Polymer Structure

STRUCTURE OF POLYMERS

Introduction• Poly & mers Poly & mers Greek ; meros=part; polymer=many parts Greek ; meros=part; polymer=many parts• Natural polymer derived from animals & plants Natural polymer derived from animals & plants wood, rubber, cotton, wood, rubber, cotton,

wool, leather, and silkwool, leather, and silk• Other natural polymers such as proteins, enzymes, starches, and cellulose Other natural polymers such as proteins, enzymes, starches, and cellulose • this group of materials and the development of numerous polymers this group of materials and the development of numerous polymers

synthesized from small organic molecules. synthesized from small organic molecules. • Many of our useful plastics, rubbers, and fiber materials are synthetic Many of our useful plastics, rubbers, and fiber materials are synthetic

polymers. polymers. • It can be produced inexpensively, and their properties can be managed to It can be produced inexpensively, and their properties can be managed to

the degree that many are superior to their natural counterparts. the degree that many are superior to their natural counterparts. • In some applications metal and wood parts have been replaced by plastics In some applications metal and wood parts have been replaced by plastics

that have satisfactory properties and may be produced at a lower cost.that have satisfactory properties and may be produced at a lower cost.• Most of polymers are organic in origin & based on hydrocarbon (H & C)Most of polymers are organic in origin & based on hydrocarbon (H & C)

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 bondeach 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

• 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

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.

• 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

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:

• 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:

• 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.

• 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.

• 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).

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)

• 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: 1) the number-ave MW, 2) the weight-ave MW, and 3) degree of polymerisation

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

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

• A typical molecular weight distribution along with these molecular weight averages

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

Example

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

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

• b) Table for weight-average MW 23,200 g/mol

c) PVC 2 C, 3 H & 1 Cl

Molecular Structure

• 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

Example of Linier Polymer

• Polyethylene HDPE• PVC• Polystyrene• Nylon• fluorocarbon

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.

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

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.

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)

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.

• 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

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

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

• 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 4) Grafted copolymer homopolymer side branches homopolymer side branches of one type may be grafted of one type may be grafted to homopolymer main chain to homopolymer main chain that are composed of that are composed of different merdifferent mer