Odd–Even Effect of Local Chain Dynamics in Polyelectrolyte Multilayers

A spin label (SL) has been covalently attached to the weak polyanion poly(ethylene-alt-maleic acid) (P(E-alt-MA)) to study the dynamics of the polymer backbone in polyelectrolyte multilayers (PEMs) formed with the oppositely charged polycations branched poly(ethylenimine) (bPEI) and poly(allylamine hydrochloride) (PAH) by electron paramagnetic resonance (EPR) spectroscopy as a function of the number of layers N. The line shape of the spectra shows the superimposition of two spectral components: A slowly tumbling SL as the main component and an SL in the fast-motion regime as a minor component. The main component is assigned to chain segments of the polyanion interacting with the chain segments of the polycation. Simulations of the slow-motion spectra revealed that a pronounced odd–even effect occurred, i.e., the rotational dynamics of the P(E-alt-MA) chain segments is influenced by the polyelectrolyte in the terminating layer. The rotational mobility of the chain segments of the polyanion interacting with the oppositely charged bPEI is higher for odd number of layers N, i.e., when the polycation is on top, compared to even N. The dependence of the rotational dynamics on the number of layers N can also be observed in PEMs with PAH. It is assumed that the odd–even effect on rotational mobility is related to extrinsic sites within the PEMs. A probably large number of extrinsic sites in PEMs with an odd number of layers reduces the number of intrinsic binding sites, i.e., the density of temporary cross-links between the oppositely charged molecules decreases, which leads to increased rotational dynamics of the polyanion chain segments.