Dependence of Shell Thickness on Core Compression in Acrylic Acid

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Dependence of Shell Thickness on Core Compression in Acrylic Acid Modified Poly(N-isopropylacrylamide) Core/ Shell Microgels Clinton D. Jones and L. Andrew Lyon* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 Received March 5, 2003. In Final Form: April 2, 2003 The swelling properties of poly(N-isopropylacrylamide) (pNIPAm)-based core/shell microgels are investigated as a function of shell thickness. Core particles composed of cross-linked pNIPAm-co-acrylic acid serve as nuclei for subsequent polymerization of a cross-linked pNIPAm shell. The thickness of the shell component is varied by changing the amount of monomer present during the shell addition polymerization. Photon correlation spectroscopy results indicate that the thickness of the shell greatly impacts the swelling properties of the core as a function of polymer network density, solution pH, and temperature. Previously reported results on this core/shell polymer combination show that the core is restricted from swelling to its native volume in the presence of the shell below the polymer phase transition temperature of 31 °C. The shell also compresses the core above the polymer phase transition temperature in pH solutions above the acid pKa by inducing a volume change, despite the fact that the core is fully charged due to deprotonation. This investigation shows that as the thickness of the shell is increased, the impact of these two phenomena increases as well.

Introduction Nanoparticles possessing a core/shell type morphology have found utility in numerous scientific pursuits. Variation of the size ratio between the core and shell can finely tune optical properties,1 control quantum effects,2 and protect particles in solution.3 Shell thickness variation has been used in polymeric systems to study fundamental swelling and rheological properties of environmentally responsive polymers on a hard-sphere core.4,5 Our group has used shell thickness variation to investigate core/ shell microgel deswelling mechanisms6 and kinetics.7 This work determines how shell thickness affects the swelling behavior of the core component in poly(N-isopropylacrylamide) (pNIPAm)-based core/shell microgels. Microgels are cross-linked, spherical colloids ranging in diameter from 10 to 1000 nm and have the ability to exist in a swollen or collapsed state depending on local solution conditions.8,9 Systems composed of pNIPAm undergo an entropically favored volume change at 31 °C, the characteristic lower critical solution temperature (LCST) of the parent polymer.10 The core/shell systems presented here consist of a temperature/pH-responsive pNIPAmco-acrylic acid (pNIPAm-co-AAc) core with a pNIPAm shell of varying thickness. This particle design allows examination of volume changes isothermally as a function of pH and elucidates the impact that the added shell has on (1) LizMarzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329-4335. (2) Dabbousi, B. O.; RodriguezViejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475. (3) Graf, C.; Schartl, W.; Maskos, M.; Schmidt, M. J. Chem. Phys. 2000, 112, 3031-3039. (4) Dingenouts, N.; Seelenmeyer, S.; Deike, I.; Rosenfeld, S.; Ballauff, M.; Lindner, P.; Narayanan, T. Phys. Chem. Chem. Phys. 2001, 3, 11691174. (5) Senff, H.; Richtering, W.; Norhausen, C.; Weiss, A.; Ballauff, M. Langmuir 1999, 15, 102-106. (6) Gan, D. J.; Lyon, L. A. Anal. Chim. Acta, in press. (7) Gan, D. J.; Lyon, L. A. J. Am. Chem. Soc. 2001, 123, 7511-7517. (8) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1-33. (9) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1-25. (10) Tanaka, T. Phys. Rev. Lett. 1978, 40, 820-823.

