Saturday, May 4, 2013

The General Properties of Fiber Polymers and Fibers
Fiber Chemistry - Part II [1-2]
Art Resource

Marie-Therese Wisniowski

Preamble
This is the fifteenth post in the "Art Resource" series, specifically aimed to construct an appropriate knowledge base in order to develop an artistic voice in ArtCloth.

Other posts in this series are:
Glossary of Terms
Units Used in Dyeing and Printing of Fabrics
Occupational, Health & Safety
A Brief History of Color
The Nature of Color
Psychology of Color
Color Schemes
The Naming of Colors
The Munsell Color Classification System
Methuen Color Index and Classification System
The CIE System
Pantone - A Modern Color Classification System
Optical Properties of Fiber Materials
General Properties of Fiber Polymers and Fibers - Part I
General Properties of Fiber Polymers and Fibers - Part II
General Properties of Fiber Polymers and Fibers - Part III
General Properties of Fiber Polymers and Fibers - Part IV
General Properties of Fiber Polymers and Fibers - Part V
Protein Fibers - Wool
Protein Fibers - Speciality Hair Fibers
Protein Fibers - Silk
Protein Fibers - Wool versus Silk
Timelines of Fabrics, Dyes and Other Stuff
Cellulosic Fibers (Natural) - Cotton
Cellulosic Fibers (Natural) - Linen
Other Natural Cellulosic Fibers
General Overview of Man-Made Fibers
Man-Made Cellulosic Fibers - Viscose
Man-Made Cellulosic Fibers - Esters
Man-Made Synthetic Fibers - Nylon
Man-Made Synthetic Fibers - Polyester
Man-Made Synthetic Fibers - Acrylic and Modacrylic
Man-Made Synthetic Fibers - Olefins
Man-Made Synthetic Fibers - Elastomers
Man-Made Synthetic Fibers - Mineral Fibers
Man Made Fibers - Other Textile Fibers
Fiber Blends
From Fiber to Yarn: Overview - Part I
From Fiber to Yarn: Overview - Part II
Melt-Spun Fibers
Characteristics of Filament Yarn
Yarn Classification
Direct Spun Yarns
Textured Filament Yarns
Fabric Construction - Felt
Fabric Construction - Nonwoven fabrics
A Fashion Data Base
Fabric Construction - Leather
Fabric Construction - Films
Glossary of Colors, Dyes, Inks, Pigments and Resins
Fabric Construction – Foams and Poromeric Material
Knitting
Hosiery
Glossary of Fabrics, Fibers, Finishes, Garments and Yarns
Weaving and the Loom
Similarities and Differences in Woven Fabrics
The Three Basic Weaves - Plain Weave (Part I)
The Three Basic Weaves - Plain Weave (Part II)
The Three Basic Weaves - Twill Weave
The Three Basic Weaves - Satin Weave
Figured Weaves - Leno Weave
Figured Weaves – Piqué Weave
Figured Fabrics
Glossary of Art, Artists, Art Motifs and Art Movements
Crêpe Fabrics
Crêpe Effect Fabrics
Pile Fabrics - General
Woven Pile Fabrics
Chenille Yarn and Tufted Pile Fabrics
Knit-Pile Fabrics

The Glossary of Terms, Timelines of Fabrics, Dyes and Other Stuff, A Fashion Data Base, Glossary of Colors, Dyes, Inks, Pigments and Resins, Glossary of Fabrics, Fibers, Finishes, Garments and Yarns and Glossary of Art, Artists, Art Motifs and Art Movements have been updated in order to better inform your art practice.

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Introduction
We have dealt with the various bonding mechanisms involved in fiber chemistry. Specifically we covered the following bonding mechanisms:
(i) Covalent Bonds.
(ii) Ionic Bonds.
(iii) Hydrogen Bonds.
(iv) van der Waals Forces.
(v) Salt Linkages.

Fibers are composed of polymers and so we need to get a feel for the chemistry of fiber polymers and how the bonding mechanisms manifest themselves within this milieu.


