By: Dr. Murtaza Najabat Ali ( CEng MIMechE P.E)

1. INTRODUCTION TO POLYMERS By: Dr. Murtaza Najabat Ali (CEng MIMechE P.E)

2. POLYMERS Background • Materials are the most important basic requirement for many applications. The application of biomedical materials started from the stone age (ceramics), passed through the bronze and iron (metals age), now has entered the age of polymers. • Biomaterials field as a whole is developing step by step with the development of materials from one category to the other • However,, other categories of materials are being replaced by the youngest member (i.e. Polymers) in almost all the spheres of material applications Stone age → Bronze age → Iron age → Polymer age

3. POLYMERS Introduction • Polymer is a Greek word, meaning (Poly) ‘Many’ ------ (Mers) ‘Parts’ • They are also called macromolecules (giant or large molecules) • Large molecules of organic (mainly) constituents which are formed by covalent bonding of a large number of repeating units of identical structure • Addition or subtraction of a few such units does not change the properties • The process of formation of polymer from the reactive simple molecules is known as Polymerization, and these simple molecules, which are forming repeating units are known as Monomers (which may be self-reactive or mutually reactive)

4. POLYMERS Introduction • The number of such repeating units present in the structure of each polymer molecule is known as Degree of Polymerization (DP) • These are utilized to determine the Molecular weight (MW) of the polymer, as MW is one of the most important governing factors for their properties • This is determined by the following equation: MW(of the polymer) = Molecular Mass (of repeating unit) x DP e.g. Polyethylene (C2H4)nwith average DP (i.e. n = 1000) has the average Molecular weight MW of (1000 x 28)g/mol = 28,000 g/mol n

5. POLYMERS Introduction • It is not so simple in the case of polymers, where more than one repeating unit types are there (Copolymer will be discussed later) • In such cases, the MW of the polymer is obtained by summing up the MW of segments for each repeating unit by using the corresponding DP and MW of that repeating unit e.g. A copolymer of ethylene (C2H4) and propylene(C3H6) with average DPof ethylene unit is 500 and propylene unit is 300 in each chain molecule, then the average MW of the polymer is (500 x 28 + 300 x 42) g/mol = 26,600 g/mol Ethylene Propylene

6. POLYMERS Introduction • The concept of polymer can be easily understood from the concept of a wall of a new building, which is constructed by bricks and cement • Now, if a wall is taken as a resemblance to a polymer molecule then like the wall, a polymer molecule is constructed by the combination of so many monomer molecules (like bricks in a wall) through covalent bonding (like cement in the wall). • The number of repeating unit (DP) is the same as the number of bricks in the wall. As the number of repeating units increases, the polymer molecules become larger. • But as a wall has the desired dimension as per the requirement, so it is also necessary to obtain the chain length as per the requirement for its optimum level of properties

7. POLYMERS Uniqueness The phenomenal growth of polymers is due to their versatility with respect to structure and properties and hence their applications. The main significances of polymers are as follows: Versatile with respect to the feed stock Versatile with respect to the properties, e.g. from very rigid to flexible to very flexible (soft) material Ease of processing and fabrication High speed of mass production A wide range of color abilities Low overall cost Light in weight

8. POLYMERS Basic Differences between Polymers and other Materials • The longevity of most of the non-biodegradable polymers is very high compared to metals and ceramics • Even though, the strength properties of most of the polymers are much lower than metals/ceramics, but combined properties are better than metals/ceramics (such as flexibility, transparency, resilience etc.) • The most significant difference comes from their weight differences (low density) • The processabilityand fabrication of polymers are much easier (as they can be easily softened on heating and are soluble in different media) • The presence of different functionalities and adequate solubility in different media make polymers easily modifiable to get different sets of properties/application. • As already stated, the feedstock of any polymer offers a wide range of choice, which is not true for metals/ceramics

9. POLYMERS Glossary • Constitutional repeating unit (CRU) • The smallest constitutional unit, the repetition of which constitutes a regular macromolecule, a regular oligomer molecule, a regular block, or a regular chain. • Main chain (backbone) • That linear chain to which all other chains, long or short or both, may be regarded as being pendant. • End-group • A constitutional unit that is an extremity of a macromolecule or oligomer molecule. • Subunit • The largest main-chain (backbone) segment of the CRU that can be named as a single unit. This may be a ring or ring system, a heteroatom or a homogeneous heteroatom chain.

