Simultaneous Determination of the Surface Tension and Density of Polystyrene in Supercritical Nitrogen

Investigation of material characteristics and processing conditions effects on bubble growth behavior in a physical foaming process

Bubble growth during polymeric foam production by a physical nucleating agent is a result of rapid gas phase separation in a polymer/gas solution media. The dynamics of bubble growth is thought to be influenced by the material properties and processing conditions. However, the degree of effectiveness of each parameter has not been evaluated in earlier studies. In this work, a simplified single bubble growth in a polymeric media was modeled to specify the critical parameters affecting the bubble growth phenomenon. The predicted bubble growth profile was compared with some experimental data reported in the literature. The model was able to predict the observed bubble growth profile with acceptable precision. Therefore, it was applied to investigate the effect of each physical property of the polymer, i.e. viscosity, surface tension and diffusion coefficient as well as processing conditions, i.e. temperature and pressure release rate on the bubble growth profile. Furthermore, the impact of each factor was clarified.

Adsorption of Surface-Modified Silica Nanoparticles to the Interface of Melt Poly(lactic acid) and Supercritical Carbon Dioxide

With the purpose of fabricating polymer nanocomposite foams and preventing coalescence in foaming processes, the interfacial tension of poly(lactic acid) (PLA)-silica composites is investigated in this work. Synthesized silica nanoparticles (SNs) with a CO2-philic surface modification are used as the dispersed nanoparticles. Interfacial tension is a key parameter in processing of polymer foams since it directly affects the final foam properties, such as cell size and cell density. Interfacial tension of silica-containing PLA and supercritical carbon dioxide (CO2) is measured using axisymmetric drop shape analysis profile (ADSA-P) pendant drop method at high pressures and high temperatures. The interfacial tension between PLA and supercritical CO2 is observed to decrease as a result of the nanoparticles' adsorption to the interface. These results indicate that the reduction in interfacial tension with increasing silica content significantly deviates from a linear trend; there is a minimum at 2 wt % loading of the SNs and then the interfacial tension curve reaches a plateau. Contact angle measurements show an affinity of the SNs for the polymer-supercritical CO2 interface, and these obtained results are used to calculate the binding energy of the nanoparticles to the PLA/CO2 interface. In addition to interfacial properties, the adsorption of silica nanoparticles at the interface is also studied in detail with scanning electron microscopy.

Simha-Somcynsky Equation of State Modeling of the PVT Behavior of PP/Clay-Nanocomposite/CO2 Mixtures

The Pressure-Volume-Temperature (PVT) property of polymer nanocomposite (PNC)/gas solutions is an important fundamental property in the foaming of PNC. However, accurate data have not yet been reported. We examined the PVT behaviors of polypropylene (PP) and PP/organoclay polymer nanocomposite (PP-PNC) by monitoring the swelling changes of the polymer melt in supercritical carbon dioxide (scCO2). A model was adopted that describes the PVT behaviors of PP-PNC with and without dissolved gas. Based on the model, a PNC consists of two sections: a hard section (a nanoparticle surrounded by solidified polymer) and a soft section (neat polymer). It was observed that an infusion of nanoparticles decreased the swelling. It seems that the hard section had a minimal free volume in which to dissolve the blowing agents, and that the number of hard sections increased with the infusion of nanoparticles. As a result, the total gas absorption capacity of the system decreased, and consequently, the swelling also decreased.

Reduction of polymer surface tension by crystallized polymer nanoparticles

Self-consistent field theory is applied to investigate the effects of crystallized polymer nanoparticles on polymer surface tension. It is predicted that the nanoparticles locate preferentially at the polymer surface and significantly reduce the surface tension, in agreement with experiment. In addition to the reduction of surface tension, the width of the polymer surface is found to narrow. The reduced width and surface tension are due to the smaller spatial extent of the nanoparticles compared to the polymer. This allows the interface to become less diffuse and so reduces the energies of interaction at the surface, which lowers the surface tension. The solubility of the surrounding solvent phase into the polymer melt is mostly unchanged, a very slight decrease being detectable. The solubility is constant because away from the interface, the system is homogeneous and the replacement of polymer with nanoparticles has little effect.