Multiple crack nucleation in drying nanoparticle-polymer coatings

We performed microscopic, in situ visualizations of spontaneous cracking in drying silica-poly(vinyl alcohol) aqueous suspension coatings. The cracking event was quantified by measuring numbers of nucleation points per unit area from acquired time-lapse video images. Direct imaging revealed that the primary nucleation rate decreases obeying a power-law, whereas the secondary nucleation rate first increases and then decreases with increase in the particle-to-polymer weight ratio. The resultant cross-over between two nucleation rates leads to a drastic morphology transition from isolated star-like cracks to interconnected ladder-like cracks. The maximum crack surface density was obtained at a critical cross-over particle loading.