X-ray crystallographic study of solid-state polymerization of trioxane and tetraoxymethylene

An X-ray study has been applied to clarify the relationship between crystal structures and crystal orientations of tetraoxymethylene and polyoxymethylene obtained by solid- state polymerization of tetraoxymethylene. The polyoxymethylene crystal obtained by the solidstate polymerization of a tetraoxymethylene single crystal was found to be definitely oriented with respect to the original tetraoxymethylene single crystal without any aftertreatment; i.e., the polymer chain (the c-axis) is parallel to the b-axis of tetraoxymethylene and the a-axis of polyoxymethylene coincides to the c-axis of tetraoxymethylene.

In addition to the main orientation, other preferred crystalline orientations (“twin structure”) of polyoxymethylene were observed in polymers polymerized at lower temperatures (60-80°C); i.e., the polymer chains are parallel to either the [100], [001], or [101] direction of the original tetraoxymethylene crystal, though the amount of such oriented crystallites is small. The twin structure is different from that of polyoxymethylene obtained from trioxane. This difference is due to the crystal structures of these original materials; tetraoxymethylene occurs in the monoclinic system, while trioxane belongs to the trigonal system, as does polyoxymethylene. These characteristic orientations are explained in terms of the molecular arrangements in the crystals of the original materials.     

Formation of a grating of submicron nematic layers by photopolymerization of nematic-containing mixtures

A submicron, spatially periodic, structure consisting of a sequence of oriented layers of a nematic liquid crystal (NLC), separated by isotropic polymeric walls, was obtained experimentally. This was accomplished by polymerization induced by the interference pattern of UV laser radiation in a NLC-containing prepolymer mixture. It was established that such a structure occurs when phase separation and nematic ordering are prevented during the polymerization process. These structures are the diffraction elements, whose efficiency is much greater than that of a standard grating of polymer-dispersed liquid crystals [1–4] which is obtained in the same initial mixture. Specifically, a diffraction efficiency of 60–70% was obtained for structures with the period Δ = 0.2 μm, even in mixtures where a grating with Δ < 6 μm cannot be obtained at all by the standard technique.     

Variable morphology of PTFE-like polymer nanocrystals fabricated by oriented plasma polymerization at atmospheric pressure

Rapid formation of PTFE-like polymer nanocrystals was achieved by a novel synthesis method—oriented plasma polymerization (OPP) at atmospheric pressure. The entire process was completed within a short period of time ranging between a few seconds to several minutes through dielectric barrier discharge (DBD) at atmospheric pressure. The surface morphology of the coated organic crystal film was observed through scanning electronic microscope (SEM) and different morphology nanocrystals were found such as nanorods and nanotubes. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed the single-crystalline phase of these nanocrystals. The sizes of the nanosingle crystals were from 10 nm to 1 μm. The effects of discharge conditions such as discharge time, ratio of the monomer to carrier gas and power on the nanocrystalline morphology and crystallinity were investigated. As a result, the physical morphology and structures could be controlled through the conditions of the oriented plasma polymerization to some extent. This novel polymerization method opened a new way to nanofabricate polymeric crystallines fast and effectively.     

Poly(4-vinylpyridine) supported acidic ionic liquid: A novel solid catalyst for the efficient synthesis of 2,3-dihydroquinazolin-4(1H)-ones under ultrasonic irradiation

A novel poly(4-vinylpyridine) supported acidic ionic liquid catalyst was synthesized by the reaction of 4-vinylpyridine with 1,3-propanesultone, followed by the polymerization and the addition of the heteropolyacid. Due to the combination of polymer features and ionic liquid, it acted as a heterogeneous catalyst to effectively catalyze the cyclocondensation reaction of anthranilamide with aldehydes under ultrasonic irradiation and afforded the corresponding 2,3-dihydro-4(1H)-quinazolinones compounds in good to excellent yields. In addition, the catalyst could be easily recovered by the filtration and reused six times without significant loss of catalytic activity. More importantly, the use of ultrasonic irradiation can obviously accelerate the reaction.     

