فیزیک جوشکاری : معرفی پلیمرها مسعود مصلایی‌پور یزدی استادیار دانشکده مهندسی مواد و معدن، دانشگاه یزد 1

Polymers are formed by chemical reactions in which a large number of molecules called monomers are joined sequentially, forming a chain. In many polymers, only one monomer is used. In others, two or three different monomers may be combined. Polymers are a large class of materials consisting of many small molecules (called monomers) that can be linked together to form long chains, thus they are known as macromolecules. A typical polymer may include tens of thousands of monomers.

In the late 1830s, Charles Goodyear succeeded in producing a useful form of natural rubber through a process known as "vulcanization." Some 40 years later, Celluloid (a hard plastic formed from nitrocellulose) was successfully commercialized. Despite these advances, progress in polymer science was slow until the 1930s, when materials such as vinyl, neoprene, polystyrene, and nylon were developed. The introduction of these revolutionary materials began an explosion in polymer research that is still going on today. History of Polymers:

Characteristics of Polymers: Low Density Low coefficient of friction Good corrosion resistance Good mouldability Excellent surface finish can be obtained Can be produced with close dimensional tolerances Economical Poor tensile strength Low mechanical properties Poor temperature resistance Can be produced transparent or in different colors

The number of repeating units in one large molecule is called the degree of polymerization. Materials with a very high degree of polymerization are called high polymers. Polymers consisting of only one kind of repeating unit are called homopolymers. Copolymers are formed from several different repeating units. Repeating unit: homopolymers and Copolymers

Polymerization process: :: فرایند پلیمریزاسیون :

. There are both naturally occurring and synthetic polymers. Among naturally occurring polymers are proteins, starches, cellulose, and latex. On the other hand most of the organic substances found in living matter, such as protein, wood, chitin, rubber, and resins, are polymers. Synthetic polymers are produced commercially on a very large scale and have a wide range of properties and uses Many synthetic materials, such as plastics, fibers (; Rayon), adhesives, glass, and porcelain, are also to a large extent polymeric substances. ☼ The materials commonly called plastics, (comes from the Greek word "plastikos“), are all synthetic polymers. Many objects in daily use from packing, wrapping, and building materials include half of all polymers synthesized. Other uses include textiles, TV's, CD's, automobiles, and many other all are made from polymers. Naturally and synthetic polymers:

Polymers are classified by the characteristics of the reactions by which they are formed. Additional Polymers and Condensation polymers Classification of polymers according to their forming reaction : 1- If all atoms in the monomers are incorporated into the polymer, the polymer is called an addition polymer.

In the first stage, a substance is split into two identical parts, each with an unpaired electron. (Peroxides, which contain an O-O bond, are often used in this role.) A molecule with an unpaired electron is called a free radical. The free radical then initiates the reaction sequence by forming a bond to one of the carbon atoms in the double bond of the monomer. One electron for this new bond comes from the free radical, and the second electron for the new bond comes from one of the two bonds between the carbon atoms. The remaining electron from the broken bond shifts to the carbon atom on the far side of the molecule, away from the newly formed bond, forming a new free radical. Each half-headed arrow indicates the shift of one electron. The chain begins to grow--propagate, stage two-- when the new free radical formed in the initiation stage reacts with another monomer to add two more carbon atoms. This process repeats over and over again to form chains containing thousands to millions carbon atoms. It can be terminated--stage three--when any two free radicals combine, thus pairing their unpaired electrons and forming a covalent bond that links two chains together. Addition polymer

If all of the atoms attached to the carbons of the monomers double bond are hydrogen atoms, the initial reactant is ethylene, and the polymer it forms is polyethylene. Addition polymer

2- If some of the atoms of the monomers are released into small molecules, such as water, the polymer is called a condensation polymer. Condensation polymers are made from monomers that have two different groups of atoms which can join together to form, for example, ester or amide links. Polyesters are an important class of commercial polymers, as are polyamides (nylon).

Polymers may be classified as the mechanical response at elevated temperatures : 1. Thermoplasts 2. Thermosets Classification based upon the physical property related to heating a) Thermoplasts: Thermoset polymers soften when heated and harden when cooled. Simultaneous application of heat and pressure is required to fabricate these materials. On the molecular level, when the temperature is raised, secondary bonding forces are diminished so that the relative movement of adjacent chains is facilitated when a stress is applied. Most Linear polymers and those having branched structures with flexible chains are thermoplastics. Thermoplastics are very soft and ductile. The commercial available thermoplasts are Polyvinyl Chloride (PVC) and Polystyrene Polymethyl methacrylate Polystyrene

b) Thermosets: Thermosetting polymers become soft during their first heating and become permanently hard when cooled. They do not soften during subsequent heating. Hence, they cannot be remolded/reshaped by subsequent heating. In thermosets, during the initial heating, covalent cross-links are formed between adjacent molecular chain. These bonds anchor the chains together to resist the vibration and rotational chain motions at high temperatures. Cross linking is usually extensive in that 10 to 15% of the chain mer units are cross linked. Only heating to excessive temperatures will cause severance of these crosslink bonds and polymer degradation. Thermoset polymers are harder, stronger, more brittle than thermoplastics and have better dimensional stability. They are more usable in processes requiring high temperatures Most of the cross linked and network polymers which include Bakelite, which is used in toasters, handles for pots and pans, dishes, electrical outlets and billiard balls. Vulcanized rubbers Epoxies Phenolic Polyester resins are thermosetting. Thermosets cannot be recycle, do not melt, are usable at higher temperatures than thermoplastics, and are more chemically inert

Applications of Polymers: Agriculture and Agribusiness Polymeric materials are used in and on soil to improve aeration, provide mulch, and promote plant growth and health. Medicine Many biomaterials, especially heart valve replacements and blood vessels, are made of polymers like Dacron, Teflon and polyurethane. Consumer Science Plastic containers of all shapes and sizes are light weight and economically less expensive than the more traditional containers. Clothing, floor coverings, garbage disposal bags, and packaging are other polymer applications. Industry Automobile parts, windshields for fighter planes, pipes, tanks, packing materials, insulation, wood substitutes, adhesives, matrix for composites, and elastomers are all polymer applications used in the industrial market. Sports Playground equipment, various balls, golf clubs, swimming pools, and protective helmets are often produced from polymers.

Example of Polymers: Types of polymers Commodity plastics PE = Polyethylene, PS = Polystyrene, PP = Polypropylene, PVC = Poly(vinyl chloride), PET = Poly(ethylene terephthalate) Specialty or Engineering Plastics Teflon (PTFE) = Poly(tetrafluoroethylene), PC = Polycarbonate (Lexan), Polyesters and Polyamides (Nylon)

Some Addition polymers

Some Condensation polymers