Small Side Groups Research Articles

Thermally Stable and Transparent Polyimide Derived from Side-Group-Regulated Spirobifluorene Unit for Substrate Application.

Advancements in flexible electronic technology, especially the progress in foldable displays and under-display cameras (UDC), have created an urgent demand for high-performance colorless polyimide (CPI). However, current CPIs lack sufficient heat resistance for substrate applications. In this work, four kinds of rigid spirobifluorene diamines are designed, and the corresponding polyimides are prepared by their condensation with 5,5'-(perfluoropropane-2,2-diyl) bis(isobenzofuran-1,3-dione) (6FDA) or 9,9-bis(3,4-dicarboxyphenyl) fluorene dianhydride (BPAF). The rigid and conjugated spirobifluorene units endow the polyimides with higher glass transition temperature(Tg) ranging from 356 to 468 °C. Their optical properties are regulated by small side groups and spirobifluorene structure with a periodically twisted molecular conformation. Consequently, a series of CPIs with an average transmittance ranging from 75% to 88% and a yellowness index (YI) as low as 2.48 are obtained. Among these, 27SPFTFA-BPAF presents excellent comprehensive performance, with a Tg of 422 °C, a 5wt.% loss temperature (Td5) of 562 °C, a YI of 3.53, and a tensile strength (δmax) of 140MPa, respectively. The mechanism underlying the structure-property relationship is investigated by experimental comparison and theoretical calculation, and the proposed method provides a pathway for designing highly rigid conjugated CPIs with excellent thermal stability and transparency for photoelectric engineering.

Colorless transparent and thermally stable terphenyl polyimides with various small side groups for substrate application

Advances in flexible display technology, especially, under-screen camera and on-board display, have created an urgent need for high-temperature stable and colorless transparent polyimide (CPI), but meeting both properties at the same time is a huge challenge in molecular design. In this work, a series of rigid p-terphenylenediamines with different small side groups were synthesized, and the corresponding polyimides (PI-1 ∼ PI-15) were prepared by polymerization with 5,5′-(Perfluoropropane-2,2-diyl) bis(isobenzofuran-1,3-dione) (6FDA) and 9,9-Bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) as dianhydrides. The conformation and steric hindrance of terphenyl-based polyimide chains were affected by the type (-H, -F, -CF3, -CH3), number and substitution position of the small side groups. This feature could be used to regulate transparency and heat resistance of polyimides. Consequently, the rigid terphenyl polyimides with an average transmittance ranging from 79 to 89% in the visible region and the glass transition temperature (Tg) ranging from 339 to 458 °C were successfully obtained. PI-11 and PI-15 containing both -CH3 side groups and -CF3 side groups exhibited excellent comprehensive performance, with a yellowness index (YI) of 1.53 and 5.95, a Tg of 420 and 436 °C, a tensile strength of 148 and 164 MPa, respectively. The strategy of side-group-regulated terphenyl polyimides provides a way for designing multi-aromatic CPIs with high thermal stability and good transparency for photoelectric engineering.

N-heterocyclic carbenes – The design concept for densely packed and thermally ultra-stable aromatic self-assembled monolayers

Self-assembled monolayers (SAMs) of N-heterocyclic carbenes (NHCs) on metal substrates are currently one of the most promising systems in context of molecular-scale engineering of surfaces and interfaces, crucial for numerous applications. Interest in NHC SAMs is mainly driven by their assumingly higher thermal stability compared to thiolate SAMs most broadly used at the moment. Most of the NHC SAMs utilize imidazolium as an anchoring group for linking molecules to the metal substrate via carbene C atom. It is well established in the literature that standing up and stable NHC SAMs are built only when using bulky side groups attached to nitrogen heteroatoms in imidazolium moiety, which, however, leads to monolayers exhibiting much lower packing density compared to thiolate SAMs. Here, by combined X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and temperature-programmed secondary ion mass spectrometry analysis, we demonstrate that using NHCs with small methyl side groups in combination with simple, solution-based preparation leads to the formation of aromatic monolayers exhibiting at least doubled surface density, upright molecular orientation, and ultra-high thermal stability compared to the NHC SAMs reported before. These parameters are crucial for most applications, including, in particular, molecular and organic electronics, where aromatic SAMs serve either as a passive element for electrode engineering or as an active part of organic field effect transistors and novel molecular electronics devices.

