Influence of polymer chain architecture of poly(vinyl alcohol) on the inhibition of ice recrystallization

\u3cp\u3ePoly(vinyl alcohol) (PVA) is a water-soluble synthetic polymer well-known to effectively block the recrystallization of ice. The effect of polymer chain architecture on the ice recrystallization inhibition (IRI) by PVA remains unexplored. In this work, the synthesis of PVA molecular bottlebrushes is described via a combination of atom-transfer radical polymerization and reversible addition-fragmentation chain-transfer polymerization. The facile preparation of the PVA bottlebrushes is performed via the selective hydrolysis of the chloroacetate esters of the poly(vinyl chloroacetate) (PVClAc) side chains of a PVClAc precursor bottlebrush. The IRI efficacy of the PVA bottlebrush is quantitatively compared to linear PVA. The results show that even if the PVA chains are densely grafted onto a rigid polymer backbone, the IRI activity of PVA is maintained, demonstrating the flexibility in PVA polymer chain architecture for the design of synthetic PVA-based ice growth inhibitors.\u3c/p\u3

Main-chain chiral poly(2-oxazoline)s:Influence of alkyl side-chain on secondary structure formation in the solid state

\u3cp\u3eThe influence of the side-chains of main-chain chiral poly(2-oxazoline) s on their thermal properties was investigated using differential scanning calorimetry (DSC) and the nature of the obtained melting endotherms was further investigated by thermal annealing of the polymers. Poly(R-2-ethyl-4-ethyl-2-oxazoline) (p-R-EtEtOx) was found to be amorphous, while polymers with longer side-chains are semicrystalline. Previously we reported that the chirally ordered crystals of poly(R-2-butyl-4-ethyl-2-oxazoline) (p-R-BuEtOx) have a high melting temperature of more than 200°C. in this work we demonstrate that elongation of the side-chains frombutyl to octyl results in a decrease in the crystallization rate and melting temperature suggesting that the chirally ordered crystals of p-R-BuEtOx are based on close packing of the mainchain enhancing diploar interactions between the tertiary amide moieties. Crystallizaiton of chiral polymers with longer side-chains resultsmay then be driven by close packing of the side-chains. This is supported by the observation that further elongation of the sidechain length increases the crystallization rate. Moerover, an additional melting endothermappears for thesepolymers at a lower temperature upon annealing ascribed to a dual crystal size population. Circular dichroism (CD) measurements of the semicrystalline main-chain chiral polymer films revealed the presence of chirally ordered crystals while X-ray diffraction (XRD) patterns revealed a closer packing of the chiral poly(2-alkyl-2-oxazoline) s compared to the non-chiral polymers, suggested to result form the chiral ordering in the crystals. Grazing incidence wide angle x-ray scattering (GIWAXS) patterns indicated that the chiral crystals of p-R-BuEtOx do not form a helical structure, however, the substrate does influence the type of structure formed.\u3c/p\u3

Noncovalent synthesis of self-assembled nanotubes through decoupled hierarchical cooperative processes

\u3cp\u3eBecause of their wide number of biological functions and potential applications, self-assembled nanotubes constitute highly relevant targets in noncovalent synthesis. Herein, we introduce a novel approach to produce supramolecular nanotubes with defined inner and outer diameters from rigid rod-like monomers programmed with complementary nucleobases through two distinct, decoupled cooperative processes of different hierarchy and acting in orthogonal directions: chelate cooperativity, responsible for the formation of robust Watson-Crick H-bonded cyclic tetramers, and nucleation-growth cooperative polymerization.\u3c/p\u3

Sustainable coatings from bio-based, enzymatically synthesized polyesters with enhanced functionalities

\u3cp\u3eBio-based sorbitol-containing polyester polyols were synthesized via enzymatic polycondensation. The selectivity of the biocatalyst for primary vs. secondary hydroxyl groups allowed for the preparation of close to linear renewable polyester polyols with enhanced hydroxyl functionalities, both as pendant groups and end-groups. In some cases, the sorbitol units were homogeneously distributed in the polyester polyol chains, whereas changes in the comonomers used and experimental conditions led to inhomogeneous and unique distributions of sorbitol, implying that some polyester polyol chains contained none and others contained multiple sorbitol units. Solvent-borne coatings were prepared by cross-linking the functional polyester polyols with polyisocyanate curing agents. An increased functionality of the polyester polyols led to an enhancement of the properties of the resulting cured coatings. Furthermore, when sorbitol units were non-homogeneously distributed, a significant improvement in the chemical resistance and mechanical properties of the cured poly(ester urethane) network was noted. By employing the bio-based diisocyanate EELDI (ethyl ester l-lysine diisocyanate) as a curing agent, almost fully renewable coatings with satisfactory mechanical properties were obtained.\u3c/p\u3

