A novel technique utilizing supercritical carbon dioxide (scCO2) was used to pre-disperse nanoclays before mixing with polystyrene. This improved the dispersion and interface of clay platelets in the final nanocomposites. The effects of clay modifier, fraction, and scCO2 processing on structure and barrier properties were investigated. Results showed that scCO2 processing led to a more homogeneous dispersion and an 82.7% reduction in oxygen permeation with only 3.3% clay loading, corresponding to an effective aspect ratio increase of 16.7%.

1. Effect of clay modifier and scCO2 processing
Effect of clay fraction and scCO2 processing
In order to obtain superior nanocomposites with enhanced barrier
property, the following two aspects need to be improved.
Dispersion
Interface
Homogenous exfoliated dispersion and adjacent interface can
maximize effective interface and aspect ratio.
Supercritical Carbon Dioxide (scCO2) Processed Nanoclays and
Polystyrene/clay Nanocomposites: Structures and Barrier Properties
Fengyuan Yang1,2, Robert Kriegel3 and Rangaramanujam M. Kannan1,2
1. Johns Hopkins University, Department of Materials Science and Engineering, Baltimore, MD 21219
2. Johns Hopkins School of Medicine, Center for Nanomedicine, Wilmer Eye Institute, Baltimore, MD 21231
3. The Coca-Cola Company, Atlanta, GA 30313
Introduction
Polymer/clay nanocomposites are attractive in academic and
food packaging industrial areas due to their improved barrier
properties. However, achieving homogenous exfoliated
dispersion and improved interfacial interactions are keys to
obtain superior nanocomposites with enhanced barrier property.
In this research, a novel super critical carbon dioxide (scCO2)
processing method was used to yield pre-dispersed clays, whose
expanded flexible and puffy structure provided more available
surface area for matrix penetration leading to form superior
dispersion and interface in final nanocomposites. The effects of
clay modifier, fraction and scCO2 processing on clay dispersion
and polymer-clay interactions in nanocomposites were
investigated. Structure- barrier property relationship was studied
using several phenomenological models. Our results suggested
scCO2 processing result into a more homogenous dispersion and
achieved a nearly 82.7% reduction of oxygen permeation in
polystyrene/clay system with only 3.3 vol% clay, which
corresponding to an effective aspect ratio of 109.1± 4.4.
Changelings and Objectives
scCO2 Processing Method
Recently, a novel process utilizing scCO2 has been reported to
exfoliate nanoclays structure with or without polymer present. The
hypothesis is that, during a soaking period, scCO2 who has a
liquid like density and gas like diffusivity penetrate into the
platelet galleries, and then by following an instantaneous
depressurization, the large density change of the supercritical
fluid pushes the platelets apart [4,5,6,7].
Two scCO2 processing strategies:
 Polymer/clay mechanically mixed – scCO2 – rapid
depressurization
 as-received clay – scCO2 – rapid depressurization –clay
mixed with polymer via solution mixing or extrusion
WAXD and SEM were employed to monitor nanostructure and
morphological changes of nanoclay before and after processing.
Structures of Nanocomposites
Barrier Properties of Nanocomposites
Effect of clay modifier and scCO2 processing
Effect of clay fraction and scCO2 processing
Discussions
Conclusions
References
Different phenomenological models were fitted to study the effect
of clay fraction and scCO2 processing on gas barrier properties.
A novel technique utilizing supercritical CO2 was used to pre-
disperse commercial clay Cloisite 10A, Cloisite 20A and
Cloisite 30B. After scCO2 process the clays lost their long
region ordered layer structure, the size of particles and the
tightness of packing reduced, among different clays, those with
lower inter-modifiers interaction tend to be dispersed.
The effects of clay modifiers, fraction and scCO2 processing on
barrier property of nanocomposites were studied, pre-
dispersed clay shown obvious improvement of dispersion and
adjacent interface in nanocomposites. Based on models study,
scCO2 processing result into a more homogenous dispersion
and achieved a nearly 82.7% reduction of oxygen permeation
and a 16.7% increase of effective aspect ratio.
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[2] Sinha R. S.; Okamoto M. Prog. Polym. Sci. 2003, 28, 1539–1641.
[3] Takahashi S; Paul D. R.. Polymer 2006, 47, 7519–7534.
[4] Mihai M; Robert J. B.; Steven H.; Esin G.; Rangaramanujam M. K. Macromolecules
2008, 41, 8038–8046.
[5] Steven H; Gulay S.; Esin G.; Rangaramanujam M. K. Polymer 2006, 47, 7485–7496.
[6] Gulari, E.; Serhatkulu, G. K.; Kannan, R. U.S. Patent 7,387,749, Feb, 2004.
[7] Mielewski, D. F.; Lee, E. C.; Manke, C. W.; Gulari, E. U.S. Patent 6,753,360, Jun, 2004.
[8] Erik D.; Daniel S. Macromolecules 2010, 43, 10536–10544.
Current Methods
Melting extrusion
 Requires high temperature and shear stress
 Short processing time
 Usually results in poor to moderately dispersed nanocomposites
Solution blending
 Produces highly intercalated nanocomposites
 High order, low dispersion, high cost and residual solvent
In situ polymerization
 Complicated. Each new polymer-clay system requires different
reaction and processing
 Not flexible. Nanocomposite to be made at the time of polymer
creation [4]
Scheme 1. Structures of polymer/clay nanocomposites [1,2]
Scheme 2. Interfacial structure of polymer/clay nanocomposites [3]
Scheme 3. scCO2 processing with present of polymer phase
Scheme 4. scCO2 processing without present of polymer phase
Figure 1. WAXD patterns of different clays before and after scCO2 processing
Figure 2. SEM images of different clays before and after scCO2 processing
Figure 3. WAXD patterns and TEM images of PS/clay nanocompsoites with different modifiers (5wt%)
Figure 4. WAXD patterns and TEM images of PS/clay nanocompsoites with different fraction (Cloisite 10A)
Figure 5. Permeation of PS/clay nanocompsoites with different modifiers (5wt%)
Figure 6. Permeation of PS/clay nanocompsoites with different fraction (Cloisite 10A)
Table 1. Published assumptions, formulas and derived tortuosity factors for various models [8]
Figure 7. Experimental and model data for oxygen permeation rate in PS/clay nanocomposites
Table 2. Fitted parameters and effective aspect ratio for various models
Scheme 5. scCO2 processing exfoliate nanoclays structure
Scheme 6. scCO2 processing enhance dispersion and effective aspect ratio of polymer/clay nanocomposites
Pre-dispersion of Clays
as-received scCO2
2wt%
7wt%
15wt%
as-received scCO2
5wt%
5wt%
5wt%