POLYMERIZATION AND STRUCTURE OF POLYMERS

POLYMERa family of natural and synthetic materials made of repetition of high weight molecules in a form of flexible chain

• Collagen• Gelatin• Silk• Wool• Natural rubber• DNA

NATURAL POLYMER SYNTHETIC POLYMER• Polyethylene terephthalate

(PET)• High Density Polyethylene

(HDPE)• Polyvinyl Chloride (PVC)• Low Density Polyethylene

(LPDE)• Polypropylene (PP)• Polystyrene (PS)

POLYMER• The word polymer comes from the Greek ‘poly’ meaning

many, and ‘meros’ is parts or units.• A polymer is organic substance made up of many repeating

units or building blocks of molecules called mers.• Combine many monomers to create a polymer.• Polymer is often used as a synonym for ‘plastic’. All plastic

are polymers, but not all polymers are plastics.

Poly mers are made up of many Mono mer ↓ ↓ ↓ ↓Many Units One Unit

POLYMERIZATION chemical process where monomers linked

into polymers in repeating unit to make longer and larger molecules

• Also called additional polymerization, with aids of initiators to form benzene or paraffin.

Chain – Reaction Polymerization

• Also called condensation polymerization, dissimilar monomer joined into short groups that gradually grow with by product released.

Step – Reaction Polymerization

Additional Polymerization The straightforward addition of monomers of the same kind Homogeneous type : A +A … → A-A-A-A-…

or a different kind

Copolymer type : A +B+A+B… → A-B-A-B-…

Rapid chain reaction of chemically activated mers Each reaction sets up the condition for another to proceed Each site need a reactive site (a double carbon bond or

unsaturated molecules) Initiator is added to open the double bond between carbon 3 stages : Initiation Propagation Temination The composition of resultant molecule is a multiple of the

individual mers Most commonly produced linear structure but can produce

network structure

• Initiation free radical – a single

unit that has one unpaired electron (OH‾ molecule)

• H₂O₂ break up into 2 OH‾ molecules

• Each can act to initiate and to terminate the reaction

• Termination - recombination

Polymerization of polyethylene

Condensation PolymerizationInvolves a polymerization reaction between two monomers with the expulsion of a simple by product.

A+B → AB + simple by product

Individual chemical reactions between reactive mer that occur one step at a time

By products (water or carbon ,oxygen or hydrogen gas) is formed and condensed out

Polymer molecule growth step by step until all of one reactant is consumed

Slower than additional polymerization Need reactive functional groups No reactant species has the chemical formula of a mer repeating

unit Most commonly produce network structure but can produce linear

structures

Condensation polymerization of nylon 6,6

Structure of PolymerThe properties of the polymer depends on:i. Structures of individual polymer molecules ii. Molecule shape and size iii. Arrangement of molecules to form a polymer structure

Basic structure of polymer molecules: (a) ethylene molecule(b) polyethylene, a linear chain of many ethylene molecule(c) molecular structure of various polymers

Molecular Weight• Molecular weight of the polymer is

the sum of the molecular weights of mers in a representative chain.

• Molecular Weight Distribution (MWD) is the spread of the molecular weights in a chain

• Strong influence on the properties: Increase in molecular weight will increase:

i. Tensile & impact strength ii. Resistance to cracking iii. Viscosity of molten state

higher molecular weight,greater average chain

length

Figure 2 Effect of molecular weight and degree of polymerization on the strength and viscosity of polymers

Degree of Polymerization• The ratio of the molecular weight of the polymer to the

molecular weight of the mer (repeating unit)• Example: Polyvinyl chloride (PVC) Mer weight: 62.5, thus DP of PVC with 50,000 molecular weight is: 50,000 / 62.5 = 800

Higher DP → Higher viscosity (resistance to flow) → hard to shape → increase cost adversely. Higher DP → stronger polymers

Bonding• During polymerization, the monomers are linked together by

covalent bond forming a polymer chain (high strength at primary bond)

• The polymer chains are held together by secondary bonds (low strength):

i. Van der Waals bondsii. Hydrogen bondsiii. Ionic bonds • In polymer, the increase in strength and viscosity → the

longer the polymer chain → the greater is energy needed to overcome secondary bonds

Polymer ChainsLinear Polymers (sequential structure) • Generally a polymer consists of more

than one type of structure (a linear polymer may contain some branched and cross-linked chains; properties are changed significantly)

Branched Polymers • Side-branch chains are attached to the

main chain during the synthesis. • Interferes with the relative movement

of the molecular chains → increase in resistance to deformation and stress cracking

• Interferes with the packing efficiency of chains → density is lower than linear-chain

Branched polymers ~pile of tree branches Linear-chain polymers ~ bundle of straight logsDifficult to move branch rather than log. 3D entanglements of branches→ difficult to move→increase in strength

Schematic illustration of polymer chains.(a) Linear structure--thermoplastics such as acrylics, nylons, polyethylene, and polyvinyl chloride have linear structures.(b) Branched structure, such as in polyethylene

Cross-linked polymers • Thermosets or thermosetting

plastics 3D structure, adjacent chains linked by covalent bonds

• Increase hardness, strength, stiffness, brittleness, better dimensional stability

Network polymers• Spatial, 3D networks of three or

more active covalent bonds • Highly cross-linked polymers =

network polymer• Cross-linking thermoplastics

polymers → by high-energy radiation (UV, X-rays, e- beams) → increase in strength

CrystallinilityPolymers are generally amorphous The chain exist without long-range order (like bowl of spaghetti, or worms in a bucket, all intertwined with each other)

Crystallinity in polymers • modify the characteristics • fostered during synthesis or deformation in subsequent process

Crystallites → Crystalline region in polymers Formed when long molecules arrange themselves in an orderly manner Semicrystalline polymer 2 phase material (crystalline + amorphous)

Different degree of crystallinity can be impart by controlling: • Rate of solidification during cooling • Chain structure Degree of crystallinity affected by branching: • Linear polymer → can become highly crystalline • Highly branched → cannot (or very low level)

The higher the crystallinity, the

harder, stiffer, and less ductile the polymer.

References

1. S. Kalpakjian, S. R. Schmid, “Manufacturing Engineering and Technology”, 6th ed, 2010