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Preparation of Nanogels with Temperature-Responsive Core and pH-Responsive Arms by Photo-Cross-Linking Dirk Kuckling,* Cong Duan Vo, and Sebastian E. Wohlrab Institut fu¨ r Makromolekulare Chemie und Textilchemistry, Technische Universita¨ t Dresden, D-01062, Dresden, Germany Received December 21, 2001. In Final Form: March 27, 2002 The preparation of temperature-responsive colloidal nanogels with a pH-responsive shell could be achieved by photo-cross-linking of poly(N-isopropylacrylamide) (PNIPAAm) graft terpolymers. The graft terpolymers were synthesized from NIPAAm, poly(2-vinylpyridine) (P2VP) macromonomers, and a chromophore monomer based on dimethylmaleimide (DMIAAm). The resulting solutions of nanogel with temperatureresponsive core and chemically bounded (P2VP) arms exhibited more stablility upon heating and low pH as compared to the corresponding PNIPAAm nanogels. A large transition of the average hydrodynamic diameter of the gels could be observed by increasing either the temperature above 32 °C or the pH above 5. It was also demonstated that it is possible to obtain a response to one stimulus without interfering with the other stimulus. Dynamic light scattering (DLS), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR) were used for characterization of the core-shell nanogels.
Introduction Colloidal particles of temperature-responsive polymers with a submicrometer size range have been the subject of much recent attention as they are widely used in various fields of life science and technology such as drug delivery systems, separation technologies, and enzyme and cell immobilization.1-4 Poly(N-isopropylacrylamide) (PNIPAAm) nanogels have been the most intensively studied in this field due to their dramatic and reversible volume changes in water upon heating between 30 and 35 °C.1-18 The colloidal nanogels of PNIPAAm are commonly prepared by free radical polymerization in dilute solutions or emulsion polymerization. These suspension systems are typically prepared with NIPAAm, potassium persulfate * To whom all correspondence should be addressed: e-mail
[email protected]; tel + 49 (351) 463 33788; fax + 49 (351) 463 37122. (1) Hoffman, A. S. Macromol. Symp. 1995, 98, 645-664. (2) Murray, M. J.; Snowden, M. J. Adv. Colloid Interface Sci. 1995, 54, 73-91. (3) Okahata, Y. Macromolecules 1986, 19, 493-494. (4) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1-33. (5) Duracher, D.; Elaissari, A.; Pichot, C. Colloid Polym. Sci. 1999, 277, 905-913. (6) Duracher, D.; Elaissari, A.; Pichot, C. J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 1823-1837. (7) Huang, J.; Wu, X. Y. J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 2667-2676. (8) Murray, M. J.; Snowden, M. J. Adv. Colloid Interface Sci. 1995, 54, 73-91. (9) Senff, H.; Richtering, W. Colloid Polym. Sci. 2000, 278, 830-840. (10) Senff, H.; Richtering, W. J. Chem. Phys. 1999, 111, 1705-1711. (11) Murray, M.; Rana, F.; Haq, I.; Cook, J.; Chowdhry, B. Z.; Snowden, M. J. J. Chem. Soc., Chem. Commun. 1994, 1803-1804. (12) Snowden, M. J.; Chowdhry, B. Z.; Vincent, B.; Morris, G. E. J. Chem. Soc., Faraday. Trans. 1996, 92, 5013-5016. (13) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163-249. (14) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247-256. (15) Pelton, R. H.; Pelton, H. M.; Morfesis, A.; Rowell, R. L. Langmuir 1989, 5, 816-818. (16) McPhee, W.; Tam, K. C.; Pelton, R. H. J. Colloid Interface Sci. 1993, 156, 24-30. (17) Wu, X.; Pelton, R. H.; Hamielec, A. E.; Woods, D. R.; McPhee, W. Colloid Polym. Sci. 1994, 272, 467-477. (18) Pelton, R.; Wu, X.; McPhee, W.; Tam, K. C. The Preparation and Characterization of PolyNIPAAm Latexes. In Colloidal Polymer Particles; Goodwin, J. W., Buscall, R., Eds.; Academic Press: London, 1995; pp 81-89.
