Analysis And Characterization Of Molecular Structure Of Natural Rubber

Natural rubber(NR)consists of approximately 94%cis-1,4-polyisoprene and 6%non-rubber components(NRC).NR is widely used in national defense,industry,medical treatment,and so on,for its excellent comprehensive properties,which is an important strategic resource and basic industrial raw material.The excellent properties of NR ascribe to its special structure and composition.Therefore,characterizing the molecular structure of NR and unraveling the role of NRC on the structure of NR can provide more insightful understanding to enhance properties of NR and design bionic NR.In this study,structure of NR particles and molecular chain structure of NR were investigated.Simultaneously,the role of proteins and phospholipids on the crosslinking network structure in NR were studied.The main research works and conclusions were as follows:(1)Cryogenic transmission electron microscopy(Cryo-TEM)and stochastic optical reconstruction microscopy(STORM)were used to investigate the structure of NR particles.The results show that NR particles are essential core-shell in structure with the polyisoprene as the hydrophobic core surrounded by a mixed monolayer of proteins and phospholipids ca.4.6 nm thick.The changes of NR latex particles were in-situ visualized by freeze-drying SEM technology and AFM during film formation process.The results demonstrate that removing proteins and phospholipids can quicken NR film formation.However,the mechanical properties of the NR film decrease in the absence of proteins and phospholipids.Based on the research,a film formation mechanism for NR latex is proposed.The role of proteins and phospholipids on electric double layer structure of NR were studied by laser Doppler electrophoresis.The results show that|?|decreases after removal of proteins and phospholipids.(2)The abnormal groups on molecular chain of NR were quantified by nuclear magnetic resonance(NMR)and fourier transform infrared(FTIR)spectroscopy.The results show that the contents of aldehyde groups,carboxyl groups,epoxide group,hydroxyl and 3,4 isoprene are 0.42%,0.03%,0.24%,0.02%and 0.06%,respectively.The branched structure and network structure of NR were studied by gel-permeation chromatography coupled to a viscosity detector as well as a multiangle laser light scattering detector(GPC-DP-MALLS)and Mooney-Rivlin model theory.The results demonstrate that proteins and phospholipids can be used as crosslinking points to promote the formation of branched structure and network structure of NR,which leads to high molecular weight and wide molecular weight distribution of NR.The relationship between the particle size and molecular weight of NR was investigated.The results show that small NR particles are mostly composed of branched molecules with higher molecular weight,while large NR particles contain linear molecules with no or few branches.Meanwhile,the number of molecular chains in rubber particles with different particle sizes was investigated.The results demonstrate that the number of molecular chains in large rubber particles is greater than that of small rubber particles.The small rubber particles with an average particle size of 106 nm contain ca.620 molecular chains,while large rubber particles with an average particle size of 628 nm contain ca.339,000 molecular chains.(3)The effect of proteins on vulcanization behavior and vulcanized crosslinking network in NR were studied.The results show that proteins can decrease E_aof vulcanization process,which contributes significantly to higher curing rate.Furthermore,to understand the mechanism at molecular level,FTIR spectroscopy and XPS spectroscopy were used to characterize the interaction among molecules.The results show the existence of coordination interaction among Zn²⁺,O atom(proteins),N atom(proteins).Such coordination interaction improves solubility of Zn²⁺in matrix and contributes to crosslinking reaction.Besides,some characteristic groups(for example,hydroxyl and carboxyl)from proteins can be converted into intermediates(for example,into thiol,thioestor and dithiocarboxylic ester groups)and further participate in crosslinking process,which further contributes significantly to higher crosslinking density and mechanical properties of NR.(4)The role of phospholipids on vulcanization behavior and vulcanized crosslinking network in NR were studied.The results from differential scanning calorimetry(DSC)and rubber processing analyzer(RPA)show that phospholipids can decrease the Ea of vulcanization process and contribute to crosslinking reaction.The results from FTIR confirm the existence of coordination interaction between some characteristic groups(for example,carbonyl group,phosphate group,and choline group)from hydrolysis of phospholipids and Zn²⁺.Such coordination interaction improves the solubility of Zn²⁺in matrix,which contributes to crosslinking process.From the results of TEM,energy dissipation and G_cof NR,we conclude that phospholipids network,featured by ionic interactions,can serve as sacrificial network which preferentially ruptures to dissipate energy for the improvement of toughness.Meanwhile,some phospholipids participate in crosslinking process and can act as crosslinking points,which increases crosslinking density and improves strength for NR.