Switches based on polymer brushes: Theoretical ascpects of phase transitions and nonequilibrium behavior
Experimental and Theoretical Physics of Polymers
Final Report Abstract
In this project we have studied the properties of surface coatings made of polymer brushes, i.e., layers of macromolecules that are covalently attached to the surface with one end. Theoretical descriptions of polymer brushes often assume that all polymers have exactly the same length. In reality, however, they have a very broad length distribution as a necessary consequence of the way they are made. In one part of the project, we have investigated the influence of this distribution (the so-called polydispersity) on the structure of the coatings. We found that the ideal „monodisperse“ brush is actually a very special state where fluctuations are anomalously high, similar to a critical point in phase transitions. In this sense, polydisperse brushes are in fact much more robust than monodisperse brushes. Moreover, polydispersity allows to tune the density profiles of the organic coatings and the interactions between coated surfaces. In a second part of the project, we have developed novel strategies for making „smart“ polymer surfaces which can respond sensitively to environmental changes such as shifts in temperature, pH level, or light. The idea is to embed small amounts of adsorption-active polymers, for which the level of adsorption depends on the external stimuli. Even if these variations are smooth for individual chains, they can become very sharp if the chains are embedded in a brush. Moreover, by tuning the architecture of the adsorption-active chains - e.g., combining adsorption-active blocks and inert blocks - the properties of the transition can be tuned such that the transition is at the same time arbitrarily sharp, very fast, and free of hysteresis (the energy barrier between the two switch states vanishes). In a side project, we have revisited the properties of individual end-grafted chains which are simultaneously attracted by the surface and pulled away from the surface. By comparing different types of transitions in these chains, we have been able to conjecture a universal scaling function for the distribution of chain contacts at the critical adsorption transition, which is in very good agreement with simulations. Thus we have also been able to make fundamental progress towards a better understanding of the adsorption transition, one of the archetype phase transitions in polymer theory.
Publications
- Polydisperse Polymer Brushes: Internal Structure, Critical Behavior and Interaction with Flow. Macromolecules, 49 9665 (2016)
Qi S., Klushin L.I., Skvortsov A. M., Schmid F.
(See online at https://doi.org/10.1021/acs.macromol.6b02026) - Anomalous Critical Slowdown at a First Order Phase Transition in Single Polymer Chain J. Chem. Phys. 147 064902 (2017)
Zhang S., Qi S., Klushin L.I, Skvortsov A.M., Yan D., Schmid F.
(See online at https://doi.org/10.1063/1.4997435) - Phase Transitions in Single Macromolecule: Comparison of Loop-stretch and Adsorption Transitions in End-grafted Polymer Chains. J. Chem. Phys. 148, N4, 044903 (2018)
Zhang S., Qi S., Klushin L.I, Skvortsov A.M., Yan D., Schmid F.
(See online at https://doi.org/10.1063/1.5013346) - Tuning Transition Properties of Stimuli-Responsive Brushes by Polydispersity, Adv. Funct. Mater. 28, 40, 1800745 (2018)
Qi S., Klushin, L. I.; Skvortsov, A. M., Liu M., Zhou J., Schmid F.
(See online at https://doi.org/10.1002/adfm.201800745) - Polydispersity Effects at Interpenetration in Compressed Brushes, Macromolecules, 52, 1810, (2019)
Klushin L.I., Skvortsov A. M., Qi S., Schmid F.
(See online at https://doi.org/10.1021/acs.macromol.8b02361) - Using copolymers to design tunable stimuli-responsive brushes, Macromolecules 53,13 (2020)
Qi S., Klushin, L. I.; Skvortsov, A. M., Liu M., Zhou J., Schmid F.
(See online at https://doi.org/10.1021/acs.macromol.0c00674)
