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POLYMERS: BASIC CONCEPTSLecture-I Dr.Rabiul Hussain School of Material Science & Engineering Jimma Institute of Technolgy, Jimma University E-mail: rabiul786@gmail.com Ph.No. +251-0966882081 (Ethiopia) +91-9508832510 (India) DR.RABIUL HUSSAIN
What is Polymer? • The word polymer is derived from the Greek poly and meros, meaning many and parts, respectively. • Some scientists prefer to use the word macromolecule, or large molecule, instead of polymer. • Polymers are macromolecules built up by the linking together of large numbers of much smaller molecules. • The small molecules that combine with each other to form polymer molecules are termed monomers, and the reactions by which they combine are termed polymerizations.
How big is Polymer? • There may be hundreds, thousands, tens of thousands, or more monomer molecules linked together in a polymer molecule. • When one speaks of polymers, one is concerned with materials whose molecular weights may reach into the hundreds of thousands or millions.
Unique Features of Polymers • Versatile with respect to the feed stock. The monomers or starting materials for polymers can be obtained from the resources like petroleums, forest or even agriculture, etc. • Versatile with respect to properties. • Very light in weight. • Excellent strength properties. • Unique flexibility. • Very high specific strength *(sp. strength=strength/sp.gravity). • Corrosion resistant. • Excellent tailorability. • Ease of processing and fabrication. • Low overall cost. • Adaptability.
History of Polymers • The first synthetic polymer, a phenol-formaldehyde resin, was invented in the early 1900s by Leo Baekeland. • Wallace Carothers invented very important polymers of neoprene rubber and Nylon in 1930s which shaped the leadership of DuPont in polymer industry. • Hermann Staudinger developed theoretical explanations of remarkable properties of polymers by ordinary intermolecular forces between molecules of very high molecular weight. He was awarded the Nobel Prize in Chemistry in 1953 for this outstanding contribution. • World War II led to significant advances in polymer chemistry with the development of synthetic rubber as natural rubber was not accessible to the Allies. • Karl Ziegler and Giulio Natta won the Nobel Prize in Chemistry in 1963, jointly for the development of coordination polymerization to have controlled stereochemistry of polymers using coordination catalysts. • In 1977, Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa reported high conductivity in iodine-doped polyacetylene. This research earned them the 2000 Nobel Prize in Chemistry. Since then, the application of polymer has expanded into active functional area such as light emitting diode, sensor, solar cell, etc.
Important Terminology POLYMER:
Repeating Unit: A repeating unit is general kept inside a square bracket of a polymer structure.
Origin/source • Natural: The polymers directly obtained from natural origin like plants, animals,etc. fall in this category. They are biodegradable. • Semisynthetic:The natural polymers which are chemically modified to improve properties for different applications fall in this category. • Synthetic: They are completely made by chemical reactions from their respective monomer(s).This class of polymers is completely “man-made” and their numbers are huge.
Thermal Response Classification of Polymers Based on Thermal Response • Classification based on Thermal Response • Thermoplastic Polymers: These polymers are those which softens on application of heat and hardens on cooling without any change in chemical composition. The process of hard to soft , soft to hard by heating and cooling and vice versa is reversible. • Polyethylene, polypropylene, saturated polyesters, polyamides, polycarbonates, polystyrenes, nylons, polyacrylates, polyurethanes, etc. are all thermoplastic polymers. Thermoplastic Polymers Thermosetting Polymers
THERMOPLASTIC POLYMERS • Thermoplastic polymers are recyclable. • No cross-links between chains. • Weak attractive forces between chains broken by heating. • Change shape - can be remoulded. • Weak forces reform in new shape when cold. POLYETHYLENE, POLYPROPYLENE POLYSTRYRENE POLYCARBONATES NYLONS POLYACRYLATES POLYURETHANES POLYAMIDES POLYESTERS PVC
THERMOSETTING POLYMERS • Thermosets can be defined as those polymers which become irreversibly hard on heating or by addition of special chemicals. This hardening involves a chemical change (curing) and hence scrap thermoset cannot be recycled except as a filler material. The curing process invariably involves a chemical reaction which connects the linear molecules together to form a single macromolecule. These connections are known as crosslinks. • During curing, the small molecules are chemically linked together to form complex inter-connected network structures. This cross-linking prevents the slippage of individual chains. Therefore, the mechanical properties (tensile strength, compressive strength, and hardness) are not temperature dependent, as compared to thermoplastics. Hence, thermosets are generally stronger than the thermoplastics. • The different types of thermosets are Alkyds, Allylics, Amine, Bakelite, Epoxy, Phenolic (PF), Unsaturated Polyester, Silicone, Polyurethane (PUR), and Vinyl Ester.