Polymers and Green Chemistry

The purpose of this lab is to introduce students to some basics of the chemistry of polymers. Polymer chemistry is often an afterthought in the General Chemistry curriculum, but polymers are very much present in students’ lives. The lab introduces the relationship between molecular structure (e.g., branched vs. linear, bulky vs small side groups) and bulk properties (e.g., flexibility). This lab is also “pre-stoichiometry” in the sense that precursor ideas to stoichiometry are introduced without formal stoichiometric terminology. In one activity, the ratio of reacting alginate to calcium ions is varied to find the maximum yield of alginate gel. The lab introduces atom economy using addition and condensation polymers without formal stoichiometry. Students are likely to be at least a little familiar with recycling polymers before they even step into the lab. Building on this, using polymers as a lab topic provides an opportunity to bring Green Chemistry and Sustainability concepts into the lab course. Working with biologically-sourced, water-soluble starch or alginate polymers should be greener than working with polystyrene or nylon polymers which do not dissolve in pure water. The aforementioned atom economy example is also related to Green Chemistry principles. This document features contributions from Bozana Lojpur at Bradley University and Saskia van Bergen at the Washington State Department of Ecology.

Thermoresponsive behavior of polyrotaxanes based on α-cyclodextrins threaded on poly(2-alkyl-2-oxazoline)-poly(ethylene glycol)-poly(2-alkyl-2-oxazoline) triblock copolymers

Polyrotaxanes with poly-2-alkyl-2-oxazoline blocking moieties have been successfully synthesized. The structure of the synthesized samples was confirmed by NMR spectroscopy, and the molar masses were measured by light scattering and GPC. The density of cyclodextrin rings on polyethylene glycol did not depend on the structure of poly-2-alkyl-2-oxazoline chains and ranged from 2 to 5. A high specific optical rotation of the prepared polyrotaxanes is found which is caused by the formation of hydrogen bonds between neighboring macrocycles. The behavior of water-salt solutions of synthesized polyrotaxanes depended on the size of the poly-2-alkyl-2-oxazoline side groups. Only polymers with poly-2-isopropyl-2-oxazoline and poly-2-n-propyl-2-oxazoline blocking moieties were thermoresponsive in aqueous solutions, and the phase separation temperature decreased at passage from a polymer with poly-2-isopropyl-2-oxazoline to a sample with poly-2-n-propyl-2-oxazoline. Aqueous solutions of polyrotaxanes with smaller side groups were molecularly dispersed at all temperatures, and the polymer with poly-2-butyl-2-oxazoline blocking chains did not dissolve in water.

Adatom mediated adsorption of N-heterocyclic carbenes on Cu(111) and Au(111).

The adsorption of N-heterocyclic carbenes (NHCs) on Cu(111) and Au(111) surfaces is studied with density-functional theory. The role of the molecular side groups as well as the surface morphology in determining the adsorption geometry are explored in detail. Flat-laying NHCs, as observed experimentally for NHC with relatively small side groups, result from the adsorption at adatoms and give rise to the so-called ballbot configurations, which are more stable than adsorption on flat surfaces and provide an efficient precursor for the formation of bis(NHC) dimers. On Au(111), the resulting (NHC)2 Au complexes are purely physisorbed and thus mobile. On the more reactive Cu(111), in contrast, the central Cu atom in the (NHC)2 Cu dimer is still covalently bound to the surface, resulting in a mobility, which has to be thermally activated.

