Preparation And Rheological Study Of Ionic-Crosslinked Konjac Glucomannan Gels

Konjac glucomannan (Abbr. KGM), a natural water-soluble polysaccharide originating from the tubers of konjac, has favorable water-preserving, thickening, filming abilities and biocompatibility. The recent studies on the foundation and applications of konjac glucomannan in foods, bio-technical, petroleum chemical and some new materials fields came to front. The main contents and conclusions were as follows:(1) Thermo-reversible KGM/borax complex gels were prepared using borax as crosslinker. The mechanism was below. Gel network was formed immediately through the crosslinking reaction between borate ions dissociated from borax and the cis-diol sites on the polysaccharide chains. Dynamic viscoelastic rheology measurements and analysis shows, at lower oscillation frequency, the complex gels behave like liquid, whereas at higher frequency they relax like true gel materials with the predominant elastic relaxation response. An obvious plateau value (G_p') of storage modulus ( G' ) and a maximum and minimum in loss modulus ( G" ) were observed on the relaxation spectrum. The characteristic viscoelastic functions were studied as a function of concentrations of konjac glucomannan and borax, strain, and temperature. The experiments and analysis show the modulus obey time-concentration superposition. By using mathematical derivation of the experiment results, we got the empirical expression of quantitative G_p' to the concentration of KGM and borax. Meanwhile, the modulus of the complex gels obey time-temperature superposition, Relaxation and crosslinking energies for the gels were calculated from the superposition curves.(2) Second we prepared the thermo-reversible KGM/organic borate complex gels using organic borate as crosslinker. The gel network was formed through the crosslinking reaction between borate ions dissociated from organic borate and the cis-diol hydroxyl groups on the mannose units of polysaccharide chains. Unlike with the system of KGM/borax, borate ions have to escape from the binding of coordination agent and then can crosslink with the cis-diol hydroxyl groups, so the system of KGM /organic borate gels reflected the dynamic characters and crosslink delayed. The rheological experiments and analysis showed the critical sol-gel transition points and the gelation time can be exactly determined by the Winter-Chambon criterion. The effects of temperature and composite ratio on the shear storage modulus (G'), the loss modulus (G") and the sol-gel transition points were investigated. Temperature dependence of modulus of the complex gels showed that the characteristic temperature corresponding to the gel-sol transition, which was called gel-broken temperature can also be exactly determined by the Winter-Chambon criterion. The effect of crosslinking density on the critical gelation temperature and the elasticity of the gels were discussed. (3) KGM/organic titanium complex gels were prepared using organic titanium as crosslinker. The gel network was formed through the crosslinking reaction between titanium ions dissociated from organic titanium and the cis-diol hydroxyl groups on the mannose units of polysaccharide chains. Titanium ions have to escape from the binding of coordination agent and then can crosslink with the cis-diol hydroxyl groups, so the system of KGM /organic titanium gels reflected the dynamic characters and crosslink delayed. Dynamic viscosity behavior was studied with the rheological spectrometer. The results showed the critical sol-gel transition points and the gelation time can be exactly determined by the Winter-Chambon criterion. The effect of temperature, concentration of KGM and organic titanium on the critical gelation points and the elasticity of the gels were discussed.(4) The shear strain dependence of three kinds of gels we prepared was also studied in this paper. Two kinds of applied rheological parameters were deduced, which were called the critical strain and the yielding stress. The effect of crosslinking density on the mechanical properties of the gels were discussed.Essential scientific datas and theoretical foundation for the feasibility of the KGM-based thermo-reversible gels, which being used as water bone fracturing fluids in the oil and gas production, was provided by the results of this paper.