by Copolymer Microgel Particles of - American Chemical Society

[email protected]. † University of Bristol. ‡ Universidad del Paıs Vasco/EHU. (1) Murray, M. J.; Snowden, M. Adv. Colloid Interface Sci...
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Langmuir 2005, 21, 1209-1215

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Equilibrium and Kinetic Aspects of the Uptake of Poly(ethylene oxide) by Copolymer Microgel Particles of N-Isopropylacrylamide and Acrylic Acid Melanie Bradley,† Jose Ramos,‡ and Brian Vincent*,† School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K., and Institute for Polymer Materials and Grupo de Ingenierı´a Quı´mica, Departamento de Quı´mica Aplicada, Facultad de Ciencias Quı´micas, Universidad del Paı´s Vasco/EHU, Apdo. 1072, 20080 San Sebastia´ n, Spain Received August 13, 2004. In Final Form: October 24, 2004 The use of microgels for controlled uptake and release has been an area of active research for many years. In this work copolymer microgels of N-isopropylacrylamide (NIPAM) and acrylic acid (AAc), containing different concentrations of AAc and also cross-linking monomer, have been prepared and characterized. These microgels are responsive to pH and temperature. As well as monitoring the equilibrium response to changes in these variables, the rates of swelling/de-swelling of the microgel particles, on changing either the pH or the temperature, have also been investigated. It is shown that the rate of de-swelling of the microgel particles containing AAc is much faster than the rate of swelling, on changing the pH appropriately. This is explained in terms of the relative mobilities of the H+ and Na+ ions, in and out of the particles. It was observed that the microgels containing AAc, at pH 8, de-swelled relatively slowly on heating to 50 °C from 20 °C. This is attributed to the resistance to collapse associated with the large increase in counterion concentration inside the microgel particles. The swelling and de-swelling properties of these copolymer microgels have also been investigated in aqueous poly(ethylene oxide) (PEO) solutions, of different MW (2000-300 000). The corresponding absorbed amounts of PEO from solution onto the microgels have also been determined using a depletion method. The results, as a function of AAc content, cross-linker concentration, PEO MW, pH, and temperature, have been rationalized in terms of the ease and depth of penetration of the PEO chains into the various microgel particles and also the H-bonding associations between PEO and either the -COOH of the AAc moeities and/or the H of the amide groups (much weaker). Finally, the adsorption and desorption of the PEO molecules in to and out of the microgel particles have been shown to be extremely slow compared to normal diffusion time scales for polymer adsorption onto rigid surfaces.

Introduction Microgel particles are spherical, cross-linked polymer particles in the colloidal-size range that swell or de-swell in response to external stimuli, such as temperature, pH, osmotic pressure, ionic strength, and solvent composition.1,2 They are normally prepared by emulsion or dispersion polymerization methods. The maximum extent of swelling is controlled by the amount of cross-linking monomer that is copolymerized. Dispersions of such particles are the basis of many applications, for example, in the coatings, cosmetics, and food industries. Additionally, microgel particles have attracted much attention because of their faster kinetic response to external stimuli compared to macroscopic gel particles. Poly(N-isopropylacrylamide) (PNIPAM), cross-linked with bisacrylamide (BA), is a well-studied colloidal microgel which is temperature-responsive;1,2 it has a hydrophilic amide group and a hydrophobic isopropyl group. The lower critical solution temperature (LCST) of homopolymer PNIPAM in water is ∼32 °C, and around this temperature PNIPAM particles, dispersed in water, decrease in size. * To whom correspondence should be addressed. E-mail: [email protected]. † University of Bristol. ‡ Universidad del Paı´s Vasco/EHU. (1) Murray, M. J.; Snowden, M. Adv. Colloid Interface Sci. 1995, 54, 73. (2) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1.

