Date of Award

12-2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Materials Science and Engineering

Advisor

Zumbrunnen, David A

Committee Member

Ellison , Michael

Committee Member

Luzinov , Igor

Committee Member

Luo , Jian

Abstract

Polymer nanoclay composites have been studied extensively in the past decade due to the excellent combination of properties they can offer at low loadings of clay. Despite robust research, the potential enhancements that nanoclays are theoretically predicted to offer have not been achieved. Such potential improvements in mechanical and gas barrier properties are realized only when the internal structure of the nanocomposite is optimized in terms of arrangement and orientation of nanoclay within the matrix. The mixing-based approach and the accompanying complex flow fields of conventional processing techniques widely used to produce nanoclay composites are unable to control the internal structure. This has also impeded the documentation and verification of the effect of orientation and arrangement of clay platelets on the matrix and the nanocomposite properties. Hence, a unique processing technique based on chaotic advection developed at Clemson University and shown to controllably produce structured materials in the past was employed to produce structured nanocomposites with a high degree of clay orientation as well as localization of platelets within layers of nanoscale thicknesses.
Continuous lengths of nanocomposites with different clay contents were extruded in the form of films by feeding separately melts of virgin polyamide-6 polymer and polyamide 6-clay masterbatch into a continuous chaotic advection blender. A variety of composite structures were producible at fixed clay compositions. The internal structure was characterized by transmission electron microscopy (TEM), x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Nanocomposites with novel in-situ multi-layered structures and a high degree of platelet orientation were formed by the recursive stretching and folding of the melt domains due to chaotic advection. Clay platelets were localized within discrete regions to form alternating virgin and platelet-rich layers leading to a hierarchical structure with multiple nano-scales. The thicknesses of the layers reduced with prolonged chaotic advection, eventually leading to nanocomposites in which the multi-layering was no longer discernible. The oriented platelets appeared to be homogenously dispersed through the bulk of the nanocomposite.
Investigation of the morphology of the matrix by XRD showed that the homogeneity of the crystalline phase and the orientation of polymer chains parallel to the film surface increased with increased chaotic advection. Also, as the layer thickness reduced, the number of polymer chains restricted by clay platelets increased causing the γ-crystalline fraction to increase. While XRD results suggested a change in total crystallinity with chaotic advection and clay content but without a specific trend, no change in crystallinity was measured by DSC. Such contradictions are consistent with results of other investigators.
Concentrating and orienting the clay platelets within layers increases the path length of the diffusing molecule and hence may improve barrier properties. The effect of multi-layering and platelet orientation on the gas permeability of the nanocomposite films was investigated both experimentally and theoretically. Experimental measurements of 2% clay films showed that a multi-layered structure with oriented clay platelets gives a 40% greater reduction in oxygen permeability compared to a structure with a homogenous dispersion of oriented clay platelets. Also, the nanocomposite films with homogenous dispersion of platelets produced by chaotic advection due to their high degree of platelet alignment exhibited improved barrier properties than nanocomposites produced by mixing. The combination of high degree of orientation and multi-layering conferred to the 2 wt% clay film produced with the chaotic advection blender a relative permeability lower than a 6 wt% clay film produced with a single screw extruder.
A theoretical model was formulated to explore the barrier properties of nanocomposites comprising a wide range of clay contents and platelet aspect ratio. The model showed the importance of orientation and layered structure. Permeabilities close to the intrinsic platelet permeability (i.e., near zero) can be realized by localizing and orienting a relatively low volume fraction (4%) of very high aspect ratio platelets (≥350) in the matrix or high volume fractions (20%) of platelets with aspect ratios around 100 (typical of the montmorillonite (MMT) clay). The chaotic advection blender was unable, however, to process such masterbatches due to limitations of available screw extruders intended for polyolefins. Experiments considered low volume fractions of MMT clay less than 4%.
Other physical properties of the films important for packaging applications were also evaluated. The presence of die lines, particulate contaminations and variations in thicknesses of the films led to data scatter of measured properties. However, even with film quality substantially less than obtained in industry, the nanocomposites of this study showed a slight increase in tensile strength with clay content without sacrificing impact toughness and resistance to tear. In addition, due to the high degree of orientation parallel to the film surface, the clay platelets were able to reinforce the material in both machine and transverse directions. The nanocomposites also retained the optical clarity of the pure polymer matrix.
Experimental and modeling results suggest that high barrier properties may be attainable if improvements to the chaotic advection blending system are made such that higher quality films can be produced with only slightly higher clay content or higher aspect ratio clay platelets than considered in this study.

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