The Study Of Rubber Particles Structure And Molecular Chain Network Structure Of Natural Rubber Latex

Natural rubber (NR) is generally obtained from the NR latex coagulation, tabletting and drying, properties of which are closely related to the composition of NR latex, structure of rubber particles and structure of rubber molecular chains. Researches on the structure of rubber particles not only provide the fundamental basis for improving the quality of NR, but also understand the synthetic mechanism of NR. Therefore, there is all-important significance in both theory and practice for studying on the structure of rubber particles. In the work, we mainly focus on the influence of non-rubber component on the structure of rubber molecular chains, and the distribution of protein and phospholipid onto the surface of NR particles.Firstly, the centrifugal natural rubber latex(C-NR), deproteinization natural rubber latex (DPNR) and lipid-free DPNR latex (P-DPNR) were prepared using high speed refrigerated centrifugation, treatment of alkaline proteinase and phospholipase, respectively. Kjeldahl nitrogen method, Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance spectroscopy (NMR) and X-ray photoelectron spectroscopy (XPS) were carried out to analyze the chemical change of all samples. The results showed that nitrogen content of NR latex had a decrease from0.72%to0.13%after high speed refrigerated centrifugation, and nitrogen content of DPNR, PNR and P-DPNR also decreased to0.02%,0.12%and0.02%, respectively. The asymmetric stretching vibration absorption peak ascribed to O-P-O groups disappears. These results fully demonstrated that alkaline proteinase and phospholipase could effectively do the purification of NR latex.Secondly, in order to test metal ions concentration in all kinds of NR, inductively coupled plasma mass spectrometry (ICP-MS) was performed. Nuclear magnetic resonance crosslink density analyzer was carried out to analyze the non-vulcanized rubber. And gel permeation chromatography (SEC) had been used to investigate the molecular weight and molecular weight distribution. The results revealed that concentration of Mg2and Mn2decreased from123.05μg/g,1.35μg/g to5.23μg/g,0.09μg/g after the treatment of alkaline proteinase and phospholipase, respectively. With decreasing the content of protein and phospholipid, molecular weight decreased from5.63×105to4.09×105, whereas molecular weight distribution increased from1.16to1.34. Also, the gel content and crosslink density of NR decreased from42.9%,6.41×105mol/cm3to8.6%,3.02×105mol/cm3, respectively. Such results were due to that there existed the physical force or chemical bond between the metal ions and protein, phospholipid. Moreover, protein and phospholipid played a role of nodes in NR, contributing to the formation of intermolecular crosslink, enlarging the molecular weight and forming the intermolecular network structure.Finally, high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were utilized to analyze the morphology of NR latex particles. The results showed that NR latex particles are mostly spherical. A protein-lipid layer existed on the surface of NR latex particle, which played an important role in stabilizing the dispersion of NR particles. With the presence of surfactant, a surfactant layer would adsorb onto the surface of NR latex particles treated by alkaline proteinase and lipase, which replaced the lipid-protein layers, effectively protecting the latex particles from the aggregation. Additionally, the particle size distribution of NR latex particles was investigated by laser particle analyzer. The size was in the range from dozens of nanometers to several microns. What’s more, Zeta potential analyzer was used to analyze the stability of NR latex. With adding the surfactant into the latex, the Zeta potential of NR, C-NR, DPNR, P-NR and P-DPNR was-42,-43,-61,-45and-60mV, respectively. This phenomenon was attributed to the different distribution of protein onto these rubber particles, and suggested that protein played an important role for the stability of NR latex.