Self-Healing Polymer Films Based on Thiol–Disulfide Exchange Reactions and Self-Healing Kinetics Measured Using Atomic Force Microscopy

Abstract

Self-healing polymeric materials with branched architectures and reversible cross-linking functionalities at the periphery of branches were synthesized by atom transfer radical polymerization (ATRP). Poly(n-butyl acrylate) grafted star polymers were prepared by chain extension ATRP from cross-linked cores comprised of poly(ethylene glycol diacrylate). These polymers were further used as macroinitiators for the consecutive chain extension ATRP of bis(2-methacryloyloxyethyl disulfide) (DSDMA), in which way disulfide reversible cross-links (SS) were introduced at the branch peripheries. The SS cross-linked polymers were then cleaved under reducing conditions to form thiol (SH)-functionalized soluble star polymers. The SH-functionalized star polymer solutions were deposited on silicon wafer substrates and converted to insoluble SS re-cross-linked films via oxidation. The self-healing of prepared polymer films was studied by continuous atomic force microscopy (AFM) imaging of cuts micromachined with the AFM tip and by optical microscopy. The re-cross-linked star polymer (X3) showed a rapid spontaneous self-healing behavior, with the extent of healing dependent on the initial film thickness and the width of the cut. The self-healing behavior observed for this sample was attributed to the regeneration of SS bonds via thiol–disulfide exchange reactions. This study demonstrated the suitability of grafted multiarm polymer architectures as building blocks of self-healing polymeric materials and pointed to the importance of low intrinsic viscosity of material and high accessibility of functional groups responsible for healing.