Chem. Mater. 2006, 18, 2997-3004
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Conductive Thin Films on Functionalized Polyethylene Particles Yeon-Seok Kim,† Kang-Shyang Liao,‡ C. Jason Jan,† David E. Bergbreiter,‡ and Jaime C. Grunlan*,† Department of Mechanical Engineering and Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843 ReceiVed February 13, 2006. ReVised Manuscript ReceiVed April 22, 2006
Hyperbranched polyethylenimine (PEI) is covalently grafted to the surface of polyethylene (PE) particles in an effort to promote the growth of conductive thin films deposited using layer-by-layer (LbL) assembly. Growth of the grafting process is monitored with infrared spectroscopy and titration. LbL films are then deposited using dilute aqueous mixtures containing carbon black stabilized with PEI or poly(acrylic acid). Deposition of carbon black-filled bilayers on PEI-grafted PE shows uniform surface coverage and strong bonding after just two bilayers, while neat PE shows patchy film growth and poor adhesion, requiring eight bilayers to achieve full surface coverage. Acid-oxidized PE shows intermediate behavior with regard to deposition but shows weak bonding like neat PE. LbL films are characterized using electron microscopy and thermogravimetric analysis, which show linear growth for PEI-grafted particles and nonlinear growth for neat and acid-oxidized particles. Following carbon black deposition, the electrical conductivities of films made by compressing the coated particles were compared. No conductivity can be measured for films made with neat PE particles containing two and four bilayers, but acid-oxidized and PEI-grafted systems exhibit conductivities of 0.000 004 5 and 0.01 S/cm, respectively. Plotting conductivity as a function of carbon black concentration reveals a percolation threshold below 0.01 wt % and a conductivity of 0.2 S/cm with just 6 wt %. This combination of covalent polyelectrolyte grafting and LbL deposition could potentially be used to impart useful properties to a variety of polyolefin surfaces.
1. Introduction Layer-by-layer (LbL) assembly, formerly called electrostatic self-assembly, is a method used to impart functionality to the surface of nearly any type of substrate.1,2 Thin films are deposited by alternately exposing a substrate to aqueous mixtures (or solutions) of mutually attractive molecules or particles. In most cases the attraction between species is provided by charge and each combination of a positively and negatively charged layer is referred to as a “bilayer”. Each deposited layer is typically 1-100 nm thick depending on charge density,3,4 counterion,3 molecular weight,5 temperature,6 deposition time,7,8 and pH9,10 of the species being deposited. Concentration8 and ionic strength8,11 of the deposition mixture will also influence layer thickness. LbL-based * To whom correspondence should be addressed. Phone: (979) 845-3027. Fax: (979) 862-3989. E-mail:
[email protected]. † Department of Mechanical Engineering. ‡ Department of Chemistry.
(1) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348. (2) Decher, G.; Schlenoff, J. B. Multilayer Thin Films-Sequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim, Germany, 2003. (3) Mermut, O.; Barrett, C. J. J. Phys. Chem. B 2003, 107, 2525-2530. (4) Schoeler, B.; Poptoschev, E.; Caruso, F. Macromolecules 2003, 36, 5258-5264. (5) Sui, Z. J.; Salloum, D.; Schlenoff, J. B. Langmuir 2003, 19, 24912495. (6) Tan, H. L.; McMurdo, M. J.; Pan, G. Q.; Van Patten, P. G. Langmuir 2003, 19, 9311-9314. (7) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153-8160. (8) Zhang, H. N.; Ruhe, J. Macromolecules 2003, 36, 6593-6598. (9) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213-4219. (10) Choi, J.; Rubner, M. F. Macromolecules 2005, 38, 116-124. (11) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655-6663.
thin films can impart chemical sensing,12-14 electrically conductive,15,16 super-hydrophobic,17 electrochromic,18,19 magnetic,20 and antimicrobial behavior21,22 to a given substrate. While substrates for LbL assembly are usually flat surfaces, there is no shape limitation, and a variety of studies have demonstrated the ability to uniformly coat small spherical particles.23-26 In most cases these thin coatings are used for some form of encapsulation rather than providing an active functionality. In the present study polyethylenimine (PEI) was grafted to the surface of irregularly shaped polyethylene (12) Lee, S. H.; Kumar, J.; Tripathy, S. K. Langmuir 2000, 16, 1048210489. (13) Tian, S. J.; Liu, J. Y.; Zhu, T.; Knoll, W. Chem. Mater. 2004, 16, 4103-4108. (14) Jiang, C. Y.; Markutsya, S.; Pikus, Y.; Tsukruk, V. V. Nat. Mater. 2004, 3, 721-728. (15) Zhang, F. X.; Srinivasan, M. P. Thin Solid Films 2005, 479, 95-102. (16) Jan, C. J.; Walton, M. D.; McConnell, E. P.; Jang, W.-S.; Kim, Y. S.; Grunlan, J. C. Carbon 2006, in press. (17) Jisr, R. M.; Rmaile, H. H.; Schelnoff, J. B. Angew Chem., Int. Ed. 2005, 44, 782-785. (18) DeLongchamp, D. M.; Kastantin, M.; Hammond, P. T. Chem. Mater. 2003, 15, 1575-1586. (19) Liu, S. Q.; Kurth, D. G.; Mohwald, H.; Volkmer, D. AdV. Mater. 2002, 14, 225 ff. (20) Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530-5533. (21) Boulmedais, F.; Frisch, B.; Etienne, O.; Lavalle, P.; Picart, C.; Ogier, J.; Voegel, J. C.; Schaaf, P.; Egles, C. Biomaterials 2004, 25, 20032011. (22) Grunlan, J. C.; Choi, J. K.; Lin, A. Biomacromolecules 2005, 6, 11491153. (23) Schuetz, P.; Caruso, F. AdV. Funct. Mater. 2003, 13, 929-937. (24) Lvov, Y. M.; Price, Y. R. Colloids Surf., B 2002, 23, 251-256. (25) Sukhishvili, S. A. Curr. Opin. Colloid Interface Sci. 2005, 10, 3744. (26) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W.; Giersig, M.; LizMarzan, L. M. Chem. Mater. 2005, 17, 3268-3272.
