Constrained thermoresponsive polymers - new insights into fundamentals and applications.

Patricia Flemming,Alexander S Münch,Petra Uhlmann,Andreas Fery

doi:10.3762/bjoc.17.138

Abstract

In the last decades, numerous stimuli-responsive polymers have been developed and investigated regarding their switching properties. In particular, thermoresponsive polymers, which form a miscibility gap with the ambient solvent with a lower or upper critical demixing point depending on the temperature, have been intensively studied in solution. For the application of such polymers in novel sensors, drug delivery systems or as multifunctional coatings, they typically have to be transferred into specific arrangements, such as micelles, polymer films or grafted nanoparticles. However, it turns out that the thermodynamic concept for the phase transition of free polymer chains fails, when thermoresponsive polymers are assembled into such sterically confined architectures. Whereas many published studies focus on synthetic aspects as well as individual applications of thermoresponsive polymers, the underlying structure–property relationships governing the thermoresponse of sterically constrained assemblies, are still poorly understood. Furthermore, the clear majority of publications deals with polymers that exhibit a lower critical solution temperature (LCST) behavior, with PNIPAAM as their main representative. In contrast, for polymer arrangements with an upper critical solution temperature (UCST), there is only limited knowledge about preparation, application and precise physical understanding of the phase transition. This review article provides an overview about the current knowledge of thermoresponsive polymers with limited mobility focusing on UCST behavior and the possibilities for influencing their thermoresponsive switching characteristics. It comprises star polymers, micelles as well as polymer chains grafted to flat substrates and particulate inorganic surfaces. The elaboration of the physicochemical interplay between the architecture of the polymer assembly and the resulting thermoresponsive switching behavior will be in the foreground of this consideration.

Highlights

During the last decades, the class of stimuli-responsive materials has entered the focus of scientific research and applied polymer science [1,2,3,4,5,6,7,8,9]
While numerous studies have been performed on free polymer chains in solution, especially focussing on the lower critical solution temperature (LCST) transition, we have recently shown that an upper critical solution temperature (UCST) behavior can be induced in poly(dimethylaminoethyl methacrylate) (PDMAEMA) brushes, thereby generating a novel approach for controllable in situ nanostructuring on surfaces [300,301]
The present review focused on the correlation of the thermoresponsitivity of polymers with morphological patterns as well as arrangement and conformation of the polymer chains

Summary

Introduction

The class of stimuli-responsive materials has entered the focus of scientific research and applied polymer science [1,2,3,4,5,6,7,8,9]. Using further theoretic models and methods, such as the lattice density functional theory, the temperature-dependent swelling behavior of a polymer brush could be simulated in comparison to the corresponding phase diagrams of the free, unbound polymer chains in solution [124]. Because of the specific interactions between water and polymer chains, water-soluble polymers can have a critical point at any concentration, which often corresponds to a miscibility gap with a LCST [74], in contrast to nonpolar brushes in an organic solvent (like polystyrene in cyclohexane), which always show an UCST.

Limitations

Findings

Conclusion