The Hierarchical Structures Of Silica-Silicone Rubber Research Basing On SAS

Silicone rubber is widely used in different fields and scenes because of its excellent heat resistance,radiation resistance and weather resistance.However,due to the weak mechanical properties of pure silicone rubber,filler filled methods are often used to improve the mechanical properties of silicone rubber to adapt to more application scenarios.It is believed that the excellent mechanical properties of filler filled silicone rubber nanocomposites depend on the synergistic effect of hierarchical structures caused by fillers.However,due to the non-equilibrium,non-uniform and multi-scale characteristics of the hierarchical structures and the limitations of existing research techniques,the formation and evolution mechanism of the hierarchical structures of silica filled silicone rubber composite is not clear,and the physical mechanism of the synergistic interaction between the bound rubber network and filler network is still unclear.Small Angle scattering can clearly identify the static and dynamic structure information of nanometer to submicron scale,which is a feasible method to reveal this problem.Therefore,based on the above research background,this paper adopts the method of small angle scattering（neutron and synchrotron radiation X-ray）combined with low field NMR,and regards the silica filled silicone rubber system as the research object.The internal factors such as the properties of different fillers,the amount of addition,and the external field conditions such as strain rate were studied systematically.This work promotes the understanding of the physical problems such as the evolution of hierarchical structures involved in the processing and service of silicone rubber composites,and accumulates basic theories for the development of silicone rubber composites and engineering ideas.The main research results and conclusions of this paper are as follows:（1）Combined with the advantages of synchrotron radiation X-ray small angle scattering（SAXS）and neutron small angle scattering（SANS）as well as low field nuclear magnetic resonance technology,the hierarchical structures of composite materials was uncoupled,and the quantitative characterization of hierarchical structures was realized by exploring the fitting model.Finally,a set of in situ characterization methods for static and dynamic multilevel structures of silicone rubber composites were successfully constructed,which provided a new idea for understanding the microstructure evolution mechanism of composite materials under different conditions.（2）The static hierarchical structures of silicone rubber composite with 5-80 phr filler loading was studied by using SANS.There is a percolation threshold Φ_Si^c in the range of 10-30 phr.When the filler fraction is higher than the percolation threshold ofΦ_Si^c,the uniformly distributed aggregates form connective filler network and bound rubber network,thus giving silicone rubber composite excellent mechanical properties.When the filler fraction is lower than the percolation threshold Φ_Si^c,the aggregate exists in a relatively isolated state and cannot form the filler network,so it cannot provide a satisfactory reinforcement effect.When it is much higher than Φ_Si^c,the bound rubber increases greatly and forms a "bi-continuous" structure,which occupies more space of the matrix so that the tensile strength of the composite becomes larger and the fracture elongation becomes smaller.The three-stage hierarchical structures physical model will help to understand the microstructure evolution caused by the loading of filler.（3）By designing silica fillers with different properties and combining with SANS and low-field NMR,the evolution of the hierarchical structure caused by the internal factors of fillers was studied.Firstly,the shape of silica is a key factor to determine the formation of aggregates,and the surface chemical structure can greatly affect the formation and fusion of bound rubber networks.Secondly,branched aggregates could form a connective filler network,which is coupled with the bound rubber network to play a synergistic effect and improve the break elongation of the composite.Finally,the surface grafted molecular chains can further enhance the filler-polymer interaction,thus reducing the viscoelasticity and increasing the modulus of the composites.This work refined the effects of different filler properties on the evolution of hierarchical structure and accumulated experience for the optimization of silicone rubber composite formula.（4）The evolutionary mechanism of various hierarchical structures under different external field conditions is further studied with the in situ SAXS based on（2）.Firstly,aggregates as a rigid basic unit are hard to deform during the strain process.Secondly,in the strain direction,the correlation lengths of the filler network increase with the increase of strain.In this process,the filler network damage and dissipate energy to achieve the reinforcement effect.However,the deformation mechanism of the filler network is completely different under different filler fraction.Finally,the tensile rate has no effect on the damage of the filler network,but it will affect the reconstruction of the filler network.Therefore,the faster tensile rate leads to lower elongation at break and higher tensile strength of the composite.The dynamic evolution model of filler network is helpful to promotes the understanding of the structure evolution under different service conditions and provide guidance for the design of silicone rubber composite materials.