Polyethylene (PE) is an important polymer, used in everything from plastic bags, wrappings, bottles and even boats. The reason for the many different types of products manufactured from PE is that changing the molecular structure and weight can modify its physical properties. Crystallinity is a key property that depends on these structural parameters, and again determines the physical properties of PE samples and therefore their possible use. Work is constantly going on to improving the properties of PE. In this thesis the morphology of PE is investigated by Nuclear Magnetic Resonance (NMR) spectroscopy. A new quantitative method for determining the crystallinity of PE from the free induction decay (FID) has been investigated.
The "FID-analysis" technique involves model fitting to three phases. The crystalline part of the FID is represented by an inverse Fourier transform of the Pake function. A fast decaying exponential function is assigned to the intermediate phase, and a Weibullian function is assigned to an amorphous phase. At room temperature the crystallinity derived by the "FID-analysis" is in excellent agreement with the crystallinity obtained by other methods. It is also shown that the crystallinity obtained from conventional spectral analysis is under-estimated by approximately 8 % due to signal loss during the pre-sampling delay.
The "FID-analysis" method has been shown to give in-situ information on phase distribution during melting and crystallization. The change in intensities during crystallization was discussed in the light of the Avrami equation, and crystallization rates have been discussed within the thermodynamic frame work. The average molecular mobility within the crystalline phase as a result of crystallization and melting was obtained from second moment calculations. Moreover, a difference in mobility is found between the amorphous and molten phases of PE, and therefore also a probably difference in densities. A combined NMR and Differential scanning calorimetry (DSC) analysis of PE suggests that the intermediate phase also goes through a melting transition with an enthalpy of melting of approximately 140 J/g.
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Last modified: Desember 30, 2000. Per Eugen Kristiansen(firstname.lastname@example.org )