Stable double gyroid network phases in asymmetric linear triblock amphiphiles

Abstract

Molecular dynamics simulations are employed to elucidate the phase behavior of a series of asymmetric linear triblock amphiphiles consisting of a sugar-based center segment (A₄) containing four hydroxyl groups with the ability to form inter- and intramolecular hydrogen bonds that is flanked by two nonpolar hydrocarbon segments (C₆H₁₃ and C_xH_2x+1, denoted as B₆ and B_x with 6 ≤ x ≤ 14). In agreement with prior simulations, the B₆A₄B₆ amphiphile with symmetric hydrocarbon blocks preferentially self-assembles into hexagonally perforated lamellae, whereas those with asymmetric hydrocarbon segments and n ≤ 12 can form stable double gyroid networks. Within these double gyroid morphologies, interconnected channel domains are composed of sugar-based segments embedded in a continuous hydrocarbon matrix. Remarkably small feature sizes ranging from 1.92 to 2.26 nm are observed as the sugar volume fraction (f_A) varies from 0.23 to 0.29. Structural analyses further reveal distinct local enhancements in oxygen density—indicative of concentrated sugar-based segments—predominantly at the gyroid nodes, accompanied by reduced densities along the connecting struts. Additionally, a mean curvature analysis confirms that these fairly stiff oligomers form gyroid structures containing a higher fraction of regions with high curvature compared to idealized gyroid structures. Together with investigations of related miktoarm triblock oligomers (A_oB_pB_q), this study highlights the role of asymmetry in the molecular design of amphiphilic block oligomers, which facilitates the stabilization of network morphologies with ultra-small feature sizes and provides insight into the microscopic packing behavior of amphiphiles.