Langmuir 2000, 16, 8543-8545
Effect of Heating Rate on Nanoparticle Formation of Poly(N-isopropylacrylamide)-Poly(ethylene glycol) Block Copolymer Microgels Peng Wei Zhu*,† and Donald H. Napper School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia Received March 31, 2000. In Final Form: July 22, 2000
Introduction The self-aggregation of amphiphilic copolymers has recently attracted much attention. This is owing both to its intrinsic scientific interest and to its technological importance.1-3 The self-aggregation of an amphiphilic copolymer could result in the formation of stable block copolymer colloids (micelles or aggregate particles) with controlled structures and different morphologies. It has been found that the self-aggregation of amphiphilic copolymers and corresponding colloids depends on solvent quality, concentration, and composition. Any change in the temperature, the solvent composition, the ionic strength, or the pH can result in selective solvent conditions under which the block copolymer colloids formed are stable. For spherical particles of block copolymers in aqueous solution, aggregates of block copolymers are composed of a hydrophobic core and a hydrophilic corona. These particles may interact with each other to form networks or domain structures. The block copolymers of poly(N-isopropylacrylamide) and poly(ethylene oxide) (PNIPAM-b-PEO) and their corresponding microgels (or cross-linked micelles) have been recently studied.4,5 As it is known, PNIPAM dissolves in water at room temperature but undergoes a phase separation when heated to ∼31-32 °C.6 The coil-to-globule transition of single PNIPAM chains can be observed at ∼31-32 °C if the aggregation is prevented. The conformational changes of PNIPAM exhibit characteristics similar to those of biological macromolecules.7,8 For example, the collapse transition of PNIPAM chains proceeds through a set of intermediates consisting of a number of “cooperative units” before the compact globular state is achieved. Note that the volume phase transition of PNIPAM microgel has also received attention because the microgels can respond quickly to their environment, in contrast to macroscopic gels.9-12 The θ-temperature of PEO, on the other hand, was estimated to be 102 °C in † Present address: Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia.
(1) Muthukumar, M.; Ober, C. K.; Thomas, E. L. Science 1997, 277, 1225. (2) Almgren M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 272, 2. (3) Forster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (4) Topp, M. D. C.; Dijkstra, P. J.; Talsma, H.; Feijen, J. Macromolecules 1997, 30, 8518. (5) Yoshioka, H.; Mikami, M.; Mori, Y.; Tsuchida, E. J. Macromol. Sci., Pure Appl. Chem. 1994, A31, 109. (6) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (7) Tiktopulo, E. I.; Uversky, V. N.; Lushchik, V. B.; Klenin, S. I.; Bychkova, V. E.; Ptitsyn, O. B. Macromolecules 1995, 28, 7519. (8) Wu, C.; Zhou, S. Phys. Rev. Lett. 1996, 77, 3053. (9) Hirose, S.; Amiya, T.; Hirokawa, Y.; Tanaka, T. Macromolecules, 1987, 20, 1342. (10) McPhee, W.; Tam, K. C.; Pelton, R. J. Colloid Interface Sci. 1993, 156, 24. (11) Zhu, P. W.; Napper, D. H. Chem. Phys. Lett. 1996, 256, 51.
