Weak polyelectrolyte multilayers (PEMs) prepared by the layer-by-layer assembly technique have recently been found to demonstrate a unique contraction upon exposure to organic solvents. This response is dependent upon which organic solvent is employed, and fundamental questions have not been clarified regarding the correlation of the magnitude of the film contraction with solvent type. In this work, we used solubility parameters to analyze the response of branched poly(ethylene imine)/poly(acrylic acid) (BPEI/PAA) multilayers when exposed to a variety of solvents. BPEI/PAA multilayers were immersed in a series of 16 different organic solvents and solvent mixtures. Immersion in organic solvent caused film dehydration and therefore contraction and also induced changes in the mechanical properties of PEMs. The film thickness was the best predictor of how a film swelled in water or contracted in organic solvent when using different batches of commercially available polyelectrolytes, rather than polyelectrolyte assembly pH conditions. The degree of film contraction was correlated with Hansen and Kamlet-Taft solubility parameters as well as solvent dielectric constant. In most cases, the hydrogen bonding ability of solvents is the primary factor to determine the magnitude of film contraction. For these solvents, increasing the temperature which corresponds to decreasing the strength of hydrogen bonding, also decreases the ability to dehydrate the films. For solvents that do not follow these trends with the strength of hydrogen bonding, a stronger correlation was found between contraction and dielectric constant, indicating that both traditional solvent quality arguments and electrostatics are important to understanding the contraction of PEMs in organic solvents.

The immersion of poly(n-propyl methacrylate), PPMA, films (ca., 425 nm) in water induces swelling that is measured in-situ using spectroscopic ellipsometry. Unexpectedly, the end group of the PPMA resulting from the reversible addition-fragmentation chain transfer (RAFT) polymerization impacts the temperature dependence of swelling, despite their relatively high molecular weights (ca., 30 kDa). At 25 8C, dithiobenzoate terminated PPMA (PPMA-DB) leads to significantly less swelling (5.6 vol %) than the dodecylsulfanylthiocarbonyl terminated PPMA (PPMA-DD, 9.0 vol %). These PPMA films swell significantly more than expected due to a common carboxylic acid end group. As temperature is increased, the swelling for PPMA-DB increases and that for PPMA-DD decreases, with a crossover at approximately 35 8C-40 8C where the swelling is indistinguishable between the two polymers. The swelling kinetics exhibit two stages: an initial rapid swelling within the first minute of immersion and then a slow increase in thickness over multiple hours. The water contact angle of PPMA-DB increases on heating, while the water contact angle of PPMA-DD is invariant. This difference in the temperature dependence of the hydrophobicity is consistent with that for swelling. These results illustrate the potential unexpected consequences of residual RAFT fragment end groups on physical properties of polymers even at relatively high molecular weights.