the swelling properties of the core.11 The swelling properties are monitored via photon correlation spectroscopy (PCS) as a function of temperature and pH, showing that the shell greatly determines the phase transition behavior of the core even under solution conditions in which the core is charged (above the AAc pKa). Particle solvation is monitored as a function of the cross-linker density as well, by using core/shell systems composed of 2 and 10 mol % N,N′-methylene(bisacrylamide) in both the core and shell components. Experimental Section Materials. All reagents were purchased from Sigma-Aldrich unless noted otherwise. N-Isopropylacrylamide (NIPAm) was recrystallized from hexanes (J.T. Baker) and dried in vacuo prior to use. N,N′-Methylene(bisacrylamide) (BIS), sodium dodecyl sulfate (SDS), and ammonium persulfate (APS) were used as received. Water for all reactions, solution preparation, and polymer purification was first distilled, then deionized to a resistance of 18 MΩ (Barnstead E-Pure system), and finally filtered through a 0.2 µm filter to remove particulate matter. Core/Shell Microgel Synthesis. Core/shell microgels were prepared via aqueous free-radical precipitation polymerization in the same manner as previously reported.11,12 Core microgels were prepared by adding 0.3 mmol APS into a 100 mL stirring monomer solution in the presence of 0.1 mmol SDS. The total monomer concentration was 70 mM, and the comonomer ratio (NIPAm/AAc/BIS) (9-X:1:X) was adjusted according to the desired cross-linker concentration. The polymerization was allowed to proceed for 5 h at 70 °C under nitrogen. Subsequent shell addition was performed under monomer-flooded conditions where 7.5 mL of the core particle solution was diluted with 12 mL of H2O along with 0.02 mmol SDS and heated to 70 °C under nitrogen. The constituent shell monomers were dissolved separately in 5 mL of H2O and heated to 70 °C under nitrogen. Shell thickness was controlled by varying the total monomer concentration from 20 to 40 to 80 mM (in the final 25.5 mL solution) while the ratio (NIPAm/BIS) (10-X:X) was adjusted according to the desired cross-linker concentration. The shell monomer solution was added to the stirring core solution and allowed to mix for several minutes, and then free radical polymerization (11) Jones, C. D.; Lyon, L. A. Macromolecules 2003, 36, 1988-1993. (12) Jones, C. D.; Lyon, L. A. Macromolecules 2000, 33, 8301-8306.

10.1021/la034392+ CCC: $25.00 © 2003 American Chemical Society Published on Web 05/03/2003

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was initiated with 0.04 mmol APS. Polymerization was allowed to proceed at 70 °C under nitrogen for 5 h. All particles used for analysis were purified via dialysis (Spectra/Por 7 dialysis membrane, MWCO 10 000, VWR) against daily changes of H2O for 2 weeks at 5 °C. Photon Correlation Spectroscopy. Microgel sizes and polydispersities were determined via photon correlation spectroscopy (Protein Solutions Inc.) as previously reported.12 Samples were analyzed in a three-sided quartz cuvette into which was placed 0.5 mL of a 10 µg/mL particle solution. The sample was allowed to equilibrate at the proper temperature for 5 min before data collection. Longer equilibration times did not lead to variation in the measured particle size, scattering intensity, or polydispersity. Scattered light was collected at 90° by a singlemode optical fiber coupled to an avalanche photodiode detector. Data were analyzed with Protein Solutions’ Dynamics Software, version 5.25.44. Each data point reported here is an average of five separate size determinations. Each size determination consists of 25 total measurements, which individually have a 5 s integration time.

Results and Discussion The swelling behavior of core/shell microgels can largely be attributed to the particle preparation method of precipitation polymerization. Particle formation occurs in aqueous solution upon temperature-induced free radical initiation with ammonium persulfate at 70 °C. At this temperature, water-soluble monomers polymerize to form oligomer chains that are insoluble upon reaching a certain critical chain length due to the LCST of pNIPAm.8 During particle growth, BIS cross-linker is statistically incorporated faster than other constituent monomers leading to the creation of a radial distribution of cross-links.13 Considering this process, the core component possesses a greater polymer network density toward the interior and more loosely cross-linked chains toward the periphery. Chains close to the particle periphery have more degrees of freedom and are able to extend over longer length scales in a good solvent. However, this same population of loosely cross-linked chains will be hindered from reaching their fully extended state in the presence of added polymer (e.g., the shell).11 The average number of chains that will be perturbed will differ according to the cross-linker concentration, with lower mol % cross-linked particles having a higher average number of loosely cross-linked chains near the periphery than highly cross-linked particles (above 7 mol % BIS) that do not have a well-defined gradient morphology.14,15 Because the synthetic conditions are the same for shell addition, a radial cross-linking density gradient is expected in the shell component as well, with the greatest network density being located at the interface of the core and shell. Once added, the densely cross-linked interior portion of the shell prohibits the periphery chains in the core from expanding to their maximum extended state as in the case prior to shell addition. Recent investigations on pNIPAm-co-AAc (core)/pNIPAm (shell) microgels illustrate that the swelling ability of the core is indeed modulated by the shell.11 The pH-responsive nature of the core allows isothermal investigations of particle size changes by variation of solution conditions. Below the pKa of acrylic acid (pKa ) 4.25),16 the acidic (13) Wu, X.; Pelton, R. H.; Hamielec, A. E.; Woods, D. R.; McPhee, W. Colloid Polym. Sci. 1994, 272, 467-477. (14) Varga, I.; Gilanyi, T.; Meszaros, R.; Filipcsei, G.; Zrinyi, M. J. Phys. Chem. B 2001, 105, 9071-9076. (15) Guillermo, A.; Addad, J. P. C.; Bazile, J. P.; Duracher, D.; Elaissari, A.; Pichot, C. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 889-898. (16) The CRC Handbook of Chemistry and Physics, 74th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1994; Vol. 124.

