Effect Of Temperature Gradient On Phase Separation And Diffusion In A Polyolefin Blend

In this dissertation, a UCST-type polyolefin blend, poly(ethylene-co-hexene)(PEH) and poly(ethylene-co-butene)(PEB) was selected as model system. Opticalmicroscopy (OM), thermal stage and digital image analysis were employed to studythe effect of temperature gradient on the phase separation kinetics, phase separationstructures and polymer diffusion in the copolymer blend PEH/PEB. The results wereshown as blow.For PEH/PEB polymer blends with an upper critical solution temperature, weinvestigated the effect of temperature gradient on spinodal decomposition. We find thatthe thermal gradient greatly accelerated domain growth through the enhancement inconcentration fluctuation in the early and intermediate stages. Because of the enhancedtemperature gradient the strength of concentration fluctuation close to the critical point(above the spinodal line) is strong enough to induce phase separation even inone-phase regime of the phase diagram.For the effect of a temperature gradient on the orientation of phase-separatedstructures in a polyolefin blend system. It was found that the bicontinuous andinterconnected tubelike structure, the characteristic morphology of the spinodaldecomposition process, exhibits a preferential alignment along the direction oftemperature gradient after phase separation. The orientation of the structures graduallyincreases with phase separation time and temperature gradients, but independent oftemperature. The orientation of phase-separated domains can respond really quickly tothe change in the direction of external temperature gradient field. The results suggestthat “thermal force” induced by the temperature inhomogeneity might play an important role in aligning phase-separated domains preferentially along thetemperature gradient direction.We then investigated the effect of temperature gradient on interdiffusion betweenPEH and PEB. It was found that in polymer blends, the thermal diffusion coefficient isdependent on molecular weight, the Scoret coefficient for PEH and PEB is negativeand positive, respectively. When the direction of the “thermal force” work on eachpolymer consistent with its move direction, the temperature gradient can accelerate theinterdiffusion process, conversely, it can hinder the mutual diffusion of polymerchains.