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Temperature, pH, and Ionic Strength Induced Changes of the Swelling Behavior of PNIPAM-Poly(allylacetic acid) Copolymer Microgels Matthias Karg,†,‡ Isabel Pastoriza-Santos,§ Benito Rodriguez-Gonza´lez,§ Regine von Klitzing,‡ Stefan Wellert,†,‡ and T. Hellweg*,† UniVersita¨t Bayreuth, Physikalische Chemie I, UniVersita¨tsstrasse 30, D-95440 Bayreuth, Germany, TU Berlin, Stranski-Laboratorium, f. Physikalische and Theoretische Chemie, Strasse des 17.Juni 124, D-10623 Berlin, Germany, and Departamento de Quimica Fisica, UniVersidade de Vigo, 36310 Vigo, Spain ReceiVed September 28, 2007. ReVised Manuscript ReceiVed March 18, 2008 The volume phase transition of colloidal microgels made of N-isopropylacrylamide (NIPAM) is well-studied and it is known that the transition temperature can be influenced by copolymerization. A series of poly(N-isopropylacrylamideco-allylacetic acid) copolymers with different contents of allylacetic acid (AAA) was synthesized by means of a simple radical polymerization approach. The thermoresponsive behavior of these particles was studied using dynamic light scattering (DLS). Further characterization was done by employing transmission electron microscopy (TEM) and zeta potential measurements. TEM observations reveal the approximately spherical shape and low polydispersity of the copolymer particles. In addition, the measured zeta potentials provide information about the relative surface charge. Since these copolymers are much more sensitive to external stimuli such as pH and ionic strength than their pure PNIPAM counterparts, the volume phase transition was investigated at two different pH values and various salt concentrations. At pH 10 for the copolymer microgels with the highest AAA content, a significant shift of the volume phase transition temperature toward higher values is found. For higher AAA content, a change in pH from 8 to 10 can induce a change in radius of up to 100 nm making the particles interesting as pH controlled actuators.
Introduction 1–6
During the last three decades a large number of works and also several reviews7–9 on the subject of so-called intelligent macroscopic hydrogels were published. This class of hydrogels is of special interest, due to the volume phase transition encountered in these systems, i.e., the collapse of the gel at the point where a certain temperature (lower critical solution temperature (LCST))4,10–12,5 or ionic strength13 is reached. The first report on a hydrogel undergoing a temperature-induced volume phase transition was published by Tanaka in 1977.14 Since then, the number of investigations on this kind of “smart” materials is steadily growing. One of the most popular systems is made of N-isopropylacrylamide (NIPAM), which has a lower critical solution temperature of around 32 °C. This organic monomer is typically cross-linked with N,N-methylenebisacryl* To whom correspondence should
[email protected]). † Universita¨t Bayreuth. ‡ TU Berlin. § Universidade de Vigo.
be
addressed
(e-mail:
(1) Tanaka, T.; Fillmore, D. J. J. Chem. Phys. 1979, 70, 1214–1218. (2) Tanaka, T.; Fillmore, D.; Sun, S.-T.; Nishio, I.; Swislow, G.; Shah, A. Phys. ReV. Lett. 1980, 45, 1636–1639. (3) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379–6380. (4) Shibayama, M.; Tanaka, T.; Han, C. C. J. Chem. Phys. 1992, 97, 6829– 6841. (5) Shibayama, M.; Takata, S.; Norisuye, T. Physica A 1998, 249, 245–252. (6) Takeda, M.; Norisuye, T.; Shibayama, M. Macromolecules 2000, 33, 2909– 2915. (7) Dusek, K. ResponsiVe Gels: Volume Transitions I, 1st ed.; Advances in Polymer Science; Springer-Verlag: Berlin, 1993; Vol. 109. (8) Dusek, K. ResponsiVe Gels: Volume Transitions II, 1st ed.; Advances in Polymer Science; Springer-Verlag: Berlin, 1993; Vol. 110. (9) Shibayama, M. Macromol. Chem. Phys. 1998, 199, 1–30. (10) Shibayama, M.; Tanaka, T.; Han, C. C. J. Chem. Phys. 1992, 97, 6842– 6854. (11) Shibayama, M.; Tanaka, T. J. Chem. Phys. 1995, 102, 9392–9400. (12) Wu, C.; Zhou, S. Macromolecules 1997, 30, 574–576. (13) Shibayama, M.; Ikkai, F.; Inamoto, S.; Nomura, S.; Han, C. C. J. Chem. Phys. 1996, 105, 4358–4366. (14) Tanaka, T.; Ishiwata, S.; Ishimoto, C. Phys. ReV. Lett. 1977, 38, 771–774.
