Design of pressure-sensitive adhesives using miniemulsion polymerization

Abstract

This thesis serves to investigate how miniemulsion polymerization, as an alternative to the conventional emulsion polymerization, can be used to produce pressure-sensitive adhesives. The key advantage that miniemulsion polymerization offers is its droplet nucleation mechanism, where polymer particles are created in such a way that intra- and extra-particle diffusion resistance is maximized. This results in the in-situ production of populations of polymer particles with different properties. The main objective of this thesis was to create an adequately stabilized miniemulsion to create bimodal distributions of particle size or molecular weight for pressure-sensitive adhesives applications. The performance of pressure-sensitive adhesives is characterized by the quantification of an adhesion as well as a cohesion component. On the one hand, pressure-sensitive adhesives must be able to wet the surface and be able to flow under low stresses and on the other hand, they must be able to be removed from the surface and be able to dissipate energy. This leads to a conflicting set of properties wherein a compromise must be made in the final properties. In order to tackle this conflict, miniemulsion polymerization can potentially be used to create latexes with compensating properties at the nanoscale through the preparation of particles with different particle sizes and/or microstructures such as different molecular weights and copolymer composition. Our work started by investigating the influence of surfactant concentration, copolymer composition and chain transfer agent concentration on the resulting latex, polymer and adhesive properties of pressure-sensitive adhesives prepared with poly(2-ethyl hexyl acrylate/vinyl acetate/methacrylic acid). It was found that a combination of an anionic surfactant (sodium dodecyl sulfate) with an ethoxylated linear fatty alcohol non-ionic surfactant adequately stabilized this copolymer yielding latexes with 43 wt.-% solids content and with a maximum change in the number of particles (Np) with respect to the number of droplets (Nd) of ∼6%. Nevertheless, this copolymer showed considerable homogeneous nucleation in the water phase due to its water solubility, which unavoidably led to inadequate pressure-sensitive adhesives properties. The second part of the study employed methyl methacrylate as a substitute for vinyl acetate to produce a 2-ethyl hexyl acrylate/methyl methacrylate/acrylic acid terpolymer. It was found that the surfactant mixture, sodium dodecyl sulfate/Disponil A3065, was also able to stabilize this miniemulsion. The adhesive properties of films cast from this polymer system were systematically compared to those produced by conventional emulsion polymerization using a factorial design. Significant differences between the latexes (or films) produced by these two techniques were found due to their different particle nucleation mechanisms. The most significant difference between the films obtained was shown via a viscoelastic analysis. The analysis showed that miniemulsion-based films presented more entangled networks, i.e., larger values of Mw/M e (which is a measure of the entanglement density) compared to emulsion-based ones even though the weight-average Mw was generally lower for the latexes produced by miniemulsion polymerization. In subsequent work, a one-pot two-step miniemulsion polymerization approach was used to create poly (2-ethyl hexyl acrylate/methyl methacrylate/acrylic acid) pressure-sensitive adhesives with well-defined and predictable bimodal particle size distributions or bimodal molecular weight distributions. The "in-situ bimodal" latexes produced using this approach were compared to monomodal latexes as well as bimodal latexes produced via conventional post-polymerization blending on the basis of their viscoelastic and pressure-sensitive adhesives properties. Finally, an exploratory study of the use of Atomic Force Microscopy to investigate pressure-sensitive adhesives performance was conducted. Atomic force microscopy was used to acquire adhesion-force curves to investigate the effect of monomodal and bimodal distributions of molecular weight and particle size on the adhesion force and energy at the nanoscale. In conclusion, this is a first attempt to engineer bimodal distributions of molecular weights and particle sizes using miniemulsion polymerization specifically for the production of pressure-sensitive adhesives. Our conclusions are broadly applicable to a large class of soft materials based on deformable polymeric networks such as adhesives, coating and paints. It is expected that in the future, production of latexes with concurrent multimodal distributions of molecular weights, particle sizes and compositions can be produced using this approach.