NAMS 2002 Workshop - ICOM 2008

Packaging and Barrier Materials – 2
Friday July 18, 3:00 PM-3:30 PM, Wai’anae
Fundamental Exploration of Metal-Catalyzed Oxidation in Styrene-
Butadiene-Styrene Block Copolymers
K. Tung (Speaker), The University of Texas at Austin, Austin, Texas, USA
C. Ferrari, The University of Texas at Austin, Austin, Texas, USA
R. Li, The University of Texas at Austin, Austin, Texas, USA
K. Ashcraft, The University of Texas at Austin, Austin, Texas, USA
B. Freeman, The University of Texas at Austin, Austin, Texas, USA - freeman@che.utexas.edu
D. Paul, The University of Texas at Austin, Austin, Texas, USA
Barrier films are essential for packaging to prolong product shelf life, and films
that have exceptional oxygen barrier properties are valuable for food packaging.
One method to improve oxygen barrier properties is to incorporate reactive
groups in the membranes[1, 2]. In the presence of a transition metal catalyst,
these reactive groups capture, or scavenge O2 as it diffuses through a film.
The ultimate goal of this study is to investigate the performance of scavenging
polymers in the form of block copolymers containing a scavengable block, such
as polybutadiene, and a block, such as polystyrene, that might compatibilize the
material with a matrix barrier resin, such as polystyrene. This study will
eventually lead to the investigation of multi-layered systems of styrenebutadiene-styrene
(SBS) block copolymers and polystyrene.
The rate and amount of oxygen uptake as a function of metal catalyst
concentration, film thickness, and oxygen partial pressure were characterized
using an SBS block copolymer with the following characteristics: MW: 114,800,
12.5 % polystyrene, and 76 % vinyl structure. The experiments were performed
at 30 °C. Effect of metal-catalyzed oxidation on polymer morphology was also
studied through microscopic and spectroscopic characterization.
The experimental mass uptake data were used to characterize oxidation kinetics
as a function of catalyst (cobalt neodecanoate) loading. Induction periods were
observed in lower catalyst loadings. Mass uptake did not scale linearly with
catalyst concentration; there was an optimum catalyst loading. Different values of
optimum catalyst concentrations were found in homopolymers (i.e., poly(1,2butadiene)
and poly(1,4-butadiene)) and the SBS block copolymer. Film
thickness also had an effect on oxidation kinetics. Mass uptake values increased
as film thickness decreased; in other words, oxidation is more efficient as the film
becomes thinner. It was hypothesized that surface oxidation takes place as O2
molecules diffuse through SBS films. AFM images revealed a hard region, which
was rationalized to be the oxidized layer, at the film surface. Normalizing by film
surface area shows a close overlap of mass uptake data of various thicknesses,