Skip to main content

Mechanically Responsive Polymers

Final Report Summary - MERESPO (Mechanically Responsive Polymers)

Polymers which change their properties “on command”, that is upon exposure to a pre-defined stimulus in a selective manner, are of considerable academic interest and attractive for countless technologically relevant applications. Many examples of chemically, thermally, electrically, optically, or electrically responsive materials exist, whereas polymers that respond in a useful and predictable manner to the exposure of mechanical stress are far less known. When polymers are exposed to excessive stress, the normal response is the unspecific scission of polymer chains, which is the molecular origin of materials’ fatigue and damage. By contrast, many biological systems have developed ways to sense mechanical force and subsequently adapt. While the underlying mechanotransduction processes are complex and difficult to mimic in artificial materials, it stands to reason that the very process of translating mechanical force into useful signals, could also be utilized to bestow artificial materials with new and useful functions.

Seizing this opportunity, this experimental research program pursued the design, synthesis, processing, exploration and exploitation of mechanically responsive polymers, in which mechanical stress triggers specific pre-programmed molecular responses, which in turn bestow the new materials with useful functions. The main goals of the program were to (i) advance the understanding of the structure-property relationships that govern the mechanochemical transduction of such materials in solution and the solid state; (ii) develop new mechanically responsive motifs, or “mechanophores” which are capable of translating mechanical forces into specific and useful responses; and (iii) utilize the outcomes of these studies to design to create one-of-a-kind materials with hitherto unattainable mechanoresponsive properties.

Several types of mechanically responsive motifs, referred to as “mechanophores”, were integrated into a range of selected polymer types with different architectures, and the mechanoresponse of these materials was studied in solution and the solid state. The mechanophores utilized involved different dissociation principles, binding strengths, and response functions, including motifs that dissociate irreversibly upon cleavage of covalent bonds (such as azo groups, dithiomaleimides, and selected esters), dynamic metal-ligand complexes, conjugated chromophores that (dis)assemble in a reversible manner (such as planar oligo(phenylene vinylene)s dyes and cyclophanes), as well as a rotaxane, whose mechanically interlocked molecular architecture of a permits reversible, ultra-low-force sliding of a macrocycle along a dumbbell-shaped molecule onto which is threaded.

The mechanophores were incorporated into a range of selected polymer backbones, including poly(urethane)s, poly(methyl acrylate), poly(caprolactone), poly(ethylene-co-butylene), poly(dimethyl siloxane), and poly(styrene). The polymers were chosen to cover different morphologies (low-Tg and high-Tg amorphous, semicrystalline, micro-phase separated), architectures (linear, cross-linked), molecular weights, and contained either one mechanophore at the center of each macromolecule or several mechanophores along the chains. The response of these materials was probed by ultrasonication of solutions, mechanical deformation of solid materials, and the responses were monitored by a battery of techniques. These results of these studies generated a wealth of knowledge. Key outcomes were:

(i) The demonstration that the chain length, and not the mass, governs the mechanochemical cleavage of polymers in solution.

(ii) The development of a theoretical framework that describes the complex kinetics of the mechanochemical cleavage of multi-mechanophore polymers in solution.

(iii) The realization that the polymer architecture has a very significant influence on the stress-transfer characteristics in the solid state.

(iv) Several new mechanophores that respond to mechanical cleavage with a fluorescence turn-on or turn-off, fluorescence change, color change, or radical production.

(v) Materials that display intriguing new solid-state functions such as ultrasound-healing, mechanochromism, and mechanochromic luminescence.

(vi) A new design principle that allows transforming compounds with mechanoresponsive luminescence into supramolecular polymers display this effect.

(vii) The demonstration that rotaxanes can serve as mechanophores that permit sensing minute forces.

(viii) New concepts for polymers whose molecular weight can be reduced by optical and thermal stimulation and which are useful in adhesives that permit (de)bonding on demand and in objects with graded mechanical properties.