A Perspective on the Glass Transition and the Dynamics of Polyelectrolyte Multilayers and Complexes

Abstract

Polyelectrolyte multilayers (PEMs) or polyelectrolyte complexes (PECs), formed by layer-by-layer assembly or the mixing of oppositely charged polyelectrolytes (PEs) in aqueous solution, respectively, have potential applications in health, energy, and the environment. PEMs and PECs are very tunable because their structure and properties are influenced by factors such as pH, ionic strength, salt type, humidity, and temperature. Therefore, it is increasingly important to understand how these factors affect PECs and PEMs on a molecular level. In this Feature Article, we summarize our contributions to the field in the development of approaches to quantify the swelling, thermal properties, and dynamic mechanical properties of PEMs and PECs. First, the role of water as a plasticizer and in the glass-transition temperature (T g) in both strong poly­(diallyl­dimethyl­ammonium)/poly­(sodium 4-styrene­sulfonate) (PDADMA/PSS) and weak poly­(allylamine hydrochloride)/poly­(acrylic acid) (PAH/PAA) systems is presented. Then, factors influencing the dynamics of PECs and PEMs are discussed. We also reflect on the swelling of PEMs in response to different salts and solvent additives. Last, the nature of water’s microenvironment in PEMs/PECs is discussed. A special emphasis is placed on experimental techniques, along with molecular simulations. Taken together, this review presents an outlook and offers recommendations for future research directions, such as studying the additional effects of hydrogen-bonding hydrophobic interactions.

Counter Anion Type Influences the Glass Transition Temperature of Polyelectrolyte Complexes

“…The average enthalpy was 10.9 ± 0.5 kJ/mol, which was close to the enthalpy associated with a single water hydrogen bond (10.5 ± 2.5 kJ/mol) . This highlights the role played by water in plasticizing the PECs and further supports the theory proposed by us that T g in PECs is driven by insertion of water molecules …”

“…As mentioned before, we proposed that the T g in PECs was driven by the insertion of water into the intrinsic ion pair’s hydration environment . If we consider this insertion as the rate determining step, a lower T g should be observed for PECs exhibiting higher water diffusion because there will be more opportunities for water molecules to interact with the intrinsic ion pair.…”

“…We speculated that "a T g -like event occurs when there exists a sufficient fraction of briefly separated intrinsic ion pairs, f (f = x/[IP])". 52 Combining this with eq 2 and the van't Hoff equation yielded…”

“…The equilibrium constant between the intrinsic ion pair and the briefly separated intrinsic ion pair is given by K eq K normale normalq = x 2 ( false[ normalI normalP false] − x ) ( false[ H 2 normalO false] − x ) where [IP] and [H 2 O] represent the concentration of intrinsic ion pairs and water molecules, respectively, and x represents the change in the concentration of intrinsic ion pair due to the insertion process. We speculated that “a T g -like event occurs when there exists a sufficient fraction of briefly separated intrinsic ion pairs, f ( f = x /[IP])” . Combining this with eq and the van’t Hoff equation yielded ln ( [ I P ] [ normalH 2 O ] ) = prefix− normalΔ H R ( 1 T g ) + normalΔ S R − ln ( f 2 1 − f ) where Δ H and Δ S represent the respective enthalpy and entropy changes.…”

“…In our recent work, 52 we explained the significance of eq 1 based on the insertion of a water molecule into the hydration shell of an intrinsic ion pair, which may lead to a brief separation or weakening of the intrinsic ion pair. This idea has parallels to the hypothesis 53 that thermal transitions in PECs can result from changes in contact and solvent-separated ion pairs.…”