Feb 3 2021
At the Tokyo Institute of Technology (Tokyo Tech), researchers have created a new polymer whose properties vary significantly upon exposure to mechanical stress.
Image Credit: Tokyo Institute of Technology.
When the mechano-responsive polymer is in bulk form, it exhibits self strengthening, color changing and fluorescence abilities even under simple extension or compression. The basic results of the study are unmatched in the field of mechanochemistry and could open the door for several applications in the area of materials science.
Biological tissues like muscle exhibit a remarkable and vital potential of self strengthening and healing in response to damage by external forces. By contrast, a majority of the manmade polymers break irrevocably under sufficient mechanical stress, which renders them less beneficial for some critical applications such as the manufacture of artificial organs.
But what if it is feasible to develop polymers that react chemically to mechanical stimuli and use the energy to improve their properties?
This aim, which has been very difficult to achieve, is being scrutinized in the field of mechanochemistry. In a new study published in Angewandte Chemie International Edition, a group of researchers from Tokyo Tech, Yamagata University and Sagami Chemical Research Institute, Japan, achieved impressive progress with bulk self strengthening polymers.
Leading the study, Professor Hideyuki Otsuka explained the motivation as follows:
Furthering the development of elegant bulk systems in which a force-induced reaction causes a clear change in mechanical properties would represent a game-changing advance in mechanochemistry, polymer chemistry, and materials science.
Hideyuki Otsuka, Professor, Tokyo Institute of Technology
They realized their aim by concentrating on difluorenylsuccinonitrile (DFSN), which is a 'mechanophore' or a molecule that responds to mechanical stress.
The researchers developed segmented polyurethane polymeric chains using both soft and hard functional segments. The soft segments include DFSN molecules that serve as the 'weakest link,' where both of its halves are linked by a single covalent bond. The side chains of the soft segments are topped off with methacryloyl units.
When mechanical stress, like simple extension or compression, is applied to the polymer, the DFSN molecule breaks into two equal cyanofluorene (CF) radicals. In contrast to DFSN, these radicals turn pink in color, which makes it easy to visually detect any mechanical damage.
Specifically, the CF radicals tend to react with the methacryloyl units at the side chains of other polymers. This causes individual polymers to chemically bind to one another in a process called cross-linking. Through this phenomenon, the overall strength of the bulk material eventually increases as polymers turn more chemically intertwined.
The researchers experimentally proved that this chemical cross-linking effect becomes more evident when more compression cycles are carried out on the segmented polymer samples. This is because more DFSN molecules get disintegrated into CF radicals.
The researchers developed a slight variant of the newly developed segmented polymer that not just becomes pink but also shows fluorescence upon irradiation with ultraviolet light and applying mechanical force to it. This functionality is very useful when researchers make attempts to more precisely measure the extent of the damage caused by mechanical stress.
The appealing characteristics and functionalities of the developed polymers are beneficial, for instance, for intuitive damage detection and the development of adaptive materials.
Otsuka was excited about the findings of the study:
We successfully developed unprecedented mechanoresponsive polymers that exhibit color change, fluorescence, and self-strengthening ability, marking the first report of force-induced cross-linking reactions achieved by simply the extension or compression of a bulk film. Our findings represent a significant advance in the fundamental research of mechanochemistry and its applications in material science.
Hideyuki Otsuka, Professor, Tokyo Institute of Technology
With the development of more mechano-responsive materials with exclusive functions, further exploration of their myriad applications can be expected in different industrial and engineering fields.
Journal Reference:
Seshimo, K., et al. (2021) Segmented Polyurethane Elastomers with Mechanochromic and Self‐strengthening Functions. Angewandte Chemie International Edition. doi.org/10.1002/anie.202015196.