10. POLYMERS

11. POLYMERS Nomenclature • It is very important /essential to nomenclate a newly developed polymer to identify it and to differentiate it • There are different ways to nomenclate the polymers • These are mainly either Source-based, Structure-based or sometimes Trade names are also used. • The term “Poly” is commonly introduced in the beginning as the prefix to the name of the monomers, when their names can be expressed in a single word, e.g. Polyethylene, Polypropylene, Polystyrene etc. • When the name of the monomer is expressed by more than one word OR a word with a number, then the name of the polymer is enclosed by a parentheses preceded by the term “Poly”, e.g. poly(vinyl chloride), poly(4-chloro styrene) • The above mentioned nomenclatures are Source-based Source based: Poly(vinyl chloride) Structure Based: Poly(1-chloro ethylene) Source based: Polystyrene Structure Based: Poly(1-phenyl ethylene)

12. POLYMERS Nomenclature • Homopolymers (having only one type of monomer/repeating units) are nomenclated by writing the prefix ‘Poly” followed by the name of the monomer used • Copolymers (having more than one type of monomers/repeating units) are nomenclated by writing the prefix ‘Poly” followed by the name of the monomers. An infix called connective is placed between the two monomers, which indicates the type of sequential arrangement of the repeating units in the polymer chains • The prefix “Copoly” followed by the name of the monomers, a solid stroke “/” in between two monomers and the type of copolymer at the beginning, is sometimes used for Copolymers e.g. Poly(lactic-co-glycolic acid) and graft-copoly(isoprene/methacrylic acid)

13. POLYMERS Nomenclature • The steps to be followed when naming a polymer are:

14. POLYMERS Nomenclature • The steps to be followed when naming a polymer are:contd… • e.g. • Structure of the polymer chain • Selection of the preferred CRU The preferred CRU will, therefore be either OR Further choice is based on the locant for substitution, so that the preferred CRU is

15. POLYMERS Nomenclature • Naming the preferred CRU • The name of the preferred CRU is formed by citing the order in which they appear in the CRU. Therefore, the oxygen atom (Oxy) and the -CH2CH2- is called Ethylene, and ethylene unit is substituted with one bromine atom is called 1-bromoethylene • The preferred CRU is therefore, named as oxy(1-bromoethylene) • Naming the polymer The name of the polymer is simply the name of the preferred CRU enclosed in squarebrackets, or braces and prefixed by Poly The polymer is named poly[oxy(1-bromoethylene)]

16. POLYMERS Classification Polymers can be classified into different categories based on their: Source Mode of Formation Main Chemical Linkages Structure Thermal Response Nature of Monomer Strength and Physical Properties

17. POLYMERS Classification • Source Based on the source of the polymers they can be classified into the following three classes: Natural: They are also termed as Biopolymers, e.g. natural rubber, wool, cellulose, proteins etc. Semisynthetic: e.g. epoxidized natural rubber and nitrocellulose Synthetic: e.g. polyethylene, polypropylene, polyurethane etc.

18. POLYMERS Classification • Mode of Formation Based on this criterion, the polymers can be classified into two categories, namely: Addition: The Addition polymers are also known as Chain Growth polymers. When the monomer(s) molecules are directly added together by covalent bond to form the corresponding polymer without loss of any atoms or small molecules as By-products in the polymerization process, then the polymer is known as Addition or Chain Growth polymer. Therefore, the MW of the polymer is direct product of DP and molecular mass of monomer (repeat unit), e.g. polyethylene, polybutadiene, poly(vinyl chloride) etc. Condensation: They are also known as Step- Growth polymers. When the monomer(s) molecules are combined together by covalent bond after elimination of small molecules such as H2O, NH3, HCl, NaCl etc. as the By-products, the polymers are termed as Condensation or Step Growth polymers, as the polymer chains are formed in Step-wise manner. In this case the repeating units are different from the monomer(s) or reactant(s) molecules, thus they have different Molecular formulas. Therefore, MW is only obtained by the product of DP and molecular weight of repeating unit, not the monomer, e.g. poly(ethylene terephthalate)

19. POLYMERS Classification • Main Chemical Linkages • This type of classification is based on the main chemical linkages formed during the polymerization process of polymer • Since 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. • e.g. • Polyether (such as Poly(ethylene glycol)) • Polyhydrocarbon (such as Polypropylene) • Polyester (such as Poly(ethylene terephthalate)) • Polysilicone (such as Silicone rubbers)

