Carbon Dioxide Induced Crystallization for Toughening Polypropylene

Preparation and properties of polydimethylsiloxane-regulated oriented microporous poly (L-lactic acid) biomimetic bone repair materials

Despite the exceptional biocompatibility and degradability of Poly (L-lactic acid) (PLLA), its brittleness, low melting strength, and poor bone induction makes it challenging to utilize for bone repair. This study used a simple, efficient solid hot drawing (SHD) method to produce high-strength PLLA, using supercritical CO2 (SC-CO2) foaming technology to give PLLA a bionic microporous structure to enhance its toughness, while precisely controlling micropore homogeneity and improving the melt strength by using Polydimethylsiloxane (PDMS). This PDMS-regulated oriented microporous structure resembled that of natural bone, displaying a maximum tensile strength of 165.9 MPa and a maximum elongation at break of 164.2 %. Furthermore, this bionic structure promoted the polarization of mouse bone marrow macrophages (iBMDM), exhibiting a simultaneous pro- and anti-inflammatory effect. This structure also contributed to the adhesion and growth of mouse embryonic fibroblasts (NIH-3 T3), promoting osteogenic differentiation, which paved the way for developing degradable PLLA bone-repair load-bearing materials.

Foaming behavior of the fluorinated ethylene propylene copolymer assisted with supercritical carbon dioxide

Foaming behavior of the fluorinated ethylene propylene copolymer (FEP) and its composites assisted with supercritical carbon dioxide (scCO2) as the blowing agent were investigated. The batch foaming process was applied at temperature ranging from 250°C to 265°C and pressure ranging between 12 MPa and 24 MPa. The optimal foaming temperature and saturation pressure were obtained for both pure FEP and FEP composites with 1 wt% different-sized BaTiO3 as nucleating agent. The cell diameter of pure FEP foam ranging from 80–140 µm was observed while the cell diameter decreased to 20–40 µm after adding BaTiO3 particles. The cell density of foamed FEP with BaTiO3 increased significantly from 106 to 108 cells/cm3 and the expansion ratio ranged between 4.0 and 5.5. Moreover, a decrease in an abnormal phenomenon that expansion ratio for the pure FEP foam was observed as the saturation pressure increased. This unexpected phenomenon can be explained by the relationship between foaming and crystallization coupling processes.

Evaluation of polypropylene/clay nanocomposite foamability based on their morphological and rheological aspects

To identify the effect of rheological influence on the development of microstructure in polypropylene/clay nanocomposites and thereby the influence of the developed microstructure on the foamability of the nanocomposites, a set of nanocomposites was prepared and batch foamed using supercritical CO2. Polypropylene and nanoclay were selected for preparing nanocomposites. During foaming, the nanocomposites were saturated with CO2 gas for three different time periods and subsequently in-situ heating was done to achieve cell growth. The gas saturation was done at subcritical condition followed by the foaming at critical condition of CO2. Thermal studies of the composites were investigated through differential scanning calorimetry, and clay dispersion morphology was investigated and validated using wide-angle X-ray diffraction, transmission electron microscopy, and parallel plate rheology. The improvement in foam morphology (cell size and cell density) and subsequent reduction in foam density was analyzed. The fingerprint characteristics of nanocomposites have an enormous role on foam structure development. With the increase in clay loading, cell density increased; furthermore, with an increase in saturation time, there was a phenomenal decrease in expansion ratio of neat polypropylene due to CO2-induced crystallization which could be mitigated by the incorporation of nanoclay into the polypropylene matrix. Therefore, nanoclay could be exploited as the inhibitor of CO2-induced crystallization.

Effect of melt viscosity on the cell morphology and properties of poly(lactic acid) foams

A compression molding foaming technique was used to prepare polylactic acid foams with a chemical foaming agent. The effect of melt viscosity of the polylactic acid on its cell morphology, apparent density, void fraction, cell population density, cell diameter, mechanical properties, and thermal property were studied. The apparent density of the foamed polylactic acid first decreased with decreasing melt viscosity and then increased, whereas the void fraction showed an opposite trend. A lower melt viscosity resulted in smaller cells, a more uniform cell size distribution, and a higher cell population density until the viscosity could not support further cell expansion which subsequently could cause gas escape and cell collapse. The tensile strength of the foams first increased with decreasing melt viscosity and then decreased. Their impact strength and flexural strength were improved by decreasing the melt viscosity. The use of glycidyl methacrylate only showed a small influence on the thermal stability of foamed polylactic acid.

Unique impact behavior and toughening mechanism of the polypropylene and poly(ethylene-co-octene) alternating multilayered blends with superior toughness

In this study, the alternating multilayered PP/POE blends with different layers were successfully fabricated by micro-co-extrusion. The notable improvement of toughness in the alternating multilayered blends is ascribed to the synergetic effects of the interfaces delaminations, craze deflection, larger subcritical damage zone during the fracture process.