The dense phase of liquid-liquid phase separation (LLPS), namely, coacervates, shows unique characteristics such as low viscosity, low interfacial energy and the ability to encapsulate guest molecules (proteins, enzymes, drugs). Compared to complex polyelectrolyte coacervates, simple non-ionic thermoresponsive coacervates have better tolerance to environmental perturbations, such as change of pH, ionic strength, and biomolecules concentration. These features endow diverse applications of simple non-ionic coacervates, including underwater adhesives, drug/enzyme encapsulation, and bioreactors. Despite the growing applications of simple coacervates, the molecular level mechanism behind the simple non-ionic coacervation is not well understood. Therefore, we used non-ionic polyamide (polyoxazoline) and poly(ester amide) (poly(N-acylated-1,4-oxazepan-7-ones)) with lower critical solution temperature (LCST) behaviors as model polymers to shed light on molecular interaction governing coacervation of simple non-ionic polymer. Fourier transform infrared spectroscopy (FTIR) indicates that the coacervation was a multistep process and initiated by losing water surrounding tertiary amide/ester, followed by the formation of hydrogen bonding between tertiary amide/ester and water, eventually by partially dehydration of the hydrophobic group (CH2 and CH3). Surprisingly, we find that the emergence of interchain C=O—D-O-D—O=C cooperative hydrogen bonding with temperature elevation in coacervate systems. However, this interaction is absent in the dilute phase of coacervation and the phase separation process of poly(N-isopropyl acrylamide). In this context, the presence of interchain hydrogen bonding stabilizes the coacervates according to FTIR experiments. These findings provide molecular insight into the simple coacervation of thermoresponsive polymer and offer guidelines to tune the physical properties of simple coacervates.

Interchain hydrogen bonding drives the coacervation of non-ionic polyamides and poly(ester amide)s

ACS Spring 2022

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