Validation of Milner's visco-elastic theory of sintering for the generation of porous polymers with finely tuned morphology

• Published in: Soft matter Vol. 16; no. 7; pp. 181 - 1824
• Main Authors: Lutzweiler, Gaëtan, Farago, Jean, Oliveira, Emeline, Jacomine, Léandro, Erverdi, Ozan, Vrana, Nihal Engin, Testouri, Aouatef, Schaaf, Pierre, Drenckhan, Wiebke
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 19.02.2020
• Subjects: Condensed Matter; Interconnections; Interstices; Materials Science; Materials selection; Mathematical models; Morphology; Paraffin; Paraffins; Parameters; Physics; Polymers; Polyurethane resins; Porous materials; Scaling; Sintering; Sintering (powder metallurgy); Soft Condensed Matter; Spheres; Theory; Viscoelasticity
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/c9sm01991j

Sacrificial sphere templating has become a method of choice to generate macro-porous materials with well-defined, interconnected pores. For this purpose, the interstices of a sphere packing are filled with a solidifying matrix, from which the spheres are subsequently removed to obtain interconnected voids. In order to control the size of the interconnections, viscous sintering of the initial sphere template has proven a reliable approach. To predict how the interconnections evolve with different sintering parameters, such as time or temperature, Frenkel's model has been used with reasonable success over the last 70 years. However, numerous investigations have shown that the often complex flow behaviour of the spheres needs to be taken into account. To this end, S. Milner [arXiv:1907.05862] developed recently a theoretical model which improves on some key assumptions made in Frenkel's model, leading to a slightly different scaling. He also extended this new model to take into account the visco-elastic response of the spheres. Using an in-depth investigation of templates of paraffin spheres, we provide here the first systematic comparison with Milner's theory. Firstly, we show that his new scaling describes the experimental data slightly better than Frenkel's scaling. We then show that the visco-elastic version of his model provides a significantly improved description of the data over a wide parameter range. We finally use the obtained sphere templates to produce macro-porous polyurethanes with finely controlled pore and interconnection sizes. The general applicability of Milner's theory makes it transferable to a wide range of formulations, provided the flow properties of the sphere material can be quantified. It therefore provides a powerful tool to guide the creation of sphere packings and porous materials with finely controlled morphologies. The sintering of paraffin spheres is shown to be well described by Milner's visco-elastic model, providing a predictive tool for the generation of microporous polymers with controlled morphology via sphere templating.