Bridge-rich and loop-less hydrogel networks through suppressed micellization of multiblock polyelectrolytes

Abstract

Most triblock copolymer-based physical hydrogels form three-dimensional networks through micellar packing, and formation of polymer loops represents a topological defect that diminishes hydrogel elasticity. This effect can be mitigated by maximizing the fraction of elastically effective bridges in the hydrogel network. Herein, we report hydrogels constructed by complexing oppositely charged multiblock copolymers designed with a sequence pattern that maximizes the entropic and enthalpic penalty of micellization. These copolymers self-assemble into branched and bridge-rich network units (netmers), instead of forming sparsely interlinked micelles. We find that the storage modulus of the netmer-based hydrogel is 11.5 times higher than that of the micelle-based hydrogel. Complementary coarse grained molecular dynamics simulations reveal that in the netmer-based hydrogels, the numbers of charge-complexed nodes and mechanically reinforcing bridges increase substantially relative to micelle-based hydrogels.

Fig. 1. Schematic representation of the network structures and gelation mechanisms.

Two types of hydrogel networks. a In the traditional hydrogel network formed through the self-assembly of ABA block copolymers, the nodes are surrounded by loops. b In the proposed densely linked network, the coacervate nodes are closely linked by a large number of bridges. c Schematic of the hierarchical assembly mechanism from the unimer to the hydrogel network in three different polyelectrolytes. As the polymer concentration increased, the triblock polyelectrolytes formed micelles, leading to the formation of a tri-PEC (polyelectrolyte complex) hydrogel network with a sparsely linked structure. Pentablock polyelectrolytes and nonablock polyelectrolytes preferentially formed netmers with branched structures and produced the densely linked penta- and nona-PEC hydrogel networks, respectively.

Fig. 2. Morphology analysis in PEC.

Cryo-TEM images of the 0.2 wt% tri(LM)- (a), tri(SM)- (b), penta- (c), and nona-PEC (d). Histogram of core diameter distribution for tri(LM)- (e), tri(SM)- (f), penta- (g), and nona-PEC (h) calculated from cryo-TEM images. The total counts are 200, and the red line is a normal distribution. Core diameters are presented as mean values ± standard deviation. i SANS profiles of tri(LM)- (green open circles), tri(SM)- (olive green open circles), penta- (black open circles), and nona-PEC (blue open circles) with 0.2 wt% polymer concentration. All SANS profiles were fitted by the best model of the form factor of the core-shell sphere (red solid line). j Node diameter of four different PECs. The open rectangle represents cryo-TEM results, and the closed rectangle represents SANS results. Node diameters are presented as mean values ± standard deviation. k Calculated aggregation number of four different PECs. Schematic of the expected ionic block conformations in the coacervate node of tri(LM)- and tri(SM)-PEC (l) and penta- and nona-PEC (m). The gray circle represents the coacervate node, the solid line represents the neutral block, and the dotted line represents the ionic block.

Fig. 3. CG-MD simulation and size analysis in PEC.

Coarse-grained molecular dynamics (CG-MD) simulations of self-assembled tri(LM)- (a), penta- (b), and nona-PEC (c) in coexistence with a dilute phase; the yellow, blue, and red monomers of polymers represent the neutral, negatively charged and positively charged species, respectively. Tri(LM)-PEC showed distinct micellar structures, and penta- and nona-PEC featured largely interlinked coacervate nodes. The coacervate nodes of the PEC in a pure dense phase are shown in the top right of panels, for the sake of clarity, the neutral blocks of the PEC are not shown in the coacervate nodes visualization. d The interlinks between a pair of nodes in three PECs. e The number of loops per node in three PECs. f The node density of three PECs in the dense phase. g Corresponding q² dependence of the mean decay rate (gamma, Γ) for three PECs from multiangle DLS experiments. h Comparison of hydrodynamic diameter for three PECs. Schematic of the expected PEC cluster structure for penta- (i), nona- (j), and tri(LM)-PEC (k). CG-MD simulation data and hydrodynamic diameter are presented as mean values ± standard deviation.

Fig. 4. Rheological properties of the PEC hydrogels.

Strain sweep was performed in a strain range from 1 to 1000% at a frequency of 3 rad s⁻¹. Storage (G′, closed rectangles) and loss (G″, open rectangles) moduli of the tri(LM)- (green), tri(SM)- (olive green), penta- (black), and nona-PEC (blue) hydrogels with 3.0 (a), 4.4 (b), 7.0 (c) and 10.0 wt% (d) polymer concentrations. Angular frequency dependencies of the complex viscosity of the hydrogels from the non-equilibrium CGMD simulations (e) and experiments (f). Rheological properties of the tri(LM)-, tri(SM)-, penta-, and nona-PEC hydrogels with various polymer concentrations. G´ obtained at a 1% strain and frequency of 3 rad s⁻¹ and the dotted line represents ideal G´ calculated through the phantom network theory of rubber elasticity (g) and crossover strains of the G´ and G´´ determined from the strain amplitude sweep measurement (h). i Free-standing and stretchable performance of nona-PEC hydrogels with and without blue dye in the form of cubes. 20 g weighted cube-shaped 10.0 wt% tri(LM)- (j), penta- (k) and nona-PEC (l) hydrogels. m PEC hydrogels compared to synthetic and natural hydrogels. Gray closed labels represent synthetic block copolymer hydrogels, and gray open labels represent natural hydrogels, including chitosan, gelatin, lignin, and peptide.