1. STRUCTURE OF POLYMERS

2. Introduction Poly & mers  Greek ; meros=part; polymer=many parts
Natural polymer derived from animals & plants wood, rubber, cotton, wool, leather, and silk
Other natural polymers such as proteins, enzymes, starches, and cellulose
this group of materials and the development of numerous polymers  synthesized from small organic molecules.
Many of our useful plastics, rubbers, and fiber materials are synthetic polymers.
It can be produced inexpensively, and their properties can be managed to the degree that many are superior to their natural counterparts.
In some applications metal and wood parts have been replaced by plastics that have satisfactory properties and may be produced at a lower cost.
Most of polymers are organic in origin & based on hydrocarbon (H & C)

3. Hydrocarbon - HC Hydrocarbon: H & C Intramolecular bonds are covalent
Each C atom has 4 e to participate in covalent bonding, every H has 1 bonding e
Single covalent bondeach of 2 bonding atoms contributes 1 e; CH4
Double & triple bond 2 C atoms share 2 & 3 pairs of e; C2H4
Saturated HC  all single bond
No new atom may be joined without removal of atoms that are already bonded
Double & triple covalent bonds  unsaturated;
each C is not bonded to max atoms; other atoms are possible to be bonded to the molecule
Acetylene
ethylene

4. Some of the simple hydrocarbons belong to the paraffin family;
the chainlike paraffin molecules include methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10)
The covalent bonds in each molecule are strong, but only weak hydrogen and van der Waals bonds exist between molecules, and thus these hydrocarbons
have relatively low melting and boiling points.
HC comp with same composition but different arrangement  isomerism; affect the properties
E.g. N-buthane & isobuthane

5. Polymer molecules
Large molecule built up by repetition of small, simple chemical units
Because of their size  macromolecules
Atoms are bound by covalent interatomic bonding
For C polymer  C the backbone
Many times each carbon atom singly bonds to two adjacent carbons atoms on either side
2 remaining valence of C may involve in side-bonding with atoms/radical that are positioned adjacent to the chain
Each of the two remaining valence electrons for every carbon atom may be involved in side-bonding with atoms or radicals that are positioned adjacent to the chain.

6. Long molecules are composed of structural entities called repeat units (Mers)
Monomer: small molecule from which a polymer is synthesized
Hence, monomer and repeat unit mean different things

8. The chemistry of polymer molecules
At Tr and P  C2H4-ethylene is gas
If the ethylene gas is polymerised under appropriate conditions, it will transform to polyethylene (PE), which is a solid polymeric material.
This process begins when an active center is formed by the reaction between an initiator or catalyst species (R●) the ethylene monomer:

9. The polymer chain then forms by the sequential addition of monomer units to this active growing chain molecule.
The active site, or unpaired electron (denoted by *), is transferred to each successive end monomer as it is linked to the chain. This may be represented schematically as follows:

10. The final result, after the addition of many ethylene monomer units, is the polyethylene molecule;
This polyethylene chain structure can also be represented as
Here the repeat units are enclosed in parentheses, and the subscript n indicates the number of times it repeats.

11. other chemistries of polymer structure are possible.
For example, the tetrafluoroethylene monomer, can polymerize to form polytetrafluoroethylene (PTFE) as follows:
Polytetrafluoroethylene (having the trade name Teflon) belongs to a family of polymers called the fluorocarbons.

12. the vinyl chloride monomer is a slight variant of that for ethylene, in which one of the four H atoms is replaced with a Cl atom.
Its polymerization is represented as
leads to poly(vinyl chloride) (PVC)
Some polymers may be represented using the following generalized form:
where the “R” depicts either an atom [i.e., H or Cl, for polyethylene or poly(vinylchloride), respectively], or an organic group such as CH3,C2H5, and C (methyl, ethyl, and phenyl).
For example, when R represents a CH group, the polymer is polypropylene (PP).

