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Monomers and Polymers
Do you know what a monomer is? Do you know about polymers? What are their average molecular weights?
Subject: Monomers and Polymers
23-Feb-2012
QuimiNet
Petrochemical
Monomers are low molecular weight compounds that can bind to other small molecules (either the same or different ones) to form long chain macromolecules commonly known as polymers.
Polymers are mixtures of macromolecules of different molecular weights. Therefore, they are not pure chemical species and do not have a definite melting point. Each of the species that form a polymer has its own specific molecular weight (Mi).
Therefore, to characterize a polymer sample, the molecular weight distribution of the molecules of the species that comprise the sample is sought: the proportion (usually by weight, wi) of the strings of each "Mi" that form the mixture.
Average Molecular Weights
The molecular weight distribution is obtained by means of the SEC technique (i.e., size exclusion chromatography). Other characterization techniques provide average values of molecular weight:
|
AVERAGE |
SYMBOL |
TECHNICAL |
DEFINITION |
|
Number |
Mn |
Osmometry |
|
|
Viscous |
Mv |
Capillary viscometry |
|
|
Weight |
Mw |
Light diffusion |
|
|
z, Third average |
Mz |
Ultra-centrifugation and Diffusion |
 |
|
z+1, Fourth average |
Mz+1 |
Ultra-centrifugation and Sedimentation |
 |
with Ni the number of macromolecules of molecular weight Mi. Given that the weight fraction of each macromolecule is
 |
the average number and weight can be calculated by the expressions:
 |
 |
The mean z and z +1 are the least used. The average viscosity is close to the number average or weight average depending on the exponent, which is the parameter of the equation of the Mark-Houwink viscometer. The relation of different averages is:
Mn < Mv < Mw < Mz < Mz+1
Polydispersity Index
This is the ratio between the weighted average molecular weight and the number average:
The result is always greater than 1 and characterizes the width of the distribution of molecular weights. When it has values close to one (1
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