Saturday, April 6, 2013

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

Marie-Therese Wisniowski

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

Today is our first post on the general properties of fiber polymers and fibers. 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 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
Fiber 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

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
When you consider the different fabrics used for clothing, furnishing and in geotextiles, it is easy to identify the different properties between different types of fabrics. Understanding why different types of fabrics have different properties brings us to fiber chemistry. I will try to make this as painless as possible, but inevitably - where there is no pain there is no gain!

A generic classification of fibers is based on the type of building blocks they are made from. These building blocks are called molecules, which are composed of a collection of atoms held together by various forces. For example, cotton and linen are composed of cellulose molecules, and so have many features in common, whereas wool and silk are composed of molecules called proteins, and so the latter two fibers have similar properties, but possess properties that are different from those composed of cellulose fibers.

Electron micrograph of cotton fibers.


Forces That Hold Atoms Together To Form Molecules
The building blocks of all polymers that make up a fiber are derived from units that are molecular in nature. Molecules are composed of atoms that are held together by forces, which are generically called - bonding mechanisms. There are several types of bonding mechanisms, and these in turn impart many of the properties of molecules, which in turn manifest in the properties of fibers and that of fabrics.

Covalent Bonds
Covalent bonds are characterized by atoms that share their electrons in order to form molecules. For example, the water molecule consists of two covalent bonds - each is formed from one electron from the oxygen atom (symbol “O”) hooking onto an electron from the hydrogen atom (symbol “H”).

Schematic of a water molecule (symbol - H2O). The covalent bonds are represented in the schematic above by two lines. Each bond holds two electrons, one from the oxygen (O) atom and one from the hydrogen (H) atom.

Covalent bonds are usually very strong and so it takes a lot of energy to break them. For example, to break up one mole of liquid water (H2O(l)) into hydrogen gas (H2(g)) and oxygen gas (O2(g)) via:

H2O(l) → H2(g)) + 1/2 O2(g))

takes 68,320 calories at 25oC to achieve such a break-up.

Diamond is composed of carbon-carbon covalent bonds and it is even more stable than liquid water.

A diamond is held together by numerous covalent bonds and so can be thought of as a large covalent molecule.

In general, molecules containing single bonds are said to be “saturated” (e.g. as in saturated fats) and are generally less reactive than molecules containing double bonds (e.g. carbon dioxide contains two double bonds - it is a green-house gas).

Schematic of covalent bonding in carbon dioxide (symbol: CO2). Here each linkage, represented by a single line, is formed by one electron from oxygen atom and one electron from a carbon atom (represented by symbol C) hooking together. There is a total of eight electrons involved in four bonds, creating two double bonds. It is an unsaturated molecule.

Molecules containing double bonds are more firmly held together, but they are more susceptible to react with other chemical entities, since the most loosely held electrons are very reactive. Hence, they are called “unsaturated” (as in unsaturated fats) since they contain more than just a single bond between each atom.

Ionic Bonds
There are atoms that are hungry for loosely held electrons (i.e. have a high electron affinity or electron attraction e.g. chlorine – symbol: Cl) and there are atoms that do not have an appetite for them (i.e. have a low electron affinity or electron attraction e.g. sodium – symbol: Na). When these two unlike atoms come close to each other, the electron (which has a negative charge) migrates from the atom with a low affinity to the atom with a high affinity. Hence when sodium and chlorine get together they form a molecule, which is bonded together because of its ionic nature; that is Na+Cl-. (Note: the attraction is called an electrostatic or ionic attraction).

If we flow chlorine gas (symbol:Cl2) over sodium solid we get,

2NaSOLID + 2Cl2(gas) → 2Na+Cl-SOLID

The solid form - Na+Cl-SOLID - is known as common table salt, a perservative you sprinkle on your food. Its crystal structure is given below, where the green dots represent the chlorine atom and the blue dots the sodium atom. Note: The chlorine anion (Cl-) is physically larger in size compared to the sodium cation (Na+).

Crystal structure of sodium chloride – common table salt.

Sodium chlorine or table salt can readily dissolve in water because the water molecules surround the sodium cation (Na+) and the chloride anion (Cl-) keeping them well apart and so screening their attraction for each other (see below).

