Blending a basic
polymer (e.g., chitosan) with Nafion
can modify
some membrane properties in direct methanol fuel cell applications,
e.g., controlling methanol crossover, by regulating the morphology
of hydrophilic channels. Unraveling the mechanisms by which the channel
morphology is modified is essential to formulate design strategies
for acid–base blend membrane development. Thus, we use molecular
simulations to analyze the morphological features of a blend membrane
(at 75/25 chitosan/Nafion wt %), i.e., (i) water/polymer phase organizations,
(ii) number and size of water clusters, and (iii) quantitative morphological
measures of hydrophilic channels, and compare them to the pure Nafion
in a wide range of water contents. It is found that the affinity of
water to different hydrophilic groups in the blend membrane can result
in more distorted and dispersed hydrophilic phase and fewer bulk water-like
features compared to pure Nafion. Also, the width of the hydrophilic
network bottleneck, i.e., pore limiting diameter (PLD), is found to
be almost five times smaller for the blend membrane compared to Nafion
at their maximum water contents. Moreover, by changing the chitosan/Nafion
weight ratio from 75/25 to 0/100, we show that as Nafion content increases,
all channel morphological characteristics alter monotonically except
PLD. This is mainly due to the strong acid–base interactions
between Nafion and chitosan, which hinder the monotonic growth of
PLD. Interestingly, water and methanol diffusion coefficients are
strongly correlated with PLD, suggesting that PLD can be used as a
single parameter for tailoring the blending ratio for achieving the
desired diffusion properties of acid–base membranes.