Polymer HPLC limiting condition

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. 

The latter solubility based methods do not directly belong to chromatography, however, they can be offline combined with polymer HPLC. Moreover, the solubility effects are directly employed in some coupled chromatographic methods (compare section 11.8.4, Eluent Gradient Polymer HPLC), while some of them employ the tendencies to phase separation rather than the complete precipitation processes (see section 11.8.6, Liquid Chromatography under Limiting Conditions of Enthalpic Conditions). 

LC-tandem MS was recently used for polymer/additive characterisation. In cases of soft ionisation processes (e.g. ESI, APCI, etc.), MS/MS is often necessary to confirm the ionic species. QITMS has the potential to improve the detection limits for organotin analysis compared to QMS. HPLC-UV and LC-API-MS/MS have been employed for the characterisation of the products of photodegradation of benzotriazole-based UV absorbers (Tinuvin P/328/900) under mild conditions [642]. Among the photoproducts identified... 

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. 

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