Plastics are used on a daily basis throughout the world. The word plastic is a common term that is used for many materials of a synthetic or semi-synthetic nature. The term was derived from the Greek plastikos, which means “fit for molding.” Plastics are a wide variety of combinations of properties when viewed as a whole. They are used for shellac, cellulose, rubber, and asphalt. We also synthetically manufacture items such as clothing, packaging, automobiles, electronics, aircrafts, medical supplies, and recreational items. The list could go on and on and it is obvious that much of what we have today would not be possible without plastics.
One way plastics changed the world was in cost. It was so much cheaper to manufacture than other materials and the various ways it could be used was staggering. For instance, the use of polymers, which are substances with a higher molecule mass and which have a large number of repeating units, is common today. There are synthetic polymers, which are produced on a large scale and have many properties and uses. And there are naturally occurring polymers, which include starches, cellulose, proteins, and latex. Polymers are molecules (monomers) that join together like a chain with one or more monomers. The polymers are changed depending on the incorporation of these monomers. If the atoms in the monomers are combined with the polymer, it is called an addition polymer. When some of the atoms of the monomers are released into small molecules, as in liquid, then the polymer is called a condensation polymer. A double bond between carbon atoms is most common in addition polymers.
In the early part of the twentieth century, a big boom occurred in polymer chemistry when polymer materials such as nylon and Kevlar came on the scene. Much of the work done with polymers focuses improvement while using existing technologies, but chemists do have opportunities ahead. There is a need for the development of new applications for polymers, always looking for less expensive materials that can replace what is used now. Chemists have to be more aware of what the market yearns for, such as products with a green emphasis, polymers that break down or are environmentally friendly. Concerns such as these have brought new activity to the science arena and there are always new discoveries to be made.
The physical structure of a polymer chain is important, it determines the macroscopic properties. Conformation and configuration are terms used to describe the geometric structure of polymer. Configuration speaks to the order that is determined by chemical bonds and conformation speaks to the order that comes from the rotation of the molecules about the single bonds. Stereoregularity is the configuration of polymer chains. Three very different structures can be obtained: Isotactic, which is an arrangement of substituents that are all on the same side of the chain. A syndiotactic chain is made up of alternating groups, and atactic is a combination that is random of all the groups.
In conformation, if two atoms are joined with a single bond, then rotation around that bond is possible. It does not require breaking the bond, unlike a double bond. That an atom can rotate this way relative to the atoms that it joins is called an adjustment of the torsional angle. If the two atoms have other groups or atoms attached to them, then configurations that vary in torsional angle are called conformations. Different conformations show varying distances between the groups rotating around the bond. These distances decide the type and amount of interaction between atoms that are adjacent. There are many possible generalized conformations: Eclipsed (Cis), Anti (Trans), and Gauche (+ or -).
There are other ways that a polymer structure can vary such as with a branched polymer, which is formed when there are side chains that come out from the main chain. There can be variations on how the branches stem off. One is called star-branching and another with a high degree of branching of the polymers are dendrimers. When more than one type of monomer is involved in the synthesis reaction it is called copolymers. The three types of copolymers are: A random copolymer, which contains a main chain with multiple monomers, a block copolymer, which contains blocks of monomers that are of the same type, and a graft copolymer, which has a main chain polymer of one type of monomer with branches of other monomers.
Although hard plastics like Bakelite were used as early as 1909, it wasn’t really big until later. In the early 1920’s, German chemist Hermann Staudinger first challenged the doubts of other chemists, on the existence of molecules having molecular weights greater than a thousand. He proposed that there were macromolecules composed of at least 10,000 atoms. While his studies focused on rubber, it soon was recognized that polymeric macromolecules made up many important natural materials. This was followed by the creation of synthetic analogs with a variety of properties such as fiber materials, flexible films, paints and tough light solids. Plastics molecules are also “polar” in nature, which means that they act like magnets. Just as a compass arrow points to magnetic north, when in the presence of a field or voltage these molecules will align themselves with each other. Because of this discovery, modern society was forever changed.