Accurate torsional potentials in conjugated systems: ab initio and density functional calculations on 1,3-butadiene and monohalogenated butadienes

Accurate calculations of the torsional potentials for rotation around the carbon–carbon single bond of all conceivable monohalogenated 1,3-butadienes C₄ H ₅X, (X?∈?[F, Cl, Br]), are presented. The parent compound, 1,3-butadiene, is also included as a benchmark and reference case. Large-scale ab initio calculations were performed at the second-order Møller–Plesset perturbation theory (MP2) and the coupled cluster, CCSD(T), levels. Additionally, density functional methods were applied. In all compounds considered, the anti- or s-trans-conformation is the most stable. For all three halogens, the 2-halo-1,3-butadiene is the most stable isomer, followed by the cis-1-halo-1,3-butadiene. Depending on the position and the type of halogen, the original 1,3-butadiene torsional potential is modified in a different manner. The modifications are particularly visible in the region of the syn- or s-cis and the gauche structures and in the barrier heights.     

Structural distortion in thiourea-mixed ADP crystals

Single crystals of ammonium dihydrogen phosphate (ADP) mixed with different mole concentrations of thiourea were grown using slow evaporation solution technique at 30 °C. In order to study the effect of mixing thiourea on the structural characteristics of ADP, X-ray diffraction studies were carried out on the crystals using Shimadzu X-ray diffractometer with Cu Kα radiation. X-ray study revealed that the structures of the thiourea-mixed ADP are slightly distorted compared to the pure ADP crystal structure. Inclusion of thiourea enhances the growth of () plane of the ADP crystal. Thiourea-mixed ADP crystals were found to have maximum inclusion, as the thiourea concentration was 10 mol%.     

Entropic control over nanoscale colloidal crystals

ABSTRACT

Here the collective results of a recent body of work, which reveal how polymer architecture determines the most thermodynamically stable colloidal crystal structure in binary nanoscale colloid–polymer mixtures, are reviewed. At the nanoscale, the dimensions of the colloids, polymer segments and overall polymer size begin to converge. This may be exploited to thermodynamically stabilise a single desired crystal polymorph from a suite of competitors by leveraging the size and shape of the polymer. When each polymorph has a unique void symmetry, the entropic cost of polymer confinement in each crystal becomes significantly different. Thus, when a sufficient amount of polymer partitions into the crystal phase, the system's total free energy difference between the competing structures is significantly amplified; in some cases by up to three orders of magnitude. The focus of this discussion is primarily on selectively stabilising one of the two close-packed polymorphs over the other; however, the heuristics presented here also lend themselves to applications in other crystals. This approach to polymorph selection requires no modification of the colloids, and is entirely based on entropy. Consequently, this technique is thermodynamically complementary to many ‘bottom-up’ self-assembly approaches, which rely on energetic interactions to stabilise a single crystal structure.     

On the low-temperature chemistry of 1,3-butadiene

In this paper, species versus temperature profiles were measured during the oxidation of 1,3-butadiene in a jet-stirred reactor (JSR) at 1 atm, at different equivalence ratios (φ = 0.5, 1.0 and 2.0), in the temperature range 600 – 1020 K. Both synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC) methods were used to analyze the species. The experimental results show that a large proportion of the products are aldehydes (formaldehyde, acetaldehyde, acrolein, etc.) and ketenes (ketene, methyl-ketene), with acrolein being one of the major products. Moreover, furan, 1,3-cyclopentadiene and benzene are also present as intermediates in significant amounts. The reaction pathways leading to the formation of these species are discussed in detail. A new detailed mechanism, NUIGMech1.3, was developed to simulate these new data as well as other experimental data available in the literature. The validation results indicate that quantum calculations are also needed to explore the formation of some important species formed in the oxidation of 1,3-butadiene. Overall, the new 1,3-butadiene mechanism agrees well with various experimental data in the low- to high-temperature regimes and at different pressures. Flux and sensitivity analyses show that 1,3-butadiene shares some common reaction chemistry pathways with 1- and 2-butene via Ḣ atom and HȮ₂ radical addition to the C = C double bond in 1,3-butadiene, reactions which are important for both systems. The low temperature chemistry of 1,3-butadiene is mainly controlled by the reaction pathways of ȮH radical addition to the C = C double bond of the fuel molecule. The 1-buten-4-ol-3-yl radicals so formed subsequently add to O₂ and react via the Waddington mechanism, which is important in accurately simulating the oxidation and auto-ignition of 1,3-butadiene at engine relevant conditions.     