10.1021/cm0603607 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/25/2006
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(PE) particles in an effort to promote stable growth of carbon black (CB) thin films. Many materials can be used as a substrate for LbL deposition without the need for surface modification, but there is difficulty building layers on polyolefins.27,28 It is difficult to get water-based coatings to adhere well to polyolefins, such as PE and polypropylene, because of low surface energy and lack of polarity. Corona or plasma treatments can be used to increase the surface energy of polyolefin films.29-32 Both of these techniques generate polar functionalities on the film surface that will improve wetting and subsequent adhesion of an aqueous coating. Oxidative acid etching is another method that generates hydroxyl and carboxylic acid species to create surface polarity.33 However, these surface modification techniques only produce monolayers of functionality whose properties change upon surface reorganization. A more recent technique, used here to promote stable LbL film growth and stronger adhesion, is the growth or graft of polyelectrolytes on polyolefin.34-37 In the present study, a hyperbranched PEI surface was generated on PE particles in a manner similar to that done previously on a silica support.37 Hyperbranching yields a uniform surface coverage of polyelectrolyte down to the microscopic level, despite initial growth occurring only sparsely over the PE particle surface. The presence of PEI on the PE particle then facilitates near perfect surface coverage by alternate deposition of CB stabilized with poly(acrylic acid) (PAA) and PEI in a LbL fashion. Untreated PE exhibits very poor acceptance of the LbL film growth under the same conditions, while oxidative acid etching exhibits intermediate behavior. The CB-coated particles so formed produce highly conductive composites when compression molded. CB-filled polymer composites are useful as thermal resistors,38,39 chemical sensors,40,41 electromagnetic interference shielding,42,43 and electrostatic dissipation layers.43,44 Such electrically conductive composites are typically prepared using melt mixing45,46 or solution processing.47,48 These (27) Delcorte, A.; Bertrand, P.; Wischerhoff, E.; Laschewsky, A. Langmuir 1997, 13, 5125-5136. (28) Greene, G.; Tannenbaum, R. Appl. Surf. Sci. 2004, 233, 336-342. (29) Hougen, L. R. Nature 1960, 188, 577-578. (30) Carley, J. F.; Kitze, P. T. Polym. Eng. Sci. 1978, 18, 326-334. (31) Morra, M.; Occhiello, E.; Gila, L.; Garbassi, F. J. Adhes. 1990, 33, 77-88. (32) Nihlstrand, A.; Hjertberg, T.; Johansson, K. Polymer 1997, 38, 35813589. (33) Blais, P.; Carlsson, D. J.; Csullog, G. W.; Wiles, D. M. J. Colloid Interface Sci. 1974, 47, 636-649. (34) Bergbreiter, D. E. Prog. Polym. Sci. 1994, 19, 529-560. (35) Tao, G. L.; Gong, A. J.; Lu, J. J.; Sue, H. J.; Bergbreiter, D. E. Macromolecules 2001, 34, 7672-7679. (36) Bergbreiter, D. E.; Boren, D.; Kippenberger, A. M. Macromolecules 2004, 37, 8686-8691. (37) Bergbreiter, D. E.; Simanek, E. E.; Owsik, I. J. Polym. Sci., Polym. Chem. 2005, 43, 4654-4665. (38) Brodeur, S. A.; Huebner, W.; Runt, J. P.; Newnham, R. E. J. Mater. Res. 1991, 6, 175-182. (39) Wan, Y.; Wen, D. J. Smart Mater. Struct. 2004, 13, 983-989. (40) Kim, Y. S.; Ha, S. C.; Yang, Y.; Kim, Y. J.; Cho, S. M.; Yang, H.; Kim, Y. T. Sens. Actuators, B 2005, 108, 285-291. (41) Koscho, M. E.; Grubbs, R. H.; Lewis, N. S. Anal. Chem. 2002, 74, 1307-1315. (42) Das, N. C.; Chaki, T. K.; Khastgir, D.; Chakraborty, A. AdV. Polym. Technol. 2001, 20, 226-236. (43) Klason, C.; McQueen, D. H.; Kubat, J. Macromol. Symp. 1996, 108, 247-260. (44) Kale, V.; Moukwa, M. J. Electrost. 1996, 38, 239-248.
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methods lead to composites with high percolation thresholds because of random CB dispersion. The percolation threshold is the minimum concentration of conductive filler required to generate a conductive pathway that travels through the entire composite film.49 In many cases, the high CB concentrations (>25 wt %) required to achieve significant electrical conductivity is often accompanied by high mixing viscosity and brittle composite films with extensive porosity due to aggregated filler.50,51 LbL deposition circumvents high viscosity processing by using dilute mixtures (
