Coil-to-Globule Transition of PNIPAM Graft Copolymers with Charged Side Chains: A 1 H and 2 H NMR and Spin Relaxation Study Mihaela Rusu, † Sebastian Wohlrab, †,‡ Dirk Kuckling, ‡ Helmuth Mo 1 hwald, † and Monika Scho 1 nhoff* ,†,§ Max-Planck-Institute of Colloids and Interfaces, Am Mu ¨hlenberg 1, D-14476 Potsdam-Golm, Germany; Fachrichtung Chemie und Lebensmittelchemie, Technische UniVersita ¨t Dresden, D-01062, Dresden, Germany; and Institute of Physical Chemistry, UniVersity of Mu ¨nster, Corrensstr. 30, D-48149 Mu ¨nster, Germany ReceiVed April 13, 2006; ReVised Manuscript ReceiVed August 1, 2006 ABSTRACT: Graft copolymers of poly(2-vinylpyridine), P2VP, grafted to a backbone of poly(N-isopropyl acrylamide), PNIPAM, are investigated concerning their thermosensitive behavior in aqueous solution. 1 H NMR monitors the coil-to-globule transition by quantifying the fraction of mobile segments in the liquid state. Field gradient NMR diffusion studies reveal a decrease of the hydrodynamic radius with temperature as the lower critical solution temperature (LCST) is approached. The LCST and the width of the transition of PNIPAM are increasing with grafting density and decreasing upon salt addition, which is attributed to electrostatic forces. The grafted segments become partially immobilized only for low grafting density. For high grafting density a fraction of the backbone segments remains mobile even above the LCST. 2 H spin relaxation rates of the hydration water indicate the presence of water molecules with very slow dynamics in the transition regime, whereas above and below the transition only fast water dynamics is found. Introduction Thermosensitive polymers exhibit a coil-to-globule transition, which is of large interest for a number of applications that require the switching of material properties with temperature. Application areas are for example drug delivery, bioseparation, or microfluidics. Examples are the separation of DNA by capillary electrophoresis, 1 enzyme isolation, where a specific technique is used to separate antigens and enzymes, 2 or the buildup of temperature-switchable valves for microscopic fluid devices. 3 Furthermore, thermosensitive polymers are considered as a model to elucidate the role of water molecules in thermal denaturation of natural polymers such as proteins. 4-6 For several applications a hydrophilic/hydrophobic balance of thermosen- sitive polymers shifting the phase transition to values around room temperature or body temperature is desirable. One of the most intensively studied polymers in this field is poly(N-isopropylacrylamide), PNIPAM, which exhibits a sharp coil-to-globule transition in water at an LCST of 32 °C. Raising the temperature above the lower critical solution temperature (LCST), PNIPAM chains in solution undergo a structural transition to insoluble globular particles. The phase transition is accompanied by a release of bound water molecules from the polymer chain, which is an endothermic process. To create functional materials from molecular constituents, modern methods of self-assembly are advantageous; however, they require the introduction of interacting groups, for example, charged segments for the electrostatic layer-by-layer approach. 7-10 Previous investigations of PNIPAM statistical copolymers containing various charged comonomers, however, showed a major influence of charged groups on the phase transition properties: While at low charge density the transition is comparable with that of the homopolymer, the transition vanishes with increasing charge density. 4,11 To minimize the effect of charged comonomers on the LCST, the comonomer structure should resemble that of the NIPAM monomer. Thus, a charged co-PNIPAM, containing 10 mol % acrylamide, bearing carboxyl groups, was analyzed in solution. DSC, turbidimetry, 1 H NMR, and PFG-NMR diffusion showed a phase transition, which was equally sharp as for the homo- polymer. 12-14 Adsorbed at interfaces and subject to geometric restrictions, however, the transition of the same copolymer was very sensitive to the presence of the charged comonomers. 15 In conclusion, statistical copolymers cannot be considered ideal structures for the formation of thermosensitive layers on the basis of elec- trostatic buildup principles. A novel idea of minimizing the charge effects on the transition is based on a spatial separation of the electrostatic function from the thermosensitive function. To achieve this, thermosensitive graft copolymers have been synthesized. 16,17 The new types of copolymers combine the thermosensitive PNIPAM with a pH- sensitive component such as poly(2-vinylpyridine) (P2VP). The idea is to achieve a high flexibility of the main chain even in a stratified 2-dimensional layer arrangement since electrostatic binding can occur via the grafted chains, if they are dissociated. First DSC curves of such copolymers in solution showed a shift of the LCST to higher temperatures with increasing grafting density and in addition a broadening of the transition. 16 Successful formation of multilayers by alternating deposition with a polyanion has already been demonstrated. 18 Similar graft copolymers have also become of interest in studies of the phase transition behavior in solution. A series of recent papers deal with the effect of poly(ethylene oxide) graft chains on the coil-to-globule transition of PNIPAM 19-22 or with hydrophobic modification. 23 In the present work, two thermosensitive graft copolymers with low and high grafting density of P2VP are investigated † Max-Planck-Institute of Colloids and Interfaces. ‡ Technische Universita ¨t Dresden. § University of Mu ¨nster. * Corresponding author: e-mail schoenho@uni-muenster.de, Ph +49- 2518323419, Fax +49-2518329138. 7358 Macromolecules 2006, 39, 7358-7363 10.1021/ma060831a CCC: $33.50 © 2006 American Chemical Society Published on Web 09/19/2006