Table 1. Size Comparisons of pNIPAm-co-AAc (Core)/ pNIPAm (Shell) Microgels of Differing Shell Thickness pH 3.5a R 25 °C (nm)b

R 57 °C (nm)

pH 6.5a R 25 °C (nm)

R 57 °C (nm)

0 mM shell (core) 20 mM shell 40 mM shell 80 mM shell

2 mol % BIS 146 57 148 63 169 77 176 89

256 241 210 191

107 112 118 99

0 mM shell (core) 20 mM shell 40 mM shell 80 mM shell

10 mol % BIS 105 58 121 71 142 77 146 92

160 158 164 149

122 103 96 109

a Each pH solution has 0.001 M total ionic strength. b Radii measured via photon correlation spectroscopy.

moieties are protonated, resulting in behavior analogous to that of pNIPAm homopolymer microgels. A pH-induced volume increase is evident under solution conditions in which the acid groups are deprotonated. This effect is due both to Coulombic repulsion between charged groups and added osmotic pressure (Donnon effect) from incorporated counterions.17,18 PCS data show that the pNIPAm shell restricts the core from swelling to its native volume both above and below the pKa of acrylic acid at temperatures below 31 °C. When compared to the volume of the parent core particle at a pH above the pKa, the core/shell structure actually displays a smaller volume due to this compression effect.11 Also under these pH conditions, the parent core particle undergoes only small temperature-induced volume changes above 31 °C due to the extreme amount of Coulombic repulsion in the network. However, the addition of a pNIPAm shell results in compression of the core and induces a large volume change above the pNIPAm LCST, evidently overcoming the charge repulsion in the core to deswell the particle.11,12 Particles containing lower concentrations of BIS display greater degrees of shellrestricted swelling and compression upon the addition of an identically cross-linked pNIPAm shell. The thickness of the shell component is also thought to impact restricted swelling and compression of the core, since it is apparently the nature of the network density in the shell that modulates such behavior. The thickness of the shell can be controlled by modulating the amount of polymer added to the core during the shell synthesis, which is done by simply varying the total monomer concentration present in the shell monomer solution. It has been shown by others that these reaction conditions lead to nearly quantitative conversion of monomer to polymer, thus allowing for easy control over shell thickness by variation of the total monomer concentration.13 Core/ shell systems of vastly differing cross-link densities were chosen to illustrate the impact that shell thickness has on the swelling properties. Table 1 illustrates the results of adding differing molar amounts of shell monomer to vary the shell thickness in the two cross-linker concentrations studied. The relative concentration of BIS is the same for both the core and shell in each case. As shown in Figure 1a,b, particle size increases with increased shell monomer concentration, providing evidence of increasing shell thickness. The average particle polydispersity index for all samples varied randomly between 15% and 20% as measured by PCS for all (17) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: London, 1953. (18) Fernandez-Nieves, A.; Fernandez-Barbero, A.; Vincent, B.; de las Nieves, F. J. Macromolecules 2000, 33, 2114-2118.

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Figure 1. Photon correlation spectroscopy results for BIS-cross-linked pNIPAm-co-AAc (core)/pNIPAm (shell) particles with varying shell thickness under pH 3.5 solution conditions. Shell thickness is controlled by the initial monomer concentration present in the shell addition synthesis: (open circles) 0 mM, (filled squares) 20 mM, (open squares) 40 mM, (filled circles) 80 mM total shell monomer. (a) Loosely cross-linked microgels containing 2 mol % BIS. (b) Highly cross-linked microgels containing 10 mol % BIS.

