Phase Transition Behaviors of Hydroxypropyl Methylcellulose Induced By Small Molecular Weak Acids

In this paper, phase change material hydroxypropyl methylcellulose (HPMC) was chosen to investigate the influence of small molecular weak acids on its phase transition temperature. The results showed that all of the chosen small molecular weak acids such as acrylic acid (AA), methacrylic acid (MAA), DL-lactic acid (LA), citric acid (CA) and acetic acid (AAc) can trigger the decrease of phase transition temperature of HPMC at different pH. With the increase of pH, AA, LA, CA and AAc can further lower the phase transition temperature, on the contrary, the phase transition temperature of HPMC increased with MAA. The change range of LCST was largest around pKa compared with other pH values because triggering effect changed gradually from hydrogen bonding effect to salt effect upon the increase of pH. Besides, phase transition temperature can also be reduced by the increase of acid concentration. This is attributed to smaller distance between molecules caused by higher concentration leading to stronger hydrogen bonding effect or salt effect. This paper provided a new perspective to modulate the LCST of phase change material by small molecular weak acids at different pH.

Salt-Enhanced Oxidative Addition of Iodobenzene to Pd: an Inter-play Between Cation, Anion and Pd-Pd Cooperative Effects

Halide salts facilitate the oxidative addition of organic halides to Pd(0). This phenomenon originates from a combina-tion of anionic, cationic and Pd-Pd cooperative effects. Exhaustive computational exploration at the DFT level of the com-plexes obtained from [Pd0(PPh3)2] and a salt (NMe4Cl or LiCl) showed that chlorides promote phosphine release, leading to a mixture of mononuclear and dinuclear Pd(0) complexes. Anionic Pd(0) dinuclear complexes exhibit a cooperativity between Pd(0) centers which favors the oxidative addition of iodobenzene. The higher activity of Pd(0) dimers toward oxidative addition rationalizes the previously reported kinetic laws. In the presence of Li+, the oxidative addition to mon-onuclear [Pd0L(Li2Cl2)] is estimated barrierless. LiCl coordination polarizes Pd(0), enlarging both the electrophilicity and the nucleophilicity of the complex, which promotes both coordination of the substrate and the subsequent insertion into the C-I bond. These conclusions are paving the way to the rational use of salt effect in catalysis for the activation of more challenging bonds.

Neutralization and Salt Effect on the Structure and Mechanical Properties of Polyacrylic Acid Gels under Equivolume Conditions

The effects of neutralization and salt on the structure and mechanical properties of polyacrylic acid (PAA) gels under equivolume conditions were investigated by small-angle X-ray scattering (SAXS) measurements and tensile tests. We attained the equivolume condition by immersing a piece of PAA gel sample in an ion reservoir containing linear PAA, NaOH, and NaCl at prescribed concentrations (post-ion-tuning). The volume fraction of the linear polymer was set to be the same as that of the gel so as to satisfy the iso-osmotic pressure at the reference state. Various types of reservoirs were prepared by adding NaOH and/or NaCl with different concentrations to the reference reservoir, followed by immersing a PAA gel piece. In the SAXS measurements, a scattering peak appeared, and the scattering intensity at q = 0 decreased by neutralization, while the addition of salt increased the scattering intensity. On the other hand, Young’s modulus measured with the tensile test decreased with neutralization; however, it scarcely changed with the addition of salt. The newly developed equivolume post-ion-tuning technique may serve as a new standard scheme to study polyelectrolyte gels.