5. U NRAVELING THE 3D L OCALIZATION AND D EFORMATION OF R ESPONSIVE
5.6 Methods
Materials. N-ispropylacrylamide (NiPAm, 99%, Acros Organics), methacrylic acid (MAA, ABCR, 99%, stabilized with 100-250 ppm hydroquinone or 4-methoxyphenol), N,N’-methylenebisacrylamide (BIS, AppliChem, molecular biology grade), potassium
FRESCA CRYO-SEM
peroxodisulfate (KPS, Merck, 99%), the fluorescent label methacryloxyethyl-thiocarbamoylrhodamine B (MRB, PolySciences), sodiumdodecylsulfate (SDS, Merck, 99%) and n-heptane (Merck, 99%) were used as received. Doubly distilled Milli-Q water was used for synthesis and characterization of the microgels and preparation of microgel dispersions. For adjustment of the pH 0.1M HCl and 0.1M NaOH were used.
Microgel Synthesis. The core microgels were synthesized in a batch synthesis by emulsion polymerization. Water was heated to 80°C and was degassed with N2. NiPAm, MAA and BIS in a mass ratio of 90:5:5 and a total monomer concentration of 180 mM
were dissolved in the reaction flask under mechanical stirring. SDS was added to the solution in a concentration of 2.4 mM. The reaction was started with KPS (1.8 mM). After 5 h the reaction mixture was cooled to room temperature under constant stirring and filtered through glass wool. Then the solution was centrifuged four times at 50000 rpm for 1 h and in each cycle the supernatant was replaced with bidistilled water. The cleaned product was freeze-dried.
For the synthesis of the core-shell microgel, core microgel and shell monomers were used in a mass ratio of 1:2. 4.0 g of the dried core microgel were redispersed in 420 mL bidistilled water and degassed with N2. SDS in a total concentration of 1.6 mM was added to the mixture and it was heated to 80°C under constant mechanical stirring. NiPAm and BIS in a mass ratio of 95:5 and 0.06 wt% MRB were dissolved in 100 mL degassed water. The reaction was started by adding 5 mL of a 1.2 mM KPS solution and 20 mL of the monomer solution to the mixture. The same amounts were then added in four portions every ten minutes. 5 hours after the last addition of reactants, the mixture was left to cool down to room temperature and was filtered through glass wool. After five centrifugation cycles at 30000 rpm for 1 h the product was freeze dried.
Characterization of the Core and the Core-Shell Microgel. The hydrodynamic diameter (dw) of the microgels in bulk was determined by dynamic light scattering (DLS) in an ALV-5000 instrument with light of 633 nm wavelength. By comparing the size of the core in the collapsed state at pH 3 and 50°C (dw = 138 ± 1 nm) with the size of the core-shell microgel at 20°C (see table 1) one can estimate the shell thickness. The minimal and maximal shell thicknesses at pH 3 are 51 ± 4 nm and 120 ± 4 nm and 62 ± 6 nm and 131 ± 6 nm at pH 9, respectively for core and core-shell particles.
Electrophoretic mobility measurements were performed with a NANO ZS Zetasizer (Malvern Instruments, UK). The electrophoretic mobility measured for the core microgel was -0.09 ± 0.01 10-8 m2/Vs at pH 3 and -0.70 ± 0.03 10-8 m2/Vs at pH 9 and for the core-shell microgel -0.05 ± 0.02 10-8 m2/Vs at pH 3 and -0.22 ± 0.02 10-8 m2/Vs at pH 9. The content of MAA in the microgels was determined by pH titration. The core microgel has an MAA content of 6.3 ± 0.6 wt% and the core-shell microgel of 2.8 ± 0.5 wt%.
FRESCA CRYO-SEM
Interfacial Tension Measurements. Interfacial tension (IFT) measurements were performed on a DSA100 (Krüss GmbH) instrument equipped with a pendant drop module. A drop of 1.5 wt% microgel dispersion in water was created on top of a needle in a cuvette filled with n-heptane and the drop shape was recorded every 1 or 2 seconds until the IFT reached equilibrium. The IFT was determined by video analysis with the Drop Shape Analysis programme supplied by the manufacturer.
