Formation of asymmetric and symmetric hybrid membranes of lipids and triblock copolymers

Hybrid membranes formed by co-assembly of A_xB_yA_x (hydrophilic-hydrophobichydrophilic) triblock copolymers into lipid bilayers are investigated by dissipative particle dynamics. Homogeneous hybrid membranes are developed as lipids and polymers are fully compatible. The polymer conformations can be simply classified into bridge-and loop-structures in the membranes. It is interesting to find that the long-time fraction of loop-conformation (f_L) of copolymers in the membrane depends significantly on the hydrophilic block length (x). As x is small, an equilibrium f*_L always results irrespective of the initial conformation distribution and its value depends on the hydrophobic block length (y). For large x, f_L tends to be time-invariant because polymers are kinetically trapped in their initial structures. Our findings reveal that only symmetric hybrid membranes are formed for small x, while membranes with stable asymmetric leaflets can be constructed with large x. The effects of block lengths on the polymer conformations, such as transverse and lateral spans (d^⊥ and ^d) of bridge-and loop-conformations, are discussed as well.