Reprocessable Covalent Adaptable Networks via Free-Radical Polymerization with an Aromatic Disulfide Cross-Linker

Abstract

Aromatic disulfides have seen widespread use in covalent adaptable networks (CANs), though previous studies have exclusively used step-growth methods to integrate them into CANs. Here, we describe a case in which an aromatic disulfidebased cross-linker, bis(4-methacryloyloxyphenyl) disulfide, also called BiPheS methacrylate or BPMA, is incorporated into a CAN by nonstep-growth polymerization. Free-radical copolymerization of n-hexyl methacrylate with 5 mol % BPMA results in a CAN which exhibits full recovery of cross-link density and thermomechanical properties across multiple reprocessing cycles. The CAN rubbery-plateau storage modulus is directly proportional to absolute temperature, characteristic of a constant cross-link density, even at temperatures where the CAN is reprocessable. Indeed, the BPMA-based CAN exhibits a constant cross-link density, and thus associative dynamic character, at temperatures up to at least 200 °C, enabling it to be used in elevated-temperature applications without risk of loss of network character. Under a 3.0 kPa shear stress, the CAN exhibits almost total arrest of creep up to 180 °C and major creep suppression at its reprocessing temperature of 200 °C, overcoming a potential Achilles' heel associated with CANs. Thus, the integration of aromatic disulfides into CANs by free-radical polymerization provides a facile route to produce recyclable networks that maintain network character at very high temperature, contributing to polymer network sustainability. Finally, we determined an Arrhenius apparent activation energy of ∼100 kJ/mol for the CAN stress relaxation and creep viscosity. This value differs substantially from the BPMA bond dissociation energy but agrees with the activation energy for the alpha-relaxation of poly(n-hexyl methacrylate) (PHMA). This indicates that the temperature dependence of these viscoelastic responses in our associative-type CAN is defined by the temperature dependence of the cooperative segmental mobility of PHMA, which makes up 95 mol % of the CAN.