Theoretical Study of the Structure and Assembly of Janus Rods

Microscopic integral equation theory is applied to investigate the real and Fourier space structure and phase behavior of compositionally symmetric AB Janus rods that interact via hard core excluded volume interactions and competing repulsive and attractive tail potentials. If the spatial range of the latter are short and equal, chemical asymmetry results in attraction driven assembly into a cylindrical micellar structure of spatial periodicity between one and two rods lengths. The apparent microphase spinodal ordering temperature increases with attraction range and rod length, and the microdomain coherence length strongly grows upon cooling. However, as the tail repulsion range increases significantly, the micellar morphology is destroyed and a new repulsion driven ordering emerges at high volume fractions characterized by a much smaller spatial periodicity. We also find a tight correlation between the local attraction induced clustering (quantified by the excess number of nearest neighbors) and the microdomain-scale ordering (measured by the amplitude of the intense low wavevector peak of the collective structure factor). Moreover, the former converges to a unique value at the microphase spinodal temperature for all volume fractions, repulsion strengths, and tail repulsion ranges studied, in qualitative contrast to the behavior of the repulsive interaction driven ordering phenomena. The phase behavior trends of the integral equation approach are compared to a computationally and conceptually simpler mean field hybrid formulation based on a random phase approximation treatment of tail potentials, thereby allowing the role of physical clustering, interparticle correlations, and microdomain scale concentration fluctuations to be assessed.