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,
Ni = Number of moles of molecules
Mi = 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,
Ni = Number of moles of molecules
Mi = Molecular weight of the
repeating unit (or 1monomer)
W = Weight of polymers; wi = Weight of molecules
ð It
can be determine by viscosity method, light scattering method and
ultracentrifugal method.