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pure water.13 The question of whether PEO aggregates form in pure water is controversial.13-15 There is no doubt, however, that the change with temperature from hydrophilic to hydrophobic occurs much more gradually for PEO than for PNIPAM. Obviously, a change in temperature can result in a selective solvent condition for PNIPAM blocks in the PNIPAM-b-PEO copolymers. We have studied the colloidal behavior of PNIPAM-bPEO microgel in aqueous solution.16 The microgel was synthesized using a redox system consisting of ceric ion Ce(IV) and PEO.4,17 Lower concentrations of PNIPAMb-PEO microgel were chosen in order to avoid the formation of a bimodal size distribution. In the present work, we report on the nanoparticle formation of PNIPAMb-PEO microgel at higher concentrations. We have employed slow and fast heating rates for the formation of PNIPAM-b-PEO microgel particles. As will be seen, the formation of nanoparticles can be controlled well by changing the heating rate. Smaller particle dimensions with a narrower size distribution can be obtained by the fast heating process. Experimental Section CH3-PEO-OH (PEO) from Aldrich, with a molecular weight of 5000, was dissolved in water, and the solution was filtered through a 0.45 µm Millipore filter. N-Isopropylacrylamide (NIPAM) from Monomer-Polymer was purified by recrystallization from a 65/35 mixture of hexane and benzene. N,N′Methylenebisacrylamide (BIS) and ceric ammonium nitrate from Aldrich were used as received. Details of preparation of PNIPAM-b-PEO microgel have been given in a previous paper.16 Typically, the CH3-PEO-OH (0.5 g) and NIPAM (0.63 g) were dissolved in 40 mL of 1 N nitric acid at 50 °C. The initial molar ratio of NIPAM/PEO is r ) 0.5. The solution was stirred under positive nitrogen pressure for about 15 min, and a solution of 7.24 g ceric ammonium nitrate in 10 mL of 1 N nitric acid was added. A solution of N,N′-methylenebisacrylamide in 10 mL of 1 N nitric acid was added with a molar ratio of BIS/NIPAM 5 × 10-3 ∼8 min after the addition of ceric ammonium nitrate solution. The precipitated copolymers were found during the polymerization. After about 6 h, the precipitated copolymers and solution were respectively dialyzed by repeated changes of fresh Milli-Q water at room temperature for ∼1 day and then at 5 °C for several days. The precipitates dissolved gradually in water at the lower temperature to a transparent solution. No polymers were formed in ∼6 h at 50 °C when a solution of ceric ammonium nitrate was added to an NIPAM solution in the absence of PEO. Dynamic light scattering measurements were performed with an argon ion laser operating at wavelength 488 nm and at power 50 mW. The autocorrelation function was measured using a Malvern 4700c correlator. Hydrodynamic diameters, dh, were determined using the Stokes-Einstein relation dh ) kBT/(3πηD), where kB, T, η, and D are the Boltzmann constant, the absolute temperature, the solvent viscosity, and the diffusion coefficient. The particle size distribution was obtained using the CONTIN program. In light-scattering measurements, solutions with different concentrations, c, were prepared by diluting a stock solution and then filtered through a 0.45 µm Millipore filter directly into a (12) Matsuoka, H.; Fujimoto, K.; Kawaguchi, H. Polym. Gels Networks 1998, 6, 319. (13) Polik, W. F.; Burchard, W. Macromolecules 1983, 16, 978. (14) Polverari, M.; Van der Ven, T. G. M. J. Phys. Chem. 1996, 100, 13687. (15) Faraone, A.; Magazu, S.; Maisano, G.; Migliardo, P.; Tettamanti, E.; Villari, V. J. Chem. Phys. 1999, 110, 1801. (16) Zhu, P. W.; Napper, D. H. Macromolecules 1999, 32, 2068. (17) Nagarajan, S.; Srinivasan, K. S. V. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 2925.
10.1021/la000489+ CCC: $19.00 © 2000 American Chemical Society Published on Web 10/07/2000
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Langmuir, Vol. 16, No. 22, 2000
Notes
Figure 1. Temperature dependence of the normalized intensity of PNIPAM-b-PEO microgel at different concentrations (g/g): 0, 4.8 × 10-5; 2, 9.7 × 10-5; 4, 2.0 × 10-4; 9, 4.6 × 10-4. The molar ratio of NIPAM to PEO is 0.5, and the molar ratio of BIS to NIPAM is 5.0 × 10-3. light-scattering cell. The sample temperature was controlled within (0.2 °C. The change in the solvent viscosity with temperature was taken into account. For slow heating experiments, the samples were allowed to equilibrate at each temperature for at least 15 min before light-scattering measurements were performed. For fast heating experiments, the samples were kept at 10 °C for 60 min before being quickly heated to the designated temperatures.
