Macromolecular effect on crystal pattern formation in ultra-thin films: Molecular segregation in a binary blend of PEO fractions

In this work we report our investigation on the crystal patterns of a 50/50 blend of two polyethylene oxide (PEO) fractions with different molecular weights in ultra-thin films. Using AFM with a hot stage the samples on the surface of silicon wafer were isothermally crystallized at 20.0 ≤ T_c ≤ 60.0 °C. The crystal patterns are different from those of the two pure fractions. Co-crystallization, partial segregation and full segregation have been observed. Especially, within 47.0 ≤ T_c ≤ 54.0 °C dual thickness crystals formed. The thickness of the middle part of the crystal corresponds to the lamella thickness of 35k-PEO fraction with multiple folds, while the thickness of the edge part is nearly equal to an extended-chain lamella thickness of 5k-PEO fraction. We suppose that the appearance of the dual thickness crystals is due to molecular partial segregation. Utilizing in-situ AFM, the growth of the crystal with dual thicknesses as a function of time was monitored at T_c = 54.0 °C. Two growth processes were observed and a pattern formation mechanism was suggested on the basis of specified molecular motion and chain-fold crystallization of polymers.     

Relative stabilities of Ce(IV) and Ce(III) limiting carbonate complexes at 5-50 °C in Na aqueous solutions, an electrochemical study

The normal potential of the Ce(IV)/Ce(III) redox couple was determined by square wave voltammetry (SWV) at different temperatures in solutions with a constant ratio [CO₃²⁻]/[HCO₃⁻] ≈10 for high ionic strengths (3.29 mol dm⁻³ at 4.39 mol dm⁻³): varies from 259.5 to 198.0 mV/S.H.E. in the 15-50 °C range. Linear variations were found for versus (RT/F)ln(mCO₃²⁻), leading to the stoichiometry, Ce(CO₃)₆⁸⁻ for the Ce(IV) limiting complex. But the slopes of these linear variations were actually found in the range 1.8-1.9, not exactly 2. This was interpreted as dissociation of the Ce(IV) limiting complex following the reaction: Ce(CO₃)₅⁶⁻ + CO₃²⁻ → Ce(CO₃)₆⁸⁻ and as dissociation of the Ce(III) limiting complex following the reaction: Ce(CO₃)₃³⁻ + CO₃²⁻ → Ce(CO₃)₄⁵⁻; for which maximum possible values of log₁₀ K_IV,6 and log₁₀ K_III,4 were estimated via fitting in the 15-50 °C temperature range (log₁₀ K_IV,6 = 0.42 (0.97) and log₁₀ K_III,4 = 0.88 (7.00) at 15 °C (50 °C). The normal potential was found to decrease linearly with T, these variations correspond to , with T⁰ = 298.15 K and . The apparent diffusion coefficient of Ce(IV) was determined by direct current polarography (DCP), cyclic voltammetry (CV) and square wave voltammetry. It was found to depend on the ionic strength and to be proportional to T.     

Systematics of salt precipitation in complexes of polyethylene oxide and alkali metal iodides

Conductivity measurements in PEO₃₀MI polymer electrolytes with M=Li, Na, K, Rb, or Cs over the temperature range from about 65 to 200 °C show an increasing tendency for salt precipitation with increasing cation size. The salt precipitation in these complexes upon heating is revealed by the decrease of the dc conductivity starting at a critical temperature T_c. Whereas LiI and NaI complexes do not show precipitation effects, T_c monotonically decreases from about 140 to 65 °C when changing the salt component from KI via RbI to CsI. For the PEO-RbI system, precipitation is further investigated by nuclear magnetic resonance (NMR) and tracer diffusion experiments. NMR analysis unambiguously demonstrates the onset of RbI salt precipitation and the increase of the precipitate fraction with increasing temperature. In diffusion experiments on PEO₃₀RbI with the radiotracers and , the precipitation effect is manifested by anomalous features in the penetration profiles, however, without noticeable changes in their depth range. Combining the resulting tracer diffusion coefficients with the dc conductivity data enables us to assess crucial parameters characterizing ionic transport in PEO₃₀RbI.