The Effect of Alkyl Substitution of Novel Imines on Their Supramolecular Organization, towards Photovoltaic Applications.

Three novel conjugated polyazomethines have been obtained by polycondensation of diamines consisting of the diimine system, with either 2,5-bis(octyloxy)terephthalaldehyde or 9-(2-ethylhexyl)carbazole-3,6-dicarboxaldehyde. Partial replacement of bulky solubilizing substituents with the smaller side groups has allowed to investigate the effect of supramolecular organization. All obtained compounds have been subsequently identified using the NMR and FTIR spectroscopies and characterized by the thermogravimetric analysis, differential scanning calorimetry, cyclic voltammetry, UV–Vis spectroscopy, and X-ray diffraction. Investigated polymers have shown a good thermal stability and high glass transition temperatures. X-ray measurements have proven that partial replacement of octyloxy side chains with smaller methoxy groups induced a better planarization of macromolecule. Such modification has tuned the LUMO level of this molecule and caused a bathochromic shift of the lowest energy absorption band. On the contrary, imines consisting of N-ethylhexyl substituted carbazole units have not been so clearly affected by alkyl chain length modification. Photovoltaic activity of imines (acting as a donor) in bulk-heterojunction systems has been observed for almost all studied compounds, blended with the fullerene derivative (PCBM) in various weight ratios.

Development and characterization of multifunctional yttrium iron garnet/epoxy nanodielectrics

In the present work, a series of yttrium iron garnet/epoxy nanocomposite systems were fabricated, and the morphology of nanocomposites were tested by Scanning Electron Microscopy. The thermal stability of the developed systems was assessed by Thermogravimetric Analysis; their thermal properties were further investigated via Differential Scanning Calorimetry (DSC) and their viscoelastic response via Dynamic Mechanical Analysis (DMA). Finally, the dielectric characterization was carried out by means of Broadband Dielectric Spectroscopy. DSC curves revealed an endothermic step-like transition of all systems, attributed to the glass to rubber transition of the polymer matrix. All nanocomposites exhibit a two stages degradation profile. The presence of nanoinclusions enhances the thermal stability of the systems shifting the onset of the first degradation process to higher temperatures. Through the DSC and DMA techniques, the transition from glassy to rubbery state of the polymer matrix was observed and the characteristic Tg temperature was determined. The addition of the ceramic inclusions enhances the thermomechanical properties, as well as the dielectric response of the nanocomposites, as implied by the augmenting values of the real part of dielectric permittivity along with the storage modulus with the reinforcing phase loading. Three dielectric relaxation processes were identified: interfacial polarization, glass to rubber transition of the polymer matrix and reorientation of the small polar side groups of the polymer chain at low, intermediate and high frequencies, respectively. Furthermore, the ability of the systems to store energy was examined via the dielectric reinforcing function.

Investigating the Effect of Zn Ferrite Nanoparticles on the Thermomechanical, Dielectric and Magnetic Properties of Polymer Nanocomposites.

In this study nanocomposites consisting of an epoxy resin and ceramic zinc ferrite nanoparticles have been successfully developed and investigated morphologically and structurally by means of scanning electron microscopy (SEM) images and X-ray diffraction (XRD) spectra. The thermal properties of the nanocomposites were studied via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The thermomechanical characterization of the fabricated nanocomposites was studied via dynamic mechanical analysis (DMA) and the magneto-dielectric response was assessed by means of a broadband dielectric spectroscopy (BDS) and by employing a superconducting quantum interference device (SQUID) magnetometer. Data analysis demonstrates that the incorporation of nanoinclusions into the matrix improves both the thermomechanical and the dielectric properties of the systems, as indicated by the increase of the storage modulus, the real part of dielectric permittivity and conductivity values with filler content, while at the same time induces magnetic properties into the matrix. Zinc ferrite nanoparticles and their respective nanocomposites exhibit superparamagnetic behavior at room temperature. Three relaxations were recorded in the dielectric spectra of all systems; originating from the filler and the polymer matrix, namely interfacial polarization, glass to rubber transition of the polymer matrix and the reorientation of small polar side groups of the polymer chain.