amide (BIS) to build a three-dimensional polymer network, which can be swollen by water. Due to their size in the submicrometer range, one special type of hydrogel is mostly referred to as microgel. The main difference between macroscopic gels and their smaller counterparts lies in the response to external stimuli, which is much faster for the latter ones.1,2 This is due to the fact that the time constant of the swelling/deswelling process depends on the geometrical dimensions of the investigated gel, which was shown by Li and Tanaka.15 This property makes microgels well-suited for several applications such as sensor design16–19 and drug delivery.20–23 From an application point of view, it is important to find a way to control the particle swelling and the phase transition temperature. A powerful tool to influence the swelling behavior of smart microgels is to use copolymerization with organic comonomers such as acrylic acid, vinylacetic acid, styrene, and others.24–28,53,30–34 (15) Li, Y.; Tanaka, T. J. Chem. Phys. 1989, 90, 5161–5166. (16) Serpe, M. J.; Jones, C. D.; Lyon, L. A. Langmuir 2003, 19, 8759–8764. (17) Wiedemair, J.; Serpe, M. J.; Kim, J.; Masson, J. F.; Lyon, L. A.; Mizaikoff, B.; Kranz, C. Langmuir 2007, 23, 130–137. (18) Ho¨fl, S.; Zitzler, L.; Hellweg, T.; Herminghaus, S.; Mugele, F. Polymer 2007, 48, 245–254. (19) FitzGerald, P. A.; Dupin, D.; Armes, S. P.; Wanless, E. J. Soft Matter 2007, 3, 580–586. (20) Duracher, D.; Elaissari, A.; Mallet, F.; Pichot, C. Langmuir 2000, 16, 9002–9008. (21) Bromberg, L.; Temchenko, M.; Hatton, T. A. Langmuir 2002, 18, 4944– 4952. (22) Zha, L.; Hu, J.; Wang, C.; Fu, S.; Elaissari, A.; Zhang, Y. Colloid Polym. Sci. 2002, 280, 1–6. (23) Nolan, C. M.; Serpe, M. J.; Lyon, L. A. Biomacromolecules 2004, 5, 1940–1946. (24) Snowden, M. J.; Chowdhry, B. Z.; Vincent, B.; Morris, G. E. J. Chem. Soc., Faraday Trans. 1996, 92, 5013–5016. (25) Saunders, B. R.; Crowther, H. M.; Vincent, B. Macromolecules 1997, 30, 482–487. (26) Duracher, D.; Sauzedde, F.; Elaissari, A.; Perrin, A.; Pichot, C. Colloid Polym. Sci. 1998, 276, 219–231. (27) Duracher, D.; Elaissari, A.; Pichot, C. Colloid Polym. Sci. 1999, 277, 905–913.
10.1021/la702996p CCC: $40.75 2008 American Chemical Society Published on Web 05/20/2008
Volume Change BehaVior of Copolymer Microgels
The change of the chemical composition of the network can affect the transition temperature as well as the swelling properties and the local network dynamics.35 Hence, the use of comonomer moieties within the microgel synthesis is a pathway to design versatile systems with specific properties. Considering pure poly-NIPAM particles the swelling can be understood as a result of the balance between polymer elasticity and the osmotic pressure. Both are responsible for the state of extension of the network. An additional contribution to the osmotic pressure appears when charged functionalities are incorporated into the network.29 Like typical polyelectrolytes, such charged networks are sensitive to changes of pH, which defines the degree of dissociation and also to changes of the ionic strength, which affects the Debye screening length. For highly charged copolymers electrostatic effects cause the swelling process to be more complex.29 In this work we want to discuss the properties of a series of new copolymer microgels with various charge densities obtained by the incorporation of varying amounts of the comonomer allylacetic acid (AAA). To investigate the influence of the charged comonomer