Oligomer Chain Research Articles

Dual approach of bimodality and nano-reinforcement towards toughened PDMS based foul release coatings

It is well established that crosslinked polydimethylsiloxane (PDMS) based coatings have efficacious foul release characteristics. However, a trade-off between mechanical integrity of filled and crosslinked PDMS based coatings and their foul release performance has been a bottleneck for producing efficient and durable coatings. In the present study, we report a dual approach of nano-reinforcement of a bimodal PDMS network as a strategy to produce simultaneously reinforced and toughened PDMS networks with facile release of macrofoulants from their surfaces. The nanocomposites of both unimodal and bimodal PDMS networks were prepared using oligomeric dihydroxyl functional PDMS precursor chains differing in their molecular weights by a factor of five and commercial Cloisite-20A nanoclay as the nanofiller. The clay layers were found to be mildly intercalated in the PDMS matrix, as revealed from investigations by scattering and imaging techniques at different length scales. While the unimodal PDMS networks did show nanoclay induced simultaneous reinforcement and toughening, at equivalent clay loadings, the bimodal PDMS nanocomposites seemed to stretch further with characteristic strain hardening before fracture. Dynamic mechanical analysis (DMA) and swelling studies of the nancomposites further confirmed the nanoclay induced reinforcement effect of the bimodal PDMS host matrix. The intrinsic low surface energy characteristics of PDMS were retained by bimodal blending of long and short chains and its subsequent nano-reinforcement. Macrofouling studies by panel immersion and release force measurements revealed that the macrofoulants could be dislodged from the nanocomposite coatings with a shear force < 0.05 MPa.

Soy Protein Isolate Non-Isocyanates Polyurethanes (NIPU) Wood Adhesives

Soy-protein isolate (SPI) was used to prepare non-isocyanate polyurethane (NIPU) thermosetting adhesives for wood panels by reacting it with dimethyl carbonate (DMC) and hexamethylene diamine. Both linear as well as branched oligomers were obtained and identified, indicating how such oligomer structures could further cross-link to form a hardened network. Unusual structures were observed, namely carbamic acid-derived urethane linkages coupled with lactam structures. The curing of the adhesive was followed by thermomechanical analysis (TMA). It appeared to follow a two stages process: First, at a lower temperature (maximum 130°C), the growth of linear oligomers occurred, finally forming a physically entangled network. This appeared to collapse and disentangle, causing a decrease of MOE, as the temperature increases. This appears to be due to the ever more marked Brownian movements of the linear oligomer chains with the increase of the temperature. Second, chemical cross-linking of the chains appeared to ensue, forming a hardened network. This was shown by the thermomechanical analysis (TMA) showing two distinct MOE maxima peaks, one around 130°C and the other around 220°C, with a very marked MOE decrease between the two. Plywood panels were prepared and bonded with the SPI-NIPU wood adhesive and the results obtained are presented. The adhesive appeared to pass comfortably the requirements for dry strength of relevant standards, showing to be suitable for interior grade plywood panels. It did not pass the requirements for wet tests. However, addition of 15% of glycerol diglycidyl ether improved the wet tests results but still not enough to satisfy the standards requirements.

Graphene-alumina nanostructured hybrid: synthesis with use dodecylamine and physicochemical properties

Combination of sol–gel and sonochemical techniques is suggested for producing a nanostructured graphene–alumina hybrid, which is a promising raw material for manufacturing fine-grained ceramics. This study focuses on the use of dodecylamine both in the sol–gel synthesis of Al-containing particles and in sonochemical graphene exfoliation. It has been shown that, when forming graphene–alumina composite in a reaction mixture of Al-containing sol and graphene suspension, graphene sheets take part as a template in the formation of a hybrid structure. Graphene sheets orient sol particles on their surface due to van der Waals interaction and control growth and direction of oligomeric gel chains. The graphene sheets limit the growth of Al-containing particles, which leads to an increase in the specific surface of the hybrid as compared to an alumina powder sourced from the same sol. The proposed approach enables the preparation of chemically homogeneous composite nanopowders with uniform distribution of components.

Exploring Solvent-Related Reactions and Corresponding Band Gap Tuning Strategies for Carbon Nanodots Based on Solvothermal Synthesis.

