Heat Capacities and Thermal Properties of a Homogeneous Ethylene-1-octene Copolymer by Adiabatic Calorimetry |
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Authors: | P J van Ekeren L D Ionescu V B F Mathot J C van Miltenburg |
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Institution: | (1) Chemical Thermodynamics Group, Debye Institute, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands;(2) DSM Research, P. O. Box 18, 6160 MD Geleen, The Netherlands |
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Abstract: | Specific heat capacities of a homogeneous ethylene-1-octene copolymer were measured by adiabatic calorimetry in the temperature
range from 5 to 400 K (stepwise heating at averaged rates of approximately 1 to 34 K h–1, after cooling at rates in the range from 8 K h–1 to 4 K min–1). The glass transition takes place from roughly 205 to225 K and is centred around approximately 215 K. At the latter temperature,
also the temperature drifts during the stabilisation periods are at maximum. Clearly, with devitrification above 215 K also
melting sets in. Using two sets of reference data (one for branched and linear polyethylenes, BPE, and the other for strictly
linear polyethylene, LPE)for completely crystalline and for completely amorphous material, the crystallinity of the polymer
was calculated as a function of temperature, within the two-phase model. In heating, the crystallinity decreased from 0.254
to zero in the temperature range from 220 to 360 K, confirming earlier DSC heat capacity measurements. During the stabilisation
periods, below325 K, negative drifts were observed, related to endothermic effects caused by melting. However, in the temperature
range from 325 K up to the end melting temperature, 360 K, positive drifts were measured, reflecting exothermic effects. These
are attributed to recrystallisation phenomena. The occurrence and amount of recrystallisation depend on the thermal history
of the sample: slower cooling and a longer time spent at a temperature of annealing clearly diminish recrystallisation.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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Keywords: | adiabatic calorimetry crystallinity ethylene-1-octene copolymer heat capacity recrystallisation relaxation |
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