Molecular Weight Determination:

INTRODUCTION:

• Various types of molecular weight ( Mn,Mw,Mz,Mv).
• Type of MW depends upon the exptl. Technique and the property measured (number, mass, viscosity, contrifugatiuon ).
• Colligative/number cryoscopy , Ebulliometry and osmometry,chemical bond-Mn
• Viscosity measurement-Mv
• Light scattering, ultracentrifugation -Mw
• All techniques makes use of polymer solution, hence concentration.dependent.

MOLECULAR WEIGHT:

The sum of the atomic masses of all atoms in a molecule.

TECHNIQUES

1. Cryoscopy (Depression Of Freezing point):

2) EBULLIOMETRY ( ELEVATION O OF BOILING POINT):

THERMODYNAMIC RELATIONSHIPS:

1).MEMBRANE OSOMOMETRY:�

• widely used to determine Mn.
• Based on the phenomenon of osmosis.
• Osmosis – defination.
• Osmotic pressure(𝜋):Pressure that must be applied to the solution to totally stop the flow

𝜋/𝑅𝑇𝐶=1/Mn =BC

(𝜋/RTC)c→𝑜=1/Mn=BC

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TYPES OF OSMOMETER:

1. Fuss-Mead block osmometer.

2.Pinner-Stabin glass osmometer.

3.Stabin-shell automatic osmometer.

4.Reiff osmometer.

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1.Static-Less preferred

2.Dynamic

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Categories:

• SOLVENT WILL PASS TO SOLUTION TILL HYDROSTATIC PRESSURE MATCHES OSMOTIC PRESSURE I.E.OSMOTIC HEAD

END GROUP ANALYSIS

• Number average molecular weight is obtained .
• The polymer must have reactive functional groups at one or both end such as –COOH,-OH etc.
• If the chain contain two functional group –it means for every two functional group identified , one molecule has to be present
• Usual chemical methods are used to determine total number of functional groups.
• The number average molecular weight may be determined experimentally by measuring the total number of moles of polymer in a know weight of sample.
• Thus,the end groups can be analyzed,the number of polymer molecules may be calculated using simple stoichiometric relationships

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• This method for determining Mn is called end group analysis.

Knowledge of functionally and functional group equivalent gives MW using following equation

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Mn=Functionally/Functional group equivalent.

Size Exclusion Chromatography(SEC): Gel Permeation Chromatography(GPC):

• A method to sort molecules according to their size in solution.
• Column is filled with rigid,porous,materials,and is carried by the solvent through the packed column.
• Sample solution is injected into this column.
• Takes about 30 mint. To know MW and MWD.
• The selection of ab appropriate solvent and the column is important.
• It is necessary to have calibration curves in order to determine the relative molecular weight form a given rentention volume/time.

Viscometry:

• Viscosity is resistance to flow,
• The molecular weight of the polymer is very high.
• Polymers are big molecules i.e.HMW.they occupy more space.hence offer more resistance.
• Viscosity of pure solvent is low.

Materials Required:

• Ostwald viscometer, stop watch, suction bulb, pipette.

Solvents:

• Acetonitrile, Acetone, Water, Toluene, Benzene.

Polymer:

• Polyvinyl acetate, PMMA, polymer alcohol, polystyrene.

Procedure:

• Select the polymer.
• Select the solvent.
• Determine the time of flow of the solvent.
• Determine the time of flow of polymer solvent system at different concentration.
• From the concentration and time of flow,the inherent viscosity,and reduced viscosity are calculated the equation;

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• A graph is drawing by plotting reduced viscosity against concentration and inherent viscosity against concentration.
• Intrinsic viscosity can be obtained by graph to zero concentration.

DISTRIBUTION LAW:

• Distribution law was given by Walther Herman Nernst.
• Also known as ‘’Nernst law of distribution’’ or ‘’patient cofficient’’.
• Distribution law is given to find out the distribution of solute in two immiscible solvents.
• According to Nernst law of distribution if a solute X distributes itself between two immiscible solvents A and B at constant temperature and same molecular conditions.

Kd=CA/CB

Kd=distribution constant

CA=concentration of X in A

CB=concentration of X in B

LIMITATIONS:

• Distribution law is only applicable for very dilute solvents.
• Temperature must be constant throughout the experiment.
• Solute must be in same molecular state in both solvents.
• The concentration of solute should be measured after the achievement of equilibrium.
• Solvent must be immiscible.

Applications:

• Solvent extraction.
• Determination of degree of dissociation.
• Determination of solubility.