The Impact of Filler Geometry on Polylactic Acid-Based Sustainable Polymer Composites

Recently, biocomposites have emerged as materials of great interest to the scientists and industry around the globe. Among various polymers, polylactic acid (PLA) is a popular matrix material with high potential for advanced applications. Various particulate materials and nanoparticles have been used as the filler in PLA based matrix. One of the extensively studied filler is cellulose. However, cellulose fibres, due to their hydrophilic nature, are difficult to blend with a hydrophobic polymer matrix. This leads to agglomeration and creates voids, reducing the mechanical strength of the resulting composite. Moreover, the role of the various forms of pure cellulose and its particle shape factors has not been analyzed in most of the current literature. Therefore, in this work, materials of various shapes and shape factors were selected as fillers for the production of polymer composites using Polylactic acid as a matrix to fill this knowledge gap. In particular, pure cellulose fibres (three types with different elongation coefficient) and two mineral nanocomponents: precipitated calcium carbonate and montmorillonite were used. The composites were prepared by a melt blending process using two different levels of fillers: 5% and 30%. Then, the analysis of their thermomechanical and physico-chemical properties was carried out. The obtained results were presented graphically and discussed in terms of their shape and degree of filling.     

Clar Rules the Electronic Properties of 2D π-Conjugated Frameworks: Mind the Gap

The key electronic properties of a family of 2D frameworks structurally convergent with holey graphenes were studied. The bandgap of these materials decreases monotonically with size, showing a common trend with anthracenes and kekulenes. This was rationalized by Clar's sextet rule, which reveals a direct relationship between the molecular systems and the 2D frameworks. In addition, a detailed benchmark against experimental data showcased the high quality of the models, which reproduce accurately available electronic properties. Overall, it was shown that DFT can be used to screen and understand the intrinsic bandgaps and electrochemistry potentials for technological applications prior to the synthesis of π-conjugated porous materials.     

A facile route to electronically conductive polyelectrolyte brushes as platforms of molecular wires

A facile strategy for the synthesis of conjugated polyelectrolyte brushes grafted from a conductive surface is presented. Such brushes form a platform of molecular wires oriented perpendicularly to the surface, enabling efficient directional transport of charge carriers. As the synthesis of conjugated polymer brushes using chain-growth polymerization via a direct “grafting from” approach is very challenging, we developed a self-templating surface-initiated method. It is based on the formation of multimonomer template chains in the first surface-initiated polymerization step, followed by the second polymerization leading to conjugated chains in an overall ladder-like architecture. This strategy exploits the extended conformation of the surface-grafted brushes, thereby enabling alignment of the pendant polymerizable groups along the template chains. We synthesized a new bifunctional monomer and used the developed approach to obtain quaternized poly(ethynylpyridine) chains on a conductive indium tin oxide surface. A catalyst-free quaternization polymerization was for the first time used here for surface grafting. The presence of charged groups makes the obtained brushes both ionically and electronically conductive. After doping with iodine, the brushes exhibited electronic conductivity, in the direction perpendicular to the surface, as high as 10^–1–10⁰ S m^–1. Tunneling AFM was used for mapping the surface conductivity and measuring the conductivity in the spectroscopic mode. The proposed synthetic strategy is very versatile as a variety of monomers with pendant polymerizable groups and various polymerization techniques may be applied, leading to platforms of molecular wires with the desired characteristics.     

An effective scaling frequency factor method for scaling of harmonic vibrational frequencies: Application to 1,2,4-triazole derivatives

Scaling of harmonic frequencies of a molecule is one of the methods of improving the agreement between the calculated from a quadratic force field and experimental vibrational spectrum. An application of the recently proposed effective scaling frequency factor (ESFF) method to the complicated 1,2,4-triazole derivatives is presented. The calculations are based on the DFT/B3LYP/6-311G** quadratic force fields. It is shown that the ESFF method is capable of providing the high-quality spectra with regard to the scaled frequencies, comparable to these obtained with the well-established scaled quantum mechanical (SQM) force field method. Using the recommended scaling factors for the 11-parameter calculations, the RMS value obtained for a set of 293 vibrational modes of four compounds is only 8.7 and 8.5 cm^?1, for SQM and ESFF, respectively, provided the hydrogen bonded CO bond was excluded from the general non-hydrogen XX stretch group, and the scaling factor attributed to this bond was optimized. The new, 9-parameter set of scaling factors provides SQM- and ESFF-scaled frequencies that are of comparable quality to those of the 11-parameter calculations. In addition, it provides (on average) more reliable band splittings in the middle region of the spectrum, and the order of the scaled frequencies corresponds to that of the experimental bands. The straightforward application of the ESFF method to estimate the value of the scaled frequency is also presented.     

A new concept for the noncovalent binding of a ruthenium-based olefin metathesis catalyst to polymeric phases: preparation of a catalyst on Raschig rings

A new concept for noncovalent immobilization of a ruthenium olefin metathesis catalyst is presented. The 2-isopropoxybenzylidene ligand of a Hoveyda-Grubbs carbene is further modified by an additional amino group (7) and immobilization is achieved by treatment with sulfonated polystyrene forming the corresponding ammonium salt. In this novel strategy for the immobilization of ruthenium-based metathesis catalysts, the amino group plays a two-fold role, being first an active anchor for immobilization and second, after protonation, activating the catalysts (electron donating to electron withdrawing activity switch). The polymeric support was prepared by precipitation polymerization which led to small bead sizes (0.2-2 microm) and large surface areas. Compared to commercial resins this tailor-made phase showed superior properties in immobilization of complex 7. This concept of immobilization was applied to glass-polymer composite megaporous Raschig rings. Ru catalyst 7 on Raschig rings was used under batch conditions in various metathesis reactions, including ring-closing (RCM), cross- (CM) and enyne metathesis, to give products of high chemical purity with very low ruthenium contamination levels (21-102 ppm). The same ring can be used for up to 6 cycles of metathesis.