Microscale modeling of the growth of microgels by precipitation polymerization
Maldonado-Parra, Francisco Daniel; Marquardt, Wolfgang (Thesis advisor); Pich, Andrij (Thesis advisor)
Aachen (2019) [Dissertation / PhD Thesis]
Page(s): XVIII, 198 Seiten : Illustrationen, Diagramme
In this work, a predictive model to describe quantitatively the three main formation mechanisms, i.e., formation of oligomers in the liquid, precipitation of gel particles and individual particle growth, during the synthesis of thermally sensitive microgels from N-isopropylacrylamide (NIPAAm) and N-vinylcaprolactam (VCL) monomers by precipitation polymerization in water is presented. Monomer conversion as well as the growth evolution of individual gel particles (particle size distribution) during the synthesis in a reactor are predicted. They are modeled as a function of reaction and transport kinetics, thermodynamics of the liquid-gel particle system, as well as as a function of the process operating conditions in the reactor. The resulting mathematical model of this work can form the basis for rational and model-based decision making in the optimization of the microgel synthesis process and the control of the microgel end-product properties, in particular, particle size distribution. The present model is developed in joint work with the group of Prof. Andrij Pich at the Institute of Technical and Macromolecular Chemistry (ITMC), RWTH Aachen University and at the DWI Leibniz Institute for Interactive Materials e.V., Aachen, Germany, as well as with the group of Prof. Gabriele Sadowski at Lehrstuhl für Thermodynamik, TU Dortmund University. At the mentioned institutes, diverse experimental measurements during the microgel synthesis as well as numerical simulations of phase equilibrium between the liquid and gel particle phases have been carried out by Michael Kather  and by Markus C. Arndt , respectively, in order to support the development of the present model in this doctoral thesis. As a result of the mentioned joint work, the critical chain length η, at which the precipitation of PVCL gel particles occurs, is experimentally determined and the volume evolution of PNIPAAm and PVCL gel particles is modeled during their synthesis by applying the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state. In addition, experimental measurements of the monomer conversion and of the averaged hydrodynamic radius of gel particles during the synthesis have been carried out at ITMC and DWI to support the present predictive model . Diverse modeling approaches at different time and length scales are applied to model the polymerization and mass transfer processes in the gel particles and the liquid as well as the thermodynamics of the gel particle-liquid system. Namely, the formation of oligomers in the liquid is modeled on a low level of detail by formulating molar balances for the oligomers with chain length i (i=1,…,η) in the reactor. The precipitation of gel particles is modeled also on a low level of detail by applying population balances which simulate the number of precipitated gel particles which are born in generation g (g=1,…,Ζ) as a function of polymerization time. In this work, the use of generations of particles is the way to mathematically observe and follow the complex processes of the microgel formation mechanisms. In order to apply the framework of population balances, a particle birth rate function is developed in this doctoral thesis which models the precipitation of gel particles at the critical chain length η. The individual particle growth is modeled on a high level of detail by applying local molar balances between single gel particles and their surrounding liquid as well as the stochastic simulation algorithm (SSA) and the PC-SAFT equation of state. In addition to the local molar balances, a Monte Carlo (MC) simulation algorithm is developed in this work which numerically evaluates the oligomer absorption to the gel particles. The SSA algorithm simulates the polymerization of monomer in individual gel particles at time and length scales which are much finer than the scales covered by the molar and population balances. The molar and population balances (liquid model) as well as the MC, SSA algorithms and PC-SAFT equation of state (individual particle model) are depending on each other. For this reason, the molar and population balances of the liquid model are linked to the individual particle model which is cast into the MC, SSA algorithms and PC-SAFT equation of state. Complex integration and simulation strategies are developed in this work which performs the integration of the liquid model with the individual particle model to allow for the prediction of monomer conversion and the growth evolution of individual gel particles (particle size distribution) in the reactor as a function of polymerization time. Simulation results are compared to experimental data of monomer conversion and averaged hydrodynamic radius of gel particles evaluated at different polymerization temperatures. The influence of the polymerization temperature on monomer conversion, on the birth of gel particles as well as on the growth evolution of individual gel particles is evaluated. The polymerization rate coefficients of the synthesis of PNIPAAm and PVCL microgels by precipitation polymerization in water are estimated. The effect of other particle formation mechanisms, i.e., the absorption of oligomer molecules and their polymerization in single gel particles, on the birth and individual growth of gel particles is also studied. The experimental data and the simulation results suggest that the phenomena of absorption and polymerization of oligomers in the gel particles are important mechanisms which contribute enormously to the individual growth of PNIPAAm and PVCL gel particles during their synthesis by precipitation polymerization in water.
- REPORT NUMBER: RWTH-2019-11674