Lab.343.

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Presentation transcript:

Lab.343

Experiment (9) Molecular weight of a polymer from viscosity measurements

polymers  are a large class of materials consisting of many small molecules (called monomers) that can be linked together to form long chains.

Types of polymers: 1-Natural ex. Natural rubber, DNA and proteins 2-Industrial(synthetic) ex. plastics such as polystyrene

Polystyrene (C8H8)n Polymerization

The ratio of the viscosity (ɳ) of a solution polymer molecules to the viscosity (ɳ◦) of the solvent related to the molecular weight M of the polymer (ɳ / ɳ◦ - 1) ______________ = K Mα C K : is a constant for any given type of polymer. α : is a function of the geometry of the molecule. C : is the number of grams of polymer in 100 ml of solution.

ɳsp/C = K Mα This equation is only valid for very dilute solution. (ɳ / ɳ◦ - 1) is known as the specific viscosity ɳsp/C = K Mα This equation is only valid for very dilute solution. The graph which is drawn of ɳsp/C vs C. Intrinsic viscosity [ɳ] [ɳ]= limc 0 ɳsp/C

Intrinsic viscosity [ɳ] Molecular weight [ɳ] = K Mα

Procedure: Prepare the stock solution of the polystyrene in toluene by weighting 5gm of the polystyrene dissolved in 200ml toluene. Make 5 different concentration by diluting the stock solution to 2,4,8 and 16 times. Measure the average time of flow (t) for each of the above 5 concentrations using viscomter at 25◦C. Measure the average flow time of toluene (t◦) at 25◦C.

Results ɳsp /C ɳsp = ɳ/ɳ◦ -1 ɳ/ɳ◦ = t/t◦ t◦ =ɳ◦ t= ɳ C

ɳsp /C [ɳ] = K Mα [ɳ] C ……..=3.7×10-4 M 0.62 M 0.62 = ……/ 3.7×10-4 0.62 ln M = ln (……/ 3.7× 10-4 ) ln M = /0.62 M= g/mol ɳsp /C [ɳ] C