as the initiator, SDS as the surfactant, and N,N′methylenebisacrylamide as the cross-linker. Pelton’s research group has systematically prepared and described PNIPAAm latex in water since 1980.4,14-18 Using the similar procedures, Snowden et al.11,12 have tailored various colloidal microgels from NIPAAm, NIPAAm with acrylic acid, and other temperature-responsive polymers. Nanogels with more complex structures such as coreshell or shell cross-linked knedels have also been developed.19-22 The cores of multiresponsive microgels based on NIPAAm and acrylic acid (AAc) were synthesized via precipitation polymerization and then used as nuclei for the shell formation. The resulting core-shell particles exhibited both pH and temperature sensitivity.19,20 Coreshell nanoparticles composed of poly(γ-benzyl-L-glutamate) as the core and PNIPAAm as the shell were prepared by the diafiltration method.21 Nanospheres derived from a block copolymer of polystyrene and poly(acrylic acid) were also developed. The block copolymers were first organized into micelles in a water and THF solvent mixture and then the poly(acrylic acid) shells were cross-linked by use of various di- and multiamino linkers.22 Taking advantage of the precipitation of PNIPAAm at elevated temperature, we have recently developed a novel method for nanogel synthesis.23 The current method could offer some advantages such as overcoming gradient crosslinking in a particle, which is commonly seen in emulsion polymerization. In addition, size and swellability of colloidal particles could be controlled by varying UV irradiation time, chromophore content, and SDS concentration.23 In this paper, we report the preparation of temperatureresponsive colloidal nanogels with a more complex structure by using similar principles. The resulting nanogels with temperature-responsive core and chemically bounded (19) Gan, D. J.; Lyon, L. A. J. Am. Chem. Soc. 2001, 123, 7511-7517. (20) Jones, C. D.; Lyon, L. A. Macromolecules 2000, 33, 8301-8306. (21) Cho, C.-S.; Cheon, J.-B.; Jeong, Y.-I. Macromol. Rapid Commun. 1997, 18, 361-369. (22) Huang, H.; Kowalewski, T.; Remsen, E. E.; Gertzmann, R.; Wooley, K. L. J. Am. Chem. Soc. 1997, 119, 11653-11659. (23) Vo, C. D.; Kuckling, D.; Adler, H. J. P.; Schoenhoff, M. Colloid. Polym. Sci. 2002 (in press).
10.1021/la015758q CCC: $22.00 © 2002 American Chemical Society Published on Web 05/03/2002
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Table 1. Recipes for Preparation of Graft Copolymers graft terpolymer
P2VP macromonomer (g)
NIPAAm (g)
DMIAAm (g)
AIBN (mg)
THF (mL)
GP1 GP2
1.9 1.58
9.100 7.567
0.902 1.500
33.2 28.6
46.5 40.7
poly(2-vinylpyridine) (P2VP) arms exhibit more stability upon heating and low pH ( 4.8 and room temperature. The graft terpolymers exhibited phasetransition temperatures of 30 °C at pH 7 as determined by DSC. The phase-transition temperature of polymer solutions remained almost the same at pH 2. There was little effect on Tc due to the P2VP content either at pH 2 or at pH 7. This clearly indicated that an independent pH or temperature response can be obtained for the graft terpolymers and can also be expected for the core-shell gels. To prepare nanogels, as illustrated in Scheme 3, the first four polymer mixtures were initially dissolved in 0.01 M HCl solution (pH 2, the A series samples; see Table 3). These polymer solutions at low pH were then heated in a temperature-controlled reactor. However, precipitation was observed in all the polymer solutions as soon as the temperature increased above Tc (to 55 °C) except in sample A100/0, which contained only graft terpolymer GP 1. The precipitation was due to the instability of PNIPAAm copolymers at low pH. This resulted in aggregation of particles into larger aggregates, whereas the graft terpolymers were still stable in solution due to the repulsive forces stemming from positively charged P2VP side chains. The instability of polymer solutions might be explained on the basis of studies of the influence of chloride ions and other salts on Tc and the stability of PNIPAAm solutions13,29 as well as on conventional colloidal principles. The dispersions are stable at low ionic strength and aggregate at high ionic strength. At 55 °C and pH 2, graft terpolymer solution A100/0 became turbid, suggesting phase transition of PNIPAAm main chains. The graft terpolymer state in solution might be described in Scheme 2 with the case T > Tc and low pH. The polymer solution at this state was irradiated while stirring. The resulting solution appeared transparent at room temperature both before and after irradiation. (29) Elaissaf, J. J. Appl. Polym. Sci. 1978, 22, 873-874.
