Rigid Network Structure Research Articles

1H HRMAS NMR Study on Phase Transition of Poly(N-isopropylacrylamide) Gels with and without Grafted Comb-Type Chains

Phase transition occurring in three different types of poly(N-isopropylacrylamide) gels—normal cross-linked gel, comb-type grafted gel, and comb-type grafted gel with styrene-modified comb chains—has been investigated by variable-temperature measurements of 1H NMR spectra and spin−spin relaxation time. Three different gels exhibit distinct collapse behaviors in response to increasing temperature. For the normal gel, remarkable network shrinking occurs in a relatively narrow temperature range from 32 to 35 °C. For the styrene-modified comb-type gel, overall chain shrinkage appears in a very broad temperature range from 22 to 35 °C in which the styrene-modified comb chains shrink at lower temperatures (22−32 °C) than the backbone networks (32−35 °C). In the comb-type gel without styrene modification, however, the backbone networks shrink first (at 32−35 °C) on heating, followed by collapsing of comb chains (at 35−36 °C). During shrinkage of backbone networks the comb chains are expulsed from the main gel networks which is revealed by abnormal T2 increase of comb chains. T2 measurements also reveal that the styrene-modified comb-type gel in the equilibrium swelling state has more rigid network structure than both conventional gel and comb-type gel without styrene modification.

Colloidal poly-imidazolium salts and derivatives

This study demonstrated the design and synthesis of chiral or achiral poly-imidazolium/poly-imidazolidium spherical particles. Particles with diameters in the range of nanometers to micrometers were formed spontaneously with certain spacer groups. The novel polymer particles have a rigid main-chain network structure. The parent ionic polymer particles could be easily transformed to stable poly-N-heterocyclic carbene (poly-NHC) particles and poly-NHC-metal organometallic particles with tailored chemical and physical properties. These colloidal nanoparticles and microparticles provided a unique platform for manipulating the versatile carbene chemistry. Poly-NHC-Pd particles exhibited excellent activity and recyclability in heterogeneous Suzuki cross-coupling reactions.

Phase Behavior and Microstructures of Nonionic Fluorocarbon Surfactant in Aqueous Systems

The phase behavior and self-assembled structures of perfluoroalkyl sulfonamide ethoxylate, C8F17SO2N(C3H7)(CH2CH2O)20H (abbreviated as C8F 17EO20), a nonionic fluorocarbon surfactant in an aqueous system, has been investigated by the small-angle X-ray scattering (SAXS) technique. The C8F17EO20 forms micelles and different liquid crystal phases depending on the temperature and composition. The fluorocarbon micellar structure induced by temperature or composition change and added fluorocarbon cosurfactant has been systematically studied. The SAXS data were analyzed by the indirect Fourier transformation (IFT) and the generalized indirect Fourier transformation (GIFT) depending on the volume fraction of the surfactant and complemented by plausible model calculations. The C8F17EO20 forms spherical type micelles above critical micelle concentration (cmc) in the dilute region. The micelle tends to grow with temperature; however, the growth is not significant on changing temperature from 15-75 degrees C, which is attributed to the higher clouding temperature of the surfactant (>100 degrees C). On the other hand, the micellar structure (shape and size) is apparently unaffected by composition (1-25 wt %) at 25 degrees C. Nevertheless, addition of fluorocarbon cosurfactant of structure C8F17SO2N(C3H7)(CH2CH2O)H (abbreviated as C8F17EO1) to the semidilute solution of C8F17EO20 (25 wt %) favors micellar growth, which finally leads to the formation of viscoelastic wormlike micelles, as confirmed by rheometry and supported by SAXS. The onset sphere-to-wormlike transition in the structure of micelles in the C8F17EO20/water/C8F17EO1 system is due to the fact that the C8F17EO1 tends to go to the surfactant palisade layer so that the critical packing parameter increases due to a decrease in the effective cross-sectional area of the headgroup. As a result, spherical micelles grow into a cylinder, which after a certain concentration entangle to form a rigid network structure of wormlike micelles.

