Deformation and failure mechanism of nanostructured polymer thin films
In this research, we develop models to describe the impact of nanostructures on the elastic deformation and failure of polymer thin films. First, we use polystyrene-b-poly(2-vinyl-pyridine) as a model material to investigate the effect of nanodomains and surface terracing on the crazing process. Here, we find that well-aligned lamellar domains can alter the extension ratio of craze fibrils and delay craze growth rate. Additionally, nanoscaled surface terraces impede the craze initiation and significantly decrease the failure strain of polymer thin films.
To investigate the impact of inorganic nanodomains on the crazing process, we use a model and well-dispersed nanocomposite of polystyrene blended with surface modified cadmium selenide nanoparticles (diameter ∼3.5 nm). With this material design, enthalpic interaction is minimized and entropic interaction dominates the mechanical properties. Due to high surface-to-volume ratio of nanoparticles and nanoparticle-polymer entropic interaction, the elastic modulus of our nanocomposites decreases as a function of volume fraction of nanoparticles (V). During craze formation and growth, nanoparticles undergo three stages of rearrangement: (1) alignment along the precraze, (2) expulsion from craze fibrils, and (3) assembly into clusters entrapped among craze fibrils. This entropically-driven rearrangement leads to the altered craze morphology and an increase in failure strain by 60% at an optimal V. This optimal volume fraction is related to the balance of two mechanisms: (1) the decrease in the volume fraction of cross-tie fibrils, and (2) the decrease in extensibility of the craze.
In the last part of this research project, larger nanoparticles (diameter is ∼6.0 nm) are used to increase nanoparticle-polymer entropic interaction. Here, we find that nanoparticles segregate to the film surface during the film casting process on the substrate. This entropically-driven segregated layer of nanoparticles leads to a significant drop in the elastic modulus at very low V (0.2%) and an increase in failure strain by 100% in an optimal V.