Polymers Lecture 2 PDF

Polymers: Lecture 2 Unit-3a Classification of Polymers 1. Based on Structure/Shape of polymer Polymers can be linear, branched, cross linked or networked. secondary bonding Linear Branched Cross-Linked Network Line...

Polymers: Lecture 2 Unit-3a Classification of Polymers 1. Based on Structure/Shape of polymer Polymers can be linear, branched, cross linked or networked. secondary bonding Linear Branched Cross-Linked Network Linear polymers have monomeric units joined end to end in single chains. There may be extensive van der Waals and hydrogen bonding between the chains. Examples: polyethylene, PVC, nylon. In branched polymers, side-branch chains are connected to main chains. HDPE – high density polyethylene is primarily a linear polymer with minor branching, while LDPE – low density polyethylene contains numerous short chain branches. Greater chain linearity and chain length tend to increase the melting point and improve the physical and mechanical properties of the polymer due to greater crystallinity. In cross-linked polymers, adjacent linear chains are joined to one another at various positions by covalent bonding of atoms. Examples are the rubber elastic materials. Small molecules that form 3 or more active covalent bonds create structures called network polymers. Examples are the epoxies and polyurethanes. 2. Based on number of monomers: Polymers can be homopolymers when only one type of monomer is present (eg: polyethylene, polypropylene, polyvinyl chloride). Polymers can be co-polymer when two or more types of monomeric units are present (eg: styrene, acrylonitrile etc) Homopolymers – only one monomer (repeating unit): - A – A – A – A – A – A – A - Copolymer – more than one monomer Statistical Copolymers Also called random copolymers. Here the monomeric units are distributed randomly, and sometimes unevenly, in the polymer chain: ~ABBAAABAABBBABAABA~. Alternating Copolymers Here the monomeric units are distributed in a regular alternating fashion, with nearly equimolar amounts of each in the chain: ~ABABABABABABABAB~. Block Copolymers Instead of a mixed distribution of monomeric units, a long sequence or block of one monomer is joined to a block of the second monomer: ~AAAAA-BBBBBBB~AAAAAAA~BBB~. Prepared by Dr Arunima Nayak and Dr Brij Bhushan Polymers: Lecture 2 Unit-3a Graft Copolymers As the name suggests, side chains of a given monomer are attached to the main chain of the second monomer: ~AAAAAAA(BBBBBBB~)AAAAAAA(BBBB~)AAA~. Red Dot: A; Black Dot: B 3. Based on Origin: Synthetic organic polymers Biopolymers (proteins, polypeptides, polynucleotide, polysaccharides, natural rubber) Semi-synthetic polymers (chemically modified synthetic polymers) Inorganic polymers (siloxanes, silanes, phosphazenes) 4. Based on Tacticity: Polymers with regular substituents on the polymer chain possess a property known as tacticity. Tacticity results from the different ways in which the substituents can be arranged on the polymer backbone. When the substituents are arranged in an irregular, random fashion, the polymer is atactic (literally, no arrangement). When the substituents are all on the same side of the chain, the polymer is isotactic (literally, the same arrangement). If the substituents alternate regularly from one side of the chain to the other, the polymer is syndiotactic. Atactic polypropylen e Prepared by Dr Arunima Nayak and Dr Brij Bhushan Polymers: Lecture 2 Unit-3a Syndiotactic polystyrene Isotactic poly(vinyl chloride) 5. Based on Tacticity Elastomers: These are rubber like solids with elastic properties. the polymer chains are held together by the weakest intermolecular forces. E.g. Buna-S, Buna-N, Neoprene, etc. Fibres: Fibres are the thread forming solids which possess high tensile strength and high modulus. These characteristics can be attributed to the strong intermolecular forces like hydrogen bonding. E.g. Polyamides (nylon 6,6), Polyesters(terylene), etc. Thermoplastic polymers: These polymers possess intermolecular forces of attraction intermediate between elastomers and fibres. E.g. Polythene, Polystyrene, Polyvinyl, etc. Thermosetting polymers: These polymers are cross-linked or heavily branched molecules, which on heating undergo extensive cross-linking in molds and again become infusible. These cannot be reused. E.g. Urea- Formaldehyde resins, Bakelite, etc. Prepared by Dr Arunima Nayak and Dr Brij Bhushan  Polymers: Lecture 2 Unit-3a Addition and Condensation Polymerization: Addition Polymerization: Addition polymers are formed when an unsaturated organic compound (presence of double/triple bond) react to form long chain polymer molecule and when no small molecules are eliminated during the reaction. Some Common Addition Polymers Name(s) Formula Monomer Properties Uses Polyethylene ethylene film wrap, plastic –(CH2-CH2)n– soft, waxy solid low density (LDPE) CH2=CH2 bags electrical Polyethylene ethylene rigid, translucent –(CH2-CH2)n– insulation high density (HDPE) CH2=CH2 solid bottles, toys Poly(vinyl chloride) vinyl chloride pipes, siding, –(CH2-CHCl)n– strong rigid solid (PVC) CH2=CHCl flooring hard, rigid, clear toys, cabinets Polystyrene –[CH2- styrene solid packaging (PS) CH(C6H5)]n– CH2=CHC6H5 soluble in organic (foamed) solvents high-melting solid Polyacrylonitrile acrylonitrile rugs, blankets –(CH2-CHCN)n– soluble in organic (PAN, Orlon, Acrilan) CH2=CHCN clothing solvents non-stick surfaces Polytetrafluoroethylene tetrafluoroethylene resistant, smooth –(CF2-CF2)n– electrical (PTFE, Teflon) CF2=CF2 solid insulation Poly(methyl lighting covers, methacrylate) –[CH2- methyl methacrylate hard, transparent signs (PMMA, Lucite, C(CH3)CO2CH3]n– CH2=C(CH3)CO2CH3 solid skylights Plexiglas) Poly(vinyl acetate) –(CH2- vinyl acetate latex paints, soft, sticky solid (PVAc) CHOCOCH3)n– CH2=CHOCOCH3 adhesives Prepared by Dr Arunima Nayak and Dr Brij Bhushan Polymers: Lecture 2 Unit-3a Condensation Polymerization: The process of formation of polymer via condensation of two different bi-or poly functional monomers having functional groups which have affinity to each other. Condensation polymers form when: Ø bifunctional monomers combine to form a long chain polymer molecule. Ø small molecules like water is eliminated during the reaction. Prepared by Dr Arunima Nayak and Dr Brij Bhushan Polymers: Lecture 2 Unit-3a Difference between addition and condensation polymerization S.No Addition Polymerization Condensation Polymerization 1. Requires the presence of double Requires the presence of minimum two bond in the monomer functional groups in a single monomer which are of close proximity to each other 2. No by product is formed A byproduct like H2O, HCl etc is formed 3. Polymer formed is homopolymer Polymer formed is heteropolymer 4. Polymer is thermoplastic Polymer formed is thermosetting 5. Example: Example: a. Ethylene polymerizing to a. Hexamethylene diamine and adipic polyethylene acid polymerizing to nylon 66 b. Vinyl chloride polymerizing b. Caprolactum polymerizing to Nylon to PVC 6 c. styrene polymerizing to c. Terepthallic acid and ethylene polystyrene glycol polymerizing to polyethylene terepthallate Prepared by Dr Arunima Nayak and Dr Brij Bhushan