Characterization of acrylic-based latex blend coatings and thermodynamics of their deformation

• Main Author: Agarwal, Naveen
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1998
• Subjects: Polymers|Materials science|Plastics
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9909143

A complete characterization of the mechanical, thermal and physical properties of acrylic-based latex blends films and a thermodynamic analysis of their deformation is presented in this study. These blends are composed of a glassy poly(methyl methacrylate-co-ethyl acrylate) $\rm(T\sb{g}=45\sp\circ$C), and a rubbery poly(methyl methacrylate-co-butyl acrylate) $\rm(T\sb{g}={-}5\sp\circ$C). Blend films are prepared, in different proportions of the two copolymers, by drying at temperatures high enough to ensure complete coalescence of the latex particles. Thermo-mechanical characterization provides evidence for the phase separation of the blend components by the existence of two distinct glass transitions. Effective blend moduli and Poisson's ratios exhibit sigmodial shaped profiles with composition, indicating the transformation of a continuous rubbery phase, with dispersions of the glassy phase, to a continuous glassy phase, with dispersions of a rubbery phase. Although not precisely measured, a range of 30-40% hard phase in the blend is identified as the interval of this transformation, bridged by a co-continuous morphology. A large amount of water is absorbed by these blends, which turns them white and opaque from their transparent dry state. The impact on mechanical properties is relatively minor as absorbed water is located in separate domains. Redrying at ${-}70\sp\circ$C preserves this whiteness, while redrying at elevated temperatures returns the blends to their original transparency. A qualitative model associates the absorbed water molecules with phase separated domains of residual surfactant within the dry films. Deformation calorimetry of these blends measures the work, heat and change in internal energy of isothermal deformation. An optimal combination of stiffness and extensibility maximizes the blend toughness by a synergistic distribution of energy between the two phases in their respective energy absorbing and energy dissipating mechanisms. The work of deformation increases at higher strain rates but the change in internal energy over fixed extensions remains constant. The additional work, consequently, is dissipated as heat by rate-dependent viscous effects. In summary, these blends provide an excellent model system to study the energy balance of deformation of two phase systems. The results highlight the need of a shift in focus when designing blends for optimum toughness and stiffness, by providing for a simultaneous maximization of energy dissipation and absorption.