Rigid Units Research Articles

Negative thermal expansion of cubic silicon dicarbodiimide, Si(NCN)2, studied by ab initio lattice dynamics

We report an ab initio calculation of crystal structure and lattice dynamics of cubic silicon dicarbodiimide, Si(NCN)2, using density functional theory methods. The calculations reveal a low-energy spectrum of rigid unit modes that are shown to be associated with negative thermal expansion. Comparisons are drawn with the closely-related materials Zn(CN)2 and the cubic-cristobalite phase of SiO2. Instabilities in the spectrum of rigid unit modes point to the existence of disorder of the positions and orientations of the dicarbodiimide molecular anions.

2D-Planar decorated 3D-network enables strong synergistic mechanics and programmable shape transformations for alginate-based hydrogels

Simultaneously enhancing the strength and toughness of physically crosslinked hydrogels is imperative as structural materials, especially for those from natural sources. Herein, inspired by the principle of synergistic effect, a facile yet effective strategy is developed to construct the two-dimensional planar decorated 3D network (2D-3D) in ionic-crosslinked alginate-based hydrogels, generating the strong synergistic mechanics. The strategy is based on a novel hybrid crosslinker (trivalent/divalent cations, e.g. Fe3+/Ca2+) with very low percentage of Fe3+ (1%), which surprisingly imparts multivalent cations with a balanced complexation competition in the hydrogels and thus co-joints loose 2D and rigid 3D structure units in a tightest matching manner. Using the approach in popular Ca-alginate/polyacrylamide system, the complementary architecture induces a 6-fold increase in yield strength accompanied with improved toughness, breaking its mechanical limitations. By varying the molar percentage of each cation in the primary crosslinker, their association abilities with the hydrogels are precisely tuned, giving architectures and mechanics over a wide range. Additionally, the diverse categories of hybrid crosslinkers become the ideal candidates for the development of programmable stimuli-responsive hydrogels, e.g., interesting reverse and unidirectional shape transformations. The method offers a new path in designing metal–ligand coordinated materials, which will certainly become popular in fabricating new generation of alginate-based hydrogels.

Transparent polyamide-imide films with high Tg and low coefficient of thermal expansion: Design and synthesis

Two series of optically transparent polyamide-imides (PAIs) were prepared by copolymerization of trimelletic acid chloride (TMAc), p-phthaloyl chloride (TPC), 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine (TFDB) and 4,4’-(9-fluorenylidene) diamine (FDA). These series of PAIs displayed high glass transition temperatures (Tgs) ranging from 340 to 441 °C, 5% weight loss temperatures (T5%s) in nitrogen with 459–505 °C, and low coefficient of thermal expansion (CTE) of 21–56 ppm/K, which greatly affected by the feed ratio of rigid TPC units and cardo diamine in the polymer main chain. The transmittances at 500 nm (T500nm) of the PAI films were found in the range of 70.0–87.7%, All polymers demonstrated good organosolubility and mechanical properties with tensile strengths of 77–146 MPa, tensile moduli of 2.9–5.8 GPa and elongations at break of 2.9–8.2%. PAI-7-8 presented the best combination of high Tg of 401 °C and high tensile modulus of 5.0 GPa, low CTE value of 24 ppm/K, and high transparency (T500nm) of 87.2%.

Sky-Blue Thermally Activated Delayed Fluorescence with Intramolecular Spatial Charge Transfer Based on a Dibenzothiophene Sulfone Emitter.

Intramolecular spatial charge transfer (ISCT) plays a critical role in determining the optical and charge transport properties of thermally activated delayed fluorescence (TADF) materials. Herein, a new donor/acceptor-type TADF compound based on rigid dibenzothiophene sulfone (DBTS) moiety, STF-DBTS, was designed and synthesized. Fluorene unit was used as a rigid linker to position the rigid acceptor and donor subunit in close vicinity with control over their spacing and molecular structure and to achieve high photoluminescence quantum yield (∼53%) and TADF property. For comparison purposes, we constructed the more flexible STF-DPS with a less rotationally constrained diphenylsulphone (DPS) acceptor instead of the rigid DBTS units, and STF-DPS showed no TADF properties and lower PLQY (16.0%). Organic light-emitting diodes (OLEDs) based on STF-DBTS achieve an external quantum efficiency (EQE) of 10.3% at 488 nm, which is a fivefold improvement in EQE with respect to STF-DPS.

