Molecular Mass/ Weight of Polymers

ð  For many reasons, particularly to know more about polymer molecular systems, it is necessary to characterize them with respect to

(i)      The chemical identity of their repeat units,

(ii)     Nature of end groups present,

(iii)   Existence of branching with nature of branch units and their frequency,

(iv)    Presence of comonomer units and also copolymer composition and comonomer sequence distribution in copolymer systems,

(v)     Solubility and associated features,

(vi)   Optical properties covering clarity or degree of clarity and refractive index, R

(vii) Resistance properties with reference to thermal, mechanical and electrical resistances, photoresistance or photostability, chemical and weather resistance, corrosion resistance, and also bioresistance or resistance to biodegradation.

ð  But what is more important and fundamental is knowledge about the molecular weight of a given polymer.

ð  For molecular weight determination, it is necessary to dissolve the polymer in an appropriate solvent and begin with a dilute solution.

ð  It is very difficult to determine the molecular weight of polymers because the molecules in polymeric chain vary considerably.

ð  Therefore, the molecular weight of any polymer is determined as average values.

·         Number Average Molecular Weight (Mn)

·         Weight Average Molecular Weight (Mw)

   

Number Average Molecular Weight (Mn)

ð  It is based on number of molecules in polymeric chain.

ð  It is defined as multiple of total number of monomer (or repeating unit) with its molecular weight divided by total number of monomers.

Where, N_i = Number of moles of molecules

 M_i = Molecular weight of the repeating unit (or 1monomer)

ð  It can be determined by end group analysis, osmometry method, cryscopy and ebull ioscopy.

Weight Average Molecular Weight (Mw)

ð  It is based on the weight of monomers.

ð  It is defined as summation of multiple of molecular weight of chain length and weight of molecules of chain length divided by total weight of molecule of chain length.

Where, N_i = Number of moles of molecules

                                     M_i = Molecular weight of the repeating unit (or 1monomer)

                                     W = Weight of polymers;  w_i = Weight of molecules           

ð  It can be determine by viscosity method, light scattering method and ultracentrifugal method.