Intrinsische Schaltkinetik und Thermodynamik thermoresponsiver Polymerbürsten
Synthetic polymers represent one of the most important class of material in the 20th century and beyond. Particularly, stimuli-responsive polymers (so called smart polymers) build an outstanding and fascinating class of synthetic polymers which have become a versatile material in designing functional devices in material science, medicine and bioengineering. Controlled reaction schemes, such as ATRP (atomic transfer radical polymerization), allows one to have precise control over polymer architecture. One such example is the buildup surface-grafted polymer brush films with thicknesses down to the sub-100 nm range on almost any surface geometry. Due to their low film thickness, high sensitivities and fast response times can be achieved. However, in order to tailor efficient and powerful applications with high performance, a detailed fundamental knowledge of the switching thermodynamics and kinetics is essential. Unfortunately, kinetic and thermodynamic data of stimuli-responsive polymer brushes are still largely missing. Tanaka and co-workers discovered in the 1980s that thermoresponsive polymer gels with dimensions in the micro and millimeter range exhibit response times on a second to hours timescale due to the diffusion-limited switching process. Nevertheless, much shorter response times in the milli/microsecond range are expected for ultrathin polymer films. Although, most previous studies revealed fairly long response times of several seconds or more for ultrathin polymer brushes. Using a novel facile noncontact laser manipulation technique, this work reveals an unprecedented insight into the switching kinetics (rate constants, activation energies, frequency factors) and switching thermodynamics (change in Gibbs free enthalpy, free enthalpy, entropy and heat capacity) of thermoresponsive polymer brushes of PNIPAAm (poly(n-isopropylacrylamide)) and PDMAEMA poly(dimethylaminoethyl methacrylate). In detail, the impact of internal brush parameters such as grafting density, brush thickness and micro/nanopatterns are studied. Moreover, external parameters, i. e. the impact of salts, alcohols, gold nanoparticles and proteins (bovine serum albumin, BSA), are addressed. These results are expected to further support the progress on new innovative applications based on switchable polymer brushes and provide an essential reference on the way to unpuzzle the molecular driving forces of the switching mechanisms.
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