Chemistry Reference
In-Depth Information
20.3
Preparation of a polymer-modified surface for the stationary phase
of environmental-responsive chromatography
Polymer grafting on the stationary phase appears to be an important determinant in separations. In this
section, three types of polymer-grafting methods on silica surfaces to the preparation of chromatographic
stationary phase are described.
First, polymer-grafted silica beads are constructed using active ester-amine coupling 'grafting to' method
[11, 17, 21] (Figure 20.2a). Chain-transfer free-radical polymerization is utilized to synthesize polymers with
one carboxylic acid end group, which can be derivatized to form
N
-hydroxysuccinimide ester that can be
grafted to the aminopropyl silica supporting materials. The syntheses of semitelechelic PNIPAAm and
its copolymer were carried out by radical polymerization using 3-mercaptopropionic acid and
2,2
-azobisisobutyronitrile as a chain transfer agent and a radical initiator, respectively. This grafting method
has the advantage of a controlled molecular weight of the grafted polymer chain by adjusting the relative
ratios of the monomers to the chain-transfer agent in bulk polymerization, and a dialysis treatment to a pure
polymer [22]. Additionally, it also has the advantage of characterizing the properties such as LCST, molecular
weight, infrared absorption and the polydispersity of synthetic polymer.
A second 'grafting from' synthetic method for PNIPAAm uses a surface-immobilized azo-initiator and
cross-linker to prepare polymer layers with conventional radical polymerization [23, 24] (Figure 20.2b).
Unlike the situation in the 'grafting to' technique, the substrate surface must be modified to generate an
initiator functionality suitable for the polymer hydrogel synthesis from a surface. PNIPAAm hydrogel-
modified matrix is prepared as follows: 4,4
′
-azobis(4-cyanovaleric acid), which is a polymerization initiator,
was immobilized on aminopropyl silica beads. A mixed solution of the monomer and cross linker agent is
added to the beads and polymerized at 70°C for several hours. The PNIPAAm gel-immobilized surface
described here showed a slightly lower transition temperature than that of the PNIPAAm terminally grafted
surface. We observed a drastic and reversible surface hydrophilic/hydrophobic property alternation for a
PNIPAAm-terminally grafted surface due to rapid changes in the polymer hydration state around the
polymer's transition temperature. Compared with a terminally grafted surface, the restricted hydrodynamic
motion of a PNIPAAm segment in grafted hydrogel was considered to be due to a restricted conformational
transition. The hydrogel-modified method for preparing a thermally responsive packing material was
relatively simple and easy compared with the PNIPAAm terminally grafted stationary phase. Additionally, a
polymer layer formed on silica beads showed resistance to an alkaline solution.
A third 'grafting from' method for PNIPAAm is high-density polymer brushes on silica-bead surfaces
using controlled polymerization techniques, such as atom transfer radical polymerization (ATRP), reversible
addition-fragmentation chain transfer (RAFT) polymerization and nitroxide-mediated polymerization (NMP)
[25-27] (Figure 20.2c). By depositing the appropriate initiators, controlled polymerization can lead to
uniform brush layers, tunable brush thickness via molecular weight control, and the ability to perform
sequential polymerization steps to yield either thicker homopolymer layers or diblock copolymer layers.
Among these, ATRP is an attractive polymer grafting method because it enables the preparation of surfaces
with a dense polymer brush from surface immobilized ATRP initiators. The methodology allows the control
of the graft chain length by varying the duration of the polymerization and regulation of the graft density by
varying the concentration of the ATRP initiator on surfaces. This method incorporates a relatively large
amount of polymer onto surface compare to the above two methods.
Yakushiji
et al
. reported temperature-dependent wettability changes for PNIPAAm hydrogel modified
surfaces by aqueous dynamic contact-angle measurements [28]. The graft configuration of PNIPAAm
produced from different grafting methods greatly influences temperature-dependent aqueous wettability
changes. Okano
et al
. reported that the molecular mobility and density of a PNIPAAm chain are greatly
′
