Novel Gelling Behavior of Poly(N-isopropylacrylamide-co-vinyl laurate

L. S. Benee, M. J. Snowden*, and B. Z. Chowdhry. School of Chemical & Life Sciences/Medway Sciences, University of Greenwich (Medway Campus), Anson ...
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Langmuir 2002, 18, 6025-6030

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Novel Gelling Behavior of Poly(N-isopropylacrylamide-co-vinyl laurate) Microgel Dispersions L. S. Benee,† M. J. Snowden,* and B. Z. Chowdhry School of Chemical & Life Sciences/Medway Sciences, University of Greenwich (Medway Campus), Anson Building, Chatham Maritime, Medway, Kent ME4 4TB, U.K. Received February 25, 2002. In Final Form: May 13, 2002 Aqueous colloidal microgel particles prepared from N-isopropylacrylamide (NIPAM) co-polymerized with different ratios of the comonomer vinyl laurate have been investigated with respect to their physicochemical properties and colloid stability. The hydrodynamic diameters of microgel particles synthesized using 10% and 50% w/v vinyl laurate have been examined. The poly(NIPAM)-co-vinyl laurate microgel particles, suspended in water, show similar conformational behavior to poly(NIPAM) microgels in that they reversibly shrink and swell (undergo a reversible volume phase transition) in response to heating and cooling. Turbidity measurements have been used to study the colloid stability of poly(NIPAM) and poly(NIPAM)-co-vinyl laurate microgel particles as a function of electrolyte concentration. When heated to 40 °C in a NaCl electrolyte solution, the poly(NIPAM)-co-vinyl laurate microgels form irreversible flocs and macroscopic gels above a critical electrolyte concentration. This novel flocculation behavior is in contrast to poly(NIPAM) microgels that reversibly flocculate when heated/cooled in an electrolyte solution (above a critical electrolyte concentration). Photon correlation spectroscopy (PCS) and turbidity measurements have been used to study the swelling (volume phase transition) behavior of the poly(NIPAM)-covinyl laurate microgels and good agreement was observed between the data obtained from both these techniques.

Introduction Interest in microgels has grown rapidly over the last 20 years because of the simplicity of their preparation and potential uses in many industrial applications.1 Microgels are currently being investigated for their use as uptake and release devices2,3 (drug delivery vehicles), in improved oil recovery,4 chromatographic or separation technology, potentially as molecular binding sites,5 and as catalytic media.6 They have also been investigated for their ability to improve existing formulations such as specialist paints.7 Microgels are discrete polymeric particles that are within the size range 1 nm to 1 µm. Microgels are of both academic and industrial interest because of their ability to shrink and swell in solution by simply changing solvency conditions, such as a change in temperature, pH, electrolyte concentration, or as a result of an applied electric field.8-10 Poly(N-isopropylacrylamide) or poly(NIPAM) microgels (and comonomers thereof) are by far the most widely investigated systems. The linear polymer (poly* To whom all correspondence should be addressed: E-mail: [email protected]. Phone: + 44 (0) 208 331 9981. Fax: + 44 (0) 208 331 8305. † Current address: Research Fellow, Drug Delivery, Pfizer Global R & D, Sandwich, Kent CT13 0NJ UK. (1) Benee, L.; Snowden, M. J.; Chowdhry, B. Z. Encyclopedia of Advanced Materials; John Wiley & Sons Ltd: New York, 2002. (2) Hoffman, A. J. Controlled Release 1987, 6, 297. (3) Ramkisson-Ganorkar, C.; Liu, F.; Baudys, M.; Kim, S. W. J. Controlled Release 1999, 59 287. (4) Snowden, M. J.; Vincent, B.; Morgan, J. C. U.K. Patent GB 226 2117A, 1993. (5) Fern, G.; Silver, J.; Snowden, M. J.; Withnall, R. Unpublished data, patent pending. (6) Hampton, K. W.; Ford, W. T. Abs. Pap.-Am. Chem. Soc. 1999, 218, 32. (7) Nakayama, Y. Prog. Org. Coat. 1998, 33 108. (8) Filipcsei, G.; Feher, J.; Ford, M. J. Mol. Struct. 2000, 55, 109. (9) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1. (10) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1.