Scheme 4. Illustration of the Photo-Cross-Linking Reaction
To avoid the precipitation during heating of the solutions of polymer mixtures at low pH, all the polymer mixtures were instead dissolved in distilled water (pH 6.5). We found no precipitation for these solutions. These solutions became turbid when they were heated, suggesting the phase transition of the PNIPAAm polymers. After irradiation, all the resulting solutions remained opaque at room temperature due to the formation of cross-linked particles with collapsed P2VP chains. The photo-crosslinking reaction is illustrated in Scheme 4. The UV lamp used in the present work was installed horizontally, so its energy must be lower than that of the vertical lamp due to the scattering of radiation energy from lenses and mirrors. As a result, the time for the beginning of nanoparticle formation in the present work was longer than the time reported in previous studies under the same conditions.23 In the same study, no free polymer chains were detectable after cross-linking. All results will be discussed on the basis of a core-shell structure of the gels, because among the conceivable structures the obtained data can easily be explained by this model. Dependence of Nanogel Dimension on Temperature at Different pH Values. Figure 1 shows the average hydrodynamic diameter of the resultant nanogels after UV irradiation as a function of temperature. As can be seen, the PNIPAAm nanogel B0/100 exhibited the same behavior as the nanogels described previously.23 There was a large change in the particle dimension in the vicinity of 32 °C, which corresponds to the phase transition of linear PNIPAAm in water. The average hydrodynamic
pH- and Temperature-Responsive Nanogels
Figure 2. Temperature dependence of the average hydrodynamic diameter of nanogels prepared from GP 1 at pH 2: (0) C0/100; (O) C50/50; (4) C75/25; (3) C100/0.
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Figure 3. Temperature dependence of the average hydrodynamic diameter of nanogel prepared from GP 2: (0) F100/0 (nanogel prepared in distilled water); (O) G100/0 [nanogel prepared at pH 2 (0.01 M HCl solution)]; (4) H100/0 (F100/0 system after adjustment of pH to 2).
