Key-and-lock configuration is crucial to self-healing commodity copolymers
Humans would likely have much shorter lives if we did not possess the miraculous capability to heal cuts and scratches. Researchers have been trying to give some materials similar self-healing powers in the hope of extending their service life. Polymer scientists have achieved some limited success (e.g., in self-healing rubber), but a cost-effective process that can be scaled up to industrial quantities has remained elusive. An interdepartmental team at Clemson University headed by Marek Urban has put polymer scientists closer to success with a molecular-level explanation of how self-healing works in copolymers made from monomers of two widely produced polymers, poly(methyl methacrylate) and poly(n-butyl acrylate). The researchers reported their results in Science.
“Urban’s group has linked known concepts of why healing takes place and which molecular parameters/processes are important during healing to the outcome of molecular dynamics simulations,” says Sybrand van der Zwaag of the Technical University of Delft. “We now have some semi-quantitative evidence for the assumptions made previously. It is very nice work.”
For useful self-healing, the bonds between polymer chains must be reversible at normal operating temperatures so that they can repair themselves in situ. Previously, researchers have drawn on a variety of strategies to promote self-healing, such as mixing covalent and hydrogen bonds, incorporating additional reagents, introducing phase-separated morphologies, or even adding live organisms. Apart from their efficacy, at the industrial scale, these methods typically require modified production processes and considerable investment in new facilities by chemical manufacturers, who are often reluctant to abandon existing facilities.
“The key to our process is taking advantage of the individually weak but abundant van der Waals interactions between neighboring polymer chains and using them to create a new, self-healing material from existing commodity polymers,” says Urban. The idea for how to accomplish this trick was born a few years back. In addition to being weak but abundant, van der Waals forces are non-directional in polymers. In more crystalline materials, these interactions are directional and collectively become very strong. Thus, the motivation was to make van der Waals interactions more directional in copolymers by designing appropriate copolymer chain topologies and to do so for commodity polymers that have a proven history of industrial-scale synthesis.
For the work, the Clemson researchers chose poly(methyl methacrylate) [p(MMA)] and poly(n-butyl acrylate) [p(nBA)]. These are no strangers to the laboratory, and Urban’s group already knew from earlier work that only a narrow compositional range of copolymers of these two polymers could lead to self-healing materials. Since both of these materials are thermoplastics that soften and become more pliable upon heating, they also knew that there were just two primary kinds of molecular-level damage for which self-healing is an option: chain cleavage in which the chains break or chain slippage in which the chains slide past one another, or a combination of the two.
The researchers set about synthesizing copolymers with MMA/nBA molar ratios ranging from 30/70 to 70/30 by three methods: atom transfer radical polymerization (ATRP), statistical free radical polymerization, and colloidal polymerization. Of the three methods, ATRP allows more precise placement of monomers in the copolymer, but it is rather slow, whereas colloidal polymerization is less precise but much faster.
In this work, typically, the copolymer films were ~250 mm thick and cuts 20 µm wide and about 30 µm deep were produced by a razor blade. Optical examination of the damaged films revealed that self-healing occurred within 14 hours only for copolymers in a very narrow composition range from 45/55 to 50/50 MMA/nBA molar ratios; outside this range, self-healing was not observed even after several days, all consistent with previous work. For films cut completely through and physically reattached, healing began rapidly but regaining mechanical properties took around 80 hours. The researchers suspect that the narrow composition range favored monomer distributions along the copolymer chains that were either random or approximately alternating, in contrast to large regions of a single monomer. Their belief was confirmed by measurements of p(MMA)/p(nBA) block copolymers with overall compositions matching those of the random copolymers; these showed no self-healing under any conditions.
To discover the molecular-level events associated with self-healing (or the failure to heal), the group drew on internal reflection infrared imaging (IRIRI), proton nuclear magnetic resonance (1H NMR), and electron spin resonance (ESR). These measurements showed that reversible spectroscopic changes were only observed for copolymer compositions in which self-healing was observed optically. Finally, to more closely look at the monomer distributions in self-healing compositions and conditions favoring self-healing, the group conducted molecular-dynamics simulations for models containing selected sequences of MMA and nBA monomer units. Only for self-healing compositions were the highest van der Waals interactions and densest packing after physical separation observed.
The researchers attributed self-healing to what they referred to as key-and-lock interactions of directional van der Waals forces between the ends of pendant alkyl (CH2/CH3) groups between macromolecular chains, resulting in a viscoelastic response and self-recovery of neighboring chains upon separation. Their experiments indicated this behavior resulted from the specific combination of both the extended helical conformation of chains within this narrow compositional window and interchain van der Waals forces.
Urban cautions against a too-hasty assumption that the problem of self-healing is solved and that industrial-scale production is just around the corner; it needs to be scaled up. “What we have are general guidelines for tailoring copolymer structures and it will be critical to find the right combination of compatible monomers to apply to other materials,” he says. At this point it is known that acrylic-based copolymers exhibit self-healing properties; whether other copolymerized monomers, for example, styrene, represents a good combination is not known. Acrylics are important, though because they offer outstanding exterior durability.
As for the future applications, “There is a lot of industrial interest in making more durable plastics. One example is the automotive industry. Roughly ~80% of car production costs are plastics, so the use of self-healing polymers could enhance both aesthetic and functional features, especially in rental vehicles shared by multiple users; there are many other technologies that may benefit from self-healing materials,” Urban says.
Read the abstract in Science.