on the swelling properties of the obtained copolymer microgels and their sensitivity on pH and ionic strength, we prepared microgels with a nominal AAA monomer feed up to 15 mol %, which were studied under various solvent conditions using dynamic light scattering. For the class of PNIPAM copolymer microgels containing COOH groups, an interesting approach was recently published by Hoare and Pelton.36,37 This approach allows comparison of different COOH containing systems in a dimensionless plot. However, in the present work we were mainly interested in the behavior of the new AAA-PNIPAM microgels at high pH (high degree of charge). Hence, mainly measurements at pH 8 and 10 are presented. Nevertheless, for the purpose of direct comparison of the swelling behavior of the AAA copolymer particles to the well-characterized vinyl acetic acid copolymers (VAA)34,18 also the swelling behavior of VAA microgels was studied at pH 8. In recent years, organic/inorganic hybrids based on PNIPAM microgels have attracted much interest, since these can combine the fascinating properties of both materials. Depending on the type of inorganic nanoparticle, interesting optical38–42 or magnetic43 properties can be achieved. For example a hybrid system was investigated where PNIPAM microgels were covered with polyelectrolyte-coated gold nanorods.44 The coated nanorods (28) Kim, J.-H.; Ballauff, M. Colloid Polym. Sci. 1999, 277, 1210–1214. (29) Kratz, K.; Hellweg, T.; Eimer, W. Colloids Surf., A 2000, 170, 137–149. (30) Fernandez-Nieves, A.; Fernandez-Barbero, A.; de las Nieves, F. J. J. Chem. Phys. 2001, 115, 7644–7649. (31) Levin, Y.; Diehl, A.; Fernandez-Nieves, A.; Fernandez-Barbero, A. Phys. ReV. E 2002, 65, 036143-1-6. (32) Debord, J. D.; Lyon, L. A. Langmuir 2003, 19, 7662–7664. (33) Berndt, I.; Richtering, W. Macromolecules 2003, 36, 8780–8785. (34) Hoare, T.; Pelton, R. Macromolecules 2004, 37, 2544–2550. (35) Hellweg, T.; Kratz, K.; Pouget, S.; Eimer, W. Colloids Surf., A 2002, 202, 223–232. (36) Hoare, T.; Pelton, R. Langmuir 2006, 22, 7342–7350. (37) Hoare, T.; Pelton, R. J. Colloid Interface Sci. 2006, 303, 109–116. (38) Jones, C. D.; Serpe, M. J.; Schroeder, L.; Lyon, L. A. J. Am. Chem. Soc. 2003, 125, 5292–5293. (39) Kim, D. J.; Kang, S. M.; Kong, B.; Kim, W.-J.; Paik, H.-J.; Choi, I. S. Macromol. Chem. Phys. 2005, 206, 1941–1946. (40) Lu, Y.; Mei, Y.; Ballauff, M. J. Phys. Chem. B 2006, 110, 3930–3937. (41) Karg, M.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Hellweg, T. Chem PhysChem 2006, 7, 2298–2301. (42) Das, M.; Sanson, N.; Fava, D.; Kumacheva, E. Langmuir 2007, 23, 196– 201. (43) Wong, J. E.; Gaharwar, A. K.; Mueller-Schulte, D.; Bahadur, D.; Richtering, W. J. Magn. Magn. Mater. 2007, 311, 219–223. (44) Karg, M.; Pastoriza-Santos, I.; Perez-Juste, J.; Hellweg, T.; Liz-Marzan, L. M. Small 2007, 3, 1222–1229.
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have positive surfaces and stick to the microgel outside due to electrostatic attraction of the oppositely charged particles. It was shown that different surface coverages can be realized. The reachable maximum coverage seems to be controlled and limited by the surface charge of the microgel. With regard to this type of hybrid material, the presented AAA copolymers are promising candidates for the production of composites with a higher surface coverage and a different thermo-optical response. This was the main motivation for the presented synthesis and characterization of the new copolymer microgels.
