Phase Behavior of Thermoresponsive Nanoplatelets

Nanoplatelets open up a wide range of possibilities for building materials, with novel properties linked to their shape anisotropy. A challenge consists of dynamically controlling the order of positioning and orientation in three dimensions by assembly to exploit the collective properties at the macroscale. While most studies to date have focused on hard platelets that cannot be stimulated by an external trigger, in the present work, we tackle the case of core/shell platelets, composed of a mineral core (laponite) coated with a soft polymeric shell (PEO₁₀₀-PPO₆₅-PEO₁₀₀ copolymer), whose swelling can be triggered by temperature variation. We identified unambiguously the signature associated with the different phases obtained as a function of temperature and concentration by combining local (X-ray scattering, electron microscopy) and global (rheology, optical birefringence) methods of analysis. In this way, we obtained two main results. The first shows that the deposition of a soft layer onto the laponite surface enables a phase transition from isotropic liquid toward liquid suspensions of random stacks, which is not observed for bare laponite suspensions in the studied weight concentration range (ϕ ≤14 wt %). The second result shows that an increase of the nanoplatelet effective volume fraction triggered by temperature (swelling of the polymer shell) induces a phase transition from liquid suspensions of random stacks toward birefringent gels of nematic stacks. Both results agree with numerically predicted phase sequences expected by variation in particle density under similar charge screening conditions, taking into account the contribution of the copolymer layer to the particle volume fraction. We believe that these results pave the way for the control of nanoplatelet self-assembly by external action.