The reduction of Eu 3+ to Eu 2+ in air and luminescence properties of Eu 2+ activated ZnO–B 2O 3–P 2O 5 glasses

A valence change from Eu 3+ to Eu 2+ was observed in the europium ion-doped ZnO–B 2O 3–P 2O 5 glasses prepared at high temperature in air. The fluorescence emission spectrum of the sample consists of a broad emission band ascribed to the 5d–4f transition of Eu 2+ ion and sharp emission peaks assigned to the transitions of 5D o– 7F J ( J = 0, 1, 2, 3, and 4) of Eu 3+ ion, indicating that part of Eu 3+ can be reduced into Eu 2+ in the glass. A charge compensation model is proposed. The rigid tetrahedral network structure of glasses plays an important role in stability of Eu 2+. The fabrication conditions are also studied.

Cover Picture: Macromol. Chem. Phys. 3/2006

Cover: The radical deactivation readily becomes diffusion controlled and causes an acceleration in the atom transfer radical polymerization of polylpar;ethylene glycolrpar; dimethacrylate. The rate increases with a decrease in the spacer length lpar;CH2CH2Orpar;x between the vinyl moieties of the dimethacrylate due to a more rigid network structure. Further details can be found in the article by Q.Yu,* J. Zhang, M. Cheng, and S. Zhu* on page 287.

Kinetic Behavior of Atom Transfer Radical Polymerization of Dimethacrylates

AbstractSummary: The reaction behavior and kinetics of the atom transfer radical polymerization (ATRP) of poly(ethylene glycol) dimethacrylates (PEGDMA) were studied with respect to polymerization rate, vinyl conversion and the development of a crosslinked network. The polymerization rates were much slower than the corresponding conventional free radical polymerizations with the ATRP systems exhibiting milder autoacceleration. The linear relationship of the semi‐logarithmic kinetic plot of ln([M]0/[M]) vs. time did not provide good evidence for any living nature of the system because of the combined effects of diffusion controlled radical deactivation and diffusion controlled monomer propagation. The influence of the spacer length (CH2CH2O)x between the vinyl moieties of the dimethacrylates on the polymerization kinetics was examined. The polymerization rate and final vinyl conversion increased as value of x decreased from 14 to 9 to 4. These increases in rate and conversion were caused by a more rigid network structure with shorter spacer lengths, and thus more restricted diffusion of the catalyst/ligand complexes that impeded the radical deactivation. The effect of temperature on the polymerization rate and final vinyl conversion were also investigated.Apparent rate constants versus vinyl conversions for the ATRP of PEGDMA with the different spacer lengths at 100 °C.magnified imageApparent rate constants versus vinyl conversions for the ATRP of PEGDMA with the different spacer lengths at 100 °C.

Increase in the level of arabinoxylan–hydroxycinnamate network in cell walls of wheat coleoptiles grown under continuous hypergravity conditions

Changes in the amount and composition of cell wall constituents in response to continuous hypergravity stimuli were studied in wheat (Triticum aestivum L.) coleoptiles. The lengths of coleoptiles grown under hypergravity (300 g) conditions for 2–4 days from germination stage were 60–70% of those of 1 g control. However, the net amounts of hemicellulosic polysaccharides and cellulose in hypergravity‐treated coleoptiles increased progressively as much as those in the control coleoptiles. As a result, their contents per unit length of coleoptile largely increased under hypergravity conditions. In the hemicellulose fraction, the amounts of arabinose and xylose, the major components of the fraction, prominently increased in response to hypergravity. When hemicellulosic polysaccharides were separated into neutral and acidic polymers by an anion‐exchange column, the amounts of the acidic fraction consisting of (glucurono)arabinoxylans were higher in hypergravity‐treated coleoptiles than in control coleoptiles. The amounts of cell wall‐bound ferulic acid and diferulic acid (DFA) increased dramatically in both 1 g control and hypergravity‐treated coleoptiles. Particularly, the amounts of DFA in hypergravity‐treated coleoptiles were significantly higher than those in control coleoptiles during the incubation period. These results suggest that continuous hypergravity increases the rigid network structures via arabinoxylan–hydroxycinnamate cross‐links within cell wall architecture in wheat coleoptiles. These structures may have a load‐bearing function and contribute to construct the stable cell wall against the gravitational force.