Experimental Section Synthesis of PNIPAM-co-AAA Microgels. N-Isopropylacrylamide (NIPAM; Aldrich, 97%) was recrystallized from hexane. Allylacetic acid (AAA; Fluka, g98.0%), N,N′-methylenebisacrylamide (BIS; Fluka, g99.5%), and potassium peroxodisulfate (KPS; Fluka, g99.0%) were used as received. Water was purified employing a MilliQ system (Millipore). The microgel synthesis was done using a conventional surfactant-free emulsion polymerization as described elsewhere.45,46 A solution of 3.961 g of N-isopropylacrylamide (NIPAM), 0.270 g of N,N′-methylenebisacrylamide (BIS) and the desired amount of comonomer allylacetic acid (AAA) dissolved in 350 mL of MilliQ water was prepared in a three-neck flask equipped with a reflux condenser and a strong stirring device. These quantities lead to particles with an approximate molar ratio of 5% with respect to cross-linker. The respective solutions were heated to 70 °C and kept under a nitrogen atmosphere to remove oxygen. After 30 min of intense stirring at 70 °C, a solution of 1 mg of potassium peroxodisulfate (KPS) dissolved in 1 mL of MilliQ water was rapidly added. The reaction solutions became turbid within 5 min after the KPS addition, and the polymerization was allowed to progress for 4 h at 70 °C. Then the white and strongly turbid polymer dispersion was cooled to room temperature and stirred overnight. The prepared microgel particles were cleaned by intense dialysis lasting 14 days with daily solvent (MilliQ water) exchange. Previous experiments have shown that this is a very soft and well-working method to remove impurities like unreacted monomers and oligomers. Further cleaning regarding longer polymer chains above the molecular weight cutoff of the dialysis tubes (Visking, cutoff at 13500 g/mol) was not performed, since no negative influence of these impurities on the employed characterization methods could be observed. Moreover, in the transmission electron microscopy (TEM) images no small polymer globules are visible as impurity also indicating that admixtures of high molar mass can be ignored. After dialysis the microgels were freeze-dried resulting in white solids of low density and an estimated residual water content of about 10%.47 The incorporated amount of AAA monomers was determined by titration with 1 mmol/L NaOH using a Titrando 836 system (Metrohm). The respective values are given in Table 1. However, in the following sections the samples are identified by the nominal AAA content. Dynamic Light Scattering. For the dynamic light scattering experiments (DLS) we used a frequency doubled Nd:YAG-Laser (Compass series, Coherent, USA) as a light source with a wavelength of 532 nm and a constant output power of 150 mW. Measurements were done at different temperatures in a temperature range between 15 and 60 °C controlled by a thermostated toluene bath. We used a classical goniometer setup with pinhole collimation (ALV, Langen, Germany) and recorded at least three correlation functions for each temperature using an ALV-5000/E multiple-τ correlator. Most of the experiments were done at a scattering angle of 60°, except of one series of DLS measurements where angles between 30° and 60° (45) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247–256. (46) Kratz, K.; Hellweg, T.; Eimer, W. Polymer 2001, 42, 6531–6539. (47) Kratz, K. Intelligente Poly-N-Isopropylacrylamid-Mikrogele unterschiedlicher chemischer Zusammensetzung. Einfluss Von KonnektiVita¨t, Ladungsdichte und Ionensta¨rke auf das QuellVerhalten Von PNIPA-Kolloiden. Thesis, University of Bielefeld, 1999.
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Table 1. Average Particle Diameters DTEM Taken from Histograms Shown in Figure 2a
copolymer
DTEM (nm)
0% AAA 1% AAA 3% AAA 5% AAA 10% AAA 15% AAA
151 (10%) 223 (12%) 220 (13%) 247 (11%) 312 (19%) 383 (12%)
AAA content (mol %) from ζ Dh (nm) Dh (nm) titration at 25 °C at 60 °C (mV) 462 622 646 692 686 1018
198 284 290 278 266 351
-3 -15 -15 -19 -19 -21
0.57 0.49 0.76 1.53 2.99
a Values in parentheses are polydispersities as results of fits of the histograms in Figure 2 with Gaussian functions. Hydrodynamic diameters Dh measured at 25 and 60 °C at pH 8 as a result of DLS. Zeta potentials ζ at 25 °C and pH 8. The last column contains the incorporated amount of AAA monomers as obtained by titration.
differences in the mesh size of the polymer network, compared to PNIPAM-acrylic acid copolymers, can be expected. Several techniques have been used to characterize the properties of the new particles and, especially, to investigate their thermoresponsive behavior. For the latter purpose dynamic light scattering is an ideal tool because it provides the translational diffusion coefficient DT of colloidal particles in solution. To make sure that the observed decay of the intensity time correlation functions in the relevant q range is only related to the center of mass diffusion of the microgels for one temperature, several scattering angles were investigated. Figure 1 shows the obtained relaxation rates for the different microgel samples, which were prepared. For all samples four scattering angles were measured and within the experimental precision all samples show purely diffusional dynamics in the interval 30-60°. DT can be computed from the slope of these curves. This shows that for scattering angles e60° the translational diffusion coefficient of the particles can be computed from data obtained at a single angle. Hence, it is sufficient to investigate the swelling behavior, for example, by using only a scattering angle of 60°. DT is then used to determine the hydrodynamic radius Rh by applying the Stokes-Einstein equation.