diameters of the PNIPAAm nanogel at 45 and 20 °C differed by a factor of 11. This 11-fold increase in the particle diameter corresponds to about a 1330-fold increase in volume. Between 45 and 50 °C, no significant decrease of nanogel diameter was observed. On the contrary, changes in dimension of nanogel from graft terpolymers or mixtures of graft terpolymer and PNIPAAm were less significant at pH 7. Only the 1.2-2-fold decrease in average hydrodynamic diameter was seen for the graft terpolymer systems. The transition temperature was not signifiantly affected by the amount of P2VP, which was also confirmed by DSC meaurements (Table 3). By looking at Figure 1 carefully, one can see that in the absence of graft terpolymer the nanogel particles are greatly swollen, and the more graft terpolymer that was present in solution, the smaller average hydrodynamic diameters in the swollen state were observed. The lower swellability of the resulting nanogels with an increase in the numbers of P2VP side chains is likely ascribed to the hydrophobic nature of P2VP at pH > 4.8. The P2VP hydrophobic grafts may suppress the swelling of the nanogels contributing to the exclusion of water from the particles. Such behaviors could be comparable with that of PNIPAAm hydrogel, which was described previously.30 The presence of PNIPAAm grafts on the main PNIPAAm chain within the bulk hydrogel created hydrophobic regions, aiding the expulsion of water from the network during collapse. To investigate the dependence of the nanogel size on temperature at low pH, 0.1 N HCl was used to adjust the pH of the samples prepared at pH 7. Immediately after addition of HCl to the colloidal nanogels, the turbidity of graft-terpolymer-based systems disappeared, suggesting the protonation and solubility of P2VP at low pH. At 35 °C these transparent systems became turbid again, indicating the phase transition of PNIPAAm cores. Figure 2 shows the average hydrodynamic diameters of the respective nanogel particles at pH 2 as a function of temperature. The average hydrodynamic diameter of PNIAAm nanogels in the swollen state (20 °C) at pH 2 was 343 nm instead of 478 nm as measured at pH 7. The increased ionic strength resulted in a decrease in the average hydrodynamic diameter of PNIPAAm nanogel by a factor of 1.4 at 20 °C. The PNIPAAm nanogels were precipitated at 35 °C during DLS measurements, whereas
the other systems were stable up to 50 °C. The decrease in the average hydrodynamic diameter in the swollen state and the instability of PNIPAAm nanogels during heating can again be attributed to the presence of HCl in solution. Chloride ions can bind to polar amide groups of the PNIPAAm segment and/or interact with hydrated water molecules associated with polar or hydrophobic polymer segments.31 The driving forces for inter- and intramolecular hydrophobic interactions increase, resulting in less swellability and instability for PNIPAAm nanogels. As expected at pH 2, the average hydrodynamic diameters of the nanogels from graft terpolymers increased significantly due to the protonation of the P2VP grafts. This resulted in the formation of expanded P2VP arms and PNIPAAm cross-linked core. At 20 °C the average hydrodynamic diameter of the graft terpolymer nanogel C100/0 (at pH 2) is 547 nm instead of 123 nm for B100/0 (at pH 7) corresponding to a 88-fold volume increase. Although a decrease in size caused by the presence of chloride ions for PNIPAAm chains was expected (Figure 2), other nanogels formed from mixtures of graft terpolymer and linear copolymer at pH 2 also show larger diameters than those measured at pH 7 under the same conditions. A sharp and large change in swelling was also observed upon heating in the vicinity of 32 °C. For example, the diameter of sample C100/0 decreased from 547 nm at 20 °C to 122 nm at 45 °C, which corresponds to a volume change by a factor of 90. In general, the more graft terpolymer in the nanogels, the larger changes in swelling were seen. The nanogel particles were expanded, which can be attributed to an increasing number of hydrophilic arms located at the periphery of the particles. The repulsive force from positively charged hydrophilic P2VP arms could also prevent particles from aggregating. In fact, no precipitation was seen for nanogels based on graft terpolymers at low pH and high temperature during DLS measurement. To obtain nanogels with an increased 2VP/NIPAAm ratio, a graft terpolymer with longer P2VP arms was synthesized. The graft density of both graft terpolymers was approximately the same (26 and 19 P2VP grafts/ polymer for GP 1 and GP 2, respectively). The nanogels of graft terpolymer GP 2, which were synthesized from P2VP macromonomer with Mn ) 8800 and D ) 1.38,
(30) Yoshida, R.; Uchida, K.; Kaneko, Y.; Sakai, K.; Kikuchi, A.; Skurai, Y.; Okano, T. Nature 1995, 374, 240-242.
(31) Park, T. G.; Hoffman, A. S. Macromolecules 1993, 26, 50455048.
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Figure 4. SEM images of different nanogels at pH 7: (1) B0/ 100; (2) B75/25; (3) B100/0.
