Modular Synthesis and Patterning of High-Stiffness Networks by Postpolymerization Functionalization with Iron-Catechol Complexes

Abstract

Bioinspired iron-catechol cross-links have shown remarkable success in increasing the mechanical properties of polymer networks, in part due to clustering of Fe³⁺-catechol domains which act as secondary network reinforcing sites. We report a versatile synthetic procedure to prepare modular PEG-acrylate networks with independently tunable covalent bis(acrylate) and supramolecular Fe³⁺-catechol cross-linking. Initial control of network structure is achieved through radical polymerization and cross-linking, followed by postpolymerization incorporation of catechol units via quantitative active ester chemistry and subsequent complexation with iron salts. By tuning the ratio of each building block, dual cross-linked networks reinforced by clustered iron-catechol domains are prepared and exhibit a wide range of properties (Young's moduli up to ∼245 MPa), well beyond the values achieved through purely covalent cross-linking. This stepwise approach to mixed covalent and metal-ligand cross-linked networks also permits local patterning of PEG-based films through masking techniques forming distinct hard, soft, and gradient regions.

Figure 1

Modular processing of dual cross-linked networks leads to a highly tunable and patternable material following the steps from left to right of the (a) precursor resin, (b) active ester network, (c) catechol substitution, and (d) iron complexation with (bottom) images of catechol networks before and after Fe³⁺ complexation.

Scheme 1. Polymerization of Networks, Substitution of Active Ester Groups with Catechol Moieties, and Subsequent Complexation with Fe³⁺

Figure 2

FTIR-attenuated total reflection (ATR) of random copolymer networks. From top to bottom: the blue trace is a 4.5/95.5 mol % PEGDA/PEGMEA network without PFPA monomer incorporation, the yellow trace a 4/61/35 mol % PEGDA/PEGMEA/PFPA network before substitution, and the red trace a PFPA-containing network after reaction with dopamine in MeOH. Key resonances from PFPA (yellow highlighted peaks) at 1780, 1520, and 985 cm^–1 disappear after substitution with dopamine, with an amide stretching band appearing near 1660 cm^–1.

Figure 3

Raman spectra of 3/39/58 mol % PEGDA/PEGMEA/PFPA films after substitution with dopamine. From top to bottom: the black trace is film after complexation with Fe³⁺, and the red trace is the corresponding film without iron treatment. Resonances at 630, 590, and 520 cm^–1 indicate the presence of Fe³⁺–catechol bis-complexes.

Figure 4

Small-angle X-ray scattering of water-swollen polymer networks and representative cartoons of the polymer microstructure. (a) Scattering of 100 mol % PEGDA network, (b) scattering of 4.5/95.5 mol % PEGDA/PEGMEA network, (c) scattering of 4/61/35 mol % PEGDA/PEGMEA/PFPA network, and (d) scattering of 4/61/35 mol % PEGDA/PEGMEA/catechol network after substitution with dopamine and subsequent Fe³⁺ complexation.

Figure 5

Young’s moduli, E, from beam bending measurements of dry samples vs theoretical maximum cross-linking density (mmol/cm³) for catechol-free PEGDA networks (unfilled blue diamonds) and Fe³⁺–catechol networks with constant PEGDA covalent cross-linker content of ∼3–4 mol % (filled orange diamonds). Error bars represent standard deviations from replicate measurements. Note that some error bars are within the bounds of the markers (see the Supporting Information for specific values).

Figure 6

Optical microscopy images of cross sections of films with varied covalent cross-linker content, illustrating differences in iron diffusion rates. (a) 3/39/58 mol % PEGDA/PEGMEA/catechol, (b) 7/33/59 mol % PEGDA/PEGMEA/catechol, and (c) 15/24/61 mol % PEGDA/PEGMEA/catechol.

Figure 7

SEM EDS characterization of Fe³⁺ distributions across film thicknesses. (a) Fe³⁺ distribution vs dimensionless thickness for 1 mm thick films with ∼3–4 mol % PEGDA and varied catechol content after exposure to Fe³⁺ solution for 72 h. (b) Comparison of Fe³⁺ distribution vs dimensionless thickness for 0.5 and 1 mm thick films of 3/39/58 mol % PEGDA/PEGMEA/catechol film after exposure to Fe³⁺ solution for 72 h.

Figure 8

(a) 3D rendering of the generalized schematic of the Fe³⁺ patterning approach. (b–d) Images of catechol-containing films (3/39/58 mol % PEGDA/PEGMEA/catechol) after selective patterning with Fe³⁺ through soft lithography styled masking displaying (b) a QR code, (c) a butterfly, and (d) a demonstration of text.

Figure 9

(a) Continuous film of 3/39/58 mol % PEGDA/PEGMEA/catechol with a central section functionalized with Fe³⁺–catechol groups. (b) Comparison of folded film strips between iron-patterned and iron-free networks. (c) Continuous film of 3/39/58 mol % PEGDA/PEGMEA/catechol with no patterning or introduction of Fe³⁺–catechol groups.