The role of the solvent in the solvothermal synthesis of carbon nanodots (CDs) is a long-standing puzzle for comprehensively understanding the CD formation mechanism and the relevant band gap tuning phenomenon. The controversy regarding solvent effect continues, largely because the importance of solvent-related reactions (SRs) has been ignored. In this Letter, we report that the SRs, i.e., oxidation-assisted solvent-involved dehydration and solvent-promoted oligomerization of phenylene/pyridinediamine (PD/PyD), are observed and dominate the formation of corresponding CDs by producing highly C2H4/C2H4O-featured species of precursors and PEG-like oligomer chains, which is very different from previously proposed theories. Moreover, our findings reveal that the photoluminescence (PL) of as-synthesized CDs originates from fluorophores formed by SRs and can be tailored by fluorophore engineering. Based on the SRs theory, three strategies for PL band gap tuning have been demonstrated.

Polymer Conformations, Entanglements and Dynamics in Ionic Nanocomposites: A Molecular Dynamics Study.

We investigate nanoparticle (NP) dispersion, polymer conformations, entanglements and dynamics in ionic nanocomposites. To this end, we study nanocomposite systems with various spherical NP loadings, three different molecular weights, two different Bjerrum lengths, and two types of charge-sequenced polymers by means of molecular dynamics simulations. NP dispersion can be achieved in either oligomeric or entangled polymeric matrices due to the presence of electrostatic interactions. We show that the overall conformations of ionic oligomer chains, as characterized by their radii of gyration, are affected by the presence and the amount of charged NPs, while the dimensions of charged entangled polymers remain unperturbed. Both the dynamical behavior of polymers and NPs, and the lifetime and amount of temporary crosslinks, are found to depend on the ratio between the Bjerrum length and characteristic distance between charged monomers. Polymer–polymer entanglements start to decrease beyond a certain NP loading. The dynamics of ionic NPs and polymers is very different compared with their non-ionic counterparts. Specifically, ionic NP dynamics is getting enhanced in entangled matrices and also accelerates with the increase of NP loading.

Influence of oligomeric unsaturated polyketone on the vulcanization of elastomeric compositions in the presence of sulfenamide C

Effect of the oligomeric unsaturated polyketone (UPK) additive on the rheological properties, vulcanization, and structural characteristics of elastomeric compositions based on butadiene and nitrile-butadiene rubbers containing N-cyclohexyl-2-benzothiazolylsulfenamide (sulfenamide C) as vulcanizing accelerator is investigated. It was demonstrated using IR and NMR spectroscopy that sulfenamide C reacts with polyketone. A mechanism for such interaction, which includes the sequential addition of benzothiazyl sulfide and amine fragments to the oligomeric chain, was proposed.

A Track-Based Molecular Synthesizer that Builds a Single-Sequence Oligomer through Iterative Carbon-Carbon Bond Formation

Summary We report an artificial molecular machine that moves along a track, iteratively joining building blocks to form an oligomer of single sequence with a continuous backbone of carbon-carbon bonds. The rotaxane features a macrocycle bearing an aldehyde-terminated chain and an axle containing different phosphonium ylides separated by rigid spacers. Each ylide is large enough to block the passage of the macrocycle, trapping the ring between the stopper at the terminus of original threading and the next ylide along the track. Once a building block is reachable, it is removed from the track through a Wittig reaction that adds it to the terminus of the growing chain. Operation on a four-barrier tetra(phosphonium salt) track produces a tetra(diphenylpropane) of single sequence linked through alkene bonds. The prototype extends the principle for molecular machines that build polymers by moving along tracks to the synthesis of sequence-encoded chains with continuous carbon backbones.

Flow-Induced Formation of Thin PEO Fibers in Water and Their Stability After the Strain Release

Recently, we have shown that a tensile stress applied to chains of poly(ethylene oxide) (PEO) in water reduces the solubility and leads to phase separation of PEO chains from water with the formation of a two-phase region. In this work, we further elucidate the generic mechanism behind strain-induced phase transitions in aqueous PEO solutions with concentrations of 50-60 wt % by performing all-atom molecular dynamics simulations. In particular, we study the stability of oriented PEO fibers after removing stretching forces. We found that the size of the PEO bundle increased with time, which is associated with the dissolution of PEO chains on the fiber surface due to the reformation of hydrogen bonds between the outer PEO molecules and water. For precise characterization of the fibers, the scattering patterns (small- and wide-angle X-ray spectra) for configurations taken at different relaxation times are calculated. The tendency of the oligomer chains to be peeled off from the surface of the bundle eventually might lead to a complete dissolution of the PEO fiber. We conclude that either entanglement constraints or a quick drying process are necessary to conserve the fiber structure in a quiescent state. The scattering results show that external strain induced a liquid-liquid phase separation first. On long time scales, this can be a precursor for crystallization of the fiber.

