bluuebunny/crossref_metadata_2025_split huggingface dataset:
The kinetics of the cyclodehydration reaction of polyhydrazides
Authors: Gebben B. , Mulder M. H. V. , Smolders C. A. 
Published: March 2011

AbstractThe kinetics of the thermal cyclodehydration reaction of poly‐(1, 3‐phenyl‐1, 4‐phenyl)‐hydrazidea) to poly‐(1, 3‐phenyl‐1, 4‐phenyl)‐oxadiazoleb) have been studied by thermo‐gravimetrical analyses. Both an isothermal method and the non‐isothermal Ozawa method have been used to evaluate the kinetic parameters. The kinetics seem to depend upon the physical state of the polymer i.e. the rubbery or glassy state. During isothermal conversions at elevated temperatures the polymer is expected to be initially in its rubbery state followed by a rubber‐glass transition showing a considerable transition in the observed kinetics. The results obtained from the non‐isothermal Ozawa method and the isothermal method agreed well for the polymer in its glassy state. With the non‐isothermal method the process can be followed as a function of the degree of conversion. In this way a parallel reaction could be observed which starts at the end of the conversion process. The morphological state or the history of the polymer seems to have a considerable influence on the kinetics. The film form of the polymer shows a much higher conversion rate than the powder form.
Etude par diffusion Raman de la transition ferro‐paraelectrique dans les copolymeres de fluorure de vinylidene et de trifluorethylene p(VF2‐TrFE)
Authors: Abénoza Michel , Armengaud Alain , Chouh Saíd 
Published: March 2011

AbstractThe ferro‐paraelectric transition in the 80/20 and 70/30 mol/mol copolymer of vinylidene fluoride and trifluoroethylene is studied by Raman spectroscopy. The Curie transition temperature is depending on the composition and the orientation of the material. At the transition drastic modifications occur in the observed Raman spectra: vanishing of the 842, 1300 and 1350 cm−1 Raman band intensities; strong variation of the band polarizations, this change being not completely reversible when lowering the temperature.This study shows that the Raman spectra evolution is consistent with a conformational change between the extended “all trans” form and a rich gauche‐sequence configuration. Moreover, polarization changes along the studied temperature cycle show that the paraelectric phase is highly disordered and that after the cycle is carried out a part of this chain disorientation remains in the ferroelectric phase.
Molecular order in poly(3‐tert‐butyloxetane)
Authors: Bello A. , Pérez E. , Fatou J. G. 
Published: March 2011

AbstractThe properties of poly(3‐tert‐butyloxetane), an asymmetric polymer obtained by ring‐opening polymerization of the corresponding oxetane, have been studied. Although the solution NMR spectrum shows that the polymer is atactic, well‐defined melting endotherms and sharp X‐ray diffraction peaks were obtained. From these studies, the equilibrium melting temperature and the enthalpy of fusion are estimated.
Experimental and theoretical data on excluded volume effect of ultrahigh‐molecular‐weight polymethacrylates
Authors: Simionescu Cristofor I. , Ioan Silvia , Simionescu Bogdan C. 
Published: March 2011

AbstractA comparison of theoretical predictions on excluded volume effect with experimental data for ultrahigh‐molecular‐weight polymethacrylates in dilute solutions is presented. The excluded volume parameter Z, determined on the basis of different theoretical expressions, has high values. The analysis of the validity criteria of the theories on the excluded volume effect and of the experimental data for the expansion factor shows that for magnified image, the experimental data for Z and for the interpenetration function φ (Z) are in good agreement with those predicted by the Kurata‐Yamakawa‐Tanake theory and in an even closer agreement with those of the Douglas‐and‐Freed theory.
Transitions and relaxations in amorphous polymers
Authors: Cebeillac P. , Chatain D. , Megret C. , Lacabanne C. , Bernes A. , Dupuis P. 
Published: March 2011

AbstractIn amorphous homopolymers, the relaxation modes observed by Thermally Stimulated Current (TSC) around and above the glass transition (Tg) have common features:– The TSC peak isolated around Tg is constituted of elementary processes characterized by relaxation times obeying an Arrhenius equation. Moreover, the activation enthalpies and entropies follow a compensation law. This mode is the dielectric manifestation of the glass transition.Since the polarization can be taken as order parameter, the width of the distribution of the relaxation time characterizes the distribution of the local order.Data obtained on polystyrenes with various orientation show the relationship between order parameter and orientation.The TSC peak observed some 50 °C above Tg is characterized by relaxation times well described by the Fulcher‐Vogel equation. This mode has been associated with the dielectric manifestation of the liquid‐liquid transition (Tll) observed by other non‐flow techniques such as adiabatic or differential scanning calorimetry. This transition is missing in chemical or physical networks.In amorphous copolymers, polymerization conditions play a key role on the relaxation spectra. A comparative study of butyl acrylate/styrene has been done.– The semicontinuous introduction of monomers gives relatively homogeneous statistical copolymers, with a DSC trace analogous with the one of a monophasic polymer. The TSC study confirms this result. Indeed, the TSC peak observed around Tg, is constituted of elementary processes following a single compensation law.The introduction by steps of monomers leads to a block copolymer, with a broad transition region on a DSC thermogram. Two TSC peaks are observed in the same temperature region. Their fine structure shows the existence of two compensation phenomena and so, confirms the presence of two well‐resolved mechanisms. These results are coherent with the existence of a core shell structure.
Relaxations and molecular motions in the glass‐transition region of glassy polymers
Authors: Colmenero J. , Alegría A. , Del Val J. J. , Alberdi J. M. 
Published: March 2011

