Quantitative Study On The Interface Of Silica-solubilized Styrene Butadiene Rubber Nanocomposites

The strength of pure rubber is very low,and its comprehensive properties are difficult to meet the needs of industry and people’s daily life.Therefore,in the rubber industry,various nanoparticles are used to improve and enhance the properties of rubber materials.Silica has the advantages of easy modification,large specific surface area and high strength.It does not rely on petroleum resources and is an ideal green environmental protection reinforcing agent.Solution polymerized styrene butadiene rubber(SSBR)is the synthetic rubber with the best comprehensive performance and the largest output.As the bond between rubber matrix and inorganic nanoparticles,interfacial phase affects the dispersion of fillers and determines the stress transfer.Undoubtedly,the interfacial interaction is the key factor ’to determine’ the reinforcement’ of nanoparticles.Therefore,’ it is o’f great significance to understand the microstructure of solution polymerized styrene butadiene reinforced by silica.However,at present,the research on interfacial phase mostly stays in the qualitative or semi quantitative stage,and lacks the visual and quantitative understanding of interfacial phase.In this paper,quantitative nanomechanical·technique of atomic force microscopy(AFM-QNM)was used to visualize the interface structure of rubber nanocomposites,and to quantitatively characterize the interface properties(interface thickness and interface nanomechanical properties).The main research contents are as follows:(1)Based on AFM-QNM,the high-resolution height map,modulus map and adhesion diagram of SiO2/SSBR nanocomposites were obtained.The force curve obtained by AFM-QNM was successfully fitted by using JKR mechanical contact model,which is more suitable for analyzing elastomer materials.The more accurate modulus of samples was calculated and obtained.Combined with the“scribing method",the accurate modulus distance curve of the interface region was obtained,and the interface characterization method of rubber nanocomposites was optimized.In addition,based on the existence of two different modulus gradients in the interface region,the steep gradient of high modulus(tight binding layer TBR)and the gentle gradient of low modulus(loose binding iayer LBR),the two layer structure of the interface phase was confirmed.(2)Based on the above quantitative characterization methods,the effects of SiO2 with different hydroxyl content on the interfacial properties of SiO2/SSBR nanocomposites were investigated.The interfacial thickness and Young’s modulus of SiO2/SSBR nanocomposites were characterized by AFM-QNM.The results show that the interfacial thickness of SiO2/SSBR nanocomposites increases from 9.8±0.2 nm to 13.0±0.3 nm with the decrease of hydroxyl content on the surface of SiO2.The thickness of LBR layer increases from 5.7±0.3 nm to 8.9±0.3 nm,and the thickness of TBR layer is almost the same,which is approximately 4 nm.The young’s modulus of TBR layer increases from 14.9±0.5 MPa to 17.0±0.4 MPa.However,Young’s modulus of LBR layer was not significantly increased,just froml 1.0±0.3 MPa to 11.4±0.3 MPa.The hydroxyl content on the surface of SiO2 has a significant effect on the modulus of TBR layer,but has little effect on the thickness of TBR layer,which.indicates that the hydroxyl content on the surface of SiO2 affects the number of contact points between molecular chainand filler surface,resulting in a great difference in the packing density of chain(TBR layer)directly contacting with SiO2 surface.Therefore,the modulus of TBR layer has a significant change,and the thickness of TBR layer may be controlled by the properties of rubber molecular chain(chain length,stiffness,etc.),so the thickness of TBR layer remains unchanged.Secondly,the hydroxyl content on the surface of SiO2 has a significant effect on the thickness of LBR layer,but has little effect on the modulus of LBR layer.This is because LBR layer is formed by physical entanglement with molecular chains in TBR layer.The weak physical entanglement has no significant effect on the molecular chain packing density of LBR layer.Therefore,the modulus of LBR layer is basically the same,but the higher chain density of TBR layer can interact with more rubber chains.Therefore,the thickness of LBR layer around TBR layer with higher modulus is thicker.All these indicate that the interfacial interaction between silica and rubber molecular chain increases with the decrease of hydroxyl content on silica surface.The results of differential scanning calorimetry(DSC)and low field nuclear magnetic resonance(LF-NMR)show that with the decrease of hydroxyl content on the SiO2 surface,the restriction of the SiO2.surface on the movement of SSBR chain increases gradually;more SSBR chains can be adsorbed on the SiO2 surface,and the mass fraction of the molecular chains in.the interface immobile layer increases from 22.3%to 33.4%,which is consistent with the AFM-QNM results.In addition,the adhesion energy between filler and rubber increases with the decrease of hydroxyl content.All the above results indicate that the interfacial interaction between SiO2 and SSBR increases with the decrease of hydroxyl content on SiO2 surface.(3)Based on the above quantitative characterization method,the influence of SSBR functionalization on the interfacial properties of SiO2/SSBR nanocomposites was investigated,and the interfacial thickness and Young’s modulus of the composites were determined quantitatively.The results show that the interface thickness of the composites increases from 9.6±0.3 nm to 12.4±0.6 nm,the thickness of TBR layer is 4.4±0.2 nm,the thickness of LBR layer increases from 5.2±0.3 nm to 8.0±0.4 nm,the young’s modulus of TBR layer increases from 13.6±0.6.MPa to 19.1±0.8 MPa,and the average young’s modulus of LBR layer increases from 7.2±0.6 MPa to 11.4±0.4 MPa.The young’s modulus of the matrix increases from 4.8±0.1 MPa to 8.1 ±0.1 MPa.The increase of interfacial modulus and thickness is due to more adsorption sites between SSBR and SiO2 surface after functionalization,and more chains are adsorbed on SiO2 surface,which increases the stacking density of chains on SiO2 surface and leads to the increase of TBR layer modulus.The dense molecular chains in TBR layer can entangle more free rubber chains,resulting in the increase of LBR layer thickness.The increase of Young’s modulus in LBR layer may be due to the increase of matrix modulus,and the functionalization of SSBR,which makes it easier to entangle with the molecular chain in TBR layer,and then increase the chain density.In addition,the results of macroscopic kinetics and thermodynamics also show that the interaction between SiO2 and functionalized SSBR interface is stronger.