Effect Of Particle Size On Physicochemical Properties Of High Amylose Maize Starch And Construction Of Starch-based Composite Film

High amylose maize starch(HAMS)has been widely used in the production of starch composite films for its structural stability,high gel strength and good film formation.However,the poor compatibility,hydrophobicity and dampness between starch and polyester severely limit its application in starch composite films.In this paper,HAMS as the research object,from the perspective of granularity effect,to explore the effect of particle size on its granule structure,modification activity and film formation.Took HAMS as the base material,with polybutene adipate-butyl terephthalate(PBAT),polylactic acid(PLA)melt blend starch/polyester composite film.On this basis,gradient composite films were prepared by in-situ extrusion technique,and the tensile and barrier properties of starch/polyester composite films were gradually optimized.The related mechanism was preliminarily discussed,which provided theoretical guidance for the development of starch-based composites and related application research in the future.The main research contents and results are as follows:Firstly,the structure and physicochemical properties of RMS and HAMS with large,medium and small particle sizes obtained by jet milling were characterized to explore the granularity effect on starch particles.With the decrease of granule size,the specific surface area of RMS and HAMS increased,but the relative molecular weight,crystallinity and gelatinization enthalpy decreased.The specific surface area of RMS-S and HAMS-S are 354.4 m~2/kg and506.4 m~2/kg,respectively.The relative crystallinity is 20.83%and 17.24%,and the enthalpy of pasting is 5.56 and 8.84 J/g.Compared with RMS-S,HAMS-S has higher specific surface area,lower relative molecular weight and apparent viscosity,which obtain more stable viscous flow in molten state.Therefore,the small size effect of HAMS is more conducive to plasticizing and film forming.Secondly,in order to evaluate the influence of particle size effect on the modification activity and film formation of HAMS,the different particle sizes of HAMS were modified with hydroxypropyl,and starch film was prepared by casting method with glycerol as plasticizer.At30%epoxide etherification level,hydroxypropyl substitution degree of HAMS-L,HAMS-M and HAMS-S reached 0.10,0.14 and 0.19,respectively.The substitution degree of hydroxypropyl increased with the decrease of particle size.The relative crystallinity of HAMS increases with the improve of etherification level,proving that the hydroxypropyl reaction occurs in amorphous region.The results of film forming test showed that,compared with HAMS-L,HAMS-S had better film forming property,with tensile strength of 18.24 Mpa and elongation at break of 40.21%.After hydroxypropyl modification,the transparency and elongation at break of starch film increased,but the tensile strength decreased.Thirdly,based on the casting film,HAMS-S,PBAT and PLA were blended to prepare starch/polyester composite film to improve the tensile and moisture resistance properties.With the increase of HAMS-S content,the relative crystallinity and surface smoothness of the composite film decreased,and the aggregation degree of starch granules increased.The starch/polyester composite film with 25%HAMS-S content has the best comprehensive performance,with tensile strength of 14.19 Mpa,elongation at break of 600%and water contact Angle of 96°on both sides of the film at 60s.The reduction in ductility caused by the introduction of HAMS-S,which reduced the 25%elongation at break by 72.6%compared to the control,was not good enough to meet the ductility requirements of its various applications.Finally,on the basis of the above starch/polyester composite film,starch/polyester gradient composite film was prepared by in-situ extrusion for many times to improve the compatibility of starch and polyester to increase the elongation at break of the composite film.SEM results show that the size of HAMS-S phase was reduced and the crystallinity and thermal stability were improved after in situ extrusion.In situ extrusion is helpful to improve the compatibility of starch and polyester.The internal structure of the film shows a continuous change from sparse to dense from the center to the surface,which proved the formation of gradient structure.When the number of in situ extrusion was three times,the elongation at break of the gradient composite membrane in the MD and TD directions increased by 8.01%and 11.02%,respectively,the initial water contact angle increased from 94.7°to 101.7°.The kinematic ability of starch and polyester segments was increased,and the ductility and surface hydrophobicity of the composite films were significantly improved.