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Polymer Fundamentals:
Fiber Applications

Polymer science addresses the chemistry and physics of large, chain-like molecules. As with the molecules themselves, this technical pursuit is diverse and complicated. The following discussion provides an introduction to the manufacture and use of synthetic organic polymers for those with some knowledge of basic science. More advanced tutorial information on polymers is contained in the links displayed at the end. Click here to go directly there.

What is a Polymer?
The term “polymer” is derived from the Greek “poly”, meaning “many”, and “mer”, meaning “parts” — thus polymers are substances made of “many parts”. In most cases the parts are small molecules which react together hundreds, or thousands, or millions of times. A molecule used in producing a polymer is a “monomer” — mono is Greek for single, thus a monomer is a “single part”. A polymer made entirely from molecules of one monomer is referred to as a “homopolymer”. Chains that contain two or more different repeating monomers are “copolymers”.

The resulting molecules may be long, straight chains, or they may be branched, with small chains extending out from the molecular “backbone”. The branches also may grow until they join with other branches to form a huge, three-dimensional matrix. Variants of these molecular shapes are among the most important factors in determining the properties of the polymers created.

The size of polymer molecules is important. This is usually expressed in terms of molecular weight. Since a polymeric material contains many chains with the same repeating units, but with different chain lengths, average molecular weight must be used. In general, higher molecular weights lead to higher strength. But as polymer chains get bigger, their solutions, or melts, become more viscous and difficult to process.

Proteins and Carbohydrates
Life as we know it could not exist without polymers. Proteins, with large numbers of amino acids joined by amide linkages, perform a wide variety of vital roles in plants and animals. Carbohydrates, with chains made up of repeating units derived from simple sugars, are among the most plentiful compounds in plants and animals. Both of these natural polymers are important for fibers. Proteins are the basis for wool, silk and other animal-derived filaments. Cellulose as a carbohydrate occurs as cotton, linen and other vegetable fibers. The properties of these fibers are limited by the form provided in their natural state. Some, like linen and silk, are difficult to isolate from their sources, which makes them scarce and expensive. There are, of course, many other sources of proteins and cellulose, such as wood pulp, but natural polymers tend to be very difficult to work with and form into fibers or other useful structures. The inter-chain forces tend to be strong because of the large number of polar groups in the molecular chains. Thus, natural polymers usually have melting points that are so high that they degrade before they liquefy.

The most useful molecules for fibers are long chains with few branches and a very regular, extended structure. Thus, cellulose is a good fiber-former. It has few side chains or linkages between the sugar units forcing its chains into extended configurations. However, starches, which contain the same basic sugar units, do not form useful fibers because their chains are branched and coiled into almost spherical configurations.

Synthetic Polymers
Synthetic polymers offer more possibilities, since they can be designed with molecular structures that impart properties for desired end uses. Many of these polymers are capable of dissolving or melting, allowing them to be extruded into the long, thin filaments needed to make most textile products. Synthetic polymer fibers can be made with regular structures that allow the chains to pack together tightly, a characteristic that gives filaments good strength. Thus, filaments can be made from some synthetic polymers that are much lighter and stronger than steel. Bullet-proof vests are made from synthetic fibers.

There are two basic chemical processes for the creation of synthetic polymers from small molecules (1) condensation, or step-growth polymerization, and (2) addition, or chain-growth polymerization.

Step-Growth Polymerization
In step-growth polymerization, monomers with two reactive ends join to form dimers (two “parts” joined together), then “trimers” (three “parts”), and so on. However, since each of the newly formed oligomers (short chains containing only a few parts) also has two reactive ends, they can join together; so a dimer and a trimer would form a pentamer (five repeating “parts”). In this way the chains may quickly great length achieve large size. This form of step-growth polymerization is used for the manufacture of two of the most important classes of polymers used for textile fibers, polyamide (commonly known as nylon), and polyester.

There are many different commercial versions of polyester in a wide variety of applications, including plastics, coatings, films, paints, and countless other products. The polymer usually used for textile fibers is poly(ethylene terephthalate), or PET, which is formed by reacting ethylene glycol with either terephthalic acid or dimethyl terephthalate. Antimony oxide is usually added as a catalyst, and high vacuum is used to remove the water or methanol byproducts. High temperature (>250oC) is necessary to provide the energy for the reaction, and to keep the resultant polymer in a molten state.

PET molecules are regular and straight, so their inter-chain forces are strong — but not strong enough to prevent melting. Thus, PET is a “thermoplastic” material; that is, it can be melted and then solidified to form specific products. Since its melting point is high, it does not soften or melt at temperatures normally encountered in laundering or drying. Another important property of PET is its Tg, or “glass transition temperature”. When a polymer is above its glass transition temperature, it is easy to change its shape. Below its Tg, the material is dimensionally stable and it resists changes in shape. This property is very important for textile applications because it allows some fibers, and the fabrics made from them, to be texturized or heat-set into a given shape. This can provide bulk to the yarn, or wrinkle resistance to the fabric. These set-in shapes remain permanent as long as the polymer is not heated above its Tg. Because its chains are closely packed and its ester groups do not form good hydrogen bonds, polyesters are also hydrophobic (i.e., they do not absorb water). This property also requires special dyeing techniques.

