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PHYS 342: Materials Science: Natural Polymers

Connects students in PHYS 342 with information on Materials Science and resources in the library.

Amber

Amber


Structure

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Properties

Amber is heterogeneous in composition, but consists of several resinous bodies more or less soluble in alcohol, ether and chloroform, associated with an insoluble bituminous substance. Amber is a macromolecule by free radical polymerization of several precursors in the labdane family, e.g. communic acid, cummunol, and biformene. These labdanes are diterpenes (C20H32) and trienes, equipping the organic skeleton with three alkene groups for polymerization. As amber matures over the years, more polymerization takes place as well as isomerization reactions, crosslinking and cyclization.

Heated above 200 °C (392 °F), amber suffers decomposition, yielding an oil of amber, and leaving a black residue which is known as "amber colophony", or "amber pitch"; when dissolved in oil of turpentine or in linseed oil this forms "amber varnish" or "amber lac"


Applications

info to come

Cellulose

Cellulose


Structure 


Properties

Cellulose has no taste, is odorless, is hydrophilic with the contact angle of 20–30, is insoluble in water and most organic solvents, is chiral and is biodegradable. It can be broken down chemically into its glucose units by treating it with concentrated acids at high temperature.

Cellulose is derived from D-glucose units, which condense through β(1→4)-glycosidic bonds. This linkage motif contrasts with that for α(1→4)-glycosidic bonds present in starch, glycogen, and other carbohydrates. Cellulose is a straight chain polymer: unlike starch, no coiling or branching occurs, and the molecule adopts an extended and rather stiff rod-like conformation, aided by the equatorial conformation of the glucose residues. The multiple hydroxyl groups on the glucose from one chain form hydrogen bonds with oxygen atoms on the same or on a neighbor chain, holding the chains firmly together side-by-side and forming microfibrils with high tensile strength. This confers tensile strength in cell walls, where cellulose microfibrils are meshed into a polysaccharide matrix.

Cotton fibres represent the purest natural form of cellulose, containing more than 90% of this polysaccharide.

Compared to starch, cellulose is also much more crystalline. Whereas starch undergoes a crystalline to amorphous transition when heated beyond 60–70 °C in water (as in cooking), cellulose requires a temperature of 320 °C and pressure of 25 MPa to become amorphous in water.

Several different crystalline structures of cellulose are known, corresponding to the location of hydrogen bonds between and within strands. Natural cellulose is cellulose I, with structures Iα and Iβ. Cellulose produced by bacteria and algae is enriched in Iα while cellulose of higher plants consists mainly of Iβ. Cellulose in regenerated cellulose fibers is cellulose II. The conversion of cellulose I to cellulose II is irreversible, suggesting that cellulose I is metastable and cellulose II is stable. With various chemical treatments it is possible to produce the structures cellulose III and cellulose IV.

Many properties of cellulose depend on its chain length or degree of polymerization, the number of glucose units that make up one polymer molecule. Cellulose from wood pulp has typical chain lengths between 300 and 1700 units; cotton and other plant fibers as well as bacterial cellulose have chain lengths ranging from 800 to 10,000 units. Molecules with very small chain length resulting from the breakdown of cellulose are known as cellodextrins; in contrast to long-chain cellulose, cellodextrins are typically soluble in water and organic solvents.

Plant-derived cellulose is usually found in a mixture with hemicellulose, lignin, pectin and other substances, while bacterial cellulose is quite pure, has a much higher water content and higher tensile strength due to higher chain lengths.

Cellulose is soluble in Schweizer's reagent, cupriethylenediamine (CED), cadmiumethylenediamine (Cadoxen), N-methylmorpholine N-oxide, and lithium chloride / dimethylacetamide. This is used in the production of regenerated celluloses (such as viscose and cellophane) from dissolving pulp. Cellulose is also soluble in many kinds of ionic liquids.

Cellulose consists of crystalline and amorphous regions. By treating it with strong acid, the amorphous regions can be broken up, thereby producing nanocrystalline cellulose, a novel material with many desirable properties. Recently, nanocrystalline cellulose was used as the filler phase in bio-based polymer matrices to produce nanocomposites with superior thermal and mechanical properties.


