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1. Introduction and Aims 7 1. Introduction and Aims “An expert is a man who has made all the mistakes he can in a very narrow field.” Niels Bohr The Kinetics of Electrosterically Stabilized Emulsion Polymerization Systems 8 Chapter 1 1.1. Polymers and Emulsion Polymerization A polymer is a macromolecule that comprises of covalently linked repeat units known as monomers, typically visualized as beads on a piece of string. Polymers can span many orders of magnitude of molecular weight (from 103 to 106-8 Da), and are both naturally occurring (natural rubber and polysaccharides such as starch as examples) as well as easily synthesized in the laboratory. Because of the variety of synthetic monomers with different functional groups and properties, the synthesis of polymers for industrial applications (e.g. paints, plastics, adhesives, films, barrier products, specialty medical applications and many more) has become the most widely performed chemical reaction in the world today, with polymer-based products pervading every aspect of modern society. A typical free-radical polymerization reaction (initiated by the thermal decomposition of a molecule to form radicals) is typically robust to many different reaction conditions,1 including a variety of different organic and aqueous solvents as well as tolerance to trace amounts of impurities. This is a massive technical advantage for industrial polymer synthesis; however the major shortcoming with bulk or solution polymerization is the relatively low percentage conversion and/or percentage solids obtainable before sample handling becomes extremely difficult (due to a significantly increased sample viscosity). As a result, emulsion polymerization (the heterogeneous polymerization of an organic monomer in an aqueous dispersed phase under constant shear, forming sub-micron scale polymer particles stabilized by surfactant) is the method of choice to synthesize polymer easily on an industrial, multi-tonne scale;2 the aqueous phase provides excellent heat dissipation, the final sample retains a relatively low viscosity and extremely high reaction rates and molecular weights are attainable. A schematic representation of the emulsion polymerization process is given in Figure 1.1, and the resultant polymer colloid is typically known as a ‘latex.’ The Kinetics of Electrosterically Stabilized Emulsion Polymerization Systems Introduction 9 surfactant initiator monomer stirring heat water polymer particles Figure 1.1. Schematic representation of the emulsion polymerization process. Despite the ease of implementing this technique in the laboratory, the kinetics and mechanism that govern polymerization in emulsion are complicated by numerous interfacial processes, as well as the kinetics of the fundamental chemical reactions within any polymerization. As a result, the study of the many aspects of emulsion polymerization kinetics (e.g. particle formation, radical entry and exit) has become a significant academic pursuit – albeit one with important industrial implications. In order to successfully model an emulsion polymerization (with a view to improving reaction conditions and synthesizing novel and desirable polymer products), a detailed knowledge of the mechanisms governing every process taking place simultaneously is required. Naturally the synthesis of polymer on a multi-tonne scale must be modelled to prevent ‘disastrous’ reaction conditions (for example, an uncontrollable exotherm or substantial coagulation), and to ensure the success of these reactions the knowledge of the mechanisms that control particle formation and particle growth are of fundamental importance. Through carefully designed experiments many of these key governing mechanisms have been elucidated over the past fifty years,3 giving the industrial chemist a solid base to work from. The Kinetics of Electrosterically Stabilized Emulsion Polymerization Systems 10 Chapter 1 One extremely significant area of study in emulsion polymerization revolves around a component that represents only a tiny fraction (by weight) of an emulsion – the stabilizing species that imparts colloidal stability for the formed polymer particles. On a small scale in the laboratory, it is common to use ionic surfactants (such as sodium dodecyl sulfate, SDS) as the stabilizer; the charge on the sulfate head group provides colloidal stability via electrostatic repulsion. The overall stability of an electrostatically stabilized latex depends on the total interaction energy profile, which is the sum of the attractive (van der Waals) and repulsive (electrostatic) terms as a function of separation distance, known as DLVO theory.4 This in general is very good at predicting the overall stability of a polymer latex stabilized in this manner. Much work also has been done to