Hydrogen Bonds
Liquid water is composed of water molecules that are held together due to the water covalent bonds being unequally shared by the oxygen and hydrogen atoms. The electrons in the covalent bond spend more time close to the oxygen atom and so less time near the hydrogen atoms since the oxygen atom has a higher electron affinity compared with the hydrogen atom.

The uneven electron affinity means that there is a partial negative pole on the oxygen atom and a partial positive pole on the hydrogen atoms. Hence the water molecule can be represented as below.

Polarity in the water molecule. The unequal time spent by electron on each atom is represent by a “+” and “–“ sign. A “+” sign represents less time spent by the electrons and so it creates a partial positive pole, whereas “-“ sign means more time spent and so it creates a partial negative pole.

When two of more water molecules are near to each other, they orientate themselves to maximize the forces of attraction between them, forming what is called “hydrogen bonds”.

Two water molecules orientating themselves in order to maximize their attraction to each other. Note: The δ sign indicates a partial loss or gain of the electrons’ time near the respective atom. The negative pole is attracted to the positive pole forming a hydrogen bond. The hydrogen bond is the dashed line between a negative pole oxygen atom (O) and a positive pole hydrogen atom (H).

Liquid water is made of millions-upon-millions of water molecules positioning themselves to maximize their attraction to each other. It is the millions upon millions of hydrogen bonds that make water a liquid and not a gas.

Orientation of a section of a water droplet. Note how the water molecules position themselves in water to maximize their attraction. Here for clarity we have not shown the partial negative and positive poles.

Millions of water molecules make up a drop of liquid water.

Hydrogen bonding is strong, since it takes 100oC in order to break the hydrogen bonds in liquid water in order to produce steam (which is composed of individual water molecules).

Water molecules can effectively break down ionic compounds such as table salt by separating the ions from each other and shielding them from each other. This severely reduces the positive sodium ions (Na+) from electrostatically being attracted to the negative chloride ions (Cl-).


The blue dots are the oxygen atoms and the red dots are the hydrogen atoms in the water molecule. The silver and green dots are the sodium cation and the chloride anion of NaCl, which is common table salt. Note: Table salt is broken down by liquid water since the polarity of the water, enables the water molecules to surround each ion and so negate the attraction of the sodium cations to the chloride anions.

Hydrogen bonds may occur within other molecular systems, such as in ethanol - the alcohol you drink!.

Ethanol is a liquid due to hydrogen bonding. It is completely miscible (i.e. totally soluble in all proportions) in water. That is, there will be no visible layer between a mixture of ethanol and water, whereas there is a visible layer between petrol and water.

In summary, hydrogen bonds are formed between hydrogen and oxygen atoms, and hydrogen and nitrogen atoms on adjacent molecules or in polymer systems when they are less than 0.5 x 10-9 meters apart. It should be noted that the hydrogen-oxygen hydrogen bond is stronger than the hydrogen-nitrogen hydrogen bond. The bond strength is of the order of 20.9 kJ mol-1. Compared to ionic or covalent bonds its relative bond strength is very weak. It is a bonding mechanism that occurs between molecules, or more specifically between polymers of natural, regenerated cellulose, nylon, polyvinyl alcohol, polyester, protein and secondary cellulose acetate fibers.

Hydrogen bonds are mainly responsible for the tenacity and the elastic-plastic nature of natural, regenerated cellulose, nylon, polyvinyl alcohol and protein fibers. They contribute significantly towards the heat setting property of nylon and protein fibers.

Hydrogen bonds occurring in a polymer system of polyester fibers are very weak and not considered to be important. Insignificant hydrogen bonds are formed in the polymer system of secondary cellulose fibers. There is no doubt about hydrogen bond formation in the polymer systems of acrylic and mod-acrylic fibers.

van der Waals Forces
van der Waals forces are very weak forces of attraction, named after the Dutch physicist Johannes Diederik van der Waals, who first postulated their existence when studying the weak attractions associated with gas molecules.

They are a particular form of a more generic force of attraction called dispersion forces. To explain these forces accurately we need to resort to quantum mechanics, a field of study well outside the scope of most fiber artists!

