Polymer groups temperature sensitive

For the last 10 years the fundamentals of a MEMS technique based on temperature-sensitive polymers have been developed by our group at the Technische Universitat Dresden. In 2008 the first highly integrated hydrogel-based MEMS named artificial skin was presented (Richter and Paschew 2009). 

A second group of techniques may be called lineshape analysis. Simple methods entail the measurements of linewidths or second moments as a function of temperature. More sophisticated methods involve the analysis or the model fitting of spectral lineshapes. A prominent method is ID lineshape analysis for deuterium-labeled polymers, which is sensitive to motions in the frequency range of lO -lO s (149). The 2D wideline separation NMR (WISE) experiment permits correlation of the high resolution spectrum with the wideline spectrum, which provides dipolar information (11,150). The linewidth is a function of the frequency of the polymer motion relative to the time scale of dipolar couplings. 

Studies carried out by Yoshida and coworkers have coupled this phenomena with oscillating chemical reactions (such as the Belousov-Zhabotinsky, BZ, reaction) to create conditions where pseudo non-equilibrium systems which maintain rhythmical oscillations can demonstrated, in both quiescent (4) and continuously stirred reactors (5). The ruthenium complex of the BZ reaction was introduced as a functional group into poly(N-isopropyl acrylamide), which is a temperature-sensitive polymer. The ruthenium group plays it s part in the BZ reaction, and the oxidation state of the catalyst changes the collapse temperature of the gel. The result is, at intermediate temperature, a gel whose shape oscillated (by a factor of 2 in volume) in a BZ reaction, providing an elegant demonstration of oscillation in a polymer gel. This system, however, is limited by the concentration of the catalyst which has to remain relatively small, and hence the volume change is small. 

The polymer network of temperature-sensitive hydrogels is made up of smaller repeating units called monomers. In addition to monomers, the so-called cross-linker groups are present in the polymer network. The cross-linker groups form coimections between different strands of polymer inside the polymer network. As a result a single giant polymer molecule is formed which is incapable of dissolving. 

ROP and RAFT polymerization techniques were combined to synthesize multiarm star-block copolymers having PeCL inner blocks and PDMAEMA outer blocks. A hyperbranched polyester core was used as a multifunctional initiator. It was calculated that the functionality of the star-blocks was equal to 19. Temperature and pH-responsive micelles were obtained in aqueous solutions. Equilibrium between unimolecular and mulrimolecular micelles was observed at pH 6.58 by dynamic LS and TEM measurements. In low-pH solutions, the PDMAEMA chains were fully protonated and therefore highly stretched, leading to maximum Rh values. When the pH was increased, the micellar Rh decreased as a result of the deprotonation of the dimethylamine groups. PDMAEMA is also a temperature-sensitive polymer, as it exhibits lower critical solution temperature (LCST) behavior. It precipitates from neutral or basic solutions between 32 and 58 °C. At pH 6.58, the Rh values were found to decrease with increasing temperature, due to the gradual collapse of the PDMAEMA outer blocks. 

Due to the relative ease of control, temperature is one of the most widely used external stimuli for the synthesis of stimulus-responsive bmshes. In this case, thermoresponsive polymer bmshes from poly(N-isopropylacrylamide) (PNIPAM) are the most intensively studied responsive bmshes that display a lower critical solution temperature (LOST) in a suitable solvent. Below the critical point, the polymer chains interact preferentially with the solvent and adopt a swollen, extended conformation. Above the critical point, the polymer chains collapse as they become more solvophobic. Jayachandran et reported the synthesis of PNIPAM homopolymer and block copolymer brushes on the surface of latex particles by aqueous ATRP. Urey demonstrated that PNIPAM brushes were sensitive to temperature and salt concentration. Zhu et synthesized Au-NPs stabilized with thiol-terminated PNIPAM via the grafting to approach. These thermosensitive Au-NPs exhibit a sharp, reversible, dear opaque transition in solution between 25 and 30 °C. Shan et al. prepared PNIPAM-coated Au-NPs using both grafting to and graft from approaches. Lv et al. prepared dual-sensitive polymer by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from trithiocarbonate groups linked to dextran and sucdnoylation of dextran after polymerization. Such dextran-based dual-sensitive polymer is employed to endow Au-NPs with stability and pH and temperature sensitivity. 

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