Forster energy transfer studies of polyelectrolyte heterostructures containing conjugated polymers: A means to estimate layer interpenetration

Using a sequential adsorption process, thin film multilayer assemblies of polymers which photophysically interact via the Forster energy transfer mechanism have been fabricated and characterized in order to determine the level of interpenetration between layers. The assemblies consisted of layers of poly(phenylene vinylene) (PPV) which were separated from layers of a sulfonatopropoxy anion derivatized poly(p-phenylene) [(-)PPP] by nonconjugated polyelectrolyte spacer bilayers. The spacer bilayers were composed of poly(allylamine hydrochloride) (PAH) with a polyanion of either poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), or poly(styrenesulfonate) (PSS). An estimate of the level of interpenetration of the layers was made for each type of spacer bilayer by correlating the relative amount of quenching of the (-)PPP photoluminescence with the measured total thickness of the spacer bilayer(s) utilizing a diffuse layer model which assumed a Gaussian distribution of polymer segments. Using this approach, the level of interpenetration for the assemblies with the PMA/PAH spacer bilayers was estimated to be between 15 and 53 Angstrom (1-2.5 bilayers). The heterostructure assembly which used spacer bilayers of PAA/PAH demonstrated that one sufficiently thick bilayer (greater than or equal to 57 Angstrom) could prevent the energy transfer from the(-)PPP to the PPV. The failure of the (-)PPP photoluminescence to be fully restored even with eight spacer bilayers (> 53 Angstrom) for the assemblies containing PSS/PAH spacer bilayers indicated that, for the processing conditions used, significant layer mixing was obtained. Overall, this work demonstrated that nonradiative energy transfer offers a valuable tool for probing the internal structure of sequentially adsorbed polyelectrolyte films and that the level of interpenetration appears to be dependent upon the system being examined.