Filamentous Viruses Grafted with Thermoresponsive Block Polymers: Liquid Crystal Behaviors of a Rodlike Colloidal Model with True Attractive Interactions

Understanding how attractive interactions among rigid polymers or rodlike particles influence their liquid crystal (LC) phase behavior is of fundamental and practical importance. This question has not been fully answered yet, mainly due to the shortage of model systems with true pairwise attractions on a single particle level while with excellent colloidal stability. Herein, we report on a well-defined rodlike system that fulfills such criteria, through covalently grafting the free end of the thermoresponsive PNIPAM block of poly(ethylene glycol)-block-poly(N-isopropylacrylamide) (PEG-b-PNIPAM) onto the classic rodlike model system-the fd or M13 virus-which is the hallmark in understanding the LC behaviors of rigid polymer or rodlike particles. Increasing temperature induces dehydration and collapse of the PNIPAM chains onto the virus surface and therefore introduces attractions among the viruses, while the outer hydrophilic PEG block offers steric stabilization to prevent interparticle aggregation, gelation, or other dynamically arrested states. The influence of the temperature, and consequently of the attraction strength between rodlike particles, on the LC phase behaviors of hard rodlike particles was thoroughly investigated via a forced phase separation assisted by a low-speed centrifuge, leading to an apparent phase diagram in the space of attractive strength and isotropic-nematic coexisting LC phases. At T << LCST, the block polymers are in the fully hydrophilic state and the rodlike system behaves as hard rods. Its LC behaviors can be quantitatively described by the flexibility-corrected Onsager's hard rod theory. Although the forced phase separation is not truly in phase equilibrium, increasing temperature to induce the collapse of the PNIPAM blocks does lead to theoretically predicted widening of the isotropic-nematic coexisting concentrations with increasing temperature. Fitting our experimental data with advanced theories reveals several physical parameters that probably characterize the LC phase of rigid polymers or rod systems with attractive interactions in general.