Preparation and Properties of Monodisperse Thermo-responsive Microgels
2.2 Submicron-Sized Monodisperse Thermo-responsive Core-Shell Hydrogel Microspheres Fabricated via
Surfactant-Free Emulsion Polymerization
In this section, preparation of submicron-sized thermo-responsive core-shell hydro- gel microspheres with poly(N-isopropylacrylamide-co-styrene) [P(NIPAM-co-St)]
cores and poly(N-isopropylacrylamide) (PNIPAM) shells will be introduced, in which the core seeds are prepared by a surfactant-free emulsion polymerization method and shell layers are fabricated by a seed polymerization method [39].
2.2.1 Preparation of P(NIPAM-co-St) Seeds
The P(NIPAM-co-St) seeds are prepared by an emulsifier-free emulsion polymer- ization method. A mixture of styrene and N-isopropylacrylamide is dissolved in 185 ml of deionized water in a 250 ml four-necked round-bottom flask equipped with a condenser, a nitrogen inlet, a thermometer, and a stirrer. Nitrogen is bubbled into the solution, and the mixture is stirred for 30 min to remove oxygen from the monomer solution. Polymerization is initiated by adding 15 ml of aqueous solution containing a certain amount of K2S2O8at 70ıC. The reaction is allowed to proceed for 24 h at 70ıC under stirring. The resulting P(NIPAM-co-St) microsphere samples are dialyzed and purified by repetitive centrifugation, decantation, and redispersion and then freeze-dried.
2.2 Submicron-Sized Monodisperse Thermo-responsive Core-Shell Hydrogel. . . 27
St + NIPAM + (K2S2O8)
NIPAM + (K2S2O8) Core
Shell
Emulsifier-free emulsion polymerization
Seed polymerization Fig. 2.1 Schematic
illustration of preparation procedure of the core-shell microspheres with PNIPAM shell layers (Reproduced with permission from Ref. [39], Copyright (2004), American Chemical Society)
2.2.2 Preparation of Core-Shell Microspheres with PNIPAM Shell Layers
The PNIPAM shell layers are fabricated on the as-prepared core seeds by a seed polymerization method. The preparation procedure of the core-shell microspheres is schematically illustrated in Fig.2.1. When the above-mentioned reaction for the P(NIPAM-co-St) seeds has not been stopped, another 15 ml of aqueous solution containing certain amount of NIPAM is added, and the polymerization is allowed to continue for 22 h under stirring at 200 rpm. By this method, the reaction manner is graft polymerization because of some living radicals on the surfaces of core particles [28, 29], and the resulting shell layers of the core-shell microspheres are hairy because no cross-linker is used. The prepared core-shell microsphere samples are also dialyzed and purified by repetitive centrifugation, decantation, and redispersion and then freeze-dried. The morphology of both core seeds and core-shell microspheres is observed by a scanning electron microscope (SEM S- 450, Hitachi). All specimens for SEM observations are sputtered with gold at fixed conditions (time 150 s, current 20 mA, voltage 2 kV).
Fig. 2.2 SEM photographs of the core-shell microspheres with P(NIPAM-co-St) cores and PNIPAM shell layers (Reproduced with permission from Ref. [39], Copyright (2004), American Chemical Society)
2.2.3 Monodispersity of Core-Shell Microspheres
with P(NIPAM-co-St) Cores and PNIPAM Shell Layers
Figure 2.2 shows the SEM photographs of the monodisperse core-shell micro- spheres with P(NIPAM-co-St) cores and PNIPAM shell layers. The PNIPAM shell layers are fabricated on the core seeds by a seed polymerization method. By this method, the reaction manner is graft polymerization because of some living radicals on the surfaces of core particles. Because the number of core seeds per unit volume is constant and monomer NIPAM contributes only to the PNIPAM shell formation on the seeds, the mean diameters of the core-shell microspheres became larger simply with increasing the NIPAM dosage. On the other hand, the free-radical density of smaller particles is relatively larger, which is helpful for the smaller particles to absorb more monomers or polymers with low molecular weight onto their surfaces to form larger particles. Therefore, the core-shell microspheres with P(NIPAM-co-St) cores and PNIPAM shell layers are highly monodisperse.
2.2.4 Thermo-responsive Characteristics of the Core-Shell Microspheres with PNIPAM Shell Layers
The hydrodynamic diameters of the prepared core-shell hydrogel microspheres at different temperatures are determined by temperature-programmed photon corre- lation spectroscopy (TP-PCS; Brookhaven BI-9000AT). This technique has been applied extensively to the characterization of such material, as it allows for in situ size characterization of soft material that cannot be reliably sized by electron micro- scopes due to deformation and/or dehydration under vacuum [2]. The dispersed
2.3 Positively Thermo-responsive Submicron-Sized Monodisperse. . . 29
200 250 300 350 400 450
22 25 28 31 34 37
Temperature [οC ]
Hydrodynamic diameter [nm]
[NIPAM] = 1.0 g [NIPAM] = 2.0 g
Fig. 2.3 Effect of the NIPAM dosage in the preparation of shells on thermo-responsive swelling characteristics of the core-shell microspheres (Reproduced with permission from Ref. [39], Copyright (2004), American Chemical Society)
particles in water are allowed to equilibrate thermally for 10–15 min before mea- surements are taken at each temperature. The hydrodynamic diameters of particles are calculated from diffusion coefficients by the Stokes-Einstein equation, and all correlogram analyses are performed using the manufacturer-supplied software. In the data presented here, each data point at a given temperature represents the average valve of 15–20 measurements, with a 20 s integration time for each measurement.
Figure2.3shows the effect of NIPAM dosage in the preparation of shell layers on the thermo-responsive swelling characteristics of the core-shell microspheres.
With increasing the NIPAM dosage in the fabrication of the shell layers, the thermo-responsive swelling ratio of the hydrodynamic diameters of the core-shell microspheres at temperatures below the LCST of PNIPAM to those above the LCST increases. The hydrophilic groups of hairy PNIPAM chains on the core- shell microsphere surfaces form hydrating layers by hydrogen bond with water.
The longer the PNIPAM chains result from increasing NIPAM dosage, the thicker the hydrating layer and then the larger the hydrodynamic diameter. The thickness of the hydrating layer decreases because of the breakage of hydrogen bonds with increasing temperature. When temperature approaches to the LCST, hydrogen bonds are broken seriously, which leads the thickness of hydrating layer to decrease rapidly, and then the linear PNIPAM polymer chains collapse quickly, resulting in a rapid decrease in the hydrodynamic diameters of the core-shell microspheres.