Figure 5. SEM images of different nanogels at pH 2: (1) A100/ 0; (2) C75/25; (3) C100/0.
showed an average hydrodynamic diameter lower than those from GP 1, which were prepared from P2VP macromonomer with Mn ) 1700 and D ) 1.38, although the former has much longer P2VP arms. In this case, the degree of cross-linking possibly influences the nanogel dimensions more significantly than the chain length of the P2VP graft. Similar relationships between pH and diameter were also observed when the pH of the solution was decreased (Figure 3). Comparison of the different states of the core-shell gels according to Scheme 2 showed, as expected, that the nanogels were highly swollen at pH 2 and 20 °C (both core and shell were swollen). The smallest dimensions of the nanogels were obtained at pH 7 and 50 °C, when both core and shell were collapsed. The two intermediate states, when one component was swollen and one collapsed, had the same order of magnitude. However, nanogels with a collapsed core (pH 2 and 50 °C) generally showed smaller average hydrodynamic diameters than nanogels with a collapsed shell (pH 7 and 20 °C). This is reasonable because the same volume effect influences the core diameter more strongly than the shell diameter. Additionally, all gels contained a higher amount of core-forming polymer than shell-forming polymer.
Morphology of Nanogel Particles by SEM. SEM images of different samples after irradiation prepared at pH 7 are shown in Figure 4. The nanogel particles appeared to be spherical and the particle size distributions were quite broad. The inhomogeneous polymer compositions were likely contributed to the large particle size distribution. The flock coagulation as could be seen in the SEM images might be due to the high concentrations of colloidal nanogels and surface tension effects caused by the drying process. The average diameters of nanogels determined from the SEM images were much higher than those determined by DLS where the nanogels were in the shrunken state (>40 °C). For example, the average diameter of sample B100/0 determined from the SEM image was 9 times larger than that determined by DLS at 40 °C. The reasons for this observation have been suggested in a previous study: first, the soft particles might be flattened on the support (aluminum foil) during sample preparation; and second, SEM images are only a local representation so smaller particles that have a significant influence on the final average diameter might not be counted.23 A combination of both these factors likely caused the differences in the particle dimensions of two methods.
pH- and Temperature-Responsive Nanogels
SEM images of the same samples at pH 2 were taken to see changes in morphology of the nanogel particles as compared to pH 7. As can be seen in Figure 5, the boundaries of particles were less sharp than those of nanogels at pH 7. This was due to hydrophilic P2VP arms forming a film around each nanogel particle during drying. The particles prepared under low-pH conditions that were seen by SEM must be the PNIPAAm core of the nanogels. The particle dimensions are smaller than those of their counterparts prepared at high pH. However, due to the large particle size distribution, a direct comparison cannot be made with the size calculated from SEM images. The differences in the morphologies between high and low pH confirm the formation of core-shell structures in the of resulting nanogels. Particle morphologies of nanogels based on GP 2 also show the same properties, as can be seen in Figure 6. Conclusions Colloidal nanogels with a temperature-responsive core and pH-responsive shell have been successfully prepared by UV irradiation of temperature-responsive photopolymers in aqueous solutions at pH 2 and 7. This is evidenced by the increase in dimension and the stability of nanogel systems based on graft terpolymer at low pH as well as changes in particle morphologies shown in SEM images. Nanogels from mixtures of graft terpolymers and linear PNIPAAm copolymers could only be prepared at pH 7. The size of the complex nanogels depends on the ratios of graft terpolymer to PNIPAAm copolymer. The current
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Figure 6. SEM image of nanogel prepared from GP 2 (sample H100/0).
work may extend the scope of preparing colloidal nanogels with more complex structures from photo-cross-linkable polymers. Acknowledgment. The financial support from the Graduiertenkolleg Forschungsgemeinschaft StrukturEigenschafts-Beziehungen bei Heterocyclen der Deutschen Forschungsgemeinschaft (DFG) is gratefully acknowledged. We are thankful to Professor K.-F. Arndt (Institute for Physical Chemistry and Electrochemistry, TU Dresden) for use of DLS. LA015758Q
