A study of fundamentals in emulsion templating for the preparation of macroporous polymer foams

Abstract

This thesis describes a series of styrene (ST) and divinylbenzene (DVB) emulsion templated polymer foams prepared via low, medium and high internal phase emulsion templates (L/M/HIPE templates). The emulsion templates were stabilized using different commercially available technical surfactants and surfactant mixtures. Since the chemical nature of the chosen technical surfactants is unknown, the surfactants where characterized by means of Fourier Transform Infrared (FT-IR) and Nuclear Magnetic Resonance (NMR) spectroscopy, Electro Spray Ionization Mass- (ESI-MS) and Matrix-Assisted-Laser-Desorption-Ionization-Time-of-Flight-Mass Spectrometry (MALDI-TOF-MS). Additionally their adsorption at the water/ST:DVB interface was studied. The investigation regarding the preparation of surfactant stabilized emulsion templates and their polymerization products revealed that the most commonly used surfactant Span 80 is not the best suited surfactant to stabilize styrene/divinylbenzene emulsion templates which is why different surfactants were used in the thesis at hand. All successfully prepared poly(merized)HIPEs proved to have interconnected, open porous polymer foam structures. In contrast, the pore structure of polyMIPEs was open, closed or non-droplet shaped, depending on the surfactant used to stabilize the corresponding emulsion template. The mechanical compression properties of all prepared polyHIPEs were similar and independent of the HIPE formulation from which they were produced but the mechanical properties of polyMIPEs differed significantly. The influence of the surfactants on the morphology and mechanical properties of the resulting macroporous polymers will be discussed in detail. Furthermore, the relationship between the relative density (porosity) of the polymer foams and the mechanical response under compression was investigated. The semi-empirical models developed by Gibson and Ashby were applied and additionally modified to provide a more accurate description of the mechanical behaviour over a larger relative density range of polymer foams prepared via emulsion templating (polyL/M/HIPEs). This allows a prediction of the mechanical properties as a function of the relative density of the respective polymer foams and vice versa for the specified emulsion template formulation. It is obvious that the surfactant type and the internal phase volume ratio of the emulsion template used to produce macroporous polymer foams significantly determine their resulting mechanical properties, as clear transition states for polyH/M/LIPEs were identified in which the mechanical properties of these materials changed dramatically. The effect of the surfactant on the mechanical properties and the polymer foam morphology is discussed in terms of the surfactant’s solubility in the polymer and thus in terms of its role as plasticizer. Finally, the influence of the pore size on the mechanical properties was investigated. It was found that the preparation process (emulsification and polymerization) of the emulsion templates is very crucial for the mechanical properties of the resulting polymer foams (reproducibility). More precisely, it was found out that the emulsion templates need to ‘equilibrate’ after emulsification. It was only for these emulsions that average pore sizes and mechanical properties could be reproduced.