Dynamic Surface Properties of Poly(N-isopropylacrylamide) Solutions B. A. Noskov, † A. V. Akentiev, † A. Yu. Bilibin, † D. O. Grigoriev, ‡ G. Loglio, § I. M. Zorin, † and R. Miller* ,‡ Chemistry Department, St. Petersburg State University, Universitetsky pr. 2, 198904 St. Petersburg, Russia, MPI fu ¨ r Kolloid- und Grenzfla ¨ chenforschung, Forschungscampus Golm, D14476 Golm, Germany, and Dipartimento di Chimica Organica, Universita degli Studi di Firenze, Via della Lastruccia 13, 50019 Sesto Fiorentino, Firenze, Italy Received May 11, 2004. In Final Form: August 1, 2004 The dynamic surface elasticity of aqueous solutions of poly(N-isopropylacrylamide) (pNIPAM) has been measured by the oscillating barrier and capillary wave methods as a function of time and concentration. While the real and imaginary parts of the surface elasticity almost did not change with the concentration, their kinetic dependencies proved to be nonmonotonic. Simultaneous measurements of the film thickness and adsorbed amount by null-ellipsometry showed that the pNIPAM adsorption can be divided into two steps corresponding to the formation of a concentrated narrow region close to the air phase and a region of tails and loops protruding into the bulk liquid. The local maximum of the elasticity can be observed in the course of the first step when the adsorbed macromolecules do not form long loops and tails. The results are in agreement with recent data on the nonequilibrium surface properties of solutions of other nonionic homopolymers and the theory of dilational surface viscoelasticity. Introduction The temperature-sensitive solubility of poly(N-isopro- pylacrylamide) (pNIPAM) in water is probably the main reason for the increasing interest in this polymer. When heated above the so-called low critical solution temper- ature (LCST), aqueous pNIPAM solutions undergo a phase separation. The proximity of the LCST (about 32 °C) to the corporal temperature opens a possibility of various biological applications. Another distinguishing feature of pNIPAM solutions consists of low surface tension, 1 which is characteristic to solutions of strong surfactants and not of the other widely studied nonionic homopolymers such as poly(ethylene oxide) (PEO) and poly(vinylpyrrolidone) (PVP). Although the static surface tension of pNIPAM solutions depends only slightly on the temperature and concentration, neutron reflectivity gives evidence of a strong increase in the adsorbed amount and strong changes of the surface layer structure in the temperature range close to the LCST. 2-5 Richardson et al. concluded from these results that the surface tension is not a good predictor of the total amount of polymer adsorbed at the air-water interface and, consequently, of the surface structure. 5 This is in accordance with data on the dynamic surface elasticity of aqueous solutions of other nonionic polymers. 6-8 The latter quantity can change with the polymer concentration by an order of magnitude while the surface tension remains almost constant. According to our knowledge, the concentration dependence of neutron reflectivity for pNIPAM solutions has not been measured, probably because of the slow adsorption kinetics at concentrations below 0.01 wt %. Kawaguchi et al. mea- sured the surface layer thickness by ellipsometry but only in a limited concentration range (0.0003-0.001 wt %) because of limitations in the equipment. 9 Most of the published results on the surface properties of pNIPAM solutions correspond to static conditions. 2-5,10 Among nonequilibrium surface properties only the dy- namic surface tension has been studied in detail. 9,11,12 First comparison of the experimental data with the results of calculation according to the Ward-Tordai equation 13 showed that the pNIPAM adsorption kinetics was con- trolled by diffusion in the bulk phase. 9 A more careful comparison with other kinetic models confirmed this conclusion only for the initial step of adsorption. 12 The subsequent adsorption steps could not be described by any of the existing adsorption kinetics models. Data obtained from the oscillating drop method suggested the contribution of loops and tails to the adsorption process of pNIPAM: “polymer loops and tails lay down upon a newly formed surface during drop expansion”. 12 However, the relative oscillation amplitude of the drop surface area was large (about 25%) so that the linear theory and the concept of dilational dynamic surface elasticity could not be applied. Huang and Wang applied to pNIPAM solutions the surface quasi-elastic light scatteringsa relaxation tech- nique with very small amplitudes of surface area oscil- lations (,1%). 14 These authors did not try to calculate the * To whom correspondence may be addressed. † St. Petersburg State University. ‡ MPI fu ¨ r Kolloid- und Grenzfla ¨ chenforschung. § Universita degli Studi di Firenze. (1) Zhang, J.; Pelton, R. Langmuir 1996, 12, 2611. (2) Lee, L. T.; Jean, B.; Menelle, A. Langmuir 1999, 15, 3267. (3) Jean, B.; Lee, L. T.; Cabane, B. Langmuir 1999, 15, 7585. (4) Jean, B.; Lee, L. T. Colloid Polym. Sci. 2002, 280, 689. (5) Richardson, R. M.; Pelton, R.; Cosgrove, T.; Zhang, J. Macro- molecules 2000, 33, 6269. (6) Noskov, B. A.; Akentiev, A. V.; Loglio, G.; Miller, R. J. Phys. Chem. B 2000, 104, 7923. (7) Noskov, B. A.; Akentiev, A. V.; Miller, R. J. Colloid Interface Sci. 2002, 255, 417. (8) Noskov, B. A.; Akentiev, A. V.; Bilibin, A.Yu; Zorin, I. M.; Miller, R. Adv. Colloid Interface Sci. 2003, 104, 245. (9) Kawaguchi, M.; Hirose, Y.; Kato, T. Langmuir, 1996, 12, 3523. (10) Pelton, R.; Richardson, R. M.; Cosgrove, T.; Ivkov, R. Langmuir 2001, 17, 5118. (11) Zhang, J.; Pelton, R. Colloids Surf., A 1999, 156, 111. (12) Zhang, J.; Pelton, R. Langmuir 1999, 15, 5662. (13) Ward, A. F. H.; Tordai, L. J. Chem. Phys. 1946, 14, 453. 9669 Langmuir 2004, 20, 9669-9676 10.1021/la048836t CCC: $27.50 © 2004 American Chemical Society Published on Web 09/25/2004