(NIPAM)) is a nonionic water soluble polymer with a lower critical solution temperature (LCST) of ∼31 °C in water.11 Poly(NIPAM) microgels are thermosensitive “spongelike” particles that shrink and swell in response to a change in temperature. The thermoreversible “conformational” or “volume phase” transition of poly(NIPAM) microgel particles occurs at 34 °C in water.12 When dispersed in water at room temperature, the microgel particles have a swollen conformation with water occupying the interstitial spaces within the porous microgel structure. Microgel particles are swollen due to the interaction of the microgel with water molecules through hydrogen bonding. However, when heated the particles shrink as polymer-solvent interactions are dramatically reduced (as the hydrogen bonds are broken), and the microgels form a hard sphere type structure as the polymer chains prefer to interact with each other. The shrinking and swelling of microgels give rise to its volume phase transition, which has been measured by techniques such as turbidity13 (since the sample becomes more opaque as the temperature passes through the LCST) and photon correlation spectroscopy (PCS).9,14 As an example, the mean hydrodynamic particle diameter for a typical poly(NIPAM) microgel undergoes a 4-fold decrease from ∼800 nm at 25 °C to ∼200 nm at 50 °C. This corresponds to a 60-fold volume decrease,13 or approximately 90% of the water is driven out of the microgel.15 The swellability of a microgel is dependent on the type of microgel, e.g., its affinity for the solvent, which in turn is dependent on monomer (and/or comonomer) composition/concentration, as well as the degree of cross linking.14 (11) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247. (12) Snowden, M. J.; Chowdhry, B. Z. Chem. Br. 1995, 31 (12), 943. (13) Murray, M.; Rana, F.; Haq, I.; Cook, J.; Chowdhry, B. Z.; Snowden, M. J. J. Chem. Soc., Chem. Commun. 1994, 1803. (14) Murray, M. J.; Snowden, M. J. Adv. Colloid Interface Sci. 1995, 54, 73-91. (15) Wu, C.; Zhou, S. J. Macromol. Sci., Phys. 1997, B36, (3), 345.

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Another physicochemical property of poly(NIPAM) microgel particles is their ability to reversibly flocculate when heated above their LCST in an electrolyte solution. Snowden and Vincent16 have shown that poly(NIPAM) microgel particles at 40 °C flocculate at a sodium chloride concentration above 0.08 M. These particles were found to redisperse when cooled back to 25 °C. The Hamaker constant (and hence, VA, the van der Waals attractive force) of the microgel particles at 25 °C is low, as a result of the particles being swollen with solvent molecules. At 40 °C, the microgel has collapsed and the solvent molecules from the interstitial spaces within the microgel are forced out of the microgel and VA subsequently increases. At the same time, the electrostatic repulsion between particles increases upon heating as the charge density increases when the particles shrink, but the presence of electrolyte reduces VR (the electrostatic repulsive force), and overall the microgel particles aggregate as van der Waals forces now dominate. The transition temperature of the microgels was also found to decrease with increasing electrolyte concentration, since the hydrogen bonding between the microgel and water molecules is further disrupted by the addition of electrolyte. Microgel particles have also been shown to undergo heteroflocculation when cationic poly(NIPAM) microgel particles are mixed with anionic polystyrene latex particles at 40 °C. Microgel particles can acquire a positive or negative charge depending on the choice of ionic initiator used during synthesis. Islam et al.17 have studied this flocculation behavior and found that at 25 °C the dispersion of latex and microgel particles was stable because of the low Hamaker constant (closely matching the solvent; low VA) of the microgel particles at this temperature, resulting in weak electrostatic forces between the polystyrene latex particles and the microgel particles. On heating, however, to 40 °C the mixed charge dispersion flocculates. When heated, the microgel particles have a much higher Hamaker constant (now much less solvent matched; higher VA) and the charge density of the microgel also increases. The increase in charge density promotes greater electrostatic attraction between the latex and microgel particles, which results in heteroflocculation. The nature of the monomer used in the preparation of the microgel will usually dictate the overall properties of the final microgel produced. The addition of comonomers with different functionalities to the microgel can create particles with a range of different physicochemical properties. For example, a number of poly(NIPAM) microgels have been prepared with comonomers such as acrylic acid (AA), methacrylic acid (MAA) and 2-methyl-2-acrylamido propanesulfonic acid (AMPS).18-19 All of these microgels are sensitive to changes in pH. The work reported in this paper involves the first example of the preparation and physicochemical characterization of a novel colloidal microgel system based on NIPAM co-polymerized with the hydrophobic comonomer vinyl laurate, [CH3(CH2)10COOCHdCH2], prepared with an anionic or cationic initiator. Normally, the addition of a hydrophobic comonomer to the synthesis of microgel particles results in colloidal particles with a diameter of around 150 nm,9 this is a result of steric hindrance of the hydrophobic moiety restricting access of the free monomers (16) Snowden, M. J.; Vincent, B. J. Chem. Soc., Chem. Commun. 1992, 1103. (17) Islam, A. M.; Chowdhry, B. Z.; Snowden, M. J. J. Phys. Chem. 1995, 99 [39] 14205. (18) Huglin, M. B.; Lui, Y.; Velada, J. L. Polymer 1997, 38 (23), 5785. (19) Snowden, M. J.; Chowdhry, B. Z.; Vincent, B.; Morris, G. E. J. Chem. Soc., Faraday Trans. 1996, 92 [24] 5013.