Surface structure of 1,4-benzenedithiol on Au(111)

The structure of self-assembled monolayers of 1,4-benzene dithiol (1,4-BDT) oligomers on Au(111) has been investigated using scanning tunneling microscopy (STM) measurements in ultrahigh vacuum combined with density function theory (DFT) calculations. The dominant adsorption geometry of 1,4-BDT oligomer chains involves an Au adatom linker forming a complex with two trans anchoring groups, resulting in a zigzag structure. STM measurements show preferential growth from step edges where gold adatoms can be extracted to form the oligomers. Ladder-like structures involving two parallel oligomer chains are observed at medium coverages, suggesting a stabilizing interaction between neighboring chains and reduced mobility following oligomerization. At high-coverages, the gold surface is covered with close-packed domains of oligomers. DFT STM simulations were performed using the Tersoff-Hamann method in order to elucidate structures observed in STM measurements.

α-Chymotrypsin catalyses the synthesis of methotrexate oligomers

The enzymatic synthesis of methotrexate (MTX) catalysed by α -chymotrypsin was studied for the first time. The proteolytic enzyme displayed activity for the synthesis of MTX oligomers composed by 6 repeating units (DPavg = 1.5). For longer oligomers, molecular dynamics simulations confirmed that as the oligomeric chain grows its accommodation in the enzymes’ active site is hindered, which is evidenced by a decrease of the binding energy associated. The full characterization of the oligomers produced was performed by nuclear magnetic resonance (NMR, 1H and 13C), matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF), electrospray ionization (ESI) and differential scanning calorimetry (DSC).

Model Lignin Oligomer Pyrolysis: Coupled Conformational and Thermodynamic Analysis of β-O-4′ Bond Cleavage

The abundance, carbon content, and functionalized nature of lignin make it a promising candidate for targeted valorization to fuels and polymer composites. While lignin modeling by the application of computational chemistry is an active area of research, electronic structure methods have been limited mainly to structures in the dimeric or trimeric range. In this study, we have modeled a lignin structure composed of 10 β-O-4′ linked guaiacyl (G) units, such that this work represents, to the best of our knowledge, the largest structure that has been examined to date using quantum mechanical calculations. As such, this work can provide information on a model, the size of which is more representative of the lignin polymer than has been previously reported. We have calculated bond dissociation enthalpy (BDE) for the homolytic cleavage reaction between each G unit in our model lignin oligomer, which occurs as one of the initial reactions during lignin pyrolysis. The objective of the current work was to determine how or if reactivity within the oligomer changes as a function of bond cleaving position within the chain. The methods used were classical molecular mechanics for conformational sampling and quantum mechanically based density functional theory (DFT) calculations. We have developed a novel and robust method for conformational sampling, which maps the conformational energy landscape efficiently and provides multiple low-energy structures that are then used to determine the BDE values by DFT. Our results for BDE calculations of lignin exhibit significant position dependence along the oligomer chain. To the best of our knowledge, we have reported for the first time the calculated standard thermodynamic properties including enthalpy of formation, heat capacity, entropy, and Gibbs free energy. Despite using a simplified model lignin oligomer structure, our calculated values for standard thermodynamic properties have a remarkable agreement with the experimental values.

Supramolecular Oligourethane Gel with Multicolor Luminescence Controlled by Mechanically Sensitive Hydrogen-Bonding.

We report a new type of mechanically sensitive multicolor luminescent oligourethane gel (OUA-gel). The conformation of the oligomeric chains can be controlled by changing the strength of hydrogen bonds. The optical properties of the oligomers are highly dependent on the conformations which vary in response to mechanical stresses and phase transitions. The design relies on the introduction of a single mechanical chromophore, aurintricarboxylic acid, with propeller-like, spatially crowded, and highly twisted conformations, and the presence of three carboxyl groups, which provide multidirectional hydrogen-bonding opportunities. Introducing dimethylsulfoxide (DMSO) as an additional H-bond acceptor molecule leads to a viscous OUA-gel which exhibits multiemission colors because of changes in the chain conformation within the matrix, which are induced by different strengths of H bonds. The conformation can be adjusted by mechanical force or temperature, both of which influence the H-bonding. The multifunctional and multicolored mechanochromism of the OUA-gel has great promise in sensing applications. The results represent a substantial step toward understanding the mechanism of polychromism in soft materials and the molecular design of advanced smart materials.

