Preparation and Properties of Monodisperse Thermo-responsive Microgels
2.3 Positively Thermo-responsive Submicron-Sized Monodisperse Core-Shell Hydrogel Microspheres
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.
2.3 Positively Thermo-responsive Submicron-Sized
them have been prepared with PNIPAM and some with poly(N-vinylcaprolactam) (PVCL). In certain applications, however, an inverse mode of the thermo-responsive phase transition behavior is preferred. In this chapter, a strategy will be introduced to prepare monodisperse thermosensitive core-shell hydrogel microspheres featured with attractive monodispersity and positively thermo-responsive volume-phase transition characteristics, i.e., the particle swelling is induced by an increase rather than a decrease in temperature [27,40].
2.3.1 Preparation of Positively Thermo-responsive
Submicron-Sized Monodisperse Core-Shell Hydrogel Microspheres
The schematic illustration of the preparation of the positively thermosensitive core- shell hydrogel microsphere is illustrated in Fig.2.4. The microsphere is composed of a poly(acrylamide-co-styrene) (poly(AAM-co-St)) core and an interpenetrating polymer network (IPN) shell of poly(acrylamide) (PAAM) and poly(acrylic acid) (PAAC). It is known that PAAM and PAAC form polycomplexes in solution through hydrogen bonding. By the cooperative “zipping” interactions between the molecules that result from hydrogen bonding, a positively thermosensitive volume- phase transition characteristic of PAAM-/PAAC-based IPN macroscopic hydrogels in water has been found [47]. When the environment temperature is lower than the upper critical solution temperature (UCST) of the PAAM-/PAAC-based IPN gel, PAAC forms intermolecular hydrogen bonds with PAAM, and the IPN hydrogels keep shrinking state by the interaction between two polymer chains or the so-called chain-chain zipper effect; on the other hand, when the environment temperature is higher than the UCST of the IPN gel, PAAC dissociates intermolecular hydrogen bonds with PAAM, and the IPN hydrogels keep swelling state by the relaxation of the two polymer chains. Therefore, the microspheres shrink at temperatures below the UCST due to the complex formation by hydrogen bonding and swell at temperatures above the UCST due to PAAM/PAAC complex dissociation by the breakage of hydrogen bonds.
The core-shell microspheres with PAAM-/PAAC-based IPN shells are fabricated in a three-step process [27]. In the first step, monodisperse poly(AAM-co-St) seeds are prepared by emulsifier-free emulsion polymerization. In the second step, PAAM or poly(acrylamide-co-butyl methacrylate) (poly(AAM-co-BMA)) shells are fabri- cated on the microsphere seeds by free-radical polymerization. In the third step, the core-shell microspheres with PAAM-/PAAC-based IPN shells are finished by a method of sequential IPN synthesis. After repetitive centrifugation, decantation, and redispersion with well-deionized water, the core-shell seeds with PAAM or poly(AAM-co-BMA) shells are immersed in aqueous acrylic acid (AAC) solution containing potassium persulfate (KPS) and methylenebisacrylamide (MBA) as
2.3 Positively Thermo-responsive Submicron-Sized Monodisperse. . . 31
Fig. 2.4 Schematic illustration of the preparation of the positively thermosensitive core-shell hydrogel microsphere. (a) Monodisperse poly(acrylamide-co-styrene) (poly(AAM-co-St)) seeds prepared by emulsifier-free emulsion polymerization, (b) microsphere seeds with PAAM shells prepared by free-radical polymerization, (c) microspheres with PAAM-/PAAC-based IPN shells prepared by a method of sequential IPN synthesis, (d) thermosensitive PAAM-/PAAC-based IPN microspheres in the swollen state at temperatures above UCST due to PAAM/PAAC complex dissociation by the breakage of hydrogen bonds, and (e) microspheres in the shrunken state at temperatures below the UCST due to the complex formation by hydrogen bonding (Reproduced with permission from Ref. [27], Copyright (2003), Wiley-VCH Verlag GmbH & Co. KGaA)
initiator and cross-linker, respectively, to swell for 24 h, and the monomer AAC is subsequently polymerized and cross-linked within the initial PAAM matrix gels of the seed shells to form PAAM-/PAAC-based IPN.
2.3.2 Morphological Analyses of the Microspheres
Figure2.5shows SEM images of the poly(AAM-co-St) seeds with PAAM shells and corresponding microspheres with PAAM-/PAAC-based IPN shells, from which it can be seen that both the core-shell seeds and the resulting microspheres are highly
Fig. 2.5 SEM images of seeds with PAAM shells and the resulting microspheres with PAAM-/PAAC-based IPN shells. (a, c) seeds and (b, d) corresponding microspheres. (a, b) No butyl methacrylate (BMA) is added in the preparation of PAAM shells on seed cores (the second step), and [MBA]/[AAC]D1 wt% in the synthesis of IPN; (c, d) [BMA]/([BMA]C[AAM])D34.4 wt% in the preparation of PAAM shells on seed cores, and [MBA]/[AAC]D1.5 wt% in the synthesis of IPN. Scale bar 1m (Reproduced with permission from Ref. [27], Copyright (2003), Wiley-VCH Verlag GmbH & Co. KGaA)
monodisperse. Because the seeds with PAAM shells are the initial matrix for the IPN synthesis, the monodispersity of the final IPN microspheres is dependent on that of the core-shell seeds consequently. Another interesting phenomenon is that the SEM images show that the mean particle size of the IPN microspheres appears to be a little smaller than that of the core-shell seeds. This is due to the freeze-drying method for the preparation of SEM samples. Because the environment temperature is lower than the UCST of the PAAM-/PAAC-based IPN gel, the IPN microspheres shrink when they are treated by freeze-drying, but the seeds do not. Therefore, the IPN microspheres appear to be a little smaller than the seeds in the SEM micrographs.
