Polyelectrolyte uptake by PEMs: Impacts of molecular weight and counterion Xingjie Zan a , David A. Hoagland b , Tian Wang a , Bo Peng a , Zhaohui Su a, * a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China b Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003, USA article info Article history: Received 22 April 2012 Received in revised form 27 August 2012 Accepted 3 September 2012 Available online 7 September 2012 Keywords: Polyelectrolyte multilayers Polyelectrolyte Diffusion abstract Presented with an elevated level of aqueous salt, a polyelectrolyte multilayer (PEM) swells and subse- quently can undergo large mass uptake when exposed to a salt solution containing the capping poly- electrolyte. Features of the uptake depend on the properties of polyelectrolyte (molecular weight, polydispersity, concentration), solvent (salt concentration and ion identities), and PEM (number of layers, polyelectrolyte identities). Here, poly(diallyldimethylammonium chloride)/poly(styrene sulfonate) (PSS) PEMs capped with PSS are investigated by Quartz Crystal Microbalance with Dissipation during successive challenges by salt and PSS. Only when the salt level is substantially higher than at PEM construction, leading to large PEM swelling, does PSS uptake become significant. Steady state uptake grows and then levels off with PSS molecular weight; the crossover to molecular weight independence falls with salt concentration. Across the dilute PSS range, uptake mechanisms are not affected by solution concentration, although at very dilute concentration, the rate limiting process becomes diffusion in solution rather than in PEM. Swelling of PEM by salt, and hence rate of uptake and ultimate uptake, correlate with the salt anion’s position in the Hofmeister series. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The uptake of polyelectrolytes by permeable substrates such as porous glasses [1], crosslinked networks [2,3], and fibers [4e6] is central to a range of technologies, including chromatography, paper fabrication, and controlled release [1,7,8]. Subtrate permeability distinguishes polyelectrolyte uptake from polyelectrolyte adsorp- tion, but locally, the interactions controlling the two phenomena are similar. In both cases, the capture of polyelectrolyte by charged substrate corresponds to a cooperative ion exchange reaction that forms salt bonds between polyelectrolyte and substrate as small counterions are released. While the release affords a major ther- modynamic impetus, uptake nonetheless reflects a delicate balance of electrostatic (and possibly other) actions, some favorable and some disfavorable [9]. For example, the increase of polyelectrolyte charge density promotes attraction to substrate, favoring uptake, but the increase also causes polyelectrolyte stiffening, disfavoring uptake. Such trade-offs, and difficulty in distinguishing between thermodynamic and kinetic control, makes outcomes difficult to predict. Consequently many experiments and theories have been developed to explain the influence of ionic strength, molecular weight, and charge density. Self-consistent mean-field theories [9,10], for example, predict that uptake decreases with ionic strength due to the reduced polyelectrolyte-surface electrostatic attraction under enhanced electrostatic screening. The same theories indicate that surface overcharging emerges in the “screening enhanced regime” if a non-electrostatic interaction between polyelectrolyte and surface is present [11]. It has also been proposed that overcharging occurs due to lateral correlations between attached polyelectrolytes [11e 13]. The ratio of pore size to polyelectrolyte coil size is another crucial factor, affected not just by the polyelectrolyte’s molecular weight but also by its charge density and concentration. Several interactions are possible when a polyelectrolyte multi- layer (PEM) constructed by the layer-by-layer (LbL) deposition method is challenged with a concentrated salt solution containing similarly charged polyelectrolyte. In one, attractive (non- electrostatic) interactions between polyelectrolyte and PEM cause an initial PEM capture of the polyelectrolyte, the captured poly- electrolyte subsequently diffusing through the PEM’s outer layers to find the first available region of opposite charge, there becoming fixed [4e6]. In another, starting with the same attachment step, polyelectrolytes distribute across the PEM through a “relay race” mechanism in which the polyelectrolytes displace inward by a cascade of inter-polyelectrolyte exchange reactions [3]. Polyelectrolyte uptake by like-charged soluble polyelectrolyte * Corresponding author. Tel.: þ86 431 85262854; fax: þ86 431 85262126. E-mail addresses: xjzan2000@hotmail.com (X. Zan), hoagland@ mail.pse.umass.edu (D.A. Hoagland), wangtian@ciac.jl.cn (T. Wang), pengbo@ ciac.jl.cn (B. Peng), zhsu@ciac.jl.cn (Z. Su). Contents lists available at SciVerse ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.09.011 Polymer 53 (2012) 5109e5115