Polymers

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Polymers

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Polymers

Made of long, repeating chains of monomers; large molecules made by bonding (chemically linking) a series of building blocks

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Monomer

Small molecule

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Classification of polymers based on origin of source

Natural, Semi-synthetic, Synthetic

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Classification of polymers based on structure

Linear, Branched chain, Cross-linked

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Classification of polymers based on molecular forces

Elastomers, Fibers, plastics (thermoplastics, Thermosetting)

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Classification of polymers based on mode of polymerization

Addition, Condensation

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Natural polymers

from nature (extracted from plants or animals); proteins, starch, cellulose, biodegradable (biopolymers)

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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

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Synthetic Polymers

Man-made, “plastics”, nylon and polyethylene, addition polymers/chain-growth polymers, mostly non-biodegradable

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Addition polymers

Polymers formed by linking monomer units without any change of material (synthetic polymers)

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Nylon

Polyamindes, amide because the backbone is hydrophilic, gets engaged in hydrogen bonding with water

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Polyvinyl Chloride (PVC)

construction; stronger and cheaper than metal alternatives; clothing, insulation and rubber alternative

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Low-density Polyethylene (LDPE)

most common synthetic polymers; thermoplastic prepared from monomer ethylene

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Polypropylene

Polypropene; thermoplastic synthetic polymer; packaging and textiles

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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

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Linear Polymers

Long and straight chains and repeat units; PVC and HDPE; flexible, Van Der Waals and hydrogen bonding

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Branched-chain polymers

Linear chains of a polymer form branches (LDPE); chain packing efficiency is reduced because of side branches; lowered density

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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

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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

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Properties of Polymers

Physical, Chemical, Optical

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Physical Properties

Chain length and cross-linking increases, tensile strength increases; do not melt – change state from crystalline to semi-crystalline

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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

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Optical Properties

Ability to change their refractive index with temperature for applications in spectroscopy and analytical applications

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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

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Fibres

High tensile strength and high modulus; strong intermolecular forces of attraction (hydrogen bonding); lead to close packing and impart crystalline nature

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Thermoplastic

intermolecular forces of attraction are moderate; easily moulded by heating; no cross-linking

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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)

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Polymerization

chemical reaction in which high molecular mass molecules are formed from monomers; 1. Chain-reaction (addition); 2. Step-reaction (Condensation)

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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

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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

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Homomer

Single type of monomer unit is present

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Homopolymerization

produces homopolymers

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Heteropolymer/Co-polymer

Different Monomer units (Nylon etc.); derived from more than one species of monomer

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Copolymerization

Polymerization of monomers into copolymers

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Bipolymers

Copolymers obtained by copolymerization of two monomer species

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Types of Copolymers

  1. Random; 2. Alternating (ABABABABAB); 3. Block (AAAA-BBBB), 4. Graft (monomer grafted to a long chain)

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Defects on Polymers/ Material Defects

Problems inherent to the polymer compound itself

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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

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Average Molecular weight

Chain lengththrough number-average or weight-average

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Polydispersive index

Ratio of weight average molecular mass and number average molecular mass of polymers (Mw/Mn)

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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)

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