Light Tracking and Light Guiding Fiber Arrays by Adjusting the Location of Photoresponsive Azobenzene in Liquid Crystal Networks

AbstractThe integration of light tracking and light guiding within fiber arrays is an intriguing challenge. In this study, an advanced drop casting/drawing method is applied to fabricate a fiber array capable of not only performing sunflower‐inspired light tracking, but also light guiding. The fiber arrays are constructed with various liquid crystal networks by adjusting the location of photoresponsive azobenzene moieties in the polymer network to understand the correlation between the extent of photoresponse and azobenzene location. Incorporating the azobenzene in the main chain oligomer renders these fibers with advanced photoresponse in both air and water. The fibers are able to track a light source and to guide the collected light, making these responsive actuator arrays potentially attractive for advanced photovoltaic and optical elements.

Effect of surface oxygen groups in the electrochemical modification of multi-walled carbon nanotubes by 4-amino phenyl phosphonic acid

Electrochemical functionalization of pristine Multi-walled Carbon Nanotubes (MWCNTs) and oxidized MWCNTs by nitric acid with 4-aminophenyl phosphonic acid (4-APPA) has been studied. Electrochemical modification has been carried out by cyclic voltammetry using different upper potential limits, what results in the incorporation of N and P functionalities through oligomerization and covalent attachment. The electrochemical characterization shows that there are important differences among the functionalized materials derived from the pristine MWCNTs or the oxidized materials. For the pristine MWCNTs, well-defined redox processes are observed. Characterization by X-Ray photoelectron spectroscopy, Raman spectroscopy and electron microscopy, show that oligomerization is easier for the pristine MWCNTs and the presence of oligomer chains are clearly distinguished for the lowest oxidation potentials. However, the presence of surface oxygen groups in the oxidized MWCNTs modify the interaction of the 4-APPA with the surface promoting mainly covalent attachment.

Crystal structure of the mouse endonuclease G

Endonuclease G (EndoG) is a mitochondrial enzyme that responds to apoptotic stimuli by translocating to the nucleus and cleaving the chromatin DNA. The molecular mechanism of EndoG still remains unknown in higher organisms. Here, we determined the crystal structure of mouse EndoG at ∼1.96 Å resolution. The EndoG shows an altered dimeric configuration in which N-terminal region of one subunit interact to the other subunit in dimer. The deletion of this region that is highly conserved in mammalian EndoGs resulted in a monomer with significantly reduced activity suggesting the association of the dimeric arrangement into the nuclease activity. Furthermore, we observed a large conformational change in the loop of the active site groove in EndoG, which corresponds to the DNA binding region. Intriguingly, EndoG dimers are linked by oxidation of the reactive cysteine 110 in this flexible loop to form a long oligomeric chain in the crystal lattice. The structural analysis and ensuing biochemical data suggest that this flexible loop region in the active site is important to the regulation of EndoG nuclease function in mouse.

One-pot facile sol-gel synthesis of W, N, C and S doped TiO2 and its application in the photocatalytic degradation of thymol under the solar light irradiation: Reaction kinetics and degradation mechanism

The ions W6+, N3−, C4+ and S6+ were incorporated into TiO2 lattice (WNCST) by sol-gel method. Sodium tungstate and l-cysteine were the precursors for these dopant ions. Energy dispersive X-ray studies confirmed the stoichiometry of the WNCST samples. Powder X-ray diffraction studies confirmed the anatase structure for TiO2, W6+-doped TiO2 and WNCST samples. The preferential sites for the occupation of dopant ions in the TiO2 lattice, the oligomeric octahedral chain growth mechanism and the mechanism of retention of anatase phase in the WNCST photocatalysts was explained in detail. The WNCST samples showed extended absorption in the visible region due to the creation of various dopant/defect energy levels. The plot of crystal density versus dopant concentration implied that the WNCST samples were omission solid solutions containing various defects such as neutral/ionized vacancies of Ti and O. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the presence of carbonate and sulfate groups on the surface of WNCST samples. Photoluminescence confirmed the optimum dopant concentration of cysteine as 0.17 mol %. The enhanced visible light photocatalytic activity of WNCST samples was mainly attributed to extended light absorption in the visible region due to the presence of 5d energy levels of W6+ ions and 2p states of S6+ and N3− dopant ions, surface adsorbed electron withdrawing sulfate groups and surface adsorbed carbonaceous content acting as a sensitizer. The thymol degradation reaction intermediates were identified using high performance liquid chromatography, UV–visible absorption spectroscopy and gas chromatography-mass spectrometry.