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to the growing polymer chains thus limiting particle growth. The structure of the vinyl laurate means that it can partake in microgel synthesis and can react with other co/monomer groups and can therefore be incorporated into the gel matrix. Moreover vinyl laurate is a molecule with both a low HLB and low cloud point. Both of these physicochemical properties are important in relation to the partitioning of the vinyl laurate comonomer into the growing microgel particles, especially in terms of the synthesis temperature of the colloidal suspension. Experimental Section Materials. 0.5% w/w microgels containing 10% and 50% copolymer were prepared using the following reagents. 10% Copolymerized Microgels. A 4.5 g sample of monomer [N-isopropylacrylamide (Aldrich)], 0.5 g of copolymer [vinyl laurate (Fluka)], 0.5 g initiator (see below), and 0.5 g of cross linker (N,N-methylene-bisacrylamide purchased from Aldrich) were used. 50% Copolymerized Microgels. A 2.5 g sample of monomer [N-isopropylacrylamide], 2.5 g of copolymer [vinyl laurate], 0.5 g of initiator (see below), and 0.5 g of cross linker (N, N′-methylenebisacrylamide) were used as received. Initiators. Potassium persulfate (AnalR) was used to prepare the anionic microgel particles, and V50 or [2-2-azobis-(2amidinopropane) dihydrochloride] (Wako Chemicals Ltd.) was used to prepare the cationic microgel particles. Stability tests were carried out using sodium chloride (Fisons), and the microgels were heated in 3.6% w/v synthetic seawater (SSW; a mixed electrolyte solution) made from a BDH test corrosion kit. Microgel Preparation. Many methods14,20-23 have been used to synthesize microgel particles, but emulsion polymerization is by far the most popular method used. The microgel particles investigated here were prepared by surfactant free emulsion polymerization according to the method of Pelton and Chibante.11 The initiator was weighed and then placed into a five-necked round-bottom flask followed by 800 mL deionized water. [Note: a silanated flask was used to prepare the cationic microgel particles.] The flask was then immersed in a water bath that was preheated to 70 °C. A water-cooled condenser was connected to the flask and a paddle stirrer inserted through the central hole in the flask. The sample was stirred continuously at approximately 100 rpm throughout the reaction. Nitrogen was supplied through a third outlet of the flask, and flowed out through the top of a condenser to an oil reservoir. Bubbles generated in the oil reservoir were used to monitor the flow of nitrogen through the flask. The system was purged with nitrogen, while the sample reached 70 °C. N-Isopropylacrylamide, vinyl laurate, and methylene bisacrylamide were weighed, transferred to a beaker, and then 200 mL deionized water was added. The mixture was stirred gently on a magnetic stirrer. Once dissolved, the reagents were transferred to the reaction flask containing the initiator. The reaction was carried out under an inert atmosphere of nitrogen with continuous stirring (∼100 rpm) for 6 h. When the reaction was complete the microgel was filtered through glass wool and transferred to well-boiled visking dialysis tubing. The dialysate was changed at least twice daily until the conductivity of the dialysate was consistently below 1 µS cm-1. Physical Measurements. Microgel particle hydrodynamic diameter and electrophoretic mobility were measured on a Malvern Zetasizer 3000, equipped with a helium-neon laser (λ ) 632 nm) with a detector placed at 90° to the sample. Size measurements were carried out in deionized water, while samples were diluted with 1 × 10-4M NaCl for electrophoretic mobility measurements. Turbidity (n Values). The aggregation behavior of the microgel particles was determined by measuring their n value, which is (20) Frank, M.; Burchard, W. Makromol. Chem. Rapid Commun. 1991, 12, 645. (21) Antionetti, M.; Pakula, T.; Bremser, W. Macromolecules 1995, 28 4227# (22) Neyret, S.; Vincent, B. Polymer 1997, 38, 6129. (23) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20 247.