DT )
Figure 1. Angular-dependent results of DLS experiments of samples with different contents of AAA at pH 8. All data points of each sample show a linear dependency within the experimental error. The measurements were done in the angular range from 30 to 60°. The data follow Γ ) DTq2 and the observed dynamics is pure center of mass diffusion of the microgels.
were chosen. The analysis of the data was done by inverse Laplace transformation of the field time correlation functions g1 (CONTIN48,49). g1 is obtained from the experimental intensity time correlation functions using the Siegert relation.50 Electron Microscopy. TEM images were recorded with a JEOL JEM 1010 microscope working with an acceleration voltage of 100 kV. Specimens were prepared by drying of some droplets of a highly diluted dispersion on copper grids (coated with a carbon membrane). Zeta Potential Measurements. Zeta potential measurements were carried out with a Malvern Zetasizer 2000 using highly diluted dispersions at pH 8 without additional salt. The temperature during the measurements was 25 °C.
Results and Discussion A series of new thermo- and pH-responsive copolymer microgels made of NIPAM and the co-monomer allylacetic acid (AAA) with a constant nominal cross-linker molar ratio of 5% (BIS) were synthesized. AAA was used as co-monomer, because like acrylic acid or vinylacetic acid28,29,34,51,18 it adds charged functionalities to the polymer and therefore influences the swelling behavior. Particularly, the sensitivity with respect to parameters such as ionic strength and pH is strongly enhanced compared to pure PNIPAM microgels. Furthermore, due to its alkyl chain length AAA has a rather hydrophobic character. Therefore, (48) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213–217. (49) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 229–242. (50) Berne, B. J.; Pecora, R. Dynamic Light Scattering; John Wiley & sons, Inc.: New York, 1976. (51) Hoare, T.; Pelton, R. Langmuir 2004, 20, 2123–2133.
kT 6πηRh
(1)
Here, η is the solvent viscosity. Due to the size of the microgels, internal interference might occur at higher scattering angles. An important measure for the state of swelling is the so-called swelling ratio R, which is defined as the ratio between the volume of collapsed particles Vshrunken and the volume of the particles in the totally swollen state Vswollen.
R)
()
Vshrunken Rh ) Vswollen R0
3
(2)
Since, we are dealing with microgels of near spherical shape in solution, one can use the radii in order to calculate R. The surfactant-free emulsion polymerization, which we used for the synthesis45 led to spherical microgels with rather low polydispersities as can be seen in Figure 2. The shown TEM images have been used to determine the particle size, which is given in the histograms. These histograms stem from the analysis of the diameter of 150 different particles observed on different TEM images taken for the individual copolymer microgels. The average diameters as well as the polydispersities are listed in Table 1. In contrast with the DLS experiments where the hydrodynamic radii are measured, the TEM images represent the particles in a rather dry and collapsed state (see diameters in Table 1). The diameters obtained from TEM are by a factor of 2.5 smaller compared to the Rh values in bulk solution. In some TEM images one can observe a regular pattern with periodic interparticle distances. We believe that this effect can only be explained by the method of preparation. During the drying process on the TEM grids, the particles form a monolayer with hexagonal packing and keep their positions during the continued drying process. Therefore, the microgel spheres are placed in an ordered way. This corresponds to previous investigations of PNIPAM microgels using scanning electron microscopy.52,46 Some of the particles are bridged, which might be due to entanglements of so-called “dangling ends”, which look like linkers between two spheres and manifest (52) Kratz, K.; Lapp, A.; Eimer, W.; Hellweg, T. Colloids Surf., A 2002, 197, 55–67. (53) Kratz, K.; Hellweg, T.; Eimer, W. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1603–1608.
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Figure 2. TEM images (JEOL JSM 1010 working at 100 kV) and histograms for particle diameters for copolymers with different amounts of co-monomer AAA. For each histogram the diameter of 150 different particles was determined from different TEM images. The TEM images indicate an increase in particle size with growing AAA content.