Self-assembly of a poly(phenyleneethynylene) on multiwall carbon nanotubes: Correlation of structural and optoelectronic properties towards solar cells application

In this work, we report on the preparation of processable nanocomposites of multiwall carbon nanotubes (MWCNTs) and a conjugated copolymer, hereafter called pPET3OC12-sqS bearing a phenyleneethynylene trimer and a dithio disulfide moiety. This specific alternation of rigid and flexible units allows self-assembly of the conjugated backbone on the nanotubes wall through π- π interactions, without affecting the effective conjugation length of the polymer as found by 1HNMR, UV–Vis and fluorescence spectroscopy. The flexible sequences favour the stabilization of the composite by wrapping the MWCNTs wall as observed by electron transmission microscopy. As a result of this supramolecular arrangement, the absorption and emission maxima of the polymer are maintained but a quenching from 0.55 to 0.27 of the polymer fluorescence is found, which likely arises from efficient photoinduced electron transfer from the pPET3OC12-sqS to the MWCNTs. The incorporation of the nanotubes in the polymer matrix increases the electrical conductivity of the thin films in one order of magnitude and lowers the LUMO value, opening the possibility of a future application of the composite as electron acceptor material in organic photovoltaic devices.

The magneto-elastic and optical properties of multiferroic GaFeO3-δ

Temperature-dependent Raman spectroscopy, X-ray absorption near-edge spectroscopy (XANES), optical absorption spectroscopy (OAS) and ac-susceptibility measurements have been carried out on GaFeO3 samples to understand the effect of sintering temperature (TS) on optical and magneto-elastic properties. The temperature-dependent ac-susceptibility measurements suggest the systematic scaling in the values of ferri-magnetic transition temperature (TC) with TS. The observed variation has been understood in terms of enhancement in the Goodenough-Kanamori-Anderson coupling. Instead of expected anharmonic behavior in Raman spectra around TC, several vibrational modes of rigid FeO6 units were found to show anomalous behavior, which suggests the significant magneto-elastic coupling contributions to phonon frequencies. The strength of magneto-elastic coupling has been quantified by using the phonon renormalization function (Δw) and is found to scale systematically with TS. Further, optical absorption study demonstrates the increase in intensity and red shift in charge-neutral d-d transitions, which infers the increase in oxygen non-stoichiometry with TS. The oxygen non-stoichiometry has been further quantified by the XANES measurements. The present investigations on GaFeO3 samples reveal that TC can be tuned by varying the oxygen stoichiometry and can be very beneficial for spintronic devices applications.

Insights into Actin Polymerization and Nucleation Using a Coarse-Grained Model

We studied actin filament polymerization and nucleation with molecular dynamics simulations and a previously established coarse-grained model having each residue represented by a single interaction site located at the Cα atom. We approximate each actin protein as a fully or partially rigid unit to identify the equilibrium structural ensemble of interprotein complexes. Monomers in the F-actin configuration bound to both barbed and pointed ends of a short F-actin filament at the anticipated locations for polymerization. Binding at both ends occurred with similar affinity. Contacts between residues of the incoming subunit and the short filament were consistent with expectation from models based on crystallography, x-ray diffraction, and cryo-electron microscopy. Binding at the barbed and pointed end also occurred at an angle with respect to the polymerizable bound structure, and the angle range depended on the flexibility of the D-loop. Additional barbed end bound states were seen when the incoming subunit was in the G-actin form. Consistent with an activation barrier for pointed end polymerization, G-actin did not bind at an F-actin pointed end. In all cases, binding at the barbed end also occurred in a configuration similar to the antiparallel (lower) dimer. Individual monomers bound each other in a short-pitch helix complex in addition to other configurations, with several of them apparently nonproductive for polymerization. Simulations with multiple monomers in the F-actin form show assembly into filaments as well as transient aggregates at the barbed end. We discuss the implications of these observations on the kinetic pathway of actin filament nucleation and polymerization and possibilities for future improvements of the coarse-grained model.

