Comparative Studies of the viscoelastic properties of polyethylene, polyisoprene and polyvinyl chloride subjected to constant loads at different constant temperatures

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Viscoelastic properties of selected polymers subjected to constant loads at different temperatures have been studied. The prepared samples were mounted in a furnace and subjected to a tension under constant loads at a specific temperature. Initial strain rates were determined. The change in elongation of the polymeric materials were measured and recorded. The loads were then increased for the same temperature and also at different temperatures. Graphs of strain against time were then plotted and the equations for the curves obtained were differentiated and the times substituted into the equations to obtain the strain rates. Generally, the initial strain rates were relatively higher due to chain uncoiling in the polymeric materials. Under a load of 0.5 kg, at temperatures of 50oC, 100oC and 150oC, polyethylene recorded minimum strain rates of 0.00346 min-1, 0.01128 min-1, 0.00346 min-1respectively with corresponding times of 28.06 mins, 18.16 mins and 6.52 mins.Polyisoprene, on the other hand, recorded comparatively low values of minimum strain rates as follows: under a load of 0.5 kg, at temperatures of 50oC, 100oC and 150oC respectively, the minimum strain rates were 0.0032989 min-1, 0.003478 min-1, 0.015146 min-1with corresponding times of 39.4 mins, 29.51 mins and 9.52 mins. PVC recorded the highest values of minimum strain rates under the same conditions with values 0.0131794 min-1, 0.024498 min-1and 0.0301 min-1 with corresponding time values of 24.21 mins, 14.24 mins, 12.26 mins.This can be attributed to PVC having the lowest heat distortion temperature compared to polyisoprene and polyethylene. Polyisoprene recorded the least strain rates under similar conditions of load or temperature due to the presence of crosslinks and double bonds in its molecular structure. Polyisoprene exhibited the highest stiffness and consequently the highest resistance to heat flow. The viscoelastic properties of the polymers used were characterized by temperature, loading, minimum strain rates and the time for the occurrence of minimum strain rates. The minimum strain rates moved to shorter times as temperature increased. At low temperatures, the deformation of the polymer was slow. However, at high temperatures, the deformation was fast.
A thesis Submitted to the Department of Physics, Kwame Nkrumah University of Science and Technology,In partial fulfilment of the requirements for the Degree of Master Of Science (Materials Science), June-2013