Influence of Nanoconfinement on the pKa of Polyelectrolyte Functionalized Silica Mesopores

AbstractFunctionalized ordered mesoporous materials are relevant in technologies, such as drug release, sensing, and separation. To design functionality, the silica framework can be functionalized with responsive molecules or polymers. Often, the pH value in those hybrid materials determines performance. Even though pH/pKa differences between polymers in bulk solutions and nanoscale confinement have been observed, the influence of confinement on pH‐ and pore filling dependent polyelectrolyte oligomer chain charge has yet not been investigated systematically. Here, mesoporous silica films are functionalized with (2‐dimethylamino)ethyl methacrylate) (DMAEMA) and 2‐(methacryloyloxy)ethyl phosphate (MEP) oligomers using photoiniferter initiated polymerization. This approach allows a controlled and environmentally friendly mesopore functionalization in water. The obtained oligomer functionalized pores are tunable with respect to pore filling. For both, poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(2‐(methacryloxy)ethyl phosphate) (PMEP), the charge generation inside mesopore confinement is significantly delayed toward harsher pH conditions resulting in pKa shifts of 1–2 pH units. Polymer amount and ionic strength show to further influence the pKa of PDMAEMA in mesopores. The technological importance of the pH value in confinement and its effect on enzyme stabilization is demonstrated. Lipase from Aspergillus oryzae loses its activity upon encapsulation in silica nanoparticles at pH values where the enzyme is stable in bulk solution.

One-pot lipase-catalyzed esterification of ε-caprolactone with methyl-d-glucopyranoside and its elongation with free 6-hydroxyhexanoate monomer units.

One-pot synthesis of sugar-functionalized oligomeric caprolactone was carried out by lipase-catalyzed esterification of ε-caprolactone (ECL) with methyl-d-glucopyranoside (MGP) followed by the elongation of functionalized oligomer chain. Functionalization was performed in a custom-fabricated glass reactor equipped with Rushton turbine impeller and controlled temperature at 60°C using tert-butanol as reaction medium. The overall reaction steps include MGP esterification of ECL monomer and its subsequent elongation by free 6-hydroxyhexanoate monomer units. A ping-pong bi-bi mechanism without ternary complex was proposed for esterification of ECL and MGP with apparent values of kinetic constant, namely maximal velocity (Vmax ), Michaelis constant for MGP (KmMGP ), and Michaelis constant for ECL (KmECL ) at 3.848×10-3 MH-1 , 8.189×10-2 M, and 6.050M, respectively. Chain propagation step of MGP-functionalized ECL oligomer exhibits the properties of living polymerization mechanism. Linear relationship between conversion (%) and number average molecular weight, Mn (gmol-1 ), of functionalized oligomer was observed. Synthesized functionalized oligomer showed narrow range of molecular weight from 1,400 to 1,600gmol-1 with more than 90% conversion achieved. Structural analysis confirmed the presence of covalent bond between the hydroxyl group in MGP with carboxyl end group of ECL oligomer.

Highly efficient supported AlCl3‐based cationic catalysts to produce polyα‐olefin oil base stocks

AbstractThe main aim of this research is to decrease the amount of AlCl3 content that is very corrosive and hazardous in the catalytic system, required for the α‐olefin oligomerization without substantial change of final oil features. This was successfully achieved by supporting AlCl3 on different carriers. More precisely, a series of supported bimetallic catalysts was synthesized by immobilization of AlCl3 and TiCl4 onto Al2O3, SiO2, and mixed supports, that is, Al2O3/FeCl3 and SiO2/FeCl3. It was found that silica and alumina‐based catalysts had higher catalytic activities compared to support free AlCl3; however, this enhancement for silica‐based supports was more significant. According to gel permeation chromatography (GPC) results, the use of single supports, that is, Al2O3 and SiO2, increased oligomer's molecular weight, while the application of mixed supports resulted in the decrease of molecular weight of the oligomers. Viscosity characteristics of the synthesized oligomers have also been studied at two different temperatures of 40 and 100°C (KV40 and KV100). The viscosity index (VI) values, derived from KV40 and KV100, of the prepared oligomers were in the range of 126–145. The molecular weight and termination mechanisms of the oligomers were studied by 1H‐NMR spectroscopy. The obtained results disclosed that the employed reaction conditions led to the production of oligomer chains with various structures including vinylidene (Vd), and di and three‐substituted vinylene (2Vn, 3Vn) structures.

Structure–activity relationships in well-defined conjugated oligomer photocatalysts for hydrogen production from water

Oligomer chain length and backbone twisting were found to have a strong effect on optoelectronic properties but a trimer of dibenzo[b,d]thiophene sulfone was found to have high photocatalytic activity approaching that of its polymer analogue.