Microstructure Characterization Of Unfilled Styrene-butadiene Rubber System During Tensile Orientation

Styrene-butadiene rubber(SBR)is widely used and plays an important role in China’s strategic resources.Unvulcanized SBR has low modulus and strength,which usually requires filler reinforcement and chemical crosslinking.When the crosslinked SBR is stretched,the molecular chains are aligned along the stretching direction,and the orientation occurs.The high strength of SBR is largely attributed to the high orientation of molecular chains during stretching.At present,there are many studies on the microstructure changes such as orientation of SBR/filler system during tensile process,but few reports on microstructure changes such as orientation of unfilled SBR system during tensile process.In this paper,the relationship between microstructural evolution and the change of mechanical properties of the unfilled peroxide(dicumyl peroxide,DCP)-crosslinked and sulfur-crosslinked SBR systems during tensile process was studied intuitively and quantitatively by atomic force microscopy(AFM).Besides,the effect of the type and dosage of vulcanizing agent on the tensile orientation behavior of SBR was also revealed by AFM.In addition,the tensile orientation behavior of the unfilled peroxide(DCP)-crosslinked and sulfur-crosslinked SBR systems was further compared by infrared dichroism method.The main results are as follows:(1)According to the analysis of mechanical properties,within the range of 0.1-2phr peroxide(DCP)content,the unfilled SBR with 0.2 and 0.3 phr peroxide(DCP)had higher tensile strength.Therefore,the relationship between microstructural evolution and the change of mechanical properties of the unfilled SBR with 0.2 and 0.3 phr peroxide(DCP)during tensile process was studied intuitively and quantitatively by atomic force microscopy(AFM).Besides,the effect of peroxide(DCP)content on the tensile orientation behavior of SBR was also revealed by AFM.The results showed that,the coarse fibrous orientation structure gradually appeared during the tensile process of the unfilled SBR with 0.2 and 0.3 phr peroxide(DCP).Overall,with the stress increasing,the area ratio of amorphous phase of SBR with 0.2 and 0.3 phr peroxide(DCP)showed a decreasing tend,besides,the area ratio of low-orientation phase and high-orientation phase showed a increasing tend,and the modulus had the tend of decreasing first and then increasing.Within the appropriate peroxide(DCP)content range,the SBR with high peroxide(DCP)content was more prone to tensile orientation behavior and had higher orientation degree.The correctness of the AFM study results was confirmed by the Mooney-Rivlin model fitting.(2)According to the analysis of mechanical properties,within the range of 1-2phr sulfur content,the unfilled SBR with 1 and 2 phr sulfur had more representative stress-strain curves.Therefore,the relationship between microstructural evolution and the change of mechanical properties of the unfilled SBR with with 1 and 2 phr sulfur during tensile process was studied intuitively and quantitatively by atomic force microscopy(AFM).Besides,the effect of sulfur content on the tensile orientation behavior of SBR was also revealed by AFM.The results showed that,the slender orientation structure gradually appeared during the tensile process of the unfilled SBR with 1 and 2 phr sulfur.Overall,with the stress increasing,the area ratio of amorphous phase of SBR with 1 phr sulfur showed a decreasing tend,besides,the area ratio of low-orientation phase and high-orientation phase showed a increasing tend,and the modulus had the tend of decreasing first and then increasing.However,for the SBR with 2 phr sulfur,the area ratio of amorphous phase showed a decreasing tend,besides,the area ratio of low-orientation phase and high-orientation phase showed a increasing tend,and the modulus had the tend of decreasing.Within the appropriate sulfur content range,the SBR with high sulfur content was more prone to tensile orientation behavior and had higher orientation degree.The correctness of the AFM study results was confirmed by the Mooney-Rivlin model fitting.(3)The tensile orientation behavior of the unfilled peroxide(DCP)-crosslinked SBR system and the unfilled sulfur-crosslinked SBR system was compared by AFM study results.The area percentage and the DMTModulus median of different phase regions of DCP/SBR and S/SBR samples at different strains were quantitatively counted according to the peak separation images of AFM LogDMTModulus distribution.The statistical results showed that,the orientation degree of the unfilled peroxide(DCP)-crosslinked and sulfur-crosslinked SBR systems increased as the strain increased during the stretching process;In the case of similar crosslink density,the unfilled sulfur-crosslinked SBR system was more prone to tensile orientation behavior,and the orientation degree was much larger than that of the unfilled peroxide(DCP)-crosslinked SBR system.In addition,the tensile orientation behavior of the unfilled peroxide(DCP)-crosslinked and sulfur-crosslinked SBR systems was further compared by infrared dichroism method and Mooney-Rivlin model.The results of infrared dichroism method and Mooney-Rivlin model were consistent with the results of AFM study,which confirmed the correctness of AFM study results.