FreSCa Cryo-SEM. 0.5 µL of a microgel suspension at 0.1 wt% were placed inside a custom-made copper holder with a 200 µm deep central cavity. Prior to use, the sample holders were cleaned in sulphuric acid (95%) and ethanol for several minutes. Their inner surface was roughened and exposed to a negative glow discharge to improve adhesion during freezing. Successively, a 3.0 µL droplet of the non-polar phase was carefully placed on top to create the liquid-liquid interface and then the holder was closed with a flat copper plate. The “sandwich” holder was frozen in a liquid propane jet freezer (Bal-Tec/Leica JFD 030, Balzers/Vienna) with a cooling rate of 30000 Ks-1 to avoid water crystallization. After freezing, the samples were mounted under liquid nitrogen onto a double fracture cryo-stage and transferred under inert gas in a cryo-high vacuum airlock (< 5×10-7 mbar Bal-Tec/Leica VCT010) to a pre-cooled freeze-fracture device at -140°C (Bal-Tec/Leica BAF060 device). The samples were then fractured and partially freeze-dried at -110°C for 3 min to remove deposited residual water condensation and ice crystals, followed by unidirectional tungsten deposition at an elevation angle α = 30° to a total thickness δ = 2 nm at -120°C and by additional 2 nm with a continuously varying angle between 90° and 30°. The second deposition is needed in order to avoid charging of the shadow during imaging which may compromise image stability at high magnifications. Freeze-fractured and metal-coated samples were then transferred for imaging under high vacuum (< 5×10-7 mbar) at -120°C to a pre-cooled (-120°C) cryo-SEM (Zeiss Gemini 1530, Oberkochen) for imaging either with an in-lens or secondary electron detector.
5.7 References
[1] J. Bibette, F. Leal-Calderon, Curr. Opin. Colloid Interface Sci. 1996, 1, 746-751.
[2] B. P. Binks, Curr. Opin. Colloid Interface Sci. 2002, 7, 21-41.
[3] E. Dickinson, J. Chem. Soc.-Faraday Trans. 1998, 94, 1657-1669.
[4] R. Aveyard, B. P. Binks, J. H. Clint, Adv. Colloid Interface Sci. 2003, 100-102, 503-546.
[5] A. Boeker, J. He, T. Emrick, T. P. Russell, Soft Matter 2007, 3, 1231-1248.
[6] T. Ngai, S. H. Behrens, H. Auweter, Chem. Commun. 2005, 331-333.
[7] B. Brugger, W. Richtering, Adv. Mater. 2007, 19, 2973-2978.
[8] B. Brugger, B. A. Rosen, W. Richtering, Langmuir 2008, 24, 12202-12208.
FRESCA CRYO-SEM
[9] B. Brugger, W. Richtering, Langmuir 2008, 24, 7769-7777.
[10] B. Brugger, S. Ruetten, K.-H. Phan, M. Moeller, W. Richtering, Angew. Chem., Int.
Ed. 2009, 48, 3978-3981.
[11] Z. Li, T. Ming, J. Wang, T. Ngai, Angew. Chem., Int. Ed. 2009, 121, 8642-8645.
[12] B. Brugger, J. Vermant, W. Richtering, Phys. Chem. Chem. Phys. 2010, 12, 14573-14578.
[13] G. Sun, Z. Li, T. Ngai, Angew. Chem., Int. Ed. 2010, 49, 2163-2166.
[14] C. Monteux, C. Marliere, P. Paris, N. Pantoustier, N. Sanson, P. Perrin, Langmuir 2010, 26, 13839-13846.
[15] Z. Li, T. Ngai, Colloid Polym. Sci. 2011, 289, 489-496.
[16] S. Schmidt, T. Liu, S. Rütten, K.-H. Phan, M. Möller, W. Richtering, Langmuir 2011, 27, 9801-9806.
