Thermal Hydrogen Bombardment (HHIC) Improves The Barrier Properties Of Polylactic Acid Films

In the past decades,more and more attentions have been paid to bio-based poly-mers due to the energy crisis and environmental problems.Poly(lactic acid)(PLA)is a biocompatible and bio-degradable aliphatic polyester derived from renewable re-sources(such as corn starch and vegetable oil)and shows high stiffness,high tensile strength and good processing ability.It is becoming one of the most promising bio-based polymers for food packaging materials to replace traditional petrochemi-cal-based polymers plastics.However,compared to conventional petrochemical-based polymer films,PLA has poor resistance to water vapor and oxygen.Constructing a dense barrier layer on the PLA film surface is a good way to im-prove the water resistance of PLA films.In this study,a new type of surface modifica-tion technique,Hyperthermal Hydrogen Induced Cross-linking(HHIC),was used to construct a dense cross-linking layer on PLA surface.The polar polypropylene acid(PAA)was grafted onto the PLA films by HHIC technology to improve the adhesion force between the PLA substrate and the assembled coating.On this basis,a multilay-er composite coating was grown by alternating poly(ethylene imine)(PEI),hydro-phobic fluorinated polymer(Nafion),and montmorillonite(MMT)layers with layer-by-layer(LBL)approach on PLA and PLA/PAA films.Structural characteriza-tion and the properties of the modified PLA films have been tested.The main results are listed as follows:(1)HHIC technology was employed to construct a stable and dense cross-linking layer on PLA films,which can serve as a dense barrier layer to water vapor diffusion.The water vapor permeability(WVP)of the PLA film significantly decreased from 26.74 to 22.96 g/m2·day by 14%after 2 min HHIC treatment.Besides,the cross-linking surface of the PLA film exhibited an improved hydrophobic state for water-proof due to the increased surface roughness in nanoscale.HHIC treatment process is shown to be a clean and mild(room-temperature reaction)method for mod-ifying polymer films with energy-controlled neutral hyperthermal hydrogen without breaking other bonds and sacrificing their original physical properties.(2)Polyacrylic acid(PAA)can be easily grafted on the surface of PLA films using HHIC technique,which was beneficial to improve surface wettability of PLA films.A stable chemical bond was formed between the PAA coating and the substrate after 60 s HHIC treatment.(3)The PAA transition layer significantly improved the binding force between PLA membrane and the coating,and enhanced the hydrophobic properties of the overall coating of PLA films(the water contact angle on the surface of PLA films in-creased from the initial 23 to the final 94 degrees).Thus the additional post processing of the composite material which was traditionally assembled to obtain high vapor barrier was avoided.(4)The layer-by-layer method was used to assemble alternating layers of PEI,Nafion,and MMT on the PLA and PLA/PAA surfaces with the aim of obtaining low gas permeability.When the number of[PEI/Nafion]2/PEI/MMT units increased to 10 layers,HHIC modified films presented excellent barrier properties.The oxygen transmittance decreased from 150 cc/(m2·day)to 0.5 cc/(m2·day)(reduced by 99.7%,almost three orders of magnitude)and water vapor transmittance decreased from 32.93 g/(m2·day)to 16.25 g/(m2·day)(reduced by 50.7%).