High Strength Nacre-like Composites From Hyperbranched Polymers

The natural nacre is of excellent properties, such as lightweight, high strength and high toughness. They can all be attributed to the unique "brick" and "mortar" structure. Inspired by the hierarchical structure of nacre, researchers are able to fabricate high-strength nacre-like composites by taking different strategies. However, the nacre-like composites with high toughness are still rare although great progress has been achieved in fabrication of robust nacre-like composites. It is therefore a challenge in construction of nacre-like composites combining high strength and toughness.In the previous reports, linear polymers were always used in fabrication of nacre-like composites, such as poly(vinyl alcohol) (PVA), poly(methyl methyacrylate) (PMMA), poly(diallyldimethyl ammonium) choloride (PDDA), chitosan, etc. There are abundant polar groups on these linear polymers, so that they can form strong hydrogen bonds with the inorganic nanosheets, leading to high strength of the obtained composites. However, the covalent bonds in these linear polymers are not able to reversibly break and reform. Therefore, the fracture energy cannot be dissipated sequentially, resulting in poor toughness. To solve this problem, dynamic interactions (like hydrogen bonds, π-π interaction, Van de Waals force, ionic bonds etc) should be introduced into the artificial nacre-like composites.Aliphatic hyperbranched polymers are very favorable to be applied in fabrication of high toughness composites considering their molecular structures. There are a lot of polar groups in the molecular backbones of hyperbranched polymers, for example, amino groups, amide groups, hydroxyl groups etc. Hydrogen bonds can be easily formed among these groups. Moreover, the hyperbranched polymers are of highly branched structure with large voids inside their molecules. Consequently, these polymers are of large free volumes and strong molecular mobility. Even when being confined between the clay nanosheets, they are expected to be of good mobility. All these are the foundations of the dynamic interactions.To further construct high-strength, high-toughness nacre-like composites from hyperbranched polymer, it is necessary to cross-link the composites properly. Many previous reports indicated that the mechanical strength of the nacre-like composites could be improved dramatically after cross-linking. It can be speculated that high-strength, high-toughness nacre-like composites from hyperbranched polymer can also be fabricated after the composites are cross-linked.Here in this work, hyperbranched poly(amido amine) (HPAMAM) were synthesized by Michael addition with methenelbiacrylamide and 1-(2-Aminoethyl) piperazine as the monomers. The HPAMAM was assembled with nanoclay or graphene oxide, respectively. Both composite films with layered structure were successfully fabricated. Later, these composite films were cross-linked by genipin. The mechanical property test showed that they were of high strength and high toughness. For the genipin cross-linked HPAMAM/clay composites, the tensile strength was as high as 159 MPa, and the toughness was about 2.5 MJ m"3; for the genipin cross-linked HPAMAM/GO composites, the tensile strength was 165 MPa, and the elongation at break was more than 10%. These kinds of composite films are of great potential in biomedical applications, such as tissue engineering, ligament replacement or bone reparation, due to the good biocompatibility of all the components.