Engineering Materials Polymeric materials Polymer == many parts Polymeric material:- is a material that contains many chemically bonded parts or units that themselves are bonded together to form a solid. In this subject we shall study some aspects of the; 1- Structure 2- Properties 3- Processing, 4- applications, of two industrially important polymeric materials Polymeric materials 1. Plastics Thermoplastics Thermosetting plastics(Thermosets) 2. Elastomers (rubbers) Elastomers (rubbers) Can be elastically deformed when a force is applied to them and can return to their original shapes (or almost) when the force is released.

Engineering Materials Polymeric materials Polymer == many parts Thermoplastics A plastic martial that requires heat to make it formable, and after cooling, remains its shape. Thermoplastic can be repeatedly softened when heated and harden when cooled. Typical thermoplastics are polyethylene, vinyls, acrylics, cellulosics, and nylons. thermoplastic pens thermoplastics objects thermoplastic resin.

Engineering Materials Polymeric materials Polymer == many parts Thermosetting plastics Usually formed into a permanent shape by heat and pressure during which time a chemical reaction takes place that bonds the atoms together to form a rigid solid. Some thermosetting reactions take place at room temperature without the use of heat and pressure. Thermosetting plastic cannot be remelted after they are set or cured and upon heating to a high temperature, they degrade or decompose. CAN NOT BE RECYCLED . Typical thermosetting plastics are phenolics, unsaturated polyesters and epoxies. The linear chains are crosslinked - strongly chemically bonded. This prevents a thermoplastic object from being melted and reformed

Engineering Materials Polymeric materials Polymer == many parts Polymerization reactions Polymerization reactions:- a chemical reaction in which many small molecules are covalently bonded together to form very long molecular chains. Monomers:- a simple molecular compound that can be covalently bonded together to form long molecular chains (polymers). For example ethylene. Polymer:- is the long chain molecule formed from the monomer units, example polyethylene.

Engineering Materials Polymeric materials Polymer == many parts Polymerization reactions Covalent bonding structure of an ethylene molecule and activated ethylene molecule Unstaturated molecule: A carbon containing molecule that has one or more Carbon-carbon double bonds, example ethylene molecule C2H4. C=C H single covalent bond double When the ethylene molecule is activated so that the double bond between the two carbon atoms is (opened up), the double covalent bond is replaced by a single covalent bond. C C H Half covalent Bond or free electron single covalent bond

Engineering Materials Polymeric materials Polymer == many parts Polymerization reactions General reaction for the polymerization of poly and the degree of polymerization: General reaction for the chain polymerization of ethylene monomer into ployethylene may be written as; H Mer Polyethylene (polymer) Ethylene (monomer) Mer: a repeating unit in a chain polymer molecule. Degree of polymerization(DP) : is equal to the number of subunits or mers in the polymer molecular chain. Its equal to the molecular mass of a polymer chain divided by the molecular mass of its mer. The average DP for the polyethylene ranges from about 3500 to 25,000, corresponding to average molecular masses ranging from about 100,000 to 700,000 g/mol.

Engineering Materials Polymeric materials Polymer == many parts Polymerization reactions Example. If a particular type of polyethylene has a molecular mass of 150,000 g/mol, what is its degree of polymerization (DP)? Solution/ the repeating unit or mer for polyethylene is –[CH2–CH2]–. This mer has a mass of 4atoms x 1g =4g for the hydrogen atoms plus a mass of 2 atoms x 12g=24 g for the carbon atoms, making a total of 28 g for each polyethylene mer. H Mer Polyethylene (polymer) Ethylene (monomer)

Engineering Materials Polymeric materials Polymer == many parts Polymerization reactions Problem1. A high molecular weight polyethylene has an average molecular weight of 410,000 g/mol. What is its average degree of polymerization? Problem2. If a type of polyethylene has an average degree of polymerization of 10,000, what is its average molecular weight?

Engineering Materials Polymeric materials Polymer == many parts Polymerization reactions Chain polymerization steps:- the polymerization reaction can be divided into the following steps; 1- initiation 2- propagation and 3- termination. 1. Initiation:- To start the polymerization reaction we need a catalyst called organic peroxide that act as free radical formers. Free Radical is an atom/molecule that has a free electron that can covalently bond to a free electron of another atom/molecule Free electrons

Engineering Materials Polymeric materials Polymerization reactions Chain polymerization steps: 1. Initiation:- One of the free radicals created by the decomposition of the organic peroxide can react with an ethylene molecule to form new longer chain free radical. H Initiator catalyst for the polymerization of ethylene

