Polymer Essay: Plexiglass

Many of the glass screens and windows we see today are not really made of glass. They are made of plexiglass. Plexiglass is not related to the chemical structure of glass at all. It is an organic polymer. Today this polymer is used in all sorts of applications. The formation of plexiglass was begun in 1930, at McGi11 University by William Chalmers, a research student. He polymerized methacrylic ethyl ester and methacrylic nitirile, creating a clear plastic paste. Later, in 1936, Walter Bauer and Otto Rohm accidentally developed plexiglass by pouring the gum between two sheets of glass. The gum hardened and separated cleanly from the glass sheets. They created plexiglass as we know it today. Since its discovery, plexiglass has found more and more applications and uses. Its important physical properties are due to the chemical structure of this polymer. Plexiglass has a glass-like appearance. It is completely transparent and clear, even clearer than glass. However, unlike glass, it is not easily shattered. Plexiglass can take a lot of force before it cracks. Due to this, plexiglass is often referred to as “shatterproof glass”. Thus it is not surprising that many of the uses of plexiglass are often as a substitute for glass. One of the first uses of plexiglass was in making the cockpit windows for airplanes. Plexiglass could take the high air pressure without shattering. Bulletproof plexiglass is used in banks. The barriers around a hockey rink are also made of plexiglass. Because the visibility through plexiglass remains clear even if it is very thick, it is also ideal for making an aquarium. Plexiglass is also used for making display cases. These useful properties of plexiglass are due to its chemical structure. The chemical name for plexiglass is poly (methyl methacrylate). It belongs in a family of polymers called the acrylates, or commonly known as the acrylics. The acrylics come under many common names. Diakon, Lucite, Perspex and Plexiglas are some examples. All acrylates have similar properties, such as their insolubility in water. Acrylates are distinguished from other polymers by the monomers that make them. An acrylate monomer is an ester with double bonded carbon atoms. One of these two carbon atoms is bonded to the carbonyl carbon atom and is referred to as the a-carbon atom. Some acrylates—including plexiglass—have an extra methyl group attached to the a- carbon atom. Hence there is the prefix “meth” in methyl methacrylate. This small methyl group is responsible for the hard and clear form of plexiglass at room temperature. The extra methyl group reduces “chain mobility”—the ability of the polymer chains to bend and flex around each other. The more mobility the chain has the softer the polymer is, like rubber. The methyl group decreases the chain mobility of plexiglass, giving it a more rigid structure, and thus its hard glass-like properties. Besides structure, the chain mobility also depends on temperature. Decreasing the temperature causes the polymer to have less energy and become more and more rigid. For this reason in very cold temperatures plastic becomes more and more brittle. Increasing the temperature on the other hand, makes the polymer more and more flexible. Each polymer (except pure crystalline polymers) has a certain temperature above which it will be soft and rubbery, and below which it will be hard and firm. This temperature is called the glass transition temperature or TG for short. (TG is not the same thing as the melting point. Above and below the TG the polymer remains a solid.) The TG for plexiglass is very high. Consequently, at room temperature plexiglass is solid and glass-like. At very low temperatures the plexiglass becomes brittle. It is easily shattered. Thus the unique chemical structure of plexiglass gives it its very useful physical properties—clear visibility and hardness. These physical properties make plexiglass an ideal material in a large number of applications, making plexiglass a very widely used polymer in today's society.