Air Plasma-Initiated Melt Crosslinking Of Low Density Polyethylene

Crosslinking polymer materials have been widely used in many application fields,such as vulcanization of rubber and high voltage cable insulation.The study on the correlation between crosslinking network structure with properties can provide the guide for developing new crosslinking materials.In this thesis,plasma technology was used to introduce active functional groups to LDPE surface.Furthermore,rheological behaviors,crystallizaition behaviors,and mechanical properties of slightly crosslinked LDPE were investigated to reveal the influences of crosslinking network.The peroxide radical concentration on the surface-treated LDPE was quantitatively determined using 1,1-Diphenyl-2-picrylhydrazyl(DPPH)method and the maximum of concentration was 5.48?10^-6 mol/g with the air plasma and the maximum discharging voltage(80V).The functional groups of LDPE surface were analyzed by attenuated total internal reflectance Fourier transform infrared spectroscopy(ATR-FTIR)and X-ray photoelectron spectroscopy(XPS),which proved the formation of C-O,C=O and O-C=O after surface treatment.The rheological analysis and tensile tests showed that the plasma treatment could etch the LDPE surface,but it could not change the bulk structure of the LDPE.The rheological behaviors of slightly crosslinking LDPE were investigated by small amplitude oscillatory shear(SAOS)experiments,including viscosity,storage modulus,loss modulus,loss angle,Han curve and Cole-Cole curve after crosslinking.Moreover,the results of tensile experiments showed that formation of slightly crosslinking structure could improve mechanical properties of LDPE.Compared with un-crosslinked LDPE,the yield strength(?y1 and?y2)of slightly crosslinking LDPE with the gel fraction of 4.1%increased from 7.0 MPa and 8.4 MPa to 7.5 MPa and 9.5MPa,respectively,and the tensile strength(?b)increased from 13.2 MPa to 15.7 MPa.DSC results showed that the formation of crosslinking network could slightly decrease the crystallinity of LDPE and decrease the memory effect.In the meantime,the use of wide-angle X-ray diffraction(WAXD)further verified the results of DSC,demonstrating that the crosslinking decreases crystallinity but does not change crystal modification.The crystallization kinetics of the quiescent and shear-induced isothermal crystallization of slightly crosslinking LDPE was investigated by the rheological method.The results showed that the slightly crosslinking structure could accelerate the crystallization rate of LDPE for both quiescent and shear-induced crystallization.