Unlike traditional wet-adhesives, dry adhesives are bioinspired solid polymers that owe their adhesion to hierarchical structures. Gecko’s are the most recognizable living creatures relying on this concept to climb walls. A close look at the gecko’s fingers reveals a hierarchical system of micro-and nanoscale filaments. These ensure excellent adaptability and contact to (rough) surfaces and the formation of a large amount of close-range van der Waals surface interactions. The reversible nature of these bonds allows for reversible and repeatable adhesion. This hierarchical system has been identified as the main reason for dry adhesion and has been object of intense research to manufacture man-made solid adhesives. Despite the many efforts, there are some unclear aspects resulting from contradictory scientific reports and a strong focus on a particular polymer chemistry used to make the hierarchical structures. This has left the effects of the material choice on dry adhesion largely neglected. In this research project, we aimed to shed some light on the role of different polymer architecture features on dry adhesion. Most available research has focused on the use of commercial siloxane elastomers that offer almost no control on the polymer synthesis. Instead, we opted to use thermoplastic polyurethane chemistry due to the large versatility this chemistry offers in terms of modification of relevant polymer architecture features. In the absence of available works on thermoplastics a new manufacturing process of such hierarchical structures had to be developed relying on heating, pressure and vacuum to respectively melt the polymer, overcome the high viscosity and impede oxidation. We studied the effects of the polymer architecture and properties on adhesion using a single micropillar architecture and varying the chemical composition (polyol length, aromatic content, hard/soft block ratio), the testing temperature and the pull-off speed. As to the polymer architecture, the best results were found with a short polyol and the lowest hard block fraction that still guaranteed structural integrity. Next to that, it was found that having aromatic rings in the hard segments was crucial, likely due to its beneficial effect on nanophase separation. With those compositions, values exceeding those of state-of-the-art dry adhesives were found, with a maximum of 440 kPa at the Tg and high retraction speeds. Furthermore, we show that while existing models are valid for thermoplastic polyurethanes well below their Tg, once they exhibit viscoelastic behaviour, the loss factor is a much more reliable indicator of performance than the reported stiffness. The effect of the surface energy was also evident but minimal compared to the mechanical properties of the polymers.

Self-healing materials and, in particular, polymers are characterized by their ability to partially or fully restore their original properties or functions after damage. Amongst the healable polymer classes, thermoplastic polyurethanes have received increasing attention in the last years. This is mainly due to their broad application areas, the variety of chemical platforms that can be used, and the potential use of hydrogen bonding to assist the healing process. However, the combination of good mechanical properties and healing at near room temperature remains challenging. In order to learn how to find a better balance between healing and mechanical properties, dedicated studies on the impact of the monomer chemical architecture on the overall polymer performance are needed.In this work we study the impact of monomer mixtures with different chemical composition on the overall mechanical and healing behaviour of self-healing thermoplastic polyurethanes based on CroHeal™ 2000, biopolyol. The polymers are synthesized via one-shot technique, starting from three constituents: a long chain diol (CroHeal™ 2000), a chain extender (EHD) and a diisocyanate (aromatics - MDI and PPDI, and aliphatic - HMDI) or diisocyanate mixtures (MDI/HMDI and MDI/PPDI in different ratios).Fracture tests using Single Edge Notch Tension specimens show a clear increase in mechanical properties and interfacial strength recovery after damage for short healing times at room temperature when using higher aromatic content in the isocyanate mixture. Furthermore, IR mapping showed that initial compositional heterogeneity is decreased when more aliphatic isocyanates are used but also when longer annealing times are applied in the case of aromatic diisocyanates. The presence of compositional heterogeneities does not influence the healing response under the conditions studied.