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Figure 1. Hydrodynamic diameter measurements for 50% anionic poly(NIPAM)-co-vinyl laurate microgel samples as a function of temperature, measured using photon correlation spectroscopy. Heating run (9) and cooling run (0). Table 1. Hydrodynamic Diameter and Electrophoretic Mobility Parameters for Each Microgel

% vinyl laurate

hydrodynamic diameter (nm) (swollen), 20 °C

hydrodynamic diameter (nm) (collapsed), 50 °C

electrophoretic mobility (×10-8 m2/Vs), at 25 °C

0 10 (anionic) 10 (cationic) 50 (anionic) 50 (cationic)

800 ( 10 693 ( 5 713 ( 6 506 ( 5 736 ( 7

200 ( 3 282 ( 3 274 ( 2 288 ( 3 295 ( 3

-0.5 ( 0.05 -0.86 ( 0.03 +0.42 ( 0.02 -1.10 ( 0.05 +0.70 ( 0.03

the wavelength dependence of the turbidity of the dispersion where24 n ) -[d log turbidity/d log wavelength]. Absorbance measurements were conducted using a Perkin-Elmer Lambda 2 UV spectrophotometer. n values range from -4 to 0, and the nearer the n value is to 0 the more flocculated are the particles within a system. A significant discontinuity in the data is indicative of a critical flocculation concentration (CFC). Turbidity (at 547 nm). The variation in turbidity of a sample was measured (in terms of a change in the percentage transmittance) as a function of temperature at a wavelength of 547 nm. Measurements in water were made on a Perkin-Elmer Lambda 2 UV spectrophotometer, and measurements in electrolyte were made by placing a turbidity probe (λ ) 555 nm) directly into the sample heated by a hotplate. Samples were diluted (1:5, microgel: deionized water) for turbidity measurements.

Results and Discussion A. Microgel Size and Electrophoretic Mobility. The average particle size and electrophoretic mobility for each microgel is summarized in Table 1. The hydrodynamic data shows that the poly(NIPAM)co-vinyl laurate microgel particles are typically of the size of poly(NIPAM) microgels (without vinyl laurate) showing that the nonpolar comonomer does not interfere (sterically hinder) with the reaction mechanism i.e., it does not prevent particle growth. If the molecules of vinyl laurate were to self-associate in the vicinity of the growing oligomer chain, then it would be anticipated that the polymerization reaction would be sterically hindered. Figure 1 shows a graph of hydrodynamic diameter measurements for the 50% anionic poly(NIPAM)-co-vinyl laurate microgel particles as a function of temperature. This transition is less sharp than for the homopolymer poly(NIPAM)16 which may be attributed to the presence of hydrophobic comonomers at the core of the particle making conformational rearrangements, during the heating cycle, occur over a wider temperature range. However, (24) Long, J. A.; Osmond, D. W. J.; Vincent, B. J. Colloid Interface Sci. 1973, 42 545.

