Evidence for spin coating electrostatic self-assembly of polyelectrolytes

The influence of molecular mass on the formation of polyelectrolyte multilayers on an oppositely charged surface of a rotating substrate was explored for a combination of strong cationic and anionic polyelectrolytes, such as poly(1-(N-benzylpyridinio-2-yl)ethylene bromide) (PVP-2B) and i-carrageenan. UV/visible spectroscopy and ellipsometry measurements confirmed that the amount of material deposited on a substrate is inversely proportional to the logarithm of the molecular weight of PVP-2B at a low concentration of 1 mM in the spin-coated as well as the solution-dipped multilayer assemblies. A quantitative evaluation of the data shows that an increase of the molecular weight M-w of PVP-2B from 3.8K to 6.0M leads to 50 and 23% decreases in the average amount and thickness per bilayer deposited using the spin coating electrostatic self-assembly technique, respectively. Studies were also carried out to determine the effect of polymer concentration and spin speed on the adsorption rate of high molecular weight (HMW) and low molecular weight (LMW) polyelectrolytes. Increase of the spin speed leads to almost the same decrease in the average deposition rate of LMW and HMW polymers on a substrate. In sharp contrast, increasing the concentration of the polymer solution causes a higher increase in the average amount of the layer pair for HMW polyelectrolytes than for LMW. It was also found that there exists a critical concentration in which an equivalent amount of polymer is adsorbed on a solid substrate for two different HMW and LMW polymers; more LMW polyelectrolyte is adsorbed until a critical concentration, above which the trends are reversed. The observation would indicate the fact that the spin coating is less and less determined by the self-assembly process with increasing the concentration, causing enhanced nonspecific interactions among polymer chains. Particularly, some evidence is found for the self-assembly process that plays an essential role in the formation of polyelectrolyte multilayers on a rotating substrate.