A laser temperature-jump technique is used to probe the impact of sodium halides on the temperature-dependent switching kinetics and thermodynamics of poly(N-isopropylacrylamide) brushes. An analysis on the basis of a two-state model reveals van't Hoff enthalpy and entropy changes. Sodium halides increase the endothermicity and the entropic gain of the switching process below and above Tc following the Hofmeister series: NaCl > NaBr > NaI. In contrast, enthalpic and entropic changes at Tc remain virtually unaffected. This provides an unprecedented insight into the underlying switching energetics of this classic stimuli-responsive polymer. Because of its model character, these results represent an essential reference on the way to unpuzzle the molecular driving forces of the Hofmeister effect.
Titania microspheres with narrow size distribution and diameters of about 1 µm were prepared and subsequently functionalized using surface‐initiated atom transfer radical polymerization (ATRP) of N‐isopropylacrylamide. The ATRP initiator was immobilized on the particle surface via acylation of surface hydroxyl groups with α‐bromoisobutyryl bromide. Subsequently, an established ATRP reaction system was used for the preparation of titania surface‐grafted poly(N‐isopropylacrylamide) (PNiPAAm). Characterization was performed with electron microscopies, X‐ray diffraction, infrared spectroscopy and dynamic light scattering. It was found that the particle size in aqueous dispersions changed reversibly with temperature as expected for a shell of PNiPAAm, a polymer with a lower critical solution temperature at 32 °C. This confirmed the successful preparation of functional, stimuli‐responsive TiO2 microparticles via a straightforward controlled surface‐initiated polymerization method.
This work describes how the control of grafting density and grafted chain length of a thermo-responsive polymer in membrane pores can be utilized to tune the pore size and the switchability of size-based selectivity in the ultrafiltration range.
Stimuli-responsive polymer brushes are well known for their unique switching properties. In their Communication on page 4513 ff., N. Hartmann and co-workers describe an all-optical stroboscopic technique that provides unprecedented insight into the temperature-dependent intrinsic switching kinetics of such ultrathin polymer coatings. In contrast to hitherto reported approaches, response times range from the micro-to the millisecond time scale.
Laserstimulierte Polymerbürsten: Die temperaturabhängige Schaltkinetik von oberflächengebundenen thermoresponsiven Polymerbürsten wurde durch ein stroboskopisches Mikromanipulations‐/Mikrocharakterisierungsverfahren zur Echtzeitmessung untersucht (siehe Bild). Intrinsische Schaltzeiten liegen im Mikro‐ und Millisekundenbereich; diese Ergebnisse könnten zur Herstellung nanodimensionierter Aktuatoren und Sensoren mit bisher unerreichtem Ansprechverhalten führen.
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