It has been shown3-5 that the cross-linker distribution within PNIPAM microgel particles is, in general, heterogeneous. It would seem that the cross-link density decreases from the center to the periphery. This has been explained by McPhee et al.6 in terms of the reactive ratios of the two monomers. They have shown that the reactivity of the BA is greater early on in the microgel synthesis, and, hence, the BA is consumed at a faster rate, earlier than the NIPAM, resulting in a higher cross-linked core. It is possible to increase the functionality of microgel particles by including monomers in the particle synthesis that are responsive to other stimuli. For example, the result of copolymerizing NIPAM with an ionizable monomer, such as acrylic acid (AAc), is a microgel that is responsive to both temperature and pH. Again it is likely that the distribution of these functional groups is nonuniform. However, their introduction can lead to specific interactions of the microgel particles with various types of molecular species7,8 and even very small nanoparticles.9 On the basis of this concept, there has been growing interest in recent years in using microgel particles to (3) Crowther, H. M.; Vincent, B. Colloid Polym. Sci. 1998, 276, 46. (4) Daly, E.; Saunders, B. R. Phys. Chem. Chem. Phys. 2000, 2, 3187. (5) Guillermo, A.; Addad, J. P. C.; Bazile, J. P.; Duracher, D.; Elaissary, A.; Pichot, C. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 889. (6) McPhee, W.; Tam, K. C.; Pelton, R. J. Colloid Interface Sci. 1993, 156, 24. (7) Bromberg, L.; Temchenka, M.; Hatton, T. A. Langmuir 2002, 18, 4944. (8) Murthy, N.; Xu, M.; Schuck, S.; Kunisawa, J.; Shastri, N.; Frechet, J. M. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 4995. (9) Han, M.; Gao, X.; Su, J. Z.; Nie, S. Nat. Biotechnol. 2001, 19, 631.

10.1021/la047966z CCC: $30.25 © 2005 American Chemical Society Published on Web 01/15/2005

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encapsulate “active” molecules for controlled release purposes. For example, Murthy et al.8 have encapsulated proteins in microgel particles by preparing the particles in the presence of the protein concerned. The cross-linker used was chosen to degrade at low pH, leading to release of the protein. Morris et al.10 investigated the potential use of PNIPAMco-AAc microgels for the removal of heavy metal ions from aqueous media. It was found that the diameter of the anionic microgel particles was reduced in the presence of Pb(II), at neutral and higher pH values. This was postulated to be due to complexation of Pb(II) with the anionic carboxylate moieties of the AAc moeities. Essentially no absorption of Pb(II) was found at pH values ∼ pH 3, i.e., significantly lower than the pKa of AAc (∼4.5). Hence, subsequent desorption of Pb(II) from the microgel particles could be achieved by reducing the solution pH. Several studies have investigated the interaction between anionic surfactants (e.g. sodium dodecyl sulfate, SDS) and PNIPAM microgels.11-13 It was shown that microgels swell in the presence of SDS and that the surfactant binds (as small aggregates or “pseudomicelles”) to the isopropyl groups of the PNIPAM microgel particles. Crowther et al.14 have also studied the binding of SDS to (quaternized, hence, strongly cationic) polyvinylpyridine microgel particles. The interaction of polymers with microgel particles has also been investigated.15,16 It was found that, on addition of a polymer to a dispersion of microgel particles, if little or no interpenetration by the polymer chains into the microgel particles occurred, then osmotic de-swelling of the microgel particles takes place. In this paper the interaction of poly(ethylene oxide) (PEO) with a series of PNIPAM-co-AAc microgel particles is described. AAc was chosen as the comonomer since, at low pH, PEO interacts with the AAc moieties, via strong H-bonding between the ether oxygen of the PEO and the -OH groups of the AAc. The main variables investigated include pH, PEO molecular weight, cross-linker, and AAc concentrations in the microgel particles. Dynamic light scattering has been used to determine changes in particle diameter, and the absorption of PEO by the microgel particles has also been determined. Both kinetic and equilibrium aspects are reported. Materials and Methods N-Isopropylacrylamide (NIPAM; Fisher) and N,N′-methylenebisacrylamide (BA; Fisher) were recrystallized from hexane and methanol, respectively. Acrylic acid (Fisher) and potassium persulfate (KPS; Fisher) were used as supplied, without further purification. “Milli-Q” grade water was used throughout the work. The various poly(ethylene oxide) samples used, with different average molecular weights (2000, 20 000, 100 000, and 300 000), were supplied by Fluka. Synthesis of the Microgel Particles. Single-step emulsion polymerization reactions were carried out in a 500 mL reaction vessel, fitted with a reflux condenser, an overhead stirrer, a thermometer, and a glass nitrogen inlet tube. The vessel was (10) Morris, G.; Vincent, B.; Snowden, M. J. Colloid Interface Sci. 1997, 190, 198. (11) Mears, S. J.; Deng, Y.; Cosgrove, T.; Pelton, R. Langmuir 1997, 13, 1901. (12) Wang, G.; Pelton, R.; Zhang, J. Colloids Surf., A 1999, 153, 335. (13) Woodward, N. C.; Chowdhry, B. Z.; Leharne, S. A.; Snowden, M. J. Eur. Polym. J. 2000, 36, 1355. (14) Crowther, H. M.; Morris, G. E.; Vincent, B.; Wright, N. G. in Role of Interfaces in Environmental Protection; Barany, S., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2003; p 169. (15) Saunders, B.; Vincent, B. J. Chem. Soc., Faraday Trans. 1996, 92, 3385. (16) Saunders, B. R.; Vincent, B. Colloid Polym. Sci. 1997, 275, 9.

