Influence of Polymer Molecular Weight on the Parabolic and Linear Growth Regime of PDADMAC/PSS Multilayers

The buildup of polyelectrolyte multilayers is investigated in solution with multiple angle null-ellipsometry. Polyanion poly(styrenesulfonate) (PSS) and polycation poly(diallyldimethylammonium) (PDADMAC) are adsorbed sequentially from 0.1 M NaCl solution. First the films grow parabolically. After N trans deposited PDADMAC/PSS layer pairs a transition from a parabolic to a linear growth occurs. For molecular weights above a threshold (M w(PSS) > 25 kDa and M w(PDADMAC) > 80 kDa), N trans is 15, the thickness per layer pair in the linear growth regime is 12.3 nm. If either the PDADMAC or the PSS molecular weight is decreased below the threshold value, N trans either falls (for PDADMAC, lowest value observed is 8) or rises (for PSS, highest value observed is 33), respectively. Simultaneously, in the linear growth regime, the thickness per layer pair decreases (down to 4.3 nm) or rises (up to 25 nm). Furthermore, for low molecular weight PSS, three growth regimes are observed: exponential, parabolic, and linear. The opposite effect of PDADMAC and PSS molecular weight reduction is discussed in terms of persistence lengths and linear charge density. The data suggest that molecular weight provides a way to control growth and internal structure of polyelectrolyte multilayers.