Shape Memory Assisted Self-Healing Coating

In this communication, we report the preparation and characterization of new shape memory assisted selfhealing (SMASH) coatings. The coatings feature a phaseseparated morphology with electrospun thermoplastic poly(εcaprolactone) (PCL) fibers randomly distributed in a shape memory epoxy matrix. Mechanical damage to the coating can be self-healed via heating, which simultaneously triggers two events: (1) the shape recovery of the matrix to bring the crack surfaces in spatial proximity, and (2) the melting and flow of the PCL fibers to rebond the crack. In controlled healing experiments, damaged coatings not only heal structurally, but also functionally by almost completely restoring the corrosion resistance. We envision the wide applicability of the SMASH concept in designing the next-generation self-healing materials. M corrosion has long been a major problem for industries in the U.S. and worldwide. According to a landmark study conducted by Battelle Memorial Institute and the Specialty Steel Industry of North America, the annual cost of corrosion in the U.S. was approximately $442 billion in 2007 or 3.1% of the Gross Domestic Product (GDP). One of the major methods employed to prevent corrosion is to apply a “barrier” organic, usually a polymeric coating, on the metal surface. However, most polymeric coatings are susceptible to impairments induced by environmental degradation and mechanical damage, which, if not repaired properly, can lead to compromised corrosion resistance or even macroscopic failure of the coating. With conventional coating technologies, repair of a coating is tedious at best and often involves extensive labor as well as high cost. There has been a constant market demand for coating materials that can “self-heal”, that is, possessing an intrinsic ability to heal damage with no or minimum external intervention. The field of self-healing polymers has attracted a significant amount of research efforts during the past decade. Selfhealing is increasingly becoming an important concept in the design of polymeric materials and composites. Broadly speaking, three major strategies have been established to incorporate self-healing functionality to polymer systems: (1) damage-initiated in situ polymerization of monomeric “healing agents”, (2) reversible chemistry based reconstruction of the molecular network, and (3) incorporation of fusible thermoplastics in a thermoset host. At least two of these approaches have also been implemented in coatings. For example, Cho et al. and Park and Braun reported self-healing coatings with liquid, reactive healing agents hosted in microcapsules and core/sheath fibers, respectively. Using the reversible Diels− Alder reaction, Wouters et al. developed thermoset coatings that can be self-healed via a thermal treatment, although the healing process involved a complete liquification of the coating material. Over the last several years, a new concept has emerged that explores the use of shape memory materials to improve the selfhealing process by providing a mechanism to partially or fully close the crack. This concept, which we term shape memory assisted self-healing (SMASH), has been demonstrated in at least two approaches. The first approach utilizes pretensioned shape memory alloy (SMA) wires or shape memory polymer (SMP) fibers that, when activated, exert a contractual force that pulls the crack surfaces closer. An apparent shortcoming in this approach is the fact that the SMA wires or SMP fibers have to be positioned locally and perpendicular to the crack in order to be effective, which is challenging to achieve in practical applications. The second SMASH approach utilizes “bulk” shape memory from the material to close the crack. One example is the poly(εcaprolactone) (PCL) based molecular composite system recently reported by our group. That particular material was a single-phase, two-component blend composed of a thiol−ene cross-linked PCL network (n-PCL) and a high Mw linear PCL (l-PCL) interpenetrating the network. The n-PCL exhibits “reversible plasticity”, a form of shape memory where large plastic deformation at room temperature (below the shape memory transition temperature) is fully recoverable upon heating. This shape memory from n-PCL assists in closing any cracks and damage, whereas the mobile l-PCL chains (the selfhealing agent in this case) tackify the crack surfaces via diffusion Received: January 14, 2013 Accepted: January 28, 2013 Letter pubs.acs.org/macroletters © XXXX American Chemical Society 152 dx.doi.org/10.1021/mz400017x | ACS Macro Lett. 2013, 2, 152−156 and ultimately rebond the crack to restore mechanical strength. Both crack closure and rebonding are achieved by a single heating step with no additional intervention. In this communication, we introduce a new SMASH strategy detailing the preparation of a new