Molecular Weight Dependence of Polymer Chain Mobility within Multilayer Films Li Xu, † Victor Selin, † Aliaksandr Zhuk, † John F. Ankner, ‡ and Svetlana A. Sukhishvili* ,† † Department of Chemistry, Chemical Biology and Biomedical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States ‡ Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States * S Supporting Information ABSTRACT: Fluorescence recovery after photobleaching has been applied to determine, to our knowledge for the first time, the molecular weight (M w ) dependence of lateral diffusion of polymer chains within layer-by-layer (LbL) films. As shown by neutron reflectometry, polyelectrolyte multilayers containing polymethacrylic acid (PMAA, M w /M n < 1.05) of various molecular weights assembled from solutions of low ionic strengths at pH 4.5, where film growth was linear, showed similar diffusion of PMAA in the direction perpendicular to the film surface. At a salt concentration sufficient for unfreezing electrostatically bonded chains, layer intermixing remained almost unaffected (changes <1.0 nm), while the lateral diffusion coefficient (D) scaled with the PMAA molecular weight as D ∼ M w −1±0.05 . P olyelectrolyte multilayers (PEMs) 1 are multicomponent materials with promising applications for enhanced photoluminescence, 2 improved antireflection coatings, 3 or multistage delivery of therapeutic compounds from surfaces. 4 Many of these applications rely on internal film layering, resulting from the inherently nonequilibrium nature of these films. Indeed, films whose thickness increases linearly with the polyelectrolyte (PE) adsorption cycle during layer-by-layer (LbL) deposition usually have a layered internal structure, arising from multisite, strong sequential adsorption of polymer chains. In the case of weak interpolyelectrolyte interactions, however, chain intermixing can occur during film deposition 5 or postassembly. 6−8 Significant chain mobility can be induced in strongly bound, stratified PEMs by addition of salt and monitored by atomic force microscopy (AFM), 6 fluorescence recovery after photobleaching (FRAP), 7,8 or neutron reflec- tometry (NR). 10−12 Understanding the factors affecting molecular motions within PEM films is critical for potential applications of these films that rely on internal film structuring. Apart from ionic strength, the mobility of polymer chains within multilayer films is also affected by solution pH, 13,14 type of salt, 7 and temperature, 7,9 as well as properties of polymer chains, such as charge density, chain rigidity, hydrophobicity, 7,9 and steric bulk at charged units. 12 Paradoxically, the effect of another fundamental parameterpolymer molecular weight on chain mobility within PEM films is much less under- stood. 13−16 In our earlier work, we have explored molecular weight dependence of the release rate of polyelectrolytes from PEMs induced by pH changes. 14 Very recently, Char and co- workers also reported that changes of film internal structure and the disintegration mode of PEM films in response to pH changes were dependent on the PE molecular weight. 15 An interesting counterintuitive dependence of the vertical diffusion of a polyanion on a polycation molecular weight has been recently found by Helm and co-workers. 16 No experiments have been yet carried out, to the best of our knowledge, to probe molecular weight dependence of lateral motions of polymer chains within PEMs. Lack of knowledge of the fundamental laws behind motions of polymer chains assembled within PEM films is in strong contrast with the abundant literature on polymer diffusion in melts, 17 dilute solutions, 18 and even the less understood, more recently studied diffusion of polymers at the solid−liquid interface. 19 This communication aims to fill this gap by reporting experiments to quantitatively access the role of PE molecular weight on lateral chain mobility within PEM films. By applying the fluorescence recovery after photobleaching (FRAP) technique to stratified LbL films assembled with PEs of varied molecular weights and low polydispersity indices (PDIs), we have determined that the center-of-mass di ffusion coefficient of PE chains in the direction parallel to the substrate, D ∥ , scales with the inverse of the polymer molecular weight. By applying neutron reflectometry (NR) techniques to the same PEM systems exposed to the same solutions, we found that during significant displacement in the lateral direction chain motion in the direction perpendicular to the surface was much more sluggish, suggesting persistent film layering during annealing in salt solutions. PEM films were constructed using poly(2-(dimethylamino)- ethyl methacrylate) as a polycation (PC, M w 30 kDa, M w /M n ≈ 1.10) and polymethacrylic acid (PMAA) with M w /M n 1.02− 1.05 of various molecular weights. PC was synthesized by atom Received: August 6, 2013 Accepted: September 12, 2013 Letter pubs.acs.org/macroletters © XXXX American Chemical Society 865 dx.doi.org/10.1021/mz400413v | ACS Macro Lett. 2013, 2, 865−868