Citation: Robin, C.; Lorthioir, C.; Fall, A.; Ovarlez, G.; Amiel, C.; Le Coeur, C. Unexpected Slow Kinetics of Poly(Methacrylic Acid) Phase Separation in the Semi-Dilute Regime. Polymers 2022, 14, 4708. https://doi.org/10.3390/ polym14214708 Academic Editors: Cristian-Dragos Varganici and Dagmar R. D’hooge Received: 28 September 2022 Accepted: 27 October 2022 Published: 3 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Unexpected Slow Kinetics of Poly(Methacrylic Acid) Phase Separation in the Semi-Dilute Regime Clément Robin 1 ,Cédric Lorthioir 2 , Abdoulaye Fall 3 , Guillaume Ovarlez 4 , Catherine Amiel 1 and Clémence Le Coeur 1,5, * 1 Institut Chimie et Materiaux Paris Est, Université Paris Est Créteil, CNRS, 2 Rue Henri Dunant, 94320 Thiais, France 2 Laboratoire de Chimie de la Matiere Condensee de Paris, Sorbonne Universite, Cnrs, College de France, 4 Place Jussieu, 75005 Paris, France 3 Laboratoire Navier, UMR 8205-Université Gustave Eiffel, Ecole des Ponts, CNRS, 77420 Champs sur Marne, France 4 LOF UMR 5258 (CNRS-Solvay-Université de Bordeaux) 178, Avenue du Dr Schweitzer, 33608 Pessac, France 5 LaboratoireLeon Brillouin, Cea-Cnrs (Umr-12), Cea Saclay, Universite Paris-Saclay, 91191 Gif-Sur-Yvette, France * Correspondence: clemence.le-coeur@cnrs.fr Abstract: Poly (methacrylic acid) (PMAA) solutions are known to exhibit a lower critical solution temperature (LCST). A temperature-composition phase diagram of PMAA has been constructed by standard cloud point determination through transmittance measurements, and also by studying the steady states reached under phase separation. This allows us to reconstruct the binodal curve describing the phase behavior of PMAA for both low and high concentration regimes, and to determine accurately the LCST temperature. In a second step, the structures formed following a temperature jump above the cloud point and their evolution in time have been investigated at the nanoscale using small angle neutron scattering (SANS). This approach shows that the formation of phase-separated nanostructures is a slow process, requiring more than 12 h. The formed structures are then shown to depend on the amplitude of the temperature jump above the cloud point. An original mechanism of phase separation is identified in the semi-dilute regime. The growth of micrometric-size droplets with an inner structure displaying the rheological properties of a gel leads to the formation of a percolating network which hinders the influence of gravity. Such a result can explain the slow kinetics of the PMAA LCST transition. Keywords: poly(methacrylic-acid); LCST polymer; SANS 1. Introduction In the past few years, stimuli-responsive (temperature, pH, ionic strength, pressure) polymers have drawn considerable interest due to their potential for a wide range of appli- cations. Such polymers can be used in biomedical applications such as on-demand drug delivery [1–7], diagnostics [4], bioadhesion [8–12] or tissue engineering [3,13,14] due to their thermally reversible coil-to-globule transition. Polymers with a thermoresponsive behavior may display either an upper (UCST) or a lower (LCST) critical solution tem- perature. Among the LCST polymers, the structure as well as the thermal properties of poly(N-isopropylacrylamide) (PNIPAm) have been extensively investigated [15–21] and this latter is now used in many materials mainly related to biological applications [22–26]. Poly (methacrylic acid) (PMAA) is a thermo-responsive polyelectrolyte which exhibits a LCST behavior as reported for the first time by Eliassaf and Siberberg [27]. This polymer has been far less investigated than PNIPAm even though the chemical structure of the PMAA repeat unit results in specific and interesting properties in water. In particular, the mechanism underlying the LCST phase separation for PMAA significantly differs from Polymers 2022, 14, 4708. https://doi.org/10.3390/polym14214708 https://www.mdpi.com/journal/polymers