Engineering Materials Polymeric materials Polymerization reactions Chain polymerization steps: 2. Propagation:- is the process of extending the polymer chain by the successive addition of monomer units. H Initiator catalyst for the polymerization of ethylene

Engineering Materials Polymeric materials Polymerization reactions Chain polymerization steps: 3. Termination:- can occur by the addition of a terminator free radical or when two chains combine. Another possibility is that trace amount of impurities may terminate the polymer chain. H Initiator catalyst for the polymerization of ethylene

Molecular weigh range ( g/mol) Engineering Materials Polymeric materials Polymerization reactions Average molecular weight for thermoplastics: Example:- calculate the average molecular weight for a thermoplastic material that has the mean molecular weight fractions for the molecular weight ranges listed in the following table: Molecular weigh range ( g/mol) weight fraction 5000 – 10,000 0.11 10,000- 15,000 0.17 15,000- 20,000 0.26 20,000- 25,000 0.22 25,000-30,000 0.14 30,000 – 35,000 0.10

Engineering Materials Polymeric materials Polymerization reactions Functionality of a monomer: The number of active bonds a monomer has is called functionality of a monomer. A monomer that uses two active bonds for the polymerization of long chain is called bifunctionality. Ethylene C2H4 is an example of a bifunctionality monomer. A monomer that uses three active bonds to form a network polymeric material is called trifunctionality. Phenol (C6H5OH) is an example of a trifunctionality monomer and is used in the polymerization of phenol and formaldehyde.

Engineering Materials Polymeric materials Polymerization reactions Homopolymers and Copolymers Homopolymers:- are polymeric materials that consist of polymer chain made up of single repeating units. If A is a repeating units, a homopolymer chain will have a sequence of AAAAAA …. in the polymer molecular chain. Copolymers:- are polymeric materials that consist of polymer chain made up of two or more chemically different repeating units that can be in different sequences. Random copolymers:- Different monomers are randomly arranged within the polymer chains. If A and B are different monomers, the arrangement might be, AABABBBBAABBBBBAAABAAA…..

Engineering Materials Polymeric materials Polymerization reactions Homopolymers and Copolymers Alternating copolymers:- Different monomers show a definite ordered alternation as ABABABABABABABABA….. Block copolymer:- Different monomers arranged in relatively long blocks of each monomer as, AAAAA - BBBBB -….. Graft copolymer:- Appendages of one type of monomer are grafted to the long chain of another as, AAAAAAAAAAAAAAAAAAAAAA B B

Engineering Materials Polymeric materials Polymerization reactions Homopolymers and Copolymers Example 3:- A copolymer consists of 15 wt% polyvinyl acetate (PVA) and 85 wt% polyvinyl chloride (PVC). Determine the mole fraction of each component. Molecular weight of the PVA mer and PVC mer can be calculated from; Structural formulas for the mers of polyvinyl chlorid (PVC) Structural formulas for the mers of Polyvinyl acetate (PVA) Example 4:- Determine the mole fractions of vinyl chloride and vinyl acetate in a copolymer having a molecular weight of 10,520 g/mol and a degree of polymerization of 160. Example 5:- If a vinyl chloride - vinyl acetate copolymer has a ratio of 10:1 vinyl chloride to vinyl acetate mers and a molecular weight of 16,000 g/mol, what is its degree of polymerization ?

Engineering Materials Polymeric materials Polymerization reactions Industrial polymerization methods Bulk polymerization:- Monomer plus activator are mixed in a reactor that is heated and cooled as required.

Engineering Materials Polymeric materials Polymerization reactions Industrial polymerization methods Solution polymerization:- The monomer is dissolved in a non reactive solvent that contains a catalyst. The heat released by the reaction is absorbed by the solvent.

Engineering Materials Polymeric materials Polymerization reactions Industrial polymerization methods Suspension polymerization:- The monomer is mixed with a catalyst and then dispersed as a suspension in water. The heat released by the reaction is absorbed by the water. After polymerization, the polymerized product is separated and dried. This process is commonly used to produce polyvinyl chloride (PVC), polystyrene, polyacrylonitrile.

Engineering Materials Polymeric materials Polymerization reactions Industrial polymerization methods Emulsion polymerization:- Is similar to the suspension process since it is carried out in water. However, an emulsifier is added to disperse the monomer into very small particles.

Engineering Materials Polymeric materials Polymerization reactions Industrial polymerization methods Solution polymerization:- The monomer is dissolved in a non reactive solvent that contains a catalyst. The heat released by the reaction is absorbed by the solvent. Solution polymerization of ethylene, using Ziegler-Natta catalysts