Study On The Fresh-keeping Effect Of Poly(L-lactic Acid)Film With Topological Structure On Winter Jujube

The purpose of this paper is to preserve the fresh jujube and extend its preservation period,and it aims to prepare a biodegradable film that matches the respiration rate of the jujube.Star-shaped 4-arm-polyethylene glycol(4-PEG)and 8-arm-polyethylene glycol(8-PEG)were used as initiators to prepare polyethylene glycol with a set molecular weight through the ring-opening reaction of L-lactide.Poly(ethylene glycol)/poly(L-lactic acid)(PEG/PLLA)topological star copolymer.The packaging performance of the film was investigated using nuclear magnetic,Fourier infrared,differential thermal scanning analysis,tensile test and water vapor transmission performance test.The results show that PLLA is successfully inserted into 4-PEG and 8-PEG,and has a higher molecular weight;the addition of star polyethylene glycol improves the brittleness of the copolymer,among which 4-PEG/PLLA7 with different PLLA block molecular weights,The glass transition temperature(Tg)of 4-PEG/PLLA6 and 4-PEG/PLLA5 is lower than that of pure PLLA,8-PEG/PLLA6 and 4-PEG/PLLA6 with the same molecular weight of PLLA block,as the number of PEG arms increases,Tg showed a downward trend,which was lower than that of PLLA;but because the sample of 8-PEG/PLLA6 was in an amorphous state,it could not form a film for the next mechanical and water vapor test.4-PEG/PLLA7,4-PEG/PLLA6,and 4-PEG/PLLA5 elongation at break increased to 101%,121%,and 152%,respectively,and Young’s modulus decreased to 144,103,and 15MPa,respectively.In addition,the introduction of 4-PEG gives the material better water vapor permeability.The transmission coefficients(WVP)of 4-PEG/PLLA7,4-PEG/PLLA6 and 4-PEG/PLLA5 are 2,2.5 and 3 times that of PLLA.4-PEG/PLLA5 can meet the moisture permeability requirements that match the respiration rate of large-capacity packaging fruits and vegetables.In order to improve the gas selective permeability of the material.The 8-PEG/PLLA6 topological copolymer that is easy to crosslink is reacted with hexamethyl diisocyanate as an initiator to prepare a network 8-arm-polyethylene glycol/polylactic acid(NET-8-PEG/PLLA6)copolymer,after Add 5%,10%and 10%8-PEG/PLLA6 and NET-8-PEG/PLLA6 to blend with PLLA respectively to prepare blended film and network interpenetrating film.Characterized by nuclear magnetic resonance,Fourier infrared,differential heat scanner,tensile test and differential pressure transmission.The interpenetrating network structure gives the film better gas permeability and selective permeability.The selective permeability of CO2/O2 increases with the increase of the interpenetrating network structure.The selective transmission ratio of CO2/O2 is as high as 6.1,which is more suitable spontaneous atmosphere of fresh fruits and vegetables.It was further applied to the balanced modified atmosphere packaging of winter jujube.The results showed that the atmosphere in the packaging was maintained at 2%O2,5%after 28 days due to the higher CO2/O2 selective transmission ratio of the NET-PEL 20%material.CO2 is more suitable for keeping fresh jujube.It maintains better sensory quality,hardness and color difference during storage.The gas component in the packaging bag is more suitable for preservation of winter jujube.Among them,the preservation effect of NET-PEL20%packaging group is more significant,and the preservation period is extended to 40 days.