Bradley et al. Table 1. Quantities Used in the Preparation of the Microgel Particles reagent water NIPAM (main monomer) BA (cross-linking monomer)

mass/g 500 2.50

reagent

mass/g

AAc (comonomer) potassium persulfate (initiator)

0-0.25 0.25

0.15-0.25

immersed in a water bath, initially at room temperature. The initial quantities of the reactants which were used are given in Table 1. The reaction mixture was stirred for 20 min, with a nitrogen purge to remove the dissolved oxygen. The water bath temperature was raised to 70 °C, and the initiator solution was then added. The polymerization reaction was allowed to continue for 24 h, under an atmosphere of nitrogen. The microgel dispersion was allowed to cool and then exhaustively dialyzed against distilled water to remove any unreacted monomer and other impurities (changing the dialysate twice daily for a week). Characterization of the Microgel Particles. The solids content and hence the yield of the various microgel preparations were determined gravimetrically. When required, the pH was adjusted by addition of small quantities of HCl or NaOH. The diffusion coefficients of the various microgel particles were determined by photon correlation spectroscopy (PCS), using a Brookhaven Instruments Zeta Plus dynamic light scattering apparatus, fitted with a 15 mW laser (λ ) 678 nm) and with the detector set at 90°. The Stokes-Einstein equation was then used to calculate the hydrodynamic diameter of the particles. This required input of the viscosity of the PEO solution used, at the appropriate concentration and temperature. These values were obtained using a standard capillary viscometer. Particle concentrations of 0.1 and 0.5 wt % were used for the 8 and 10 wt % cross-linked particles, and the 6 wt % cross-linked particles, respectively. The size of the microgel particles is expressed here in terms of the swelling ratio, S, defined as S ) (d/do)3, where do is the diameter of the microgel particles in water at 20 °C. PEO Absorption by the Microgel Particles. Equilibrium absorbed amounts of PEO from solution into the microgel particles were determined using a depletion method. Microgel dispersions were added to PEO solutions of known initial concentration (with the same total particle concentration used to determine the hydrodynamic diameter) and left to equilibrate for 7 days. The dispersion was then centrifuged at 5000 rpm, for 20 min, using a Sorvall RC 5b Plus centrifuge. The equilibrium PEO concentration in the supernatant was determined by complexation of the PEO present in the supernatant with phosphomolybdic acid. Details of this technique can be found elsewhere.17 Swelling Kinetics of the Microgel Particles. When microgel particles swell, their light absorbance (abs) decreases, since their refractive index moves closer to that of the continuous phase. This effect may be utilized to follow their swelling (or de-swelling) kinetics, when the local conditions are changed. To this end, stopped-flow kinetic experiments were carried out using a HiTech instrument, with an SF3 support unit and SF30C control unit, as discussed in an earlier paper from this group.18 Swelling or de-swelling was induced by changes in temperature or pH or addition of a given concentration of PEO. In this case, the swelling/ de-swelling is expressed as (abs/abs0), where abs0 is the initial absorbance value (at λ ) 350 nm). The particle concentration used to measure the swelling kinetics was 0.05 wt %.

