Chapter 15. Polymer Structures
Polymers are common in nature, in the form of wood, rubber, cotton, leather, wood, silk, proteins, enzymes, starches, cellulose. Artificial polymers are made mostly from oil. Their use has grown exponentially, especially after WW2. The key factor is the very low production cost and useful properties (e.g., combination of transparency and flexibility, long elongation).
15.2 Hydrocarbon Molecules
Most polymers are organic, and formed from hydrocarbon molecules. These molecules can have single, double, or triple carbon bonds. A saturated hydrocarbon is one where all bonds are single, that is, the number of atoms is maximum (or saturated). Among this type are the paraffin compounds, CnH2n+2 (Table 15.1). In contrast, non-saturated hydrocarbons contain some double and triple bonds.
Isomers are molecules that contain the same molecules but in a different arrangement. An example is butane and isobutane.
15.3 Polymer Molecules
Polymer molecules are huge, macromolecules that have internal covalent bonds. For most polymers, these molecules form very long chains. The backbone is a string of carbon atoms, often single bonded.
Polymers are composed of basic structures called mer units. A molecule with just one mer is a monomer.
15.4 The Chemistry of Polymer Molecules
Examples of polymers are polyvinyl chloride (PVC), poly-tetra-chloro-ethylene (PTFE or Teflon), polypropylene, nylon and polystyrene. Chains are represented straight but in practice they have a three-dimensional, zig-zag structure (Fig. 15.1b).
When all the mers are the same, the molecule is called a homopolymer. When there is more than one type of mer present, the molecule is a copolymer.
15.5 Molecular Weight
The mass of a polymer is not fixed, but is distributed around a mean value, since polymer molecules have different lengths. The average molecular weight can be obtained by averaging the masses with the fraction of times they appear (number-average) or with the mass fraction of the molecules (called, improperly, a weight fraction).
The degree of polymerization is the average number of mer units, and is obtained by dividing the average mass of the polymer by the mass of a mer unit.
Polymers of low mass are liquid or gases, those of very high mass (called high-polymers, are solid). Waxes, paraffins and resins have intermediate masses.
15.6 Molecular Shape
Polymers are usually not linear; bending and rotations can occur around single C-C bonds (double and triple bonds are very rigid) (Fig. 15.5). Random kings and coils lead to entanglement, like in the spaghetti structure shown in Fig. 15.6.
15.7 Molecular Structure
Typical structures are (Fig. 15.7):
15.8 Molecular Configurations
The regularity and symmetry of the side-groups can affect strongly the properties of polymers. Side groups are atoms or molecules with free bonds, called free-radicals, like H, O, methyl, etc.
If the radicals are linked in the same order, the configuration is called isostatic
In a stereoisomer in a syndiotactic configuration, the radical groups alternative sides in the chain.
In the atactic configuration, the radical groups are positioned at random.
Copolymers, polymers with at least two different types of mers can differ in the way the mers are arranged. Fig. 15.9 shows different arrangements: random, alternating, block, and graft.
15.10 Polymer Crystallinity
Crystallinity in polymers is more complex than in metals (fig. 15.10). Polymer molecules are often partially crystalline (semicrystalline), with crystalline regions dispersed within amorphous material. .
Chain disorder or misalignment, which is common, leads to amorphous material since twisting, kinking and coiling prevent strict ordering required in the crystalline state. Thus, linear polymers with small side groups, which are not too long form crystalline regions easier than branched, network, atactic polymers, random copolymers, or polymers with bulky side groups.
Crystalline polymers are denser than amorphous polymers, so the degree of crystallinity can be obtained from the measurement of density.
15.11 Polymer Crystals
Different models have been proposed to describe the arrangement of molecules in semicrytalline polymers. In the fringed-micelle model, the crystallites (micelles) are embedded in an amorphous matrix (Fig.15.11). Polymer single crystals grown are shaped in regular platelets (lamellae) (Fig. 15.12). Spherulites (Fig. 15.4) are chain-folded crystallites in an amorphous matrix that grow radially in spherical shape grains.