Effects And Mechanisms For LDPE/MWCNTs Composites Irradiated With Low Energy Protons And Electrons

Low energy protons and electrons, which could cause serious damage tomicroelectronic devices used in space, are important parts of space charged particlesradiation environment. Polyethylene exhibits excellent space radiation shieldingefficiency. However, its mechanical and thermal properties are insufficient. Aimingat the requirements of radiation shielding for microelectronic devices used in space,the mechanical and thermal properties and the corresponding mechanisms ofpolyethylene/multi-walled carbon nanotubes (LDPE/MWCNTs) compositesirradiated with low energy protons and electrons were studied systematically, whichhas important academic value and engineering significance.In this paper, tensile test, differential scanning calorimetry (DSC) analysis andthermogravimetic analysis (TGA) were used to reveal the mechanical and thermalproperties of LDPE/MWCNTs composites before and after low energy protons andelectrons irradiation. The synchrotron radiation X-ray small angle scattering (SAXS)and wide angle diffraction (WAXD) were used to reveal the microstructureevolution real-time during the process of stretching and temperature changing. Themechanisms of mechanical and thermal behaviors and irradiation effects andmechanisms were analyzed. Besides, the proton and electron shielding efficiency ofLDPE/MWCNTs composites was evaluated.Experimental results show that, the tensile deformation behavior for LDPEafter low energy protons and electrons irradiation is different from that beforeirradiation. Tensile deformation behavior for LDPE irradiated with low energyprotons still shows three stage characteristics, while that for LDPE irradiated withlow energy electrons exhibits two stage characteristics. With increasing fluence,tensile strength increases, while elongation at break decreases. Low energy electronsirradiation can inhibit the formation of new crystals in LDPE to some extent.Crosslinking of amorphous molecular chains in LDPE caused by low energy protonsand electrons irradiation is the dominant mechanism for changes of tensiledeformation behavior. Under low energy protons and electrons irradiation,MWCNTs exhibit both displacement and ionization effects. The former leads tocontraction of MWCNTs, the latter is manifested as formation of steady unpairedelectrons.Tensile deformation behavior for LDPE/MWCNTs composites is significantlydifferent from LDPE. With the addition of MWCNTs from2%to8%, the tensilecurves of LDPE/MWCNTs composites change to two stage characteristics fromthree stage characteristics, and the stress increases significantly while the strain at break decreases significantly. Basing on the SAXS and WAXD analyses, it is foundthat MWCNTs can hinder the deformation of LDPE matrix, constrain thefragmentation of original lamellae, the formation of new crystals and the rotation oflamellae, meanwhile introduce orientation and interface strengthening mechanisms.After irradiated with low energy protons and electrons, the strain hardening rate forLDPE/2%MWCNTs composites increases significantly, resulting in increase oftensile strength and decrease of elongation at break. With increasing fluence andstrain, the aberrance degree of interfacial layer between LDPE and MWCNTsincreases. During tensile deformation process, the main strengthening mechanismsare crosslinking strengthening effect in LDPE and enhanced strengthening effect ofMWCNTs induced by irradiation (mainly interface strengthening).Basing on the SAXS and WAXD analyses, it is shown that low energy electronsirradiation can slow down the amorphous region expansion during melting, theformation of mesophase and the amorphous region contraction during crystallization.Low energy protons irradiation has no effect on the melting process of LDPE, whileslows down the formation of mesophase and the amorphous region contractionduring crystallization. MWCNTs can hinder the movement of amorphous molecularchains, inhibit the initial melting of lamellae, but promote the melting process afterthe initial melting begins, and promote the formation of mesophase, but inhibit thegrowing up of crystals. Low energy electrons irradiation may slow down theamorphous region expansion, the initial melting of lamellae, the melting process oflamellae, the formation of mesophase, the growing up of crystals and the amorphousregion contraction. Low energy protons irradiation has no effect on the meltingprocess of LDPE/MWCNTs composites, but slows down the formation ofmesophase, the growing up of crystals and the amorphous region contraction. TGAresults show that low energy electrons and protons irradiation can improve thethermal stability of LDPE. MWCNTs could enhance the initial degradationtemperature of LDPE/MWCNTs composites. Low energy protons irradiation canimprove the thermal stability of LDPE/MWCNTs composites, while low energyelectrons irradiation can weaken the thermal stability of LDPE/MWCNTscomposites, but still much higher than LDPE.The experiments indicate that LDPE/MWCNTs composites exhibit excellentshielding efficiency for6.2MeV proton and1MeV electron irradiation, which isbetter than Al. With increasing loading of MWCNTs, the shielding efficiency can befurther improved.