3814
Langmuir 2009, 25, 3814-3819
Oligo(ethylene glycol)-Based Thermoresponsive Core-Shell Microgels Chenglin Chi, Tong Cai, and Zhibing Hu* Department of Physics, UniVersity of North Texas, Denton, Texas 76203 ReceiVed NoVember 22, 2008. ReVised Manuscript ReceiVed January 8, 2009 Oligo(ethylene glycol)-based thermoresponsive core-shell microgels were synthesized by a two-step polymerization method: The core particles mainly consisted of poly(ethylene glycol) ethyl ether methacrylate (PEGEEMA), while the shell mainly consisted of a copolymer of PEGEEMA, poly(ethylene glycol) methyl ether methacrylate (PEGMEMA), and poly(acrylic acid). The copolymerization of the shell resulted in a higher volume phase transition temperature than that of the core. The mass of a single microgel particle was determined by both the static light scattering method and a new method using UV-visible spectroscopy. Core-shell microgels in water self-assembled into crystalline structures with iridescent colors, which were the result of Bragg diffraction. The melting kinetics of microgel crystals was studied by using UV-visible transmission spectroscopy.
Introduction To obtain multiresponsive and special swelling properties, microgels have been prepared to possess core-shell architectures by incorporating responsive polymers for the core and/or the shell.1-3 Most current work has focused on poly-N-isopropylacrylamide (PNIPAM) polymer as the main component in constructing core-shell microgels. The PNIPAM can be applied to both the core and the shell. Temperature- and pH-responsive core-shell microgels were composed of PNIPAM as a core and PNIPAM-acrylic acid (AAc) as a shell.2 Microgels consisting of a PMIPAM core and a PNIPAM shell had doubly thermosensitive properties.3 On the other hand, the PNIPAM-AAc4 or PNIPAM5,6 was only applied to the shell, while the core of the particles was solid polystyrene. Some core-shell microgels were further incorporated with inorganic materials. Specifically, gold nanoparticles were synthesized in situ, using the cationic sites in the inner shell as the nuclei for particle growth.7 Core-shell structure microgels have many applications due to their unique structures. Proteins can filter through the shell to tie with the core-bound ligand.8 Using degradable PNIPAM as the core and nondegradable phenylboronic acid (PBA)-conjugated P(NIPAM-PBA) as the shell, a dispersion of microgels may be used for glucose detection.9 Furthermore, microgels with a pHresponsive core (polyvinylpyridine) and a temperature-responsive shell (PNIPAM) have been used for the uptake and release of an anionic surfactant.10 Recently, Lutz et al. have reported that oligomers composed of a (meth)acrylate moiety connected to a short poly(ethylene glycol) (PEG) chain have similar thermoresponsive properties as PNIPAM polymer.11-14 The atom transfer radical copolymerization of two oligo(ethylene glycol) methacrylates of different chain lengths leads to the formation of thermoresponsive * Corresponding author. E-mail:
[email protected]. (1) Pelton, R. AdV. Colloid Interface Sci. 2000, 85, 1. (2) Jones, D. C.; Lyon, L. A. Macromolecules 2000, 33, 8301–8306. (3) Berndt, I.; Richtering, W. Macromolecules 2003, 36, 8780–8785. (4) Kim, J. H.; Ballauff, M. Colloid Polym. Sci. 1999, 277, 1210–1214. (5) Crassous, J. J.; Wittemann, A.; Siebenbuerger, M.; Schrinner, M.; Drechsler, M.; Ballauff, M. Colloid Polym. Sci. 2008, 286, 805–812. (6) Hellweg, T.; Dewhurst, C. D.; Eimer, W.; Kratz, K. Langmuir 2004, 20, 4330–4335. (7) Suzuki, D.; McGrath, J. G.; Kawaguchi, H.; Lyon, L. A. J. Phys. Chem. C 2007, 111, 5667–5672. (8) Nayak, S.; Lyon, L. A. Angew. Chem., Int. Ed. 2004, 43, 6706–6709. (9) Zhang, Y.; Guan, Y.; Zhou, S. Biomacromolecules 2007, 8, 3842–3847. (10) Bradley, M.; Vincent, B. Langmuir 2008, 24, 2421–2425.
