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Polymers

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Unit III Part III

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Polymers
Made of long, repeating chains of monomers; large molecules made by bonding (chemically linking) a series of building blocks
Monomer
Small molecule
Classification of polymers based on origin of source
Natural, Semi-synthetic, Synthetic
Classification of polymers based on structure
Linear, Branched chain, Cross-linked
Classification of polymers based on molecular forces
Elastomers, Fibers, plastics (thermoplastics, Thermosetting)
Classification of polymers based on mode of polymerization
Addition, Condensation
Natural polymers
from nature (extracted from plants or animals); proteins, starch, cellulose, biodegradable (biopolymers)
Semi-synthetic Polymers
Derived from naturally occurring polymers but undergo further chemical modification/treatment to improve physical properties; cellulose nitrate, cellulose acetate, vulcanized rubber, gun cotton, viscose/rayon
Synthetic Polymers
Man-made, “plastics”, nylon and polyethylene, addition polymers/chain-growth polymers, mostly non-biodegradable
Addition polymers
Polymers formed by linking monomer units without any change of material (synthetic polymers)
Nylon
Polyamindes, amide because the backbone is hydrophilic, gets engaged in hydrogen bonding with water
Polyvinyl Chloride (PVC)
construction; stronger and cheaper than metal alternatives; clothing, insulation and rubber alternative
Low-density Polyethylene (LDPE)
most common synthetic polymers; thermoplastic prepared from monomer ethylene
Polypropylene
Polypropene; thermoplastic synthetic polymer; packaging and textiles
RECYCLING CODES FOR PLASTIC
1 PETE/PET; 2 HDPE/PE-HD, 3 PVC/V, 4 LDPE/PE-LD; 5 PP, 6 PS, 7 O/ N/A
Linear Polymers
Long and straight chains and repeat units; PVC and HDPE; flexible, Van Der Waals and hydrogen bonding
Branched-chain polymers
Linear chains of a polymer form branches (LDPE); chain packing efficiency is reduced because of side branches; lowered density
Cross-linked Polymers
Bifunctional and trifunctional monomers (adjacent linear chains joined to one another); stronger covalent bond; crosslinking achieved during synthesis or nonreversable chemical reaction; accomplished by additive atoms; rubber elastic materials
Network Polymers
highly crosslinked materials; atom valencies are satisfied by bonds that result in a 3d structure (three or more active covalent bonds); distinctive mechanical and thermal properties
Properties of Polymers
Physical, Chemical, Optical
Physical Properties
Chain length and cross-linking increases, tensile strength increases; do not melt – change state from crystalline to semi-crystalline
Chemical Properties
enabled with hydrogen bonding and ionic bonding for better cross-linking strength; dipole-dipole bonding for high flexibility; Van der Waals forces are weak but low melting point
Optical Properties
Ability to change their refractive index with temperature for applications in spectroscopy and analytical applications
Elastomers
Polymer chains held by weakest intermolecular forces allowing it to be stretched; a few cross-links are introduced in between chains to help the polymer go back to its original position
Fibres
High tensile strength and high modulus; strong intermolecular forces of attraction (hydrogen bonding); lead to close packing and impart crystalline nature
Thermoplastic
intermolecular forces of attraction are moderate; easily moulded by heating; no cross-linking
Thermosetting polymers
made from low molecular mass semi-liquid polymers; when heated in a mould because infusible and insoluble hard mass; improve mechanical properties (chemical and heat resistance)
Polymerization
chemical reaction in which high molecular mass molecules are formed from monomers; 1. Chain-reaction (addition); 2. Step-reaction (Condensation)
Addition Polymerization
Small monomer units joined to form a giant polymer; repeated addition of monomers that possess double or triple bonds; no by products; molecular weight of polymers is a multiple of monomer’s molecular weight; PVC, PET
Condensation Polymerization
Small molecules (H20, CO, NH3) are eliminated; organic compounds containing bifunctional groups undergo this; must have two similar or different functional groups; Nylon, silicon
Homomer
Single type of monomer unit is present
Homopolymerization
produces homopolymers
Heteropolymer/Co-polymer
Different Monomer units (Nylon etc.); derived from more than one species of monomer
Copolymerization
Polymerization of monomers into copolymers
Bipolymers
Copolymers obtained by copolymerization of two monomer species
Types of Copolymers
1. Random; 2. Alternating (ABABABABAB); 3. Block (AAAA-BBBB), 4. Graft (monomer grafted to a long chain)
Defects on Polymers/ Material Defects
Problems inherent to the polymer compound itself
Reasons for Defects
Improper choice of polymer or grade of polymer, inadequate fillers/stabilzers, excessive regrind: include both material processing errors and structural fabrication errors
Average Molecular weight
Chain lengththrough number-average or weight-average
Polydispersive index
Ratio of weight average molecular mass and number average molecular mass of polymers (Mw/Mn)
Degree of Polymerization
Dp or Xn; number of monomer units in the polymer; ration of the molecular weight of a polymer and molecular weight of the repeat unit; (Mn/m)