Retrogradation Behavior,Structure And Properties Of Thermoplastic Cassava Starch And It's Composites

Starch is a renewable,degradable green natural polymer material.The process of starch retrogradation is mainly the ordering of the starch molecular chains.The phenomenon of retrogradation occurs during the processing,use or storage of starch,which changes the properties of the material such as mechanical properties,crystallization,and hardness.Therefore,the research on the retrogradation behavior of starch/silica composites has theoretical and practical significance.At present,the research of starch retrogradation is mainly concentrated in the food field.In this paper,three kinds of starch composites of thermoplastic tapioca starch?TPS?/SiO₂,TPS/PVA and TPS/PVA/SiO₂ were prepared by melt blending of silica?SiO₂?and polyvinyl alcohol?PVA?to study the retrogradation behavior of starch composites and analyze the structure and performance of the retrogradation process.By differential scanning calorimetry?DSC?,Fourier transform infrared?FTIR?,X-ray diffraction?XRD?,polarized light microscopy?PLM?,scanning electron microscopy?SEM?,and thermogravimetric analysis?TG?to investigate the retrogradation behavior,structure and properties of starch composites at different retrogradation time,different amounts of SiO₂ and PVA,different SiO₂ surface characteristics and different SiO₂ particle sizes.The following conclusions can be draw from the paper:There are great differences in the degree of retrogradation,structure and performance of cassava starch composites at different retrogradation times.With the increases of the retrogradation time,the starch composites of the three systems will continue to form intermolecular hydrogen bonding,the number of crystals will increase,the degree of retrogradation will gradually increase,and will reach the maximum at the 21 st day.In addition,the crystalline crystals constantly change,and a stable Vh type is formed at the 21 st day.The thermal stability of which will be reduced.The tensile strength of starch composites at different retrogradation time varies due to the effect of starch water absorption.The more water is absorbed,the lower the tensile strength will be.There is a stable hydrogen bond will be formed between SiO₂ and starch,which will weaken the hydrogen bonding between starch molecules.The addition of SiO₂ will inhibit the retrogradation of starch composites.The effect of its amount on the starch composites isdetermined by the agglomeration of SiO₂ and the hydroxyl groups it carries.The use of 5 parts of SiO₂ will promote the degree of retrogradation of starch composites and reduce its thermal stability,but a composite material with higher tensile strength and better water resistance is obtained.The agglomeration effect of 7 parts of SiO₂ was obvious,which weakened the hydrogen bonding of starch composites and had the effect of inhibiting the retrogradation of starch composites.The modified SiO₂ can effectively control the retrogradation of the starch composite materials,has better compatibility and dispersibility with the starch.Which can prevent the SiO₂ particles from agglomerating and falling off in the matrix material and improve the tensile strength and water resistance of the starch composite materials,while their thermal stability reduce.The particle size of SiO₂ determines its surface activity,which in turn affects its effect in the polymer.The 200 nm particle size SiO₂ can effectively inhibit the retrogradation of starch,and the starch composite materials with better thermal stability can be obtained.The starch composite material with better water resistance and mechanical properties can be obtained when the particle size is 20 nm.PVA will form hydrogen bond with starch,weaken the hydrogen bonding between starch molecules,accordingly inhibit the degree of starch retrogradation.The use of 40 parts of PVA will reduce the melting enthalpy of the TPS/PVA composites,resulting in TPS/PVA composites with higher tensile strength and contact angle but lower thermal stability.PVA will destroy the birefringence behavior of starch,and make its polarized cross structure not obvious.The application of stress will promote the short-term retrogradation of starch composites,while there is a certain inhibitory effect on long-term retrogradation.The stress caused a blue shift of the hydroxyl peaks in the FTIR spectra,destroyed the polarized cross structure of starch,and increased the compatibility and binding of SiO₂ with starch.The SiO₂ particle size of 23?m greatly promoted the retrogradation of TPS/SiO₂ composites.