Understanding the limitations of the solvent-free enzymatic synthesis of sorbitol-containing polyesters

\u3cp\u3eEnzymatic catalysis is an attractive approach toward the synthesis of sustainable polyesters, which also provides advantages in terms of selectivity compared to conventional methods. Furthermore, the use of immobilized enzymes allows for solvent-free, ecofriendly polycondensation routes, but also leads to some limitations in terms of applicability to certain systems. A systematic study has been performed on the synthesis of close to linear aliphatic polyesters from biobased, commercially available sorbitol, 1,10-decanediol, and dimethyl adipate. Polycondensation reactions were carried out in the melt using SPRIN liposorb CALB (trade name for the immobilized form of Candida antarctica lipase B) as catalyst, targeting a number-average molecular weight between 4 and 6 kg/mol, and an amount of pendant and terminal hydroxyl groups within the range commonly used for coating applications. The efficacy with which the increasing amounts of sorbitol were built into the polyester backbone was studied in detail via \u3csup\u3e13\u3c/sup\u3eC NMR spectroscopy. In addition, the particular selectivity for primary vs secondary hydroxyl groups of the biocatalyst was confirmed via \u3csup\u3e31\u3c/sup\u3eP NMR spectroscopy. Extensive structural characterization was carried out via MALDI-ToF-MS analysis, which also provided further insights into limitations of the system related to sorbitol incorporation. Differential scanning calorimetry and X-ray diffraction analysis revealed that the melting temperature and crystallinity of the materials are lower when increased amounts of sorbitol are incorporated into the polyesters.\u3c/p\u3

Effect of high-temperature treatment on Fe/ZSM-5 prepared by chemical vapor deposition of FeCl3. I. Physicochemical characterization

The effect of severe (hydrothermal) treatment on Fe/ZSM-5 prepared by sublimation of FeCl3 is studied by a combination of high-resolution TEM, EXAFS, 57Fe Mössbauer spectroscopy, IR, UV–vis, nitrogen adsorption, 27Al NMR spectroscopy, and low-temperature nitrous oxide decomposition. The heterogeneous nature of Fe/ZSM-5 is stressed with a preponderance of iron oxide particles on the external zeolite surface. Additionally, neutral iron oxide nanoparticles and charge-compensating iron complexes are located in the micropores. Severe calcination at 973 K induces the growth and ordering of the iron oxide aggregates. Moreover, some of the occluded neutral iron oxide nanoparticles are transformed into charge-compensating iron complexes upon a protolysis reaction with the Brønsted protons. These effects are more pronounced in the case of steaming at 973 K, additionally resulting in the removal of Al from framework positions. Despite the low dispersion of iron oxide in Fe/ZSM-5, relatively low Fe---Fe coordination numbers were derived from the EXAFS data for Fe/ZSM-5; high-temperature treatments increased this number. This low value points to the disordered nature of the iron oxide aggregates rather than to the presence of an abundant fraction of binuclear iron clusters. Titration of sites active in nitrous oxide decomposition shows that their amount increases upon increasing severity of treatment of Fe/ZSM-5. Their number, however, remains very small (a few percent of the total iron) and appears to correlate to the amount of Fe2+ present after room temperature exposure to vacuum conditions. A comparison to a commercial HZSM-5 zeolite with a very low iron content is made. The catalytic performance of these materials is discussed in a companion paper (J. Catal. (2003))

Solvent Selectivity Governs the Emergence of Temperature Responsiveness in Block Copolymer Self-Assembly.