Figure 2. n values for a anionic poly(NIPAM) microgel as a function of sodium chloride concentration at 25 °C (×), 40 °C (0), then cooled back to 25 °C (4).

the comonomer microgel is still able to undergo shrinking and swelling despite being co-polymerized with vinyl laurate. All of the poly(NIPAM)-co-vinyl laurate microgels exhibit similar behavior and all samples remained dispersed over the temperature range (25-50 °C) studied. B. Turbidity (n Values). The aggregation behavior of the poly(NIPAM)-co-vinyl laurate microgel particles has been measured by turbidity as a function of sodium chloride concentration, at 25 °C, 40 °C, and then when cooled back to 25 °C (to determine whether the microgel redisperse on cooling). As a comparison, the stability of an anionic poly(NIPAM) microgel, i.e., not containing any vinyl laurate was also investigated. Figure 2 shows the n values obtained for the anionic poly(NIPAM) microgel as a function of electrolyte concentration and temperature. The n values show that at 25 °C the microgel particles are stable at all sodium chloride concentrations studied. This is because at this temperature the microgel particles are swollen with solvent molecules occupying the interstitial spaces within the microgel, and hence the particles have very low attractive van der Waals forces, i.e., the Hamaker constant for the microgel is closely matched to the solvent. With increasing electrolyte concentration, the repulsive forces originating from the initiator are reduced, since they are screened by electrolyte ions. The microgel particles do not aggregate because the contribution to the overall driving force for coagulation of the microgel particles due too van der Waals forces is too small. Heating the microgel to 40 °C results in an aggregation of the microgel particles above a certain critical electrolyte concentration. The critical electrolyte concentration or critical flocculation concentration (CFC) for the anionic poly(NIPAM) microgel particles is 0.1 M NaCl. This value is in agreement with data reported in the scientific literature.17,25 When cooled back to 25 °C, the n values show that the microgel particles fully redisperse since they have returned to their original value of -2.3. This behavior is typical for poly(NIPAM) microgel particles and has been reported elsewhere.17 Figures3 and 4 show the n values for the 10% anionic and cationic poly(NIPAM)-co-vinyl laurate microgel particles as a function of electrolyte concentration at 25 °C, 40 °C, and then cooled back to 25 °C. Similar trends are observed in both sets of data at all temperatures studied. At 25 °C and at low electrolyte concentrations (between 0.001 and 0.01 M NaCl) the n values are at approximately -2.3 indicating stability (dispersed particles) at these electrolyte concentrations. As the concentration of sodium chloride increases there is a gradual decrease in the value (25) Daly, E.; Saunders, B. R. Langmuir 2000, 16, 5546.

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Figure 5. 10% poly(NIPAM)-co-vinyl laurate microgel particles in water. A: cationic microgel particles. B: anionic microgel particles. C: a mixed charge dispersion. A, B, and C are at 25 °C. D: cationic microgel particles. E: anionic microgel particles. F: a mixed charge dispersion. D, E, and F are at 60 °C. Figure 3. n values for a 10% anionic poly(NIPAM)-co-vinyl laurate microgel as a function of sodium chloride concentration at 25 °C (×), 40 °C (0), and then cooled back to 25 °C (4).

Figure 6. 10% poly(NIPAM)-co-vinyl laurate microgel particles in SSW. G: cationic microgel particles. H: anionic microgel particles. I: mixed charge dispersion. G, H, and I are at 25 °C. J: cationic microgel particles. K: anionic microgel particles. L: a mixed charge dispersion. J, K, and L are at 60 °C. Table 2. CFCs for the Various Microgel Particles at 40 °C % vinyl laurate in microgel

Figure 4. n values for a 10% cationic poly(NIPAM)-co-vinyl laurate microgel as a function of sodium chloride concentration at 25 °C (×), 40 °C (0), and then cooled back to 25 °C (4).