Magneto-Dielectric Behaviour of M-Type Hexaferrite/Polymer Nanocomposites.

In the present study two sets of nanocomposites consisting of an epoxy resin and BaFe12O19 or SrFe12O19 nanoparticles were successfully developed and characterized morphologically and structurally via scanning electron microscopy and X-ray diffraction spectra. The dielectric response of the nanocomposites was investigated by means of broadband dielectric spectroscopy and their magnetic properties were derived from magnetization tests. Experimental data imply that the incorporation of the ceramic nanoparticles enhances significantly the dielectric properties of the examined systems and their ability to store electrical energy. Dielectric spectra of all systems revealed the presence of three distinct relaxation mechanisms, which are attributed both to the polymer matrix and the nanoinclusions: Interfacial polarization, glass to rubber transition of the polymer matrix and the re-orientation of small polar side groups of the polymer chain. The magnetic measurements confirmed the ferromagnetic nature of the nanocomposites. The induced magnetic properties increase with the inclusion of hexaferrite nanoparticles. The nanocomposites with SrFe12O19 nanoparticles exhibit higher values of coercive field, magnetization, magnetic saturation and remanence magnetization. A magnetic transition was detected in the ZFC/FC curves in the case of the BaFe12O19/epoxy nanocomposites.

Magnetic nanoparticles – polymer matrix nanodielectrics: Manufacturing, characterization and functionality

In the present study, series of nanocomposites consisting of magnetic hexaferrite (BaFe12O19, SrFe12O19) nanoparticles as reinforcing phase and an epoxy resin acting as matrix were manufactured and characterized by multiple techniques. The morphological characterization was conducted via Scanning Electron Microscopy (SEM). The thermomechanical properties were studied using Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Dynamic Mechanical Analysis (DMA) and tensile tests. The dielectric response of the nanocomposites and the related relaxation phenomena were examined by means of Broadband Dielectric Spectroscopy (BDS). Finally, the magnetic characteristics of the nanocomposites were investigated by employing a Superconducting Quantum Interference Device (SQUID) magnetometer.Experimental data demonstrate fine dispersions of nanoinclusions in all types of nanocomposites and an improvement in both the electrical and the mechanical properties of nanocomposites, increasing the filler concentration. Three different relaxation processes were observed: Interfacial Polarization, glass to rubber transition of the matrix and re-orientation of small polar side groups of the polymer chain. Furthermore, the specimens present ferromagnetic behaviour at room temperature and magnetization increases with filler content.

Kinetic Study on Achiral-to-Chiral Transformation of Achiral Poly(diphenylacetylene)s via Thermal Annealing in Chiral Solvent: Molecular Design Guideline for Conformational Change toward Optically Dissymmetric Structures

Achiral poly(diphenylacetylene)s (PDPAs: pMe3, pEt3, piPr3, pMe2O1, pMe2OD1, and mMe3) with different alkyl side chains at the para or meta position of the side phenyl ring were prepared to examine achiral-to-chiral transformations upon thermal annealing in a chiral solvent. All the para-substituted PDPAs showed significant circular dichroism (CD) enhancement upon annealing, whereas the meta-substituted polymer, mMe3, was inert to the same treatment. To investigate the kinetics, the asymmetric conformational change was monitored by observing the changes in the magnitude of circular polarization (gCD) or optical rotation. PDPAs with bulkier, round-shaped side groups (pEt3 and piPr3) had greater gCD values at equilibrium than pMe3 with a smaller side group. Moreover, the activation energy for the forward reaction (Eaf) was lower in the bulkier polymers than in pMe3 owing to enhanced miscibility with the chiral solvent. Similarly, the long alkyl chains of pMe2O1 and pMe2OD1 acted as internal plasticizers to ...

