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POLYMERS
The field of polymers is so vast and the applications so varied, that it is important to understand how polymers are made and used.
Since there are over 60,000 different plastics searching for a place in the market. Companies manufacture over 30 million tons of plastics each year
Polymers are macromolecules formed by the linking of a large number of smaller molecules. These smaller molecules (repeating units of polymers) are called monomers.
Polymerization Reactions
• The chemical reaction in which high molecular mass molecules are formed from monomers is known as polymerization.
(or) the fundamental process by which low molecular weight compounds are converted into high molecular weight compounds
Types of polym ers
O ccur in na tu reex. w oo l, ce llu lose , p ro te in ,
R ubber, S ta rch , nucle ic ac ids
N atura l
F ibresN ylon, te rry lene
synthe tic rubbersN eoprene
P lastics
S ynthe tic
P o lym ers
• The monomers in a polymer can be arranged in a number of different ways.
• Both addition and condensation polymers can be linear, branched, or cross-linked.
• Linear polymers are made up of one long continuous chain, without any excess appendages or attachments.
• Branched polymers have a chain structure that consists of one main chain of molecules with smaller molecular chains branching from it.
• Chains with only one type of monomer are known as homopolymers.
• If a mixture of two or more monomers is allowed to undergo polymerization the process is known as co-poymerization and product obtained is known as copolymer
• If two or more different type monomers are involved, the resulting copolymer can have several configurations or arrangements of the monomers along the chain.
Copolymer configurations
Produced by special techniques to give certain specific properties
to the product
• Copolymerization is an advantageous process because through it polymers can be made having different properties than either of the constituent homo-polymers
• There are two basic types of polymerization - chain-reaction (or addition) and step-reaction (or condensation) polymerization.
• Chain-Reaction Polymerization
• One of the most common types of polymer reactions is chain-reaction (addition) polymerization.
• This type of polymerization is a three step process involving two chemical entities.
• The first, known simply as a monomer, can be regarded as one link in a polymer chain.
• It initially exists as simple units. In nearly all cases, the monomers have at least one carbon-carbon double bond.
• Ethylene is one example of a monomer used to make a common polymer.
if X were a methyl group, the monomer would be propylene and the polymer, polypropylene.
In free radical polymerization, the entire propagation reaction usually takes place within a fraction of a second. Thousands of monomers are added to the chain within this time. The entire process stops when the termination reaction occurs.
• Step-Reaction Polymerization • Step-reaction (condensation) polymerization is another common
type of polymerization. • This polymerization method typically produces polymers of
lower molecular weight than chain reactions and requires higher
temperatures to occur. • Unlike addition polymerization, step-wise reactions involve two
different types of di-functional monomers or end groups that
react with one another, forming a chain. • Condensation polymerization also produces a small molecular
by-product (water, HCl, etc.). • Ex. Nylon 66, a common polymeric clothing material, involving
one each of two monomers, hexamethylene diamine and adipic
acid, reacting to form a dimer of Nylon 66.
• Essential condition for polymerization is the presence of 2 or more reactive groups or double bond in the monomers
• At this point, the polymer could grow in either direction by
bonding to another molecule of hexamethylene diamine or
adipic acid, or to another dimer.
• As the chain grows, the short chain molecules are called
oligomers.
• This reaction process can, theoretically, continue until no
further monomers and reactive end groups are available.
• The process, however, is relatively slow and can take up to
several hours or days.
• Typically this process breeds linear chains that are strung out
without any cross-linking or branching, unless a tri-functional
monomer is added.
• Thermosetting and Thermoplastic resins
• Thermosetting - Under the influence of heat and pressure it
becomes soft can be moulded into different shapes but on
further heating it becomes hard and infusible due to chemical
change and also cannot be re-moulded
• Ex. Phenol formaldehyde and Urea formaldehyde
• Thermoplastic – Under the influence of heat and pressure it
can be moulded into different shapes and on cooling they
retain their shape – can be re-moulded on further heating
• Ex. Polystyrene and cellulose acetate
• Molecular Weight Determination
• Most important measurement – mechanical properties depends
on mol. wt
• Almost all the properties of polymer changes with degree of
polymerization and hence its application
• Polymer – complex mixture of molecules of different mol.wts
- Polydisperse and heterogenous in composition
• Mol.wt of polymer – Average of mol.wts of constituent
molecules
• Different types of average mol.wts can be obtained
• Number Average - Weight Average - Viscocity average
Mn Mw Mv
• Mn = [N1M1 + N2M2 + N3M3 + -----]
N1 + N2 + N3 + ----
Where N1, N2, N3 – No. of molecules
M1, M2, N3 – Mol. wts
• Mw = [N1M12 + N2M2
2 + N3M32 + -----]
N1M1 + N2M2 + N3M3 +--
• Mv = NiMi(1+) 1/ - Constant
iMi
If But, ranges from 0.5 – 0.9, so,
For a polydisperse system For monodisperse system
NiMi2/iMi
Mv = Mw
Mv < Mw
Mw
NiMi/i
Mw > Mv > Mn Mw = Mn
• Molecular weight determination
• Mn - A colligative property which depends on number of molecules is measured to get Mn.
• Some methods are osmometry, end group analysis, vapour pressure osmometry.
• Mw – Light scattering measurement• Mv – Measurement of viscosity of polymer solution relative to
solvent
Q1. Equal number of molecules with M1=10,000, M2=100,000 are
mixed. Calculate Mn and Mw.
Q2. Equal masses of polymer molecules with M1 = 10,000, M2=
100,000 are mixed. Calculate Mn and Mw.
It can be assumed in polymer synthesis, each chain reacts independently.Therefore, the bulk polymer is characterized by a wide distribution of molecular weights and chain lengths. The degree of polymerization (DP) refers to the number of repeat units in the chain, and gives a measure of molecular weight. Many important properties of the final result are determined primarily from the distribution of lengths and the degree of polymerization.
In order to characterize the distribution of polymer lengths in a sample, two parameters are defined: number average and weight average molecular weight. The number average is just the sum of individual molecular weights divided by the number of polymers. The weight average is proportional to the square of the molecular weight. Therefore, the weight average is always larger than the number average.
The molecular weight of a polymer can also be represented by the
viscosity average molecular weight. This form of the molecular
weight is found as a function of the viscosity of the polymer in
solution (viscosity determines the rate at which the solution flows -
the slower a solution moves, the more viscous it is said to be - and
the polymer molecular weight influences the viscosity).
The degree of polymerization has a dramatic effect on the
mechanical properties of a polymer. As chain length increases,
mechanical properties such as ductility, tensile strength, and
hardness rise sharply and eventually level off.
However, in polymer melts, for example, the flow viscosity at a
given temperature rises rapidly with increasing DP for all polymers
• Threshold Molecular Weight
• Lower molecular weight polymer – Brittle and less mechanical
strength
• Very high mol. wt. Polymer – Tough and intractable cannot be
easily handled
• Polymerization proces should be controlled after certain stage
depending upon the application
• Threshold molecular weight is the minimum molecular weight
that a polymer must attain to develop the properties needed for a
particular application
• Physical properties of polymer such as tensile strength and
impact resistance are related to mol. wt and mol. wt distribution
of the polymer
• Variation of mech. Properties with DOP• Tensile strength and impact resistance increases with increase in
degree of polymerization upto a point after which slow increase is there
• Melt flow viscosity initially increases slowly then rapidly after the polymer has attained a certain degree of polymerization
• RI, colour, density, hardness and electrical properties are independent of the molecular weight of polymer
Tensile strength
Impact res.
Meltviscosity
property
Deg. of polymerization