RESUMEN
We report the experimentally determined phase diagram for an aqueous solution of isotactic-rich poly(N-isopropylacrylamide) (PNiPAM) composed of the sol-gel transition curve and the cloud-point curve. The meso diad content of isotactic-rich PNiPAM is 64%, and it is soluble in water at low temperatures, but undergoes a sol-to-gel transition with increasing temperature in the investigated concentration range of 1.8 wt. %-6.0 wt. %. With a further increase in temperature, the system becomes turbid. The gel formation and clouding behavior are thermally reversible. This is the first observation of thermoreversible gelation under the cloud-point temperature for an aqueous solution of PNiPAM. On the basis of the determined phase diagram, we carried out light scattering experiments to characterize the sol-gel transition behavior as a function of temperature.
RESUMEN
Self-assembly of acetylated dextran (Ac-DEXs) was investigated with a modified dextran with acetic anhydride in the presence of pyridine. The effect of acetylation degree on solution properties has been investigated by static and dynamic light scattering (DLS). Molecular weight (Mw) and the radius of gyration (Rg) of dextran significantly increased with acetylation degree due to the aggregates formation. However, those aggregates noticeably reduced with further increase of acetylation degree. It suggested that the aggregates have shrinkage. The aggregate formation is clearly confirmed by DLS analysis in the presence of the bimodal relaxation distribution for Ac-DEXs. The hydrodynamic radius (Rh) of fast and slow mode is distinctly corresponded with single dextran macromolecules and aggregates, respectively. The Rh of aggregates varied slightly with increasing acetylation degree. The aggregates of Ac-DEXs represent hard spherical nanoparticles whereas the random coil structure is found in dextran. Formation of gel nanoparticles was monitored at the highest acetylated substitution.
RESUMEN
We report the experimental results of dielectric relaxation spectroscopy for the systems of poly(N-isopropylacrylamide) [PNiPAM] in various solvents in the frequency range of 40 kHz to 20 GHz at the solution temperature of 25.0 °C. The solvents used in this study were protic solvents (water, methanol, ethanol, and 1-propanol) and aprotic solvents (acetone, methyl ethyl ketone, and dimethyl sulfoxide). Two relaxation processes were observed at frequencies of approximately 1 MHz and 10 GHz in all the solutions. The origins of the two relaxation processes are considered to be the reorientation of dipoles of the PNiPAM chains at middle frequencies (m-process) and that of solvent molecules at higher frequencies (h-process). For the PNiPAM solutions composed of protic solvents except for 1-propanol, the relaxation time of the h-process increased with increasing PNiPAM concentration, whereas that of the h-process for the 1-propanol decreased with increasing PNiPAM concentration. In contrast, the relaxation times of the h-process for the aprotic solvents were independent of the density of hydrogen-bonding sites. For the m-process, which is attributed to the local chain motion of PNiPAM, the extrapolated relaxation time to zero polymer concentration τ(m0) was scaled by the solvent viscosity for all the protic solvents, whereas for the aprotic solvents τ(m0) showed no correlation with the solvent viscosity. The dynamics of polymer chains and solvent molecules in their solution state are clarified in terms of cooperative motion, which is associated with the interactions through hydrogen bonding at the molecular level.