Results and Dicussion Equilibrium Swelling/De-swelling of the Microgel Particles. A series of microgel particles was prepared, with variable AAc and cross-linker concentrations (wt %). The equilibrium hydrodynamic diameters of these particles are shown in Figures 1 and 2, as a function of pH and temperature, respectively. In Figure 1 it may be seen that, as expected, the microgel particles containing AAc (17) Shar, J. A.; Obey, T. M.; Cosgrove, T. Colloid Surf., A 1998, 136, 21. (18) Loxley, A.; Vincent, B. Colloid Polym. Sci. 1997, 275, 1109.

Uptake of PEO by Copolymer Microgel Particles

Figure 1. Hydrodynamic diameter as a function of pH for 10 wt % cross-linked PNIPAM microgel particles with varying amounts of AAc [(9) 0 and (b) 10 wt %] and for 10 wt % AAc PNIPAM microgels with varying amounts of cross-linker [(2) 6 and (1) 8 wt %].

Figure 2. Hydrodynamic diameter, as a function of temperature, for 10 wt % cross-linked PNIPAM microgel particles with 0 wt % AAc [(9) pH 3 and (0) pH 8] and 10 wt % AAc [(b) pH 3 and (O) pH 8].

swell with increasing pH. At a given pH value, the microgel particles swell more with decreasing cross-linker concentration. Without AAc present, the diameter of the PNIPAM microgel particles is independent of pH (Figure 1), and the de-swelling temperature is centered around 35 °C (Figure 2), and is again independent of pH. For the microgels containing 10 wt % AAc the transition temperature is increased, to being centered around 40 °C at pH 3, while at pH 8 the increase in the de-swelling temperature was beyond the range studied here. These results have to do with the increase in the hydrophilic nature of the microgel particles and are in good agreement with those reported previously by this group19 and by Jones.20 Kratz et al.21 reported that, for PNIPAM/AAc copolymer microgels, at high pH, a second de-swelling process appeared to occur at higher temperature. The effect of cross-linker concentration on the deswelling transition temperature of PNIPAM microgels has also been studied. For example, Woodward et al.22 reported, from turbidity and particle size studies, that the de-swelling transition of PNIPAM occurred over a broader temperature range with increasing cross-linker concentration (up to 30 wt %). However, over the range (19) Snowden, M.; Chowdhry, B.; Vincent, B.; Morris, G. J. Chem. Soc., Faraday Trans. 1996, 92, 5013. (20) Jones, M. S. Eur. Polym. J. 1999, 35, 795. (21) Kratz, K.; Hellweg, T.; Eimer, W. Colloids Surf. 2000, 170, 137. (22) Woodward, N.; Chowdery, B.; Snowden, M.; Leharne, S.; Griffiths, P.; Winnington, A. Langmuir 2003, 19, 3202.

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Figure 3. Relative light adsorption as a function of time for the swelling of PNIPAM microgel particles, on changing from pH 3 to pH 8: (1) 4 wt % AAc, 10 wt % cross-linker; (2) 10 wt % AAc, 6 wt % cross-linker; (b) 10 wt % AAc, 10 wt % crosslinker.

