Influence of Polymer Molecular Weight on the Parabolic
and Linear Growth Regime of PDADMAC/PSS Multilayers
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Abstract
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 <i>N</i><sub>trans</sub> deposited PDADMAC/PSS layer
pairs a transition from a parabolic to a linear growth occurs. For
molecular weights above a threshold (<i>M</i><sub>w</sub>(PSS) > 25 kDa and <i>M</i><sub>w</sub>(PDADMAC) >
80 kDa), <i>N</i><sub>trans</sub> 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, <i>N</i><sub>trans</sub> 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