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Langmuir 2003, 19, 7580-7586
Hydrophobic Polymer Modification with Ionic Reagents: Polystyrene Staining with Water-Soluble Dyes Melissa Braga, Carlos A. P. Leite, and Fernando Galembeck* Institute of Chemistry, Universidade Estadual de Campinas, P.O. Box 6154, CEP 13084-862, SP, Brazil Received February 15, 2003. In Final Form: June 19, 2003 Homopolymer polystyrene films tinted with methylene blue and other hydrophilic cationic dyes are made following a new two-step procedure: dye adsorption in latex particles, followed by polymer plasticization with a suitable solvent. The dyed polymer is soluble in nonpolar solvents, and the dyes do not phase separate, even in nonsolvents for the pure dye salts. Microanalytical data on the particles and films were acquired using energy-loss spectroscopy imaging (ESI-TEM) and scanning electric potential microscopy (SEPM). The results are interpreted considering that (i) the polystyrene latex particles have a mild ionomer character, due to the charged sulfate groups arising from the polymerization initiator and bound to the chain ends; (ii) there is formation of ion pairs made out of dye cation and chain-end sulfate; (iii) the cationic dye self-associates, as evidenced by a pronounced metachromatic behavior. A nonpolar polymer can thus be made compatible with a basic dye without any special chain modification, just by using this new procedure based on dye sorption followed by diffusion.
The rules for hydrophobic polymer dyeing are fairly simple and well-known: a dye or pigment is added to the polymer by satisfying one of the following requirements: (i) the dye and polymer are miscible1,2 following, for example, Hildebrand theory of regular solutions; (ii) the dye and the polymer are immiscible but the dye particles are finely dispersed in the polymer melt, often using strong mechanical action and amphiphilic compatibilizing agents;3,4 (iii) the dye (or even a pigment) is formed in-situ in the polymer, following sorption and reaction of a precursor, which is in turn miscible with the polymer;5,6 (iv) the dye chromophore is covalently bound to the polymer chain;7-10 (v) the dye is firmly bound at the polymer surface, often in sites formed by surface-modification procedures.11 Polymer-dye mixtures are most often prepared following procedures that satisfy one or both of the two first requirements listed above. This poses serious limitations to the number of combinations between polymer and dye that can be successfully used. For instance, sorption of a precursor followed by in-situ transformation into a dye or pigment is useful in a few unusual cases, as in the fabrication of poly(tetrafluoroethylene) loaded with iron oxide nanoparticles.12 * Corresponding author. Phone: +55-19-3788-3080. Fax +5519-3788-3023. E-mail:
[email protected]. (1) Schild, H. G. Macromol. Rapid Commun. 1995, 16, 197-200. (2) Lupo, D.; Ringsdorf, H.; Schuster, A.; Seitz, M. J. Am. Chem. Soc. 1994, 116, 10498-10506. (3) Homola, T. J. Mol. Cryst. Liq. Cryst. 2000, 344, 63-68. (4) Knuttel, F. U.S. Patent No 1.064.490, 1964. (5) Galembeck, F. J. Polym. Sci.: Polym. Chem. 1978, 11, 30153017. (6) Yates, M. Z.; Birnbaum, E. R.; McCleskey, T. M. Langmuir 2000, 16, 4757-4760. (7) Andruzzi, L.; Altomare, A.; Ciardelli, F.; Solaro, R.; Hvlsted, S.; Ramanujam, P. S. Macromolecules 1999, 32, 448-454. (8) Peterson, J. I. U.S. Patent No. 4,194,877, 1980. (9) Schuhmacher, A.; Ehrhardt, A.; Hansen, C. P. EP Patent No. 1.022.652, Germany, 1962. (10) Ito, H.; Ikeda, T.; Ichimura, K. Makromol. Chem. 1993, 194, 2215-2224. (11) Rubira, A. F.; da Costa, A. C.; Galembeck, F.; Escobar, N. F. L.; Da Silva, E. F.; Vargas, H. Colloids Surf. 1985, 15, 63-73. (12) Galembeck, F.; Ghizoni, C. C.; Ribeiro, C. A.; Vargas, H.; Miranda, L. C. M. J. Appl. Polym. Sci. 1980, 25, 1427-1433.
The use of covalently bound dyes leads to very important but expensive products, which are important for optoelectronic and information technology13-15 but not for large-scale applications. Dyeing by adsorption is useful for the characterization of polymer surfaces,16-18 but it does not often lead to uniformly tinted polymer. Many interesting dyes are water-soluble salts, and they are thus insoluble in hydrophobic polymers. These dyes are adsorbed to polymer colloids made by emulsion polymerization, carrying at least one charge bound to each polymer chain, which imparts to them a modest ionomer character. If ionic comonomers such as acrylic and other acids are used, the ionomer properties of the emulsion polymers become more pronounced and large densities of ionic groups are obtained. Recent microchemical work using ESI-TEM (electron spectroscopy imaging in the transmission electron microscope) showed that these charges are well dispersed throughout the polymer dry latex,19-21 in agreement with the well-known “burying” of titratable groups in polymer colloids. Charges dispersed throughout a hydrophobic polymer were also observed in natural rubber.22 Following ESI-TEM data on latex particles and films, Na+, K+, and other cationic species are also found dispersed in the particle bulk, thus showing some degree of miscibility with the polymer, probably mediated by ionpair formation with the covalently bound sulfate and carboxylate anions. (13) Somani, P. R.; Radhakrishnan, S. Mater. Chem. Phys. 2003, 77, 117-133. (14) Suzuki, H. Thin Solid Films 2001, 393, 352-357. (15) Tawa, K.; Zettsu, N.; Minematsu, K.; Ohta, K.; Namba, A.; TranCong, Q. J. Photochem. Photobiol. A 2001, 143, 31-38. (16) Da Costa, R. A.; Coltro, L.; Galembeck, F. Angew. Makromol. Chem. 1990, 180, 85-94. (17) Zinger, B.; Shier, P. Sens. Actuators, B 1999, 56, 206-214. (18) Mubarekyan, E.; Santore, M. Langmuir 1998, 14, 1597-1603. (19) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1998, 14, 3187-3194. (20) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1999, 15, 4447-4453. (21) Braga, M.; Costa, C. A. R.; Leite, C. A. P.; Galembeck, F. J. Phys. Chem. B 2001, 105, 3005-3011. (22) Rippel, M. M.; Leite, C. A. P.; Galembeck, F. Anal. Chem. 2002, 74 (11), 2541-2546.
10.1021/la034258p CCC: $25.00 © 2003 American Chemical Society Published on Web 07/25/2003
Polystyrene Staining with Hydrophilic Dyes
The present work describes a new procedure for the incorporation of water-soluble dyes to polystyrene, by using a two-step procedure: dye adsorption on the polymer particles, followed by plasticization. Experimental Procedure Latex Preparation. The latex used in this work is a homopolymer polystyrene colloid initiated with K2S2O8 (Sigma). It was prepared and characterized according to a previous work, using Brij 35 (Sigma) and sodium dodecyl sulfate (SDS, Quimis) as the emulsifiers.23 Yield was 94% with 14.3% solids content, and the amount of coagulated material was
