| A hydrogel is described as a soft,solid,or liquid-like unique condensed material that has a three-dimensional network,it is htdrophilic,and it can swell rapidly in water and retain large volumes of water in the swelling state without dissolving.These 3D network condensed materials usually form through chemical and physical crosslinking.It shows great water absorption,water retention and excellent biocompatibility.At the same time,it also has a certain mechanical strength.The hydrogel has a wide range of applications in agriculture,horticulture,sewage treatment,biological medicine and other fields.However,traditional hydrogels still have many problems,such as poor water retention and low mechanical strength.Moreover,most hydrogels are prepared based on synthetic polymer materials,which are difficult to degrade in nature,overuse of synthetic polymer materials will cause a series of environmental problems.In order to solve these problems,the combination of natural polymer materials and synthetic polymer materials to prepare composite hydrogels has become a promising research areas.In this study,nano-cellulose and starch,these two kinds of natural polymer materials,were added into the hydrogel constructed with acrylamide,and nano-sized calcium carbonate was used to modify them to prepare nano-sized calcium carbonate reinforced composite hydrogel.Reducing the amout of monometer and crosslinking agent.Nano-sized calcium carbonate was used as the condensation center to form hydrogen bonds with the molecular chains of nano-cellulose,starch and polymer,and the distance between the molecular chains was closer,the uniformity of crosslinking and mechanical strength were improved.The pore size,swelling property and mechanical strength of hydrogels can be changed by changing the addition amount of nano-sized calcium carbonate.Nano-cellulose is a promising nanomaterial with the advantages of low density,large specific surface area,good mechanical properties,etc.However,the preparation process of nano-cellulose is complex,energy consumption is really big,and the price is high.Although starch has poor mechanical properties,it comes from a wide range of sources,and it has a simple preparation process and a low price,which can be directly introduced into the hydrogel system only by gelatinization.Therefore,this study considered combining the two,using nanocellulose to improve the strength and starch to reduce the cost,so as to prepare a lightweight nanocellulose/starch composite hydrogel with a mass fraction of only 5%.The main research contents are as follows:(1)Nano-cellulose for hydrogel(TOCN)were prepared by Tempo oxidation method with fully bleached KP needle pulp as raw material.Discuss the reaction time and dosage of NaClO on the oxidation degree of fibers.The prepared TOCN was characterized by FT-IR and TEM.The results showed that the content of carboxyl group increased rapidly in 1～2 hours,slowly increased in 2～4 hours,and tended to be stable after 4 hours under different oxidant dosage.1 gram of dry pulp corresponds to 0.016 g TEMPO,0.16 g Na Br and 20 mmol NaClO,keep the p H in10.5 during the reaction process.After 4 h oxidation,the content of TOCN carboxyl group was about 1.851 mmol/g.Fibers were ultrasound treated for half an hour,TOCN with length of 300-400 nm and width of 5-10 nm could be obtained.(2)The content of amylopectin in tapioca starch(ST)is high,the cost of it is low.The preparation process of TOCN is mature and the ratio of length to diameter is relatively high,and these two are used as the framework of hydrogel.Three groups of hydrogels were prepared by mixing TOCN,ST,TOCN/ST(1:1)solution with acrylamide(AM)at the ratio of 1:4,respectively:PAM-TOCN hydrogel,PAM-ST hydrogel and PAM-TOCN-ST hydrogel.Using N,N ’-methylene diacrylamine(MBA)as the crosslinking agent and ammonium persulfate(KPS)as the initiator,the nanometer calcium carbonate suspension was uniformly dispersed in the hydrogel by ultrasonic wave to obtain the crude product,which was soaked in ethanol solution(80% volume fraction)for6 h to strengthen the hydrogen bonding effect.Finally,a porous hydrogel with certain strength was obtained.The dosage of nano-sized calcium carbonate was controlled as 0%,2%,4%,6%,8% and10%(relative to the mass of nano-cellulose and starch),and the effects of nano-sized calcium carbonate dosage on the shape,swelling property,mechanical property,rheological property and thermal stability of hydrogels were studied without changing the dosage of monomer and crosslinking agent.BET was used to detect the specific surface area and average pore size of the hydrogel.The results show that PAM-ST hydrogel preparation process is simpler and cost is low,only need to simple starch gelatinization pretreatment,can with acrylamide radical polymerization crosslinking form interpenetrating network,water quantity balance moisture content is higher,and higher elongation at break,but PAM-ST hydrogel with poor mechanical strength become a bigger problem.In comparison,the pore distribution of PAM-TOCN hydrogel is more uniform,the pore size is smaller,and the water retention rate,compression strength and tensile strength are higher,but the water absorption performance of PAM-TOCN hydrogel is poor,and the elongation at break is really low.The properties of PAM-TOCN-ST hydrogel are between PAM-TOCN and PAM-ST.Compared with PAM-ST hydrogel,the addition of TOCN improved the water retention and mechanical strength of the hydrogel.Compared with PAM-TOCN hydrogel,the addition of starch(ST)reduced the cost,increased the water uptake,equilibrium water content and elongation at break.Therefore,PAM-TOCN-ST hydrogel has better comprehensive performance.Appropriate addition of nano-sized calcium carbonate can make the pore wall of the hydrogel become thinner and the pore diameter become larger,improve the homogeneity of pore distribution,and improve the swelling property and mechanical strength of the hydrogel.However,the excessive addition of nano-sized calcium carbonate will make the pore size of the hydrogel to be too large,up to about 60 μm,and the pore wall to be too thin,which will lead to the decrease of the toughness of the hydrogel and the water retention.Therefore,when the addition of nano-sized calcium carbonate is 4%,it has a great influence on the pore size,mechanical properties,rheological properties,swelling properties of the three groups of hydrogels.Especially,PAM-ST hydrogel was greatly improved,the compressive strength and tensile strength of PAM-ST hydrogel were increased by 107.5% and 68.9%.Among the three groups of hydrogels,the PAM-TOCN-ST hydrogels with 4% nano-sized calcium carbonate addition have the best comprehensive performance.The pore size of the PAMTOCN-ST hydrogels is uniformly distributed in the range of 20-30 μm,the water retention is267.443%,compressive strength is 80.543 KPa,tensile strength is 84.745 KPa,and strain at break is 399.4%.This hydrogel has better mechanical properties and swelling properties... |
PDF Full Text Request