Estimating Lc in carbon fibers

A procedure is described for estimating L_c, the fibril dimension perpendicular to graphitic planes in carbon fibers. The procedure is based on a lubricostratic structural model. The model consists of graphitic planes, within which carbon atoms are arranged as in a graphite crystal. The horizontal component of the translation of neighboring planes is arbitrary. The distribution of the spacing, z, between nearest-neighboring graphitic planes is assumed to be represented by α²(z?b)exp[?a(z?b)]. where z is restricted to values equal to or greater than b = 3.354 Å( = $\text{c}{}_0\text{2}$ for graphite). This representation proves to be particularly advantageous, since both the number of such planes (hence L_c) and the distribution parameter, α can be determined from the {0002} reflection.     

Dependence of the lattice thermal expansion perpendicular to the layer planes of pyrocarbons on their structural properties

The lattice thermal expansion perpendicular to the layer planes (α_c) has been measured between room temperature and 1200°C on a large variety of pyrocarbon structures deposited in a fluidized bed as coatings on nuclear fuel particles. A correlation of α_c with the structural and physical properties characterizing the deposits indicated that a linear relationship exists between α_c and the apparent stack size L_c, α_c increasing with L_c. High resolution electron micrographs of such pyrocarbons show that the crystallites are spherulite-shaped particles with the layer planes bent parallel to the external surface. Polygonization of the spherulites associated with growth is believed to furnish indicative elements for an explanation of the dependence of α_c on L_c.     

Study on soapless emulsion copolymerization methyl methacrylate and n-butyl acrylate

The soapless emulsion copolymerization of methyl methacrylate (MMA) and n-butyl acrylate (n-BuA) at four levels of monomer feed composition (f₁₀) was studied. Conversion (X), average particle diameter (D_p), molecular weight distribution (MWD), surface charge density, and glass transition temperature (T_g) of the copolymer as a function of reaction time (t) were measured. The copolymers obtained even at low conversion, except for the run of (f₁₀) = 90 wt. percent MMA, exhibit two T_gs in their DSC thermograms. Phase separation is found to occur in the latex particles during polymerization. The heterogeneous distribution of monomers in particles, in which a relatively rich MMA region exists in the shell and a relatively rich n-BuA region exists in the core of the particles, is assumed to arise from phase separation. The average copolymer composition and the fraction of the two domains are estimated. The polymerizatrion course and particle size growth follow the linear X vs. t² and D_p^3/2 vs. t relationships, respectively. Although the coagulation of particles happens after around 30 percent conversion, the polymerization behaviors, except for increasing rates, are not affected.     

Structural Analysis of Nano Ferrites Synthesized by Combustion and Microwave Methods

We suggest a new approach to synthesis of nano nickel ferrites via combustion synthesis in electric and microwave ovens. The oxidation of organic compounds, decomposition of salts, and their degradation were explored by TGA–DTA. As-prepared nanopowders were characterized by X-ray powder diffraction. Structures of both ferrites were slightly different. Detailed material parameters such as crystallite size (D), lattice constant (a), micro strain (ε), X-ray density (Δ_X), dislocation density (ρ_X), hopping lengths (L_A and L_B), bond lengths (A–O and B–O), ionic radii (r_A and r_B), texture coefficient [TC(hkl)], and mechanical properties were measured and comparatively analyzed.     

Synthesis and Structural Analysis of Co–Zn–Cd Ferrite by Williamson–Hall and Size–Strain Plot Methods

The work aimed at studying the structural properties of Cd-doped cobalt zinc ferrite (Co_0.5Zn_0.5Cd_0.2Fe_1.8O₄) prepared by simple, low-cost solid-state reaction method and characterized by XRD techniques. The X-ray analysis confirms the formation of ferrite particles with a cubic spinel structure. Crystallite size D, lattice constant a, micro strain ε, X-ray density Δ_X, dislocation density ρ _D , hopping lengths (L_A and L_B), bond lengths (A–O and B–O), ionic radii (r_A and r_B), texture coefficients [TC(hkl)], and mechanical properties are also reported.     

