Temperature-dependent relationship between the structure and mechanical strength of volatile organic compound-free latex films prepared from poly(butyl acrylate-co-methyl methacrylate) microspheres

Latex films that are formed by evaporating dispersions in the absence of volatile organic compounds (VOCs) typically suffer from poor mechanical strength compared to solution-cast latex films. In our previous work, we discovered that this disadvantage can be overcome by using microspheres crosslinked with rotaxanes, which consist of a crown ether wheel and an axle. In the present study, to obtain tougher latex films, we investigated the relationship between the mechanical properties and the nanostructures of films prepared at different film-formation temperatures (FFT), i.e., FFTs above and below the glass-transition temperatures (T_g) of the microspheres. Tensile tests revealed that the films showed the highest fracture energies when the film was formed at a temperature higher than the T_g of the microspheres and followed by annealing. In addition, the interfacial thickness (t_inter), which is an indicator of the magnitude of the relationship between the t_inter of neighboring microspheres, was correlated with the fracture energy as a function of annealing time. Thus, tough latex films could be obtained without the use of any additives by increasing the FFT during the formation and subsequent annealing of the film. This study may lead to new applications, e.g., VOC-free coatings for biomaterials.