Polymer HPLC critical conditions

The concept of entropy-enthalpy compensation resulting in the critical conditions of enthalpic interactions and the molar mass independent sample retention turned out useful also for the understanding several other coupled methods of polymer HPLC. It is accepted [195,196] that the polymer species tend to elute at the critical conditions also when either eluent strength or quality change within the HPLC system in the course of the HPLC experiment that is in the continuous and local gradient methods (Sections 16.5.3, 16.5.4, and 16.5.6). Irrespective of the problems and limitations of LC CC, its concept belongs to the important breakthroughs in polymer HPLC. 

All above homopolymers are used also for the identification of suitable conditions for the coupled polymer HPLC techniques. Typical examples are liquid chromatography under critical (LC CC) and limiting (LC LC) conditions, and eluent gradient liquid chromatography (EG LC). For the development of latter methods, several defined statistical and block copolymers are available. 

To sununarize, the processes taking place in the colunm of polymer HPLC cause either acceleration or deceleration of separated macromolecules in relation to elution rate of mobile phase molecules. Only in case of critical conditions the elution rate of macromolecules and molecules of mobile phase is equal. 

Until recently, standard HPLC methods for polymers, e.g., gradient chromatography or chromatography at critical conditions (LCCC), were limited to ambient or slightly elevated temperatures [134, 135]. The majority of published HPLC separations were conducted at operating temperatures of a maximum of 80°C. These temperatures are too low for the dissolution of polyolefins, which require at least 120°C for dissolution due to their mostly semicrystalline nature. It was, therefore, a challenge to develop HPLC methods for the separation of polyolefins that operate at temperatures of 120°C and higher. 

Eluent gradient adsorption chromatography is one of the main HPLC techniques used in polymer fractionation including deposition-dissolution, chromatography under critical conditions, temperature gradient adsorption LC and precipitation dissolution LC [24-30]. 

An additional technique that has been found useful in analysis of the composition of polymers and blends is liquid chromatography. Pasch and Rode used the critical point of adsorption of the least polar component of a blend to determine the liquid chromatographic conditions for separating blends of polymethacrylates into components (50). High pressure liquid chromatography (HPLC) in combination with mass spectroscopy was used to analyze the components of an epoxy resin (51). HPLC has also been used with a precipitation-redissolution technique to separate polymer molecular weights for several polymers as a shorter technique compared to SEC (52). Reverse-phase liquid chromatography with UV detection was useful in qualitative determination of brominated flame retardants in polymeric waste materials (53). 

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