Gel phase formation in dilute triblock copolyelectrolyte complexes

Abstract

Assembly of oppositely charged triblock copolyelectrolytes into phase-separated gels at low polymer concentrations (<1% by mass) has been observed in scattering experiments and molecular dynamics simulations. Here we show that in contrast to uncharged, amphiphilic block copolymers that form discrete micelles at low concentrations and enter a phase of strongly interacting micelles in a gradual manner with increasing concentration, the formation of a dilute phase of individual micelles is prevented in polyelectrolyte complexation-driven assembly of triblock copolyelectrolytes. Gel phases form and phase separate almost instantaneously on solvation of the copolymers. Furthermore, molecular models of self-assembly demonstrate the presence of oligo-chain aggregates in early stages of copolyelectrolyte assembly, at experimentally unobservable polymer concentrations. Our discoveries contribute to the fundamental understanding of the structure and pathways of complexation-driven assemblies, and raise intriguing prospects for gel formation at extraordinarily low concentrations, with applications in tissue engineering, agriculture, water purification and theranostics.

Figure 1. Scattering from di- and triblock copolyelectrolyte self-assemblies.

Neutron scattering profiles (I(q) versus wave vector q) from self-assemblies comprising oppositely charged diblock (circles) and triblock (squares) copolyelectrolytes at various polymer concentrations. The polyelectrolytes were oppositely charged functionalized forms of (a) PAGE₂₇-PEO₄₅₅-PAGE₂₇ and PEO₂₂₇-PAGE₂₆ and (b) PAGE₃₀-PEO₂₂₇-PAGE₃₀ and PEO₁₁₄-PAGE₂₇. In (a) diffraction peaks corresponding to arrangements of complex domains in a BCC lattice are illustrated for the 40% by mass gel. In (b) various power law slopes are indicated by corresponding lines. Error in the data are typically smaller than the symbols and therefore not shown in the figure.

Figure 2. Structure evolution of complexation-driven assemblies.

Diblock copolyelectrolytes assemble into star-like micelles. At low polymer concentration, discrete micelles remain suspended in the solution. With increasing polymer concentration, micelles jam, thus forming viscoelastic solids, and eventually order in cubic lattice structures. Triblock copolyelectrolytes are expected to form flower-like micelles at extremely low concentrations (the expected flower-like micelle is shown in the inset), but instead form interconnected gels that phase separate from the solution. Increasing polymer concentrations leads to growth of the networks until they percolate through the solution resulting in viscoelastic solids, followed by a disorder-order transition of the PEC domains. In the corresponding jammed/percolated and ordered phases, PEC domains have very similar size and arrangements, leading to nearly identical scattering patterns.

Figure 3. Structure of the interconnected gels.

(a) Structure factor S(q) for di- and triblock copolyelectrolyte self-assemblies at various polymer concentrations. (b) Inter-domain distance d, and (c) SR (=(d–2R_c)/R_e-e,0) for the neutral midblock as a function of polymer concentration for various triblock copolyelectrolyte gels. In (a), successive S(q) plots for different concentrations are shifted vertically by 1 unit each, and collectively by 2 units for different polymer architectures. In (b), closed and open symbols denote data obtained from neutron and X-ray scattering, respectively.

Figure 4. MD simulations reveal triblock copolyelectrolyte assembly into interconnected gels.

(a) Snapshots of the simulation box showing self-assembled structures comprising oppositely charged di- and triblock copolyelectrolytes. The polycation, polyanion and neutral blocks are depicted by red, blue and yellow coloured beads, respectively. (b) Fraction of PEC cores forming isolated flower-like micelles as a function of polymer concentration for triblock copolyelectrolyte assemblies. The fraction approaches a near zero value at φ=0.36% by mass. Insets: Snapshots of the simulation box depicting triblock copolyelectrolyte assemblies at various polymer concentrations. The polycation, polyanion and neutral blocks are depicted by red, blue and grey coloured beads, respectively.

Figure 5. Neutral block conformations from MD simulations.

(a) Distribution of the end-to-end distances of the neutral block in the interconnected triblock copolyelectrolyte gels at φ=0.36 and 3% by mass. (b) Fractions of neutral midblocks forming loops or bridges and (c) normalized end-to-end distance of the neutral midblocks forming bridges or loops as a function of polymer concentration in the interconnected gels. In (b) and (c), open symbols correspond to the corresponding data from assembly of uncharged, amphiphilic triblock copolymers.