Self-assembling multiblock amphiphiles: Molecular design, supramolecular structure, and mechanical properties

Abstract

We perform off-lattice, canonical ensemble molecular dynamics simulations of the self-assembly of long segmented copolymers consisting of alternating, tunably attractive and hydrophobic {\em binder} domains, connected by hydrophilic {\em linker} chains whose length may be separately controlled. In such systems, the molecular design of the molecule directly determines the balance between energetic and entropic tendencies. We determine the structural phase diagram of this system, which shows collapsed states (dominated by the attractive linkers' energies), swollen states (dominated by the random coil linkers' entropies) as well as intermediate network hydrogel phases, where the long molecules exhibit partial collapse to a {\em single molecule network} state. We present an analysis of the connectivity and spatial structure of this network phase, and relate its basic topology to mechanical properties, using a modified rubber elasticity model. The mechanical properties are further characterized in a direct computational implementation of oscillatory rheology measurements. We find that it is possible to optimize the mechanical performance by an appropriate choice of molecular design, which may point the way to novel synthetics that make optimal mechanical use of constituent polymers.