Transparent and Recyclable PDMS Adhesive Enabled by Dynamic Diels-Alder Cross-linking

When designing a consumer device, recyclability is not usually a primary consideration. Thus, packaging and encapsulation of interior and exterior components is achieved using robust materials─e.g., metals, glasses, ceramics, and adhesives bearing irreversible cross-links. If recycling is attempted, the high value components are most often liberated by incineration or destructive mechanical processing. Controlled debonding using a reversible adhesive is an approach to circumvent some of this destruction. This paper describes a reversibly cross-linked interlayer polymer ("ReCLIP") that cures by way of a thermally reversible Diels-Alder (DA) reaction between furan-functionalized silicone chains and an aromatic bismaleimide. Two-dimensional ¹H-¹H COSY NMR experiments were used to distinguish between the exo and endo diastereomers of the furan-maleimide DA adduct. The exo-endo ratio exerted a pronounced influence on the thermal stability and adhesion strength of the resulting polymer network. With one-dimensional ¹H NMR monitoring, we established that the Diels-Alder cross-linking proceeded most rapidly at 80 °C, while complete conversion and a high proportion of the exo adduct (∼93%) were also observed at 60 °C with extended reaction times. One-dimensional ¹H NMR also revealed that the onset of the reverse (retro-Diels-Alder) reaction occurred at ∼90 °C. Using a lap-shear geometry, ReCLIP exhibited a maximum shear strength of 286 ± 50 kPa, comparable in magnitude to values seen for other silicone adhesives. Films exhibited excellent transparency and stability over time under ambient conditions.