Natural polymeric fibers

in order to understand the processing, structure, and properties of polymeric fibers, the main focus of Chapters 3 and 4, it will be useful to review some general and basic concepts regarding the structure of polymeric materials, and 

Polymers, natural or synthetic, are characterized by an extended chain structure. Giant molecules called macromolecules are formed by joining the chain elements in different ways. Each unit or building block of the chain is called a monomer and a polymer results when we join many monomers to form a chain. We describe below some important classes of polymers, their structure, and some of their important attributes. 

Polymers can be classified in many ways. An easy way to classify polymers is based on the processing used to make them. There are two main types of polymerization process  

We can also classify polymers based on their structure. The basic structural unit of a polymer is a flexible macromolecular chain (Fig. 3.1a). When there is no long-range molecular order and the flexible molecular chains are arranged 

Yet another type of classification of polymers is based on the type of repeating unit. A homopolymer has one type of repeat unit. Copolymers are polymers that have more than one type of monomers or repeat units. If the monomers in a copolymer are distributed randomly along the chain, it is called a regular or random copolymer. If, on the other hand, a sequence of one type of monomer is followed by a sequence of another type of monomer, it is called a block copolymer. If the main chain is one type of monomer and the branch chains are of another monomer, it is called a graft copolymer. 

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In much the same way, natural polymeric fibers like wool, cotton, silk, etc., are often touted as superior to anything that is man-made or synthetic. But is this fair There is no doubt that natural fibers have a unique set of properties that have withstood the test of time (e.g., it is difficult, but not impossible, to match silk s feel or cotton s ability to breathe ). On the other hand, consider Lycra , a completely synthetic fiber produced by DuPont (Figure 1-12) that has a truly amazing set of properties and is the major component of Spandex (a material that keeps string bikinis on ). Or consider the wrinkle-free polyester fibers used in clothing and the stain proof nylon and polyacrylonitrile polymers used in carpets. The point here is that polymers, be they natural" or synthetic, are all macromolecules but with different chemical structures. The challenge is to design polymers that have specific properties that can benefit mankind. 

Natural polymeric fibers can originate in the animal kingdom or vegetable kingdom. We give below a brief description of these. 

Natural polymeric fibers find extensive applications in all walks of life. We give some examples. 

Natural polymeric fibers are used for a variety of textile applications ranging from clothing to upholstery to sacks and cordage. Frequently, these are blended with synthetic fibers to obtain an optimum set of properties. Many naturally occurring fibers can be and are used in composites, but mostly in applications involving not very high stresses (Chawla, 1976 Chawla and Bastos, 1979 Roe and Ansell, 1985 Chand and Rohatgi, 1994)... 

Although naturally occurring ceramic or mineral fibers are not as common as natural polymeric fibers, there are two important examples, asbestos and basalt fibers. Asbestos fiber has some health hazards associated with it which have curbed its use. We provide a summary of these two natural ceramic fibers before describing synthetic ceramic fibers in more detail. 

Naturally occurring polymeric fibers possess interesting properties and have found widespread use in passive and active applications long before man-made polymers began to flourish. Well-known examples included cellulose, silk, cotton, etc. The AFM investigation of cellulose will be discussed as a prime example for these important natural polymeric fibers. 

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