Applications

Silk

Silk


Structure


Properties

Silk fibers from the Bombyx mori silkworm have a triangular cross section with rounded corners, 5–10 μm wide. The fibroin-heavy chain is composed mostly of beta-sheets, due to a 59-mer amino acid repeat sequence with some variations. The flat surfaces of the fibrils reflect light at many angles, giving silk a natural sheen. The cross-section from other silkworms can vary in shape and diameter: crescent-like for Anaphe and elongated wedge for tussah. Silkworm fibers are naturally extruded from two silkworm glands as a pair of primary filaments (brin), which are stuck together, with sericin proteins that act like glue, to form a bave. Bave diameters for tussah silk can reach 65 μm. Silk emitted by the silkworm consists of two main proteins, sericin and fibroin, fibroin being the structural center of the silk, and serecin being the sticky material surrounding it. Fibroin is made up of the amino acids Gly-Ser-Gly-Ala-Gly-Ala and forms beta pleated sheets. Hydrogen bonds form between chains, and side chains form above and below the plane of the hydrogen bond network. The high proportion (50%) of glycine allows tight packing. This is because glycine's R group is only a hydrogen and so is not as sterically constrained. The addition of alanine and serine makes the fibres strong and resistant to breaking. This tensile strength is due to the many interceded hydrogen bonds, and when stretched the force is applied to these numerous bonds and they do not break. Silk is resistant to most mineral acids, except for sulfuric acid, which dissolves it. It is yellowed by perspiration. Chlorine bleach will also destroy silk fabrics.


Applications

Wool

Wool


Structure 


Properties

Wool is a complicated weak fibre. The low tensile strength is because of comparatively fewer hydrogen bonds. When it absorbs moisture, the water molecules steadily force sufficient polymers apart to cause a significant number of hydrogen bonds to break. The water molecules also hydrolyze several salt linkages in the amorphous regions of the strand. Breakage and hydrolysis of these inter-polymer forces of attraction are explicit as swelling of the fibre and result in loss of strength of the wet woolen material. This is elastic and resilient. Covalent bonds can stretch, but they are strong. The disulphide bonds in the amorphous parts of the strand or fibre are able to stretch when the strand is extended. When the strand is released the disulphide bonds pull the protein molecules back into their original positions. If there are too few disulphide linkages as when the strand has been weakened by alkali or if the extension is great enough to break some of the covalent bonds, then some polypeptide chains will slide past one another. This causes a permanent extension of the wool. The natural crispness of the fibre also supports it to regain its real shape. It has the very absorbent nature because of the polarity of the peptide group, the salt linkages and the amorphous nature of the polymer system. The peptide groups and salt linkages easily attract water molecules which enter the amorphous polymer system of the fibre. In comparatively dry weather wool may develop static electricity. This is since these are hot enough. Water molecules in the polymer system support to distribute any static electricity which might develop. It has a comparatively low density and therefore fibres are light with regard to their visible weight. It has a low conductivity of heat and therefore makes it ideal for cold weather. The resiliency of the fibre is significant in the warmth properties of the fabric. Wool fibres do not pack well in yarns because of the crimp and scales, and this makes wool fabric process and capable of inserting much air. Air is one of the best insulators since it keeps body heat close to the body. The medulla of the wool fibre comprises air spaces that increase the insulating power of the fibre. This strand can take up moisture in vapor form. Absorbency is a factor also in the warmth of clothing. In winter, when people go from a dry indoor atmosphere into the damp outdoor air, the heat developed by the fibre in absorbing moisture keeps to protect their bodies from the impact of the cold atmosphere. It has poor dimensional stability and therefore shrinks easily. Felting or shrinkage results since under mechanical action, such as agitation, friction and pressure in the presence of heat and moisture, it tends to move root wards, and the edges of the scales interlock prohibiting the fibre from returning to its original position. This results in the fabric becoming thicker and smaller, that is it shrinks or felts.


