Research On Strengthening-Toughening Design Of Dynamic Polysiloxane Materials Based On Sacrificial Zn(Ⅱ) Coordination Bonds And The Catalysis Of Zn(Ⅱ)

The polysiloxane elastomer,which exhibits excellent properties such as good transparency,heat resistance,and solvent resistance,is widely used in diverse fields.However,as a permanently crosslinked polymeric material,it is generally not recyclable and cannot be reused.Introducing dynamic bonds into the material is an effective way to endow it with dynamic properties like malleablility and reprocessability.Nevertheless,due to the weak intermolecular interaction between polysiloxane macromolecules,the reported dynamic polysiloxane materials generally have poor mechanical properties,which cannot meet the requirements of practical applications.The traditional reinforcement approach of polymers is the addition of nanoparticles,which,however,often limits the movement of polymer chains,and reduces the dynamic properties.Therefore,it is of great significance to develop new strengthening-toughening methods for dynamic polysiloxane materials.In recent years,the sacrificial bond strategy,which is inspired by the natural biomaterials with high strength and toughness,has attracted much attention of the researchers because of its remarkable strengthening-toughening effect.In view of the above background,this thesis aims to obtain polysiloxane materials with both good mechanical and dynamic properties.Firstly,dynamic polysiloxane networks are prepared.Then,considering that the Zn（Ⅱ）ion is non-toxic,colorless,and inexpensive,and has moderate coordination strength,sacrificial Zn（Ⅱ）coordination bonds are constructed in the networks to strengthen and toughen the materials.In particular,this thesis systematically proposes to use the Zn（Ⅱ）ion in sacrificial bonds as the catalyst to facilitate the rearrangement of dynamic covalent networks for the first time.The main contents are as follows.（1）Strengthening-toughening system based on Zn（Ⅱ）-carboxyl ionic coordination bonds.Colorless and transparent dynamic polysiloxane materials were successfully prepared by introducing Zn（Ⅱ）-carboxyl ionic coordination bonds via aza-Michael reaction in one pot.The aggregation of ionic species could be adjusted by changing the ion content.Finally,a dynamic crosslinking system with the synergy of strong and weak ionic aggregates was constructed.The strong aggregates imparted the elastomers with high strength,while the weak ionic aggregates acted as sacrificial bonds to enhance the toughness.The toughness of the best sample was more than 15 MJ/m³,and it also had good self-healing and reprocessing properties.（2）Strengthening-toughening system based on Zn（Ⅱ）-amino coordination bonds.Considering that dynamic covalent polymers maintain the covalent crosslinking structure of traditional thermosetting polymers,we prepared three kinds of dynamic polysiloxane materials based on vinylogous ureas,imines and silyl ethers.Then,by simply adding Zn Cl₂ during the preparation,Zn（Ⅱ）-amino coordination bonds were constructed in the polymer networks.The results showed that the integration of coordination bonds could remarkablely enhance the mechanical properties of the three dynamic covalent polymers.The tensile strength was increased by 9,24 and 5 times,respectively.Moreover,the toughness was increased by 49,98 and 35 times,respectively.Coordination bonds could dissipate energy through reversible rupture and reconstruction during stretching,significantly enhancing the tensile strength and toughness of materials.Among a variety of metal ions,Zn（Ⅱ）had the best sacrificial bond strengthening-toughening effect because of its moderate coordination ability.In particular,Zn（Ⅱ）could also catalyze the bond-exchange reactions of vinylogous ureas,imines,and silyl ethers,thus facilitating the network rearrangement significantly.Therefore,the resulted materials had good reprocessing properties under heating.（3）Strengthening-toughening system based on the synergy of nanoparticle reinforcement and sacrificial bonds.Carbon black nanoparticles were added to the dynamic polysiloxane networks with dual crosslinks of Zn（Ⅱ）-amino coordination bonds and imine bonds.Therefore,two kinds of strengthening-toughening mechanisms of sacrificial bonds and nanoparticles were introduced into the system simultaneously.The materials were also endowed with electrically conductive function.The integration of sacrificial bonds could balance the decrease of elongation caused by nanoparticles;moreover,nanoparticles could promote the rupture and energy dissipation of sacrificial bonds.Therefore,the overall mechanical properties of the materials could be further improved through the synergy of the two mechanisms.In addition,as an effective catalyst of the Schiff-base reaction,Zn（Ⅱ）could reduce the gel time during preparation,inhibit the aggregation of carbon black nanoparticles,and improve the electrical conductivity of nanocomposites.The electrical conductivity of the best sample was 6.48S/m,which could also be reshaped,healed and reprocessed under heating.