Crystallization during polymerization with special reference to synthetic polypeptides

In order to pursue the possibilities of the extended chain crystals formed in the course of polymerization, kinetics of the heterogeneous polymerization of L-and DL-alanine NCA in acetonitrile and the morphologies of the resultant polymers were studied. Electron microscopy revealed that the fibers formed in the beginning of polymerization thickened as the polymerization proceeded and that the growth mechanism of the polypeptide crystals is quite similar to that of polyoxymethylene crystals during the polymerization of trioxane in cyclo-hexane. X-ray diffraction patterns and IR spectroscopy revealed that the growing chain molecules, when they attained a certain degree of polymerization (DP = -3 to 4) beyond which they lost solubility in the system, crystallized into the form of ribbon-like fibers in which molecules, with the β-conformation, were aligned perpendicularly to the fiber axis. As the growing chains attained a critical length by further polymerization, they began to take the α-helical conformation which seemed to be more stable than the β-conformation in the later stages. The crystals grew successively thereafter, taking the α-helical conformation through addition of the monomer molecules onto the reactive chain ends.

The grown polypeptide chains were loosely packed but extended α-helices in the crystals throughout the polymerization. Possible mechanisms are proposed for the nucleation and the subsequent growth of the crystals in the course of polymerization.

The polymerization of N-carboxy anhydrides of glycine and β-alanine has also been studied in acetonitrile with n-butylamine as initiator. As was expected from the similarity of the backbone chain structure to nylons, the polymer precipitated in the form of a spherulite consisting of ribbon-like crystalline units just as in the case of nylons previously reported. The molecules in the ribbon-like crystals were arranged antiparallel, forming a hydrogen-bond sheet.     

Comparative study on BIS thiourea cadmium acetate crystals using HRXRD, etching, microhardness, UV-visible and dielectric characterizations

〈1 1 1〉 oriented bis thiourea cadmium acetate (BTCA) crystal of diameter 15 mm and length 45 mm was grown for the first time by the unidirectional Sankaranarayanan-Ramasamy (SR) method. The conventional and SR method grown BTCA crystals were characterized by using high-resolution X-ray diffraction (HRXRD), chemical etching, Vickers microhardness, UV-vis, dielectric studies and differential scanning calorimetry. The HRXRD analysis indicates that the crystalline perfection of SR method grown crystal is good without having any low angle internal structural grain boundaries. The transmittance of SR method grown BTCA is 14% higher than that of conventional grown crystal. The dielectric constant was higher and the dielectric loss was less in SR method grown crystal. The crystals grown by SR method possess less dislocation density and higher microhardness.     

Crystallization and Melting Behavior of Monodisperse Oligomers of ϵ-Caprolactone

Monodisperse ?-caprolactone (CL) oligomers with different end groups (t-butyldimethylsilyl, benzyl, hydroxyl, and carboxylic acid) and different numbers of repeating units (4–64) have been studied by differential scanning calorimetry and small-angle X-ray scattering (SAXS) in order to gather information regarding the melting temperature, long period, and melting enthalpy. Oligomers crystallized at their maximum temperatures (different for the different oligomers) to full crystallinity yielded extended-chain crystals for oligomers with 4, 8, and 16 repeating units with the important exception of the oligomers with four and eight repeating units and hydroxyl and benzyl end groups that showed double-layer crystals. Oligomers with 32 and 64 repeating units exhibited remarkably stable once-folded (32-mer) and thrice-folded (64-mer) crystals. Only the oligomer with 16 repeating units showed two crystallization temperature regimes resulting in once-folded crystals (low temperatures) and extended-chain crystals (high temperatures). The end groups had a profound effect on the structures. Hydrogen-bonding groups promoted the formation of crystal bilayers and led to a very high melting enthalpy (150 J g^?1) exceeding the melting enthalpy of 100% crystalline poly (?-caprolactone). The bulky end groups, in particular t-butyldimethylsilyl, reduced the crystallinity and favored chain tilting and probably preventing the unfolding of crystal stems in the oligomers with 32 and 64 repeating units. Melting temperatures of mature crystals obeyed a linear relationship with inverse CL stem length. The intercept (equilibrium melting temperature) was in the range of 350 to 357 K.     