Figure 3. Zeta potentials of dilute AAA-copolymer microgel suspensions at pH 8 and a temperature of 25 °C, where all particles are in a swollen state. Here, the incorporated amount of AAA is given on the abscissa.
the soft and fuzzy character of the polymer network of the microgels. However, these observations strongly depend on the particle concentration used for the preparation of the TEM samples. The incorporation of charges in the network was studied by titration of the microgel samples. The obtained values for the incorporated amount of comonomer are also given in Table 1. The yield with respect to the incorporation of the AAA monomers is about 15-20% at least for the higher nominal AAA feeds. This is similar to the found incorporation rates of vinyl acetic acid (VAA)18 but significantly lower compared to acrylic acid.29 Moreover, the particles were compared with respect to their surface charge by measurements of the zeta potential at pH 8. Even when these measurements do not provide any reliable absolute value for the surface charge of such rather big microgel particles, they reveal the increase of the surface charge with increasing contents of AAA. Figure 3 gives the results of the zeta potential measurements for the series of prepared copolymer microgels. The data are plotted versus the real AAA content in the different samples as determined by titration. Charges, which stem from the anionic radical initiator potassium peroxodisulfate (KPS), can be neglected in all samples due to the low amounts (0.1 mol %) we used to initialize the polymerizations. Therefore, as expected the pure PNIPAM microgel has a negligible zeta potential. However, already a low amount of incorporated AAA leads to a non-negligible zeta potential and the values are decreasing with higher AAA contents.
Figure 4. Results of temperature-dependend pH measurements for a sample with a nominal AAA content of 15 mol %.
Influence of the AAA Content and the pH on the Swelling Behavior. Measurements of the swelling behavior of PNIPAM-AAA copolymer microgels are problematic, because also added salt can influence the swelling behavior. Hence, in the experiments discussed in this section the pH was adjusted by addition of small amounts HCl and NH3. Therefore, only small ionic strength effects have to be considered. However, these effects are smaller than the changes of ionic strength, which would be caused by the use of a buffer. In addition, this procedure produces a minor imprecision with respect to the pH value. Figure 4 shows the change of the pH value of a PNIPAM-AAA copolymer microgel-solution as a function of temperature. The pH value was initially adjusted to 8.1 and goes slightly down to approximately 7.3 when the microgel collapses due to the raised temperature. The collapse obviously leads to significant proton release. This effect is expected to be similar for all samples having a similar microgel concentration with a comparable number COOH groups. Therefore, we think that all relative changes discussed in the following are not significantly perturbed by this proton release. Figure 5 shows swelling curves of the AAA copolymer particles at two different pH values (for one sample at three different values) as obtained from temperature-dependent DLS measurements at a scattering angle of 60°. All PNIPAM-co-AAA microgel samples undergo a volume phase transition which was found to be fully reversible for all cases. The increasing amount of incorporated AAA in the investigated series of copolymer microgels leads to a strong increase in the
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Figure 6. Comparison of the swelling behavior of copolymer microgels with a nominal content of 15 mol % of AAA and also a nominal content of 15 mol % of VAA (vinylacetic acid) as co-monomer at pH 8. From titrations the effectively built-in amount of co-monomer is known and the number of COOH groups is found to be similar for both samples (see Table 1 and ref 18). Shown is the hydrodynamic radius Rh and the reciprocal swelling ratio R-1 as a function of temperature. The transition temperatures are 46 °C (AAA) and 41 °C indicated by the dashed lines. Despite of the fact that the AAA microgels were made with 5 mol % of cross-linker and the VAA particles only contain 2 mol % BIS, the AAA particles swell and deswell more than the VAA particles. This is remarkable since usually an increase of the cross-linker content leads to a decrease of the swelling capacity.
