Exploring the limits of the gas dissolution foaming towards new applications for porous polymers

Resumen

Cellular polymers are biphasic materials composed of both gaseous and solid phases, being gas dissolution foaming one of the most employed fabrication methods. Their physical properties as well as the density reduction promoted by the gas phase introduction allow employing the cellular polymers in many industrial sectors while reducing the material costs. However, one of the most pursuit challenges related to cellular polymers has not been accomplished, i.e., the formation of the solid skins in the borders of the cellular polymers during the gas dissolution foaming. This drawback promotes a reduction in the physical properties and the limitation to be employed in some applications which require the exposition of the cellular structure. This thesis has been focused on the development of a new approach to enhance the fabrication of cellular polymers by gas dissolution foaming. The motivation of this thesis is to improve the physical properties of the cellular polymers by avoiding the precedent limitations regarding to their fabrication process (i.e., the solid skins formation). In addition, one of the aims of this thesis is also to extend the range of applications of the cellular polymers by avoiding the formation of the solid skins in their edges. In order to solve the mentioned issues, the formation of the solid skins has been investigated to deepen into the problem. Gas diffusivity and solubility has been studied in poly(methyl methacrylate) (PMMA) and poly(styrene) (PS) to analyse how gas concentration influences the growth of the cellular structure. Once understood that the key feature of the solid skins formation, a new approach to solve that limitation is proposed for first time in this thesis. Unlike to the previous attempts to remove the solid skins, the gas diffusion barrier approach allows to obtain whole foamed samples without any disadvantage. A polymer with significant low CO2 diffusivity is located on the major surfaces of the polymer in order to keep a suitable gas concentration in the edges of the polymer. The use of the gas diffusion barrier approach has also enabled the possibility of foaming thin films and reduced-size systems, something impossible so far. Thus, foamed thin films and hollow microfibres have been successfully obtained by using this technique in gas dissolution foaming. Furthermore, another challenge of this thesis has been to produce and optimize open-cell structures by employing the block copolymer approach. In this line, a complete study by employing different grades of PMMA and MAM copolymer was performed, analysing the influence of the polymer nanostructuration and its viscoelastic properties on the cell growth. The interesting achievements of this thesis to the cellular polymers field go beyond the foaming features. Thanks to this novel approach, the range of applications of the cellular polymers has been broadened by evaluating three different applications for the produced materials. Hollow microfibres were tested as drug delivery systems obtaining a slow and controlled drug release in contrast to the solid fibres, that delivered the drug too fast according to the requirements of this application. Same hollow fibres have been evaluated as electrochemical sensors by introducing a conductive polymer. The porosity created by the foaming resulted to better performances of the sensors. Finally, the production of open-cell structures produced in thin films without solid skins has been enabled their use as gas separation membranes. The gas diffusion barrier approach allowed controlling the thickness of the dense layer in the edges, enhancing the permeability properties for that application. The most interesting challenge for these membranes is performing into a sustainable cycle for CO2 recovery in gas separation that would be used to fabricate new membranes by CO2 dissolution foaming.