WAXS study of the structural reorganization of semi-crystalline polylactide under tensile drawing

This work deals with the in situ and ex situ WAXS study of the strain-induced structural changes in relation to the mechanical behavior of a semi-crystalline polylactide containing 4% of d-stereomer units. The crystals isothermally generated at 120 °C were in the α form. The draw temperature was in the range 65 °C < T_d < 90 °C, i.e. just above T_g. In this T_d range isothermal crystallization was so slow that only strain-induced crystals could be generated during the time scale of the experiments. This allowed studying samples with various initial levels of crystallinity, X_cr. For X_cr = 5%, the early occurrence of strain-hardening contrasted with the case of the amorphous material. This was due to the physical crosslinking of the macromolecular network by the crystallites that prevented chain relaxation since the onset of drawing. Strain-induced α′ crystals developed in parallel with the mesophase, the respective amount of the two phases being dependent on T_d. For X_cr = 24%, the drawing behavior was that of a crosslinked and filled elastomer. Again, both strain-induced α′ crystals and mesophase occurred in various proportions in relation to T_d. For X_cr = 40%, it was shown that the initial α crystals underwent partial destruction and subsequent reorganization into mesophase and/or α′ crystals. Whatever T_d and X_cr, the total amount of ordered phases, i.e. crystal + mesophase, did not exceed the value of 40% that was the maximum accessible crystal content for the present material. A mechanism of plastic deformation involving twinning is proposed from the WAXS analysis of the crystalline texturing in the case X_cr = 40%. AFM observations as a function of strain corroborate the proposed deformation scheme.     

Modification of poly(vinyl alcohol) using maleic acid and its application to the separation of acetic acid-water mixtures by the pervaporation technique

The ‘solution technique’ modification of poly(vinyl alcohol) (PVA) using maleic acid was carried out with the help of triethanolamine/water catalysts. The resulting PVA membranes were characterized by differential scanning calorimetry, infrared spectroscopy, and tensile studies to investigate the reaction between PVA polymer and maleic acid. It was found that the resulting PVA membranes had two portions, branched and crosslinked, and there were no more branched than cross-linked portions. For the pervaporation separation of the acetic acid-water system, two reaction densities (X_cr = mole maleic acid per mole monomeric unit of PVA) of 0.05 and 0.1 were studied for the separation of the entire range of mixture compositions at 25° C. The separation factors of the X_cr = 0.05 modified PVA membranes were higher than those of the X_cr 0.1 modified ones and the highest separation factor of 7.80 was obtained at 70wt% water in the feed with the X_cr = 0.05 modified PVA membrane.     

Separation characteristics of pervaporation membrane separation processes using modified poly(vinyl alcohol) membranes

Modified poly(vinyl alcohol) (PVA) membranes prepared by the ‘solution technique’ were tested for ethanol-water mixtures by varying the reaction density (X_cr = 0.05, 0.1) at various temperatures. The results are compared with those of PVA membranes (X_cr = 0.05) prepared by the technique of the GFT Company, Germany.     

Preparation of supramolecular corrosion-inhibiting nanocontainers for self-protective hybrid nanocomposite coatings

The copolymers were prepared by the photo-induced copolymerization of methyl methacrylate (MMA, M ₁ ) with 4-methacryloyl-1,2,2,6,6-pentamethyl-piperidine (MPMP, M ₂ ) in benzene solution at ambient temperature. The reactivity ratios for these monomers were measured by running a series of reactions at various feed ratios of initial monomers, and the copolymer compositions were determined by ¹H NMR spectroscopy. The reactivity ratios of the monomers were found to be r ₁ ?=?0.28 and r ₂ ?=?1.63 by the application of extended Kelen-Tüdos method. The results showed that, MMA preferred to copolymerization while MPMP tended to homopolymerization in the copolymerization system. The sequence structures of the MMA/MPMP copolymers were characterized by ¹H NMR spectroscopy. The backbone structures of the MMA/MPMP copolymers were mainly composed of a syndiotactic and an isotactic configuration. Three kinds of the sequences of rr, rr’ and lr’ in the syndiotactic configuration and two kinds of the sequences of km’ and mm’ as the isotactic configuration were found. The sequential distribution and the sequence-length distribution in the MMA/MPMP copolymers were also obtained.     

