The nano/micro-morphology of a material is a qualitative property that is typically articulated after inspection of SEM images. An extreme case of that is encountered with sol-gel derived polyurea aerogels, which, depending on the monomer and catalyst concentration and the solvent, display an extremely rich variety of complicated nanostructures ranging from random assemblies of nanoparticles, to nanoworm-like structures of variable aspect ratios, to entangled nanofibers, to microspheres entrapped in hair-like nanofibers. In order to develop deterministic procedures that relate morphology to synthetic conditions, it is necessary to define the nano/micro-structure quantitatively. Working with three independent variables in 8 different solvents, we prepared 188 different polyurea aerogels guided by statistical design of experiments methods. A close look at SEM images revealed that the nanostructure can be described in terms of openness and texture; openness is quantified by porosity (∏), and texture is related to the contact angle (θ ) of water droplet resting on the material (hydrophobicity). It was then discovered that all polyurea nanomorphologies could be put in eight different groups, which were uniquely related to a morphology descriptor that we refer to as the K-index (= θ /∏ ). A first validation of the K-index came from samples compressed to different strains: the porosity decreased, but also the water contact angle, and K-index remained constant. The predictive power of the K-index was then confirmed with new polyurea aerogels prepared in 22 new binary solvent mixtures. Finally, using empirical models the K-index was correlated to the monomer concentration and the Hansen solubility parameters of the sol, thus enabling accurate prediction of synthetic conditions required for any nanomorphology of interest.

K-index: A quantitative tool that correlates complex polymeric nanomorphologies with synthetic conditions

256th National Meeting (2018)

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