Web Release Date: December 18,
Effects of Graft Densities and Chain Lengths on Separation of Bioactive Compounds by Nanolayered Thermoresponsive Polymer Brush Surfaces
and
Institute of Advanced Biomedical Engineering and Science, Tokyo Women's Medical University, 8-1 Kawadacho, Shinjuku, Tokyo 162-8666, Japan, Department of Materials Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan, and Department of Physical Pharmaceutical Chemistry, Kyoritsu University of Pharmacy, 1-5-30 Shibakoen, Minato, Tokyo 105-8512, Japan
Received June 21, 2007
In Final Form: August 18, 2007
Abstract:
We have prepared various poly(N-isopropylacrylamide) (PIPAAm)-grafted silica bead surfaces through surface-initiated atom transfer radical polymerization (ATRP) by changing graft densities and brush chain lengths. The
prepared surfaces were characterized by chromatographic analysis using the modified silica beads as chromatographic
stationary phases. ATRP initiator (2-(m,p-chloromethylphenyl)ethyltrichlorosilane) density on silica bead surfaces
was modulated by changing the feed composition of the self-assembled monolayers (SAMs) of mixed silane coupling
agents consisting of ATRP initiator and phenethyltrichlorosilane on the surfaces. IPAAm was then polymerized on
SAM-modified silica bead surfaces by ATRP in 2-propanol at 25 
C. The chain length of the grafted PIPAAm was
controlled by simply changing the ATRP reaction time at constant catalyst concentration. The thermoresponsive
surface properties of the PIPAAm-grafted silica beads were investigated by temperature-dependent elution behavior
of hydrophobic steroids from the surfaces using Milli-Q water as a mobile phase. On the surfaces grafted with shorter
PIPAAm chains, longer retention times for steroids were observed on sparsely grafted PIPAAm surfaces compared
to dense PIPAAm brushes at low temperature, because of hydrophobic interactions between the exposed phenethyl
groups of SAMs on silica surfaces and steroid molecules. Retention times for steroids on dilute PIPAAm chain columns
decreased with temperature similarly to conventional reverse-phase chromatographic modes on octadecyl columns.
This effect was due to limited interaction of solutes with the PIPAAm-grafted surfaces. Retention times for steroids
on dilute PIPAAm brush surfaces with longer PIPAAm chains became greater above the PIPAAm transition temperature.
At low-temperature regions, hydrated and expanded PIPAAm at low temperatures prevented hydrophobic interactions
between the phenethyl group of SAMs on the silica bead surfaces and steroid molecules. Retention times for steroids
on a dense PIPAAm brush column increased with temperature since solvated polymer segments within the dense brush
layer undergo dehydration over a broad range of temperatures. In conclusion, PIPAAm graft density has a crucial
influence on the elution behavior of steroids because of the interaction of analytes with silica bead interfaces, and
because of the characteristic dehydration of PIPAAm in dense-pack brush surfaces.
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