Chemical composition and molecular structure

Polyolefins

Many polymers are derived from the olefins, a family of hydrocarbon compounds—that is, compounds containing hydrogen (H) and carbon (C)—which are produced from the refining of petroleum and natural gas. An olefin contains one double bond between two carbon atoms. The general chemical formula can be represented as CH₂=CHR, with R representing any of several possible atoms or groups of atoms. As the repeating unit of a polymer, the compound has the following chemical structure:

Here the brackets signify that the compound is a repeating unit, and n represents the number of times the unit is repeated in the polymer.

When R in the above structure represents a methyl group (CH₃), the polymer obtained is polypropylene. Polypropylene is a material of moderately high melting temperature (176 °C, or 349 °F) that can be melt-spun into fibres useful for several types of clothing, upholstery, carpets, and nonwoven fabrics. When R is hydrogen (H), the polymer is polyethylene, a relatively low-melting material (137 °C, or 279 °F) that finds use as a fibre in industrial applications—e.g., nonwoven fabrics—but not in most household applications.

Still another variation is found when R represents a cyano, or nitrile, group (C≡N), containing carbon and nitrogen linked by a triple bond. In this case the polymer obtained is polyacrylonitrile, an acrylic that does not melt without decomposition and therefore must be solution-spun into fibres used in clothing, drapes, and carpets.

It can be observed from the structural variations noted above that the methyl and cyano groups in polypropylene and polyacrylonitrile raise melting points and alter solubility. At the same time, however, they are known to have a detrimental effect on tensile properties. For example, although fibres made from polypropylene can be very strong, their tensile strength is only about one-fourth that of the high-modulus polyethylene fibres.

Polyesters and polyamides

As noted in industrial polymers, chemistry of: Step-growth polymerization, one important route to the formation of polymers is the reaction of dicarboxylic acids with alcohols to form esters (containing CO―O groups) and with amines to form amides (containing CO―NH groups). The difference in properties produced by reacting with alcohols as opposed to amines can be illustrated by two structures.

In the first structure (above), when X represents oxygen (O), the polyester polyethylene terephthalate (PET) is obtained. Having a melting point of 265 °C (509 °F), PET can be melt-spun into very practical and cheap fibres that are widely employed in clothing, furnishings, carpets, and tire cord under such trademarked names as Dacron and Terylene. On the other hand, when X is an amine group (NH), a polyamide with a melting point greater than 400 °C (750 °F) is formed. This compound, polyethylene terephthalamide, can only be spun from solution, using costly solvents; therefore, it is not made into fibres.

In the second structure (above), when X represents oxygen, a very low-melting polyester called polyhexamethylene adipate, unsuitable for fibres, is obtained. When X represents an amine group, however, a useful polyamide, polyhexamethylene adipamide (nylon 6,6), is obtained. With a melting point of 265 °C (509 °F), nylon 6,6 can be melt-spun readily into fibres employed in apparel, carpets, and tire cord.

From the above illustrations, it is clear that the amide (CO―NH) groups produce much higher melting points than do the ester (CO―O) groups, even when the overall structures of the polymers are otherwise identical. The reason for this is that the CO―NH combinations are capable of a type of chemical bonding known as a hydrogen bond. Hydrogen bonds can produce bonds between polymer chains that are similar to the covalently bonded cross-links found in network polymers. They are not covalent bonds, however, and do not form true cross-links. In particular, the strength of the hydrogen bonds diminishes with the application of heat or solvent, allowing the polymers to be spun from the melt or from solution.

Very high melting points and oxidatively stable bonds can be produced when the CO―NH groups of the polyamide structures illustrated above are combined with aromatic hydrocarbons. When these stiff, ring-shaped molecules take the place of the more flexible CH₂ groups, very high-melting aromatic polyamides, or aramids, are obtained. Better known by the trademarks Kevlar and Nomex, aramids are made into flame-resistant clothing, bulletproof vests, tire cord, and stiffening reinforcement for composite materials used in large structures such as boat hulls and aircraft parts. The structures of these two compounds are shown below.

Cellulose-based polymers

Cellulose, a complex carbohydrate that is the basic structural component of the plant cell wall, is the most abundant polymer on earth. The basic structure of cellulose and its derivatives is shown below.

In unaltered native cellulose, X represents hydrogen, forming a number of pendant hydroxyl (OH) groups. Hydroxyl groups, like amides, are capable of forming hydrogen bonds. Partly as a result of such bonds, native cellulose behaves much like a cross-linked polymer, melting only with chemical decomposition—and therefore precluding melt-spinning into fibres. On the other hand, cellulose can be spun from solution when the OH groups are converted to other groups. For instance, rayon fibres can be formed by converting the OH groups to xanthate groups (e.g., O―CS―S―Na; an organic salt containing oxygen, carbon, sulfur, and sodium) in a basic solution prior to spinning and then converting the xanthate groups back to OH groups by spinning the dissolved compound into an acidic bath. Substitution of an acetyl group (O―CO―CH₃) for the OH group leads to a material that can be spun from a simple solvent such as acetone. These fibres are known as cellulose acetate, or simply acetate.