In highly selective solvents, block copolymers (BCPs) form association colloids, while in solvents with poor selectivity, they exhibit a temperature-controlled (de)mixing behavior. Herein, it is shown that a temperature-responsive self-assembly behavior emerges in solvent mixtures of intermediate selectivity. A biocompatible poly-ethylene(oxide)-block-poly-ε-caprolactone (PEO-PCL) BCP is used as a model system. The polymer is dissolved in solvent mixtures containing water (a strongly selective solvent for PEO) and ethanol (a poorly selective solvent for PEO) to tune the solvency conditions. Using synchrotron X-ray scattering, cryogenic transmission electron microscopy, and scanning probe microscopy, it is shown that a rich temperature-responsive behavior can be achieved in certain solvent mixtures. Crystallization of the PCL block enriches the phase behavior of the BCP by promoting sphere-to-cylinder morphology transitions at low temperatures. Increasing the water fraction in the solvent causes a suppression of the sphere-to-cylinder morphology transition. These results open up the possibility to induce temperature-responsive properties on demand in a wide range of BCP systems

Solvent Selectivity Governs the Emergence of Temperature Responsiveness in Block Copolymer Self-Assembly.

In highly selective solvents, block copolymers (BCPs) form association colloids, while in solvents with poor selectivity, they exhibit a temperature-controlled (de)mixing behavior. Herein, it is shown that a temperature-responsive self-assembly behavior emerges in solvent mixtures of intermediate selectivity. A biocompatible poly-ethylene(oxide)-block-poly-ε-caprolactone (PEO-PCL) BCP is used as a model system. The polymer is dissolved in solvent mixtures containing water (a strongly selective solvent for PEO) and ethanol (a poorly selective solvent for PEO) to tune the solvency conditions. Using synchrotron X-ray scattering, cryogenic transmission electron microscopy, and scanning probe microscopy, it is shown that a rich temperature-responsive behavior can be achieved in certain solvent mixtures. Crystallization of the PCL block enriches the phase behavior of the BCP by promoting sphere-to-cylinder morphology transitions at low temperatures. Increasing the water fraction in the solvent causes a suppression of the sphere-to-cylinder morphology transition. These results open up the possibility to induce temperature-responsive properties on demand in a wide range of BCP systems

The origin of high activity of amorphous MoS\u3csub\u3e2\u3c/sub\u3e in the hydrogen evolution reaction

\u3cp\u3eInvited for this month′s cover is a collaborative research effort involving experiments by L. Wu, A. Sharma, M. Hendrix, A. A. Bol, E. Hensen, and J. P. Hofmann (Eindhoven University of Technology) as well as A. Longo (European Synchrotron ESRF), and theoretical modelling from N. Dzade and N. De Leeuw (Utrecht and Cardiff Universities). The Communication itself is available at 10.1002/cssc.201901811.\u3c/p\u3

The origin of high activity of amorphous MoS2 in the hydrogen evolution reaction

\u3cp\u3eMolybdenum disulfide (MoS \u3csub\u3e2\u3c/sub\u3e) and related transition metal chalcogenides can replace expensive precious metal catalysts such as Pt for the hydrogen evolution reaction (HER). The relations between the nanoscale properties and HER activity of well-controlled 2H and Li-promoted 1T phases of MoS \u3csub\u3e2\u3c/sub\u3e, as well as an amorphous MoS \u3csub\u3e2\u3c/sub\u3e phase, have been investigated and a detailed comparison is made on Mo−S and Mo−Mo bond analysis under operando HER conditions, which reveals a similar bond structure in 1T and amorphous MoS \u3csub\u3e2\u3c/sub\u3e phases as a key feature in explaining their increased HER activity. Whereas the distinct bond structure in 1T phase MoS \u3csub\u3e2\u3c/sub\u3e is caused by Li \u3csup\u3e+\u3c/sup\u3e intercalation and disappears under harsh HER conditions, amorphous MoS \u3csub\u3e2\u3c/sub\u3e maintains its intrinsic short Mo−Mo bond feature and, with that, its high HER activity. Quantum-chemical calculations indicate similar electronic structures of small MoS \u3csub\u3e2\u3c/sub\u3e clusters serving as models for amorphous MoS \u3csub\u3e2\u3c/sub\u3e and the 1T phase MoS \u3csub\u3e2\u3c/sub\u3e, showing similar Gibbs free energies for hydrogen adsorption (ΔG \u3csub\u3eH*\u3c/sub\u3e) and metallic character. \u3c/p\u3