self-healing material tailored for coating/corrosion-inhibition applications. Unlike the singlephase n-PCL/l-PCL system, the coatings exhibit a phaseseparated morphology wherein the thermoplastic healing agent exists as randomly oriented, nonwoven nanoand microfibers evenly distributed in a shape memory thermoset matrix. Conceptually, this morphology enables more significant flow of the liquefied thermoplastic compared to the limited diffusion distance in the case of a miscible, single-phase blend, therefore, allowing the healing of larger cracks and defects. Yet the high aspect ratio fibers are more efficient than many other geometries (such as spheres) in creating a large interfacial area and providing more sustained healing agent delivery. The overall concept is further illustrated in Figure 1. Typical damage to a polymeric coating usually contains two forms: plastic/permanent deformation, indicated as the shaded area surrounding the crack tip, and cracks involving the creation of free surfaces. In severe cases, some portions of the material may even be permanently removed from the coating, leaving voided space. Self-healing is initiated by heating the damaged coating to a temperature higher than both the liquefying temperature of the fibers (in this case, melting temperature or Tm) as well as the transition temperature (glass transition temperature or Tg) of the SMP matrix. Two events take place simultaneously: (1) recovery of the SMP matrix that releases the stored strain energy in the plastic zone and closes the crack, that is, bringing crack surfaces into spatial proximity, and (2) melting and flow of the thermoplastic to rebond the crack. The most significant advantage of SMASH is that the crack closure minimizes the healing agent needed. Therefore, healing of large cracks and voids becomes possible. The new self-healing coating was prepared via a two-step process. The first step involved direct solution electrospinning of PCL (Tm ca. 60 °C) onto a steel substrate (3 × 3 cm) using the setup shown in Figure 2A,B (more details in Experimental Methods). This led to a uniform, fibrous coating on the steel substrate, as shown by the photograph in Figure 2C. The SEM image (Figure 2E) reveals a structure of continuous PCL fibers that are randomly oriented, with an average fiber diameter (measured by image analysis) of 1.38 ± 0.87 μm. In the second step, a shape memory epoxy formulation consisting of an equimolar mixture of diepoxide, diglycidyl ether of bisphenol A (DGEBA), neopentyl glycol diglycidyl ether (NGDE), and poly(propylene glycol)bis(2-aminopropyl) ether (Jeffamine D230) was spin-coated onto the PCL-coated steel substrate. The liquid epoxy could easily wet the PCL fibers due to favorable surface energetics but does not lead to any dissolution or swelling of PCL (this has been thoroughly studied in our previous publication). One advantage of the spin-coating technique is the automatic removal of the excess epoxy by the centrifugal force from high-speed spinning. The epoxy-coated specimens were allowed to fully cure first at room temperature for 72 h and then at 40 °C for 24 h. This particular epoxy formulation led to a glassy SMP with a Tg of about 50 °C. 21 After curing, a translucent, void-free and rigid coating was formed on the steel substrate (Figure 2D,F). The average PCL weight fraction in the final coatings was measured gravimetrically to be about 12%. One important structural variable in the current self-healing coating system (or any self-healing coatings) is the coating thickness, mainly because it determines the amount of healing agent available for damage of a given size. In our case, the coating thickness can be controlled simply by the electrospinning time. As discussed above, PCL acts essentially as a “primer” that retains the necessary amount of epoxy to form a continuous matrix, while any excess amount of epoxy is removed by spinning. Therefore the total coating (epoxy/PCL) thickness is dictated only by the fiber (PCL) layer thickness, which in turn can be controlled (other conditions remaining the same) by the time of electrospinning. This was confirmed by measuring final coating thicknesses of various electrospinning times. As shown in Figure 3, the coating thickness Figure 1. Schematic illustration of the coating morphology and the shape memory assisted self-healing (SMASH) concept. Figure 2. Process to prepare the SMASH coatings. (A) Schematic illustration of the electrospinning setup; (B) photograph showing the actual electrospinning process; (C) PCL-coated steel substrate after electrospinning for 10 min; (D) steel substrate after spin-coating of epoxy, (E) SEM image of the PCL-coated surface, and (F) SEM image of the final coating. ACS Macro Letters Letter dx.doi.org/10.1021/mz400017x | ACS Macro Lett. 2013, 2, 152−156 153 increases linearly with electrospinning time, with a slope of 11