of cross-linker concentrations studied in this work (6-10 wt %), no discernible differences in the de-swelling transition temperature were observed. Kinetics of Swelling and De-swelling of the Microgel Particles. In Figure 3 the changes in the light absorption ratio with time, after changing the pH of the microgels from 3 to 8 (i.e., swelling) in the stopped-flow apparatus, are shown (duplicate runs) for three microgels containing varying amounts of AAc (4 or 10 wt %) and cross-linker (6 or 10 wt %), all at 20 °C. There is not a strong dependence on AAc content, but a marked dependence on cross-linker content: the lower the cross-linker content, the faster the swelling. Relaxation rates are typically about a few seconds. This is comparable with data reported earlier from this group, for the swelling of polyvinylpyridine microgel particles, on changing the pH.17 Interestingly, the de-swelling rates, induced by changing from pH 8 to 3 (data not shown here), were much faster, with relaxation times approximately a few milliseconds (just detectable with the stopped-flow equipment used). These differences in swelling and de-swelling response may well have to do with the relative mobilities of the (faster moving) protons and the (slower moving) Na+ counterions, and the relative distances over which these two species have to diffuse in the two processes. In deswelling, it is the incoming protons which have to move further, than the outgoing Na+ ions, while in swelling it is the reverse process. Hence, de-swelling will be much faster. If this is the correct explanation, then it would seem that water diffusion, in and out of the particles, is not the rate-determining step. This is probably because, at 20 °C, the microgels are already reasonably swollen (see Figure 1), and, moreover, any changes in the bulk solution water concentration are negligible, whereas changes in the proton and Na+ ion concentrations in bulk solution are more significant. Unfortunately, attempts to measure the corresponding response to temperature changes (at fixed pH) were hampered by the fact that temperature equilibration of the sample was now a consideration. Changing the temperature from 20 to 50 °C was estimated to take up to a few tens of seconds for equilibration. De-swelling of all the microgels studied at pH 3 (when the AAc groups, if present, are in the neutral form) was shown to be complete within this time scale. However, Figure 4 shows some temperature-induced (20-50 °C) de-swelling rates for PNIPAM/AAc microgel particles at pH 8, where the AAc groups are now in the ionized form. Now the relaxation rates are approximately a few minutes (i.e. very much

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Figure 4. Relative light absorption, as a function of time, at pH 8, for PNIPAM microgel particles with 10 wt % cross-linker, undergoing de-swelling with a temperature change from 20 to 50 °C: (1) 4 wt % AAc; (4) 6 wt % AAc; (O) 10 wt % AAc.

Figure 5. Swelling ratio as a function of PEO (2000) concentration for 10 wt % cross-linked PNIPAM microgel particles with 0 wt % AAc [(9) pH 3 and (0) pH 8] and 10 wt % AAc [(b) pH 3 and (O) pH 8].

slower than the temperature equilibration process). The absolute changes in light absorbance are small (cf. the open circles in Figure 2), but the slower equilibration rate, at pH 8, compared to pH 3, must have to do with the increase in concentration of the counterions within the microgel particle, associated with the de-swelling process, which reduces the flow rate of water out of the microgel particles. Equilibrium Interaction of PNIPAM-co-AAc Microgel Particles (10 wt % Cross-Linker) with PEO 2000. In Figure 5 the equilibrium swelling ratio is shown, as a function of PEO (2000) concentration, for pure PNIPAM, and for PNIPAM-co-AAc (10 wt %), microgels at pH 3 and pH 8. The cross-linker concentration was the same in both cases (10 wt %), and these data were obtained at 20 °C, when PNIPAM is in its swollen state (see Figure 1). All the plots (with the exception of the pure PNIPAM microgel at pH 8) show an initial increase in swelling, up to some given PEO concentration, followed by a decrease in swelling. The likely explanation is that, at low concentrations, the PEO molecules are entering the microgel particles, causing swelling. For the PNIPAMco-AAc (10 wt %) microgels, the swelling is much greater at pH 3 than at pH 8. At pH 3 the AAc moeities are in their undissociated form, such that the PEO moieties can H-bond to the -COOH groups, whereas at pH 8 this H-bonding is disrupted. However, there is still a very limited amount of swelling, even at pH 8. With the pure NIPAM microgels, there is again some swelling at pH 3. These last two observations suggest that there may be some H-bonding of PEO with the H on the amide groups

Bradley et al.

Figure 6. Absorbed amount of PEO as a function of PEO (2000) concentration at pH 3 for 10 wt % cross-linked PNIPAM microgels with varying amounts of AAc: (9) 0 and (b) 10 wt %.