These forces are very weak in nature since they involve the electrostatic attraction of neutral molecules (e.g. attraction between organic solids such as in fibers). They become an important attraction mechanism when the molecules that make up the fibers are held by mechanical and other means, very close to each other. For example if a dye pops into an empty space (called a void) between fibers, the molecules that make up the fiber can attract the dye by this electrostatic mechanism. Fiber molecules (called polymers – see below) need to be at least 2x10-10 meters apart in order for this force to come into play.

van der Waals’ forces formed between both sets of atoms in the curly brackets: (a) Two adjacent or very closely aligned polyvinylidene chloride polymers (see below); (b) Two adjacent or closely aligned polyethylene polymers.

The influence of the difference in strength of van der Waals’ forces in the above two fibers is illustrated by their melting points. In general, the stronger the inter-molecular forces of attraction the higher the melting points of the fiber systems. Hence,

Fiber Melting Point Range
Polyethylene 110 – 140oC
Polyvinyl Chloride 170 – 200oC

van der Waals’ forces are also formed between the fiber molecules and dye molecules when these molecules come close enough together. In this way, van der Waals’ forces contribute to color-fastness of dyed or printed fabric fibers.

In summary, van der Waals’ forces are formed between atoms along the length of adjacent polymers (see below) when these are less than 0.3 x 10-9 meters apart but no closer than 0.2 x 10-9 meters. The strength of the interaction is of order of 8.4 kJ mol-1. It occurs between the molecules or more specifically, the polymers of all fibers. They are the only inter-molecular or more precisely the inter-polymer force of attraction existing in the polymer system of polyethylene, polypropylene, polyvinylchloride, polyvinylidene chloride, primary cellulose acetate and 100% polyacrylonitrile fibers. These forces are considered to be the predominate inter-polymer force of attraction in the polymer system of attraction in the polymer system of acrylic, mod-acrylic and polymer fibers.

Salt Bridges
These bonding mechanisms or forces are also called salt linkages. They are electrovalent or ionic bonds. Salt linkages occur between positively and negatively charged radicals on very close or adjacent fiber polymers. Radicals are entities that have an unpaired electron available for further possible reaction and because electrons are a negatively charge entity, radicals are shown with a small negative sign. Ozone is a radical molecule.

In the formation of a positively charged radical, an electron is lost by the radical, leaving a remaining unpaired electron. Losing electrons causes the radical to become positively charged. The number of small positive signs shown on the radical indicates not only its positive charge but also the number of electrons it has lost. In this case the radical is called an ion.

Salt bridges can be formed between a carboxyl radical (-C-O-) on one fiber polymer and a positively charged or protonated amino group (-NH3+) on an adjacent polymer.

Salt bridge between two fiber polymers.

These salt bridges are said to be ionic bonds or electrovalent bonds. The latter is a composite word “electro” meaning “electric”, because of the strongly positive or negative charges, which must always be present. The “valent“ part of the word springs from the fact that these charges arise from the most outer part of the electronic system of the molecular entity.

If the radicals of the fiber polymers carry only one charge (i.e. one negative and one positive sign as shown above) then they will form only one electrovalent bond or salt linkage.

They are called salt linkages because ionic or electrovalent bonds are formed between the ions and/or radicals of chemical compounds called salts (see table or common salt). They form strong bonds, typically having bond energy or bond strength of 54.5 kJ mol-1. The bonding mechanism occurs between polymers of protein and nylon fibers. They contribute towards the tenacity, elastic-plastic nature and durability of the fiber. They attract water molecules and so enhance the hygroscopic nature of fiber (e.g. hydrophilic nature of the fiber, making it more absorbent and hence, more comfortable to wear). They also attract the anion of acid dyes and so they are very good dye sites. However, they may make the fiber’s polymer system liable to chemical degradation, since salt bridges make the fiber system more reactive than those without these inter-polymer forces of attraction.


Conclusion
You now have some rudimentary understanding of the main bonding mechanisms that occurs between and within polymers that make up fibers and which are responsible for the properties that they possess. Hopefully, you have not been mentally damaged by this brief journey into the world of Chemistry!

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).