To make the red-alert plastics, researchers placed small ring-shaped molecules that they call “mechanophores” in the center of polymer chains. In response to mechanical force, these rings break, changing the color of the polymer [ScienceNOW Daily News].
In the study, published in Nature, lead researcher Nancy Sottos tested the stretchy plastic material her team had created, which stretched about as much as a Stretch Armstrong doll, by pulling on it until it broke. The bright red color appeared a few seconds before the material snapped, suggesting the molecules acted as an early warning sign that the material had incurred damage. In another test, the researchers mimicked repetitive stress by repeatedly stretching and relaxing the soft polymer. “After a few cycles of that, we got this brilliant color change in the material, without it breaking,” Sottos says [Science News]. But the color change was not permanent, and exposure to light or heat can interfere with the reaction.
Researchers say this was a proof of concept experiment that demonstrates how far they have to go before the technology can be used in practical applications. However, they can’t help speculating on how such a material could be used: Sottos says the material could eventually be used to make solid objects including rollerblade wheels, thin films such as coats of paint, or even thin fibers that could turn red when tiny deformations develop [Science News]. Those fibers could potentially be woven into climbing ropes and parachute cords, two lifelines that can snap unexpectedly.
Beyond sensing stress, researchers say that a mechanoresponsive material would have many applications beyond being a simple force sensor. One can imagine a polymer that provides structural reinforcement once it experiences too much stress, or a material that can release a drug when it suffers a certain level of pressure [Ars Technica]. Sottos is also interested in triggering self-healing reactions in plastics.
A polymer is simply a plastic. Polymers (plastics) are used a lot in our modern lifestyle and is made from one main raw material crude oil.
Polymers are simply large molecules in a chain made from many small molecules called monomers (Poly - Many, Mono - One). These monomers make the polymers by joining together in a process called polymerisation. Many of the polymers that we use in our everyday life are made from alkenes.
Alkenes are used to make polymers because the double bond in alkenes can be broken and that bond can be used to join the molecule to another alkene molecule.
When the alkene (In this example ethene) is heated at high pressures and heat in the presence of a catalyst, the molecules in ethene react with each other to form poly(ethene).
Depending on how the alkene was heated the plastic made will be high density (Strong plastic) or low density (flexible plastic). If ethene is
Heated: 200°C
Pressure: 2000 atm
Catalyst:trace of oxygen
A low density poly(ethene) is formed. It is low density because the molecules are loosely packed because the molecules in the chains are more branched.
If ethene is
Heated: 60°C
Pressure: 2 atm
Catalyst:Ziegler-Natta
A high density poly(ethene) is formed. It is high density because the molecules are tightly packed together and the polymer branches are more compact making the plastic more rigid.
The uses of the low density poly(ethene) is to make plastic bags and cling film as the polymer made is flexible and soft.
The uses of the high density poly(ethene) is to make buckets, bins and bottles(anything upright and strong) because the properties of the high density poly(ethene) is that it is harder and stiffer than low density poly(ethene)
source: http://www.scienceray.com/Chemistry/Explanation-of-Polymers.695573
Recently a number of people have been asking about the difference between dry and conditioned data for polyamide (nylon) materials. In a nutshell, "dry" refers to data that is obtained from a sample of material with equivalent moisture content as when it was molded (typically <0.2%). "Conditioned" on the other hand, refers to data obtained from a sample of material that has absorbed some environmental moisture at 50% relative humidity prior to testing. The vast majority of dry and conditioned data is seen with polyamide (Nylon) materials. To understand the need for two sets of data it's important to understand a little about the structure of Polyamide.
Polyamide is classified as a crystalline polymer, but it is only mostly crystalline; some amorphous regions do exist. The crystalline regions form because of the amide group of the polymer being polar. The electrons shared between some atoms aren't shared equally resulting in regions of slight positive and slight negative charges in the polymer chain. These charged regions are attracted to one another causing the polymer chain to fold over itself again and again. These crystalline regions are illustrated in Figure 1.
Figure 1. Polyamide Crystalline Regions
It is from these crystalline regions that polyamide receives much of its strength. These crystalline regions resist being pulled apart. Add more polymer chains to the mix, and you have a lot of semi-crystalline polymer chains attracted to one another. Although these regions haven't technically experienced cross-linking, the result is similar, higher strength and higher stiffness.
The polar nature of the amide group in the polymer is not only polyamide's strength, but is a weakness as well. Water is also a polar molecule, and water is everywhere. As mentioned previously some atoms don't share electrons equally. In this case the selfish atoms are Oxygen and Nitrogen. When a water molecule comes into contact with polyamide, weak bonds form between the two (see Figure 2). As this process continues, the water molecules diffuse though the material seeking out any charged areas and forcing polymer chains apart along the way. This is why polyamide parts swell after being exposed to moisture. The separation of the polymer chains reduces the polar attraction between chains and allows for increased chain mobility, resulting in diminished mechanical properties. It is within the amorphous regions that the water bonds to the polymer chain. Luckily the crystalline regions of polyamide resist being pulled apart by the water because the bonds between the amide groups are stronger than the attraction to water. Were this not the case polyamide would dissolve in water. Instead, water acts as a plasticizer rather than an out right solvent.
Figure 2. Polyamide Interaction with Water
Plasticizers cause a polymer to swell and soften. Both of these effects are exhibited by polyamides when they are exposed to moisture and must be taken into account when designing a part. This is why both dry and conditioned data for polyamides are given. The tensile strength and stiffness of the material will diminish, while the flexibility and impact toughness will increase with moisture absorption. The extent to which the different properties change depends a great deal on the chemistry of the polymer itself. Polyamide 12, for example, doesn't absorb as much moisture as Polyamide 6, so Polyamide 12's properties don't fluctuate as much with moisture.
