ma6b00742_si_001.pdf (4.53 MB)
Effect of Water on the Thermal Transition Observed in Poly(allylamine hydrochloride)–Poly(acrylic acid) Complexes
journal contribution
posted on 2016-09-30, 17:55 authored by Yanpu Zhang, Fei Li, Luis D. Valenzuela, Maria Sammalkorpi, Jodie L. LutkenhausHere,
we present the thermal behavior of polyelectrolyte complexes
(PECs) containing weak polyelectrolytes poly(allylamine hydrochloride)
(PAH) and poly(acrylic acid) (PAA) as influenced by water content
and complexation pH. Modulated differential scanning calorimetry (MDSC)
reveals a glass-transition-like thermal transition (Ttr) that decreases in value with increasing hydration
and with decreasing complexation pH. We show the collapse of all Ttr values into a single master curve when plotted
against the ratio of water molecules per intrinsic (PAH+–PAA–) ion pair for all pH values explored.
Remarkably, this master curve is linear when the natural log of the
water to intrinsic ion pair ratio is plotted against the inverse of Ttr. This strongly indicates that the thermal
transition is heavily influenced by water at the intrinsic ion pair
site. Other water–solvent mixtures are also explored, for which Ttr appears to depend only on water content,
regardless of the added solvent. These results suggest that water
plays a dual role in PAH–PAA complex: first by participating
in the hydrogen-bonding network within and second by plasticizing
the PEC. A hypothesis for the thermal transition is proposed in which
hydrated PECs undergo a two-step thermal transition caused by an initial
restructuring of the water–polyelectrolyte hydrogen-bonding
network, followed by chain relaxation.