Aloy Polyp a Anima Ar

8/11/2019 Aloy Polyp a Anima Ar

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Methods for Molecular weight Determ

4

th

Yr Chem


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Determines many physical properties.

For eg,Temperatures for transitions from liquids to waxes

Mechanical properties such as stiffness, strength,

Viscoelasticity, toughness, and viscosity.

If molecular weight is too low, the transition temperatures and the properties will generally be too low for the polymer material to have commercial applications


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The molecular weight of a polymer is not one unique value.

Rather, a given polymer will have a distribution of molecular weights.

The distribution will depend on the way the polymer is produced

For polymers we should not speak of a molecular weight, but rather of the dof molecular weight, P(M), or of the average molecular weight,

The type of property being studied will determine the desired type of averagmolecular weight.


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The simplest, most common molecular weight is the number-averagemolecular weight (n)

end-group analysis or colligative properties (b.p. elevation, osmoticpressure, etc)

others commonly used are weight-average molecular weight (w), z-

average molecular weight (z) and viscosity-average molecular weig

(u)

light scattering (w), sedimentation equilibrium (z) and solution viscosi

(u)


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Consider a property which is only sensitive to the number of molecules presentproperty that is not influenced by the size of any particle in the mixture ,for eg:Belevation, Freezing point depression, and Osmotic pressure.

For such properties, =

. =

The more useful, alternate form is:

Wi=weight fraction of polymer i divided by total polymer weight.


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The number average molecular mass of apolymer can be determined by gelpermeation chromatography

High Number-Average Molecular MassPolymersmay be obtained only with ahigh fractional monomer conversionin thecase of step-growth polymerization, as per

the Carothers' equation.


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For property which depends not just on the number of polymer molecules bsize or weight of each polymer molecule.

For eg:Light scattering.

Comparing with Number average,Mn is the average Mi weighted accordingfractions and that Mw is the average Mi weighted according to weight fracti


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