Polymers

Unit III Part III

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)