The Push Drill Mechanism as a Novel Method to Create 3D Mechanical Metamaterial Structures

The push drill mechanism is used to produce a novel fully auxetic structure. The mechanism works by converting linear to rotational motion and vice versa, a feature that it has in common with that found in two major auxetic mechanisms, namely the rotating rigid units and the chiral systems. Farrugia and co-workers (see article number 2000125) provide a practical application of its use by connecting 2D layers of rotating squares with the push drill system, thus extending the original 2D auxetic structure to three dimensions. Depending on the connectivity, both conventional and auxetic behavior can be attained. It is also shown that the push drill system can act as a novel auxetic mechanism on its own in order to design structures exhibiting a negative Poisson’s ratio. One such scheme is presented on the cover.

Divalent Triazole‐Linked Carbohydrate Mimetics: Synthesis by Click Chemistry and Evaluation as Selectin Ligands

Starting from an enantiopure 3‐amino‐substituted pyran derivative, the synthesis of a series of divalent 1,2,3‐triazole‐linked carbohydrate mimetics is described. The preparation of the required 3‐azido‐substituted pyran proceeds smoothly by copper‐catalyzed diazo transfer. Using different conditions for the Huisgen‐Meldal‐Sharpless cycloaddition, this azide reacts with several diynes to furnish the desired divalent carbohydrate mimetics bearing rigid or flexible linker units. The in situ generation of the 3‐azidopyran in the presence of Cu/C as catalyst followed by the reaction with the alkyne allows a direct one‐pot transformation from the 3‐aminopyran to the desired click products. We also examined the Sakai‐Westermann method that transfers primary amines with the aid of α,α‐dichlorotosylhydrazones into 1,2,3‐triazoles. These copper‐free click conditions were applied for the first time to the preparation of a divalent compound. The O‐sulfation of the carbohydrate mimetics was achieved using the SO3‐DMF complex under careful 1H‐NMR control. Five polysulfated compounds could be obtained in pure form and these were tested by surface plasmon resonance spectroscopy as inhibitors of L‐selectin giving IC50 values between 45 nm and 50 µm.

Selectable and Releasable Noncovalent Functionalization of Semiconducting SWCNTs by Biethynyl‐2,5‐bis(dodecoxy)benzene Unit‐Containing Conjugated Copolymers

AbstractA rigid structural unit diethynyl‐2,5‐bis(dodecoxy)benzene (DDB) is proposed to functionalize single‐walled carbon nanotubes (SWCNTs) with the assistance of pyrimidine ring. The effectiveness of conjugated polymer extraction of semiconducting SWCNTs is demonstrated by absorption and Raman spectroscopy. The protonated pyrimidine ring by the trifluoroacetic acid is used to reduce the interaction between polymer and SWCNTs that the bound wrapping polymer can be removed from the sorted SWCNTs, and molecular dynamics simulations of the binding ability of DDB and pyrimidine units to SWCNTs are conducted to further illustrate the strong binding ability of DDB units to SWCNTs. Therefore, this work provides an effective structural and theoretical reference for polymer separation or modification of semiconducting SWCNTs.

