Polymers: a chemical point of view

Introduction: What is a polymer?Examples of polymersThe synthesis of polymerThe morphology of polymer The glass transitionSurface: Example with hydrophilic and hydrophobic properties

Definition

Molecules whose molecular weight (or size) is in the range of several thousand or more

Repetition of a unit ...-A-A-A-A-A-A-A-...

But We can have different arrangements!

Linear

Branchedand a lot of others possibilities...

Mixing, Molecular weight

Many possibilities with polymers: Alternating

-A-B-A-B-A-B-A-B-A-

Random

A-B-B-A-B-A-A-B-A-

Block A-A-A-A-B-B-B-B-B-

And then various configurations (tacticity,...)

Different definitions of molecular weight

Types of polymers

Thermoplastics✔ can be processed many

times without a chemical change between the chains of the polymer

✔ can be melted again and again and reformed as many times as desired

Thermosets✔ chemical change

during production that cannot be repeated or reversed : one big molecule!

cross-linking

Common polymers

Example of polymers

Polyethylene

Polypropylene

Poly(vinyl chloride)

Polystyrene

Polyesters

Nylon

Vinyl polymerswith a C=C in

monomer

Synthesis of polymers (1)

Monomer? Types of reactions: Addition Polymerization

Entire monomer becomes part of the polymer

Condensation Polymerization

Some atoms of the monomer don't end up in the polymer

Molecule which react with other molecules of the same type to form a polymer

Monomer Polymer

Synthesis of polymers (2)Example of vinyl polymers

3 majors ways of synthesis Anionic Cationic Free Radical (example of LDPE below)

Initiation

Propagation

Termination ?

Crystallised/Amorphous Zones

Proportion of Crystallized zones depend on polymer structure Example LDPE/HDPE

Linear (HD) : Highly crystallized

Branched (LD) : lowly crystallized

Polymer is never 100% cristal

The glass transition

Important parameter: viscositySome polymers can have a Tf and a Tg2nd order transition involves a change in

heat capacity, but does not have a latent heat

1 leads to amorphous state 2 leads to crystalline state

Surface

– Relationships between surface chemical (or morphological) structure and surface properties:

•Length of the chain (mobility)

•Type of the groupsThe pendant groups

Hydrophobic/ Hydrophilic

Type of pending chain: Hydrophobic

✔ CH3, C

2H

5, Cl, F,...

Hydrophilic ✔ Groups with oxygen (acids, =O),....

Fluor: decrease the surface energy, the polymer becomes very hydrophobic (example: Teflon)

Polymers are in general hydrophobic. This necessitates their surface modification/treatment to render them adhesionable.

Conclusion

Things that makes polymers different : Chain Entanglement Summation of Intermolecular Forces Time Scale of MotionSurface properties depend on the pendants

groups