Microstructures within Polymers


Concepts Shown:

Microstructures within Polymers


Soft garbage bag (or something else made of low density polyethylene (LDPE)). Detergent container or pop bottle that is made of high density polyethylene (HDPE). Cigarette lighter.


Pass around the garbage bag and detergent container so that people can see that although the two are made from the same material, i.e. polyethylene, the materials in the both objects seem to possess very different properties. For instance the PE used in the garbage bag seems less stiff than the one used in the pop bottle. Now heat the garbage bag using the lighter, and watch it shrivel up. Heat the pop bottle for the same period of time and notice that the 'shriveling up' process is not observed. Present the numerical dada on the table that shows the difference between HDPE and LDPE. Also begin to explain that the differences in properties arises from he differences in crystallinity between the two. Warning: Heat the materials for only a short period of time.


Common plastics such as polystyrene and polyvinyl chloride are amorphous. As a result, they lack any conventional microstructure. Although original methods of making polyethylene produced a noncrystalline product (LDPE), even PE may be sufficiently crystalline to give a definite microstructure. Much of the crystallinity in polymers arises, not through the bundling of a large number of parallel molecules, buy by the folding of molecular chains on themselves. Amorphous regions remain, since the folding process is not perfect. Crystallization by chain folding is shown in the diagram below. [eq]. The consequences of partial crystallinity are shown in the table below. LDPE which has approximately zero percent crystallinity is less dense than HDPE, which has about 50 percent crystallinity. Notice that the crystallinity in HDPE gives it a higher elastic modulus and a higher tensile strength than the LDPE. Probably equally important is the change in heat resistance; the high-density polyethylene can withstand boiling water for sterilization. [eq]. Crystallization is impeded in LDPE, which uses the original, high-pressure processing method that permits some of the mers to enter the chain 'incorrectly'; whereas the catalyzed reaction that is used for making HDPE leads to the more idealized polymer chain without irregularly spaced -CH3 side units. In the latter, the molecules can mesh together more favorably for the long-range order of a crystal. As a result, there is greater density and less free space in the product. A chain with spatial irregularities crystallizes less rapidly. Chains with large side radicals do not crystallize easily. Polystyrene and polyvinyl chloride are such chains. The benzene ring of the former, and the large -Cl of the latter, interfere with chain folding in the crystallization process. Crystallization is however, facilitated by the presence of polar groups. For example crystals form more readily in nylon than in polyethylene. this is because the single hydrogen with the nitrogen, N-H, forms a bond with the side oxygen, O=C, in the adjacent molecule. The periodic presence of this hydrogen bridge helps to align adjacent molecules into the ordered crystalline pattern. In PE however, there is negligible opportunity for bonding. In fact, the hydrogen atoms, which are only exposed protons on the ends of covalent bonds, mutually repel each other. [eq].

Remarks: This is a very simple demonstration to show how crystallinity changes the properties of a polymer. It is also inexpensive since garbage bags and empty pop bottles cost nothing. There is no real safety equipment required. However, be careful about heating the plastics for too long.


Rahul Pinto

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