What are the main Structural features of polymers?

The term polymer simply means “ many mers”, where mer is the building block of the long-chain or network molecule. Fig. 1 shows how a long-chain structure results from the joining together of many monomers by chemical reaction.

Fig. 1. Polymerization is the joining of individual monomers (e.g., vinyl chloride, C₂H₃Cl) to form a polymer [(C₂H₃Cl)_n] consisting of many mers (again, C₂H₃Cl).

Structural features of polymers:

l First polymer structure aspect – length of the polymeric molecule.

Example: how large is n in (-C₂H₄-)_n ?

In general, n is termed the degree of polymerization and is also designated DP.

It is usually determined from the measurement of physical properties, such as viscosity and light scattering.

For typical commercial polymers, n can range from approximately 100 to 1000, but for a given polymer, the degree of polymerization represents an average.

l Directly related to molecular length is molecular weight, which is simply the degree of polymerization (n) times the molecular weight of the individual mer.

l For network structures, there is, by definition, no meaningful one-dimensional measure of length. For linear structures, there are 2 such parameters. First is the root-mean-square length, L, given by L = l m (1)

where l is the length of a single bond in the backbone of the hydrocarbon chain and m is the number of bonds. Eq. (1) results from the statistical analysis of a freely kinked linear chain as illustrated in Fig. 3.

Fig. 3 The length of kinked molecular chain is given by Equation (2), due to the free rotation of the C-C-C bond angle of 109.5⁰.

Each bond angle between three adjacent C atoms is near 109.5 ⁰, but as seen in Fig. 3. this angle can be rotated freely in space. The result is the kinked and coiled molecular configuration. The root-mean-square length represents the effective length of the linear molecule as it would be present in the polymeric solid.

l The second length parameter is a hypothetical one in which the molecule is extended as straight as possible (without bond angle distortion),

L _ext = ml sin 109.5 ^o /2 (2)

were L _ext is the extended length. The “sawtooth” geometry of the extended, molecule is illustrated in Fig. 4.

Fig. 4. Sawtooth geometry of a fully extended molecule. The relative sizes of carbon and hydrogen atoms are shown in the polyethylene configuration.

For typical bifunctional, linear polymers such as polyethelene and polyvinyl chloride, there are 2 bond lengths per mer, or m = 2n, where n is the degree of polymerization.

Remember:

1) In general, the rigidity and melting point of polymers increase with the degree of polymerization.

2) BUT: the melting point of nylon does not change with degree of polymerization.

3) Useful rule of thumb:

Rigidity & melting point increase as the complexity of the molecular structure increases.

Vulcanization – complete transition from linear to network structure is produced by cross-linking. Rubbers are the most common examples of cross-linking.

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