copolymers with a precisely tunable low critical solution temperature (LCST) in water.11 The thermoresponsive properties and the effect of side chain length on the cloud point of poly[oligo(ethylene glycol) alkyl ether methacrylate]s have been discussed in detail.15,16 The amphiphilic block copolymers containing poly[oligo(ethylene glycol) alkyl ether methacrylate] segments show the characteristic surface morphology and the excellent biocompatibility.17-20 Similar oligomers have been used for preparing microgels with a variety of particle radii by free radical polymerization.21 The PEG derivative microgels attached with vinyl groups can act as both a crystalline array and cross-linkers that connect neighboring polymer chains to stabilize the crystal structure.22 Here we show new core-shell microgels made with poly(oligo(ethylene glycol)) derivative thermoresponsive polymers. The core-shell microgels were synthesized by first preparing the oligo(ethylene glycol)-based core and then using the core as a seed for the formation of the shell. The shell mainly consisted of copolymer of poly(ethylene glycol) ethyl ether methacrylate (PEGEEMA), poly(ethylene glycol) methyl ether methacrylate (PEGMEMA), and poly(acrylic acid), resulting in a higher volume phase transition temperature than that of the core. The resultant core-shell particles have a very narrow size distribution and can self-assemble into colloidal crystals. It is significant to incorporate nonlinear PEG-based oligomers as components for core-shell microgels so that these particles will not only have thermal responsive properties like PNIPAM polymer but also have biocompatibility like nonlinear PEG polymer.11 (11) Lutz, J.-F.; Akdemir, O.; Hoth, A. J. Am. Chem. Soc. 2006, 128, 13046– 13047. (12) Lutz, J.-F.; Hoth, A. Macromolecules 2006, 39, 893–896. (13) Lutz, J.-F.; Weichenhan, K.; Akdemir, O.; Hoth, A. Macromolecules 2007, 40, 2503–2508. (14) Lutz, J.-F. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3459. (15) Han, S.; Hagiwara, M.; Ishizone, T. Macromolecules 2003, 36, 8312. (16) Takashi Ishizone, T.; Seki, A.; Hagiwara, M.; Han, S.; Yokoyama, H.; Oyane, A.; Deffieux, A.; Carlotti, S. Macromolecules 2008, 41, 2963. (17) Yokoyama, H.; Miyamae, T.; Han, S.; Ishizone, T.; Tanaka, K.; Takahara, A.; Torikai, N. Macromolecules 2005, 38, 5180. (18) Ishizone, T.; Han, S.; Hagiwara, M.; Yokoyama, H. Macromolecules 2006, 39, 962. (19) Zhang, R.; Seki, A.; Ishizone, T.; Yokoyama, H. Langmuir 2008, 24, 5527. (20) Oyane, A.; Ishizone, T.; Uchida, M.; Furukawa, K.; Ushida, T.; Yokoyama, H. AdV. Mater. 2005, 17, 2329. (21) Cai, T.; Marquez, M.; Hu, Z. B. Langmuir 2007, 23, 8663. (22) Cai, T.; Wang, G.; Thompson, S.; Marquez, M.; Hu, Z. B. Macromolecules 2008, 41, 9508–9512.
