Tailoring Lifetimes and Properties of Carbodiimide-Fueled Covalently Cross-linked Polymer Networks

Abstract

The mechanical properties of nonequilibrium polymer hydrogels obtained from the transient cross-linking of polymer chains by a chemical fuel were investigated. Aqueous polymers featuring pendant carboxylic acids were treated with a carbodiimide to give anhydride-cross-linked gels. The anhydrides spontaneously hydrolyze back to the polymer solution, and the cycle can be repeated multiple times. Oscillatory rheology was employed to study the effects of temperature, fuel concentration, chain length, and polymer composition on the storage and loss moduli of the polymeric materials as well as the time taken for the polymers to undergo decross-linking. Regardless of the temperature used, at constant carbodiimide concentration, degelation times are more sensitive to experimental temperature than are the peak storage moduli. Decross-linking times decrease with increasing temperature. As carbodiimide concentration decreases, there is a decrease in moduli and decross-linking times. Within the scope of materials studied, the polymer structure was found to have a relatively small impact on the transient properties of gel networks compared to the fuel concentration and temperature. These findings facilitate the design of tunable on-demand networks and gels.

Chemically Fueled Reinforcement of Polymer Hydrogels

“…Initially the Am to AA ratio was varied, using primary chain compositions of poly(Am50-AA50-MBAm1.3), poly(Am70-AA30-MBAm1.3), and poly(Am85-AA15-MBAm1.3), which maintains the total primary chain length of 100 units. As seen in Figure 2a, poly(Am50 -AA50-MBAm1.3) has the highest G'max and longest transient network lifetime, while poly(Am85-AA15-MBAm1.3) has the lowest G'max and shortest transient network lifetime; however, all networks had transient network lifetimes in the order of 20,000 s. A similar trend of increased G'max with higher AA density was observed in previous studies 14,15 that focused on EDC-fueled sol-gel-sol transitions. The G'max values are similar for poly(Am50-AA50-MBAm1.3) and poly(Am70-AA30-MBAm1.3).…”

“…[9][10][11] Applying fuels to polymer materials is a useful first step toward practical applications of these nonequilibrium systems. We recently showed that the carbodiimide-driven formation of anhydrides, developed concurrently by Boekhoven and our group, 12,13 can be used to generate temporary crosslinks in polymer systems; 14,15 similar materials have since been adapted by Wang for potential medical applications. 16 Walther has used carbodiimide chemistry to induce phase separation by increasing the hydrophobicity of polyacid microgels, 17 and has studied time-dependent phase separation of poly(norbornene dicarboxylic acid).…”

“…The previously established behavior of Am/AA copolymers and the timescales of the experiments are consistent the formation of temporary anhydride crosslinks (Scheme 1b). 14,15 After consumption of EDC, the anhydride linkers eventually hydrolyze, returning the material to the original state.…”

Chemical crosslinking in ‘reactive’ multicomponent gels

“…Recently, this concept has been applied to create transient covalent crosslinking via anhydride formation in polymeric materials. [24][25][26] Here we show that, with appropriate designing of the gel system, the EDC-triggered pathway can be employed to achieve a permanent covalent crosslinking between the components. To achieve this, we design a multicomponent hydrogel comprising of gelators with a carboxylic acid (1) and amine (2) functional groups (Fig.…”

“…Overall, we have shown that with appropriate gelator design, hydrolysis of EDC can be used to achieve a permanent covalent cross-linking through amide bond formation between the components in a multicomponent gel. Unlike traditional EDC-fueled systems where transient covalent crosslinking leads to materials with a short lifetime [24][25][26] potentially imposing limitations on their practical applications, our method allows preparing crosslinked gels with enhanced mechanical properties. The ability of pH-responsive charge complementary multicomponent systems to achieve different degrees of ionization upon pH change provides a further level of control on the final stiffness of the materials.…”