Revisiting the Makishima–Mackenzie model for predicting the young's modulus of oxide glasses

The theoretically-derived Makishima-Mackenzie (MM) model expresses the Young's modulus of glass in terms of two determining factors, namely, the inter-atomic bonding strength (dissociation energy) and the ways in which atoms are packed (atomic packing fraction). This simple model offers a clear physical picture to understand the compositional dependence of the stiffness of glasses, but it generally underestimates the actual Young's modulus for many glasses, especially in the high value range. In this study, we argue that the inadequacy of the MM model mainly arises from its definition of the atomic packing fraction—which is defined as the ratio between the volumes of the atoms and the actual macroscopic volume of the glass. Such a definition results in a considerable amount of spacing within the basic building units being counted as free volume, which eventually leads to low packing fractions and, consequently, to low Young's modulus values. Here, we propose a more suitable packing metric, the Rigid Unit Packing Fraction (RUPF), which defines the basic building units as fully-filled, whole polyhedra made of “touching” oxygen atoms with no interstitial free volume. Young's moduli of 155 oxide glasses predicted from our revised MM model show a significantly improved level of agreement with respect to experimental data as compared to the original MM model. This study not only improves the ability of the physics-based MM model to yield accurate predictions of the Young's modulus, but also supports the relevance of rigid-unit theory, which could be applied as a basis to decipher other property-structure correlations.

The Push Drill Mechanism as a Novel Method to Create 3D Mechanical Metamaterial Structures

Two of the most prominent 2D auxetic mechanisms, namely, the rotating rigid unit and the chiral systems, work, at least partly, by converting linear to rotational motion. This property can be harnessed by introducing a system that can convert back the rotational motion to a linear displacement so as to induce an expansion or a compression in the out‐of‐plane direction. The push drill mechanism offers an effective way of providing this capability. It is, hence, possible to couple 2D rotating rigid unit or chiral structures to the push drill mechanism to create novel 3D auxetic structures. The resultant Poisson's ratio in the out‐of‐plane direction can be both negative and positive depending on the connectivity. Herein, the concept is illustrated with the use of rotating squares. An analytical model is derived to determine the parameters on which Poisson's ratio and Young's moduli depend. Subsequently, the analytical results are compared with those obtained from experimental testing of different sample structures with the two showing good agreement. Further considerations indicate that it is possible to design 2D and 3D auxetic systems with the push drill as the only deformation mechanism.

Effect of pendent phenyl and bis-phthalazinone moieties on the properties of N-heterocyclic poly(aryl ether ketone ketone)s

A series of novel N-heterocyclic copoly (aryl ether ketone ketone)s containing pendent phenyl groups and bis-phthalazinone moieties (P-BNP) with high molecular weight were successfully synthesized via solution condensation from 4-(3′-phenyl-4′-hydroxyphenyl)-2,3-phthalazin-1-one, 4,4′-di(4′-1(2H)-phthalazinone)-1,1′-biphenyl and 1,4-bis(4′-fluorobenzoyl)benzene. The properties of P-BNPs were identified by thermogravimetric analysis (TGA), differential scanning calorimetry analysis (DSC), and dynamic thermomechanical analysis (DMA) etc. The polymer exhibits excellent thermo-stability in both nitrogen and air. The glass-transition temperature (Tg) is in the range of 240–290 °C, copolymers performing a presentable thermo-resistance. Moreover, copolymers can be dissolved in chloroform and N-methyl-2-pyrrolidone (NMP), exhibiting excellent film-forming property. Simultaneously, copolymers possess favorable mechanical properties. The introduction of pendent phenyl moieties enhances the mobility of molecular chains and solubility of copolymers. The introduction of rigid bis-phthalazinone units enhances the thermo-resistance and mechanical properties of copolymers. Rational allocation of rigid units and side groups content will optimize the comprehensive properties of high-performance polymers.

Synthesis and study of physicochemical properties of relatively high birefringence liquid crystals: Tolane-type with symmetric alkoxy side groups

A Liquid Crystal (LC) consisting of rigid unit, tolane, and flexible alkoxy side groups, R = OC6H13, is synthesized. The purity of the product is verified by elemental analysis. Its chemical structure is characterized by, NMR and FT-IR spectroscopy. Their physicochemical properties are investigated by Polarizing Optical Microscope (POM) and Differential Scanning Calorimetry (DSC). The compound exhibits nematic phase, with good thermal stabilities and high birefringence. In addition, it has two smectic phases. The sequences of phases in the obtained results of POM are in good accordance with the results realized by DSC.