10.1021/la803866z CCC: $40.75 2009 American Chemical Society Published on Web 02/16/2009
ThermoresponsiVe Core-Shell Microgels
Langmuir, Vol. 25, No. 6, 2009 3815 Table 1. Core and Shell Compositions
core (g) PEGEEMA CS1 CS2 CS4 CS3 CS5
3.65 3.27 3.27 3.62 3.62
PEGMEMA
shell solution (g) AA
G13
core R (nm)
PEGEEMA
PEGMEMA
0.10 0.10
0.23 0.26 0.26 0.23 0.23
60 65 65 80 80
1.36 1.38 2.07 2.07 2.07
1.62 1.98 2.98 2.50 3.50
0.44 0.44
Experimental Section Materials. Poly(ethylene glycol) ethyl ether methacrylate (PEGEEMA, Mn ∼ 246 g mol-1, Aldrich), poly(ethylene glycol) methyl ether methacrylate (PEGMEMA, Mn ∼ 300 g mol-1, Aldrich), glycerol 1,3-diglycerolate diacrylate (G13, cross-linker, Aldrich), acrylic acid (AA, 99%, Aldrich), sodium dodecyl sulfate (SDS, ultrapure bioreagent, J.T. Baker), and potassium persulfate (KPS, >99%, Aldrich) were used as received. Water for sample preparation was distilled and deionized to a resistance of 18.2 MΩ, by a Millipore system, and filtered through a 0.22 µm filter to remove particulate matter. Preparation of Core Microgels. The core microgels were synthesized using a precipitation polymerization method.21,22 A certain amount of PEGEEMA, PEGMEMA, AA, and G13, as shown in Table 1, and 0.03 g of SDS were added into a reactor with 195 mL of deionized water. The solution was heated up to 70 °C under nitrogen gas bubbling for about 40 min, and 0.07 g of KPS dissolved in 5 mL of deionized water was added to initiate the reaction. The reaction was carried out at 70 °C for more than 6 h. After cooling the solution to room temperature, the final dispersion was exhaustively dialyzed in a dialysis tube for 7 days while the deionized water was exchanged twice a day. Preparation of Core-Shell Microgels. The shell prepolymer solution was prepared first. A certain amount of PEGEEMA, PEGMEMA, AA, G13, as shown in Table 1, 0.03 g of SDS, and 0.07 g of KPS were added into a beaker with 200 mL of deionized water, and the solution was bubbling with nitrogen gas for about 30 min. The core particle dispersion (200 mL) was put into a reactor and heated up to 70 °C under nitrogen environment for 40 min. Then the shell solution was dripped into the reactor. It took about 30 min to finish this process. The reaction was carried out at 70 °C for more than 6 h. After cooling the solution to room temperature, the final reaction dispersion was exhaustively dialyzed in a dialysis tube for 7 days while the deionized water was exchanged twice a day. Scheme 1 shows the synthesis processes. Light-Scattering Characterization. A laser light scattering (LLS) spectrometer (ALV) equipped with an ALV-5000 digital time correlator was used with a helium-neon laser (Uniphase 1145P, output power of 22 mW and wavelength of 632.8 nm) as the light source. The incident light was vertically polarized with respect to the scattering plane, and the light intensity was regulated with a beam attenuator (Newport M-925B). The scattered light was conducted through a thin optic fiber leading to an active quenched avalanche photodiode (APD), the detector. In static light scattering (SLS), the angular dependence of the excess Rayleigh ratio Rvv(q) of a dilute microgel dispersion is measured. Rvv(q) is related to the weight average molar mass Mw, the second virial coefficient A2, and the z-average root-mean-square radius of gyration 〈Rg2〉z1/2 (or simply 〈Rg〉) by23
KC 1 1 1 + (Rg2)q2 + 2A2C = Rvv(q) Mw 3
(
)
(1)
where K ) 4π2n2(dn/dC)2/(NAλ04) and q ) (4πn/λ0) sin(θ/2) with NA, n, C, λ0, and θ being Avogadro’s constant, the solvent refractive index, the solid concentration (g/cm3 or g/g), the light wavelength in the vacuum, and the scattering angle, respectively. In SLS, the samples were scanned from angle 30° to 120° with a 5° interval. (23) Zimm, B. H. J. Chem. Phys. 1948, 16, 1099.
AA
G13
core-shell R (nm)
0.15 0.25 0.25 0.25
0.23 0.20 0.27 0.35 0.28
114 145 198 167 229
Dynamic light scattering (DLS) is related to the fluctuation of the scattered intensity with time t. The intensity-intensity time correlation function G(2)(t, q) in the self-beating mode can be expressed by24,25
G(2)(t,q) ) 〈I(φ, q) I(t,q)〉 ) A[1 + β|g(1)(t,q)|2]
(2)
where t is the delay time, A is the measured baseline, and β is the coherence factor. For a polydispersed sample, |g(1)(t,q)| is the firstorder electric field time correlation function E(t,q) and is related to the line-width distribution G(Γ) by
g(1)(t,q) )
〈E(φ,q) E*(t,q)〉 ) 〈E(φ,q), E*(φ,q)〉
∫0 ∞G(Γ)e-Γτ dΓ
(3)
G(Γ) can be calculated from the Laplace inversion of g(1)(t,q). g(1)(t,q) was analyzed by cumulant analysis to get the average line width 〈Γ〉. The extrapolation of Γ/q2 to q f 0 led to the translational diffusion coefficient (D). This corresponding analysis of this function yields the diffusion coefficient D, which can be translated into the hydrodynamic radius Rh by the Stokes-Einstein relation:
Rh )
kBT 6πηD
(4)
where kB, η, and T are the Boltzmann constant, the solvent viscosity, and the absolute temperature, respectively. The DLS measurements were carried out at θ ) 60°. UV-Visible Spectroscopy Measurements. The Bragg diffraction turbidity of the microgels was measured as a function of wavelength using a diode array UV-visible spectrometer (Agilent 8453) by calculating the ratio of the transmitted light intensity (It) to the incident intensity (I0): A ) -(1/d) ln(It/I0), where d is the thickness (1 mm) of the sampling cuvette. The polymer concentration of a microgel dispersion was obtained by completely drying the dispersion at 60 °C and then weighing it.