of n to around -1.8, which suggests possible signs of the onset of instability. Visually, all of the samples looked dispersed. Turbidimetric analysis of the microgel particles at 40 °C showed that the 10% anionic and cationic poly(NIPAM)co-vinyl laurate microgel particles also exhibit a CFC. At 40 °C, the 10% anionic poly(NIPAM)-co-vinyl laurate microgel particles are stable up to an electrolyte concentration of 0.04 M NaCl, while the 10% cationic poly(NIPAM)-co-vinyl laurate microgel particles are stable up to an electrolyte concentration of 0.01 M NaCl. Above the CFC, the microgel particles are aggregated. On cooling back to 25 °C the particles that aggregated remained aggregated and do not redisperse, i.e., the microgel particles undergo irreversible flocculation upon heating. This is in contrast to the anionic poly(NIPAM) microgel particles (without vinyl laurate), showing that the vinyl laurate must have an effect on the stability of the microgel particles. On heating, there is an increase in the Hamaker constant (and hence an increase in van der Waals attractive forces) and electrostatic repulsion is small due to the presence of electrolyte, so the microgel particles flocculate. Therefore, the reason for the irreversible flocculation in the poly(NIPAM)-co-vinyl laurate microgels must arise from the presence of vinyl laurate since this aggregation behavior does not occur with poly(NIPAM) microgel particles. The vinyl laurate groups are, of course, hydrophobic and when the microgels flocculate, the vinyl laurate chains are able to come into close enough contact to interact (by hydrophobic association) with other polymer chains on nearby particles (either NIPAM or vinyl laurate chains). This aggregation behavior only occurs above a critical electrolyte concentration, i.e., when the electro-

0% (anionic) 10% (anionic) 10% (cationic) 50% (anionic) 50% (cationic)

CFC in NaCl (M) 0.1 0.04 0.01 0.04 0.01

static repulsion is sufficiently small. On cooling back to 25 °C, the microgels re-swell slightly (as seen by the increase in n value from -0.5 to -1.3) but they do not return to their original (preheated) swollen conformation n value of -2.3. This suggests that the hydrophobic association of vinyl laurate chains with other polymer chains/microgel particles is too strong to be overcome by simply cooling the microgel (as is the case with conventional poly(NIAPM) microgels). Energetically it is more favorable for the vinyl laurate chains to interact with each other and/or the polymer chains than to interact with solvent molecules, which may not be surprising considering the hydrophobic character of vinyl laurate. The increase in n value at 25 °C (cooled) appears to be as a result of re-swelling of the poly(NIPAM) portion of the microgel when the temperature is decreased. Identical behavior (including irreversible aggregation) was also observed for the 50% anionic and cationic poly(NIPAM)-co-vinyl laurate microgel particles with increasing electrolyte concentrations at 25 °C, when heated to 40 °C, and when cooled back to 25 °C. These microgel particles also exhibit the small increase in n value (microgel reswelling) when the aggregated particles are cooled back to 25 °C. The aggregated particles do not redisperse since the n value does not return to its original (preheated) value. Table 2 shows the CFC’s of all of the microgel samples examined. Figures 5-7 can help explain the unusual n values obtained for the cooled aggregated microgel particles. Figure 5 shows 10% poly(NIPAM)-co-vinyl laurate microgel particles in water. A is anionic, B is cationic, and

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Figure 7. Cooled microgel “gel” particles in SSW (J, K, and L are at 25 °C).

C is a mixed charge dispersion; all 3 samples are shown at 25 °C. D is anionic, E is cationic, and F is a mixed charge microgel dispersion; all three samples are shown at 60 °C. At 25 °C both the anionic and cationic microgels are dispersed but the mixed charge sample has weakly flocculated as a result of electrostatic attraction between the negatively and positively charged particles. D, E, and F have been heated in an oven at 60 °C for about 20 min, and there is an increase in turbidity of the anionic and cationic samples (due to the microgel shrinking). Both samples D and E are dispersed but the mixed charge sample (F) has aggregated and formed a macroscopic “gel” that has the appearance of a small tablet. This is unusual behavior for microgel particles in water. Figure 6 shows 10% poly(NIPAM)-co-vinyl laurate microgel particles in SSW. G is anionic, H is cationic, and I is a mixed charge dispersion; all three samples are at 25 °C. J is anionic, K is cationic, and L is a mixed charge dispersion; all three samples are at 60 °C. G, H, and I are all dispersed including the mixed charge microgel particles that were flocculated in deionized water at 25 °C. The mixed charge microgel particles are stable (dispersed) because the electrolyte ions screen the particle charge (and hence the electrostatic repulsion), which prevents flocculation from taking place. Attractive forces at this temperature will also be small since the microgels are swollen with solvent molecules. At 60 °C all microgel particles form small collapsed “gels”. Again, this gelation on heating is unusual for microgel particles. The heated microgels shown in Figure 6 were then cooled back to 25 °C. Figure 7 shows the cooled 10% poly(NIPAM)-co-vinyl laurate microgel particles in SSW at 25 °C. The microgel particles maintain their “gel” structure but are swollen in solution when compared to the same sample in Figure 6. This explains why the microgels had n values of around -1 in Figures 2 and 3. The microgel particles are aggregated but are also swollen, and thus have an n value between the two extremes. The particles reswell when cooled to 25 °C as a result of the increased polymer-solvent interactions of the poly(NIPAM) portions of the microgel but the particles stay as a single large aggregate (or maintain their shape) due to the hydrophobic association of vinyl laurate chains with other polymer molecules. Under these conditions it is not possible to obtain an n value for a single, albeit large, aggregate. Other polymers such as methylcellulose also exhibit this type of gelation and are known to reversibly gel in water with an increase in temperature.26 The gelling of methylcellulose on heating is a result of dehydration of the polymer chains followed by hydrophobic interaction. (26) Henderson, A. Cellulose Ethers-The Role of Thermal Gelation. In Gums and Stabilisers for the Food Industry 4; Philips, G. O., Wedlock, D. J., Williams, P. A., Eds.; IRL Press Ltd.: 1988; p 265.

