Building tunable degradation into high-performance poly(acrylate) pressure-sensitive adhesives

Pressure-sensitive adhesives (PSAs) based on poly(acrylate) chemistry are common in a wide variety of applications, but the absence of backbone degradability causes issues with recycling and sustainability. Here, we report a strategy to create degradable poly(acrylate) PSAs using simple, scalable, and functional 1,2-dithiolanes as drop-in replacements for traditional acrylate comonomers. Our key building block is α-lipoic acid, a natural, biocompatible, and commercially available antioxidant found in various consumer supplements. α-Lipoic acid and its derivative ethyl lipoate efficiently copolymerize with n-butyl acrylate under conventional free-radical conditions leading to high–molecular-weight copolymers (Mn > 100 kg mol–1) containing a tunable concentration of degradable disulfide bonds along the backbone. The thermal and viscoelastic properties of these materials are practically indistinguishable from non-degradable poly(acrylate) analogues, but a significant reduction in molecular weight is realized upon exposure to reducing agents such as tris(2-carboxyethyl)phosphine (e.g. Mn = 198 kg mol–1 → 2.6 kg mol–1). By virtue of the thiol chain ends produced after disulfide cleavage, degraded oligomers can be further cycled between high and low molecular weights through oxidative repolymerization and reductive degradation. Transforming otherwise persistent poly(acrylates) into recyclable materials using simple and versatile chemistry could play a pivotal role in improving the sustainability of contemporary adhesives.

Methods
1H nuclear magnetic resonance spectroscopy: Solution state 1H nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance III HD 400 MHz or Bruker Avance NEO 500 MHz. Chemical shifts (δ) are reported in ppm relative to residual protio solvent in CDCl3 (7.26 ppm).

Size-exclusion chromatography instrumentation: Size-exclusion chromatography (SEC) was performed on a Waters instrument using a differential refractive index detector and two Tosoh columns (TSKgel SuperHZM-N, 3 μm polymer, 150 × 4.6 mm) with THF at 35 °C or chloroform containing 0.25% TEA at 35 °C for the mobile phase. Molar masses and molar mass dispersities (Đ) were determined against narrow PS standards (Agilent).

Mechanical analysis: Frequency sweeps and isochronal temperature sweeps were obtained on a TA Instruments ARES G2 in oscillatory shear mode with an 8 mm parallel-plate geometry and a nitrogen-purged forced–convection oven. For all experiments, oscillatory shear was applied within the linear viscoelastic regime as verified from isochronal strain sweeps at a fixed temperature. Differential scanning calorimetry (DSC) was performed with a TA Instruments DSC Q2000 at a heating/cooling rate of 10 °C/min using 3–5 mg of sample in a sealed aluminum pan. Lap shear and 180° peel tests were performed on a TA.XTplusC texture analyzer equipped with A/MTG tensile grips. 180° Peel test samples were performed with a peel rate of 100 mm min–1 over a 20 mm displacement. Lap shear experiments were extended at a rate of 0.5 mm min–1 until cohesive failure.