Controlled growth behavior of chemical vapor deposited Ni nanostructures

Isolation of four distinct nanostructured Ni products is demonstrated in a well-controlled chemical vapor deposition process. These nanostructures include core–shell Ni–NiO nanowires, horizontally oriented nanowires, vertically oriented nanowires, and fully isometric cubic crystals all obtained upon an amorphous SiO₂|Si growth substrate from an identical metal halide precursor. Transmission electron microscopy indicates the horizontally- and vertically-oriented nanowire products to be high-quality single crystals with a preferred growth axis along the ?001? direction while the Ni–NiO core–shell nanowires are polycrystalline metal at the center and surrounded by an outer oxide. The differing crystal structures are reflected in the magnetic response of each nanowire type, as evidenced by magnetoresistance measurements. Detailed discussion of the formation mechanisms leading to each of the four nanostructured Ni products is presented along with a discussion of the general applicability of this non-epitaxial growth process to other material systems.     

Electron irradiation effects on optical properties of semiorganic antimony thiourea bromide monohydrate single crystals

Antimony thiourea bromide monohydrate (ATBM) single crystals were grown by solution growth technique at room temperature for the first time. The UV–vis, FT-IR and fluorescence spectra were recorded and electron irradiation effects on these properties were studied. The optical absorption edge of the UV–vis spectrum shifts towards lower wavelength with the increase of irradiation. The fluorescence quantum yield is increased for electron irradiated ATBM crystals. The FT-IR analysis shows that the water of crystallization is weakly bonded in as-grown and electron irradiated ATBM crystals.     

Structure and crystallization of molecular complexes between poly(ethylene oxide) and hydroxybenzenes

The phase diagrams of the binary systems poly(ethylene oxide) (PEO)-resorcinol and poly(ethylene oxide)-p-nitrophenol show the presence of molecular complexes with well-defined stoichiometries. The crystal structure of these two molecular complexes has been determined from wide-angle x-ray diffraction patterns of stretched films and spherulites. The morphology of the two complexes crystallized from the melt is investigated by differential scanning calorimetry and small-angle x-ray scattering. The crystallization of the PEO-resorcinol complex from the melt gives integral-folded crystals with either extended chains (EC) or n-folded chains (n-FC). As observed for PEO oligomers, the fraction of EC crystals of the PEO-resorcinol complex increases with the crystallization temperature to give finally only EC crystals but in a larger range of crystallization temperatures than for pure PEO. On the other hand, the PEO-p-nitrophenol complex crystallizes over all the studied crystallization range as stable nonintegral-folded (NIF) crystals. Two proposals related to the crystal structure of these complexes and their mode of growth are invoked to explain these two greatly different morphologies at the lamellar level.     

The pressure-dependence of the dimerization of 1,3-butadiene: Experimental and theoretical volumes of activation and reaction

Abstract

The volume of activation found for the [4+2]-Diels-Alder cyclodimerization of 1,3-butadiene leading to 4-vinylcyclohexene turned out to be substantially lower than that found for the competing [2+2]-cyclodimerization leading to trans-divinylcyclobutane (ΔΔ₀ ^#= ?12cm³mol^?1). The [4+2]-cyclodimerization of Z, Z-1,4-dideuterio-l,3-butadiene shows only 3 % loss of stereochemistry at 1 bar and <1 % at 6.8–8.0 kbar. These findings show good evidence for a stereospecific pericylic Diels-Alder mechanism competing with a small amount of nonstereospecific stepwise reaction which is almost completely suppressed at high pressure. The volumes of activation and reaction for the various dimerization pathways of 1,3-butadiene are calculated by a Monte-Carlo computer simulation. The good agreeement between experimental and simulated data confirms the hypothesis that configurational effects (e.g. different packing of cyclic and acyclic states) are important for the explanation of activation and reaction volumes.     

Ethylene-Oxygen Reactions Induced by Exposures to a Continuous CO2 Laser

The reactions of ethylene induced by irradiation with a relatively low power, continuous CO₂ laser were studied. Ethylene and ethylene oxygen mixtures were irradiated using a laser beam of power (a) 25 watts and (b) 40 watts. Ethylene irradiated with a 25 watts laser produced propylene as the major product in a slow reaction. An 80:100 ethylene-oxygen mixture with a laser power of 25 watts produced a faster reaction, with acetaldehyde, 1,3-butadiene and benzene as major products. Both ethylene and an 80:20 ethylene:oxygen mixture produced methane propylene and 1,3-butadiene as major products when irradiated with 40 watts of, laser power.

