Van der Waals Bond The van der Waal bonds occur to some extent in all materials but are particularly important in plastics and polymers. These materials are made up of a long string molecules consisting of carbon atoms covalently bonded with other atoms, such as hydrogen, nitrogen, oxygen, fluorine. 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 or a thermosetting material. Thermoplastic materials can be easily remelted for forming or recycling and thermosetting material cannot be easily remelted. In 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, can not be remelted and formed as easily as thermoplastics. Solid State Structure In the previous pages, some of the mechanisms that bond together the multitude of individual atoms or molecules of a solid material were discussed. These forces may be primary chemical bonds, as in metals and ionic solids, or they may be secondary van der Waals' forces of solids, such as in ice, paraffin wax and most polymers. In solids, the way the atoms or molecules arrange themselves contributes to the appearance and the properties of the materials. Atoms can be gathered together as an aggregate through a number of different processes, including condensation, pressurization, chemical reaction, electrodeposition, and melting. The process usually determines, at least initially, whether the collection of atoms will take to form of a gas, liquid or solid. The state usually changes as its temperature or pressure is changed. Melting is the process most often used to form an aggregate of atoms. When the temperature of a melt is lowered to a certain point, the liquid will form either a crystalline solid or and amorphous solid. Amorphous Solids A solid substance with its atoms held apart at equilibrium spacing, but with no long-range periodicity in atom location in its structure is an amorphous solid. Examples of amorphous solids are glass and some types of plastic. They are sometimes described as supercooled liquids because their molecules are arranged in a random manner some what as in the liquid state. For example, glass is commonly made from silicon dioxide or quartz sand, which has a crystalline structure. When the sand is melted and the liquid is cooled rapidly enough to avoid crystallization, an amorphous solid called a glass is formed. Amorphous solids do not show a sharp phase change from solid to liquid at a definite melting point, but rather soften gradually when they are heated. The physical properties of amorphous solids are identical in all directions along any axis so they are said to have isotropic properties, which will be discussed in more detail later Crystalline Solids More than 90% of naturally occurring and artificially prepared solids are crystalline. Minerals, sand, clay, limestone, metals, carbon (diamond and graphite), salts ( NaCl, KCl etc.), all have crystalline structures. A crystal is a regular, repeating arrangement of atoms or molecules. The majority of solids, including all metals, adopt a crystalline arrangement because the amount of stabilization achieved by anchoring interactions between neighboring particles is at its greatest when the particles adopt regular (rather than random) arrangements. In the crystalline arrangement, the particles pack efficiently together to minimize the total intermolecular energy. The regular repeating pattern that the atoms arrange in is called the crystalline lattice. The scanning tunneling microscope (STM) makes it possible to image the electron cloud associated individual atoms at the surface of a material. Below is an STM image of a platinum surface showing the regular alignment of atoms.
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