High Toughness And Permselective Poly(Lactic Acid)Membranes And Its Beneficial Effect Of MAP Packaging Of Fruits And Vegetables

In order to improve the toughness,processability,CO₂ permeability,CO₂ and O₂ permselectivity and water vapor permeability of poly?L-lactide??PLLA?.Flexible poly?ethylene glycol??PEG?and poly??-caprolactone??PCL?,which has good CO₂ permselectivity,were used as midblock to prepare PLLA-PEG-PLLA?PLGLxGy?and PLLA-PCL-PLLA?PLCLxCy?triblock polymers based on the molecular design.PLLA films with good toughness and characteristics can be prepared by adjusting the block length and compatibility.The modify film was more suitable for the preservation of cherry tomato and strawberry due to its good permeability and permselectivity,which can adjust the atmosphere in package to optimum low O₂ and high CO₂ state automatically.?1?First,the PLGLxGy copolymer was prepared by ring-opening polymerization of lactide with di-hydroxyl PEG as initiator.The effect of the molecule weight of PEG middle block and the chain length of PLLA,as well as the film forming conditions on the mechanical property and crystallization behavior of PLLA were investigated.Highly crystallization and amorphous samples was prepared respectively by hot press with different curing temperatures.Both DSC and WAXD results showed that the PLLA and PEG block in has good compatibility in the amorphous samples.However,phase separation was observed in crystallized samples.It should be noticed that the PEG was in amorphous sate in both crystallized and amorphous samples.The maximum elongation at break of amorphous samples reaches 600%.The maximum elongation at break of crystallization samples was lower than amorphous samples,but still higher than 30%.The crystallization rate of PLGLxGy copolymer was increased and the molding cycle times of PLLA decreased to 6 min.?2?Combined the closed and the permeable system methods to determinate the respiration rate of cherry tomato and strawberry.Based on the measurement of respiration rate,the optimum CO₂ and O₂ permeability and permselectivity,as well as the effective area of films were estimated by the Michaelis-Menten type equation coupled with dynamic balance equation.Further,PEG was chose and used to modify the gas permeability and permselectivity of PLLA film.The result showed that the PLGL75G20 copolymer film has higher H2O,CO₂ and O₂ permeability and permselectivity than PLLA.The simulation results according to equations also proved that when the gas permeability is certain,use a certain area film for the MAP packaging of cherry tomato and strawberry can keep the package an optimum CO₂ and O₂ level.In addition,when use the same size film for the MAP packaging,the atmosphere in the PLGL75G20 package was more close to the optimum condition than PLLA package.According to all those results,it was assumed that the MAP packaging effect of PLLA can be well modified by PEG.?3?PCL with a molecular weight about 20000 was synthesized using ethylene glycol as an initiator.Further,the di-hydroxyl was used as initiator to prepare the PLCLxCy copolymer.Two different compatible system copolymer films were prepared by solution casting,the effect of middle PEG/PCL on the packaging performance including mechanical property,phase separation,H2o and CO₂ permeability,as well as CO₂/O₂ permselectivity were investigated.The DSC,ATR-FTIR,WAXD and TEM showed that both PCL and PEG blocks were in amorphous state in films.The crystallinity of PLLA blocks were only 4.3?7.2%and 5.4?11.5%in two kinds of films.That is to say both PLGLxGy and PLCLxCy films were almost in amorphous state.In addition,the PCL block has more significant impact than PEG on the compatibility of copolymers.PLLA and PCL blocks in PLCLxCy copolymer showed more serious phase separation,the size was about 8 nm.However,the size was about 3 nm in PLGLxGy copolymer.The extent of phase separation increases with increasing molecular weight of middle segments.The PLLA block was more ordered arrangement in PLGLxGy film than PLCLxCy.The PLGLxGy film exhibits quite good CO₂ permeability due to the excellent CO₂ dissolve and diffuse abilities of PEG,but the O₂ permeability was hardly affected by PEG.The PLGLxGy copolymer film shows remarkable CO₂/O₂ permselectivity in low temperature,the permselectivity decreases with increasing temperature,while increases with increasing molecular weight of middle block.-The CO₂/O₂ selective permeable ratio of neat PLLA film was about 3.the selective ratio of PLCL35C20 and PLGL35G20 films increased to 6.7 and 12.4,respectively.This is because there was a lot of micro-phase separation structure in those films,which greatly improve the toughness of PLLA.Those special structures also increase the gas permeability "gate" with high CO₂/O₂ permselectivity.?4?Based on the the oretical derivation,the PLGxG20 copolymers with high CO₂/O₂ permselectivity were chose and used for EMAP packaging of cherry tomato in cold storage.It was found that the perseveration of cherry tomato affected by ripening,senescence and storage environment.Compared with PLLA package,the copolymer film keeps the package a relative low O₂ and high CO₂ level,which can well slow the decline in fruit quality and inhibit the proliferate of microorganism.The O₂ level in the package almost not affected by the variation of PEG in PLGLxGy copolymers and restrained a steady-state.However,the CO₂ level decreased with increasing PEG.This is because the insert of PEG segment was hardly affect the O₂ permeability of copolymer films,but significant affect the CO₂ permeability of films.The atmosphere in PLGL35G20 package reach a steady-state with 3?5%CO₂ and 3?4%O₂ on the 6th day and the cherry tomato keep good sensory quality and nutrition constituent.?5?Uniaxial tensile PCL film was prepared by casting extruder machine.It was found that the tensile film has good gas permeability and high CO₂/O₂ permselectivity.When used for EMAP packaging of strawberry,the package restrains a steady-state with 1.2?4.2%CO₂ and 3?7%O₂ during 28 storage.Compared with PA/PE and PE,the strawberry keeps good quality and nutrition constituent within 20 days in cold storage.The PLCLxCy copolymer was prepared with PCL as middle block.The PLCL35C20 film,which has good packaging characteristics,was used for EMAP packaging of strawberry with the PLLA and PLGL25G20 as control groups.The quality of strawberry in PLLA package dropped swiftly under extremely high CO₂ and low O₂ level.The CO₂ in PLGL35G20 and PLCL35C20 packages shows slowly decrease at first and reach a steady-state level about 5?10%till the end of storage.The CO₂ level in PLCL35C20 package was slightly higher than PLGL35G20,while the O₂ reach a dynamic balance about 5-8%after 12th day.Both PLGL35G20 and PLCL35C20 packages keep optimum high CO₂ and low O₂ level due to the suitable permeability of films.However,the weight loss of strawberry in PLCL3520 package was only half that of PLGL35G20 package.As a whole.the respiration rate strawberry was well inhibited and the shelf life has been greatly extended.