The influence of Morphology on the Transport and Mechanical Properties of Polyethylene

Abstract: The sorption/desorption behaviour of n-hexane in high molarmass linear polyethylene (PE) and branched PEs with 0.39 and5.09 hexyl branches per 100 main chain C atoms andcrystallinities between 4 and 82% at 298 K has been studied.Crystal core contents determined by Raman spectroscopy werealways lower than those determined by density measurements. Then-hexane solubilities in the copolymers depended in anon-linear manner on the content of penetrable polymercomponent and were lower for homogeneous copolymers than forheterogeneous copolymers at the same contents of penetrablecomponent. The solubility of hexane in the linear PE sampleswas proportional to the volume fraction of the penetrablepolymer and the solubility was low in comparison with that ofthe branched PE of the same crystallinity. TheCohen-Turnbull-Fujita (CTF) free volume theory was capable ofdescribing the desorption process in the PEs studied. Theconcentration dependence of the thermodynamic diffusivitypredicted by the CTF free volume theory was confirmed by thedata obtained by the differential method, and the differencesbetween the results obtained by the integral and differentialmethods were within the margins of experimental error. Thedependence of the fractional free volume of the penetrablephases on the phase composition suggests that mass transporttakes place from the liquid-like component to the interfacialcomponent and that the penetrant molecules are trapped at theinterfacial sites. The linear PE samples showed a physicallyrealistic trend with a decrease in the geometrical impedancefactor (t) with decreasing degree of crystallinity, whereas theopposite trend was obtained for the copolymers. The decrease int with increasing crystallinity in the copolymers may beexplained by the presence of wide crystal lamellae in the lowcrystallinity samples.A novel melt-extrusion method was used to createcircumferential chain orientation in pipes of crosslinked PE.The microstructure of the pipes was characterized usingdifferential scanning calorimetry (DSC), density measurements,X-ray diffraction, infrared dichroism and contractionmeasurements. The mechanical properties were assessed byuniaxial tensile tests. The maximum degree of circumferentialorientation was obtained at the inner wall of the orientedpipe. The oriented pipe material exhibited a 5-15% higherdegree of crystallinity and a greater crystal thickness thanconventionally crosslinked pipe. The circumferential and axialmoduli of the oriented, crosslinked pipe were greater than thecorresponding moduli of the non-oriented crosslinked pipe.Blends of single-site materials of linear PE andethyl-branched PE were prepared using solution- and melt-mixingmethods. The thermal properties of the blends were studied byDSC and results obtained by the two mixing methods werecompared. Data obtained for heats of melting andcrystallization, melting and crystallization peak temperaturesand melting and crystallization temperature profiles wereessentially the same for the samples obtained by the two mixingmethods. The heat associated with the high temperature meltingpeak of the blend samples obtained by both preparation methodsafter crystallization at 398 K was higher than that of thelinear polyethylene included in the blends, suggesting that apart of the branched polyethylene crystallized at 398 K.Key words:n-Hexane diffusion, polyethylene, free volume,solubility, sorption, desorption, mechanical properties,orientation, thermal properties, blend.