Sulfur-urushiol copolymer: A material synthesized through inverse vulcanization from renewable resources and its latent application as self-repairable and antimicrobial adhesive

Abstract

The growing cost of petroleum derivatives and the increasingly stringent environmental regulations have led the adhesive industry to re-seek solutions from natural resources in the past decades. However, most currently available natural adhesives (e.g. polysaccharides) are conflicted with low performances such as weak adhesion and easy microbial degradation, etc. On the other hand, owning self-repairability has become a popular way for a new material to extend its lifetime, to reduce its environmental footprint, and to meet the increasing demands of smart functions. This is also true to adhesive industry with self-repairable adhesive highly desirable.In this work, plant resourced urushiol and industrial waste of elemental sulfur (S) was used to prepare sulfur-urushiol copolymers (IVurux) as a high-strength, self-repairable and antimicrobial adhesive through inverse vulcanization, a facile and green approach free of solvent and catalyst. The polymerization between unsaturated bonds of urushiol and sulfur in forming a reversibly crosslinked network with polysulfide segments has been verified by FTIR and 1H NMR analysis. The adaptable covalent S-S bonds from polysulfide segments endow the copolymer networks with a distinct self-healing property, while the rich catechol groups from urushiol moiety guarantee the material with an outstanding adhesive property.The adaptable covalent networking was well validated by a distinct rubbery plateau in rheological measurements of both temperature ramping test and frequency sweep test. The crosslinking density was revealed to increase with sulfur content, with frequency scaling exponent of G’ in the plateau regime varying from G’∼ω0.27 in IVuru70 (S content 30 wt%) to G’∼ω0 in IVuru50 (S 50 wt%) and IVuru40 (S 60 wt%), while when S >60 wt% few unreacted sulfur presented in the product as indicated from XRD and DSC analysis. Moreover, the self-healing property was well demonstrated in a repeated breaking-healing cycle test with 92.7% recovery obtained after 4 breaking cycles for IVuru50, and the dynamic S-S bond exchange was studied by stress relaxation to be temperature dependent with an activation energy of metathesis ∼100 kJ/mol. Furthermore, the P(sulfur-r-urushiol)s were examined to exhibit a considerable adhesion strength ∼3.0 MPa, about 6 times of a commercial hide glue, with IVuru60 being the best as a balanced contribution between catechol groups and covalent networking for adhesive and cohesive bonding. Meanwhile, a very good self-repairability of the adhesive was demonstrated with heating being the trigger for S-S metathesis. Ultimately, the P(sulfur-r-urushiol) copolymers were found to be hydrophobic with a contact angle ∼ 85° and have an excellent antimicrobial activity against S. aureus and S. cerevisiae with a high inhibition efficiency >80% and >52%, respectively. Such a high-strength, self-repairable and antimicrobial adhesive material foresees promising applications in broad fields in light of its advantages of fabrication simplicity, environmentally friendliness and good durability.