Applications

Natural Rubber

Natural Rubber


Structure

Latex is the polymer cis-1,4-polyisoprene – with a molecular weight of 100,000 to 1,000,000 daltons. Typically, a small percentage (up to 5% of dry mass) of other materials, such as proteins, fatty acids, resins, and inorganic materials (salts) are found in natural rubber. Polyisoprene can also be created synthetically, producing what is sometimes referred to as "synthetic natural rubber", but the synthetic and natural routes are completely different. Some natural rubber sources, such as gutta-percha, are composed of trans-1,4-polyisoprene, a structural isomer that has similar, but not identical, properties. Natural rubber is an elastomer and a thermoplastic. Once the rubber is vulcanized, it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both; i.e., if it is heated and cooled, it is degraded but not destroyed. The final properties of a rubber item depend not just on the polymer, but also on modifiers and fillers, such as carbon black, factice, whiting, and a host of others.


Properties

Rubber exhibits unique physical and chemical properties. Rubber's stress-strain behavior exhibits the Mullins effect and the Payne effect, and is often modeled as hyperelastic. Rubber strain crystallizes.

Due to the presence of a double bond in each repeat unit, natural rubber is susceptible to vulcanisation and sensitive to ozone cracking.

The two main solvents for rubber are turpentine and naphtha (petroleum). The former has been in use since 1764 when François Fresnau made the discovery. Giovanni Fabbroni is credited with the discovery of naphtha as a rubber solvent in 1779. Because rubber does not dissolve easily, the material is finely divided by shredding prior to its immersion.

An ammonia solution can be used to prevent the coagulation of raw latex while it is being transported from its collection site.

On a microscopic scale, relaxed rubber is a disorganized cluster of erratically changing wrinkled chains. In stretched rubber, the chains are almost linear. The restoring force is due to the preponderance of wrinkled conformations over more linear ones.

Cooling below the glass transition temperature still permits local conformational changes but a reordering is practically impossible because of the larger energy barrier for the concerted movement of longer chains. "Frozen" rubber's elasticity is low and strain results from small changes of bond lengths and angles: this caused the Challenger disaster, when the AmericanSpace Shuttle's flattened o-rings failed to relax to fill a widening gap. The glass transition is fast and reversible: the force resumes on heating.

The parallel chains of stretched rubber are susceptible to crystallization. This takes some time because turns of twisted chains have to move out of the way of the growing crystallites. Crystallization has occurred, for example, when, after days, an inflated toy balloon is found withered at a relatively large remaining volume. Where it is touched, it shrinks because the temperature of the hand is enough to melt the crystals.

Vulcanization of rubber creates disulfide bonds between chains, which limits the degrees of freedom and results in chains that tighten more quickly for a given strain, thereby increasing the elastic force constant and making the rubber harder and less extensible.


Applications

Polymers

Polymers



 

Introduction

A polymer is a large molecule, or macromolecule, composed of many repeated subunits. Because of their broad range of properties, both synthetic and natural polymers play an essential and ubiquitous role in everyday life.

Learn more about polymers here and/or here. 


Applications

Elastomers - Rubber is the most important of all elastomers. Natural rubber is a polymer whose repeating unit is isoprene. This material, obtained from the bark of the rubber tree, has been used by humans for many centuries. It was not until 1823, however, that rubber became the valuable material we know today. In that year, Charles Goodyear succeeded in "vulcanizing" natural rubber by heating it with sulfur. In this process, sulfur chain fragments attack the polymer chains and lead to cross-linking. The term vulcanization is often used now to describe the cross-linking of all elastomers.

Plastics - Americans consume approximately 60 billion pounds of plastics each year. The two main types of plastics are thermoplastics and thermosets. Thermoplastics soften on heating and harden on cooling while thermosets, on heating, flow and cross-link to form rigid material which does not soften on future heating. Thermoplastics account for the majority of commercial usage.

Fibers - Fibers represent a very important application of polymeric materials, including many examples from the categories of plastics and elastomers. Natural fibers such as cotton, wool, and silk have been used by humans for many centuries. In 1885, artificial silk was patented and launched the modern fiber industry. Man-made fibers include materials such as nylon, polyester, rayon, and acrylic. The combination of strength, weight, and durability have made these materials very important in modern industry.

Information on the industrial applications of polymers can be found here


Processing

Once a polymer with the right properties is produced, it must be manipulated into some useful shape or object. Various methods are used in industry to do this. Injection molding and extrusion are widely used to process plastics while spinning is the process used to produce fibers.

Learn more about polymer applications and processing here

Download Ailton De Souza Gomes' book, New Polymers for Special Applications, here

 

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