Supramolecular engineering of adaptive bioinspired nanocomposites

• Supramolekulares Maßschneidern von adaptiven, bioinspirierten Nanokompositen

Zhu, Baolei; Möller, Martin (Thesis advisor); Plamper, Felix Alois (Thesis advisor)

Aachen (2016)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

Biological materials such as nacre, bone and crustaceans fascinate us with their synergistic combination of strength, stiffness, high toughness and light weight. Their high mechanical performance originates from the combination of soft and hard building blocks, high fraction of inorganic reinforcements, and perfectly ordered structures. Replication of those structural features and transferring the high mechanical properties, especially the combination of high stiffness and high toughness would undoubtedly benefit a wide field of areas. Nacre is among the most extensively studied biological materials, due to its high mechanical performance and unique structure. Different approaches have been employed to mimic the inorganic/organic brick and mortar structure, and in this work we utilize the most recently developed ‘self-assembled nacre mimetics’, in that it is easy and simple and allows for large area production of thick films. Well defined polymers, despite its low fraction (usually below 5 vol% in nacre), play very important roles in the mechanical properties, such as integrating the inorganic reinforcements, providing appropriate frictional sliding between the platelets, and giving sacrificial bonds and hidden length mechanisms to enhance the toughness. However, all of the previous work only concentrated on commercially available, high Tg polymers, and no efforts have so far been devoted to careful macromolecular engineering of the polymer phase. I am going to address this challenge in the first part of my PhD work. Dynamic polymers were designed with low glass-transition temperature and bonded by quadruple hydrogen-bonding motifs, and subsequently assembled them with high-aspect-ratio synthetic nanoclays to generate nacre-mimetic films. The high dynamics and self-healing of the polymers render transparent films with a near-perfectly aligned structure. Varying the polymer composition allows molecular control over the mechanical properties up to very stiff and very strong films (E ≈ 45 GPa, σUTS ≈ 270 MPa). The amount of supramolecular bonds in the nacre mimetic material governs the mechanical properties in a large extent. Stable crack propagation and multiple toughening mechanisms occur in situations of balanced dynamics, enabling synergistic combinations of stiffness and toughness. In the second part, I transfer the supramolecularly engineered nacre mimetic composites into a light adaptive material via doping a small fraction of reduced graphene oxide. Supramolecular interactions of the nanoconfined polymer phase govern the mechanical tensile properties of all nacre-mimetic films. The materials containing higher molar amount of supramolecular motifs are very stiff and strong, whereas those with lower amount realize interesting combination of stiffness and toughness/ductility. Co-assembly of 1 wt% of RGO imparts a strong photo-thermal effect, the material quickly reach a steady state temperature where heat generation and dissipation are balanced. The amount of supraomolecular bonds and more importantly the laser intensity governs the stress relaxation mechanism in the RGO doped nacre mimetic materials. In situ digital image correlation (DIC) analysis shows that we can modulate the strain field at will by using localized laser irradiation. Most importantly, the material is light adaptive. The bulk material turns from strong/stiff to soft/tough when we globally irradiate it and readily opens up the supramolecular bonds. In the third part, I explore other possibilities of our supramolecular copolymers as the soft phase of a different type of bioinspired nanocomposite materials. The synthesized low Tg, hydrophilic copolymers with varying functionalization of supramolecular bonding were self-assembled with cellulose nanocrystals, to give ordered cholesteric phases with characteristic photonic stop bands. The dimensions of the helical pitch are controlled by the ratio of polymer/CNC. We demonstrate that the supramolecular motifs regulate the swelling when exposing the biomimetic hybrids to water, and they allow engineering the photonic response. Moreover, the amount of hydrogen bonds and the polymer fraction are decisive in defining the mechanical properties. The molecular engineering allows us to span an unprecedented mechanical property range from highest inelastic deformation (strain-to-failure, εb up to ∼13%) to highest stiffness (E ∼ 15 GPa) and combinations of both.