Smart rubber is a polymer that is able to "heal" when torn. It can heal itself at room temperature and can be repeated multiple times. The supramolecular self-healing rubber can be processed, re-used, and recycled. The reason this can happens is because you can tear the rubber apart and lightly place it back together and it will start to self-heal. The smart rubber depends only on hydrogen bonds which makes this all possible. This process doesn't depend on covalent bonding or ionic bonding, which are present in normal rubber. Unlike covalent and ionic bonding, hydrogen bonding will occur simply by pressing the two faces of a substance together.
The rubber’s stress strain curve resembles that of soft rubbers. The breaking point is more than 500% strain. After stretching to 300% at a 2.5 mm per min and letting go your residual strain is less than 5%. This polymer is able to return to the original size and shape when it is deformed but only after you release the stress. The longer the self-healing rubber is in contact with the recently broken piece the better the healing process is. Even with contact though of fifteen minutes the smart rubber can be stretched or deformed up to 200%
The places that are healed back together are not visible to the human eye but the rubber will break at the scar unless you allow longer healing times.
The self-healing process is efficient since there are large numbers of ready to go groups that want to link together. After a certain point you cannot bond and during this time your thermal equilibrium is reached.
The time varies with temperature the rubber is stored at. The rubber was tested by storing the cut pieces in varying temperatures and then were dropped back to room temperature to test the self-healing properties.
- It was found that as temperature went up the maximum waiting time went down.
This is the Table of values that shows the temperature at which the samples were stored at right after being cut and the maximum amount of time before you reach thermal equilibrium and you cannot bond the cut or torn samples back together.
Temperature (°C) Time (hrs) 23 168 40 48 60 1.6 90 0.25 120 0.08
When smart rubber is cut and put back together it will reach initial strength after 3 hours. There are non-associated bonds that make the elastomer self-healing remain in a free state for a certain amount of time until it comes in contact with the same cut edge or another cut edge with non-associated hydrogen bonds waiting to be healed. After 6 hours or 18 hours of being cut the non-associated hydrogen bonds take longer to bond and are less effective. FTIR spectroscopy shows the breaking and reassociation of hydrogen bonds in the rubber. This made it possible to see the free and bound N-H bonds which had been heated for 10 minutes at 125°C. This made the hydrogen bonds free and was proceeded to be quenched. The reassociation was about 10^4 seconds for the N-H bonds.
Compared to Rubber
When compared to rubber which is covalently cross-linked smart rubber it cannot continually hold stress without undergoing some slow plastic deformation and the strain recovery is slow.
Very good compromises that has been obtained while keeping the self-mending property and the high extensibility is when you use water as the plasticizer. Normally you use dodecane, but using water makes the glass transition drop to -15 degrees C and there is also a 500% strain recovery within seconds of when you let the stress go. Another is that when you stretch and hold the samples for up to 10 hours you will not get much creep.
This can be used in the future to make stockings, children's toys, shoes, protective coatings, vehicle fan belts, washing-up gloves, and self-healing seals.
Real World Versions
Materials of this type are still in development, for example a type invented at the Industrial Physics and Chemistry Higher Educational Institution ESPCI in Paris, France. Their translucent, yellowish-brown rubber is synthesized from fatty acids and urea. It is still too soft for most anticipated practical applications, like rubber gloves and shoe soles.
Among the potential problems of the material is that it would also bond to itself any other way it came into contact, like upon closing the fingers or hand of a glove. This may be solved by coating objects made from the rubber with some other polymer, so that the smart rubber is only exposed when torn.
The way to make the smart rubbers is to graft H-bonding moieties onto backbones that are already synthesized. This process happens when you use oligocondensation of a mixture of fatty mono, di-, and triacids with diethylenetriamine (DETA). When you change the degree of the backbone branching and the molecular weight by changing the di- and triacids and also by end-capping before the condensation reaction some acid groups with 2-aminoethylimidazolidone (UDETA)which is what helps in the bonding process. The additional H-bonding units are introduced by grafting urea onto all secondary amines of DETA. Analogous synthetic routes with different H-bonding moieties or building blocks of branched backbones can of course be envisaged.
- In short you graft UDETA onto fatty acids. Then you add DETA. Urea is reacted on the secondary amines of DETA to make 1,1-Dialkylureas. The stoichiometries of DETA and UDETA with the fatty acids give us the control to make the molecular weight different, the branching degrees, and the number of H-bonding moieties for each molecule.
The smart rubber is only made when you synthesize with the highest molecular weight of materials and it also has a particularly long relaxation time. These behave like supramolecular self-healing rubbers or otherwise know as smart rubber.
To get a fine tuned smart rubber with characteristics that you want you just use different fatty acids made from vegetable oils and you also have the option to change the ratio of diacid/triacid.
- Wietor, Jean-Luc; Sijbesma, Rint P. (13 October 2008). "A Self-Healing Elastomer". Angewandte Chemie International Edition 47 (43): 8161–8163. doi:10.1002/anie.200803072.
- Montarnal, Damien; Tournilhac, François; Hidalgo, Manuel; Couturier, Jean-Luc; Leibler, Ludwik (17 June 2009). "Versatile One-Pot Synthesis of Supramolecular Plastics and Self-Healing Rubbers". Journal of the American Chemical Society 131 (23): 7966–7967. doi:10.1021/ja903080c.
- Montarnal, Damien; Cordier, Philippe; Soulié-Ziakovic, Corinne; Tournilhac, François; Leibler, Ludwik (15 December 2008). "Synthesis of self-healing supramolecular rubbers from fatty acid derivatives, diethylene triamine, and urea". Journal of Polymer Science Part A: Polymer Chemistry 46 (24): 7925–7936. doi:10.1002/pola.23094.
- "Self-healing rubber promises products that mend themselves". Retrieved 6 May 2013.
- "Smart rubber promises self-mending products". newscientist.com. Retrieved July 15, 2012.