Linear Polymers (the lampchain model)

Location:

Concepts Shown:

Properties of linear organic polymers

Equipment:

The length of lamp chain whose beads will crudely represent the carbon atoms in a polymer.

Procedure:

The length of lamp chain can be thrown into the audience. Alternately, the instructor can walk around the class unwinding the roll. This will enable the students to see the length of an average polymer on a much larger scale.

Science:

Macromolecules or polymers as they are better known as, may contain many atoms with strong intramolecular bonds. A typical example of a macromolecule is polyethylene, which may contain molecules with an average of 50,000 atoms with thousands of such bonds. Such macromolecules form the basis of plastics and related products. Polymers (many units) are produced b joining many single units (monomers). In the case where polymer chains have linear backbones, there is strong covalent bonding that exists along the length of the backbones. Polyvinyls and polyesters are commercial polymers with such linear backbones. Polyvinyl chloride, polystyrene and polyethylene are examples of the polyvinyl family. The monomers of polyvinyls have monomers as shown below. The R is a chlorine atom (Cl) in polyvinyl chloride, a benzene ring in polystyrene and another hydrogen atom in polyethylene. [eq]. The particular type of architecture the molecules possess, controls the properties of the material. For instance, thermoplastics are linear or branched polymers which can be melted upon the application of heat. They can be molded and remolded using conventional techniques and now make up the largest bulk of polymers used. Properties of molecules differ according to their respective sizes. The polyethylene macromolecule, a solid, is made up of an ethylene monomer. Ethylene however, which contains two carbon atoms, is a liquid. As more carbons are added, by the joining of these monomers, the chain begins to demonstrate properties of a solid. The minimum number of carbon atoms required to form a stable material with solid properties is about 10,000-12,000. Such molecules are synthesized by the process of polymerization. Therefore, mechanical, chemical, and service stabilities are also size dependent. [eq]. Molecular size can be reported in terms of the degree of polymerization, n. By knowing the weight of the molecule and the mer weight of the polymer, n is calculated: N = M/m = molecular weight/mer weight One could calculate the average linear length of a polymer given the degree of polymerization, n, and the data that the c-c bond is 0.154nm long. For instance, the polyethylene molecule with n=500 would have a length of 154nm, as each monomer as 2 carbon atoms. However, since the carbon atoms are not linear, but at a 120 degree angle, some correction has to be made. The sawtooth length would be the linear length times sin 60 degrees (sin 60 = sin 120), or in the case of the polyethylene, 135nm. The average, root-mean-length-square length, L, in non-crystalline polymers, can be calculated on a statistical basis as L=1*sqrt(x), where l is the individual bond length, and x is the number of bonds in the chain. The RMS is based on the random or drunkard walk problem. Remarks: Each inch of the chain has 7 beads on it, each representing a carbon atom. Since there is roughly 200ft. of the chain there will be approximately 16,800 beads. Although the chain may seem to represent a very long polymer, in reality it represents one of very average length.

References:

Author:
Rahul Pinto
Credits:


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