Diffusion of liquids into semicrystalline polyethylene

doi:10.1016/S0730-725X(98)00032-0

Magnetic Resonance Imaging

Volume 16, Issues 5–6, June–July 1998, Pages 647-649

https://doi.org/10.1016/S0730-725X(98)00032-0 Get rights and content

Abstract

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) and microimaging experiments have been performed to study the diffusion of liquid alkanes into a variety of semicrystalline polyethylene (PE) samples. The results highlight the importance of the crystalline phase in controlling the diffusion process in terms of both the geometric impedance imposed by the presence of impenetrable crystals and their effect on the mobility of the polymer chains comprising the amorphous material through which the penetrants migrate.

Introduction

An understanding of the diffusion of small molecular penetrants through a polymer matrix is required for a variety of technological applications, including the design of materials for separation processes, controlled pharmaceutical and agrochemical release devices, and packaging materials. This paper summarises the results of a study of the transport characteristics of a number of alkane penetrants within nine different semicrystalline polyethylene (PE) samples. In a semicrystalline polymer, liquid penetration is believed to occur within the amorphous material;1 a semicrystalline polymer can therefore be considered a porous material, with the amorphous material representing the pore space and the solid matrix represented by the crystallites.

Section snippets

Materials and methods

PE plaques (15 cm² and 2.8 mm thick) were prepared from commercially available PE pellets by moulding in a hot press held at 150°C for 10 min. Four high density polyethylene (HDPE) and five linear low density polyethylene (LLDPE) samples were studied in this work. The PE samples were characterised in terms of crystallinity and lamellar thickness using Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and relaxation time analysis, differential scanning calorimetry, and x-ray

Results and discussion

Typical profiles for hexane diffusion into a HDPE and a LLDPE sample are shown in Fig. 1. The liquid concentration profiles are in excellent agreement with gravimetric mass uptake experiments. There is very little swelling in the samples; the total density of the system must therefore be increasing on absorption of the penetrant. The maximum liquid concentration at the surface was found to increase with time, a result of the viscoelastic response of the polymer to the penetrating liquid.

Figure

Conclusions

The results described above indicate the importance of polymer crystallinity in determining the diffusivity of simple organic penetrants in semicrystalline PE. The effects of annealing of the polymer on the liquid diffusivity have been investigated.

Acknowledgements

S.H. thanks the Ministry of Agriculture, Fisheries, and Food (MAFF) for financial support.

References (3)

M Hedenqvist et al.
Diffusion of small-molecule penetrants in semicrystalline polymers
Prog. Polym. Sci.
(1996)

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