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Chromatography Separation
of Polymers
 Chromatographic
separation is a consecutive equilibrium process of analytes between the mobile and stationary phase. A variety
of chromatography methods exists and they are categorized according to the nature
of the equilibrium process, which is essentially determined by the choice of
the mobile and stationary phase. The equilibrium constant is a function of two
thermodynamic parameters, △Ho
and △So, associated
with the solute partition process between the mobile phase and the stationary
phase.
 ,(Analyte
concentration in mobile phase: Cm,
stationary phase: Cs )
Size exclusion
chromatography (SEC) is the most widely used method in the characterization
of polymers, in which the mobile and stationary phases are identical solvent
but differ in their physical environments: interstitial space between porous
packing materials and the pores. In an ideal SEC separation of flexible chain
polymers, the distribution of solute molecules between the interstitial and
the pore volume is entirely due to the conformational entropy of polymer chains
in the two different environments. Larger (higher molecular weight) polymer
chains distribute more preferentially in the interstitial space since they
suffer more than small molecules from the entropy penalty to be located in the
pore space.(△Ho =
0 and △So < 0)
Therefore a very large polymer chain cannot penetrate into the pores and elute
first after sweeping the interstitial volume (Vi) only
while small molecules (injection solvent, for example) elute last after sweeping
the pore volume (Vp) as well as the interstitial volume.
Therefore, in SEC mode, all the analytes elute between the interstitial volume
and the total void volume (Vo = Vi
+ Vp): VR = Vi
+ KVp, 0 ≤K ≤ 1.
 On the other hand, in the
Interaction chromatography (IC), the retention is mainly due to the interaction
between the stationary phase and the solute molecules dissolved in the mobile
phase. The interaction can be in many different nature; adsorption, partition,
ion exchange, hydrophobic interaction, etc. The interaction strength is often
proportional to the polymer chain length, which is known as the Martin's rule.
Martin's rule
: 
( n: degree of polymerization ), then
In results, the retention of polymer molecules increases exponentially with
the degree of polymerization. In order for a polymer sample with a wide molecular
weight distribution to elute in a reasonable experimental period, it is necessary
to control K
during the elution. One way to control the retention is to change △Go itself: Solvent gradient interaction chromatography.
The other way to control the retention is to change column temperature: Temperature
gradient interaction chromatography (TGIC)
1. TGIC is superior to SEC in resolution and sample loading capacity,
and its retention is sensitive to molecular weight unlike SEC which is sensitive
to chain size.
2. TGIC also has an advantage over solvent gradient HPLC
because it permits the use of refractive index sensitive detection method such
as differential refractometry and light scattering by virtue of its isocratic
elution nature.
3. We have applied TGIC for the precise analysis of several polymer systems
that cannot be done by SEC satisfactorily; precise analysis of MWD of anionically
polymerized polymers, precise characterization of branched polymers, simultaneous
TGIC/SEC characterization of binary polymer mixtures, fractionation of individual
blocks of block copolymers, end group analysis of hydroxy terminated polystyrenes
and so on.
TGIC separation of 14 polystyrene standards.
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