Crystallization Properties Of Amylose Nanoparticles And Application In Thermoplastic Starch Composites

As a renewable resource,starch has drawn much attention in recent years.The small size and large surface area of starch nanoparticles have made them widely used in the area of food,drug and waste water treatment.Starch nanocrystals prepared by acid hydrolysis can be used as reinforcements in thermoplastic starch composites to improve mechanical properties and barrier properties due to their high crystallinity.However,the time consuming and low yield of starch nanocrystals limited their applications.The nanoprecipitation method can prepare starch nanoparticles efficiently,but the crystallinity of the obtained starch nanoparticles is low.Efficient preparation of starch nanoparticles with a high crystallinity is desirable for their applications.In this study,amylose was used as raw material to prepare amylose nanoparticles by nanoprecipitation,and effect of drying conditions and precipitation conditions on crystalline structure and crystallinity of amylose nanoparticles were investigated,aiming at increasing crystallinity of the amylose nanoparticles.At the same time,amylose-fatty acid complex nanoparticles were prepared and used as reinforcements in thermoplastic starch composites prepared through casting method,mechanical properties and barrier properties were investigated.The crystalline structure and crystallinity of amylose nanoparticles dried by freeze-drying,hot air drying and room temperature drying were investiged.X-ray diffraction analysis?XRD?revealed that the freshly prepared amylose nanoparticles did not show any diffraction peaks,and the dried amylose nanoparticles presented the typical V-type crystalline structure,drying condition including relative humidity,temperature and time had significantly affects on crystallinity of the amylose nanoparticles.It has been found that the temperature cycles drying at 4?and 40?significantly increased crystallinity of amylose nanoparticles.Dried at 11%relative humidity for 24h?4/40?,12h/12h?,the crystallinity of amylose nanoparticles could be up to 50.05%.The average particle size?nm?and polydispersity index?PDI?of amylose nanoparticles obtained by dynamic light scattering?DLS?analysis indicated that the drying process caused some aggregates,but the dried amylose nanoparticles were well dispersed in water.Influence of the preparation conditions of amylose nanoparticles,temperature of the amylose paste and the ethanol dropping rate,on crystallinity of the amylose nanoparticles was investigated.Dropping ethanol of 20?into the amylose paste of90?at a rate of 5mL/min promoted association of amylose and ethanol molecules into a helical complex in the precipitated amylose nanoparticles,then dried at temperature cycles?4/40?,12h/12h?,the crystallinity of amylose nanoparticles was significantly increased?60.72%?.Thus,the optimum preparation conditions for preparing high crystallinity amylose nanoparticles were obtained.Acid hydrolysis?2mol/L HCl?and ultrasonic treatment were used to change the length of amylose molecular chains.The effect of amylose molecular chain length on the crystallinity of amylose nanoparticles was studied.XRD results indicated that the crystallinity of amylose nanoparticles gradually decreased from 60.72%to 36.10%during the acid hydrolysis time of 0-8h.When acid hydrolysis times were 24h and48h,the crystallinity of amylose nanoparticles began to increase again.Ultrasonic treatment could make scission of amylose chains,XRD showed that the crystallinity of the obtained amylose nanoparticles gradually decreased from 60.72%to 40.67%with the increasing of ultrasonic times,the change trend of crystallinity was the same as that of acid hydrolysis for 2h and 8h.It can be seen that shorter amylose chains can be unfavorable to the formation of single helix structure between ethanol and amylose with a range of molecular weights.Amylose-fatty acid complex nanoparticles were prepared by amylose and fatty acid.Amylose was complexed with fatty acids at 90?and then ethanol was used to precipitate the complex.The amylose-fatty acid complex nanoparticles were dried under temperature cycling conditions.The effect of the amount of added fatty acid,fatty acid carbon acid chain length and chain length of amylose on the size,crystallinity and morphology of complex nanoparticles were studied.FT-IR spectra suggested that amylose and fatty acids had compkex rather than physical mixing.The complex index showed that the effect of the amount of added fatty acid and fatty acid carbon chain length on complexing degree.XRD showed that all the complexes showed V-type crystal structure,however,the longer the carbon chain length of fatty acid,the smaller crystallinity of the complex.At the same time,the result of ¹³C CP/MAS NMR analysis showed that the palmitic acid was contained more?18.99%?in the complex among the three amylose-fatty acid complex nanoparticles.Amylose-fatty acid complex nanoparticles have been used as reinforciements in thermoplastic starch.The mechanical properties,moisture barrier properties moisture absorption properties,color and crystallinity of the composite films were investigated.The addition of amylose-fatty acid complex significantly increased the tensile strength,elongation at break,and Young's Modulus of thermoplastic starch composite films,and changed the moisture barrier properties and moisture absorption properties,the crystallinity of the reinforced thermoplastic starch composite films of the amylose-palatate and lauric acid complex nanoparticles had higher crystallinity,but did not significantly change the color of the films.