Poly(N-isopropylacrylamide) (PNIPAAm)-modified silica with varied grafting density was prepared via surface-initiated atom transfer radical polymerization (SI-ATRP). The grafting density of PNIPAAm on silica surface was modulated by controlling the ATRP-initiator density immobilized on surface which was regulated by changing the feed ratios of 2-bromo-2-methylpropionyl bromide to acetyl chloride. The grafting density and amounts of PNIPAAm on surface were characterized by elemental analysis, thermogravimetric analysis (TGA), and gel permeation chromatography (GPC), respectively. The results demonstrated that shorter PNPAAm chains with higher grafting density and larger amounts formed on silica surface while longer PNIPAAm chains with smaller amounts were sparely grafted onto silica surface. The influence of grafting conformation on temperature-responsive property of stationary phase surface was investigated by evaluating the elution behavior of five steroids using water as a mobile phase. On thin grafting surface, shorter retention time, narrower peak width, and better resolution were observed compared with thick PNIPAAm grafted surface at temperatures both above and below phase transition temperature, i.e., 32 �C. This is because analyte molecular more readily diffused among expanding polymer chains at temperatures below 32 �C and relative weak hydrophobic interaction allowed for the fast and excellent elution at temperatures above 32 �C on sparely grated PNIPAAm surface. The two interactions influence the retention factor alteration across 32 �C. On dilute PNIPAAm grafted surface, temperature-dependent retention time changes were more remarkable. PIPAAm grafting density has a crucial influence on separation efficiency of thermosensitive chromatographic stationary phase because the interaction of analytes with silica surfaces was greatly affected by the grafting structure of PNIPAAm chains.

Inferences of grafting densities of poly(N-isopropylacrylamide) on hydrophilic surface on its thermosensitivity and separation efficiency of steroids

245th National Meeting (2013)

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