Figure 2. Photon correlation spectroscopy results of BIS-cross-linked pNIPAm-co-AAc (core)/pNIPAm (shell) particles with varying shell thickness under pH 6.5 solution conditions. Shell thickness is controlled by the initial monomer concentration present in the shell addition synthesis: (open circles) 0 mM, (filled squares) 20 mM, (open squares) 40 mM, (filled circles) 80 mM total shell monomer. (a) Loosely cross-linked microgels containing 2 mol % BIS. (b) Highly cross-linked microgels containing 10 mol % BIS.

temperatures investigated. These data were taken under pH 3.5 solution conditions (0.001 M total ion), where the core component is not significantly charged. Thus, the volume phase transition behavior of each particle should be nearly identical to that of typical pNIPAm homopolymer microgels. Indeed, the volume phase transition temperature (VPTT) is approximately the same for the core and core/shell particles represented in Figure 1. Recent results using a nonresponsive poly(butyl methacrylate) core with varying thickness of a pNIPAm shell also show independence of the VPTT as a function of shell thickness when interrogated via PCS.6 The loosely cross-linked (2 mol %) system in Figure 1a also shows a systematic particle size increase at temperatures above 31 °C as the shell monomer concentration increases (see Table 1). Similarly, the particle size increases at temperatures below 31 °C. However, these size increases are not as dramatic as those observed for deswollen particles, thus indicating that the shell restricts the swelling ability of the core, even under conditions where the acid groups are protonated. Similar behavior is shown for the highly cross-linked system in Figure 1b. In this case, the core has a higher network density, the swelling of which is perturbed to a lesser degree in the presence of the added shell; a steady size

increase is now evident for each increasing shell thickness at all temperatures. The data in Figure 2a,b show the most dramatic results of modulated swelling behavior in the presence of increasing shell thickness. These data were collected under pH 6.5 solution conditions (0.001 M total ion), where the acidic groups are fully deprotonated and thus the core component is charged. The loosely cross-linked system shown in Figure 2a displays interesting behavior below ∼32 °C; the particle volume decreases as the shell thickness increases (see Table 1). In each case, the size decrease is greater than the apparent shell thickness observed in Figure 1a, indicating that increasing the thickness of the shell concurrently decreases the ability of the parent core to swell to its original volume. Similar behavior is shown in Figure 2b for the highly cross-linked system. Core compression is not as pronounced in this case due to the already constrained nature of the high-density network structure. These data indicate that the network structure in the shell changes with shell thickness, thus affecting the swelling ability of the core and the entire core/shell particle. The cross-linking density gradient in thicker shells may have a more gradual persistence toward the

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particle periphery, thereby decreasing the overall hydrodynamic radius as interrogated via PCS. Compression of the core as a function of temperature is also related to shell thickness as shown in Figure 2a. The VPTT decreases with increasing shell thickness as the particles become more pNIPAm-like in nature. The thin 20 mM shell has little impact on the deswelling behavior as a function of temperature, with the core/shell data almost overlapping the volume phase transition curve of the charged core. The 40 and 80 mM shells greatly impact the particle phase transition behavior and compress the charged core beginning at ∼32 °C. A similar trend is shown in Figure 2b for the highly cross-linked system. In summary, we have demonstrated that shell thickness has an impact on the swelling behavior of acrylic acid modified pNIPAm core/shell microgels. The shell restricts the swelling ability of the core and acts to compress the core in volume even under conditions that are normally

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unfavorable. Thicker shells impact these phenomena by producing greater degrees of restricted swelling and compression. This is most likely due to the polymer network structure in the shell that is created during particle synthesis. Acknowledgment. L.A.L. gratefully acknowledges financial support from the National Science Foundation, Division of Materials Research (DMR-0203707). C.D.J. gratefully acknowledges partial support from the Georgia Institute of Technology Molecular Design Institute under prime contract N00014-95-1-1116 from the Office of Naval Research, as well as a National Science Foundation Trainee Fellowship in Environmental Sciences and support from the Polymer Education Research Center at the Georgia Institute of Technology. LA034392+