Polymerization
Fibers are made up of molecules. Fiber molecules belong to a special class of molecules called polymers. The word itself is derived from the Latin poly meaning “many” and mer meaning “unit”. Hence a polymer is composed of many molecular units. The unit part of a polymer is called a monomer which is also derived from the Latin word mono meaning “one”.

Electron microscope image of a wool fiber.

At the molecular level polymers are extremely long and linear (like a piece of string). Nevertheless they can be branched (as in trees). A monomer is a very small portion of a polymer. Monomers are usually chemically reactive whereas polymers tend to be unreactive. Nevertheless, this does not prevent polymers being subsequently attached by chemicals and susceptible to other degrading agents such as UV light. It should be noted that polymerization is a chemical reaction, which causes monomers to join end-to-end to form a polymer.

The polymerization of ethene to form poly(ethene).

The length of a polymer is most important. All fibers, both man-made and natural, have long to extremely long polymers. The length of a fiber polymer is difficult to measure. Estimates of the length of a polymer can be made by determining the degree of polymerization. The mathematical expression for this is:

Degree of polymerization = (average molecular weight of the polymer)/(molecular weight of the repeating in the polymer)

Note: Some fiber molecules may have been polymerized from two or more different monomers; thus, the repeating unit is the combination or segment formed by one, two or more different monomers, which repeat regularly along the length of the polymer.

Polymers and their repeating units. PP= Polypropylene (see below). MR is the molecular weight of the repeating unit.

To give this formula some substance, the degree of polymerization of cotton is 5,000. This is interpreted as that on an average each cotton polymer consists of about 5,000 repeating units. The repeating unit of cotton is a monomer called cellobiose.

Chemical structure of cellobiose.

Imagine the cotton polymer being as thick as an ordinary 8mm pencil, the polymer length would be about 50m long. Of course polymers do not have this actual size, but rather it gives you an idea of the relative width-to-length dimension of a polymer.

Although it is not yet known how cellulose and keratin are polymerized in nature, the polymerization of man-made synthetic monomers to polymers is well understood. The manufacture of the synthetic, polymeric materials, which will be extruded to form the synthetic, man-made filament is categorized by two types of polymerization - addition and condensation polymerization.

Addition polymerization requires that monomers add or join end-to-end without liberating any by-product on polymerization. Some fibers consisting of addition polymers are acrylic, mod-acrylic, polyethylene or polyethene (abbreviation PE), polypropylene or polypropene (PP), polyvinyl alcohol and chlorofibers, namely polyvinyl chloride (PVC) and polyvinylidene chloride.

Addition reaction to produce polyvinyl chloride. Note: The right hand formula has an "n" as a subscript. This "n" is a large integer number and so illustrates that this unit is repeated a large number of times to form the polymer.

Condensation polymers are monomers that join end-to-end and liberate the by-product. This by-product is usually a simple compound – generally water – but it may alternatively be hydrogen chloride or ammonia, depending upon the specific monomers involved. Some fibers consisting of condensation polymers are elastomeric, nylon and polyester.

Condensation reaction producing polyamide nylon. Note: In this reaction H2O is the by product.

Not all fibers fit into these two classifications. The polymers of acetate, cotton, flax, silk, triacetate, viscose and other regenerated cellulose fibers and wool, do not fit readily into the above classification, because not enough is known as yet about the way their polymers are synthesized in nature.


Types of Polymers
There are various types of polymers that make up fibers used to fashion fabrics and cloths.

Homopolymer
Such a polymer is polymerized from the same or only one kind of polymer (note: homos is Greek for “same”). Some homopolymer fibers are: nylon 6, nylon 11, polyethylene, polypropylene, plyvinyl chloride, polyvinylidene chloride, polyacrylonitrile (as distinct from acrylic and mod-acrylic).

Repeating pattern of a homopolymer. For simplicity sake, we will represent the monomer by the letter "A". A homopolymer is therefore a chain composed of letters "A".

