Determination of the crystallinity of polyethylene/α‐olefin copolymers by thermal analysis: Relationship of the heat of fusion of 100% polyethylene crystal and the density

Abstract

The heat of fusion measured with differential scanning calorimetry (DSC) is typically divided by a constant value of the heat of fusion of 100% polyethylene (PE) crystal (⌬H m o ) for the estimation of the fraction crystallinity of PE copolymers, regardless of the density [i.e., the short-chain branching (SCB) concentration]. In this work, values of ⌬H m o of about 288 J/g were determined with a combined DSC and X-ray diffraction (XRD) method for a series of PE copolymers containing SCB from 0 to 50 Br/1000 C (density ϭ 0.965-0.865 g/cc). There was no systematic change in ⌬H m o observed across this density range. This result supports the suitability of determining the fraction crystallinity of PE of any density by the simple division of the observed heat of fusion determined by DSC by a constant value of ⌬H m o . This DSC method yielded values of PE crystallinity in good agreement with corresponding values determined by XRD for a series of PE copolymers. The determination of ⌬H m o involved a small precision error for higher density (lower SCB) PEs, but the precision error increased for lower density (i.e., higher SCB) PEs. This was due to the difficulty in measuring the heat of fusion for lower density PEs, which exhibited low values of the heat of fusion and melted only slightly above room temperature, and due to the difficulty of measuring lower values of crystallinity by XRD. The crystal thickness measured by small-angle X-ray scattering for this series of PE copolymers decreased exponentially from about 280 to 6 Å.

Polyethylene loss of ductility during oxidation: Effect of initial molar mass distribution

“…where ΔH m and ΔH m100 are respectively the melting enthalpy of the investigated sample and the melting enthalpy for an ideally 100% crystalline PE sample, which is equal to 288 J g −1 [15].…”

Characterization of Ultrafine Cellulose-filled High-Density Polyethylene Composites Prepared using Different Compounding Methods

“…where ΔHf is the heat of fusion of the neat HDPE and composites, ΔH°f is the heat of fusion for 100% crystalline HDPE (ΔH100=287.3 J/g), and ω is the mass fraction for HDPE in the composites (Mirabella and Bafna 2002). The results were calculated as the mean of three samples.…”

FTIR Microanalysis and Phase Behaviour of Ethylene/1‐Hexene Random Copolymers

“…The observed T m values decreased from 134.7 8C for the homopolymer to 124 8C for the copolymer with the highest 1-hexene content, and the corresponding DH m changed from about 135 to 27 J Á g À1 . From the latter values a change of the crystallinity degree from 46 to 22% was found for the copolymers, as calculated by the ratio of observed DH m to that of 100% crystalline PE, assumed as DH8 m ¼ 293 J Á g À1 [18] (crystallinity values calculated assuming DH8 m ¼ 288 J Á g À1 for PE [24] were found to diverge by less than 2%). The DSC crystallinities were found in good agreement with those calculated from wide-angle X-ray spectra, which displayed a continuous decrease in the intensity of crystalline peaks at about 2u ¼ 218 and 2u ¼ 248 (corresponding respectively to 110 and 200 reflections of orthorhombic PE crystals) with an increase in the 1-hexene content.…”