Parallel-plate rheology of latex films bonded to wood

Abstract

Abstract A parallel-plate rheological analysis was developed for two types of cross-linking poly(vinyl acetate) latex films: solid freestanding films (neat), and solid films bonded directly to wood substrates (composites). The composite sample glass transition was increased by 5°C relative to the neat films, suggesting a strong interaction through polymer adsorption. Time-temperature equivalence was used to evaluate the mechanical response and the distribution of relaxation times in both systems. The resulting master curves revealed significant differences in the mechanical response during the glass transition: the neat films exhibited a 2.5–3-decade change in storage modulus and a damping factor of 1.65, while the composite samples showed a much smaller transition (approx. 0.3 decades and tan δ=0.4, respectively). The coupling model of Plazek and Ngai adequately described segmental relaxations in the glass transition. It was found that wood caused a small but significant decrease in segmental coupling (n=0.32±0.01) relative to that of the neat PVAc films (n=0.37±0.01). This finding was independently confirmed by directly fitting the Gram-Charlier statistical model to evaluate the distribution breadth. That wood caused an increase in the poly(vinyl acetate) glass transition, but a decrease in the related segmental coupling seems counterintuitive. The decrease in segmental coupling may result from reduced cross-linking in the composite films relative to the neat films; perhaps wood absorbs water-soluble reactive chains that would otherwise increase cross-linking in the neat films.