Copolymer
Such a polymer is polymerized from two or more different monomers. These include such sub-categories as:
(i) Alternating copolymer – usually two monomers polymerized in alternating sequence. Some alternating copolymers are nylon 6.6 and polyester.
(ii) Block polymer – polymerized in blocks or segments before linking up to form the polymer. Block polymers are still largely experimental.
(iii) Graft polymer – polymerized in such manner that a segment, polymerized from two or more different monomers used, attaches itself as a side-chain or forms a branch of the polymer. Zefran, an acrylic graft polymer fiber that has dye-receptive groups on a back-bone of heat-resistant polyacrylonitrile.
(iv) Random copolymers – polymerize in no particular order. They tend to be polymerized mainly from only two different monomers. Some random co-polymers are acrylic or mod-acrlic polymers.

Repeating pattern of a particular copolymer.
1. Homopolymer.
2. Alternating polymer.
3. Random polymer.
4. Block polymer.
5. Graft polymer.


Intra-polymer Bonding
Intra-polymer bonding means the bonds that hold atoms together to make up the fiber polymer. The fiber polymer is the smallest portion of a substance capable of existing independently and retaining the properties of the original substance.

Fiber polymers are mostly organic compounds, the exceptions being the molecules comprising the man-made inorganic fibers and the natural mineral fibers. Describing fiber polymers as organic compounds signifies that they are predominately composed of carbon (C) and hydrogen (H) atoms, with some oxygen (O), nitrogen (N), chlorine (Cl) and/or fluorine (F) atoms.

The main bonding mechanisms between atoms in fiber polymers are covalent in nature (see Part I). There are certain categories of covalent bonds that can be characterized in terms of chemical groups as follows.

Amide or Peptide Groups
When these collections of atoms are present in polymers it is called the amide group. When this same chemical group is present in silk, wool, mohair and all animal or protein fibers it is called a peptide group. The group is formed via an acid and alkaline hydrolysis.

The formation of an amide or peptide group in a fiber polymer. Note: The loss of water (H2O) to form the peptide bond.

The hydrolysis of a peptide group represents the most common chemical destructive attack upon the amide or peptide group during the life of the polymer, and hence the fiber.

Alkaline hydrolysis or saponification of the amide or peptide group: hydrolysis occurs at a single covalent bond, which exists between carbon (C) and nitrogen (N) atoms.

Phenyl Groups
Phenyl groups are based on the benzene molecule, which is a hexagon and is often referred to a ring structure. It is composed of carbon and oxygen atoms and because of the outer most electrons being delocalized over the entire ring, it makes this structure very stable and so unreactive. On the other hand, side groups attached to the ring may be very reactive.

Monomer unit of a polymer. The CH2CH- is a side group to the benzene like group. The circle inside the hexagon indicates the most loosely held electrons are delocalized over the ring.

Ether Linkages
The ether linkage may be found in such polymers as cellulose, elastomeric, ester-cellulose and polyesters. It exists between carbon and oxygen atoms as follows.

Ether linkage involves a carbon (C) atom linked to an oxygen atom (O), which is in turn linked to a carbon atom.

The linkage obtains its name from family of organic compounds called ethers – the most well known to the public is anaesthetic ether.

Ethers are chemically quite unreactive basically because of the great stability of the carbon-oxygen bond found in every ether molecule. In general, an ether linkage in fiber polymer tends to be the most durable and least affected by degrading agents. There is one important exception to this and that is the ether linkage in cellulose known as the glucoside link, which links glucose units. Under acidic conditions the glucoside linkage will undergo hydrolysis and part as shown below.

Acid hydrolysis of cellulose polymer.

This is the reason why acids have such a destructive effect on cellulosic fibers.

Ester Group
Esters may be regarded as organic equivalents of salts. In fiber polymers this is usually the reaction between:
(i) A carboxyl group (i.e. –COOH). It is an organic group typically occurring in organic acids such as acetic acid, formic acid, citric acid (known generically as carboxylic acids).
(ii) A hydroxyl group (i.e. –OH), which is the characteristic group of alcohol (e.g. ethanol has one –OH group per molecule). In polyhydric alcohols, such as cellulose, have more than one –OH group per molecule.

