Borate Ester Bond-based Self-healing/ Shape Memory Polymer Materials

Over the past few years,there has been a great deal of interest in the development of polymer materials with tunable structural and mechanical properties,which exhibit rapid and autonomous self-healing and shape memory properties.In this paper,we design and prepare the new self-healing and shape memory polymer materials utilizing the reversible and dynamic borate ester bonds.A fast self-healing double network(DN)hydrogel based on PVA and borax is presented,consisting of a dynamic borate ester bond as the first network and PVA crystalline domains as the second network.The dynamic PVA-borax network will provide the hydrogel with the selfhealing property,and the crystallization of PVA occurs at low temperature will provide an additional platform to bear stress and firm the structure,thus improving strength and toughness.As a result,the DN hydrogel shows an excellent self-healing ability(almost 100% recovery of strength in 10 min)at room temperature without external stimulus and robust mechanical strength(tensile strength ? 0.2MPa).DN hydrogel also exhibits an outstanding self-healing performance underwater,which will have a great significance in broadening their application fields.In addition,since the double networks of the hydrogel consists of dynamic covalent bonds and crystallization,both of which are temperature-sensitive,the hydrogel can be reshaped upon heating.Self-healable hydrogels based on the dynamically reversible boronate ester or borate ester bonds are usually prepared by reacting boronic acid or boric acid with diol compounds or polymer-like poly(vinyl alcohol)bearing a hydroxyl group in each monomer unit.Herein,we report a finding that not only facilitates the preparation but also extends the range of selfhealable hydrogels of this kind.By simply copolymerizing commercially available N,Ndimethylacrylamide and 2-hydroxyethyl acrylate(8:2 weight ratio)in the presence of boric acid in a one-pot fashion,the resulting random copolymer can gel in aqueous solution at pH = 9,giving rise to a solid hydrogel(tensile strength >0.5 MPa at water content of 30%)that,on the one hand,can autonomously self-heal(near 100% fracture stress recovery within 48 h in air at room temperature)and,on the other hand,shows the characteristics of elastomer(little stress relaxation under loading and small residual deformation after unloading upon repeated 300% elongation cycles)and water-induced shape memory property.The results reveal that it can be sufficient to have a random copolymer with comonomer units bearing hydroxyl groups for reacting with boric acid to generate dynamically reversible borate ester bonds.This finding thus points out a general,facile,and cost-effective method to obtain and explore new borate ester bond-based self-healable hydrogels.