Results and Discussion The average hydrodynamic radius distribution function f(Rh) of PEG derivative microgels was characterized using a laser light scattering spectrometer. The PEG core and PEG core-shell microgels (CS3) are compared as shown in Figure 1a. Both the core and the core-shell particles are diluted to 1.0 × 10-5 g/mL with distilled deionized water. The core particles were narrowly distributed with Rh around 80 nm, while the core-shell particles had Rh around 167 nm. Increased particle size and their narrow distribution demonstrate that the shell is successfully added onto the core particle. On the basis of TEM imaging of the PNIPAM system, Lyon et al. have already demonstrated that synthesis for the shell at 70 °C has two advantages: First, the collapsed core particles serve as nuclei for further polymerization of the shell, Scheme 1. Synthesis of Core-Shell Microgels
3816 Langmuir, Vol. 25, No. 6, 2009
Chi et al.
Figure 3. Temperature dependence of hydrodynamic radii (Rh) of PEG derivative core-shell microgels with various chemical compositions.
Figure 1. (a) Hydrodynamic radius distributions (f(Rh)) of CS3 core and core-shell microgels in deionized water at pH 5.0 at 20 °C. (b) Comparison of particle radii at pH ) 3.5, 5.0, and 6.5 for the dispersion of CS3 core-shell microgels at 20 °C.
Figure 4. Hydrodynamic radii of the core and the core-shell microgels for the sample CS3 are plotted as a function of temperature. The left dashed purple line relates the first decrease in size for the core-shell particles to the volume phase transition of the core near 23 °C, while the right dashed purple line points to the second decrease in size due to the volume transition of the shell near 42 °C.
Figure 2. Hydrodynamic radius distributions (f(Rh)) of PEG derivative core-shell microgels in deionized water at 20 °C. The pH values for the samples in this figure and figures below are about 5. The average core radii for CS1, CS2, CS3, CS4, and CS5 are 60, 65, 80, 65, and 80 nm, respectively.
so the formation of new particles can be avoided. Second, the collapsed core particles hinder the shell polymer fromn interpenetrating into the core area. As a result, there is a clear boundary between the core and the shell.2 The baselines in Figure 1a also shows that the formation of new homoshell particles during core-shell synthesis is negligible. Figure 1b compares the particle sizes at various pH values. Upon the increase of pH value, the particle size increases. This is because a significant amount of poly(acrylic acid) has been incorporated into the core-shell particles. As the pH value increases, the PAA becomes ionized to make the microgel swell more in water. In the following discussions, the pH values of the dispersions of the PEG core-shell microgels are adjusted to 5.0. Figure 2 shows typical results of the hydrodynamic radius distribution of PEG core-shell microgels prepared with different chemical compositions, as shown in Table 1. All core-shell microgel particles have a narrow size distribution. As the oligomer concentrations increase, both core and core-shell microgels sizes increase. Adding a small amount of AA, compared to PEG oligomers, increases the size of the shell significantly, as shown in Table 1. (24) Berne, B. J.; Pecora, R. Dynamic Light Scattering; Wiley & Sons: New York, 1976. (25) Chu, B. Laser Light Scattering; Academic Press: New York, 1974.