Production of finely ground natural graphite particles with high electrical conductivity by controlling the grinding atmosphere

Natural graphite particles with high crystallinity sieved to obtain a particle size range of under 63 μm were ground with a ball mill, under various well-controlled grinding atmospheres such as N₂, O₂, He, H₂, and vacuum. The ratio, X_dif50/X_st50, i.e. between the 50 wt.% Stokes diameter and the 50 wt.% laser diffraction diameter, of the ground particles, was used as an index of the flakiness of the particles. The specific resistance of films composed of the ground graphite particles was systematically measured. The rate of reduction in the size of the particles by grinding was slow under an O₂-rich atmosphere such as 100% O₂ and dry air. On the other hand, it was relatively fast in vacuum, or under an N₂ or He atmosphere, and a gas mixture of 99% N₂ and 1% O₂. The rate of size reduction by grinding under a H₂ atmosphere was intermediate. In our experimental conditions, the flakiness of the ground particles increased with the decrease in the particles’ sizes. The electrical conductivity of the ground particles, however, tended to decrease with the decrease in their sizes. Under the condition that the Stokes diameter of the ground particles remains constant, the electrical conductivity of films made from the ground particles increases with the increase in the flakiness of the particles. It was finally determined from our systematic grinding experiments that small flaky particles, which had a size, X_st of ∼1 μm, with a high electrical conductivity can be produced by grinding in a gas mixture of 99% N₂ and 1% O₂. In this case, the flaky shape of the ground particles was visually confirmed by scanning electron microscopy.     

Rubber modification of unsaturated polyester resin with core–shell rubber particles: Effect of shell composition

Poly(butyl acrylate)/poly(vinyl acetate‐co‐methyl methacrylate) PBA/P(VAc‐co‐MMA) core–shell rubber particles with various shell compositions, i.e., VAc/MMA weight ratios, were used to toughen unsaturated polyester. The morphology and surface‐free energy of the rubber particles were determined by transmission electron microscopy (TEM) and contact angle measurements, respectively. The effect of shell structure on the dispersion state of rubber particles inside the unsaturated polyester resin was studied by scanning electron microscopy and TEM. Increasing MMA units in the shell changed the particle dispersion state from small agglomerates or globally well‐dispersed particles to large aggregates in the cured‐resin matrix. For the blends that contain 5 wt% rubber, the highest un‐notched impact toughness, stress‐intensity factor (K_IC), and fracture energy (G_IC) were observed for the blend containing PVAc shell particles. The results showed that by increasing the particle level from 5 to 10 wt%, the highest K_IC and G_IC values were obtained for the blend containing rubber particles with VAc/MMA (80/20 wt/wt) copolymer shell. The crack‐tip damage zone in the neat and rubber‐modified unsaturated polyester resins was observed by means of transmission optical microscopy. In addition, using PVAc shell particles exhibited a minimum reduction in the volume shrinkage and tensile properties of the rubber‐modified resin. POLYM. ENG. SCI., 52:1928–1937, 2012. © 2012 Society of Plastics Engineers     

Two-dimensional assembling of 4,4′-bipyridine and 4,4′-azopyridine bridged iron (II) linear coordination polymers via hydrogen bond

Novel two-dimensional polymers, [Fe(L¹)(H₂O)₂(NCX)₂] · L¹ (L¹ = 4,4′-bipyridine (bipy)) (1, 2) and [Fe(L²)(CH₃OH)₂-(NCX)₂] · L² (L² = 4,4′-azopyridine (azpy)) (3) and X = S (1,3),Se (2), have been synthesized and characterized by X-ray crystallography. The structures reveal the formation of trans-L-bridged [Fe(NCX)₂(Y)₂] where Y = H₂O, CH₃OH linear chains assembled into two-dimensional networks by hydrogen bonds between the uncoordinated ligand L and the coordinated solvent molecules.     