The reaction was found to result from a direct laser radiation-ethylene interaction and was not due to thermal effects. A study was undertaken to determine the reaction intermediates of these systems. The products suggested the formation of ethylene diradical at 25 watts and triplet methylene at 40 watts. These results were surprising since the energy available from a photon with 10.6 μλ was insufficient to generate such radicals directly.     

Investigation on mechanical properties of some thiourea complex crystals: Important nonlinear optical materials

In this work the authors have grown good quality single crystals of zinc thiourea sulphate, bis thiourea cadmium acetate, bis zinc thiourea acetate and bis thiourea zinc chloride were grown from slow evaporation solution growth method at ambient temperature and studied their mechanical properties. The crystal system and lattice parameters were confirmed by powder X-ray diffraction analysis. Vickers microhardness of the grown crystals was investigated by using Leitz-Wetzlar (Miniload 2) hardness tester up to an applied load of 120 g. It was observed that, upto 40 g applied load the hardness of the grown crystals increases with increasing the load and thereafter it is practically independent of the indentation load. Meyer's law and Hays–Kendall's law fail to explain the observed load variations. But the variation could be satisfactorily explained by PSR model proposed by Li and Bradt. Classification of cracks and their transition from Palmqvist to median types is explained. The average value of fracture toughness, brittleness index, Young's modulus and yield strength were calculated using expressions for Palmqvist and median types of cracks. Anisotropic nature of the grown crystals was studied using Knoop indentation technique.     

Synthesis of nanostructured polymer materials with different morphologies: Nanoporous ordered networks and hollow capsules

Nanostructured polymer materials with interesting morphological variation, which include three dimensionally interconnected uniform nanoporous network arrays (volume- and surface-templated ordered arrays) and hollow core spheres were synthesized by inducing different polymerization process of phenol and formaldehyde as a precursor over silica templates (ordered silica colloidal crystals or individual silica particles). The pore sizes of the resulting nanostructured polymer materials can be easily controlled by monitoring the sizes of silica spheres, while their morphologies were modulated by controlling the initiation sites of the acid-catalyzed condensation reaction of the same polymer precursor and by modifying silica templates.     

Phase separated composite films of liquid crystals

Phase separation of liquid crystals from a solution with polymers has long been studied and used to prepare polymer stabilized and polymer dispersed structures. They are formed by spatially isotropic phase separation. A new mode, in which the phase separation proceeds anisotropically, has recently been discovered. Known as phase separated composite films (PSCOF), the resultant structures are made of adjacent parallel layers of liquid crystal and solidified polymer. PSCOFs have been made with nematic, ferroelectric (FLC), and antiferroelectric (AFLC) liquid crystals. Liquid crystals in PSCOFs exhibit electro-optical properties not observed in devices prepared by conventional methods, polymer dispersion, or polymer stabilization methods. Devices incorporating FLCs possess grey scale and switch 100 times faster at low fields than conventional surface stabilized devices. This method makes it possible to prepare very flexible devices and devices with liquid crystal film thickness comparable to optical wavelengths with great ease.     

Morphology and Structure of Poly(p-phenylene Benzobisthiazole) Crystals

The rigid polymer poly(p-phenylene benzobisthiazole) (PBZT) was crystallized from dilute solution. Electron microscopy showed that, on quenching, flat fibrils several nanometers thick were produced. Subsequent heat treatment in a solvent changed the morphology from fibrillar into “segmented ribbon” structure. Isothermal crystallization at a temperature of about 30°C below the dissolution temperature, in general, resulted in aggregation of rod crystals. The polymer chains were oriented normal to the rod crystals. The width of the rod crystal increased with average molecular length, but saturated to a value much smaller than the average molecular length. In the shorter molecular length range, the rod crystals clustered in a fanned-out manner, while with a medium molecular length (ca. 70–120 nm), all rods crystals in a cluster aligned parallel to each other and were of the same length. With longer molecular length (more than ca. 180 nm), the rod growth slowed because of small diffusion constants of molecular chains to the growing face. Based on observation of the morphology and the crystallization process, an isothermal crystallization mechanism is proposed. Because of the rigidity and wide length distribution of polymer chains, the chain ends were inevitably included within the crystals, resulting in crystal defects such as axial shifts, lattice curvatures, and edge dislocations, all of which were observed directly by lattice imaging.