The figure below shows the formula of the ester group.

Alkali hydrolysis or saponification of an ester group in a fiber polymer. Note: saponification means soap making and the sodium carboxylate group is typical a water-soluble end-group of all soaps made from natural oils or fats with sodium hydroxide.

The group is susceptible to attack by alkalis (which is called saponification or alkaline hydrolysis). The saponification of the ester group produces water-soluble and reactive end-groups, leading to further degradation of the polymer and hence the fiber (see above figure).

Alkaline hydrolysis or saponification of the polyacrylonitrile fiber polymer; the above indicates how polyacrylonitrile in acrylic and mod-acrylic fibers can become degraded by alkalis.

Hydroxyl Goup
It is designated as –OH group and is attached by a single covalent bond as a side group to fiber polymers (see cellubiose).

The presence of this group on fiber polymers has a two-fold significance:
(i) The –OH group is polar and will therefore attract water molecules, which are also polar, thereby promoting moisture absorbency of the fiber and hence its comfort when worn.
(ii) The polarity of the –OH groups will give rise to the formation of hydrogen bonds (see above) and so these bonds will promote the coherence of the fiber’s polymer system.

The presence of the –OH groups are so important that these groups are artificially introduced in hydrophobic synthetic fiber polymers.

Nitrile Group
These are designated by –CN and are a polymer side group of acrylic and mod-acrylic fibers. In general the nitrile group does not react with acids or break down in acidic conditions. However, it is subject to alkaline hydrolysis or saponification, as would occur during normal laundering. Under normal circumstances this is not very noticeable, because of the crystallinity or “good” orientation of the polymer system of the acrylic fiber allows such hydrolysis to occur only on the surface of the fiber.


Inter-polymer Bonding
Fibers are composed of polymers. The coherence of the polymer system of a fiber is due to inter-polymer forces of attraction. We have already dealt with three of them in Part 1 namely:
(i) Hydrogen bonding.
(ii) van der Waals forces.
(iii) Salt linkages.

These inter- or shall we say “in-between polymer attractions” together help to constitute a fiber. The only one we have not yet covered is cross linkages, which are very important inter-polymer bonding mechanisms.

Cross-links
These linkages occur between the polymers of elastomeric and protein fibers – but not silk. Cross links are single covalent bonds that link carbon atoms, occasionally joined by oxygen and nitrogen atoms, to form the back-bone of the fiber polymer systems.

Single covalent bonds linking atoms that form the backbone of the fiber polymer system.

Single covalent bonds occur not only within fiber polymers, but also at selected points between the polymers in such fibers as elastometric, wool, mohair, but not in silk. When single covalent bonds link adjacent polymers, they are called cross-links.

The number of cross-links between polymers in a polymer system is termed by the degree of cross-linking. Thus the degree of cross-linking in the polymer system of a fiber significantly influences the fiber’s elastic-plastic nature and tenacity. In general, the greater the degree of cross-linking, the stiffer, less flexible and more rigid the fiber. Fibers such as wool have a low degree of cross-linking and have good to very good elastic properties. When the degree of cross-linking is even lower such as in elastomerics, the elastic property will be between very good to excellent.

Synthetic resins, applied to fabrics to make them more easy-care, have very highly cross-linked polymers. Such resins make the fabric very stiff in order to achieve an easy-care property. In fact, the cross-linking in these resins are so high that they can be thought of as three-dimensional cross-linkages (length, breadth and depth).

Cross-linkages can be broken and reformed under controlled conditions for heat and chemical setting of wool and other protein fibers but not silk.

Cross-links are formed between adjacent polymers of wool due to the di-sulfide bond (S-S) or cross-link. Note: 0.1 nm is 1 x 10-10 meters.


Conclusion
We have now completed all the fiber chemistry you will ever need! I hope you have survived this onslaught.

References:
[1] E.P.G. Gohl and L.D. Vilensky, Textile Science, Longman Cheshire, Melbourne (1989).
[2] A Fritz and J. Cant, Consumer Textiles, Oxford University Press, Melbourne (1986).

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