First example of near-infrared (NIR) luminescent Yb4(Salen)4-containing metallopolymer through radical copolymerization with MMA (methyl methacrylate)

Through the radical copolymerization of MMA (methyl methacrylate) with a vinyl-containing complex monomer {[Yb₄(L)₂(HL)₂(μ₃-OH)₂(NO₃)₂]·(NO₃)₂} (1; H₂L = N,N′’-bis-(5-(3′-vinylphenyl)-3-methoxy-salicylidene)cyclohexane-1,2-diamine), the first example of PMMA-supported and Yb₄(Salen)₄-containing metallopolymer Poly(1-co-MMA) exhibits significantly improved physical properties besides relatively higher NIR quantum yields (Φ_Yb^Yb = 1.18% and Φ_L^Yb = 0.86%) than those (Φ_Yb^Yb = 1.02–1.04% and Φ_L^Yb = 0.73–0.75%) of complex monomer 1 in solution and 1@PMMA in solid state.     

Synthesis of tercopolymer BA–MMA–VTES and surface modification of nano‐size Si3N4 with this macromolecular coupling agent

A new macromolecular coupling agent butyl acrylate (BA)‐methyl methacrylate (MMA)‐vinyl triethoxy silane (VTES) tercopolymer was synthesized using solution polymerization initiated by free radical initiator benzoyl peroxide (BPO) and dicumyl peroxide (DCP). Dodecylthiol is choosed as the chain transfer to control the molecule weight of this tercopolymer. The terpolymer's molecular structure was confirmed by FTIR and NMR, and its average molecular weight was determined by GPC. In this work, the tercopolymer BA–MMA–VTES is used for surface modification of silicon nitride (Si₃N₄) nanopowder. The structure surface properties and thermal stability of modified nano‐Si₃N₄ were systematically investigated by FTIR, TGA, TEM, and size distribution analyzer. The results show that the macromolecular coupling agent bonds covalently on the surface of nano‐sized Si₃N₄ particles and an organic coating layer is formed. The optimum loading of this macromolecular coupling agent BA–MMA–VTES tercopolymer is 5% (wt %) of nano‐sized Si₃N₄. TEM also reveals that modified nano‐Si₃N₄ possesses good dispersibility. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A new approach to diagnostics of ideal and non-ideal flow patterns: I. The concept of reactive-mixing index (REMI) analysis

A new approach is proposed allowing to characterize the hydrodynamic regime in the presence of chemical reactions, through small pulse-response experiments using a reacting component as a tracer instead of inert components which are used in the traditional Danckwerts-type experiment.Theoretically, our approach is based on the escape time defined as the mean exit age of molecules that escaped all reactions, and uses the original concept of the reactive mixing index (REMI), which is the ratio of the normalized difference between the escape time (ET(X)=M₁(X)/M₀(X)) in the presence and in the absence of reaction, to conversion X: REMI(X)=(1-ET(X)/ET(0))/X.Experimentally, the observed values are the numbers of additional exiting molecules per second as a function of time, F(t) (mol/s). Based on these values, the moments M₀(X) and M₁(X) can be computed and REMI calculated.This REMI is a global, macroscopic, external, model-free characteristic and to our knowledge has not been introduced or studied before. This value is identical to 1 for perfectly mixed continuous-flow reactors, and identical to zero for ideal plug flow reactors (PFR). For non-ideal PFR, the REMI depends on two independent characteristic dimensionless numbers, the Péclet number and the conversion or, equivalently, the Damköhler number. Expressions for these dependencies are developed.The smallness of the perturbations allows to use a linearized kinetic model. The theory of this new index is developed; application to experimental data will be the subject of future articles.     

Kinetics studies of two-stage soapless emulsion polymerization of butyl acrylate and methyl methacrylate

In this work, butyl acrylate (BA) and methyl methacrylate (MMA) were used as monomers and K₂S₂O₈ was used as the initiator to study the kinetics of two-stage soapless emulsion polymerization. The first stage of the reaction was to synthesize polyBA (PBA) seeds, and the second stage of the reaction was a seeded polymerization of MMA. The results showed that an increase of initiator concentration would increase the rate of polymerization and the number of polymer particles, but would decrease the size of the polymer particles and the weight-average molecular weight of the polymers. On the other hand, a decrease of the weight ratio of BA/MMA caused a decrease in the reaction rate and the weight-average molecular weight of the polymers in the second stage of the reaction. The morphology of emulsion particles was observed from transmission election microscopy (TEM). The polymer particles were very uniform in size and showed coreshell morphology with PBA as a core and poly MMA (PMMA) as a shell. © 1994 John Wiley & Sons, Inc.     

