Significant slip can occur in the flow of a blend of two immiscible polymers due to reduced entanglements at their interface. The slip is of practical importance because of its effect on morphology and adhesion in, for example, disordered two-phase blends or multilayer films. Interfacial slip was quantified using two polymer pairs each with closely matched viscosity and elasticity but different miscibility ( χ ) : polypropylene (PP)/polystyrene (PS) χ = 0.04 and polyethylene (PE)/fluoropolymer (FP) χ ≅ 0.1 . To control the amount of interfacial area, we prepared alternating layers by coextrusion. The number of layers of PP/PS ranged from 20 to 640 while that for PE/FP was 80. Nominal viscosity of the multilayer samples was measured with three types of rheometers: an in-line slit-die rheometer, rotational parallel-disks, and sliding plate. Good agreement was found between the three methods. The nominal viscosity as well as shear normal stresses of the multilayer samples decreased with the number of layers. From the viscosity reduction, an apparent interfacial slip velocity V slip was calculated. The detectable slip, about 1 μ m / s , occurred at the same level of stress normalized by average plateau modulus τ / G N , av 0 = 0.01 for both pairs. However, V slip did not show a sudden increase at a critical shear stress as predicted by Brochard-Wyart and de Gennes [C. R. Acad. Sci., Ser. II: Mec., Phys., Chim., Sci. Terre Univers317, 13 (1993)] but rather increased V slip ∝ τ n , where n ≈ 3 for PS/PP and n ≈ 6 for PE/FP. The steeper dependence of slip on stress for PE/FP may be related to its higher χ value and with its narrower molecular weight distribution.

This paper elucidates the effects of cell density on the volume expansion behavior of polypropylene (PP) foams blown with butane in extrusion. The cell density was controlled by varying the talc content, and foam expansion was observed at a fixed blowing agent content while varying the melt and die temperatures. As observed in our previous studies, the curve of the final expansion ratio of PP foam versus temperature showed a typical mountain shape for each talc content, confirming that the two governing expansion mechanisms were gas loss and stiffening of melt. As the talc content increased, the expansion curve skewed towards the lower temperature, which showed that the expanded foams with a high talc content were more susceptible to gas loss at elevated temperatures. This indicates that the processing temperature should be decreased to have a large expansion ratio from the extruded PP foams at a high talc content. In other words, the optimum temperature to maximize expansion decreased at a higher talc content. It is believed that the shift of the expansion curves was caused by the promoted expansion rate of extruded foams with a greater talc content because of the reduced diffusion distance of gas molecules to the nearest stabilized nucleus. On the other hand, the increased cell density at a high talc content increased the number of cell layers in the cross section of the extruded foam, and thereby the gas loss was localized to the cells on the surface which acted favorable for the final expansion ratio to a certain degree.

Manufacture of thermoplastic foams with a fine cellular structure (a higher expansion ratio, a higher cell density, and smaller cell sizes) is challenging work due to the weak viscoelastic behavior and the unsuitable crystallization behavior of common thermoplastic materials. In this work, a novel method of making microcellular foams with micro‐/nano‐fibrillar reinforced polymeric composites (M/NFC) is introduced, which shows various advantages compared to conventional foams. The M/NFC foams have improved cellular structures, excellent mechanical properties, and enhanced thermal insulation properties, which make them popular candidates for structural applications and insulative products. Various methods to manufacture of M/NFC foam are summarized. To understand the fundamental mechanisms of the foaming enhancement by incorporating micro‐/nano‐size fibrils, the rheological and crystallization behavior of the M/NFC are analyzed. It is shown that the micro‐/nano‐fibrils can strengthen the melt strength, induce faster crystallization, and increase the number of crystals. Due to the improvement of the cell morphology and the stiffness of the cell walls, the reinforced foams have superior mechanical properties. A hierarchically porous structure in high expansion ratio reinforced foams has also been developed. It is believed that the nano‐size holes in the cell walls can further reduce the thermal conductivity of the foams.

Highly porous poly (ϵ-caprolactone) microfiber scaffolds can be fabricated using electrospinning for tissue engineering applications. Melt electrospinning produces such scaffolds by direct deposition of a polymer melt instead of dissolving the polymer in a solvent as performed during solution electrospinning. The objective of this study was to investigate the significant parameters associated with the melt electrospinning process that influence fiber diameter and scaffold morphology, including processing temperature, collection distance, applied, voltage and nozzle size. The mechanical properties of these microfiber scaffolds varied with microfiber diameter. Additionally, the porosity of scaffolds was determined by combining experimental data with mathematical modeling. To test the cytocompatability of these fibrous scaffolds, we seeded neural progenitors derived from murine R1 embryonic stem cell lines onto these scaffolds, where they could survive, migrate, and differentiate into neurons; demonstrating the potential of these melt electrospun scaffolds for tissue engineering applications.