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Lec-05

Stereo Isomerism

In a syndiotactic configuration, the R

groups alternate sides of the chain while

for random positioning, the term atactic

configuration is used.

Syndiotactic configuration

Atactic configuration

2. Stereo Isomerism

Another example of isotatctic polymers is polypropylene chains where all

methyl CH3 substituent are on the same side as shown in Figure.

But due to steric repulsion between adjacent methyl groups, the chain would

fold into a new orientation, in which some of the methyl groups take the

upper and lower side of the backbone carbon chain, which is syndiotactic.

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2. Stereo Isomerism

If the random arrangement results, it will be called atactic.

Another example is PVDF with α-phase and β-phase which are

stereoisomers.

Syndiotactic

Isotactic

3. Structural Isomerism

Or geometric isomerism, occurs where mer units have double bond between

carbon atoms. For example consider the isoprene repeat structure

In which the CH3 group and the H atom are positioned on the same side of

the double bond. This is termed a cis-structure, and the resulting polymer,

cis-polyisoprene, which is natural rubber.

In trans-structure, the CH3 and H reside on opposite

sides of the double bond. Trans-polyisoprene (gutta percha)

properties are different from natural rubber.

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Thermoplastic Polymers

The thermoplastics softens and can be made to flow when it is heated. This

allows them to be fabricated into the required component and become solid

when cooled to RT.

Thermoplastics consists of linear polymers which can not cross-link (although

may be branched) to form a rigid network. The bonding between chains is due

to the weak van der Waals forces.

Thermoplastics may be repeatedly heated and cooled, hence recyclable but

may degrade with repeated cycling unless stabilised.

Examples: Nylon(polyamide), polystyrene, polyethylene, polycarbonate, PMMA

(Perspex), PVC and PET

Thermosetting Polymers

Thermosets readily cross-link/network during curing by heat/pressure or by

a catalyst/curing agent/hardener. Bonding is covalent which restricts the

movements of chains past each other under stress or temperature resulting in

increase in the glass transition temperature Tg to above room temperature.

Due to the cross-linking, thermosets can not be re-shaped/resoftened by

reheating, rather they degrade on reheating. Thermosets are also found brittle

at room temperature. Ccomparably, thermosets are more resistant to chemical

attacks.

Examples of thermosetting polymers are Bakelite, Epoxies, Polyester,

synthetic Rubber, Polyimide etc.

Polyethylene and poly(vinyl chloride) wire coverings and pipe can be

converted to thermoset structures by cross-linking their molecules

photochemically (by high-energy radiation) or by decomposition of peroxides.

The main advantage of this cross-linking is enhanced dimensional stability

under load and elevated temperatures.

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Which polymers are these?

Rubber

PE

PMMA

PS

Bakelite

PVC PET

Molecular Weight of Polymers

During polymerisation, not all polymer chains will grow to the same length.

This results in a distribution of chain lengths or MW’s. Therefore the

molecular weight of polymers is expressed as average. There are two ways

to express it.

Number Average Molecular Weight

Is obtained by dividing the chains into series of size ranges and then

determining the number fraction of chains within each size range. It is

expressed as given by;

Where Mi is the mean molecular weight of size range i, and xi is the fraction of

the total number of chains within the corresponding size range.

iin MxM

nM

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Molecular Weight of Polymers

Weight Average Molecular Weight

Is based on the weight fraction of the molecules within various size ranges.

Where wi denotes the weight fraction of the molecules within the same size range.

Degree of Polymerization (DP)

Is the average number of repeat units in the polymer chain. DP is related to the number-average molecular weight by the equation

Where ‘m’ is the molecular weight of the repeat unit or mer.

Polydispersity Index of the polymer

Tells us how broad the molecular size/weight distribution is

iiw MwM

wM

m

MDP

n

nM

n

w

M

MPDI

Polydispersity Index

Mw/Mn = 1 monodisperse

Polymer sample consisting of molecules all of which have the same

If chain length Mw/ Mn > 1 polydisperse

Polymer consisting of molecules with the variety of chain length

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Molecular Weight of Polymers

What we need to do is to make a list of Mi and xi to calculate the number

average MW as follows.

Molecular Weight of Polymers

Mi (kg/mol) xi xiMi Wi=xiMi/∑xiMi

7.5 0.05 0.37 0.37/21.14=0.02

12.5 0.16 2.0 2.0/21.14=0.09

17.5 0.22 3.85 3.85/21.14=0.18

22.5 0.27 6.07 6.07/21.14=0.29

27.5 0.2 5.50 5.5/21.14=0.26

32.5 0.08 2.60 2.6/21.14=0.12

37.5 0.02 0.75 0.75/21.14=0.03

No. average ∑xiMi=21.14

kg/mol

Weight avg.

Calculate wiMi , Mw , PDI and DP?