Figure 8. Turbidity data as a function of temperature for the 10% anionic poly(NIPAM)-co-vinyl laurate microgel in water. Heating run (9) and cooling run (0).

It has also been reported27 that when a solution of surfactant and a polymer that clouds at its LCST (i.e., become less polar with increasing temperature) are heated, thermo-reversible gels may be formed. The microgel particles only form a gel in water when a mixed charge dispersion is used, but gels are formed by all microgel particles in electrolyte (when the samples are heated). This suggests that the irreversible gelling of the microgel particles only occur as a result of hydrophobic interactions of the vinyl laurate chains with other polymer molecules. The hydrophobic interactions only take place, however, once the particles have come into close enough contact (i.e., when they have flocculated). Once the hydrophobic interaction of the vinyl laurate chains has taken place, it cannot be reversed. Energetically there is little reason for the vinyl laurate chains to interact with water molecules, and they prefer to remain in contact with other polymer molecules. The gelation and irreversible flocculation of the poly(NIPAM)-co-vinyl laurate microgels is a direct result of the co-polymerization of vinyl laurate, since poly(NIPAM) microgel particles do not exhibit this behavior. Re-swelling of the gel is observed on cooling as the poly(NIPAM) portions of the microgel hydrogen bond with water molecules, therefore the overall gel structure swells but does not redisperse. C. Turbidity (at 547 nm). Turbidity has been used by Murray et al.13 to measure the change in the light scattering intensity of the dispersion with a change in the hydrodynamic diameter of the poly(NIPAM) microgel particles as a function of temperature. The turbidity of the dispersion is measured against deionized water, between the temperature range 20-55 °C at a rate of approximately 1 °C/min. Turbidity heating and cooling curves for the 10% anionic poly(NIPAM)-co-vinyl laurate microgel particles in water are shown in Figure 8. The shrinking and swelling of the microgel particles, in water, is a result of an increase in the Flory parameter (χ), with an increase in temperature. As the temperature increases there are more polymer-polymer interactions, and the spherical spongelike microgels adopt a hard sphere type structure. The decrease in particle size with an increase in temperature also gives rise to the change in turbidity of the sample since there is an increase in the refractive index of the sample relative to the refractive index of the solvent. This enables turbidity to be used as a technique to measure the change in particle size with a change in temperature. The shrinking and swelling of poly(NIPAM) microgel particles is a reversible process so that on cooling, (27) Jonsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; Wiley & Sons: New York, 1988.

