Study On Retrogradation Behavior Of Thermoplastic Cassava Starch/SiO₂ Composites Under External Field

Starch has the advantages of low cost,regeneration,wide sources and good biodegradability,which is a promising natural polymer material,and widely applied in packaging,textile,biomedical and agricultural fields.However,starch has a general retrogradation phenomenon,which changes the mechanical properties,thermal properties,crystallization properties,and hydrophobic properties of the materials.During the processing or use of starch products,under the action of external fields such as stretching,ultrasound,light,temperature field and compound field,the internal retrogradation heat behavior and retrogradation structure are changed.Therefore,the study on the effect of external fields on the retrogradation behavior of starch composites provides important guiding significance for the exploration of the structure of starch composites,the revealing of the retrogradation mechanism,and the improvement of performance.Taking cassava starch as the research object,thermoplastic cassava starch（TPS）/silica（SiO₂）composites were prepared to change the amount,particle size and surface properties of SiO₂.Apply tensile,ultrasonic,tensile and ultrasonic composite fields to the composites,study the effects of different external field conditions on their thermal properties and structure,and establish the correlation between thermal properties and material structure,in order to provide a theoretical basis for the material changes in the use process and reveal the regeneration mechanism under the external field.The relationship between thermal properties and material structure is established,aiming to provide a theoretical basis for material changes in the use process and reveal the regeneration mechanism under the action of external fields.Differential scanning calorimeter（DSC）and thermogravimetric analyzer（TG）were used to study the melting enthalpy,melting peak temperature and thermal degradation temperature of the composites in the process of regeneration under the action of external field;The retrogradation structures of TPS/SiO₂ composites under different external fields were studied by means of polarizing microscope（PLM）,contact angle analyzer（CA）,Fourier infrared analysis（FTIR）,X-ray diffraction（XRD）,scanning electron microscope（SEM）and dynamic thermomechanical analyzer（DMA）.The main results are as follows:By studying the melting enthalpy,melting peak temperature,retrogradation kinetics and thermal degradation behavior of TPS/SiO₂ composites under the action of tensile external field,it was found that the melting enthalpy first increased and then decreased with the increase of retrogradation time,and the melting enthalpy was retrograded for 14 days.With the addition of SiO₂,the crystallization rate constant of starch became smaller,and the inhibitory effect on the formation of starch crystals increased.With the increase of SiO₂ content,the melting enthalpy decreased,the melting peak temperature increased,and the thermal degradation carbon residue rate increased.The melting enthalpy increases and the melting peak temperature decreases with the increase of SiO₂particle size.The melting enthalpy of the composites with hydrophilic SiO₂（H10）is smaller than that of other composites with hydrophobic modified SiO₂,the melting peak temperature is also the highest,the residual carbon rate of thermal decomposition is the highest,and the thermal stability is the strongest.In this paper,the thermal behavior and thermal stability of materials under tensile field are revealed.By studying the structural changes of TPS/SiO₂ starch materials when subjected to a tensile external field,it is found that when subjected to a tensile external field,cracks will form on the surface of the material,and SiO₂ will fall off from the material;the V_A-type crystals in the material disappear,and the starch can be recovered in a short time.A-type double helical crystals were produced.With the increase of time,the number of spherulites increased,and the number of spherulites began to decrease after 14d retrograde.As the amount of SiO₂ increases,the contact angle increases and the number of spherulites decreases.The crystal types formed by adding SiO₂with different surface properties are the same,but the glass transition temperature（Tg）of SiO₂（L12）composites modified by hexamethyldisilazane decreases and the glass transition temperature of TPS/H10 composites increases.This study explores the change law of the structure of starch composites under stretching,which has guiding significance for the research and application of the tensile external field on the structure and properties of starch film materials.The thermal behavior of TPS/SiO₂ composites under the action of ultrasound was studied.It was found that the action of ultrasonic external field promoted the short-term retrogradation of starch,and there was an obvious melting peak after being subjected to ultrasound for 14 hours.The retrogradation degree increased with the increase of time,but the melting enthalpy did not change much.,the degree of retrogradation is the highest at 14d;the melting peak temperature decreases with the increase of time,and the thermal stability of the material decreases.Ultrasound promotes the movement of SiO₂.The smaller the particle size,the stronger the movement ability.As the particle size of SiO₂decreases,the melting enthalpy decreases,the crystallization rate constant decreases,and the inhibitory effect on starch retrogradation is enhanced;SiO₂ with a particle size of200 nm forms a network with starch.The structure improves the thermal stability of the material.Compared with the composites with hydrophobically modified SiO₂,the hydrogen bond between H10 and starch is the most obvious,the melting enthalpy of TPS/H10 is the smallest,the melting peak temperature is the highest,and the thermal stability of the material is the strongest.Through this study,the thermal behavior and thermal stability of the material under the ultrasonic external field are revealed.The effect of ultrasound on the retrogradation structure of TPS/SiO₂ composites was studied,and it was found that ultrasound accelerated the crystallization rate of starch,and could induce more EH-type crystals in a short time,and C-type crystals in the material disappeared.obvious.As the amount of SiO₂ increases,the contact angle becomes larger and the hydrophobicity improves.With the addition of nano-SiO₂,the particle size decreases,the agglomerated SiO₂ on the surface of the material decreases,the number of spherulites decreases,and the contact angle increases.Ultrasound makes the starch hydrogen bond peaks in the infrared spectrum of the composites with different surface properties SiO₂ red-shifted,and the hydrogen bonding between SiO₂ and starch increases,which limits the formation and growth of starch crystals,and the mobility of starch is restricted.increased Tg.The thermal behavior and structural changes of TPS/SiO₂ composites under the combined effect of tensile and ultrasonic external fields were investigated.It was found that the application of the first stretching and then ultrasonic external field and the application of the first ultrasonic and then stretching external field can promote the retrogradation of starch,and the melting enthalpy increases with the increase of time.With the increase of SiO₂ particle size,the retrogradation rate of starch accelerated,and the inhibitory effect on starch retrogradation became poor;with the increase of particle size,the melting enthalpy increased,the thermal degradation carbon residual rate decreased,and the thermal stability became worse.The crystallization rate of starch was accelerated by the combined action of stretching and ultrasonic external field,and more spherulites were observed within 14 h of retrogradation,and the number of spherulites increased with time.The SiO₂is uniformly dispersed by the action of the first stretching and then the ultrasonic external field,which enhances the compatibility of starch and SiO₂,and produces starch A-type crystals.In the infrared spectrum,the hydroxyl peak of water moved to a high wavenumber,and the water content increased,which further plasticized the tapioca starch,and the Tg peak of the tapioca starch in the material disappeared.