Impact of extreme softness on the crystallization of ultra-low crosslinked microgels in two and three dimensions

Ultra-low crosslinked poly(N-isopropylacrylamide) microgels are used to elucidate the effect of softness on crystallization in two- and three-dimensions. 2D systems are produced by compressing and depositing microgels from oil-water interfaces. Ex-situ analysis by atomic force microscopy reveals that no crystals form upon compression, as opposed to the case of stiffer microgels. Once adsorbed at a fluid interface, microgels spread into disk-like objects whose size and stiffness are a strong function of polymer content and crosslinking. Consequently, interfacial confinement exacerbates size and mechanical polydispersity, effectively suppressing crystallization. In contrast, in 3D, the microgels crystallize at high concentrations, indicating that softness per se is not sufficient to suppress the liquid-solid phase transition, unless it is coupled to induced polydispersity. Furthermore, small-angle neutron scattering with contrast matching reveals that interpenetration is dominant over deswelling and deformation when these microgels are in 3D-overcrowded environments.