Gelation and Re-entrance in Mixtures of Soft Colloids and Linear Polymers of Equal Size

Abstract
Liquid mixtures composed of colloidal particles and much smaller non-adsorbing linear homopolymers can undergo a gelation transition due to polymer-mediated depletion forces. We now show that the addition of linear polymers to suspensions of soft colloids having the same hydrodynamic size yields a liquid-to-gel-to-re-entrant liquid transition. In particular, the dynamic state diagram of 1,4-polybutadiene star-linear polymer mixtures was determined with the help of linear viscoelastic and small-angle X-ray scattering experiments. While keeping the star polymers below their nominal overlap concentration, a gel was formed upon increasing the linear polymer content. Further addition of linear chains yielded a re-entrant liquid. This unexpected behavior was rationalized by the interplay of three possible phenomena: (i) depletion interactions, driven by the size disparity between the stars and the polymer length scale which is the mesh size of its entanglement network; (ii) colloidal deswelling due to the increased osmotic pressure exerted onto the stars; and (iii) a concomitant progressive suppression of the depletion efficiency on increasing the polymer concentration due to reduced mesh size, hence a smaller range of attraction. Our results unveil an exciting new way to tailor the flow of soft colloids and highlight a largely unexplored path to engineer soft colloidal mixtures.

Citation
Parisi, D., Truzzolillo, D., Slim, A. H., Dieudonné-George, P., Narayanan, S., Conrad, J. C., Deepak, V. D., Gauthier, M., & Vlassopoulos, D. (2023). Gelation and Re-entrance in Mixtures of Soft Colloids and Linear Polymers of Equal Size. Macromolecules. https://doi.org/10.1021/acs.macromol.2c02491

Acknowledgements
We would like to thank Professor Nikos Hadjichristidis (KAUST) for providing a linear polymer used in this work and Dr. Julian Oberdisse for fruitful discussions. This research was partly funded by the EU [European Training Network Colldense (H2020-MCSA-ITN-2014), grant number 642774]. A.S. and J.C.C. acknowledge support from the Welch Foundation (E-1869) and the US National Science Foundation (CBET-2004652). This research used resources of the Advanced Photon Source, a user facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.

Publisher
American Chemical Society (ACS)

Journal
Macromolecules

DOI
10.1021/acs.macromol.2c02491

Additional Links
https://pubs.acs.org/doi/10.1021/acs.macromol.2c02491

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