Effect of Silica Nanoparticles on Interfacial Tension and Crystallization of Poly(lactic acid) in Supercritical Carbon Dioxide

Abstract

In this thesis, the effect of silica nanoparticles on two of the most important parameters in the foaming of poly (lactic acid) (PLA) was studied: interfacial tension and crystallization. According to classical nucleation theory, the nucleation rate is inversely related to the exponential cubic power of interfacial tension, similarly the critical nucleation cell size to interfacial/surface tension. A decrease in surface tension decreases the energy barrier for cell nucleation and consequently increases the number of cells, leading to an exponential increase in cell density and smaller cell size. Solid nanoparticles, such as those made of silica, can be adsorbed at the interface and decrease the interfacial tension between polymer melt and surrounding fluid. They can also prevent coalescence through repulsion between two similar particles at the interface of two growing cells or increase in elasticity of the interface. Furthermore, nanoparticles can act as nucleating agents for the foaming of polymers by increasing local stress variations around the particles. In particular, nanoparticles can improve PLA crystallization, which is one of the approaches to address the low melt strength of PLA, one of the barriers for PLA foaming. In addition to melt strength, crystallization and crystallinity can further improve the mechanical properties of PLA. At first, the interfacial behavior of the PLA/ supercritical carbon dioxide (CO2) under foaming conditions was studied using Axisymmetric Drop Shape Analysis Profile (ADSA-P). The results showed a decrease in interfacial tension with increasing temperature and pressure, and a decrease in dependency of interfacial tension on temperature at high pressures. As the next step, the interfacial tension of PLA composites made with surface- modified silica is studied. The interfacial tension between PLA and supercritical CO2 decreased as a result of nanoparticles’ adsorption to the interface. There was a minimum at 2 wt. % loading of the nanoparticles and the interfacial tension curve reached a plateau afterwards. The lateral capillary force of the adsorbed aggregates of the nanoparticles to the PLA-CO2 interface was considered the reason for the observed increase in interfacial tension. Contact angle measurements at high pressures showed the affinity of the surface-modified nanoparticles to the polymer-supercritical CO2 interface. A comparison of calculated binding energy of the nanoparticles to the PLA-CO2 interface with thermal energy (kBT) showed that the adsorption was irreversible. Last, the crystallization behavior of PLA/surface-modified silica nanocomposites under isothermal, non-isothermal, and isothermal with compressed CO2 conditions were studied. A significant improvement in crystallization rate was observed after introduction of amine-modified silica nanoparticles. A modified Hoffman-Lauritzen nucleation theory showed that the low surface energy of the modified nanoparticles and interfacial energy between polymer/nanoparticle facilitated the crystallization. Avrami exponents obtained from isothermal investigation of the nanocomposites indicated the sporadic formation of three-dimensional spherulites in the PLA matrix, which shift into the range of two-dimensional at higher temperatures. In the presence of compressed CO2, crystallization rate increases, but at pressures higher than 21 bar no significant effect was observed. Nanocomposites of PLA samples with lower molecular weight and higher stereoregularity also showed a significant increase in crystallization rate with no change in crystallization mechanism in presence of the nanoparticles.