Results and Discussion The intrinsic stability of the PNIPAM-b-PEO microgel in water was checked over several hours at 33 and 60 °C, respectively. In the investigated range of microgel concentrations, the solutions appeared blue at the higher temperature, indicating the formation of stable nanoparticles. No precipitation was observed for the samples which had been thoroughly dialyzed. If the samples were not carefully dialyzed, a progressive precipitation occurred and the solutions turned milky. The results imply that Ce(III) ions produced during the polymerization can interact with the PNIPAM-b-PEO microgel or the microgel aggregates to form complexes which precipitated at higher temperatures. Figure 1 shows the temperature dependence of the normalized intensity of PNIPAM-b-PEO microgel at different concentrations. An increase in the normalized intensity was referred to as the onset of a microgel aggregation. The critical aggregation temperature, CAT, of PNIPAM-b-PEO microgel increases with a decrease in the microgel concentration. At c ) 4.8 × 10-5, the lightscattering intensity of the solution is almost constant when the temperature is lower than 38 °C, which is taken as an indication of no interparticle interaction. An increase of light-scattering intensity at 38 °C signals the beginning of microgel aggregation. When the concentration is greater than 4.6 × 10-4, the temperature at which the microgel aggregation starts is 32 °C, irrespective of the microgel concentration. The effect of heating rate on the formation of the microgel aggregates was studied above the critical aggregation temperature. Figure 2a shows typical plots of the intensity-weighted hydrodynamic diameter distribution as a function of temperature at the concentration 4.6 × 10-4 g/g. The sample was heated by a step-by-step increase in temper-
Figure 2. Temperature dependence of the hydrodynamic diameter distribution of PNIPAM-b-PEO microgel particles at a concentration of 4.6 × 10-4 (g/g). The molar ratio of NIPAM to PEO is 0.5, and the molar ratio of BIS to NIPAM is 5.0 × 10-3. (a) The sample was heated step-by-step from 20 °C to the designated temperatures. (b) The sample was quickly heated from 10 °C to the designated temperatures. The curves were vertically shifted for clarity. The light-scattering angle is 60°.
ature from 20 to 60 °C. The formation of a bimodal distribution is found at temperatures which are a few degrees higher than the CAT in the slow heating experiments. It is noted from Figure 2a that the diameter of microgel aggregates increases when the temperature increases from 36 to 40 °C and then decreases upon a further increase in temperature from 40 to 60 °C. Figure 2b shows the hydrodynamic diameter distribution of microgel aggregates when the same sample was quickly heated from 10 °C to the designated temperatures. In contrast to Figure 2a, where the size distribution above the CAT has two broadly distributed peaks, Figure 2b shows a monomodal size distribution at all of the temperatures studied. It is apparent from a comparison of Figure 2a and b that the fast heating can produce smaller aggregate particles with narrower distributions. For example, the aggregate particles with an average diameter of 55 nm can be obtained at 60 °C by means of the fast heating, while the slow heating to the same temperature leads to the formation of microgel aggregates with two
Notes
Langmuir, Vol. 16, No. 22, 2000 8545
Table 1. Particle Diameters at Different Conditionsa avg diameter of microgel particles (nm) conc, c (g/g)
40 °C
50 °C
60 °C
9.7 × 10-5 2.0 × 10-4 4.6 × 10-4 1.05 × 10-3
s, f 80, 50 105, 66 197, 76 232, 85
s, f 62, 50 115, 52 202, 58 240, 62
s, f 60, 50 118, 50 200, 52 238, 52
a s and f represent the slow and fast heating, respectively. The particle diameters from the slow heating are the average values of two average diameters, except for c ) 9.7 × 10-5 (g/g).