There are also many important classes of synthetic polyamides (nylons) and they have a wide variety of commercial uses. These are usually distinguished from each other by names based on the number of carbon atoms contained in their monomer units. As with polyesters, polyamides are formed by step-growth polymerization of monomers possessing two reactive groups. Here, the reactive functions are acids and amines. The monomers used may have their two reactive functions of the same chemical type (both acids, or both amines), or of different types. Thus, nylon 6,6 — a very common fiber polymer — is made by reacting molecules of adipic acid (containing six carbons in a chain, with an acid function at each end) with hexamethylene diamine (also six carbon atoms, with amine functions at each end). In another variant the diamine contains ten carbons atoms, the product designated nylon 6,10.

The other common polyamide fiber polymer is nylon 6. Its monomer has six carbons in the chain, with an amine at one end and an acid at the other. Thus only one form of monomer is needed to conduct the reaction. Commercial production of nylon 6 makes use of caprolactam, a derivative which provides the same result.

As with the polyesters, nylons have regular structures to allow good inter-chain forces that impart high strength. Both nylon 6 and nylon 6,6 have melting points similar to PET but they have a lower Tg. Also, since the amide functions in nylon chains are good at hydrogen bonding, nylons can be penetrated by water molecules. This allows them to be dyed from aqueous media, unlike their polyester counterparts.

In addition to nylon, there is another commercially important group of synthetic polyamides. These are the aramids, which contain aromatic rings as part of their polymer chain backbone. Due to the stability of their aromatic structures and their conjugated amide linkages, the aramids are characterized by exceptionally high strength and thermal stability. Their usefulness for common textile applications is limited by their high melting points and by their insolubility in common solvents. They are expensive to fabricate, and they carry an intrinsic color that ranges from light yellow to deep gold.

Other step-growth polymers — the polyurethanes — are produced by the reaction of polyols and polyisocyanates. For fiber purposes, this class of linear polymers is formed from glycols and diisocyanates. Usually, the reactions are carried out to form block copolymers containing at least two different chemical structures — one rigid, and the other flexible. The flexible segments stretch, while the rigid sections act as molecular anchors to allow the material to recover its original shape when the stretching force is removed. Varying the properties of the segments, and the ratio of flexible to rigid segments controls the amount of stretch. Fibers made in this way are classified as spandex and they are used widely in apparel where stretch is desirable.

Chain-Growth Polymerization
Chain-growth polymerization occurs when an activated site on a chemical, such as a free radical or ion, adds to a double bond, producing a new bond and a new by activated location. That location then attacks another double bond, adding another unit to the chain, and a new reactive end. The process may be repeated thousands, or millions, of times, to produce very large molecules. This is usually a high energy process and the intermediate species are so reactive that, in addition to attacking available monomer, they also may attack other chains, producing highly branched structures. Since these branches prevent the molecules from forming regular structures with other molecules, their inter-chain forces are weak. The resulting polymers tend to be low-melting and waxy.

The breakthrough in making chain-growth polymers useful for fibers and for most commercial plastics came with the development of special selective catalysts that drive the production of long, straight polymer chains from monomers containing basic carbon-to-carbon double bonds.

Ethylene and propylene form the simplest chain-growth polymers. Since their polymer chains contain no polar groups, these polyolefins must rely on close contact between the molecular chains for strength. Thus, the physical characteristics of polyethylene are very sensitive to even a small number of chain branches. Very straight chains of polyethylene can form strong crystalline structures which exhibit exceptional strength. Protective fabrics made from this type of highly structured polyethylene are virtually impossible to penetrate or cut.

Polypropylene is more complicated. Even without chain branching, each monomer unit adds one methyl group pendant to the chain. The arrangement of these side groups is described as the “tacticity” of the polymer. A random arrangement is considered “atactic”, or without tacticity. Regular arrangement with all side groups on one side of the chain is “isotactic”, and a regular alternating structure is “syndiotactic”. Polypropylene molecules can only pack closely in an isotactic arrangement. Synthesis of these polymers was a major challenge, but several stereoselective catalysts are now available, and high-density polypropylene has become a commodity product. Fibers made from it are lightweight, hydrophobic and highly crystalline. Their resistance to wetting gives them good moisture wicking and anti-staining properties. This also makes them virtually undyeable, except when the dye is applied to the polymer in its molten state — a process know as “solution dyeing”.

By contrast, the pendant nitrile functions in polyacrylonitrile are sufficiently polar to produce very strong inter-chain forces. Pure homopolymers from acrylonitrile are non-thermoplastic and difficult to dissolve or dye. Thus, for most commercial acrylonitrile polymers, small amounts of other monomers with bulky side chains are introduced to force the chains apart, to reduce the inter-chain forces. Common co-monomers for these fiber applications include vinyl chloride, vinyl acetate, acrylic acid, and methyl acrylate.

Specialty fiber polymers
There are also a number of complex, specialty fiber polymers with methods of synthesis that are not easily classified. These materials are occasionally used in high performance materials where the complex structures impart exceptional strength, thermal stability, electrical conductivity, and others desirable properties. They include PBI (polybenzimidazole) and sulfar. Their chemistry is beyond the scope of this introductory discussion.

Polymer Links
For more extensive information on polymers go to the following sites:
 The Macrogalleria  Polymer Chemistry Hypertext
 Molecular Arts Corporation  Polymer Liquid Crystals
 Polymers Dot Com  
   
. . . and for other relevant information go to our links page.

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