tuned to optimise the physical aging, mechanical stability, and permselectivity of nanocomposite films. As a basis for future work, the findings presented here should accelerate the advancement of MMM technologies and realise membranes’ potential to reduce the energy cost of the world’s key separation processes.