These materials are made up of a long string of molecules consisting of carbon

These materials are made up of a long string of

This preview shows page 43 - 45 out of 79 pages.

These materials are made up of a long string of molecules consisting of carbon atoms covalently bonded with other atoms, such as hydrogen, nitrogen, oxygen, fluorine, etc. The covalent bonds within the molecules are very strong and rupture only under extreme conditions. The bonds between the molecules that allow sliding and rupture to occur are called van der Waal forces. When ionic and covalent bonds are present, there is some imbalance in the electrical charge of the molecule. Take water as an example. Research has determined the hydrogen atoms are bonded to the oxygen atoms at an angle of 104.5°. This angle produces a positive polarity at the hydrogen-rich end of the molecule and a negative polarity at the other end. A result of this charge imbalance is that water molecules are attracted to each other. This is the force that holds the molecules together in a drop of water. This same concept can be carried on to plastics, except that as molecules become larger, the van der Waal forces between molecules also increases. For example, in polyethylene the molecules are composed of hydrogen and carbon atoms in the same ratio as ethylene gas. But there are more of each type of atom in the polyethylene molecules and as the number of atoms in a molecule increases, the matter passes from a gas to a liquid and finally to a solid. Polymers are often classified as being either a thermoplastic, a thermosetting material or an elastomer. Thermoplastic materials can easily be remelted for forming or recycling but thermosetting materials cannot be easily remelted. Thermoplastic materials consist of long chainlike molecules. Heat can be used to break the van der Waal forces between the molecules and change the form of the material from a solid to a liquid. By contrast, thermosetting materials have a three-dimensional network of covalent bonds. These bonds cannot be easily broken by heating and, therefore, thermosetting materials cannot be remelted and formed as easily as thermoplastics. There are three types of secondary bonds.
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~ Page 44 of 79 ~ These are discussed below. Dispersion Bonds Distribution of electrons in a molecule is symmetrical around its nucleus. Electrons are always in motion. Sometimes the distribution of electrons is not symmetrical around their axes. This creates an electronic imbalance of the charge which is known as polarization. This is fluctuating in nature and is known as dispersion effect. This results in a weak force of attraction being developed between the two molecules of the same element. Due to this force a bond is formed between the molecules of the same element which is called dispersion bond. The molecules of noble gases like helium, argon, etc. which consist of single atom type are held together by dispersion bonds when they are solidified at very low temperature.
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