The temperature-dependent hydrodynamic radii (Rh) of the core-shell microgels with DLS are shown in Figure 3. The oligomers of PEGEEMA and PEGMEMA were chosen because their lower critical solution temperatures (LCST) were at 24 and 61 °C, respectively. Properly mixing these two oligomers can lead to a LCST that is close to the physiological temperature.12 Samples CS3 and CS5 show two transition regions. In order to understand these two transition regions, hydrodynamic radii of the core and the core-shell microgels for the sample CS3 are plotted as a function of temperature, as shown in Figure 4. The left dashed green line relates the first decrease in size for the core-shell particles to the volume phase transition of the core near 23 °C, while the right dashed green line points to the second decrease in size due to the volume transition of the shell near 42 °C. For samples with smaller core size (CS1, CS2, and CS4), the first size decrease is not apparent. The volume phase transition of PEG derivative core-shell microgels is not as sharp as PNIPAM microgels. Static light scattering was carried out for dilute CS3 microgel dispersions. Figure 5 shows the Zimm plot at 20 °C, where dn/ dC ) 0.134 cm3/g for PEG was used.26 From the extrapolation of KC/Rvv(q) in eq 1 to θ ) 0 and C ) 0, Mw, A2, and 〈Rg〉 were obtained to be 3.15 × 108 g/mol, 1.93 × 10-9 mol dm3/g2, and 156 nm, respectively. The small positive A2 value indicates that water is a good solvent for the PEG analogue microgels at 20 °C. By combining DLS and SLS results, we obtained Rg/Rh ) 0.93 for PEG analogue core-shell microgels at 20 °C. This value is slightly higher than the theoretical value of (3/5)0.5 for uniform hard spheres. (26) Brandrop, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook, 4th ed.; John Wiley & Sons, Inc.: New York, 1999.
ThermoresponsiVe Core-Shell Microgels
Langmuir, Vol. 25, No. 6, 2009 3817
Figure 5. Zimm plot of static light scattering for CS3 at 20 °C. The polymer concentrations used were 2.0, 4.0, 6.0, 8.0, and 10.0 × 10-6 g/mL from left to right, respectively.
The core-shell microgels have been concentrated by using ultracentrifugation at a speed of 10 000 rpm for 2 h. The microgel dispersion was then diluted to different concentrations. These dispersions were heated up above LCST and then cool down slowly to 20 °C. These core-shell particles can form colloidal crystals quickly. The crystalline phase of core-shell particles (CS3) shows iridescent colors, as shown in Figure 6 at 20 °C for the polymer concentrations between 3.8 and 6.3 wt %. In this range, microgel spheres not only have sufficient interaction but also have enough freedom to form colloidal crystals. The iridescent colors of the core-shell microgel dispersions are the result of Bragg diffractions from differently oriented crystalline planes. Figure 7a shows UV-vis spectra of the PEG analogue core-shell microgel (CS3) dispersions at various polymer concentrations. The Bragg peak wavelength decreases from 625 to 530 nm as the polymer concentration increases from 3.8 to 6.3 wt %. The diffraction wavelength is related to interplane spacing according to the Bragg equation, mλ ) 2nd sin θ, where n is the mean refractive index of the dispersion, θ is the diffraction angle, d is the lattice spacing, m is the diffraction order, and λ is the wavelength of diffracted light. Considering polymer concentration in the dispersion as C ) m/V,
C)
m 4mp a ) 3 , d111 ) V a √3
(5)
2n (4mp)1⁄3C-1⁄3 √3
(6)
We have
λ)
where C is the polymer concentration, mp is the mass of one particle, m is total mass of all particles, V is the total volume,
and a is the crystal cell size. From the structure characterizations of PNIPAM microgels by neutron scattering27 and of PEG derivative microgels by AFM,22 the current core-shell microgels are assumed to form an fcc structure with four particles per unit cell and they align with the (111) plane to parallel with the surface of the glass cell. From eq 6, one can see that the Bragg diffraction wavelength λ is proportional to C-1/3. The value of λ is plotted as a function of C-1/3, as shown in Figure 7b. The dashed line in Figure 7b is the best fit to the data with the relationship of λ ) 54 + 192C-1/3. Comparing the fitted slope with the one in eq 6, we have obtained the mass for a single microgel particle mp ) 2.9 × 108 g/mol, which is very close to the value (3.15 × 108 g/mol) obtained by static light scattering. This shows that we have developed a new method for measuring the mass of a