Research On The Multi-scale Structure Of Hydrophobic Starch-based Nanocomposites And Plasticizer Migration

Starch presents the advantages of environmental protection,low price and extensive sources,and thus starch-based packaging material,as a hotspot in food packaging and pharmaceutical industry,has taken the place of petroleum-based plastics gradually.However,poor processing performance of natural starch restricts the development of starch-based material,which could be overcome by some processing aids such as plasticizer to improve the properties and expand application fields.Nevertheless,as well as in conventional plastics,plasticizer would diffuse and migrate inevitably when starch-based food packaging material contacts with food system.This undesired migration would arouse untoward effects such as deterioration of food quality and packaging performance.Therefore,it is necessary to control plasticizer migration in order to achieve the reasonable application of biodegradable food packaging material.Nowadays,much attention has been paid to nanofiller that could strengthen mechanical and barrier property of packaging material.Based on the interrelation between packaging material structure and plasticizer migration,the effect of nanofiller on multi-scale structure and performance of composites would affect the plasticizer migration.In this dissertation,hydrophobic starch-based nanocomposites were prepared to probe into the interaction of nanofiller/starch/plasticizer,and the effect of macromolecular structure on the microstructure of nanocomposites and plasticizer migration were discussed.The findings established the foundation for controlling plasticizer migration of starch-based packaging material and further designing desirable material.Cornstarch samples with different amylose/amylopectin ratio(waxy,normal,G50 and G80)were esterified with substitution degree(DS)of 2.61,2.36,2.40 and 2.50,respectively.Modern analytical techniques were applied to study the multi-scale structure and thermal stabilities of starch esters.Results showed that morphology of corn starches were destroyed after esterification and generated some holes even aggregates.Along with the increasing amylose/amylopectin ratio,the prepared starch ester was observed with more ordered structure and strengthened thermal stability.Hydrophobic starch-based materials were prepared and triacetin was chosen as plasticizer.The microscopic morphology,crystalline structure,ordered aggregation and dynamic thermo-mechanical property of composites were discussed.With plasticization of triacetin,smoothness and homogeneity of fractured surface were ameliorated,and the flatness of material with higher amylose/amylopectin ratio was descended because of linear starch molecular rearrangement.Further,due to the diversity of rearrangement and aggregation of amylose and amylopectin after the plasticization,the motility of linear starch molecular was more readily enhanced to form microcrystalline and ordered structure.The plasticization was more intensive in “plasticizer-rich domain” which was generated from formation of crystallite,and the amorphous starch chain displayed relaxation behavior at lower temperature.On the other hand,the ordered structure restricted the relaxation behavior of amorphous chain and weakened the phase transition.With more obvious mobility,the glass transition temperature of starch-based material with more linear macromolecules was decreased compared to the materials with more amylopectin.Based on the research above,starch-based nanocomposites were prepared by adding organically modified montmorillonite(Dellite 72T).Modern analytical techniques were applied to study the variation of multiscale structure and dynamic thermo-mechanical property.Starch-based nanocomposite containing macromolecules with more branches structure displayed an amorphous matrix,and the nanofiller were dispersed homogeneously to form exfoliated structure and the lattice structure of nanofiller was also destroyed.With the amylose content increasing,the interplanar spacing of nanofiller within the corresponding material was enlarged to form the intercalation,and simultaneously,the crystallite of starch-based material was changed to lower degree.With the increase of nanofiller content,the aggregation of nanoparticles was observed and phase separation in nanocomposites was formed.In addition,the glass transition temperature of nanocomposites was ascended compared to the control,and the nanocomposites with higher linear macromolecules showed lower glass transition temperature.When starch-based film contacted with distilled water,the plasticizer migration in all hydrophobic nanocomposites obeyed the first-order kinetics for overall releasing process,and obeyed the Fick diffusion law in the short-term migration.The nanocomposites with exfoliation displayed the smallest diffusion coefficient of plasticizer migration due to the homogeneous film matrix,and comparatively,the intercalation and phase separation of within nanocomposites caused more fast migration.Along with plasticizer migration,the interaction among starch molecules was enhanced,which promoted aggregation further and increased the roughness of fractured surface.Meanwhile,the enhanced interaction between nanofiller and starch ester,as well as the rearrangement of macromolecules,simultaneously motivated new exfoliation from intercalation progressively,which contributed to the nanocomposites were detected as amorphous state.In this dissertation,a pattern was proposed that multi-scale structure difference of starch-based nanocomposites attributed to different starch molecular structure,and the effect on plasticizer migration further.It was expected to provide theoretical basis for efficient control to plasticizer migration and reasonable design of starch-based food packaging material.