Study of Vibrational Spectra of Interlayer Water in Sodium Beidellite by Molecular Dynamics Simulations

To elucidate the molecular structure and water-clay interaction of interlayer water in sodium type smectite (hydrated layered alminosilicate), vibrational spectra of water were investigated using molecular dynamics (MD) simulations based on the free flexible force field model. Vibrational spectra of water were obtained by Fourier transformation of the velocity autocorrelation function of the hydrogen atom. Two distinct bands were found at 3365 and 3500 cm - 1 in the stretching vibrational spectrum of interlayer water. The former band was assigned to an O-H bond unbound to the clay surface, while the latter was attributed to O-H vibrations bound to the clay surface through hydrogen bonding. The hydrogen bond distance (the H...O distance) between water and the clay surface (H w a t e r ...O c l a y = 0.22 nm) was larger than that between water molecules (H w a t e r ...O w a t e r = 0.19 nm). Detailed comparison of simulation results with IR spectroscopic observations indicated good agreement. The hydrogen bond structure and the vibrational spectrum of interlayer water suggest no rigid network structure of water molecules (icelike water) near the smectite surface.

Ultra-thin tetrahedral amorphous carbon films with strong adhesion, as measured by nanoscratch testing

Amorphous carbon can form hard and dense films, which find applications for tooling components, magnetic recording, etc. The challenge is to prepare hard layers that have good adhesion to the substrate. Moreover, mechanical characterisation of sub-20 nm layer is still difficult and semi-quantitative. The paper is addressing these issues. We prepared 10-nm and 50-nm thick tetrahedral amorphous carbon (ta-C) and hydrogenated amorphous carbon (a-C:H) films, respectively, by filtered cathodic arc deposition (FCVA) and plasma enhanced chemical vapour deposition (PECVD). The films were characterised by nanoindentation and nano-scratching. With such hard films, blunting of the diamond tip can affect the measurements and we used a hardness slope ratio protocol, which gave more accurate results. The ta-C films were the hardest and more wear resistant, i.e. for the 50-nm thick ta-C film on Al 2O 3–TiC; H (25 nm)=51 GPa and residual depth=4 nm for a 5 mN scratch load. This is typical of the FCVA conditions, i.e. a relatively high energy, mono-energetic (approx. 25 eV) non-hydrogenated ion flux, resulting in formation of a dense sp 3 network. Ramping the scratch load to observe delamination events, we measured the highest critical loads (approx. 6 mN) again for the ta-C films. We used SEM/EDX thickness measurements to confirm these delaminations. Again, the ta-C films show the best adhesion. We believe that, the FCVA beam is sufficiently energetic to provide a dense and rigid network structure and a good ion beam mixing in the substrate but, appropriately, not enough to produce the large internal stress observed at −100 V substrate bias.

Electrical and Heating Properties of Nickel-coated Carbon Filler filled Plane Heater

Polymer composites filled with electrical conductive fillers are used in various industrial fields, and some of these materials show a positive temperature coefficient resistivity (PTC). This phenomenon is ascribed to volume expansion of the matrix polymer. By applying the PTC effect, these electrically conductive materials are able to be used as self-temperature control plane heater.In this study, new type of plane heater, different from ordinary plane heaters, with a rigid network structure filled with conductive fillers was prepared by a new processing method. First of all, the characteristics of the matrix urethane resin such as curing conditions and thermomechanical properties were investigated. This resin mixed with short nickel coated carbon fibers having different fiber lengths or aspect ratios was processed in to thin sheets and cured. The relationship between resistivity and fiber content and the influence of environmental temperature on resistivity of these composites were investigated experimentally. The relationship between heating properties and resistivity of the materials under supplied voltage and also the influence of environmental temperature on heating temperature were also discussed.

