What makes polymers flexible

A free radical is a molecule with a single unpaired electron. Or, to get technical, a molecule with an unpaired electron in its outermost valence shell is an unstable molecule. Either way, the lone electron is going to want to pair up with another electron. It attacks the double bond joining the two carbons in the ethylene molecule and swipes an electron.

The other carbon, previously happily paired, now has an unpaired electron. It has become a free radical, with an unpaired electron eager to join up with another to make a pair. A second ethylene molecule is introduced. The newly created free radical breaks the carbon-to-carbon bond, swiping an electron, and creating a new free radical with a single unpaired electron on the end.

This continues, as a chain reaction, with a long chain forming as more ethylene molecules are added. The process keeps going until free radicals meets another free radical, completing the chain. Now we have our polymer, polyethylene, made up of the monomer repeating unit ethylene. Some other examples of polymers formed in this way are polychloroethylene PVC , used to make things like plumbing pipes and insulation for electrical cables, and polypropylene, used in products such as rubber ducks and other toys and, when processed into fibres, carpets.

The way molecules are arranged gives different polymers different properties. Polyethylene, for example, has long polymer chains side by side. When they cool down, the chains interact and become entangled. Polyethylene can be melted and reformed into a new shape over and over again. These meltable, reshapable polymers are known as thermoplastics. Other examples include polystyrene and polypropylene.

The strength of polymers also varies depending on how the molecules are arranged. To use our paperclip analogy, you may decide to have some paperclips branching off your main line. They result in a polymer with a lower density.

Low-density polyethylene LDPE —the squishy material that plastic bags and wrap like the kind you might wrap your sandwich in —is an example.

The resulting polymer is stronger and has a higher density. An example is high-density polyethylene HDPE , used to make things like plastic bottles, food containers and plumbing pipes. In contrast to thermoplastic polymers are thermosetting polymers. It is useful, though, for things like car tyres, since a tyre that melts in the heat is going to make for a pretty interesting drive to the beach. Glues and electrical components are also thermosetting polymers.

As well as the arrangement of molecules, the properties of a polymer are also determined by the length of the molecular chain. In a nutshell, longer equals stronger. This is because, as a molecule gets longer, the total binding forces between molecules are greater, making the polymer chain stronger. When more than a thousand carbon atoms line up in a chain of ethylene monomers, for example, the resulting polymer, polyethylene, is strong and flexible.

As chains flex and bend against each other, various attractive and repulsive forces affect how polymer chains arrange themselves, either more orderly or less orderly. Degree of crystallinity is directly related to whether a polymer melts like a typical solid or whether it transitions between glassy and rubbery states. Highly crystalline polymers have a more traditional melting point, so when they are heated, they reach a certain temperature at which the orderly arrangement of their long-chain structure transitions to a random and disorganized arrangement.

This value is usually a specific number, designated as the melting point, or T m. Instead, they reach a range of temperatures over which the material becomes less glassy and more rubber-like or vice versa. The glass transition temperature of a specific polymer may be listed as a single temperature, but this number is a representative value representing a range of temperatures.

To explain glass transition temperature in terms of molecular motion, we would say that, at temperatures below T g , the amorphous polymer chains cannot rotate or move in space the cooked spaghetti is frozen and cannot move. This produces the glassy state, which is hard, rigid, and brittle. When the temperature rises above T g , the entangled chains can move small portions of the spaghetti noodles can move around.

This produces a rubbery state, when an amorphous polymer is soft and flexible. Although percent and 0 percent crystallinity are rare, some polymers fall close to either extreme. Those that tend toward high crystallinity are rigid, have high melting points, and are less affected by solvent penetration.

Those that tend toward high amorphousness are softer, have glass transition temperatures , and are penetrated more by solvents than are their crystalline counterparts. Here are some examples, along with their key properties:. To understand why, it helps to realize that polymers can have multiple configurations. A polymer has a main backbone with small clusters of atoms, called pendant groups, coming off of the chain.

If all of the pendant groups are on the same side of the chain, the polymer is isotactic. If the pendant groups come on alternating sides of the chain, the polymer is said to be syndiotactic. If the pendant groups are on both sides, but in no particular order, the polymer is atactic. These structural differences can have a significant impact on the properties of a polymer. Many applications of polymers and polymer coatings need flexibility at low to ambient temperatures. Conversely, when hardness and rigidity are required, a polymer with greater crystallinity may be preferred.

Mallard Creek Polymers specializes in the design and manufacturing of amorphous polymers. Yet they have very different properties. Starch will dissolve in water and can be digested. The only difference between these two polymers is how the glucose monomers have been linked together.

Living things build proteins — a particular type of polymer — from monomers called amino acids. Although scientists have discovered some different amino acids, animals and plants use only 20 of them to construct their proteins.

In the lab, chemists have many options as they design and construct polymers. Chemists may build artificial polymers from natural ingredients. Or they can use amino acids to build artificial proteins unlike any made by Mother Nature. More often, chemists create polymers from compounds made in the lab. Polymer structures can have two different components.

All start with a basic chain of chemically bonded links. This is sometimes called its backbone. One of these attachments may be as simple as a single atom. Others may be more complex and referred to as pendant groups. Sometimes pendant groups, instead of hanging loose from one polymer chain, actually connect two chains together. Think of this as looking like a rung that stretches between the legs of a ladder. Chemists refer to these ties as crosslinks.

They tend to strengthen a material such as a plastic made from this polymer. They also make the polymer harder and more difficult to melt. The longer the crosslinks, however, the more flexible a material becomes. A chemical bond is what holds atoms together in a molecule and some crystals. Once oxygen forms two bonds, it ibecomes stable.