Condensation Polymers Condensation polymers are formed by a series
Classification based on Line structure • Linear Structure: The monomers are linked together to form linear chains . The vanderwaals interaction takes place between the linear chains. Some common examples are nylon,high density polyethylene, polyvinyl chloride, polyester. • Branched Structure: The monomers are joined to form long chains with branches of different lengths . For examples: low density polyethylene, glycogen and starch. • Cross-linked Structure: The monomers are combined together to form cross-linked chains. Chains are connected by covalent bonds. For example: rubber, bakelite.
Tacticity • Isotactic polymers have highest strength and lowest solubility. • Atactic polymers have lowest strength and highest solubility. • Syndiotactic have intermediate strength and solubility.
Based on Type of Repeating Unit • This classification is based on the different types of repeating units present in the molecular chain of the polymers.According to this criterion, the polymers are classified into two different classes: • Homopolymers: When a polymer is formed from only one type of monomer i.e. in its structure only one type of repeating unit is present, then the polymer is a homopolymer. Polyethylene, poly(vinyl chloride), polybutadiene, etc. are examples of such class of polymers. • Copolymers: when polymers are formed from more than one type of monomers or more than one type of repeating units are present in the structure of the polymers, then they are termed as copolymers.This type of polymers are also known as heteropolymers. The examples of copolymers are poly(ethylene-co-propylene), poly(styrene-co-butadiene), poly(acrylonitrile-co-butadiene-co-styrene), etc.
Type of Repeating Units • Depending on the arrangements of the repeating units in the structure of the copolymers, they can again be classified into four different classes. • Random Copolymers: when two or more numbers of repeating units (say A and B) are arranged randomly in the chain of the polymers, they are known as random cpolymers. • Alternating Copolymers: When two or more repeating units (say A and B) are alternately arranged in the chain of the copolymers, then they are termed as the alternating copolymers. The preparation of such copolymer is very difficult. • Block Copolymers: When the repeating units (say A and B) are arranged as block in the polymer chain, the copolymers are termed as block copolymers. • Graft Copolymers: This type copolymers essentially consist of two or more homopolymers in such a fashion that one homopolymer is covalently attached (grafted) as a long chain branch , to the backbone of the other homopolymer.
Classification based on Physical Properties • On the basis of their physical properties, polymers can be classified into three classes: • Rubber or Elastomer: This class of polymers can easily undergo very high and reversible elongation under relatively low applied stress, i.e. they have long range of elasticity. This is the reason they are also known as elastomers. Characteristics of elastomers are as follows: • They are high molecular weight amorphous polymers with low Tg (below room temperature), high chain flexibility and low secondary forces among the chain molecules. • The mechanical strength of rubbers is also low because of low intra/inter molecular force of attraction. • The dimensional stability can be obtained through chemical crosslinking, called vulcanization or through blending with other plastic , called thermoplastic elastomerfor their end applications. • Natural Rubber(NR), polybutadiene (PB), polychloroprene (CR), styrene butadiene copolymer (SBR), etc. are few examples of such class of polymers.
Based on Physical Properties • Plastic: This polymers possess much higher intramolecular force of attraction than rubbers, but extensibility is lower than elastomers. • The strength properties of plastics are higher but elongation at break is lower than rubbers. • They have sufficient dimensional stability and hence, there is no need to crosslink them for their end-uses. • They possess certain degree of crystallinity and Tg is above room temperature. • Their processing is comparatively easier than elastomers as they become softer on heating without decomposition and transformed to dimensionally stable, hard materials on cooling. • They can be easily recycled and reused. • Examples are: PE, PP, PS, PVC, etc.
Based On Physical Properties • Fibers: These polymers have the highest strength and crystallinity but the lowest elongation among these classes of polymers. • These polar polymers have very high intra/inter molecular attraction forces among the chain molecules through polar-polar, polar-induced polar, H-bonding, etc. along with the van der Waals forces. • The Tg is much higher than the room temperature. • They must be drawn into filaments (fibres) by solution or by melt spinning technique. Thus fibre forming polymers are highly symmetric, linear and high molecular weight polymers. • Nylon 6, nylon6,6, kevlar, PET etc. are few examples of fibre forming polymers.
Based on type of Linkages • The main chemical linkage formed during the polymerization process of polymer is the basis of this classification. As the linkages formed are very high in a polymer, so the ultimate properties of the polymer are largely influenced by the nature of the linkages. • Based on this criterion, the polymers can be classified into different classes which are tabulated below:
Class of Polymers according to main linkages CONTINUED….