also. Figure 6 shows equilibrium absorption isotherms for PEO 2000, at pH 3, for the two microgels. It may be seen that, indeed, PEO does absorb into both microgels, although to a greater extent for the AAc-containing microgels. It is interesting that the absorbed amount is still increasing at PEO concentrations beyond where the maximum in swelling occurs (Figure 5). One possible explanation is that the initial PEO molecules which enter the microgel penetrate deeper and have the biggest swelling effect, whereas, at higher PEO concentrations, the later-entering PEO molecules are more associated with the periphery of the microgels where they will cause less or even no effective swelling. The “shape” of the isotherms in Figure 6, lends some credence to this argument. They are initially low affinity, suggesting that at pH 3, for both microgels, the first PEO molecules enter with some difficulty, but once they are in, and have started to swell the microgel particles, then it is easier for more PEO molecules to enter later. The pure PNIPAM microgel particles at pH 8 are much less swollen than the ones containing 10 wt % AAc at pH 8 (see Figure 1), so it is going to be more difficult for PEO chains to enter them anyway, and the driving force for entry is much smaller. So, for PNIPAM, there is a decrease in swelling ratio at all PEO concentrations. This also occurs for the other systems at higher PEO concentrations (see Figure 5). This has been attributed by one of us before15,16 to the increase in free polymer concentration in the bulk solution (i.e. outside the microgels) and, hence, the reduction in the osmotic pressure difference between the inside and the outside of the microgel particles. Figure 7 shows the effect of raising the temperature from 20 to 40 °C, for the PNIPAM microgel particles with 10 wt % AAc, at pH 3 and 8. At both pH’s it was postulated earlier in this paper that H-bonding plays an important role as the driving force for PEO entry into these microgels, although at pH 8, where the swelling is much less (Figure 5), this is more likely to be between the PEO and the H on the amide groups. As may be seen in Figure 7, increasing the temperature leads to significantly less swelling at 40 °C than at 20 °C, at both pH values. Indeed, at pH 8 and 40 °C, it seems that no PEO entry occurs for this microgel. Effect of Cross-Linker Concentration on the Equilibrium Interaction of PNIPAM-co-AAc Microgel Particles (10 wt % AAc) with PEO 2000. Figure 8 shows the effect of varying the microgel cross-linker concentration (from 6 to 10 wt %) on the swelling ratio of the PNIPAM microgel particles with 10 wt % AAc at pH 3 and 20 °C, as a function of PEO concentration. The results

Uptake of PEO by Copolymer Microgel Particles

Figure 7. Effect of temperature on the swelling ratio as a function of PEO (2000) concentration for 10 wt % cross-linked, 10 wt % AAc PNIPAM microgel particles: at 20 °C, (b) pH 3 and (O) pH 8; and at 40 °C (9) pH 3 and (0) pH 8.

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Figure 9. Effect of cross-linker concentration on the absorbed amount of PEO as a function of PEO (2000) concentration, at pH, for PNIPAM-co-AAc (10 wt %) microgels with varying amounts of cross-linker: (2) 6, (1) 8, and (b) 10 wt %.

Figure 8. Effect of cross-linker concentration on the swelling ratio as a function of PEO (2000) concentration, at pH 3, for PNIPAM-co-AAc (10 wt %) microgels with varying amounts of cross-linker: (2) 6, (1) 8, and (b) 10 wt %.

imply that, as expected, the lower the cross-linker concentration the more readily the microgels swell initially on adding PEO 2000. In all cases, at higher PEO concentrations, the particles eventually de-swell, to just less than their original dimensions, as the free PEO concentration in solution builds up (this is true even for the 10 wt % cross-linked microgel, if one looks at Figure 5, where the PEO concentration range is more extended, i.e., to 20 g L-1). The PEO concentration at which this de-swelling becomes dominant moves to much lower PEO concentrations, the lower the cross-linker concentration in the microgel particles. This is a very large effect and again must reflect the relative ease of swelling/de-swelling of the lower cross-linked microgels. Figure 9 shows the corresponding equilibrium absorption isotherms for the systems shown in Figure 8. The most notable differences are that, for the 6 wt % crosslinked microgel particles, the isotherm is high affinity initially but the absorbed amount only reaches a relatively low value at higher PEO concentrations, whereas for the 8 and 10 wt % cross-linked microgel particles the isotherms are of the low affinity type initially, as was discussed earlier, but they attain a much higher absorbed amount than the 6 wt % cross-linked microgel at higher PEO concentrations. The higher, initial affinity of the 6 wt % cross-linked microgel particles comes back to the point that, with the more open pores of this more weakly crosslinked microgel, the PEO molecules can penetrate more easily, but for the “tighter”, higher cross-linked microgels, PEO entry is more difficult initially. However, the higher absorbed amount for the 8 and 10 wt % cross-linked

