Effect of particle morphology on film morphology and properties (original) (raw)

One of the greatest challenges facing modern society is the reduction of climate change and the development of sustainable materials. With this in mind, the production of protective and decorative coatings, which has traditionally been dominated by solvent based products, is gradually being shifted towards the use of waterborne dispersions. Such coatings imply the formation of a film which often needs to satisfy several seemingly contradictory requirements with regards to mechanical properties. One common example is the necessity for outdoor coatings to be both capable of forming a film at relatively low temperatures whilst being hard at the same temperature. Solvent based coatings easily fulfill these requirements. Matching their performance with waterborne coatings is not trivial and often potentially toxic additives, such as coalescing agents, have to be added to the formulation. Efforts are now devoted to synthesize latexes presenting good properties whilst adding a minimum of additives. One way to do this is through the use of a blend of two latexes with different glass transition temperatures (Tgs). One latex with a low Tg to form a film at low temperature and the other one with a higher Tg, to provide film hardness. However, blending leads to non homogenous films and the properties of the film are affected. 1 A more uniform distribution of the two phases in the film can be obtained by using heterogeneous polymer particles (also called hybrid particles). In this case, two different polymers are present in the same particle which offers the opportunity to combine the Chapter 1 2 positive properties of their constituents. Although such polymers are typically incompatible, their phase separation during film formation can be controlled by grafting reactions. 2-6 Recent works showed that controlling the morphology of hybrid particles is not trivial. 1,7-13 Interestingly, much less attention has been paid to the fact that application properties are really determined by the film morphology and that the relationship between the particle morphology and the film morphology is not evident. The goal of this PhD Thesis is to shed some light on this relationship. Therefore, as it is expected that the film morphology is affected by the particle morphology and the film formation process, the knowledge available about these aspects will be summarized in the next sections. 1.2 Synthesis of hybrid particles A variety of chemical and physical methods exists for the synthesis of structured polymer particles with different morphologies. Of these, emulsion polymerization is the most common for the synthesis of waterborne polymer-polymer hybrids due to its versatility to control the properties of the final product. Miniemulsion polymerization is more adequate when hydrophobic monomers, preformed polymers and inorganic particles should be included in the polymer particles. 14-19 12 Miniemulsion is also used in the synthesis of polymer-inorganic material hybrid because no transport through the water phase is needed. Aguirre et. al. incorporated cerium oxide (CeO2) in acrylate latex particles hybrid by semi-batch emulsion polymerization. 55,56 The seed was produced by miniemulsion polymerization containing the whole load of the metal oxide. It was found that the nanoparticles were preferentially located at the surface of the seed particles. They obtained a single aggregate of CeO2 particles per polymer particle due to the incompatibility between the CeO2 and the polymer (see Figure 1-4 a)). The hybrid dispersion led to UV protective coatings. Another example is the work of de San Luis et al. 57. They used miniemulsion to encapsulate quantum dots into acrylic particles. This hybrid system can be used to improve the electronic transmission in electronic devices such as light emission diodes or solar cells. 58-60 They showed that inefficient encapsulation led to a loss of fluorescent intensity of the quantum dots. This issue was solved by formation of cross-linked core/shell particles composed by a core of cross-linked polystyrene and quantum dots and a shell of cross-linked polymethyl methacrylate produced by seeded semi-batch miniemulsion polymerization (Figure 1-4 b)). Sun et al. 15 encapsulated Laponite clay into polystyrene particles. Hydrophobicity of the clay is the key point for the success of the encapsulation and the stability of the latex. A quaternary ammonium salt was mixed with the clay in the monomer phase in order to disintegrate the agglomerates in the clay under sonication. Then, the miniemulsion was prepared by ultrasonication. The salt helped to stabilize the system with a nonionic surfactant. Gong et al. 61 encapsulated magnetite into cross linked PS latex particles. After optimization of the formulation (surfactant and crosslinker amount), a homogeneous distribution of the magnetite in the particles was obtained (see Figure 1-4 c)). The magnetic