The National Science Foundation's (NSF) Tokyo Office periodically receives and disseminates reports on research developments in Japan that are related to the Foundation's mission. NSF-sponsored researchers currently working in Japan prepare many of these reports. These reports present information for use by NSF program managers and policy makers; they are not statements of NSF policy. .
Mr. Linwood Blanton Muire, a Ph.D. candidate in the Department of Chemical Engineering,
Clemson University prepared the following report. Mr. Muire
was a participant in the 1999 Monbusho Summer Program sponsored
by NSF and the Ministry of Education, Science, Sports and Culture
(Monbusho). Dr. Yasuo Ito of the Research Center for Nuclear Science
and Technology,University of Tokyo, Japan, hosted Mr. Muire.
Mr. Muire can be reached via email at: lmuire@clemson.edu
Introduction:
Linear Low-Density Polyethylene (LLDPE) film is commonly used in packaging applications for consumer products. Plain LLDPE film, however, is difficult to process because of its high coefficient of friction (COF); often additives called slip agents are coextruded with LLDPE. These slip agents then diffuse to the surface, where they lower the COF. Common slip agents are long-chain amides, such as erucamide.
It has been found that in a large roll of film, film from near the core of the roll is more difficult to process than film from the outer layers. It was found that the near-core layers had lower COF's than the outer layers. It has also been estimated that the applied external pressure at the core of such a film roll can be up to 1000 pounds per square inch (psi). Because erucamide's migration to the surface is a diffusion process, the relation between applied external pressure and its surface diffusion was studied. These studies showed that as the applied pressure increases the equilibrium surface concentration of erucamide decreases.
The reason for this decrease in diffusion with pressure remains unexplained. It was theorized that under high applied pressures the free volume of the film was being decreased by compression, and that this hindered erucamide diffusion. The purpose of this research was to use positron annihilation lifetime spectroscopy (PALS) to validate or invalidate the above hypothesis by measuring the free volume distributions of various LLDPE/Erucamide films.
When a positron is emitted from a radioactive source, a gamma ray of certain energy is released. If the positron enters some medium, it will reach thermal equilibrium with that medium and then diffuse through it. Because the positron is positively charged, it is repelled by the nuclei of the atoms that make up the medium and finds its way to voids (a.k.a. free space or free volume). In these voids the positron eventually pairs with an electron to form positronium, a meta-stable particle similar to hydrogen. Eventually positronium annihilates, releasing another gamma ray with energy different from the first. Because of this difference in gamma ray energies, we can precisely time how long a positron has survived in the medium; in other words, we can calculate its lifetime. Since it is expected that positrons will "live longer" if they reside in larger holes (where there is less of a chance for them to find a free electron), this lifetime is correlated to the size of free volume in the medium, using substances with known void size as a calibration and the assumption that free volume holes are basically spherical.
Results:
The free volume size and distribution are not functions of film thickness, erucamide concentration, or pressure treatment. Erucamide does not dissolve in the LLDPE film, but rather resides in the free volume holes. Because there is an equilibrium amount of erucamide in films initially containing the additive, they show a slight decrease in free volume fraction.
Contact Information:
I (Linwood Muire) am currently working on my M.S. in Chemical Engineering at Clemson University in Clemson, SC. My host scientist for this program was Dr. Yasuo Ito of the University of Tokyo. I can be contacted at lmuire@clemson.edu