pHYSICAL cHEMISTRY - Ii

Experiment 4

Aim of the Experiment

Determine the molecular weight of a high polymer, say polystyrene, from viscosity measurements.

Principle

The viscosity of a high polymer solution is related to the molecular weight of the polymer by Mark-Kuhn-Houwink (MKH) equation.

[ η ] = KM^a …………..(i)

Where [ η ] is the intrinsic viscosity and M is the average molecular weight of the polymer: K and a are the constants for a given solvent-polymer system at a particular temperature.

The constant a lies in most cases between 0.6 and 0.8

Intrinsic viscosity is defined by

Where C is the concentration in g of polymer in 100 cm³ (dl-decilitre) of the solvent.

Intrinsic viscosity can be obtained by plotting ηsp / C against concentration (abscissa) and extrapolating the curve nto C= 0; the intercept on the ordinate axis is the intrinsic viscosity.

Thus, molecular weight of a polymer can be determined from viscometric measurements using eq. (i), provided K and a are known. These are obtained from the samples of same polymer of known molecular weight, determined by other methods.

Materials required

Apparatus: Ostwald’s viscometer or better a suspended level viscometer, stop watch, thermostat at 25 ^oC,

Chemicals : Polystyrene, toluene or benzene as the solvent.

Procedure

(i) Prepare a solution containing about 0.5 g (but accurately weighed) polystyrene in 25 g of toluene. This gives 2% solution. Take a suitable volume of the solution in viscometer thermostated at 25 ^oC and determine the time of flow 3 to 4 times.

(ii) Withdraw the solution from the viscometer and by means of graduated pipette mix equal volumes of this solution and the solvent, and determine the time of flow of the same volume of the resulting solution (1%). Viscometer must, of course, be rinsed with this solution before filling it.

Similarly, prepare 0.50 and 0.25 % solution by serial dilutions and determine the time of flow at each dilution.

(iii) Finally, determine the time of flow of the same volume of the pure solvent.

(iv) Determine the densities of pure solvent and the solutions.

Use of Suspended Level Viscometer

(i) Take 20 Cm³ of the solvent in the viscometer and determine the flow time

(ii) Empty the viscometer and dry it. Then introduce into it 20 Cm³ of the polystyrene solution (2%) using a pipette. Determine the flow time following the usual precautions.

Observations (i) (ii) (iii) Mean

(1) Flow time of the solvent

(2) Density of the solvent ρ₀

Temperature of thermostat = ^oC

Observation Table

Thus, calculate and tabulate the values of η / η0 , η sp and η sp /C .Plot a graph between and the concentration (abscissa) and from the intercept of the straight line (when C=0),calculate the intrinsic viscosity.

Then,using the expression (i) calculate the intrinsic viscosity of the polymer, taking known values of K and a. ;molecular wt.can be determined.

M = Antilog 1 /a [ log η - log K ]

Result

Molecular weight of a polystyrene is

Questions

1.State different methods for determination of mol.wt. of polymers .

2.How viscosity is related with molecular wt.of polymer?

3.Why did you use toluene? Can you use another polar solvent?

4.How does Ostwald viscometer work.

5.What is intrinsic Viscosity?

Reference Material

1.Advanced Practical Physical Chemistry by J.B. Yadav

2.Polymer Science by V R Gowarikar,N V Viswanathan,Jayadev Sreedhar

3.Textbook of Poymer Science by Fred.W.Billmeyer