Evolution with heat treatment of crystallinity in carbons

The variation with heat treatment of the dimensions L_a and L_c of the graphite-like crystallites of graphitizable and non-graphitizable carbons is studied. The increases of L_a and L_c with heat treatment temperature (HTT) owing to three processes (crystallite growth in-plane, coalescence of crystallites along the c-axis and coalescence of crystallites along the a-axis) are functionally separated. The evolution with HTT of the number of crystallites (N_cr), the mean volume of the crystallites (v_cr) and the total volume occupied by the crystallites (V_cr) are determined in terms of the changes of L_a, L_c and d₀₀₂. Since among other characteristics the crystallites form the electrical and thermal conducting phase of the carbon, N_cr, v_cr and V_cr are important parameters in many physical properties of these materials. The developed expressions were applied to a non-graphitizable and to a graphitizable carbon.     

Effect of swelling on spatial inhomogeneity in poly(acrylamide) gels formed at various monomer concentrations

The effect of swelling on the spatial inhomogeneity in poly(acrylamide) (PAAm) gels has been investigated with the static light scattering measurements. Four sets of gels were prepared using N,N′-methylenebis(acrylamide) (BAAm) as a crosslinker at various initial monomer concentrations. The crosslinker ratio X (the mole ratio of BAAm to the monomer acrylamide) was fixed at 1/50, 1/61.5, 1/66, and 1/100 in each set of gels. The gels, both at the state of preparation and at the equilibrium swollen state in water, exhibit a maximum degree of spatial gel inhomogeneity at a critical monomer concentration (ν_2,cr⁰). ν_2,cr⁰ shifts toward smaller concentrations as X is decreased or, as the gel swells beyond its swelling degree after preparation. Swelling enhances the extent of spatial inhomogeneity in PAAm gels and, this enhancement mainly occurs at low crosslinker ratios. The theoretical prediction of the Panyukov-Rabin theory was found to be in qualitative agreement with the experimental findings. It was also shown that three different effects, namely crosslinker, concentration, and swelling effects determine the extent of inhomogeneities in gels formed at various monomer concentrations.     

Synthesis,crystal structure and photoluminescence of a new Cd(II) coordination polymer with unusual geometry

A new coordination polymer 1 [CdL₂(NO₃)₂]_n (L: 3, 6-di-(1, 2, 4-triazole-1-yl)-N-ethylcarbazole) has been synthesized by self-assembly reaction and characterized by elemental analysis and IR spectroscopy. The single-crystal X-ray diffraction shows that the [CdL₂(NO₃)₂]_n crystallizes in the monoclinic system and C2/m space group and the Cd(II) atom is four-coordinated. The coordination polyhedron can be described as a rectangle which is rather infrequent. The coordination bonds, hydrogen bonds and π–π interactions shaped the 3D network structure. A possible formation mechanism was discussed. The optical property was also studied.     

A kinetic investigation of thermally initiated emulsion copolymerization of styrene and methylmethacrylate without conventional initiators

The thermally initiated emulsion copolymerization of styrene (ST) and methyl methacrylate (MMA) was carried out in the absence of conventional initiators. The hydroperoxide (HPO) concentration in the monomers, sodium dodecyl sulfate (SDS), deionized water, and the formulation of those for emulsion copolymerization were measured. The HPO concentration in ST and MMA increased with the storage time, and were considered to be the major sources of HPO. The thermal decomposition of hydroperoxide in monomers, the thermal initiation of ST by Mayo mechanism, and the complex formation between SDS and the monomers were proposed to be three main sources of the radical generation. It was confirmed that new polymer particles were generated throughout the polymerization process, and consequently resulted in a broader distribution of polymer particle size, compared with that for conventional emulsion polymerization. Approximately 80 wt % of monomer conversion was obtained in the presence of SDS at 373 K in 24 h. The initiation rate of the 30 wt % monomer charge was faster than those of 10 wt % and 20 wt % monomer charge. The latex instability at higher solid content was improved by adding electrolyte to promote the electrostatic repulsion force between the polymer particles. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 455–467, 2002; DOI 10.1002/app.2343