NMR Investigations into Heterogeneous Structures of Thermosensitive Microgel Particles A. GUILLERMO, 1 J. P. COHEN ADDAD, 1 J. P. BAZILE, 1 D. DURACHER, 2 A. ELAISSARI, 2 C. PICHOT 2 1 Laboratoire de Spectrome ´trie Physique, CNRS UMR C5588, Universite ´ Joseph-Fourier, Grenoble-I, BP 87, 38402 Saint Martin dHe `res Cedex, France 2 Unite ´ Mixte, CNRS-BioMe ´rieux, ENS de Lyon, 46 Alle ´e d’Italie, 69364 Lyon Cedex 07, France Received 28 July 1999; revised 20 December 1999; accepted 22 December 1999 ABSTRACT: The internal properties of submicron poly(N-isopropylmethacrylamide) la- tex particles were investigated as a function of the methylene bisacrylamide (MBA) concentration used as a crosslinker. Two experimental approaches were performed. First, quasi-electric light scattering measurements provided the size variation of the particles as a function of temperature, from which the swelling capacity of the particles as a function of MBA were estimated. In addition, the broadening and lowering effects of the volume phase transition temperature were detected from the turbidity of the solutions versus the MBA concentration. Second, observations of the transverse relax- ation of protons gave evidence for heterogeneous structures inside the particles; several structural parts were discriminated from one another from different proton mobilities detected through magnetic relaxation rates. Corresponding to the concentration gradi- ents of the crosslinker, the internal particle structures were looser and looser from the core to the shell. The state of the gelation of the polymer particles was governed by the initial amount of the crosslinker introduced into the latex recipe. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 889 – 898, 2000 Keywords: poly(N-isopropylmethacrylamide) latex particles; NMR; light scattering INTRODUCTION The synthesis and characterization of thermosen- sitive microgel latex particles have received in- creasing attention as illustrated by numerous re- cently published studies. Thermosensitive micro- spheres are mainly used in two domains. In the field of wastewater treatment, poly(N-isopropyl- acrylamide) [poly(NIPAM)] microgel particles are applied to the elimination of heavy metal ions, a procedure based on complexation processes. 1,2 In the biomedical domain, the thermosensitive prop- erty of such particles can be used for concentrat- ing proteins; Kawaguchi et al. 3,4 showed that these biomolecules could be adsorbed onto or de- sorbed from poly(NIPAM) particles, whether the temperature was greater than or less than the volume phase transition temperature ( T VPT ), re- spectively. In addition, poly(NIPAM) particles bearing cationic groups have also been used for specific extractions of nucleic acids (DNA and RNA) from biological systems through the control of pH, ionic strength, and temperature variations, as reported by Elaissari et al. 5 Due to their ver- satility in many applications, numerous studies have been dedicated to the colloidal characteriza- tion of these stimuli-responsive latexes, mostly with poly(NIPAM)-based particles. Most studies reported on the colloidal stability, 6 electrokinet- ics, 7 and swelling behavior 8,9 observed as a func- tion of temperature. In contrast, only a few exper- Correspondence to: J. P. Cohen-Addad (E-mail: jean-pierre. cohen-addad@ujf-grenoble.fr) Journal of Polymer Science: Part B: Polymer Physics, Vol. 38, 889 – 898 (2000) © 2000 John Wiley & Sons, Inc. 889