Adhesive curing through low-voltage activation

Abstract

Instant curing adhesives typically fall within three categories, being activated by either light (photocuring), heat (thermocuring) or chemical means. These curing strategies limit applications to specific substrates and can only be activated under certain conditions. Here we present the development of an instant curing adhesive through low-voltage activation. The electrocuring adhesive is synthesized by grafting carbene precursors on polyamidoamine dendrimers and dissolving in aqueous solvents to form viscous gels. The electrocuring adhesives are activated at -2 V versus Ag/AgCl, allowing tunable crosslinking within the dendrimer matrix and on both electrode surfaces. As the applied voltage discontinued, crosslinking immediately terminated. Thus, crosslinking initiation and propagation are observed to be voltage and time dependent, enabling tuning of both material properties and adhesive strength. The electrocuring adhesive has immediate implications in manufacturing and development of implantable bioadhesives.

Figure 1. Concept of electrocuring adhesive.

Low-voltage activation allows polymer and substrate crosslinking.

Figure 2. Electrochemical behaviour of aryl-diazirine.

(a) Cyclic voltammograms recorded for 1 mM aryl-diazirine in the absence and presence of 1.0% acetic acid (proton donor) in acetonitrile containing 0.1 M tetraethylammonium perchlorate at a glassy carbon electrode (d. 3 mm). Scan rate: 100 mV s⁻¹. (b) i_pa/i_pc values over a range of scan rates. The i_pa represents anodic peak current and i_pc represents cathodic peak current. (c) Absorbance spectra of 1 mM aryl-diazirine in acetonitrile containing 0.1 M tetraethylammonium perchlorate on various electrostimulation times with an applied voltage of −2.0 V versus Ag/AgCl at a glassy carbon electrode (diameter, 10 mm).

Figure 3. Characterization of PAMAM-g-diazirine conjugates.

(a) Schematic illustration of the synthesis route of PAMAM-g-diazirine conjugate. (b) SEC–MALLS–UV characterization of the fifth generation PAMAM (control) and the subsequently modified PAMAM-g-diazirine conjugates. (c) Electrochemical behaviour of PAMAM (control) and PAMAM-g-diazirine conjugates in PBS at a glassy carbon electrode (diameter, 3 mm). Scan rate: 100 mV s⁻¹. (d) Absorbance spectra of PAMAM-g-diazirine in PBS on different electroactivation times (0, 6 and 30 min) with an applied voltage of −2.0 V at a glassy carbon electrode (diameter, 10 mm).

Figure 4. Electrorheological properties of PAMAM-g-diazirine solutions.

(a) Scheme of real-time oscillatory dynamic rheometry of electro-activated adhesives. (b) Storage modulus (G′) with respect to time before and after an applied voltage of −2.0 V versus Ag/AgCl on the disposable Zensor chip. PAMAM-g-diazirine solutions (25 wt% in PBS in all figures) with different conjugation degrees (5, 10 and 15%) were electrocured for 20 min. (c) Kinetics of storage modulus (G′) and loss modulus (G″) with respect to magnitude of applied potential versus Ag/AgCl. Gelation point is defined where G′=G″. (d) Kinetics of storage modulus with respect to temporal activation of a −2.0 V applied to PAMAM-g-diazirine (15%).

Figure 5. Shear adhesion of electro-activated PAMAM-g-diazirine conjugates.

(a) Experimental set-up of shear adhesion failure analysis. (b) Typical shear stress versus shear strain curves of electrocured PAMAM-g-diazirine (15%) conjugates in PBS with the concentration of 25 wt%. Electrocuring adhesive is sandwiched between two transparent ITO plates with various times of electroactivation at −2 V. Inset: maximum shear modulus (G) was plot against electrocuring time (t) with a high linear correlation. (c) Comparison of maximum shear stress values obtained across all tested PAMAM-g-diazirine conjugates and electroactivation times. (d) Demonstration 1 (top): tunable adhesive properties employing 100-g mass standards (0.98 N force) at 0- and 1-min electrocuring times. With no electroactivation, the inactivated adhesive fails to support the load. Demonstration 2 (bottom): breakaway tuning of electrocuring adhesive properties employing 200-g mass standards (1.96 N force). The electrocuring adhesive time for one formulations of PAMAM-g-diazirine was chosen to predict shear adhesion strength that would allow support of the stress or adhesion failure.

Figure 6. Diazirine electrochemistry and aryl-carbene formation.

Depending on the environment, several products are possible for the electrochemical reduction of aryl-substituted diazirine. In aprotic environments, the semistable diazirinyl radical is formed that can act as a strong reducing agents towards other molecules or itself (disproportionation). In acidic protic mediums, diaziridine can be protonated, explaining how the diazirine is reduced into the gem-diamine which under goes hydrolysis to the ketone. Alternatively, aryl-diaziridine can be oxidized to the original diazirine in caustic pH>11. The aryl-diazirine or one of its negatively charged intermediates (see square scheme in Supplementary Fig. 3) may be oxidized by the positively charged anode into the short lived carbene radical, which inserts itself into X–H species. Any number of elements could be ‘X' including Carbon, Nitrogen, Oxygen or Sulfur.

Figure 7. Proposed half-reactions at the cathode and anode.

Aryl-diazirine is reduced to aryl-diaziridine with two equivalents of electrons and protons. Aryl-diaziridine is then oxidized to aryl-carbene yielding two equivalents of electrons and protons and one equivalent of diatomic nitrogen.