20. POLYMERS Classification • Structure • Again, the structures of a polymer influence the properties and hence, its applications • Based on their structures, the polymers can be classified by the following criterion: Classification based on Line representation Classification based on their long range order Classification based on the chemical constituents present Classification based on Line structural representation HDPE Linear ----- the skeleton of the polymer’s structure is represented by simple line (mainly zigzag fashion) of finite lengths, e.g. linear polyethylene Branched ---- In this case, the structure can be represented by the linear finite length with short or long branch lines. This branched lines may arise due to uncontrolled reaction, deliberate grafting with other monomer or even designing of a polymer where each repeat unit generates a new branching point. This new class of 3D symmetric polymers with regular branching point is known as Dendritic polymer LDPE The branching increases the volume and thus reduces the density of the polymer.

21. POLYMERS Classification • Structure • Sometimes the ends of several polymer chains are joined together at a common center. • Polymers like this are called star polymers. • They're often used as additives or as coating materials. • Sometimes there is no backbone chain at all. • Sometimes a polymer is built in such a way that branches just keep growing out of branches and more branches grow out of those branches. • These are called Dendrimers, from the ancient Greek word for "tree".

22. POLYMERS Classification • Structure c) Cross-inked ---- They are 3D giant molecules that are formed in the finished state • Sometimes, both ends of the branch chains are attached to the backbone chains of separate polymer molecules. • If enough branch chains are attached to two polymer molecules, it can happen that all of the polymer backbone chains in a sample will be attached to each other in a giant 3-D network. • This is what happens in certain hydrogels, rubber, silicone and certain polyurethanes.

23. POLYMERS Classification • Structure 2. Classification based on their long range order • The polymer can also be classified into two different classes depending on their long range order structures, i.e. the degree of Crystallinity in the structures Crystalline ---- If the polymers have certain degree of Crystallinity in their structuresdu to the orderness of some segments of polymer chains, they are termed as Crystalline polymers Amorphous (or non-crystalline) ---- Most of the polymers do not have orderness in their structures and hence, they do not have any degree of Crystallinity. These polymers are known as Amorphous

24. POLYMERS Classification • Structure 2. Classification based on the chemical constituents present • The polymer can also be divided into two different classes depending on the chemical constituents present in their structures Organic ---- If the carbon atoms are present in the main chain of the polymers along with the other elements, such as H, O, N etc. then they are termed as organic polymers. Most of the polymers are included in this class Inorganic---- If the main chain of the polymers are formed by inorganic elements such as Si, P, S, N etc. and do not have any carbon atom in the main chain, they are known as inorganic polymers. However, in the side chain carbon containing moiety may be present. (Inorganic) Polysiloxanes or Silicone

25. POLYMERS Classification • Thermal Response The thermal response (i.e. the response of heat energy) towards any polymer is very significant compared to any other categories of materials. This is mainly due to the organic nature of polymers. Based on this criterion, they can be classified into two different classes Thermoplastic ---- These polymers are repeatedly softened to offer desired shape on heating with some precautions (the temperature of heating on specified pressure should be well below the decomposition temperature of the polymer, e.g. polyethylene, polypropylene etc. Thermosetting ---- These polymers are soluble and fusible and can be easily flowed in their intermediate state (resinous state). But once these polymers are set through chemical crosslinking reactions to form 3D network structures on exposure of heat or high energy radiation, with or without crosslinking agent, they are transformed into insoluble and infusible, dimensionally stable mass, which are never changed to their original state without chemical degradation, e.g. elastomers

26. POLYMERS Classification • Nature of monomer/ Type of repeating unit • This classification is based on the different types of repeating units present in the molecular chain of polymers. Homopolymer ---- 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, e.g. polyethylene, poly(vinyl chloride) etc. Copolymer ---- 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, e.g. poly(ethylene-co-propylene) etc.

27. POLYMERS Classification • Nature of monomer/ Type of repeating unit • Homopolymer : -A-A-A-A-A-A-A-A- • Random copolymer :    -A-B-B-A-B-A-A-B- • Alternating copolymer : -A-B-A-B-A-B-A-B- • Block copolymer :      -A-A-A-A-B-B-B-B- • Graft copolymer :     -A-A-A-A-A-A-A-A- Again monomers are further classified into: B-B-B-B-B-

28. POLYMERS Classification • Physical Properties On the basis of their physical properties, polymers can be classified into three classes: Rubber/Elastomer Plastic Fiber Rubber bands Textile yarn made of nylon fibers Plastic (rigid) basket Elastomeric (flexible) duct tape