From Heterogeneous Polymer Networks to Self-Repairing Materials
Marek Urban, The University of Southern Mississippi
Considerable challenges and significant inter-disciplinary scientific and technological interests stimulated the development of new heterogeneous polymeric solid materials that maintain useful functions and, at the same time, are capable of stimuli-responsiveness. The presence of heterogeneous regions within polymeric networks facilitates localized structural variations enabling favorable spatial and energetic conditions for spontaneous macroscopic responses to minute external or internal stimuli. Although stimuli-responsiveness is easily obtained in polymeric solutions, significant spatial restrictions in solids, near surfaces, and at interfacial regions impose severe limitations. While Brownian molecular motions can be relatively easily overcome in solutions, energetic and spatial restrictions in solids require an orchestrated design of the network in order to achieve desirable responses. These generalized concepts formulated the principles leading to the development of heterogeneous colloidal dispersions that upon coalescence form solid polymeric films with cilia-like interfacial regions capable of multi-responsiveness to temperature, pH, light, deformation, and others. Furthermore, this new generation of polymer networks containing energetically favorable substitutions and pending groups was designed to provide self-healing and repairing characteristics. One example are networks composed of hexamethylene diisocyanate (HDI) and polyethylene glycol (PEG) that contain oxetene-modified chitosan which maintain useful polyurethane properties, but upon mechanical damage and exposure of the damaged area to UV light, result in self-healing (Science, 2009, 323(5920), 1458). Along the same lines, the developments of superparamagnetic iron oxide nanoparticles (Adv. Mater. 2009) resulted in networks capable of self-repairing upon exposure to oscillating magnetic fields (OMF).