Figure 5. Swelling curves of AAA copolymers as result from dynamic light scattering experiments. The hydrodynamic radius Rh of each microgel in suspensions of pH 8 and pH 10 was determined as a function of temperature. The doted lines indicated the volume phase transition temperature of pure PNIPAM microgels. In the graph for the 15 mol % particles a curve for pH 5.5 is also included. At this pH the swelling behavior is already similar to the pure PNIPAM system. Table 2. Hydrodynamic Radii Rh at 60 °C (Collapsed State), Values in Parentheses Are Swelling Ratios r, Temperatures of Volume Phase Transition Ttr for pH 8 and pH 10 copolymer
Rh (nm) pH 10
Ttr (°C) pH 10
Rh (nm) pH 8
Ttr (°C) pH 8
0% AAA 1% AAA 3% AAA 5% AAA 10% AAA 15% AAA
95 (0.07) 153 (0.11) 140 (0.06) 134 (0.07) 148 (0.07) 210 (0.12)
32 32 35 36 47 50
99 (0.08) 142 (0.10) 145 (0.09) 139 (0.06) 133 (0.06) 176 (0.05)
32 32 34 35 39 46
size of the synthesized particles. The hydrodynamic radius goes up from 231 nm (no AAA) to 508 nm (15 mol % AAA nominally). This is similar for pH 8 and 10. The same trend can be observed on the TEM images for the partly dried particles (see also values in Table 1). Since the conditions were similar for all polymerizations, this effect might be caused by an increase of the interfacial tension between the droplets formed upon heating the reaction mixture and the surrounding aqueous phase. Such an increase of the interfacial tension might be caused by the hydrophobic character of AAA. In addition, the growing AAA content also has a significant impact on the temperature associated with the volume transition at pH 8 and 10 (see also Table 2). This shift is higher than the one observed for PNIPAM-co-acrylic acid microgels,53,29 but smaller than the one observed for macroscopic PNIPAM-co-acrylic acid gels.10,13 With respect to the pH sensitivity, two major effects can be observed: (1) the sensitivity of the particles with respect to the
pH increases with increasing AAA-content; (2) the transition temperature moves to higher values with increasing pH. Up to a nominal AAA content of 5 mol %, the impact of the pH value on the swelling curves of the particles is small. At 10% and 15% nominal molar ratio of AAA, a change from pH 8 to pH 10 shifts the transition temperature by approximately 5-10 K. This makes these copolymer particles very interesting, since a small change in pH (e.g., from 8 to 10) can induce a drastic change in size. For example the particles synthesized with 10 mol % AAA exhibit a change in hydrodynamic radius of 100 nm, when pH increases from 8 to 10. Hence, pH changes can be converted in mechanical work and movements on the nm scale. For the microgel with a nominal AAA content of 15 mol %, Figure 5 also gives the swelling curve at pH 5.5. At this pH the particles nearly exhibit the swelling behavior of pure PNIPAM microgels. Compared to pure PNIPAM particles, the transition temperature can be shifted by up to 20 K toward higher temperatures. The change in swelling behavior can be attributed to additional contributions to the osmotic pressure in these systems (Donnan contribution). This is in line with the temperatureinduced changes of the pH in solution of the AAA copolymer microgels as shown in Figure 4. These observations are in qualitative agreement with the results found for PNIPAM-coacrylic acid microgels.29 However, here no very pronounced two-step transition occurs as was observed for acrylic acid copolymer particles. Instead, only a kind of shoulder is observable in the swelling curve of the AAA 15 mol % sample (see also Figure 6). This might point to differences in the monomer distribution inside the AAA copolymer microgels compared to acrylic acid containing ones. The found swelling behavior also shows some similarities to results for other carboxylic acid containing microgels.34,51 For the sample prepared with 15 mol % AAA at pH 10, the shift of the transition temperature is as large as the shifts observed by Hoare and Pelton for methacrylic acid and vinylacetic acid copolymer microgels (see Figure 8 in ref 34). Compared to these other monomers, AAA-containing microgels apparently have a higher swelling capacity. On the basis
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Figure 7. Influence of ionic strength on the hydrodynamic radius of different AAA copolymers at 25 °C (left) and 32 °C (right).
Figure 8. Relative change of the hydrodynamic radius of different AAA copolymers at 25 °C (left) and 32 °C (right) as a function of salt concentration.