Hierarchical porous organic hyper-cross-linked polymers containing phthalazinone and carbazole moieties for gas uptake and fluorescence properties

Based on rigid and twisted heterocyclic phthalazinone unit as the core skeleton, novel hierarchical porous carbazole-containing porous organic polymers (PCMOPs) were successfully constructed through Friedel-Crafts reaction (PCMOP-Hs) and oxidation coupling reaction (PCMOP-Os), respectively, with anhydrous FeCl3 as catalyst. The structures of resultant PCMOPs were characterized by FT-IR, 13C CP-MAS NMR and WXRD. PCMOP-Hs displayed higher BET surface area (660–675 m2 g−1), CO2 uptake capacity (11.8–12.1 wt%, 273 K, 1 bar) and CO2/N2 selectivity (up to 78.46, IAST) than those of PCMOP-Os, respectively. Even though, PCMOP-Os both displayed high sensitivity and selectivity with the fluorescent quenching rate value up to 83% for 2,4-dinitrotoluene (DNT) and 70% for nitrobenzene (NB) in low concentration (10−2 mol/L). The properties along with large-scale and low-cost preparation make these hierarchical POPs potential candidates for gas storage and separation as well as fluorescence detection of explosives application.

Non-symmetric 9,10-Di(2-naphthyl)anthracene derivatives as hosts and emitters for solution-processed blue fluorescent organic light emitting diodes

A series of new asymmetric anthracene derivatives have been successfully synthesized for use in the light emitting layer of solution processed blue fluorescent organic light emitting diodes (OLEDs), which could be used both as host and dopant. Especially, new synthetic blue fluorescent materials with anthracene moieties without any alkyl chains showed high solubility in common organic solvents probably due to their severely twisted internal structures. They also show moderately high thermal stability due to their rigid internal core units. With those novel soluble blue fluorescent materials, we created moderately efficient solution-processed fluorescent OLEDs. And a new anthracene derivative [9-(9,10-di(naphthalen-2-yl)anthracen-2-yl)-9H-carbazole (DN-2-CzA)] as a host material exhibited moderately high efficiency and external quantum efficiency of up to 3.2cd/A and 1.6%, respectively, when we utilized 4,4′-bis[4-(diphenylamino)styryl]biphenyl as a dopant. Meanwhile, the [N-(naphthalen-1-yl)-9,10-di(naphthalen-2-yl)-N-phenylanthra-cen-2-amine (DN-2-NPAA)] as a dopant material showed pretty good device performance, up to 5.2cd/A and 2.2% (EQE), when we utilized 2-tert-butyl-9,10-di(naphth-2-yl)anthracene as a host.

Lignin-Based Epoxy Resins: Unravelling the Relationship between Structure and Material Properties.

Here we investigate the relationship between thermomechanical properties and chemical structure of well-characterized lignin-based epoxy resins. For this purpose, technical lignins from eucalyptus and spruce, obtained from the Kraft process, were used. The choice of lignins was based on the expected differences in molecular structure. The lignins were then refined by solvent fractionation, and three fractions with comparable molecular weights were selected to reduce effects of molar mass on the properties of the final thermoset resins. Consequently, any differences in thermomechanical properties are expected to correlate with molecular structure differences between the lignins. Oxirane moieties were selectively introduced to the refined fractions, and the resulting lignin epoxides were subsequently cross-linked with two commercially available polyether diamines (Mn = 2000 and 400) to obtain lignin-based epoxy resins. Molecular-scale characterization of the refined lignins and their derivatives were performed by 31P NMR, 2D-NMR, and DSC methods to obtain the detailed chemical structure of original and derivatized lignins. The thermosets were studied by DSC, DMA, and tensile tests and demonstrated diverse thermomechanical properties attributed to structural components in lignin and selected amine cross-linker. An epoxy resin with a lignin content of 66% showed a Tg of 79 °C from DMA, Young’s modulus of 1.7 GPa, tensile strength of 66 MPa, and strain to failure of 8%. The effect of molecular lignin structure on thermomechanical properties was analyzed, finding significant differences between the rigid guaiacyl units in spruce lignin compared with sinapyl units in eucalyptus lignin. The methodology points toward rational design of molecularly tailored lignin-based thermosets.

