Numerical Investigation of Nucleating-Agent-Enhanced Heterogeneous Nucleation

Abstract

Nucleating agents have long been employed in polymeric foaming processes to promote cell nucleation, increase cell density, and improve cell uniformity. This improvement in foam morphology is usually considered to result from the enhanced heterogeneous nucleation caused by the lower free energy barrier for cell nucleation. However, less is known about the underlying mechanisms of nucleating-agent-enhanced nucleation. In the polymer foaming process, pressure is a critical parameter that affects the degree of supersaturation of gas within a polymer-gas solution. In most previous theoretical studies on cell nucleation, a uniform pressure was assumed throughout the solution. Although this assumption may be acceptable when no particles have been added, its validity is questionable when nucleating agents are present. It has been speculated that growing cells that have already been nucleated generate local flow fields that induce tensile stresses around nearby particles, resulting in local pressure fluctuations. The discontinuity at the interface between a nucleating agent particle and the surrounding polymer melt yields local pressure and stress fields around the particle that are different from those in the bulk, which may enhance it as a potential heterogeneous nucleation site. This paper presents a numerical analysis to investigate the pressure profile in the vicinity of nucleating agents and provides new information about the underlying mechanism that promotes cell nucleation in the presence of nucleating agents.

Development of polylactide bead foams with double crystal melting peaks

“…According to the DSC analyses, after saturation at 137.5 C (6 MPa) and 110 C (17.2 MPa), 27% and 25% of high melting temperature crystallinity was induced during saturation, respectively. The large amount of induced crystallinity significantly promoted the heterogeneous cell nucleation rate through the local stress variations [30,31] around the crystals [31e35]. However, cell growth was noticeably hindered due to the increased stiffness of the PLA matrix with developed crystals with a high melting temperature.…”

“…Specifically, crystals formed during the saturation step (that is, crystals with a high melting temperature) can significantly affect the cell nucleation and expansion of the PLA bead foams. According to heterogeneous cell nucleation theory, cell nucleation can be promoted through the local stress variations [30,31] around the formed crystals [31e35]. On the other hand, the molecules connected through the crystals improve the PLA's low melt strength and consequently increase the PLA's ability to expand by minimizing the gas loss and cell coalescence [12,36].…”

Supercritical processing of CO 2 -philic polyhedral oligomeric silsesquioxane (POSS)-poly( l -lactic acid) composites

“…POSS can be applied in various biomedical applications such as drug delivery, dental applications, biomedical devices, and tissue engineering, mostly as nanocomposites that they form with polymers. In polymer processing with scCO2, such particles can act as heterogeneous nucleating agents, which stimulate higher rate of cell nucleation by decreasing the free energy barrier for nucleation and provide new sites for cell formation [5,38,39]. We used two types of POSS to observe their effects on the formation of pores in PLLA; a CO2-philic type, trifluoropropyl polyhedral oligomeric silsesquioxane (TFPOSS) [40], and a non CO2-philic type, octamethyl polyhedral oligomeric silsesquioxane (OMPOSS) [41].…”

The Thermal Behavior of Polylactide with Different D‐Lactide Content in the Presence of Dissolved CO2

“…But too high a crystallinity will also hinder cell growth due to the excessive stiffness it causes . On the other hand, heterogeneous cell nucleation can be improved through local pressure variations around the formed crystals . The enhanced crystallinity will not only develop the foaming behavior of PLA, but it will also improve the mechanical properties and the service temperature (i.e.…”