of the data in ref 34, the swelling ratio R for vinylacetic acid (VAA) copolymer microgels at similar pH can be estimated to be ∼0.3. Under the same conditions, the new AAA copolymer particles have an R ≈ 0.03. In Figure 6 we compare the swelling curves of a VAA containing and an AAA-PNIPAM copolymer microgel. The nominal comonomer content was equal to 15 mol % in both cases and also the resulting real content was similar for these two samples (VAA content 2.5 mol %; AAA content 2.99 mol %). Figure 6 also displays the reciprocal value of the swelling ratio R for these two samples. Here, we observe a smaller R for the VAA copolymer as well. The difference is less important compared to the value computed on the basis of the data in ref 34. This is due to the cross-linker content of the VAA copolymer particles studied here. They only have a cross-linker content of 2 mol %. It is known that the swelling capacity goes down with growing cross-linker content. In addition also the swelling behavior of the sample with a nominal content of 10 mol % AAA (real content 1.53 mol %) was compared to the behavior of the sample made with vinylacetic acid ((VAA content 2.5 mol %). Both samples have a very similar swelling behavior (data not shown). Hence, about 60-70% less AAA has to be incorporated in the polymer network to produce the same changes in swelling as obtained by copolymerization of NIPAM with VAA. Influence of Ionic Strength on the Swelling Behavior. The incorporation of charges into the polymer network of a microgel affects not only the sensitivity to pH but, in a stronger way, the
Table 3. Hydrodynamic Radii Rh for AAA Copolymers at 40 °C and pH 8 for Different Weight Fractions of Salt (NaCl) Rh copolymer
0.02 wt % NaCl
0.10 wt % NaCl
0.50 wt % NaCl
0% AAA 1% AAA 3% AAA 5% AAA 10% AAA 15% AAA
321 nm 155 nm 164 nm 172 nm 231 nm 360 nm
aggregation aggregation 163 nm 166 nm 202 nm 353 nm
aggregation aggregation aggregation aggregation 314 nm 276 nm
response upon changes of the ionic strength, which has been controlled by the simple addition of NaCl to the microgel dispersions in the present work. At the rather high pH values used (8 and 10), the acid groups stemming from AAA are considered to be completely deprotonated resulting in negatively charged particles. With increasing ionic strength, these charges are shielded, which on one hand induces the formation of aggregates. On the other hand the process of aggregation depends on the degree of swelling like it is shown in Figures 7 and 8. These figures represent the hydrodynamic radius of the copolymers at pH 8 as a function of the NaCl concentration at three different temperatures, corresponding to different degrees of swelling. The only weakly charged microgel with an AAA-content of 0% is nearly unaffected by the ionic strength at 25 and 32 °C. Only at 40 °C this sample forms aggregates, even at the lowest amount of added salt.
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For the charged copolymers the situation is different. Particles with higher charge react much stronger on the addition of salt. For example, for the 15 mol % AAA copolymer microgel the hydrodynamic radius decreases by ≈120 nm. In general, particles with lower charge have a stronger tendency to form aggregates. Figure 8 shows radii of the differnet samples normalized to the respective radius in salt-free solution.
Conclusions A series of new thermoresponsive copolymer microgels on the basis of NIPAM and the comonomer allylacetic acid (AAA) was synthesized. The freeze-dried copolymer samples are white solids of low density, which can be easily redispersed in water. This is similar to pure PNIPAM microgel samples and also to other copolymer particles. DLS experiments at pH 8 and pH 10 have shown that the transition temperature of the microgel particles is shifted to higher temperatures with increasing amounts of comonomer. Moreover, a similar trend can be observed for the overall particle size (see, e.g., Figure 2). Using zeta potential measurements, it was possible to proof the change in net charge of the particles with increasing AAA content. However, the values of the zeta potential only qualitatively monitor the surface charge of the particles. Nevertheless, the found tendency is in good agreement with expected results and results for other charged copolymers.29,34,51,18 This work again shows that copolymerization is a powerful tool to design microgels with specific properties. The presented
Karg et al.
copolymer particles are possible candidates in applications like smart surface coatings and transport media showing stronger pH-dependent changes compared to other COOH containing copolymer microgels. At the transition temperature, small pH changes can produce movements of the order of 100 nm. As Figure 5 shows, the transition temperature can be varied over a range of 15 °C, at least, by variations in the monomer composition. Furthermore, an application as pH-controlled actuators in, e.g., microfluidic devices might be possible. However, the differences observed in comparison to other carboxylic acid group containing microgels still remain to be understood in a more quantitative way. Hence, in the future the distribution of the AAA comonomers inside the network forming the particles will be studied by means of small-angle neutron scattering applying contrast variation using partial deuteration of the network. In the preparation of inorganic/organic hybrid materials,44 the AAA copolymer microgels might also be useful, because in their charged state it will be rather easy to adsorb a high number of oppositely charged nanoparticles on these microgels. Respective studies are being carried out. Acknowledgment. This work was funded by the Deutsche Forschungsgemeinschaft within the framework of the priority program SPP 1259. In addition partial support of COST Action D43 is acknowledged. LA702996P
