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