| Superabsorbent hydrogels(SH)are three-dimensional matrix constituted by linear or/and branched hydrophilic polymers that are chemically cross-linked,with the ability to absorb large quantities of water(swelling ratio > 100).SH are gradually being applied into agriculture as soil conditioners and nutrient carriers to support the plant growth in the past few decades.It has been demonstrated that SH could significantly improve the physical properties of soil through influencing the soil permeability,density,structure,and texture to regulate the evaporation and infiltration rate of water in soil.Particularly,the application of SH is able to reduce the irrigation frequency,stop the erosion and water run-off,and increase microbial activity.Also,the controlled release fertilizer coated with swollen SH can improve their utilization and effectiveness.However,the high production cost and the non-biodegradability of traditional petroleum based SH has restricted their application at large scale in agriculture.The introduction of natural renewable starch as a raw material and the reactive extrusion(REX)as a green processing method to the production of SH contributes a lot to great reduction of production cost and environmental protection.However,during the production of starch based SH via REX,the viscoelasticity of starch melts with temperatureand shear-dependence(non-Newtonian behavior)and the evolution of constituent and structure caused by complicated reactions between the used components,makes it difficult to control the REX process precisely.The evolution of constituent and structure of the system would bring about the change of rheological behaviors,known as rheokinetics.Hence,the study on rheokinetics for starch melt graft copolymerization makes a great difference for regulating and modeling REX to the industrial production of starch based SH.In this work,we used a mixer to simulate REX process for starch melts graft copolymerization to prepare starch based SH.Innovatively,a combination of mixer mixing and on-line monitoring of in situ synthesis allowed studying the rheokinetics of graft copolymerization,which subsequently facilitates the characterization of microstructures of starch based hydrogels.Moreover,we explored the rheological effect of starch melts with various amylose content(AC)on the graft copolymerization and corresponding hydrogels.The establishment of reaction formulation in a mixer suitable for starch melt graft copolymerization laid a foundation for the following research.The starch melt graft copolymerization showed a graft efficiency lower than 40% using ammonium ceric nitrate as an initiator,which could be ascribed to its low initiating activity to produce a certain amount of homopolymer.With increasing either the ratio of starch to monomer or the moisture content,the graft efficiency significantly decreased using ammonium persulfate as initiator.The formulations with a ratio of starch to monomer greater than 15/15 and a moisture content lower than 70% gave a graft efficiency higher than 63% and monomer conversion higher than 89%.Nevertheless,with deceasing the moisture content,both the yield and the water absorbent capacity(WAC)of hydrogels decreased,due to an increased extent of destruction of network with an increased shear stress.Starch melts prepared from the mixer were loaded onto the plate of a rheometer to monitor its reaction rheokinetics.A modified Hill equation can be used to well fit the evolution of G'(t)for starch melts in situ graft copolymerization.of hydrogels followed a linear progression with the crosslinker concentration.The study on the effect of reaction temperature and initiator content indicated that the reaction kinetics of starch melts has predominated the reaction rheokintics,which indicated that the mixtures of starch melts were completely micromixing.However,G' of corresponding hydrogels were independent on reaction temperature and initiator content,which could be ascribed to the strong chains transfer during reaction.The tendency of formation of physical network between the amylose has resulted in a different rheokintics behaviors of starch melts with different AC.In situ synthesis of hydrogels in the rheometer can further facilitate the characterization of microstructures of reaction products.The cross-linker concentration can be effectively used to regulate the crosslinking density of hydrogels.With increasing the amount of cross-linker,the micromorphology of SH transformed from gridding membrane structure to alveolate network,resulting in a significantly decreased WAC.While,the initiator concentration has little influence on WAC of SH,ranging from 948 to 1192 g/g.The relative high reaction temperature contributed to the graft copolymerization,which avoid weakening of WAC of SH.Both WCS(AC: 3%)and G80(AC: 83%)based SH showed smaller WAC,which could be ascribed to a greater cross-linking density from amylopectin and less crosslinking with formation of sol,respectively.Corn starches with different amylose/amylopectin ratios,which have diverse chain structures and rheological behaviors,were used as model materials to study their effect on the starch graft polymerization and the network structures and performance of starch based hydrogels.The viscoelastic moduli of starch melts increased with increasing AC,leading to a decreased extent of micro-mixing detected by a reduced gelation time.With increasing AC,the graft efficiency was observed to decrease but with almost the same monomer conversion(about 87.5%).The extent of retrogradation of the starch increased and two-phase separation was enhanced for starch-based hydrogels with the increase of AC.Interestingly,microscopic analysis showed the SH from NCS and G50 starch(AC: 24% and 56%)exhibited a gridding membrane porous structure,resulting in a WAC of 550g/g.This result was attributed to the moderate crosslinking and slightly greater graft content.During the starch melt graft copolymerization,the high viscosity of starch melts has resulted in the increased tendency of chains transfer and strong auto-acceleration effect.The regulation of rheological behaviors of starch melts could ensure a high graft efficiency for graft polymerization and a superior WAC of starch based SH. |
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