13. MOLECULAR WEIGHT
Extremely large molecular weights are observed in polymers with very long chains.
When all of the repeating units are the same  homopolymer
Chain may be composed of 2 or more different repeat units  copolymer
During polymerization process, not all polymer chains grow the same length, result in distribution of chain length/MW length an average molecular weight is specified
the melting or softening temperature increases with increasing molecular weight
At Tr polymers with very short chains (M ~100 g/mol)  liquid; ~ 1000 g/mol are waxy solids (such as paraffin wax) and soft resins; Solid polymers (sometimes termed high polymers), commonly have M ranging 10,000 - several million g/mol)

15. Thus, the same polymer material can have quite different properties if it is produced with a different molecular weight.
There are several ways of defining average molecular weight:
the number-ave MW,
the weight-ave MW, and
degree of polymerisation

16. 1)The number-average MW
Dividing the chains into series of size range
then determining the number fraction of chain within each size range.
Expressed as:
Mi=mean/middle MW of size range i
xi = fraction of total number of chain within the corresponding size range

17. 2) The weight-average MW
weight fraction of molecules within various size ranges.
Calculated as:
Mi=mean MW within size range i
wi =weight fraction of molecules within the same size interval

18. A typical molecular weight distribution along with these molecular weight averages

19. 3)Degree of polymerization
DP  Average chain size of polymer
DP average number of repeat units (mers) in a chain
related to the number-average molecular weight
Can be expressed as :
Mn & m = number average MW & repeat unit (mer) MW

20. Example
Figures of MW distribution are for PVC. Calculate a) number-average MW b) weight-average MW & c) degree of polymerisation

21. a) Table for number-average MW  21,150 g/mol

22. b) Table for weight-average MW  23,200 g/mol
c) PVC  2 C, 3 H & 1 Cl

23. Molecular Structure LINIER POLYMERS
Linear, branced, crosslinked, network
LINIER POLYMERS
 repeat units are joined end to end in single chains
each circle represents a repeat unit
Melt on heating, flexible
Mechanical strength increases with entangle chain

24. Example of Linier Polymer
Polyethylene  HDPE
PVC
Polystyrene
Nylon
fluorocarbon

25. BRANCHED POLYMERS
The branch considered to be part of the main chain molecules
side-branch chains are connected to the main one
May result from side reactions that occur during the synthesis
The chain packing efficiency reduces with formation of side branches lowering polymer density
By changing T, the branched polymer can be hardened or softened
Those polymers that form linear structures may also be branched.
E.g. LDPE contains short chain branches.

27. CROSSLINKED POLYMERS
Adjacent linear chains are joined one to another at various positions by covalent bonds
increase strength, reduce plasticity
Achieved during synthesis or by nonreversible chemical reaction
Often, accomplish by additive atom/molecules that are covalently bonded to the chains
The movement of adjacent chains is greatly restricted, affected the mechanical properties to a great extent
E.g. rubber elastic material

28. Networking polymer
Multifunctional monomers forming three or more active covalent bonds, make three-dimensional networks
a polymer that is highly crosslinked may also be classified as a network polymer.
These materials have distinctive mechanical and thermal properties;
E.g: epoxies, polyurethanes, and phenol-formaldehyde
Polymers are not usually of only one distinctive structural type. For example, a predominantly linear polymer might have limited branching and crosslinking.

29. Thermoplastic & Thermosetting
The response of a polymer to mechanical forces at elevated temperatures is related to its dominant molecular structure.
one classification scheme for these materials is according to behavior with rising temperature: Thermoplastics (or thermoplastic polymers) and thermosets (or thermosetting polymers)

30. Thermoplastic Polymers
Soften when heated (eventually liquefy), harden when cooled reversible & may be repeated
Plastic & flexible properties
Formed at high T, cooled, remelted & reformed into different shape without changing properties
On a molecular level, as the temperature is raised, secondary bonding forces are diminished (by increased molecular motion) so that the relative movement of adjacent chains is facilitated when a stress is applied.

31. Overheat material decomposes, irreversible degradation
Relatively soft
Most linear, some branches polymer with flexible chains
Fabricated by simultaneous heat & pressure
Example: polyethylene, polystyrene, PVC, poly(ethylene terephthalate

32. Thermosetting Network polymers
Strong bonds, often formed by condensation
Permanently hard during formation when heat applied
Do not softened/reshaped upon subsequent heating loss of part of the molecule
Further heat  burn/decompose
Generally harder, stronger & better stability than thermoplastic
Most crosslinked, in that 10 to 50% of the chain repeat units are crosslinked.
Only heating to excessive temperatures causes severance of these crosslink bonds and polymer degradation.
Ex: phenolic, vulcanized rubber, epoxies

33. Copolymers Polymers with more than 1 repeat unit
Different type depends on method synthesis & repeat unit type
Sequencing arrangement: random, alternating, block & graft copolymer
1) Random copolymer  random distribution of various mers
E.g nitrile rubber

34. 2) Alternating copolymer  2 mer units alternate chain position
3) Block copolymer identical repeat units are clustered in blocks along the chain
4) Grafted copolymer  homopolymer side branches of one type may be grafted to homopolymer main chain that are composed of different mer