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A series of TPU nanocomposites were prepared by incorporating organically modified layered silicates with controlled particle size. To our knowledge, this is the first study into the effects of layered silicate diameter in polymer nanocomposites utilizing the same mineral for each size fraction. The tensile properties of these materials were found to be highly dependent upon the size of the layered silicates. A decrease in disk diameter was associated with a sharp upturn in the stress−strain curve and a pronounced increase in tensile strength. Results from SAXS/SANS experiments showed that the layered silicates did not affect the bulk TPU microphase structure and the morphological response of the host TPU to deformation or promote/hinder strain-induced soft segment crystallization. The improved tensile properties of the nanocomposites containing the smaller nanofillers resulted from the layered silicates aligning in the direction of strain and interacting with the TPU sequences via secondary bonding. This phenomenon contributes predominantly above 400% strain once the microdomain architecture has largely been disassembled. Large tactoids that are unable to align in the strain direction lead to concentrated tensile stresses between the polymer and filler, instead of desirable shear stresses, resulting in void formation and reduced tensile properties. In severe cases, such as that observed for the composite containing the largest silicate, these voids manifest visually as stress whitening.

Polyurethane (PU) composites incorporating Cloisite 15A (15A) were prepared via melt compounding and solvent casting. The melt-compounded composites had better dispersion and a smaller silicate stack size as a result of the higher shear forces associated with twin-screw extrusion. The PU microphase separation and hard domain order were greater in the melt-processed materials. At the concentrations of 15A employed in this study (Յ7 wt %), the filler did not have an observable effect on the microphase texture of either the solvent-or melt-processed PU. The tensile properties of the melt-compounded materials were lower than those of their solvent-cast counterparts because of thermal degradation. The solvent-cast composite containing a 3 wt % loading of 15A displayed improved tensile strength and elongation, primarily because of plasticization by the silicate organic treatment. The addition of layered silicates with high aspect ratios increased the hysteresis and permanent set of this PU elastomer.

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This study determined the impact of pasteurization, high-pressure processing (HPP), and retorting on the barrier properties of nylon 6 (N6), nylon 6/ethylene vinyl alcohol (N6/EVOH), and nylon 6/nanocomposite (N6/nano) materials. The pasteurized and high-pressure treated films were coextruded with low-density polyethylene (PE) as the heat-sealing layer. The retorted films were coextruded with polypropylene (PP). Oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) of the samples were measured after pasteurization (75• C for 30 min), HPP (800 MPa for 10 min at 70• C), and retorting (121 • C for 30 min) treatments. These were compared with the thermal characteristics and morphologies of the samples using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Results showed that OTR of N6 and N6/Nano increased after HPP (16.9% and 39.7%), pasteurization (13.3% and 75.9%), and retorting (63.3% and 112.6%), respectively. For N6/EVOH, a decrease in OTR after HPP (53.9%) and pasteurization (44.5%) was observed. The HPP treatment increased the WVTR of N6 (21.0%), N6/EVOH (48.9%), and N6/Nano (21.2%). The WVTR of N6, N6/EVOH, and N6/Nano increased by 96.7%, 43.8%, and 40.7%, respectively, after pasteurization. The DSC analyses showed that the enthalpy and percent crystallinity increased (2.3% to 6.5%) in the N6/Nano when compared with the N6 material after each treatment. Retorting caused a decrease (3.5%) in the percent crystallinity of the polypropylene material. HPP did not cause major morphological changes to the samples. Results of the barrier studies were influenced by the crystallinity changes in the materials as seen in the XRD diffractograms.

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Two organically modified layered silicates (with small and large diameters) were incorporated into three segmented polyurethanes with various degrees of microphase separation. Microphase separation increased with the molecular weight of the poly(hexamethylene oxide) soft segment. The molecular weight of the soft segment did not influence the amount of polyurethane intercalating the interlayer spacing. Small-angle neutron scattering and differential scanning calorimetry data indicated that the layered silicates did not affect the microphase morphology of any host polymer, regardless of the particle diameter. The stiffness enhancement on filler addition increased as the microphase separation of the polyurethane decreased, presumably because a greater number of urethane linkages were available to interact with the filler. For comparison, the small nanofiller was introduced into a polyurethane with a poly-(tetramethylene oxide) soft segment, and a significant increase in the tensile strength and a sharper upturn in the stress-strain curve resulted. No such improvement occurred in the host polymers with poly(hexamethylene oxide) soft segments. It is proposed that the nanocomposite containing the more hydrophilic and mobile poly(tetramethylene oxide) soft segment is capable of greater secondary bonding between the polyurethane chains and the organosilicate surface, resulting in improved stress transfer to the filler and reduced molecular slippage.

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