microgel particle using UV-visible spectroscopy. In a dilute dispersion at 20 °C, the density of the CS3 particle is estimated to be mp/[(4/3)πRh3] ) 4.8 × 10-16g/[(4/3)π(167 nm)3] ) 0.025 g/cm3. This value of the core-shell CS3 is interestingly close to 0.026 g/cm3, that for a PNIPAM particle, obtained by the light scattering method.28 The UV-visible Bragg peak can provide the distance between two particles. Considering the Bragg diffraction as the firstorder, diffraction angle θ ) 90°, and the index of refraction n ) 1.33, the corresponding interparticle distances from right to left Bragg peak positions in Figure 7a have been calculated to be 286, 279, 271, 261, 260, 253, and 245 nm, respectively. Compared with 334 nm, the diameter of a microgel in a very dilute dispersion, the size of the particles in a concentrated dispersion becomes smaller. This suggests the increasing compression of the particles with increasing polymer concentration, similar to the PNIPAM microgels.29 The formation of colloidal crystals is not only dependent on polymer concentration but also on temperature. Figure 8a-A shows the iridescent patterns when PEG derivative core-shell particles with the polymer concentration of 6.5 wt % self-assemble into a crystalline phase at 20 °C. At 30 °C, the iridescent grains disappear completely and the dispersion becomes a homogeneous liquid with slight cloudiness (Figure 8a-B). When the temperature is raised further to 42 °C, the sample becomes more turbid, indicating a phase separation (Figure 8a-C) above the LCST. When the sample was cooled back to 20 °C, the microgels form colloidal crystalline structure again (Figure 8a-D). The melting kinetics was further investigated using UV-vis spectroscopy. As the sample (CS3 with polymer concentration of 6.5 wt %) in the glass cuvette was heated up from 18 to 30 °C, the Bragg peak height gradually increased (Figure 8b). This increase is due to the increase of refractive index that is caused by thermally responsive shrinkage of microgels. Once the temperature reached 30 °C for CS3, melting started. One can see
Figure 6. The colloidal crystals of CS3 core-shell microgels in water at 20 °C. The polymer concentrations are 6.3, 5.9, 5.5, 5.3, 4.6, 4.3, and 3.8 wt % from left to right.
3818 Langmuir, Vol. 25, No. 6, 2009
Chi et al.
Figure 9. Progressive appearances of the UV-visible spectrum of CS5 sample at 5.1 wt % polymer concentration. At 31.5 °C, both the peak height and the peak wavelength decrease with time, indicating melting of crystalline structures.
Figure 7. (a) UV-visible spectra for CS3 sample at various polymer concentrations: 6.3, 5.9, 5.5, 5.3, 4.6, 4.3, and 3.8 wt % (from left to right). (b) The linear relationship between the wavelength of the Bragg peak and (C)-1/3, where C is the polymer concentration of the microgel dispersions. The dashed line is the best fit to the data.
Figure 10. Comparison of melting kinetics for CS3 and CS5 samples as presented in Figures 8 and 9. (a) The Bragg peak height as a function of time. (b) The Bragg peak wavelength as a function of time.
Figure 8. (a) The dispersion of PEG derivative core-shell particles (CS3) with polymer concentration of 6.5 wt % at various temperatures: (A) 20 °C (the crystalline phase), (B) 30 °C (melting), (C) 42 °C (above the LCST), and (D) 20 °C (recrystallization). (b) Progressive appearances of the UV-visible spectrum of CS3 sample at 6.5 wt % polymer concentration. From 18 to 30 °C, the peak intensity increases due to the increase of refractive index. At 30 °C, both the peak height and the peak wavelength decrease with time, indicating melting of crystalline structures.
that the peak height decreases with time, as shown in Figure 8b. The peak height is related to the volume fraction of crystallites in the sample.30 The decrease of the peak height corresponds to the reduction of crystallites in the sample. During the melting process, it is apparent that the Bragg peak position shifted toward shorter wavelengths. The shifts for sample CS3 is about 7 nm. The blue shift of the Bragg peak is attributed to the decrease of the lattice spacing induced by the shrinking particles. In order (27) Hellweg, T.; Dewhurst, C. D.; Bru¨ckner, E.; Kratz, K.; Eimer, W. J. Colloid Polym. Sci. 2000, 278, 972. (28) Gao, J.; Hu, Z. B. Langmuir 2002, 18, 1360–1367. (29) Stieger, M.; Pedersen, J. S.; Lindner, P.; Richtering, W. Langmuir 2004, 20, 7283–7292. (30) Tang, S.; Hu, Z.; Zhou, B.; Cheng, Z.; Wu, J.; Marquez, M. Macromolecules 2007, 40, 9544–9548.
