Preparation And Properties Of High-strength Intrinsic Self-healing Silicone Elastomers

Silicone elastomer is a special elastomer formed by the vulcanization of linear polysiloxanes through silanol condensation,hydrosilylation or other crosslinking reactions.Silicone elastomer shows the incomparable properties of aging resistance,high and low temperature resistance,weather resistance and biocompatibility to most other organic polymers because of its unique organic/inorganic hybrid molecular structure.Therefore,it is widely used in the fields of transportation,medical and health,aerospace and so on.However,silicone elastomer is inevitably be damaged during production and using,resulting in adverse effects.Therefore,it is necessary to develop silicone elastomer with self-healing properties.However,the self-healing performance mainly depends on the dynamic reversibility of the crosslinked network,which often requires the construction of weak crosslinked networks.Therefore,the existing self-healing silicone elastomers usually cannot combine high mechanical properties and excellent self-healing ability,which thus limit the wider applications of self-healing silicone elastomer.Therefore,it is of great theoretical significance and application value to develop silicone elastomer with high mechanical properties and excellent self-healing ability.At present,self-healing silicone elastomer with high mechanical properties and excellent self-healing ability is usually prepared by combining dynamic covalent bonds and supramolecular interactions in its polysiloxane crosslinked network.However,the dynamic covalent bonds and supramolecular interactions are always derived from different crosslinkers,which are exisited in different chains or networks,thus leading to inhomogeneity of crosslinked networks and dynamic bonds.To effectively solve the above problems,two kinds of high-strength self-healing silicone elastomers with different asymmetric dynamic cross-linking chain structure were prepared by the crosslinking of two amino-modified polydimethylsiloxanes as matrix with p-carboxybenzaldehyde as crosslinker that has asymmetric functional groups.1.Preparation and properties of self-healing silicone elastomers based on single asymmetric dynamic cross-linked chain structure（1）Several aminopropyl modified polydimethylsiloxanes（PMAS）with different amine values were prepared by the polymerization of 3-aminopropylmethyldimethoxysilane（KH-554）,octamethylcyclotetrasiloxane（D₄）and hexamethyldisiloxane（MM）catalyzed by tetramethylammonium hydroxide（TMAH）.The structures of PMAS were characterized by fourier transform infrared spectroscopy（FTIR）and nuclear magnetic resonance hydrogen spectroscopy（~1H NMR）.（2）A self-healing silicone elastomer（PMAS-FBA）with single asymmetric crosslinking chain structure with combined dynamic ionic hydrogen bond and imine bond was prepared by crosslinking PMAS with FBA.The results show that when n（NH₂）/n（FBA）=2,with the increase of amine value,the mechanical properties changed from soft and weak to strong and tough,and finally to hard and brittle.With the increase of the amount of crosslinking agent,the tensile strength of the elastomer first increased and then decreased,and the breaking elongation first decreased and then increased.The elastomer PMAS3.5-FBA-2 showed good comprehensive mechanical properties,with tensile strength of 5.27±0.44 MPa,elongation at break of 257±32%and toughness of 11.43±1.42 MJ·m^-3.（3）Elastomers PMAS-FBA have excellent self-healing and reprocessing properties.The mechanical properties of the cutted elastomer PMAS3.5-FBA-2 can recover more than 95%of the original strength after re-splicing and self-healing at 60°C for 3 h.The pulverized elastomer PMAS3.5-FBA-2 can be reprocessed after being molded at 60°C and the pressure of 10 MPa pressure for 2 h.After three cycles of regeneration,the mechanical properties of the regenerated sample can still maintain 90%of the original strength.Furthermore,the self-healing behavior of elastomers were mainly depended on the dynamic ionic hydrogen bond and imine bond.2.Preparation and properties of self-healing silicone elastomers based on dual asymmetric dynamic cross-linked chain structure（1）Three kinds of diethylenetriaminopropyl-modified polydimethoxysilanes with different amine values were prepared by the polymerization ofγ-diethylenetriaminopropyl-methyldimethoxysilane（CG-605）,D₄ and MM under TMAH catalysis.The structure of PDETAS was characterized by FTIR and ~1H NMR.（2）A self-healing silicone elastomer（PDETAS-FBA）with dual asymmetric dynamic cross-linked chain structure combining dynamic ionic hydrogen bonds,imine bonds and aminal bonds was prepared by cross-linking through the crosslinking of PDETAS with FBA.The mechanical properties of elastomers PDETAS-FBA are also depended on the amine value and the amount of crosslinking agent.As the amine value increases,the mechanical properties of the elastomer increase.The tensile strength of the elastomer increases but the elongation at break decreases with the increase of the amount of crosslinking agent.The elastomer prepared by adding FBA into three kinds of PDETAS with different amine values in the ratio of n（NH＆NH₂）/n（FBA）=2 shows better comprehensive mechanical properties.The tensile strengths of elastomers PDETAS1-FBA-2,PDETAS2-FBA-2,and PDETAS3-FBA-2 are 2.60±0.08,6.45±0.12,and 12.02±0.11 MPa,respectively,and their toughness are 26.03±1.95,28.58±1.77,and 25.63±1.16 MJ·m^-3,respectively.（3）Elastomers PDETAS-FBA have excellent self-healing and reprocessing performance.The mechanical properties of the cutted elastomer PDETAS3-FBA-2 can recover more than95%of its original strength after re-splicing and self-healing at 100°C for 2 h.The pulverized elastomer PDETAS3x-FBA-2 can be reprocessed after being molded at 80°C and the pressure of 10 MPa for 2 h.Furthermore,this self-healing behavior of the elastomer was attributed to the synergistic contribution of dynamic ionic hydrogen bonds,imine bonds,and aminal bonds.（4）The dual asymmetric dynamic cross-linked chain structure also endows the elastomers PDETAS-FBA with redissolvability,by breaking the ionic hydrogen bonds in the dual asymmetric dynamic cross-linked chain structure.Based on this property,elastomers are used to develop reusable adhesives.The adhesion strength of the elastomers is comparable to that of some commercial adhesives,and after three bongding and debonding cycles,the adhesion strength is still close to that of the initial strength.In addition,conductive MWCNTs composites were prepared,and the addition of MWCNTs enhanced the tensile strength and modulus of the elastomers.The composite still has excellent self-healing ability,and the flexible sensor developed by this composite has sensitivity properties.Most importantly,the MWCNTs and elastomers can be separated from the composite by redissolving them in THF,and the obtained MWCNTs and elastomers retain their original morphology and tensile strength,respectively,meaning they can be reused to develop new items.