Preparation and Properties of Inherently Black Polyimide Films with Extremely Low Coefficients of Thermal Expansion and Potential Applications for Black Flexible Copper Clad Laminates

In the current work, a series of black polyimide (PI) films with excellent thermal and dimensional stability at elevated temperatures were successfully developed. For this purpose, two aromatic diamines including 4,4′-iminodianline (NDA) and 2-(4-aminophenyl)-5- aminobenzimidazole (APBI) were copolymerized with pyromellitic dianhydride (PMDA) to afford PIs containing imino groups (–NH–) in the molecular structures. The referenced PI film, PI-ref, was simultaneously prepared from PMDA and 4,4′-oxydianiline (ODA). The introduction of imino groups endowed the PI films with excellent blackness and opaqueness with the optical transmittance lower than 2% at the wavelength of 600 nm at a thickness of 25 μm and lightness (L*) below 10 for the CIE (Commission International Eclairage) Lab optical parameters. Meanwhile, the introduction of rigid benzimidazole units apparently improved the thermal and dimensional stability of the PI films. The PI-d film based on PMDA and mixed diamines (NDA:APBI = 70:30, molar ratio) showed a glass transition temperature (Tg) of 445.5 °C and a coefficient of thermal expansion (CTE) of 8.9 × 10−6/K in the temperature range of 50 to 250 °C, respectively. It is obviously superior to those of the PI-a (PMDA-NDA, Tg = 431.6 °C; CTE = 18.8 × 10−6/K) and PI-ref (PMDA-ODA, Tg = 418.8 °C; CTE: 29.5 × 10−6/K) films.

Anharmonicity and scissoring modes in the negative thermal expansion materials ScF3 and CaZrF6

We use a symmetry-motivated approach to analysing X-ray pair distribution functions to study the mechanism of negative thermal expansion in two ReO$_3$-like compounds; ScF$_3$ and CaZrF$_6$. Both average and local structure suggest that it is the flexibility of M-F-M linkages (M = Ca, Zr, Sc) due to dynamic rigid and semi-rigid "scissoring" modes that facilitates the observed NTE behaviour. The amplitudes of these dynamic distortions are greater for CaZrF$_6$ than for ScF$_3$, which corresponds well with the larger magnitude of the thermal expansion reported in the literature for the former. We show that this flexbility is enhanced in CaZrF$_6$ due to the rock-salt ordering mixing the characters of two of these scissoring modes. Additionally, we show that in ScF$_3$ anharmonic coupling between the modes responsible for the structural flexibility and the rigid unit modes contributes to the unusually high NTE behaviour in this material.

Mechanically Strong Globular‐Protein‐Based Fibers Obtained Using a Microfluidic Spinning Technique

Proteins used for the formation of light weight and mechanically strong biological fibers are typically composed of folded rigid and unfolded flexible units. In contrast to fibrous proteins, globular proteins are generally not regarded as a good candidate for fiber production due to their intrinsic structural defects. Thus, it is challenging to develop an efficient strategy for the construction of mechanically strong fibers using spherical proteins. Herein, we demonstrate the production of robust protein fibers from bovine serum albumin (BSA) using a microfluidic technique. Remarkably, the toughness of the fibers was up to 143 MJ m-3 , and after post-stretching treatment, their breaking strength increased to almost 300 MPa due to the induced long-range ordered structure in the fibers. The performance is comparable to or even higher than that of many recombinant spider silks or regenerated silkworm fibers. Thus, this work opens a new way for making biological fibers with high performance.