to retain the osmotic pressure of the crystalline phase in balance with the liquid phase in the suspension, the reduction of the particle size must be compensated by the decreasing of the lattice spacing.30 Similar melting kinetics has been observed for sample CS5, as shown in Figure 9. The polymer concentration for CS5 is about 5.1 wt %. For sample CS5, the melting temperature is slightly higher at 31.5 °C. The melting kinetics for samples CS3 and CS5 are compared in Figure 10a for the Bragg peak height and Figure 10b for the Bragg peak position. It takes about 55 s for CS3 sample to completely melt while takes about 80 s for CS5. The core size and compositions for CS3 and the CS5 are the same. But the cross-linker density of CS5 is about 0.28, which is smaller than 0.35 for CS3 sample. The radius of CS5 is about 229 nm and is larger than 167 nm for CS3. Therefore, the CS5 sample has a much thicker and looser shell than CS3 sample. Thus, CS5 core-shell is softer than CS3 particle. It shows that the softness of the CS5 particles results in the slower kinetics, compared with CS3 sample. The formation of crystalline structures from these core-shell microgels is also very fast. A typical crystallization process was recorded using UV-visible spectroscopy for CS5 core-shell particles in water as shown in Figure 11. We started with a shear-melted dispersion of microgel particles with a polymer
ThermoresponsiVe Core-Shell Microgels
Figure 11. UV-visible spectrum of the CS5 core-shell microgels at 20 °C. The polymer concentration of the microgel dispersion is 5.1%. The Bragg peak arises with the formation of the crystalline structure.
concentration equal to 5.1%. Because of the Bragg diffraction, the UV-visible spectrum exhibits a sharp attenuation peak in the crystalline phase. At 20 °C (Figure 11), a weak Bragg peak appeared about 30 s after the shear-melting was stopped. The peak intensity increased with time and reached a maximum at around 45 s.
Conclusions The core-shell microgels based on oligomers composed of a (meth)acrylate moiety connected to a poly(ethylene glycol) (PEG) chain have been synthesized by a two-step polymerization method. The core particles mainly consist of poly(ethylene glycol) ethyl ether methacrylate (PEGEEMA), while the shell mainly consists of a copolymer of PEGEEMA, poly(ethylene glycol) methyl ether methacrylate (PEGMEMA), and poly(acrylic acid). The copolymerization of the shell results in a higher volume phase transition temperature than that of the core. The average hydrodynamic radius distribution function f(Rh) of these microgels
Langmuir, Vol. 25, No. 6, 2009 3819
was characterized using a laser light scattering spectrometer. All core-shell microgel particles have a narrow size distribution. As the oligomer concentrations increase, both core and core-shell microgels sizes increase. Adding a small amount of AA, compared to PEG oligomers, increases the size of the shell significantly. Sample CS3 and CS5 show two regions. The first size decrease is due to the shrinkage of the core and the second size decrease near 42 °C is due to the shrinkage of the shell. The core-shell microgels self-assemble into a colloidal crystalline phase at 20 °C for the polymer concentrations between 3.8 and 6.3 wt %. UV-vis spectra of the PEG analogue core-shell microgel dispersions at various polymer concentrations have been measured. The sharp peak is due to Bragg diffraction and shifts from 625 to 530 nm as the polymer concentration increases from 3.8 to 6.3 wt %. The diffraction wavelength has been found to scale as C-1/3, where C is the polymer concentration. From the slope of the λ versus C straight line, we have obtained that the mass for single microgel particle is about 2.9 × 108 g/mol, which is very close to the value (3.15 × 108 g/mol) obtained by static light scattering. This shows that we have developed a new method for measuring the mass of a microgel particle using UV-visible spectroscopy. Analysis of UV-visible spectra also points that the increasing compression of the particles with increasing polymer concentration. The formation of colloidal crystals is not only dependent on polymer concentration but also on temperature. The core shell microgel crystals melt around 30 °C. The core shell particles with less cross-linker density melt slower than the ones with more cross-linker density. It shows that the softness of the core-shell particles results in the slower kinetics. Acknowledgment. We gratefully acknowledge financial support from the National Science Foundation through Grant DMR-0805089. LA803866Z