Effects of side groups on the entanglement network of cellulosic polysaccharides

The transient entanglement networks of cellulosic polysaccharides in concentrated solutions were characterized by the molecular weight between entanglements (M e) using dynamic viscoelasticity measurements. From the concentration dependence of M e, M e for the cellulosic polysaccharides in the molten state (M e,melt) was estimated as the material constant reflecting the chain characteristics. The values of M e,melt were compared among three cellulosic polysaccharides: cellulose, methylcellulose, and hydroxypropyl cellulose. Methylcellulose and hydroxypropyl cellulose were employed as cellulose derivatives having small and large side groups, respectively. It appeared that hydroxypropyl cellulose had significantly larger M e,melt compared with cellulose and methyl cellulose. However, the numbers of repeating glucose-ring units between entanglements were very close to each other among the three polysaccharides.

Structural Characterization of Chlorophyll-a by High Resolution Tandem Mass Spectrometry

A high resolution Fourier transform ion cyclotron resonance (FTICR) mass spectrometer is used for characterizing the fragmentation of chlorophyll-a. Three tandem mass spectrometry (MS/MS) techniques, including electron-induced dissociation (EID), collisionally activated dissociation (CAD), and infrared mutiphoton dissociation (IRMPD) are applied to the singly protonated chlorophyll-a. Some previously unpublished fragments are identified unambiguously by utilizing high resolution and accurate mass value provided by the FTICR mass spectrometer. According to this research, the two long aliphatic side chains are shown to be the most labile parts, and favorable cleavage sites are proposed. Even though similar fragmentation patterns are generated by all three methods, there are much more abundant peaks in EID and IRMPD spectra. The similarities and differences are discussed in detail. Comparatively, cleavage leading to odd electron species and H(•) loss both seem more common in EID experiments. Extensive loss of small side groups (e.g., methyl and ethyl) next to the macrocyclic ring was observed. Coupling the high performance FTICR mass spectrometer with contemporary MS/MS techniques, especially IRMPD and EID, proved to be very promising for the structural characterization of chlorophyll, which is also suitable for the rapid and accurate structural investigation of other singly charged porphyrinic compounds.

Study of the structure and dynamics of poly(vinyl pyrrolidone) by molecular dynamics simulations validated by quasielastic neutron scattering and x-ray diffraction experiments

Quasielastic neutron scattering, x-ray diffraction measurements, and fully atomistic molecular dynamics simulations have been performed on poly(vinylpyrrolidone) homopolymer above its glass transition temperature. A "prepeak" appears in the x-ray diffraction pattern that shows the typical features of a first amorphous halo. From an effective description of the experimentally accessed incoherent scattering function of hydrogens in terms of a stretched exponential function, we observe enhanced stretching and a momentum-transfer dependence of the characteristic time different from that usually reported for more simple polymers (main-chain polymers or polymers with small side groups). The comparison with both kinds of experimental results has validated the simulations. The analysis of the simulated structure factor points to a nanosegregation of side groups (SG) and main-chains (MC). The detailed insight provided by the simulations on the atomic trajectories reveals a partial and spatially localized decoupling of MC and SG dynamics at length scales between the average SG-SG distance and the characteristic length of the backbone interchain correlations. Anomalous behavior in correlators calculated for the SG subsystem are found, like e.g., logarithmiclike decays of the density-density correlation function. They might be a consequence of the existing large dynamic asymmetry between SG and MC subsystems. Our results suggest that, as the SGs are spatially extended and chemically different from the backbone, they form transient nanosegregated domains. The dynamics of these domains show similar behavior to that found in other systems displaying large dynamic asymmetry.

