Molecular Mass

The molecular mass is the relationship between the mass of a molecule and the unit of mass of the atoms: uma (equivalent to 1/12 of the mass of a carbon 12 atom).

It can be obtained by the addition of the Atomic mass of each atom of the molecule multiplied by their numerical index in the rough Formule or measured in experiments by Spectrometry mass.

The molar Masse is equivalent to as many grams as there are units in the molecular mass.

Molecular mass of polymers

A Polymeric making up of a distribution of chains different lengths, one must speak about average molecular mass:

That is to say X the Degree of polymerization
That is to say nx the number of Macromolecule S of degree of polymerization x.
That is to say Mx mass of such macromolécules.
That is to say M0 mass of a Monomeric

  • One defines the average molecular mass of number like:

\ bar M_n= \ frac {\ sum n_xM_x} {\ sum n_x}

  • In the same way, one defines the average molecular mass in weight like:

\ bar M_w= \ frac {\ sum n_xM_x^2} {\ sum n_xM_x}

NB: In the case of polymeric isomoléculaires, these two masses are identical.

  • One defines the index of polymolecularity by:

I= \ frac {\ bar M_w} {\ bar M_n}

The properties of polymers depend largely on these two average masses:
High mechanical properties require a high \ bar M_n while a implemented easy request a \ bar M_wmodérée.

These masses can be given by steric Chromatographie of exclusion (SEC) also called Chromatographie on permeable freezing (GPC).

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