2.3.3 Positively Thermosensitive Swelling Characteristics
Temperature-programmed photon correlation spectroscopy (TP-PCS) is used to determine the temperature dependence of the hydrodynamic diameter of
2.3 Positively Thermo-responsive Submicron-Sized Monodisperse. . . 33
Fig. 2.6 Temperature dependence of hydrodynamic diameter of microspheres with PAAM-/PAAC-based IPN shells. The sample code is defined in Table2.1 (Reproduced with permission from Ref. [27], Copyright (2003), Wiley-VCH Verlag GmbH & Co. KGaA)
Table 2.1 Experimental recipe for the preparation of microspheres with PAAM-/PAAC-based IPN shell (Reproduced with permission from Ref. [27], Copyright (2003), Wiley-VCH Verlag GmbH & Co. KGaA)
In the second step: fabrication of PAAM or poly(AAM-co-BMA) shell on the seed core
In the third step: PAAM/PAAC IPN synthesis
Sample code
[BMA]/([BMA]C [AAM]) [wt%]
[MBA]/([St]C [AAM]) [wt%]
[AAC]/([AAC]C
[AAM]) [mol/mol] [MBA]/[AAC] [wt%]
A 0 0 1.0 1
B 0 1.2 0.5 1
C 34:4 0 0.5 1
D 34:4 0 0.5 1.5
microspheres with PAAM-/PAAC-based IPN shells. Figure 2.6 shows the temperature dependence of hydrodynamic diameter of the prepared microspheres with PAAM-/PAAC-based IPN shells, in which the sample code is defined in Table2.1. All of the microspheres exhibit positively thermosensitive volume-phase transition characteristics. The hydrodynamic diameters of the PAAM-/PAAC- based IPN microspheres are about 210 nm in the temperature range 10–15ıC and increase to 260–380 nm in the temperature range 30–40ıC. A sharp transition of the hydrodynamic diameters occurs from 15ıC to 25ıC, which corresponds to the UCST of PAAM-/PAAC-based IPN hydrogels [47]. Below the UCST, PAAM/PAAC intermolecular complexes form by hydrogen bonding, and the chain-chain zipper effect makes the IPN microspheres shrunken; as a result, the mean hydrodynamic diameter is small. In contrast, the IPN microspheres are in the swollen state at temperatures above the UCST due to PAAM/PAAC complex dissociation by the breakage of hydrogen bonds, and therefore, a larger hydrodynamic diameter is the result (as illustrated in Fig.2.4).
The swelling ratios of the hydrodynamic diameters of IPN microspheres decreased when adding hydrophobic monomer butyl methacrylate (BMA) into the experimental recipe for the preparation of PAAM shells on the seeds in the second step. The reason exists in two aspects as follows: (1) The hydrophobic monomer BMA contributes to the mechanical properties of the IPNs, and (2) the hydrophobic interactions stabilize the hydrogen-bonding complexation of the IPNs. In the second step for fabricating poly(AAM-co-BMA) shell on the seed core, hydrophobic monomer BMA and hydrophilic monomer AAM had “equal chance” to participate in the free-radical copolymerization reaction. Consequently, certain contents of hydrophobic polymer are randomly distributed in the prepared poly(AAM-co-BMA) shell layer and therefore in the resulting IPN shell layer. The mechanical strength of PAAM-/PAAC-based IPN macroscopic hydrogels increases with increasing BMA content in the IPNs. The increase in the mechanical strength of the IPN shell restricts the swelling and shrinkage of the microsphere. In addition, hydrophobic interactions are proposed to increase with increasing BMA content in the IPNs. The hydrophobic interactions prevent the polymer chain complexes from dissociating, i.e., stabilize the hydrogen-bonding complexation of the IPNs. As a result, the swelling of the microsphere is depressed.
With increasing cross-linker MBA dosage in the IPN synthesis or adding MBA in the second step for PAAM shell preparation, the results show that the swelling ratios of the hydrodynamic diameters of IPN microspheres also decrease to some extent. Although cross-linker is helpful to prepare the IPNs, an overfull cross-linker dosage results in a decrease of the thermosensitive swelling ratio. By adding cross- linker MBA in the PAAM shell preparation or increasing MBA dosage in the IPN synthesis, the network of PAAM chains or that of PAAC chains becomes more rigid, and the polymer chains restrict to each other more strongly; therefore, the deformation of the IPNs becomes more difficult. As a result, the swelling ratio of the hydrodynamic diameter of the microspheres is depressed. That is to say, the positively thermosensitive swelling ratio of the prepared core-shell microspheres could be adjusted by regulating the hydrophobic monomer BMA dosage and the cross-linker MBA dosage.