Kinetics of aggregation in thermoresponsive polymer solutions upon a pressure jump

• Applicant: Professorin Dr. Christine M. Papadakis, Ph.D.
• Subject Area: Experimental and Theoretical Physics of Polymers
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 403786900

• Final Report Year: 2024

Final Report Abstract

The research project addresses the relation between the hydration of thermoresponsive polymers as well as their dynamics in aqueous solution and the kinetics of the phase transition aggregation upon pressure jumps across the phase boundary. In dependence on temperature and pressure, thermoresponsive polymers exhibit a phase diagram with an elliptical coexistence line. Using the model polymer poly(N-isopropylacrylamide) (PNIPAM), we found that, at high pressures, the hydrophobic hydration of the polymers decreases less at the phase transition than at atmospheric pressure. The kinetics of the aggregation of the polymers and of the disintegration of the aggregates could be structurally investigated using time-resolved smallangle neutron scattering (SANS) in dependence on the target pressure in a range of length scales of ca. 1-100 nm and with a time resolution of ca. 50 ms. It was demonstrated that, for the aggregation not only the thermodynamic driving force plays a role for the final size of the aggregates, but that the formation of a rigid shell hinders their growth. For the disintegration of the aggregates, two mechanisms could be identified, namely the release of single chains and the swelling of the entire aggregates, and the time scales of these processes differ significantly. These results are of interest for, among others, applications of polymeric nanoparticles for drug delivery as well as for the disintegration of microplastics. These investigations were complemented by measurements with dynamic light scattering under high pressure, which yielded information about the pressure-dependent chain conformation in the one-phase state. For the polymer poly(N-isopropylmethacrylamid) (PNIPMAM) in aqueous solution, the phase diagram was determined, and we observed that the one-phase region is strongly enlargened compared to the one of PNIPAM. SANS revealed that, at atmospheric pressure, even in dilute solution in the one-phase region, aggregates are present, and they disintegrate under pressure. The important role of the hydration of the hydrophobic groups for the phase behavior was corroborated by Raman spectroscopy. For a poly(methyl methacrylate)-b-poly(N-isopropylacrylamide) (PMMA-b-PNIPAM) diblock copolymer, the PNIPAM shell collapses significantly less at high pressures than at low pressures, which is compatible with the behavior of PNIPAM. Accordingly, the mechanisms and time scales of the collapse of the shell and the aggregation of the micelles during pressure jumps from the one-phase to the two-phase region very different. These results are of significant interest for applications of block copolymer micelles with a thermoresponsive shell, e.g. as medium for chemical reactions. Key aspects of our results were highlighted in a recent feature article. As a new development, the investigation of the co-nonsolvency effect of solutions of PNIPAM or PMMA-b-PNIPAM under pressure emerged. Furthermore, promising high-pressure experiments were carried out on perdeuterated PNIPAM.

Publications

• Kinetics of Mesoglobule Formation and Growth in Aqueous Poly(N-isopropylacrylamide) Solutions: Pressure Jumps at Low and at High Pressure. Macromolecules, 52(17), 6416-6427.
  Niebuur, Bart-Jan; Chiappisi, Leonardo; Jung, Florian; Zhang, Xiaohan; Schulte, Alfons & Papadakis, Christine M.
  
• What can Pressure Do for the Understanding of Thermoresponsive Polymers? Hellenic Neutron Association Newsletter 5, 1-5, 2019.
  B.-J. Niebuur, A. Schulte & C. M. Papadakis
  
• Pressure Dependence of the Cononsolvency Effect in Aqueous Poly(N-isopropylacrylamide) Solutions: A SANS Study. Macromolecules, 53(10), 3946-3955.
  Niebuur, Bart-Jan; Ko, Chia-Hsin; Zhang, Xiaohan; Claude, Kora-Lee; Chiappisi, Leonardo; Schulte, Alfons & Papadakis, Christine M.
  
• Nanoscale disintegration kinetics of mesoglobules in aqueous poly(N-isopropylacrylamide) solutions revealed by small-angle neutron scattering and pressure jumps. Nanoscale, 13(31), 13421-13426.
  Niebuur, Bart-Jan; Chiappisi, Leonardo; Jung, Florian A.; Zhang, Xiaohan; Schulte, Alfons & Papadakis, Christine M.
  
• Pressure Dependence of Water Dynamics in Concentrated Aqueous Poly(N-isopropylacrylamide) Solutions with a Methanol Cosolvent. Macromolecules, 54(9), 4387-4400.
  Niebuur, Bart-Jan; Lohstroh, Wiebke; Ko, Chia-Hsin; Appavou, Marie-Sousai; Schulte, Alfons & Papadakis, Christine M.
  
• Cononsolvency of the responsive polymer poly(N-isopropylacrylamide) in water/methanol mixtures: a dynamic light scattering study of the effect of pressure on the collective dynamics. Colloid and Polymer Science, 300(11), 1269-1279.
  Niebuur, Bart-Jan; Deyerling, André; Höfer, Nicole; Schulte, Alfons & Papadakis, Christine M.
  
• Cononsolvency of thermoresponsive polymers: where we are now and where we are going. Soft Matter, 18(15), 2884-2909.
  Bharadwaj, Swaminath; Niebuur, Bart-Jan; Nothdurft, Katja; Richtering, Walter; van der Vegt, Nico F. A. & Papadakis, Christine M.
  
• Thermoresponsive Polymers under Pressure with a Focus on Poly(N-isopropylacrylamide) (PNIPAM). Langmuir, 40(1), 1-20.
  Papadakis, Christine M.; Niebuur, Bart-Jan & Schulte, Alfons