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Chromatography 
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Chromatography Separation of Polymers

hplc1Chromatographic 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.




1,(Analyte concentration in mobile phase: Cm, stationary phase: Cs )

 

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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.

 

 

 

 

 

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 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 : 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.


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                             TGIC separation of 14 polystyrene standards.