Solid-state NMR studies ofcis-1,4-polyisoprene crosslinked with dicumyl peroxide in the presence of triallyl cyanurate

The network of dicumyl peroxide (DCP)/triallyl cyanurate (TAC) crosslinked cis-1,4-polyisoprene was studied by solid-state NMR techniques such as direct-polarization (DP), cross-polarization (CP), and proton T2 experiments. Line broadening and cis/trans isomerization of mobile carbons were observed in the DP experiments. The information on rigid carbons of network structures was observed with the CP technique. Motional heterogeneity was examined by proton T2 relaxation experiments. Decreases in long T2 (T2L) values from the mobile non-network structures and short T2 (T2S) values from the rigid network structures were observed with an increase in peroxide or coagent concentration. The percentage of T2S in T2 relaxation, which is related to network density, was observed to increase with peroxide and coagent addition. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1417–1423, 2000

Photoluminescence behavior of SiO 2 prepared by sol–gel processing

Two kinds of SiO 2 were prepared by the usual sol–gel process (heat-treated sample) or a sol–gel process using photoirradiation (photoirradiated sample) and their photoluminescence behaviors were investigated. All heat-treated samples had photoluminescence bands, but none of the photoirradiated samples did. Intensity of emission of heat-treated sample increased as the treatment temperature increased. This photoluminescence behavior occurred reversibly with thermal treatment and photoirradiation. The network structure of SiO 2 samples were evaluated by solid state NMR. The intensity of 17O solid state NMR peak increased with increased emission intensity. These results implied that heat-treated samples included some defects which showed photosensitivity, but the photoirradiated samples lacked such defects and formed a rigid network structure instead. These defects caused the observed photoluminescence.

Vibrational spectroscopy of single methanol molecules attached to liquid water clusters

Medium-sized water clusters [(H2O)n, n=15–20], produced by adiabatic expansion of water vapor into a vacuum, were doped with single methanol molecules by crossing the water cluster beam with an effusive methanol beam. The spectroscopy of the adsorbed methanol molecule has been studied, by exciting the C–O stretch with a CO2 laser (9.6 μm) and the C–H or O–H stretch with a Nd:YAG-laser-pumped optical parametric oscillator (2.6–3.6 μm) and monitoring the vibrational predissociation of the complex (infrared molecular beam depletion spectroscopy). It is found that the methanol molecule adopts a surface site and that it is bound to the water cluster by two or three hydrogen bonds, with the methanol molecule acting as proton donor and proton acceptor. As is evidenced by the essentially unperturbed C–H stretching spectrum, the methyl group is pointing away from the host cluster surface. Analysis of the dissociation products reveals that the water cluster does not adopt a rigid network structure. Instead it is composed of loosely bound subclusters which can be expelled from the host cluster by exciting the methanol chromophore with low-energy laser photons at moderate flux. The close resemblance to IR spectra of liquid water suggests that the doped water clusters are liquid.

Electrochemical Studies on Ion Transport in Gels with Scanning Electrochemical Microscopy

Ion transport properties of two gels made from polyacrylate and polyacrylamide (PAAM) were studied with scanning electrochemical microscopy (SECM) by using three ferrocene derivatives, which carry different charges, as the probe molecules. Ion diffusion in these gels could be nearly as fast as in aqueous electrolytes and did not adhere to the Stokes−Einstein equation. Passivation of the electrode surface in polyacrylate gels reported previously was mainly caused by the electrophoretic deposition of gel particles on the surface of the electrode. The deposited gel film can act as a cation-exchange membrane and shows permselectivity, which affects both the electrochemical and SECM behavior. No detectable film formation was found in the PAAM gel perhaps because of its neutral charge and rigid three-dimensional network structure. The small change in diffusion coefficients as a function of PAAM gel concentration and charges of the probe molecules suggest much of the transport in the gel occurs via a water-filled domain. The transient (chronoamperometric) SECM response could be used to determine diffusion coefficients from the critical transient time as a function of tip displacement without knowing the concentration of the redox mediator, the tip radius, and the number of electrons transferred in the electrode reaction.

