Molecular Dynamics Simulation And Properties Of Thermoplastic Starch

"White pollution" caused by the traditional plastics become more and more serious because of its non-biodegradable properties,while the full biodegradable materials(e.g.,PBAT,PBS,PLA,etc.)in the market are limited due to their high price.Starch has many advantages such as wide source,renewable,short regeneration cycle,low price and good film formation.As a low-cost filler,starch can be blended with other fully biodegradable materials to reduce production cost.However,there are strong inter-and intra-molecular molecules hydrogen bonds in starch,which makes the processing temperature of starch higher than its decomposition temperature.By adding plasticizer to starch and reducing the processing temperature,thermoplastic starch(TPS)can be successfully prepared.However,the effects of water content,type and number of functional groups of plasticizers on the properties of TPS have not been reported in detail based on the simulation calculation from the microscopic molecular angle.In this study,the effects of starch water content,plasticizer functional group type and plasticizer functional group number on TPS performance were compared and analyzed by combining molecular dynamics simulation and experiment.The specific research content and main results are as follows:(1)Glycerol was used as the plasticizer to explore the effect of starch water content(8%-14%)on plasticizing effect.The results showed that the diffusion coefficient of glycerol in starch increased with the increase of water content when the water content of starch ranged from 8% to 11%,and the ability of glycerol forming hydrogen bond with starch increased gradually.When the water content of starch was more than 11%,the diffusion coefficient of glycerol tended to be stable,and the crystallization peak type of TPS did not change,indicating that glycerol was fully diffused in starch and starch plasticized completely.When the water content of starch was 12%,the results showed that the capacity of hydrogen bond formation between glycerol and starch reached the maximum by the analysis of the radial distribution function(RDF)of hydrogen bond formation between glycerol and starch,the hydrogen bond energy between glycerol and starch,and the infrared test results of TPS.However,with the further increase of water content,excessive water molecules increased the hydrogen bond energy between starch and water,glycerol and water,and reduced the bonding ability between glycerol and starch,thus reducing the tensile strength of the prepared TPS.In conclusion,when the water content of starch was12%,the bonding ability between glycerol and starch was the highest,and the prepared TPS maintained certain mechanical properties(tensile strength 5.6Mpa,elongation at break 140%),and the plasticizing effect was relatively better.(2)Based on the results of(1),the water content of starch was selected as 12%,and ethylene glycol(EG),ethylene diamine(EDA)and ethylene dimethylamide(EBF)were selected as plasticizers to explore the effect of functional group types(hydroxyl,amino,amide)on starch plasticization.The results showed that the ability of plasticizer to form hydrogen bonds with starch was closely related to the type of functional groups of plasticizers.For EG and EDA,hydroxyl and amino groups mainly formed hydrogen bonds with hydroxyl groups on starch molecular chains.However,the amide group in EBF can not only form hydrogen bonds with the starch hydroxyl group,but also form hydrogen bonds with the ether bond on the starch main chain,so that EBF has a stronger ability to form hydrogen bonds with starch,and the prepared TPS has a lower glass transition temperature,smaller crystallinity and smoother microstructure.(3)According to the research needs,a new plasticizer containing three amide groups: N,N-bis(2-formylaminoethyl)formamide(BFEF)was designed and synthesized to further explore the effect of amide base order(1,2,3 amide groups)on starch plasticization.The results showed that the three amide plasticizers could not only form hydrogen bonds with hydroxyl groups on starch,but also form hydrogen bonds with ether bonds on starch backbone.The space steric hindrance increases with the increase of the number of the amide group and the molecular chain of plasticizers,resulted in a decrease of the diffusion ability of plasticizers in the chain of starch molecular,as a result,prat of the hydrogen bonds between starch molecules will be retained in the system,which ensure the good mechanical properties of TPS.The results proved that the tensile strength of TPS prepared with BFEF reached 6.8Mpa,and the elongation at break was 65%.