Figure 10. Effect of PEO MW on the swelling ratio, as a function of PEO concentration, for 10 wt % cross-linked PNIPAM-co-AAc (10 wt % AAc) microgel particles: (b) 2000; (9) 20 000; (2) 100 000; (1) 300 000. (a) pH 3; (b) pH 8.

microgels, at higher PEO concentrations, is consistent with the fact that they are more swollen at these concentrations, and therefore their absorption capacity is higher. Effect of PEO Molecular Weight on the Equilibrium Interaction of PNIPAM-co-AAc Microgel Particles (10 wt % AAc, 10 wt % Cross-Linker) with PEO. In Figure 10 the swelling ratio of PNIPAM-co-AAc (10 wt %) microgel particles is shown as a function of PEO concentration, at 20 °C, for a range of PEO molecular weights (2000-300 000), for two pH values: pH 3 (Figure 10a) and pH 8 (Figure 10b). At pH 3, where there is strong H-bonding between the PEO moeities and the -COOH groups, it would seem that the three lowest MW PEO molecules (2000, 20 000, and 100 000) cause some swelling, with the extent of swelling decreasing with increasing

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Bradley et al.

Figure 11. Effect of PEO MW on the absorbed amount, as a function of PEO concentration, for 10 wt % cross-linked PNIPAM-co-AAc (10 wt % AAc) microgel particles at pH 3: (b) 2000; (9) 20 000; (2) 100 000. Table 2. Estimated Values for the Radius of Gyration (Rg) of PEO in Aqueous Solution, at 20 °C 15 MW

Rg/nm

MW

Rg/nm

2 000 20 000

2.7 8.5

100 000 300 000

19 33

MW, but that 300 000 MW PEO causes no swelling. Also, the higher the MW of the PEO, the stronger is the osmotic de-swelling effect of the free PEO in solution at higher PEO concentrations. Similar trends are observed at pH 8 (Figure 10b), except that the initial swelling, where it occurs, is severely reduced, in line with the results and discussion presented earlier in this paper, concerning limited H-bonding at this pH between the PEO moeities and the H of the amide groups. In Figure 11 we present the PEO absorption isotherms for the three lower MW PEO samples at pH 3 and 20 °C (unfortunately no meaningful data could be obtained with the PEO 300 000 sample as these molecules appeared to centrifuge out to some extent, along with the microgel particles, during the analysis procedure). It is interesting that the final absorbed amount is highest for the 2000 MW sample, and lowest for the 100 000 MW sample. This reflects the ease of penetration of the different MW PEO samples into the microgel particles, with PEO 2000 being the easiest. This is consistent with the observation, in Figure 10, that the osmotic de-swelling is greater, at a given PEO concentration, the higher the MW of the PEO sample used. The fact that, in Figure 11, the 100 000 PEO sample gives an apparently high-affinity isotherm, whereas the lower MW PEO samples give lower affinity isotherms, reflects the fact that the larger PEO 100 000 molecules are probably only penetrating the periphery of the microgel particles. It is of interest, in the context of the penetration of the different MW PEO samples into the microgel particles, to discuss the solution radius of gyration (Rg) values for the four different MW PEO samples studied. The Rg value may be estimated15 using eq 1.

Rg ) 0.06M0.5

(1)

The values are listed in Table 2. Under the conditions of the absorption experiments shown in Figure 11 (20 °C and pH 3) the PNIPAM-co-AAc (10 wt %) microgel particles, in the absence of PEO, have a size of ∼700 nm, compared to ∼200 nm at 60 °C and pH 3 (see Figure 2), so they are quite swollen. The fact that PEO 2000 (Rg ) 2.7 nm) enters more-or-less freely, but that PEO 100 000 (Rg ) 19 nm) barely enters the particles, gives a feel for

Figure 12. Relative light absorption, as a function of time, for microgels at pH 3 upon the addition of 1 g L-1 PEO: for 6 wt % cross-linker, 10 wt % AAc PNIPAM microgel particles [(b) 2000 MW and (9) 20 000 MW] and for 10 wt % cross-linker, 10 wt % AAc PNIPAM microgel particles [(O) 2000 MW and (0) 20 000 MW].