[17] M. Destribats, V. Lapeyre, M. Wolfs, E. Sellier, F. Leal-Calderon, V. Ravaine, V.
Schmitt, Soft Matter 2011, 7, 7689-7698.
[18] M. Destribats, V. Lapeyre, E. Sellier, F. Leal-Calderon, V. Schmitt, V. Ravaine, Langmuir 2011, 27, 14096-14107.
[19] M. Destribats, V. Lapeyre, E. Sellier, F. Leal-Calderon, V. Ravaine, V. Schmitt, Langmuir 2012, 28, 2744-2755.
[20] T. Liu, S. Seiffert, J. Thiele, A. R. Abate, D. A. Weitz, W. Richtering, Procl. Nat.
Acad. Sci. 2012, 109, 384-389.
[21] A. Pich, W. Richtering, Adv. Polym. Sci. 2010, 234, 1-37.
[22] S. Nayak, D. Gan, M. J. Serpe, L. A. Lyon, Small 2005, 1, 416-421.
[23] S. Berger, H. Zhang, A. Pich, Adv. Funct. Mater. 2009, 19, 554-559.
[24] R. K. Shah, J.-W. Kim, D. A. Weitz, Langmuir 2010, 26, 1561-1565.
[25] B. R. Saunders, N. Laajam, E. Daly, S. Teow, X. Hu, R. Stepto, Adv. Colloid Interface Sci. 2009, 147-148, 251-262.
[26] A. San Miguel, J. Scrimgeour, J. E. Curtis, S. H. Behrens, Soft Matter 2010, 6, 3163-3166.
[27] B. P. Binks, S. O. Lumsdon, Langmuir 2000, 16, 8622-8631.
[28] F. Ortega, H. Ritacco, R. G. Rubio, Curr. Opin. Colloid Interface Sci. 2010, 15, 237-245.
[29] F. Bresme, M. Oettel, J. Phys.: Condens. Matter2007, 17, 413101
[30] T. S. Horozov, R. Aveyard, J. H. Clint, B. P. Binks, Langmuir 2003, 19, 2822-2829.
[31] L. Isa, F. Lucas, R. Wepf, E. Reimhult, Nat. Commun. 2011, 2, 438.
[32] C. D. Jones, L. A. Lyon, Macromolecules 2003, 36, 1988-1993.
[33] I. Berndt, J. S. Pedersen, P. Lindner, W. Richtering, Langmuir 2006, 22, 459-468.
[34] M. Stieger, W. Richtering, J. S. Pedersen, P. Lindner, J. Chem. Phys. 2004, 120, 6197-6206.
[35] K. M. Reed, J. Borovicka, T. S. Horozov, V. N. Paunov, K. L. Thompson, A.
Walsh, S. P. Armes, Langmuir 2012, 28, 7291-7298.
FRESCA CRYO-SEM
[36] K. A. Tay, F. Bresme, J. Am. Chem. Soc. 2006, 128, 14166-14175.
[37] L. Isa, E. Amstad, K. Schwenke, E. D. Gado, P. Ilg, M. Kröger, E. Reimhult, Soft Matter 2011, 7, 7663-7675.
[38] C. Stefaniu, M. Chanana, H. Ahrens, D. Wang, G. Brezesinski, H. Möhwald, Soft Matter 2011, 2011, 4267.
[39] S. Zeppieri, J. Rodriguez, A. L. López de Ramos, J. Chem. Eng. Data 2001, 46, 1086-1088.
[40] G. Gillies, K. Buscher, M. Preuss, M. Kappl, H. J. Butt, K. Graf, J. Phys.: Condens.
Matter 2005, 17, S445-S464.
[41] M. D'Souza Mathew, M. S. Manga, T. N. Hunter, O. J. Cayre, S. Biggs, Langmuir 2012, 28, 5085-5092.
[42] D. Suzuki, S. Tsuji, H. Kawaguchi, J. Am. Chem. Soc. 2007, 129, 8088-8089.
COMPRESSION ISOTHERMS