| Recently,the development of the fast-moving consumer goods industry has greatly promoted the research of buffering materials.With the increasing pollution of traditional non-degradable plastics,the design and application of degradable foams increasingly draw people’s attention.Starch is promising to work as a feedstock to develop eco-friendly foams,owing to its abundance,cost-efficiency,and biodegradability.However,the starch molecule has a large amount of free hydroxyl and is hydrophilic.During foaming,excess water molecules in the thermoplastic starch could weaken the foamed cells’strength,causing coalescence of cells,resulting in an irregular foam structure;while during storing,a strong plasticization of water to the starch from their hydrogen bonding could induce the recrystallization of starch,resulting in weak mechanical properties of starch foams.Developing starch-based foam with the incorporation of non-starch polymers is expected to improve the performance of the foam.As a natural plant fiber,citrus fiber is rich in soluble fiber(pectin)and insoluble fiber(cellulose,hemicellulose,and lignin),and has the advantages of high strength,low density,and antioxidant function.In this work,we use different technologies to systematically study the molecular interaction of citrus fiber and starch.Based on the theory of polymer chemistry and physics,a technical method for regulating the starch foaming process and modifying the structure and properties of starch-based foam through citrus fiber was established.The main results are as follows:1.Study on the molecular interaction of the soluble fiber-starch.Soluble fiber(pectin)was extracted from the citrus fiber by an acid method,and the effect of different levels of pH treatment on pectin structure was characterized.Accordingly,the pectin-starch mixture was treated in an alkaline solution at pH 9.5 to promote the compatibility of pectin and starch.After it,the molecular interaction of pectin-starch was further strengthened through a high-concentration static thermal treatment.Results showed that the processing could break the double helix structure of amylopectin,and promote polymer chains’Brownian motion,thus enhancing the intermolecular hydrogen bond of pectin-amylopectin and pectin-amylose,and improving network structure stability of starch gels stored at different temperatures.Due to this,the pectin-incorporated starch gel displayed a decreased syneresis rate from 45.9%to 4.50%after freeze-thaw cycles.2.Based on the esterification interaction mechanism of pectin and starch molecules,we explored and regulated the foaming process and properties of starch-based foams.The pectin-starch system was treated by a melt mixing treatment,and it showed that the effect of the thermal and mechanical energies could induce esterification cross-linking between chains of pectin and starch by depolymerizing starch chains and promoting the friction of pectin and starch molecules.This improved the viscosity of thermoplastic starch,thus increasing the equilibrium torque from 1.14 to2.43 N·m.Accordingly,we loaded the foaming agent and used a two-step treatment of melt mixing and hot-pressing to produce pectin(0-8wt%)-starch foams,and the effect of pectin on the modification of starch’s foaming process as well as the improvement of starch-based foam’s properties was carefully studied.3.The starch-based foam’s structures and properties were strengthened using citrus fiber.The citrus fiber(0～40wt%)-starch foam precursors were determined by the Variable-Temperature Fourier Transform Infrared Spectroscopy.It showed that the thermal energy(≥100℃)could promote esterification interaction between chains of fiber and starch by intensifying polymers’vibrations and increasing the interaction probability of carboxyl and hydroxyl groups.Accordingly,the effect of citrus fiber-starch interactions on the regulation of the starch foaming process was explored.By controlling the factors of citrus fiber’s structure,chemical composition,and loading concentration,the foaming density of the starch-based foam could be reduced to 0.14 g/cm~3,and its compression strength was increased to 1.41～6.47 MPa.The moisture resistance of the starch-based foam was improved under a high-humid storage,this was because the crystalline structure of citrus fiber was difficult for water molecules to penetrate,which caused a tortuous path effect and prolonged the transfer way of water molecules in the starch-based foam.The citrus fiber-starch foam also presented an excellent antioxidant activity.4.The mechanism of citrus fiber on the improvement of the starch-based foam’s structures and properties was studied.Citrus fiber was modified by the combined technology of alkali and homogenization.Results showed that the combined treatment could depolymerize fiber chains,destroy hydrogen bonds between cellulose chains,thus inducing hydrogen bonds between cellulose-pectin/hemicellulose,which reduced the crystallinity(from 31.97%to 23.35%),and decreased the average particle size(from 127.0 to 67.0μm)of citrus fiber.Due to the looser structure of the modified fiber and the exposure of its free carboxyl and hydroxyl groups,the esterification crosslinking between chains of the modified fiber and starch could be promoted by the melt mixing treatment at 90℃.Accordingly,this chemical crosslinking interaction modified the fiber-starch interfacial properties and improved the starch foaming process,making the starch-based foam display a more uniform cell structure,higher expansion ratio(5.9～7.1),improved recovery ratio(92.1%),and storage stability under high humidity.In conclusion,this paper systematically explored the fiber-starch interaction and regulated the fiber-starch interfacial properties,thus effectively regulating the starch foaming process and strengthening the structures and properties of the starch-based foam.The results deepened the research of fiber-starch interactions,enriched the theoretical research system of the intrinsic relationship between the molecular and macro-structural properties of foams,and provided fundamental data support for the development of degradable polymer materials. |
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