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Figure 9. First derivative plot of turbidity data (from Figure 8) for the 10% anionic poly(NIPAM)-co-vinyl laurate microgel in water. Heating run (9) and cooling run (0). Table 3. LCST of Various Microgel Dispersions in Water and in SSW % vinyl laurate

LCST in water (°C)

LCST in SSW (°C)

0 10 (anionic) 10 (cationic) 50 (anionic) 50 (cationic)

34 ( 0.1 34.2 ( 0.2 35.4 ( 0.2 34.8 ( 0.2 37.4 ( 0.2

25 ( 0.2 25.2 ( 0.2 25.6 ( 0.2 30.2 ( 0.2 28.0 ( 0.2

the microgel particles reswell as polymer-solvent interactions begin to take place as χ decreases with a decrease in temperature. Thus, the data in Figure 8 shows a reversible volume phase transition of the poly(NIPAM)co-vinyl laurate microgels in water with the heating and cooling curves being almost superimposable. The minimum of a first derivative plot of the heating curves gives the value of the LCST of the microgel. Figure 9 shows the first derivative plot for the 10% anionic poly(NIPAM)co-vinyl laurate microgel particles. Table 3 shows the LCST of each microgel in water and in SSW. The LCST of the poly(NIPAM)-co-vinyl laurate microgels in water are in good agreement with the LCST of poly(NIPAM) microgel particles14 of 34 °C, with the exception of the 50% cationic poly(NIPAM)-co-vinyl laurate microgel which has an LCST of 37.4 °C. This suggests that the incorporation of vinyl laurate has little effect on the volume phase transition temperature of the microgel particles in water. Vinyl laurate is a very hydrophobic monomer having poor/sparing solubility in water. It is likely therefore that the vinyl laurate monomer will be included in the polymerization reaction ahead of the more soluble NIPAM. This could result in a core-shell type structure where the core is largely composed of the hydrophobic vinyl laurate while the shell is the more hydrophilic NIPAM. Block copolymers of NIPAM are known to keep the LCST of poly(NIPAM), whereas random copolymers have a shifted LCST depending on the nature of the comonomer.10 In the case of vinyl laurate, the hydrophobic nature of the monomer would be expected to dramatically lower the LSCST. As the 50% vinyl laurate

copolymer has an LCST similar to the 10% and homopolymer poly(NIPAM) microgel, it would appear that the data supports a core- shell type structure analogous to a block copolymer for these microgels. The addition of electrolyte can have a dramatic effect on the dispersibility of the poly(NIPAM)-co-vinyl laurate microgel particles (as shown by the n values above) and it also results in a decrease in the microgel LCST. The addition of electrolyte (NaCl) can lower a polymer LCST since it increases χ (for water-soluble polymers) or in other terms, an electrolyte solution is a poor solvent compared to water. The electrolyte dehydrates the microgel so that the LCST occurs at a lower temperature. Daly and Saunders25 have investigated the stability of poly(NIPAM) microgels in a range of electrolytes. They have shown that the LCST of poly(NIPAM) microgel particles decreases with an increase in electrolyte (NaCl) concentration. This agrees with the findings above where there is a substantial decrease between the LCST of the microgels in water and the corresponding LCST in electrolyte. Conclusions The work presented herein provides the first reported example of the synthesis and physicochemical characterization of (NIPAM) copolymerized with, the nonpolar comonomer, vinyl laurate. The corresponding microgel particles have been shown to exhibit typical poly(NIPAM) behavior, such as a reversible conformational change (shrinking and swelling) as a function of temperature in water. The temperature at which the conformational transition occurs is in line with values of poly(NIPAM) microgels, and it has been shown that this temperature can be dramatically reduced by the addition of electrolyte. However, poly(NIPAM)-co-vinyl laurate microgel particles also exhibit some very unusual behavior as they form irreversible aggregates or “gels” when heated in an electrolyte solution above a critical electrolyte concentration. Both the 10% and 50% anionic poly(NIPAM)-co-vinyl laurate microgel particles form irreversible aggregates above 0.04 M NaCl (at 40 °C), while the 10% and 50% cationic poly(NIPAM)-co-vinyl laurate microgel particles form irreversible aggregates above 0.01 M NaCl (at 40 °C). These values are an order of magnitude lower than the values obtained for poly(NIPAM) microgel particles, showing that the incorporation of vinyl laurate into poly(NIPAM) microgels has a dramatic effect on the colloid stability of these particles. This work has also shown that these microgels form macroscopic gels (when heated in an electrolyte solution), which reswell on cooling but do not redisperse, which is also atypical poly(NIPAM) microgel behavior. Further work is required to fully understand and characterize the novel behavior of these copolymerized microgel particles. Acknowledgment. The authors wish to thank the EPSRC (Case Award) and BP Exploration for funding this work. LA025660R