From lyotropic to thermotropic behavior: solvent-free liquid crystalline phases in polymer–surfactant-conjugated rod-shaped colloidal viruses

Filamentous bacteriophages <i>fd</i> are viral particles, highly monodisperse in size, that have been widely used as a model colloidal system for studying the self-assembly of rod-shaped particles as well as a versatile template in nanoscience. In aqueous suspensions, <i>fd</i> viruses exhibit lyotropic behavior, forming liquid crystalline phases as their concentration increases. Here, we report a solvent-free system displaying thermotropic phase behavior, achieved through covalent coupling of low molecular weight PEG-based polymer surfactant onto the <i>fd</i> virus surface. Upon lyophilization of aqueous suspensions of these polymer-grafted bacteriophages and subsequent thermal annealing, a solvent-free material is obtained, exhibiting both viscoelasticity and, notably, thermotropic liquid crystalline properties. A combination of small-angle X-ray scattering and optical microscopy experiments reveals the formation of an ordered hexagonal mesophase below 30 °C, which undergoes a melting transition into an isotropic liquid at higher temperatures. Our results demonstrate an efficient approach for converting lyotropic into thermotropic phase behavior in the columnar liquid crystalline phase of filamentous <i>fd</i> colloids. This approach paves the way for extending such functionalization to other technologically relevant rod-like systems, such as carbon nanotubes and cellulose nanocrystals, enabling the introduction of thermotropic properties in anhydrous colloidal materials.