Investigation of structure and properties of novel multi-layer clay nanocomposite films produced controllably by continuous chaotic advection blending.

摘要

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 gamma-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 cont