Figure 13. Relative light absorption, as a function of time, for microgels with absorbed PEO (1 g L-1) upon changing from pH 3 to pH 8: for 6 wt % cross-linker, 10 wt % AAc PNIPAM microgel particles [(b) 2000 MW and (9) 20 000 MW] and for 10 wt % cross-linker, 10 wt % AAc PNIPAM microgel particles [(O) 2000 MW and (0) 20 000 MW].

the order of magnitude of the “pore” dimensions in the microgel particles under these conditions, but it must be remembered that these pores will have a wide distribution of sizes, reflecting the fact that the cross-link density most likely varies across the diameter of the microgel particles, as discussed in the Introduction. Kinetics of the Interaction of PNIPAM-co-AAc Microgel Particles (10 wt % AAc) with PEO. Figure 12 shows the kinetics of absorption of PEO of MWs 2000 and 20 000 (at a concentration of 1 g L-1 in both cases) into PNIPAM-co-AAc microgel particles (10 wt % AAc), at 20 °C and pH 3, for two cross-linker concentrations (6 and 10 wt %). These results were obtained using the stoppedflow apparatus. It is apparent that the absorption is faster the lower the PEO MW and the lower the cross-link density. Both trends are consistent with the discussion presented earlier concerning the ease of penetration of PEO molecules into the microgel particles. Figure 13 shows equivalent PEO “desorption” data for the same two systems as in Figure 12. These results were obtained by switching from pH 3 to pH 8 in the stoppedflow apparatus. Again, the PEO molecules appear to desorb faster, the lower the cross-linker concentration in the microgel particles. However, the 20 000 MW PEO molecules seem to desorb slightly faster. This may be because they have not penetrated as far into the microgel particles as the 2000 MW PEO molecules.

Uptake of PEO by Copolymer Microgel Particles

The data shown in Figures 12 and 13 were collected up to 1200 min, i.e., ∼1 day. The light absorption values still appear to be changing even after this time, suggesting that diffusion of PEO molecules in to and out of the interior of microgel particles is a very slow process indeed, compared to the diffusion of polymer molecules to solid surfaces in standard adsorption behavior23 of polymers at impenetrable surfaces. In the (assumed) “equilibrium” absorption experiments, reported earlier in the paper, samples were left to stand typically for 1 week, before any analysis was carried out. After that period effectively no further changes were observed. Conclusions It has been shown that the equilibrium swelling and de-swelling of PNIPAM-co-AAC microgel particles, as a function of pH and temperature, depend strongly on the cross-linker concentration and the concentration of AAc, in line with previous work. However, a new result is that the rate of de-swelling of the microgel particles, containing AAc, is much faster than the rate of swelling, on changing the pH appropriately. This has been explained in terms of the relative mobilities of the H+ and Na+ ions, in and out of the particles. The rate of response to temperature change proved to be more difficult to follow on the (23) Fleer, G. J.; Cohen, M. S.; Scheutjens, J.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993.

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millisecond time scale, but it was observed that the microgels containing AAc, at pH 8, de-swelled relatively slowly on heating to 50 °C from 20 °C. This was attributed to the resistance to collapse associated with the large increase in counterion concentration inside the microgel particles. The interaction of PEO with PNIPAM-co-AAC microgel particles has been studied in terms of the (equilibrium) swelling behavior and the amount of PEO absorption. The results, as a function of AAc content, cross-linker concentration, PEO MW, pH, and temperature, have been rationalized in terms of the ease and depth of penetration of the PEO chains into the various microgel particles and also the H-bonding associations between PEO and either the -COOH of the AAc moeities and/or the H of the amide groups (much weaker). Finally, the rate of adsorption and desorption of the PEO molecules in to and out of the microgel particles has been shown to be extremely slow compared to normal diffusion time scales. Acknowledgment. B.V. and M.B. acknowledge financial support from the EPSRC through the IMPACT Faraday Partnership (Grant GR/R90086/01). J.R. expresses his gratitude to Eusko Jaurlaritza, Etortek 20022204, Project MAAB, for support to enable him to visit Bristol. LA047966Z