A thermo softening plastic or a thermoplastic is a polymer that when heated turns to liquid or when frozen will turn very glass like. Thermoplastics in general are usually high molecular weight polymers. Unlike Bakelite, which is a thermoset and once hardened stays that way, these polymers can be melted and remolded, making them a great product for recycling. The melting point will depend on the strength of the intermolecular forces. If hydrogen bonding is present, it will lift the melting points.
An interesting idea is that of induction heating, which has great potential for plastic reflow. Because of the rapid controlled application of heat, it would be useful for bonding plastics such as PVC or attaching plastics to other surfaces. However, since plastics to not conduct current, it is necessary to create a charge, such as when one rubs a balloon on their head and creates an electrostatic charge. Similarly, the same thought process is put into the idea of induction forces with plastic. When speaking of dispersion forces it is assumed that attraction forces arise from between surfaces with spherical curvature. The radius depends on how much deformation of the plastic there is during compression. Then there are hydrogen bonds, which is an attractive interaction between a hydrogen atom and an electronegative atom, which is usually oxygen, fluorine or nitrogen. This gives the hydrogen a partial positive charge. The hydrogen needs to be covalently bonded to another electronegative atom to create the bond. The bonds can happen between molecules, or in varying parts of a single molecule.
Because of their geometry, some polymers pack together tightly when the material is hard, this is called crystalline. Used in making items such as nylon, acetyl, polypropylene and polyethylene, crystalline polymers have superior properties, but do tend to considerably shrink as they cool and re-harden. Usually, these polymers exhibit a sharp melting point, so when they are heated it takes a very low temperature to melt or become liquid, acting much like candle wax.
Amorphous materials, such as acrylics, ABS and polycarbonate, are those that do not crystallize upon becoming solid. Instead, they demonstrate a softening that happens gradually as the temperature is increased. Since they do not flow as easy, materials such as these are usually not processed as easily as the crystalline material.
The properties of plastic can also be altered. This can occur by modifying the polymers from the original, or changed by additives, colorants, reinforcement, or fillers. Additives must be compatible as there is always some sort of trade off when an additive is used. Chemists attempt to keep all of the other material properties as high as possible while still reaching the desired improvement, like a resistance to burning. Other additives may be used to improve high temperature stability, as flame-retardants or fungicides. Reinforcements improve strength with additives such as carbon, mica, glass and aramids, which may be in the form of mats, spheres, flakes, or other things such as filaments. Reinforcements in plastics allow them to be used for loads and at higher temperatures, with greater stability dimensionally. This permits for more freedom in design and significant advances in technology in both the aviation and aerospace fields.
Newtonian linear viscosity in the liquid state is one of the simplest models for the deformation behavior of an ideal material in a liquid state. While the behavior of many real materials does approximate to this idealized model, that of polymers markedly deviates. This deformation is dependent on time. It is nonlinear, and resembles some combination of viscous and elastic responses.
Plastics have become a staple product in the world; the only real concern is how they will affect the environment. Latest news speaks of plastics that release toxins; there are litter concerns and the effect on landfills and waterways. While costs had been low, there has been a steady rise due to the cost of the fuels needed for its production. The solution is still unknown, but science is working on the problem and has found some promising alternatives such as fructose and oil shale. Only time will tell what the future of plastics will be, but it is unlikely that the world will ever be without them.
Polymer Crystallinity an explanation as well as diagrams of crystalline polymers, includes supporting pages.
Hydrogen Bonds page discusses the intermolecular force of hydrogen bonding, with a comparison of bond lengths and molecular electrostatic potential energy.
Polymers an introduction to polymer science. Part of the Virtual Textbook of Organic Chemistry lecture program by William Reusch.
Chemical of the Week a page focusing on polymers of all kinds.
Power from Plastics John Hopkins news release titled: Hopkins Scientists Create All-polymer Battery.
Introduction to Polymer Chemistry Crystalline and Amorphous polymers – information and models.
Thermal Properties of Polymers explains the concept of the polymer glass transition.
History of Plastics page includes links to plastic additives, processes, thermoplastics, thermosets, transitions and more.
Ask a Scientist Chemistry archives gives a simple explanation of heating plastics.