Study On Crosslink Density And Swelling Process Of Vulcanized Natural Rubber By Nuclear Magnetic Resonance

NMR has become a common technique to measure crosslink density of rubbers.In this work,solid-state NMR is used to measure the crosslink density of natural rubbers.Swelling process of rubber is also studied by NMR.The main results include:1.Three NMR approaches for measuring crosslink density of rubbers,i.e.,¹H double-quantum?DQ?NMR,¹H Hahn echo and ¹H CPMG echo,are systemically discussed.The origin of the difference in the measured crosslink density from the different approaches is discussed.Based on this,the dependence of the measured crosslink density on the echo time in the ¹H CPMG approach is studied.2.We construct a ¹H T₂ relaxation curve by summating the Gaussian weighted¹H CPMG relaxation curves acquired using different echo times.Analyzing this Gaussian weighted relaxation curve can yield the crosslink density of rubbers.The crosslink density values obtained in this way show a greatly improved correlation between the crosslink density and the material hardness.Compared with ¹H DQ NMR,our approach provides an equal or better correlation between the crosslink density and the hardness of rubber.Note that the experimental time of our approach is only one-tenth of that of ¹H DQ NMR.3.The ¹H DQ NMR is used to measure the crosslink density of the dry rubber,the swollen rubber and the swollen rubber after the solvent is drained.The swelling process of rubber and solvent extraction process are observed successfully.