To prevent greenhouse
emissions into the atmosphere, separations
like CO2/CH4 and CO2/N2 from natural gas, biogas, and flue gasses are crucial. Polymer membranes
gained a key role in gas separations over the past decades, but these
polymers are often not organized at a molecular level, which results
in a trade-off between permeability and selectivity. In this work,
the effect of molecular order and orientation in liquid crystals (LCs)
polymer membranes for gas permeation is demonstrated. Using the self-assembly
of polymerizable LCs to prepare membranes ensures control over the
supramolecular organization and alignment of the building blocks at
a molecular level. Robust freestanding LC membranes were fabricated
that have various, distinct morphologies (isotropic, nematic cybotactic,
and smectic C) and alignment (planar and homeotropic), while using
the same chemical composition. Single gas permeation data show that
the permeability decreases with increasing molecular order while the
ideal gas selectivity of He and CO2 over N2 increases
tremendously (36-fold for He/N2 and 21-fold for CO2/N2) when going from randomly ordered to the highly
ordered smectic C morphology. The calculated diffusion coefficients
showed a 10-fold decrease when going from randomly ordered membranes
to ordered smectic C membranes. It is proposed that with increasing
molecular order, the free volume elements in the membrane become smaller,
which hinders gasses with larger kinetic diameters (Ar, N2) more than gasses with smaller kinetic diameters (He, CO2), inducing selectivity. Comparison of gas sorption and permeation
performances of planar and homeotropic aligned smectic C membranes
shows the effect of molecular orientation by a 3-fold decrease of
the diffusion coefficient of homeotropic aligned smectic C membranes
resulting in a diminished gas permeation and increased ideal gas selectivities.
These results strongly highlight the importance of molecular order
and orientation in LC polymer membranes for gas separation.
Organic ionic plastic crystals (OIPCs) are a class of soft materials showing positional order while still allowing orientational freedom. Due to their motional freedom in the solid state, they possess plasticity, non-flammability and high ionic conductivity. OIPC behavior is typically exhibited by 'simple' globular molecules allowing molecular rotation, whereas the interactions that govern the formation of OIPC phases in complex nonglobular molecules are less understood. To better understand these interactions, a new family of non-globular OIPCs containing a 15-crown-5 ether moiety was synthetized and character-ized. The 15C5BA molecule prepared does not exhibit the sought-after behavior because of its non-globular nature and strong intermolecular H-bonds that restrict orientational motion. However, the OIPC behavior was successfully obtained through complexation of the crown-ether moiety with sodium salts containing chaotropic anions. Those anions weaken the interactions between the molecules, allowing rotational freedom and tuning of the thermal and morphological properties of the OIPC.
Classical hydroformylation catalysts use mononuclear rhodium(I) complexes as precursors; however, very few examples of bimetallic systems have been reported. Herein, we report fully substituted dirhodium(II,II) complexes (C1−C6) containing acetate and diphenylformamidinate bridging ligands (L1−L4). The structure and geometry around these paddlewheeltype, bimetallic cores were confirmed by single-crystal X-ray diffraction. The complexes C3−C6 show electrochemical redox reactions, with the expected reduction (Rh 2 4+/3+ ) and two oxidation (Rh 2 4+/5+ and Rh 2 5+/6+ ) electron transfer processes. Furthermore, the bimetallic complexes were evaluated as catalyst precursors for the hydroformylation of 1-octene, with the acetate-containing complexes (C1 and C2) showing near quantitative conversion (>99%) of 1-octene, excellent activity and chemoselectivity toward aldehydes (>98%), with moderate regioselectivity toward linear products. Replacement of the acetate with diphenylformamidinate ligands (complexes C3−C6) yielded moderate-togood chemoselectivity and regioselectivity, favoring linear aldehydes.
Layer-by-layer (LbL) assembly of the alternating adsorption
of
oppositely charged polyions is an extensively studied method to produce
nanofiltration membranes. In this work, the concept of chaotropicity
of the polycation and its counterion is introduced in the LbL field.
In general, the more chaotropic a polyion, the lower its effective
charge, charge availability, and hydrophilicity. Here, this is researched
for the well-known PDADMAC (polydiallyldimethylammonium chloride)
and PAH (poly(allylamine) hydrochloride), and the synthesized PAMA
(polyallylmultimethylammonium), with two different counterions (I– and Cl–). Higher chaotropicity (PDADMAC
> PAMA-I > PAMA-Cl > PAH) translates into a reduced charge
availability
and a more pronounced extrinsic charge compensation, resulting in
more mass adsorption and a higher pure water permeability. PAMA-containing
membranes show the most interesting results in the series. Due to
its molecular structure, the chaotropicity of this polycation perfectly
lies between PDADMAC and PAH. Overall, the chaotropicity of PAMA membranes
allows for the formation of the right balance between extrinsic and
intrinsic charge compensation with PSS. Moreover, modifying the nature
of the counterions of PAMA (I– or Cl–) allows to tune the density of the multilayer and results in lower
size exclusion abilities with PAMA-I compared to PAMA-Cl (higher MWCO
and lower MgSO4 retention). In general, the contextualization
of the polyion interaction within the specific (poly)ion effects expands
the understanding of the influence of the charge density of polycations
without ignoring the chemical nature of the functional groups in their
monomer units.
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