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Polyzwitterion fast and slow mode behavior are coupled to phase separation as observed by dynamic laser light scattering



Phillip Pickett, Yuanchi Ma, Vivek Prabhu


A model zwitterionic polysulfobetaine, poly(3-(acrylamidopropyl-dimethyl-ammonium) propyl-1-sulfonate) (pAPAPS), phase separates upon cooling with upper critical solution temperature (UCST) behavior with no added salt in deuterium oxide solutions. Measurements of the collective diffusion coefficient by dynamic laser light scattering, reveals previously observed slow and fast diffusive modes, where the fast mode reflects chain diffusion, and the slow mode arises from the diffusion of multi-chain dynamic clusters. Below the overlap concentration a dynamic equilibrium between the fast and slow mode appears when studied systematically as a function of temperature. An association temperature was assigned when the slow mode appears and increases in scattering amplitude as the phase boundary is approached. The fast mode exhibits a concentration dependence above the association temperature (T*) that follows a well-known virial expansion in the collective diffusion where the sign of the first virial coefficient (kd) appears negative, even within the expected good solvent region above the expected Flory temperature (  39 °C), and with an inverse temperature dependence expected by mean field theory. Therefore, the onset of the rise in multichain clustering at T < T* appears as the solution transitions to a poor solvent domain (T < ). The short-range attractive dipolar interactions due to the sulfobetaine groups in pAPAPS are speculated to lead to the multichain clusters. The addition of 100 mM NaCl leads to a single diffusive mode quantitatively consistent with polyzwitterion chain dimensions in dilute solution and shifts the UCST out of the measurement range due to good solvent behavior. The apparent dynamic equilibrium between the slow and fast modes within the one-phase region are discussed in the context of the proximity to the measured phase diagram. We find that the concentration dependent diffusion is highly linked to the theta temperature and the emergence of dynamic clusters as the polymer goes from good to poor solvent.
The Journal of Chemical Physics


poly(sulfobetaine), Zwitterionic polymers, polymer dynamics, UCST, temperature-responsive polymers


Pickett, P. , Ma, Y. and Prabhu, V. (2023), Polyzwitterion fast and slow mode behavior are coupled to phase separation as observed by dynamic laser light scattering, The Journal of Chemical Physics, [online],, (Accessed April 24, 2024)
Created September 11, 2023, Updated September 12, 2023