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This study reports the glass transition temperature ([T.sub.g]), and sorption and diffusion of subcritical C[O.sub.2] gas in polymethyl methacrylate (PMMA) nanocomposites containing organically modified smectite clay, Cloisite 20A (C20A). A range of methods for preparing the PMMA-clay nanocomposites was investigated and a solution coprecipitation method was selected as the most appropriate. Using this method, PMMA nanocomposite containing 2, 4, 6, and 10 wt% nanoclay loadings were prepared. Wide-angle X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) indicated that the 2 wt% nanocomposite materials had a well-dispersed intercalated clay structure. The [T.sub.g] for PMMA-C20A nanocomposites, as measured by differential scanning calorimetry (DSC), was found to be independent of the clay loading. C[O.sub.2] solubility studies from 0 to 65°C and pressures up to 5.5 MPa using an in situ gravimetric technique were performed on compression-molded films. The organoclay was found to have no effect on the solubility of C[O.sub.2] in PMMA, and therefore the solubility of C[O.sub.2] in the nanocomposite can be determined from the solubility of C[O.sub.2] in the matrix polymer alone. Diffusion coefficients were determined using the appropriate transport models for these test conditions and the diffusion coefficients for C[O.sub.2] in PMMA-C20A composites were found to increase with organoclay loading. It is believed that the processing path taken to prepare the nanocomposites may have resulted in the agglomeration of the C20A organoclay, thereby preventing the polymer chains from fully wetting and intercalating a large number of clay particles. These agglomerations are responsible for the formation of large-scale holes within the glassy nanocomposite, which behave as low resistance pathways for gas transport within the PMMA matrix. POLYM. ENG. SCI., 45: 904-914, 2005. © 2005 Society of Plastics Engineers, INTRODUCTION Polymer nanocomposites are materials that contain fillers, such as montmorillonite clay, that have at least one dimension on the order [10.sup.-9] m, and can be dispersed at the nanometer [...]

The volume expansion behavior of low-density polypropylene foams in extrusion is investigated in this paper. Since escape of blowing agent from the foam would cause the foam to contract, and to have low expansion, efforts were made to prevent gas loss during foaming. The basic strategies to the promotion of a large volume expansion ratio are: to use a branched material for preventing cell coalescence; to use a long-chain blowing agent with low diffusivity; to lower the melt temperature for decreasing gas loss during expansion; and to optimize the processing conditions in the die for avoiding too-rapid crystallization. Use of a branched polypropylene resin was required to achieve large volume expansion because prevention of cell coalescence will retard gas loss from the extruded foam to the environment. The foam morphologies of linear and branched polypropylene materials at various processing temperatures were studied using a single-screw tandem foam extrusion system and their volume expansion behaviors were c ompared. Ultra low-density, fine-celled polypropylene foams with very high expansion ratio up to 90-fold were successfully produced from the branched polypropylene resins., INTRODUCTION Thermoplastic foams possess a cellular structure create y the expansion of blowing agent. This internal structure provides unique properties that enable the foamed plastics to be used effectively for [...]

This paper presents an innovative dilatometer that can measure the pressure-volume-temperature (PVT) properties of polymer/C[O.sub.2] solutions in a molten state. The basic rationale of the design is to determine the density (or equivalently, the specific volume) of a polymer/C[O.sub.2] solution by separately measuring the mass and volume flow rates of the solution flowing in an extruder at each temperature and pressure. A positive-displacement gear pump mounted on an extruder is used to measure the volume flow rate of the solution. A single-phase polymer/C[O.sub.2] solution is formed by injecting a metered amount of C[O.sub.2] into a polymer melt and completely dissolving it in the melt using a foam extrusion line. The temperature of solution was precisely controlled and homogenized by using the second extruder in a tandem system and a heat exchanger with a static mixer The pressure was controlled by the rotational speed of the screw in the second extruder. In order to reduce leakage across the gear pump, the difference between the upstream and downstream pressures was minimized using a variable resistance valve attached downstream of the gear pump. The mass flow rate was measured by directly collecting the extruded polymer melt for a fixed time after degassing C[O.sub.2]. A critical set of experiments was carried out to verify the functions of the system using pure polymer melts with known PVT data. Finally, the system was used to measure the specific volume of PS/C[O.sub.2] solutions as a function of C[O.sub.2] concentration, temperature, and pressure. [DOI: 10.1115/1.1418696]

This study demonstrates that the impact energy absorption capabilities of flexible sheets can be significantly enhanced by implementing tessellated designs into their structure. Configurations of three tessellated geometries were tested; they included a triangular-based, a rectangular-based, and a novel square-based pattern. Due to their geometrical complexity, multiple configurations of these tessellations were printed from a rubber-like material using an inkjet printer with two different thicknesses (2 and 4 mm), and their ability to absorb impact energy was compared to an unpatterned inkjet-printed sheet. In addition, the effect of multi-sheets stacking was also tested. Due to the tailored structure, the impact testing showed that the single-layer sheets were more effective at absorbing impact loads, and experience less deformation, than their two-layer counterparts. The 4 mm thick tessellated patterns were most effective at absorbing impact loads; all three thick patterns measured about 40% lower impact forces transferred to the base of the samples compared to the unpatterned counterparts. [ABSTRACT FROM AUTHOR]

A novel nanocomposite foam with microcellular structures based on poly(vinylidene fluoride) (PVDF) and titanium dioxide (TiO2) was fabricated by the combination of melt compounding and supercritical carbon dioxide (scCO2) foaming. To improve its dielectric performance, silane modified and unmodified titanium dioxide nanoparticles were added as reinforcing fillers at low weight percentages (0.5, 1, and 5 wt%) during the melt blending process. It was found that the incorporation of nanoparticles had a strong influence on cell morphology. As a result, the foaming process significantly altered the dielectric, and mechanical properties of the composite foams. The dielectric constants of the composite foams were no longer frequency dependent while tan delta was lowered at least by a factor of 10. Furthermore, the porous structure generated by foaming also assisted the α-to-β phase transformation of the PVDF matrix in a way similar to mechanical stretching. Such method is superior to other phase transformation techniques since β-phase PVDF can be produced in bulk geometries instead of a thin film configuration. [ABSTRACT FROM AUTHOR]

Complex morphology of open cell porous media makes it difficult to link microstructural parameters and acoustic behavior of these materials. While morphology determines the overall sound absorption and noise damping effectiveness of a porous structure, little is known on the influence of microstructural configuration on the macroscopic properties. In the present research, a novel bimodal porous structure was designed and developed solely for modeling purposes. For the developed porous structure, it is possible to have direct control on morphological parameters and avoid complications raised by intricate pore geometries. A semi-empirical model is developed to relate microstructural parameters to macroscopic characteristics of porous material using precise characterization results based on the designed bimodal porous structures. This model specifically links macroscopic parameters including static airflow resistivity (σ), thermal characteristic length (Λ'), viscous characteristic length (Λ), and dynamic tortuosity (α∞) to microstructural factors such as cell wall thickness (2t) and reticulation rate (Rw). The developed model makes it possible to design the morphology of porous media to achieve optimum sound absorption performance based on the application in hand. This study makes the base for understanding the role of microstructural geometry and morphological factors on the overall macroscopic parameters of porous materials specifically for acoustic capabilities. The next step is to include other microstructural parameters as well to generalize the developed model. In the present paper, pore size was kept constant for eight categories of bimodal foams to study the effect of secondary porous structure on macroscopic properties and overall acoustic behavior of porous media. [ABSTRACT FROM AUTHOR]