The key objective of this thesis is the morphology control of latex/clay nanocomposites (LCN) which are of particular interest to water-based coating and adhesive applications. Indeed, the incorporation of inorganic fillers into a polymer matrix generally leads to better performing materials. However, a good dispersion and an alignment of the clay layers as single platelets into the polymer matrix are the prerequisites for the largest property enhancement. Such requirements have been the driving force for the development of many LCN synthetic routes. The inorganic encapsulation technique, using conventional emulsion polymerization was employed in this thesis. The natural occurring montmorrilonite clay particles were used as the inorganic fillers. The primary goal was to make a start in fine tuning the dispersion and orientation of the clay into the polymer matrix by controlling the morphology of the clay-encapsulated latex particles. We realized that with so many parameters involved the potentials of high-throughput experimentation (HTE) and on-line Raman spectroscopy should be explored, so some first attempts in this direction were made. Furthermore this thesis investigated the influence of clay on the morphology of multiphase latex particles. Clay particles are hydrophilic inorganic compounds and must be rendered (partially) hydrophobic in order to be more compatible with the in-situ synthesized polymer. In this thesis, the organic modification of clay was performed using two kinds of titanate coupling agents, titanium IV, (2-propanolato)tris(2-propenoata-O), 2-(2-methoxy-ethoxy) ethanol(KR39DS) and titanium IV, 2-propanolato, tris isooctadecanoato-0 (KRTTS), where the former is unsaturated and the second contains saturated alkyl groups. In Chapter 3, a study of the hydrolytic stability of the organoclays thus synthesized showed that the titanate modifiers used were highly susceptible to hydrolysis in the emulsion polymerization conditions. From the results obtained, it was concluded that successful clay encapsulation does not require the use of unsaturated organic modifiers as previously believed. Furthermore, emulsion polymerizations carried out with pristine clay also led to successful clay encapsulation showing that the modification step could be completely omitted. In addition to the organic modification, the influence of monomer feed composition, i.e. monomer mixtures consisting of different weight ratios of methyl methacrylate/ butyl acrylate, and the process type on the morphology of two-phase LCN was studied. It was shown that the monomer feed composition added under starved conditions strongly influenced the morphology of the LCN. Indeed, when the Tg of the encapsulating (co)polymer was well above the reaction temperature (hard polymer), anisotropic latex particles containing single clay platelets were mainly observed. In contrast, the use of a soft encapsulating polymer led to armored-like latex particles. Furthermore, only starved-feed monomer addition led to successful encapsulation: a batch process generated larger aggregates of clay particles and only a few armored-like latex particles. A heat treatment of the encapsulated clay particles showed that the clay encapsulation process was mainly kinetically controlled: after the heat treatment the clay was again on the outside of the latex particles. A mechanism of encapsulation was proposed, where the clay particles act as seed in the process (polymerization carried out from the surface of the inorganic particle). A systematic study of the effects of clay loading, surfactant concentration and surfactant type on the clay/polymer interaction was performed via a design of experiments. All three parameters were found to have significant effects on the clay/polymer interaction. In Chapter 4, three-phase PMMA/PS/MMT latex particles were synthesized from clay-containing PMMA seeds via (semi-)batch emulsion polymerization of styrene. For the interpretations of the morphologies obtained, the established theories to understand the morphology development of two-phase latex particles could be applied. An interesting observation was that clay platelets could act as physical barriers preventing diffusion of second stage polymers within the seed latex particles. A methodology to successfully conduct batch emulsion copolymerization using a parallel stirred automated synthesizer is described in Chapter 5. The most challenging step for such automated reactions was sampling. Sampling operations and inhibition were found to be the main source of errors in the determination of the partial monomer conversion-time histories. A comparison of the conversion rates of the automated reactions and the analyses of the particle size distributions and the molar mass distributions of the latexes synthesized clearly showed that the automated reactions were highly reproducible. A methodology to successfully conduct batch emulsion copolymerization using an automated parallel synthesizer is described in Chapter 5. The most challenging step for such automated reactions was sampling. Sampling operations and inhibition were found to be the main source of errors in the determination of the partial monomer conversion-time histories. A comparison of the conversion rates of the automated reactions and the analyses of the particle size distributions and the molar mass distributions of the latexes synthesized clearly showed that the automated reactions were highly reproducible. Chapter 5 also clearly demonstrated the potential of a low-cost-low-resolution portable Raman spectrometer to monitor emulsion homopolymerizations. The Raman spectroscopy technique in combination with partial least regression methods requires extensive calibration steps in order to gather large and representative data sets. Low-cost low-resolution portable Raman can be used as a conversion monitoring device and might be easy to integrate in the robotic platform.
|Qualification||Doctor of Philosophy|
|Award date||24 Apr 2012|
|Place of Publication||Eindhoven|
|Publication status||Published - 2012|