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I S O L A T I O N A N D P U R I F I C A T I O N

O F P O L Y M E R S

W H Y T H E I S O L A T I O N A N D P U R I F I C A T I O N O F P O L Y M E R S?

• Polymers are prepared from solution, emulsion or suspension techniques. Thus they are contaminated with monomers, initiators, solvents, and emulsifiers. This requires isolation and purification. ❖ Filtering or centrifuging and washing ➢When polymers are precipitated from the solvent-monomer mixture as fine powder ➢When polymers are produced by suspension method in the form of beads.

❖Emulsion polymerization

• In this case the polymer can be isolated by breaking the emulsion with an electrolyte such as HCl, NaCl or alum or by freezing.
• The polymer crumbs obtained can be washed with water to free them from the contaminant emulsifier or can be purified by dissolution & precipitation. ❑Purified polymers in all the above cases can be freed from non-solvent either by drying in a vacuum at a suitable temperature ( 60 ) or by employing infrared lamps. ❖Polymer Fractionation
• The process of separating the unfractionated sample into several fractions of

progressively increasing or decreasing average molecular weight.

H O W T O S E P A R A T E T H E O R I G I N A L S A M P L E I N T O

D I F F E R E N T F R A C T I O N S?

1. Fractional precipitation
2. Partial dissolution
3. Gradient elution
4. Gel permeation chromatography

➢Experimentation

• 1  solution of the polymer is taken in a poor solvent in a large container with

provision for stirring and dropwise addition of the non-solvent.

• Set-up is kept in the thermostat.
• Fractional precipitation should be carried at a constant temperature.
• After the system attains constant temperature, the non-solvent is added dropwise

with continuous stirring.

• As non-solvent is added progressively, turbidity appears indicating the

precipitation of highest mol.Wt. species.

• When sufficient turbidity has appeared, addition of the non-solvent is stopped and the

contents are slowly heated till turbidity disappears.

• On heating, the precipitated polymer redissolves at a slightly higher temp.
• Now stirring is stopped and set up is allowed to cool down. To the original

precipitation temp.

• Turbidity reappears.
• Equilibrium between the low mol wt. species in the dissolved state and high mol wt.

species in the precipitated state is established, resulting in effective fractional

precipitation.

N D

M E T H O D O F F R A C T I O N A L P R E C I P I T A T I O N - S O L V E N T

V O L A T I L I Z A T I O N

• Polymer is dissolved in the binary mixture of solvent and non-solvent
• Solvent should be highly volatile compared to the non-solvent.
• Slow suction of the vopour phase is done & the solvent slowly evaporates.
• The solution now becomes richer in non-solvent, so polymer precipitates

P A R T I A L D I S S O L U T I O N ( E X T R A C T I O N ) T E C H N I Q U E

• Principle behind the technique is that the polymer solubility in a solvent is restricted depending on the solvent power , by its molecular weight.
• Poor solvent dissolves only low molecular weight species, whereas good solvent can dissolve higher mol. wt. species too.
• Solvent with different solvent power is taken and the polymer fractions of different mol. wt. are obtained one after the other starting from lowest fraction.

• After complete dissolution of the soluble components, stirring is stopped, filtered & filterate is collected.
• Fiterate is evaporated & the first fraction is obtained.
• To the swollen gel left undissolved by the first set of binary mixture, the next set of binary mixture with higher solvent power is added, stirred & the above process is repeated.
• Thus a series of fractions with increasing average molecular weight can be separated.

G R A D I E N T E L U T I O N T E C H N I Q U E

• This technique also works on the partial

dissolution principle.

• Here, the polymer is precipitated on glass

wool packed at the top of the column filled

with silica or alumina.

• Polymer is eluted successively with solvent/non-solvent mixtures of increasing solvent power.

G E L P E R M E A T I O N C H R O M A T O G R A P H I C T E C H N I Q U E ( G P C )

• It a quick and efficient method for polymer fractionation and simultaneous

computation of the molecular weight distribution curve.

Principle:- In any column chromatographic process as the solute travels

inside the column, from one end to another, it is transferred between two

phases-one remaining stationary and other mobile.

• In GPC, a single liquid (solvent) acts as both stationary and mobile phases.
• The column is filled with gel (in the form of fine spherical beads) made of

cross linked styrene – divinyl benzene copolymer or glass.

• Gel contains micropores of different porosity grades (ranging from 50 to

1000 Å. They are hard, incompressible.

• When the GPC column is filled with suitable solvent, the solvent occupies not only free volume between the beads(void volume) but also volume in the pores(pores volume) inside the gel beads.

G P C S E P A R A T I O N M E C H A N I S M

▪Polymer sample is prepared as a dilute solution ( 0. 5 %) in the suitable solvent and is injected at the top of the column and the column is eluted with the solvent. ▪The GPC column is packed with porous beads of controlled porosity and particle size. ▪Polymer solution along with the solvent starts streaming down the column. ▪ The mobile phase, around the gel beads contains polymer molecules. ▪ Stationary phase within the gel beads does not contain polymer. ▪ Due to this difference in polymer concentration, the polymer starts diffusing into the stationary phase so as to equalise the concentration.

• The pore size restricts the entry and allows molecules up to a certain hydrodynamic size to enter the pores. Thus molecules of bigger size are excluded from entering the pores.
• Thus the biggest sized molecules( highest mol. wt.) are eluted first from the column.
• Now, the polymer molecules are in the pores, & when the solvent is added due to differences in the concentration, a little bigger sized polymer diffuse out and are eluted from the column.
• Thus the smallest molecular weight species are retained in the column for the maximum period and hence elute last and molecules with intermediate sizes elutes in-between.