|The Structure and Properties of Polymers|
Crosslinked Polyvinyl Alcohol (PVA)
|Polyvinyl alcohol is a polymer made from vinyl alcohol. The structure is shown to the right.|
When many vinyl alcohol monomers are linked together a long polymer molecule (PVA) is formed. A fragment of the structure of the PVA polymer molecule is shown below. The PVA molecule consists of a long chain of carbon atoms (about 4400 carbon atoms per molecule) to which are attached hydrogen atoms and OH groups.
|The addition of borate ion, which has the structure shown on the right, to an aqueous solution of PVA causes dramatic changes in the properties of the solution which can be attributed to the formation of bonds (or crosslinks) between the individual polyvinyl alcohol chains by the borate ion.|
The structure of the resulting crosslinked PVA is shown below. The gray regions represent relatively weak hydrogen bonds. The effect of this crosslinking is to produce a three-dimensional network of polyvinyl alcohol chains. This network traps and immobilizes water molecules, and the resulting product is a "gel". Other examples of gels are fruit jellies, gelatin and "Jello." The properties of the gel will depend upon the amount of crosslinking. In this experiment the amount of crosslinking will be varied by varying the amount of borate ion added to the PVA solution. The effect of changing the amount of crosslinking will be measured by measuring the viscosity of the polymer solution. The viscosity of a fluid is a measure of its resistance to flow. For example, molasses is said to be viscous because it does not pour easily.
Bakelite is a commercially important polymer which is frequently used when a strong, brittle polymer is needed. Shown below is the first step in the formation of the Bakelite. Here, two molecules of resorcinol are heated with formaldehyde (H2CO) and a strong, covalent bond is formed between them.
When the mixture is heated for a longer time, many more bonds between the molecules are formed. These bonds result in the formation of a polymeric structure with considerable crosslinking. This structure is very difficult to deform because of these bonds and, as a result, is hard and brittle. A small fragment of such a Bakelite polymer molecule is shown below.