Fibrin Gel Network Characteristics and Coronary Heart Disease: Relations to Plasma Fibrinogen Concentration, Acute Phase Protein, Serum Lipoproteins and Coronary Atherosclerosis

The native fibrin gel structure formed in vitro from plasma samples was examined by liquid permeation of the hydrated fibrin gel networks in 18 men who had suffered a myocardial infarction before the age of 45 years and in 20 control subjects. Patients with an elevated plasma fibrinogen concentration had a considerably lower fibrin gel porosity (permeability coefficient, Ks) compared with patients with a normal plasma fibrinogen level and with controls. The calculated fiber mass-length ratio of the fibrin gel networks was decreased in both patient groups. Gel porosity differed markedly between individuals at a given plasma fibrinogen concentration. Fairly strong inverse correlations were found between plasma orosomucoid level on the one hand and Ks (r = -0.617, p less than 0.01) or fiber mass-length ratio (r = -0.499, p less than 0.05) on the other. The low density lipoprotein (LDL) cholesterol concentration also correlated inversely with Ks (r = -0.471, p less than 0.05) and fiber mass-length ratio (r = -0.522, p less than 0.05). Significant inverse relations, which were independent of plasma fibrinogen and lipoprotein concentrations, were detected between Ks (r = -0.519, p less than 0.05) and calculated fiber mass-length ratio (r = -0.723, p less than 0.001) and number and severity of coronary artery stenoses determined by angiography. A proneness to formation of tight, rigid and space-filling fibrin network structures with small pores thus appears to be associated with premature coronary artery disease.

A novel preparation technique for preparing hydrogenated amorphous silicon with a more rigid and stable Si network

A novel preparation technique, termed ‘‘chemical annealing,’’ was developed with the aim of making a stable and rigid Si network structure. The hydrogen content (CH) in the films and the optical gap could be reduced gradually without any change in substrate temperature by alternating deposition of a hydrogenated amorphous silicon layer several tens of Å thick and treatment with atomic hydrogen. These films showed CH of 1.5–10 at. %, and exhibited high photoconductivities in the level of 10−5–10−4 S/cm. In the films with CH of 3 at. % or less, in particular, improvement was observed in stability against illumination with light. Their photoconductivity remained at about 65% of the initial value even after illumination with white light (AM1, 100 mW) for 60 h. In addition, time-of-flight experiments revealed a significant enhancement in hole drift mobility to a value of 0.2 cm2/V s at 300 K.

Hydration—dehydration-induced conformational changes of agarose, and kappa- and iota-carrageenans as studied by high-resolution solid-state 13C-nuclear magnetic resonance spectroscopy

High-resolution, solid-state 13C-n.m.r. spectra of agarose, and kappa- and iota-carrageenans were recorded for various states, including anhydrous and hydrated powder, film, gel, and lyophilized gel. The 13C-n.m.r. spectra varied substantially with the extent of hydration. Fully hydrated samples, as in the agarose gels, gave rise to 13C-n.m.r. peaks of the narrowest line widths, whereas dehydration caused considerable line-broadening or displacement of the G1 peak (or both). These observations were interpreted in terms of hydration-dehydration-induced conformational alteration or readjustment, depending on the extent of hydration or dehydration and the variety of samples used. 13C-n.m.r. spectrometry of agarose gel by solid-state n.m.r. technique also indicated the existence of a rather rigid network structure with an anisotropic molecular tumbling.

Permeability studies with crosslinked cellophane

Evidence for the crosslinking of cellulose by chemical treatment has been discussed by numerous authors. In the present investigation an effort is made to determine whether such a structural change in cellulose may be inferred from a change in permeability. The effect of a dialdehyde starch crosslinking treatment on the permeability of cellophone, to potassium chloride ions from dilute solution is studied. Diffusion coefficients are calculated and compared. To account for sensitivity of diffusion results to the microstructure of the cellophane, it is assumed that under the experimental conditions a significant portion of the transport of ions across the membrane may take place by a process of activated diffusion directly through the polymer network. It is suggested that the crosslinking treatment blinds adjacent cellulose chains together creating a more rigid network structure which should resist penetration by diffusing ions.