Figure 3. Temperature dependence of the average hydrodynamic diameter of PNIPAM-b-PEO microgel particles at a concentration 4.6 × 10-4 (g/g). The molar ratio of NIPAM to PEO is 0.5, and the molar ratio of BIS to NIPAM is 5.0 × 10-3. The sample was quickly heated from 10 °C to designated temperatures. The light-scattering angle is 60°.
average diameters of 90 and 310 nm. The aggregate diameters obtained from different microgel concentrations and temperatures are given in Table 1. The smaller aggregate size can be obtained at the higher temperature using fast heating. Such results are typically given in Figure 3 for c ) 4.6 × 10-4 (g/g). As can be seen from Figure 3, the aggregate diameter decreases continuously from 120 nm at 33 °C to 50 nm at 66 °C. The narrower size distribution is observed at the higher temperature. It is interesting to note from Table 1 that the aggregate size obtained with the fast heating is almost independent of the microgel concentration at the higher temperatures. These results indicate that the PNIPAM-b-PEO microgel has an unusual self-aggregation behavior. Usually, the larger particle size could form in the aggregation if the solvency condition is becoming worse than the CAT, as in the aggregation behavior in the slow heating experiments in the present work. The effect of the heating rate on the microgel aggregation can be considered to be a consequence of the competition
between interparticle interaction and intraparticle coilto-globule transition.18 The interparticle aggregation occurs before the onset of a proper coil-to-globule transition of PNIPAM segments inside the microgel during the slow heating. There are several explanations for the selfaggregation of amphiphilic block copolymers.2 Hydrophobic interactions could play a crucial rule. When the sample is quickly heated to a temperature higher than the CAT, the intraparticle coil-to-globule transition of PNIPAM segments can precede the interparticle aggregation. Since the intraparticle coil-to-globule transition can result in a core-corona structure, with mainly hydrophobic PNIPAM globules as a core and hydrophilic PEO chains as a corona, the particles formed are sterically stabilized, and the interparticle aggregation is effectively restrained.19 The formation of smaller particles at the higher temperature by means of fast heating is attributed to a further collapse of the core. Although the interactions between PEO segments and water molecules could be significantly weakened as the temperature is gradually increased, the PEO remains in good solvency condition. At lower temperatures, the interactions between PEO and water can effectively restrain the PNIPAM collapse. The collapsed PNIPAM clusters are spatially segregated inside the corecorona structure, and the core density has not reached the final globular density.20-22 When the temperature is high enough, the collapsed PNIPAM clusters continue to shrink to a well-defined core-corona structure. The results also imply that the well-defined core-corona structure forms immediately after the sample is heated to 60 °C. This is evidenced by the observation that the aggregate size is almost independent of the microgel concentration at this temperature. The results show that the PNIPAM-b-PEO microgel has its unique aggregation behavior in aqueous solutions. The aggregate size and its distribution can be well controlled by the fast heating. For the concentrations given in the present work, aggregate particles with monomodal size distributions can be obtained if the sample is quickly heated to temperatures higher than the critical aggregation temperature. The fast heating can result in smaller particle sizes at higher temperatures. It is also found that the aggregate size obtained is almost independent of the microgel concentration if the temperature is high enough. The observation has been explained as a competition between intraparticle “coil-to-globule” transition and interparticle aggregation. Acknowledgment. The authors wish to thank the Australian Research Council for its financial support of the present work. LA000489+ (18) Qiu, X.; Wu, C. Macromolecules 1997, 30, 7921. (19) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (20) de Gennes, P. G. J. Phys., Lett. (Paris) 1985, 46, L-639. (21) Grosberg, A. Y.; Nechaev, S. K.; Shakhnovich, E. I. J. Phys. (Paris) 1988, 49, 2095. (22) Kuznetosv, Yu. A.; Timoshenko, E. G.; Dawson, K. A. J. Chem. Phys. 1995, 103, 4807.
