Salt-Induced Polyelectrolyte Interdiffusion in Multilayered Films: A Neutron Reflectivity Study Houssam W. Jomaa and Joseph B. Schlenoff* Department of Chemistry & Biochemistry, and Center for Materials Research and Technology (MARTECH), The Florida State University, Tallahassee, Florida 32306 Received January 12, 2005; Revised Manuscript Received June 13, 2005 ABSTRACT: Polyelectrolyte multilayers were constructed from poly(styrenesulfonate), PSS, and poly- (diallyldimethylammonium) with regularly interspersed layers of deuterated PSS. Annealing, by salt, of the fuzzy internal layering within this multilayer was followed using neutron reflectometry. A “limited source” diffusion model fit the data well and showed that polyelectrolyte migrates much more slowly within the bulk of a multilayer than at the surface. Enhanced surface mobility and a nonlinear increase in diffusion rate with salt concentration were explained by a salt doping model, where correlated short lengths of polyelectrolyte move by exchange with counterions (“extrinsic charge”) doped into the ultrathin film of polyelectrolyte complex on exposure to solutions of high ionic strength. Introduction Since the boundaries between nominal “layers” in polyelectrolyte multilayers, PEMUs, are somewhat dif- fuse, due to interpenetration between positive and negative components, these thin films of complexed polyelectrolytes tend to be amorphous. 1,2 However, because polyelectrolyte mobility has a range limited to a few nanometers, it is possible to assemble chemically or isotopically distinct layers separated by several layers of different composition. 3 Labeling of sulfonated poly- electrolyte by deuterium, for example, has permitted a detailed perspective on the “fuzzy” nature of PEMU structuring. 4-6 The linear, layer-by-layer, growth mode of PEMUs, though widely appreciated and exploited, is an inher- ently nonequilibrium process, yielding nonequilibrium structure. 7-9 In the adsorption process itself, individual molecules attach to the surface of a growing PEMU via many ion pairing contacts. 10 Unless the polyelectrolyte molecules are small, or weakly interacting (as might be the case in the presence of high salt concentration), the adsorption is kinetically irreversible on the time scale of the PEMU assembly. 10,11 Because the thickness increment (“layer” thickness) quickly becomes indepen- dent of total thickness, it is reasonably assumed that the adsorbing polyelectrolyte becomes locked in place, notwithstanding some short-range interdiffusion. Re- cent work has challenged the notion of frozen or im- mobile polyelectrolytes within multilayers. For example, nonlinear or “exponential” growth of multilayers 12-19 is evidence that one or more polyelectrolytes is able to diffuse throughout the film, rather than being held static on a local level. Exchange of multilayer for solution polyelectrolyte is further proof of mobility within multilayers capable of exponential growth. 16 Large scale rearrangements within multilayers, from phase separation 20 to decomposition, 21-24 may be in- duced by changing the interactions between polymers with variations of ionic strength or pH (if the polymer is a weak acid/base). In recent atomic force microscopy (AFM) work, 25 we showed that swelling of PEMUs by salt frees polyelectrolyte segments and allows polymer interdiffusion, leading to smoothing of the surface. The AFM work also recognized the strong nonlinear depen- dence of interdiffusion on degree of swelling, or “doping”, by salt ions. 25 The introduction of salt counterions, or “extrinsic charge”, 26,27 within PEMUs is represented by the following equilibrium: 28 where Pol + and Pol - are respective positive and negative polyelectrolyte repeat units. y is the fraction of the multilayer in the extrinsic form, and 1 - y is the intrinsic fraction. The subscript “m” refers to compo- nents in the multilayer phase. While it is clear that salt moderates interpolyelectrolyte interactions, and there- fore mobility, 25,29 as has been known for some time for both solution-precipitated polyelectrolyte complexes (PECs) 30-32 and polyelectrolyte adsorption to charged surfaces, 33,34 salt-controlled mobility of polyelectrolyte within the bulk of a PEMU remains largely unexplored. AFM measurements of PEMU topology only reveal salt- induced migration of polyelectrolyte on the surface 25,35 (which is, as will be shown below, quite distinct from bulk interdiffusion). The principal tools for evaluating layering within PEMUs have been neutron or X-ray structural stud- ies. 2-6,36-42 For example, Lo ¨sche et al. described a de- tailed neutron reflectometry study of poly(styrenesul- fonate)/poly(allylamine), PSS/PAH, multilayers. 6 In this system, deuterated PSS layers were interspersed with nondeuterated layers in a regular fashion during the multilayering process. 6 Estimates were made of the range over which a nominal layer of polyelectrolyte was distributed. It was shown that neighboring layers in- terpenetrate, leading to an amorphous structure, unless deuterated layers were separated by a sufficient number of nondeuterated layers. 6 While deposition conditions were varied to obtain a range of fuzzy layer thicknesses, postdeposition mobility within PEMUs was not evalu- ated. In the present paper, we present neutron reflec- * Corresponding author: Fax (850)644-3810, e-mail: schlen@chemmail.chem.fsu.edu. Pol + Pol m - + yNa aq + + yCl aq - h (1 - y)Pol + Pol m - + yPol + Cl m - + yPol - Na m + (1) 8473 Macromolecules 2005, 38, 8473-8480 10.1021/ma050072g CCC: $30.25 © 2005 American Chemical Society Published on Web 09/02/2005