Covalent Adaptable Networks (CANs): A Unique Paradigm in Crosslinked Polymers

Abstract

Polymer networks possessing reversible covalent crosslinks constitute a novel material class with the capacity for adapting to an externally applied stimulus. These covalent adaptable networks (CANs) represent a trend in polymer network fabrication towards the rational design of structural materials possessing dynamic characteristics for specialty applications. Herein, we discuss the unique attributes of CANs that must be considered when designing, fabricating, and characterizing these smart materials that respond to either thermal or photochemical stimuli. While there are many reversible reactions which to consider as possible crosslink candidates in CANs, there are very few that are readily and repeatedly reversible. Furthermore, characterization of the mechanical properties of CANs requires special consideration owing to their unique attributes. Ultimately, these attributes are what lead to the advantageous properties displayed by CANs, such as recyclability, healability, tunability, shape changes, and low polymerization stress. Throughout this perspective, we identify several trends and future directions in the emerging field of CANs that demonstrate the progress to date as well as the essential elements that are needed for further advancement.

Figure 1. The thermoreversible Diels-Alder (DA) reaction between furan and maleimide (panel A) is used to demonstrate CAN fabricated by formation of the reversible linkage (panel B) or by polymerizable functionality flanking the reversible linkage (panel C)

In each of the panels, the reaction between maleimide and furan are represented as complementary geometric shapes. In panel A, the DA cycloaddition between furan (left) and maleimide (middle), which forms the bicyclic compound (right) at low temperatures, is shown. This model thermoreversible reaction undergoes the retro-Diels-Alder reaction at elevated temperatures. In panel B, the network is formed by bismaleimide and trisfuran monomers that undergo a step-growth polymerization via the reversible linkage. In panel C, the reversible CAN linkage is flanked by two polymerizable groups (PG) which, as an example, are given as acrylate functional groups. These acrylate functional groups can be either i) polymerized via a radical-mediated chain-growth mechanism, or ii) co-polymerized with a multifunctional thiol monomer (e.g., pentaerythritol-tetrakis-3-mercaptopropionate) via base-catalyzed Michael addition. The insets of B and C represent the formed network structure, which contains reversible linkages that enable reversible depolymerization. It should be noted that although a thermoreversible CAN was used as an example, this demonstration of polymerization types is equally applicable for photoreversible CANs.

Figure 2. A selection of thermoreversible crosslinking addition reactions

Here, we show the thermoreversible 1) nucleophilic addition between isocyanate and imidazole, 3) carbene dimerization,2) reversible radical coupling between TEMPO and a styryl radical, and the DA cycloaddition between 4) furan and maleimide, 5) anthracene and cyanoacrylate, and 6) fulvene and cyanoacrylate (only exo product is shown).

Figure 3. Thermoreverisble homolytic cleavage of alkoxyamine producing linear chains with pendant styryl and TEMPO radicals.

In this radical crossover reaction, heating causes the capping styryl and TEMPO functional groups to dissociate from their respective linear chain, revealing a pendant TEMPO radical from one chain and a pendant styryl radical from another chain that are capable of forming a thermoreversible crosslink. The small molecular styryl and TEMPO capping species are also able to thermoreversibly combine.

Figure 4. Photodimerization of coumarin and anthracene as controlled by irradiation wavelenth

The four potential isomers resulting from the [2+2] coumarin dimerization include a) head-to-head, syn; b) head-to-head, anti; c) head-to-tail, syn; and, d) head-to-tail, anti. For clarity, one of the four potential isomers resulting from the [4+4] anthracene dimerization is shown.

Figure 5. Radical-mediated addition-fragmentation chain transfer allows for rearrangement of polymer connectivity

The thiyl radical and the allyl sulfide units in the polymer are in a dynamic pseudoequilibrium relation where the thiyl radical catalyzes the cleavage and reformation of the allyl sulfide linkages in the polymer network.

Figure 6. Photoinduced stress relaxation (i.e., permanent set) of an allyl sulfide thiol-ene network.

The sample is (I) uniaxially stretched to 1.5 (dotted), 3.0 (dot-dot-dash), 4.5 (dot-dash), 6.0 (dash), and 7.5 % (solid) strain, (II) held at constant strain for 1 min, and (III) irradiated with 365 nm light at an intensity of 40 mW/cm². The inset shows the time shifted (to the beginning of phase II) and normalized stress, which demonstrates 90% stress reduction for all the samples presented here. Adapted from reference .

Figure 7. Samples containing allyl sulfide deforming in response to stress gradient ‘written’ via light attenuation through the sample.

As shown in the inset, the samples are uniaxially stretched and irradiated (365 nm light at 40 mW/cm² for 30, 60, and 90s – left to right), triggering bond rearrangement via addition-fragmentation of allyl sulfide functional groups in the network backbone. Since the samples are optically thick (using ultraviolet light absorber), the light is attenuated, producing a gradient of active bond rearrangement and thus a gradient in stress. After irradiation, the samples deform by warping to equalize the internal stresses.

Figure 8. Elastic (closed symbols) and viscous (open symbols) moduli as a function of angular frequency for a stoichiometric bismaleimide and trisfuran monomer mixture above (triangles, 95°C), near (circles, 91°C), and below (squares, 87°C) the gel-point temperature

For all temperatures, the terminal behavior (i.e., ω → 0) approaches that of a viscoelastic liquid (G′ ~ ω² and G″ ~ ω¹). The moduli exhibit similar frequency scaling (G′ ~ G″ ~ ω^0.56) at the gel-point temperature, corresponding well to that determined by the Flory-Stockmayer gel-point conversion determined by FTIR (92.5±0.5°C). Adapted from reference .

Figure 9. The relationship between the gel-point temperature, T_gel, and conversion, p_E,gel, for the reaction between diene and dienophile (shown as furan and maleimide functional groups, respectively, where R is the connectivity to the remainder of the monomer structure) is related through the initial functional group concentration, c₀, equilibrium conversion, p_E, heat of reaction, ΔH_rxn, and entropy of reaction, ΔS_rxn

For the case that the diene and dienophile are not stoichiometric, r is the stoichiometric ratio defined as the limiting functional group concentration divided by the functional group concentration in excess, and where c₀ and p_E,gel are the limiting functional group concentration and conversion, respectively.

Figure 10. Schematic of photoinduced cinnamate crack healing.

Crack propagation in a cinnamate crosslinked network promotes the retro-[2+2] cycloaddition as evidenced by the appearance of a C=C absorption monitored by infrared spectroscopy. The subsequent photo-induced healing of the crack resulted in an increase in flexural strength as well as a reduction of the C=C absorption. Adapted from reference .

Figure 11. Using a model thiol-ene photopolymerization of a tetra-thiol (panel A, top) and an allyl sulfide divinyl ether (panel A, middle), a reduction is polymerization stress (panel B) was achieved when compared to the analogue tetra-thiol and propyl sulfide divinyl ether (panel A, bottom) thiol-ene photopolymerization

In panel B the polymerization stress is measured as a function of conversion, where the propyl sulfide-based material exhibits an increase in stress at the gel point (~58% conversion) that continues until achieving full conversion. The propyl sulfide monomer is incapable of undergoing addition-fragmentation and thus only follows the step growth mechanism shown as cycle I (inset). While the allyl sulfide-based material also exhibits an increase in stress at the gel point, the evolution in stress reaches a maximum followed by a decrease that is roughly 1/3^rd the stress of analogue. The allyl sulfide monomer is capable of both the step-growth mechanism (cycle I, inset) as well as the addition-fragmentation mechanism (cycle II, inset), which is responsible for the observed decrease in stress. Adapted from reference .