Shear modulus of polyelectrolyte gels under electric field

The shear modulus of two polyelectrolyte gels poly(acrylic acid), a weak acid with small side groups, and poly(2-acrylamide-2-methylpropane sulfonic acid), a strong acid with large side groups, have been measured with and without application of an electric field across a gel sample swollen with water to equilibrium. In the absence of electric field the shear modulus of PAMPS gels made to different crosslink degrees is shown to be inversely proportional to the swelling degree, in accord with theoretical prediction. Under a steady electric field the measured modulus of these gels is seen to reduce with time. This is explained by migration of the free counter ions and associated water towards the cathode, resulting in reduced contact between gel and rheometer due to exuded water. Theoretical prediction of G' and G'' in the presence of a thin slip layer are in good quantitative agreement with experimental observation. Preliminary measurements of the reduction of shear modulus under pulsed electric field have been obtained and an unexpected recovery of the initial modulus is seen on all subsequent applications of the field, before the continued reduction to successively lower values. When roughened platens are used a step-wise variation in measured modulus is seen, with a slightly lower modulus being recorded in the presence of an applied electric field.

Photophysics of Main-Chain Polychromophores Prepared by Acyclic Diene Metathesis Polymerization

A series of bis(vinylthienyl)silane monomers were prepared by reaction of 2 equiv of (vinylthienyl)lithium or (vinylthienyl)magnesium bromide with different dichlorosilicon dialkyls. The monomers react with the Schrock initiator Mo(NAr)(CHCMe2Ph)(OCMe(CF3)2)2 (Ar = 2,6-diisopropylphenyl), via the acyclic diene metathesis polymerization (ADMET) mechanism, to give poly(silanyl−dithienylethene) derivatives in excellent yield. GPC data suggest that the resulting polymers have short repeat sequences. Interchromophore cooperativity is evident, as a low energy emission, for polymer structures that have smaller side groups. This phenomenon is similar to the chromophore cooperativity observed in polymers containing stilbene fragments along the backbone.

Influence of molecular weight and branch content on the fracture behaviour of polyethylene

The fracture behaviour of a range of polyethylenes with different molecular weights and side group content has been studied by means of a novel high-pressure tensile testing facility. The normally ductile specimens became brittle at high pressures, e.g. 700 MPa, so that the test results could be studied by means of linear elastic fracture mechanics. Oriented specimens were prepared by die-drawing to draw ratios of 5∶1 and 9.5∶1. The performance of the isotropic and oriented (longitudinal and transverse directions) materials were assessed using their KIC values. Results were carefully compared in order to study the effects of molecular weight and low concentrations of small side groups on the fracture behaviour of polyethylene. The structure of the oriented polymers was characterised using wide-angle X-ray diffraction.

Polymer strength and chain conformation

It is found that the expected maximum strength of polymers is not proportional to the number of polymer chains per unit area. Under uniaxial stressing only part of polymer chains, i.e., overstressed chains, contribute to the strength of the polymer. The number of overstressed chains obtained here for some polymers is consistent with the results of electron paramagnetic resonance and stress-Fourier-transform IR measurements: (a) Polymers with all-trans conformation of their chains have more overstressed chains than those with helix conformation; (b) in the case of polymers with helix conformation, those with smaller side groups have more overstressed chains. Under uniaxial stress, the polymer chains change their conformation from gauche to trans which results in a relief of stress. Polymer with helix conformation thus have less overstressed chains and, hence, lower strength than polymers with all-trans conformation.

Theoretical approach for radiosensitizers. Correlation between calculated electroaffinity and sensitization factors of nitro-compounds.

The electron affinity of eight well-known nitroaromatic and nitroheterocyclic radiosensitizers is computed resorting to a quantum mechanical approach. Four of the compounds studied were nitroimidazoles; but p-nitroacetophenone and some 5-nitrofuran derivatives were also investigated for comparison. A good correlation between the theoretical electroaffinity values and the radiosensitization efficiencies is found in the case of the nitroimidazoles, in which the pi-electron system appears to be limited to the aromatic ring, except for small side groups. In the case of molecules where the pi-electron system spreads out on the side-chain the theoretical procedure seems to fail.