understand the role of the stabilizer with regards to particle formation – the ‘homogeneous nucleation3, 5, 6’ and ‘micellar nucleation7’ models for particle formation have proven extremely successful in modelling the final particle number (Np, the number of polymer particles per litre of emulsion) as a function of stabilizer concentration under a wide range of conditions. The excellent knowledge base established for systems stabilized by conventional surfactants has meant that much research has been invested in such systems – yet commercially they are of little practical interest. Added surfactants are generally retained in any final polymeric product, increasing the water permeability of polymer films and decreasing adhesion to substrates.8 Another mode of stabilization, (electro)steric stabilization, is often employed in industrial systems, and is discussed more in the subsequent section. 1.2. Steric and Electrosteric Stabilization Steric stabilization of an emulsion polymer particle is when a physical barrier is placed on the particle surface to prevent coalescence of neighbouring particles and to retain colloidal stability. This physical barrier is usually in the form of hydrophilic polymer chains (either grafted or The Kinetics of Electrosterically Stabilized Emulsion Polymerization Systems Introduction 11 adsorbed to the particle surface) that extend into the aqueous phase – typical examples are polymers of acrylic acid or ethylene oxide. The method of stabilization is essentially an entropic one; upon approach to one another, sterically stabilized particles begin to undergo chain entanglement and interaction between stabilizing blocks, which is thermodynamically unfavourable. This results in the particles repelling one another in order to maximise the entropic configuration of the stabilizing chains,9 shown in Figure 1.2. If the hydrophilic monomer can be ionized (e.g. the neutralization of acrylic acid) then the particle is stabilized both sterically and electrostatically, which is often referred to as ‘electrosteric stabilization.’ An electrosterically stabilized polymer particle is often visualized as a particle surrounded by a corona of stabilizing hydrophilic chains, commonly referred to as a ‘hairy layer.’ Entanglement of stabilizing chains - thermodynamically unfavourable Figure 1.2. Steric stabilization of emulsion polymer particles. The advantages of (electro)steric stabilization for industrial polymer synthesis are numerous. Firstly, the need for added surfactant is removed, as one can simply perform an emulsion copolymerization with a hydrophilic monomer to provide colloidal stability. Secondly, the stabilizer is ‘built-in,’ with no possibility of surfactant migration and other such problems in the final product. Finally, the new-found tools at the polymer chemist’s disposal of controlled- radical polymerization via RAFT10-14 (Reversible Addition Fragmentation Chain Transfer) and nitroxide-mediated15-17 polymerization techniques have allowed the ability to control the length The Kinetics of Electrosterically Stabilized Emulsion Polymerization Systems 12 Chapter 1 and nature of the stabilizing block, and the synthesis of novel diblock structures that can serve as effective particle stabilizers.8, 16, 18 Despite the massive industrial interest in (electro)sterically stabilized systems, very little scientific endeavour has been invested in an attempt to better understand these systems. Poly(acrylic acid)19 (polyAA) and poly(ethylene oxide)20 stabilized latexes are plagued by secondary nucleation (the formation of new particles after the establishment of an original population of polymer particles) under conditions where new particle formation should not be possible; the rate coefficients of radical entry and radical exit (that govern the growth of polymer particles) in polyAA stabilized emulsions have demonstrated significant departures19, 21 from the well-accepted mechanisms that govern these events in electrostatically stabilized systems. The use of amphiphilic block copolymers as ‘surfactants’ in ab initio emulsion polymerization experiments has also given strange results in the literature,22 with unexpected dependencies on the charge on both the hydrophilic block and the initiator. It is clear that these systems are extremely poorly understood, and that the stabilizer itself plays a crucial role in the different kinetics these systems display. Nonetheless, (electro)sterically stabilized emulsions remain (to date) a poorly examined area of polymer science. 1.3. Aims and Objectives The aim of this work is to quantitatively characterize the kinetics that govern the key processes in (electro)sterically stabilized emulsion polymerization systems – namely particle growth and particle formation. Particle growth
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