Elastic response of polymer-nanoparticle composite sponges: Microscopic model for large deformations

Abstract

We propose a minimalist coarse-grained microscopic model to investigate the mechanical response of ice-templated polymer nanocomposite sponges with large open voids. Earlier experimental work [Rajamanickam et al., Chem. Mater. 26, 5161 (2014)] has demonstrated that such systems show elastic recovery after being subjected to large compressive strains exceeding 80%, despite being comprised primarily of inorganic nanoparticles. Our model captures the essential features of the nonlinear mechanical response to uniaxial compression up to strain γ=0.8. From our simulation we identify three different regimes for the stress response: (i) the stress increases linearly with strain at low strains up to ≈0.2; (ii) at intermediate strains, such that γ is approximately in the range 0.2−0.5, we observe a plateau regime in the stress-strain data; and (iii) finally we see a sharp increase in stress at strains >0.5 .This agrees with experimental observations. The model helps us establish a correlation between the stress-strain response and the underlying microscopic reorganization of microstructure spanning multiple length scales, which leads to the emergence of the three regimes. The nature of individual void deformations was statistically analysed to demonstrate the progression of void shapes as the sponge is compressed. We also establish that nanoparticles at the interface of voids respond differently to stress as compared to those away from the interface. Our simulation model is versatile and allows us to vary parameters, which correspond to variations in the cross-link density and architecture of nanoparticle connectivity in experiments.