Film formation from aqueous suspensions of polymer nanoparticles is an important process in many environmental friendly applications and particularly for waterborne coatings. This process occurs via three mains steps: concentration, sintering, and interdiffusion. During the sintering step, the particles in close-packed morphology deform and the interstices between them close under Laplace pressure. This step is crucial in the film formation process since it is where the suspension turns into a uniform defect-free film. Most of the experimental and theoretical studies on sintering assume that the interstices close uniformly over the entire film. We use small-angle neutron scattering to probe void closure between polystyrene nanoparticles. We show that the voids close simultaneously and uniformly throughout the annealing process in large particles. For particles with a diameter smaller than 60 nm, we interpret the results to show that the interstices close heterogeneously at the nanoscopic level: in the beginning of annealing, some interstices close while others enlarge, and eventually they all vanish. The difference between the behavior of large and small particles is related to the high polydispersity of small particles compared to the larger ones.

Coalescence during drying of suspensions containing glassy polymer nanoparticles generates fascinating crack patterns, whereas coalescence of soft polymer particles gives crack-free films. This process is widely used in environmental-friendly water-based coatings, which use nanosized polymer nanoparticles in the film formation process. Numerous studies have shown that polymer nanoconfinement could lead to a drastic modification of the polymer properties such as a reduction of the glass transition temperature (T g). If such a change in the properties of polymers also exists when nanoconfined in nanoparticles in suspension, then one would expect an alteration in the film formation process, particularly a reduction in the minimum film formation temperature. In this paper we look at the effect of nanomechanical properties of polymer nanoparticles on crack patterns generated by particle coalescence. We study crack pattern morphology during drying of aqueous suspensions of PBMA nanoparticles (50 nm diameter) with different nanomechanical properties (different cross-linking ratios). We investigate the effect of temperature and relaxation time on crack morphologies in the transition from crack to crack-free films. These results were analyzed in terms of time−temperature superposition and compared to bulk behavior.