posted on 2024-02-22, 17:34authored byChikaodinaka
I. Eneh, Kevin Nixon, Suvesh Manoj Lalwani, Maria Sammalkorpi, Piotr Batys, Jodie L. Lutkenhaus
The coacervation
and complexation of oppositely charged
polyelectrolytes
are dependent on numerous environmental and preparatory factors, but
temperature is often overlooked. Temperature effects remain unclear
because the temperature dependence of both the dielectric constant
and polymer–solvent interaction parameter can yield lower and/or
upper critical solution phase behaviors for PECs. Further, secondary
interactions, such as hydrogen bonding, can affect the temperature
response of a PEC. That is, mixtures of oppositely charged polyelectrolytes
can exhibit phase separation upon lowering and/or increasing the mixture’s
temperature. Here, the phase behavior of poly(diallylmethylammonium)/poly(acrylic
acid) (PDADMA/PAA) complexes under varying KBr ionic strengths, mixing
ratios, and temperatures at a fixed pH (in which PAA hydrogen bonding
can occur) is examined. At room temperature, the PDADMA/PAA PECs exhibit
four different phase states: precipitate, coexisting precipitate and
coacervate, solid-like gel, and coacervate. Variable-temperature optical
microscopy reveals the upper critical solution temperature (UCST)
at which each phase transitioned to a solution state. Interestingly,
the UCST value is highly dependent on the original phase of the PEC,
in which solid-like precipitates exhibit higher UCST values. Large-scale
all-atom molecular dynamics (MD) simulations support that precipitates
exhibit kinetic trapping, which may contribute to the higher UCST
values observed in the experiment. Taken together, this study highlights
the significance of temperature on the phase behavior of PECs, which
may play a larger role in stimuli-responsive materials, membraneless
organelles, and separations applications.