AbstractDifferent kinds of relaxation techniques as both mechanical and dielectric spectroscopies (DMTA and DETA), as well as Thermally Stimulated Depolarization Current (TSDC) have been used in this work in order to study the temperature behaviour of the main time scales for relaxation around the glass‐transition of several glass‐forming polymeric systems. The results obtained show that both mechanical and dielectric relaxation times behave in a similar way with the temperature. Moreover, there is a crossover from a Vogel‐Fulcher behaviour (above the glass‐transition) to an Arrhenius‐like behaviour (glassy zone) for all the polymers investigated. This universal behaviour has been interpreted in the frame of a kinetic free volume model previously proposed by some of the authors. A possible extension of the model which would allow to introduce the effects of a distribution of the relaxation times is also outlined here. Finally, the new α'‐relaxation which has been recently detected at temperatures just below the glass‐transition in both, several glassy polymers and metallic glasses, can be understood in the frame of such an extended model, at least qualitatively.
Parameter estimation in emulsion copolymerization
Authors: De La Cal José C. , Adams Mary E. , Asua José M. 
Published: March 2011

AbstractAn approach for the estimation of kinetic parameters in emulsion copolymerization systems is presented. This approach is based on an algorithm for parameter estimation in differential equations and uses the time evolution of the conversions of the monomers in chemically initiated seeded emulsion copolymerizations. The approach is used to estimate the kinetic parameters of a mathematical model useful for predictive work. This mathematical model includes material balances for the monomers, population balances for particles containing i radicals and the free radical balance in the aqueous phase. Analysis of the variability of the parameters of the model during polymerization showed changes as large as 18% in the values of the parameters. These values were mainly due to changes in the particle size rather than to a shift in the composition of the reaction mixture. Combination of the present approach with methods for both parameter estimation in emulsion homopolymerization and estimation of reactivity ratios in emulsion copolymerization systems allows a reasonable good estimation of these varying kinetic parameters as constant parameters. Additionally, it was found that when the dependence of the parameters on the particle size was included in the estimation process, the parameters of the model were accurately estimated.
Miniemulsion copolymerization of styrene‐methyl methacrylate: Effect of transport phenomena
Authors: Asua J. M. , Rodriguez V. S. , Silebi C. A. , El‐Aasser M. S. 
Published: March 2011

AbstractThe mechanisms of mass transfer between miniemulsion droplets and polymer particles in the miniemulsion copolymerization of styrene‐methyl methacrylate were studied. A mathematical model was developed for the process. This model includes the mass transfer of highly water‐insoluble compounds from miniemulsion droplets to polymer particles by both molecular diffusion and collision between droplets and particles. According to previous results, the concentration of the monomers in the different phases were assumed to be under thermodynamic equilibrium. Both homogeneous nucleation and heterogeneous nucleation were accounted for. The mathematical model also includes the calculation of the average number of radicals per particle when a mixture of oil‐soluble initiator and water‐soluble initiator was used. This model was used to analyze a previously reported experimental result of seeded copolymerizations carried out in the presence of miniemulsion droplets containing a water‐insoluble inhibitor at different stirring speeds. The simulation results indicated that at 30 rpm, about 66% of the mass transfer of the highly water‐insoluble inhibitor occurred by diffusion and the rest by collision. On the other hand, at 120 rpm only about 20% of the inhibitor's mass transfer was by molecular diffusion, whereas 80% occurred by collision.
Film formation: Aspects on the coaleschence of latex particles
Authors: Leonardo Rios‐Guerrero 
Published: March 2011

AbstractThe mechanism of the formation of a latex film is discussed. Two opposite sets of forces may simultaneously act during water evaporation:To produce coalescence: Interfacial tension (Fs), Capillary forces (Fc), Van der Waals attraction forces (Fv), Force of gravity (Fg), Thermodynamic forces (Ft), Potential energy to surpass the maximum of repulsion energy (Fw).To prevent coalescence:Electrostatic or steric repulsion forces between the polymer paricles (Fe), Particle resistance to deformation (Fve).A general expression to predict latex film formation is deduced when forces to produce coalescence are higher than the forces to prevent it:
Cationic ring‐opening polymerization of lactones in the presence of alcohol
Authors: Okamoto Yoshihisa 
Published: March 2011

AbstractCationic ring‐opening polymerization of e‐caprolactone using triethyloxonium hexafluorophosphate initiator in the presence of alcohol to produce low molecular weight polyester glycol was examined. The polymerization proceeded by an activated monomer mechanism; thus the polymer molecular weight increased directly with the monomer conversion. Polymerization of lactones other than ϵ‐caprolactone were also examined, and the overall rate of polymerization was in the order δ‐valerolactone> ϵ‐caprolactone>> β‐butyrolactone> diketene> γ‐butyrolactone.
Ring‐opening polymerization and copolymerization of cyclic carbonates
Authors: Höcker H. , Keul H. , Kuhllng S. , Hovestadt W. 
Published: March 2011

AbstractThe anionic ring‐opening polymerization may result in a broad variety of homo‐ and copolycarbonates. Upon suitable selection of monomers, polymers with functional groups may be obtained which are eligible to cross‐linking reactions or exert a high degree of hydrophilicity. Copolymers with lactones are readily obtained, provided both polymerization reactions follow the same mechanism; the ring‐opening polymerization of cyclic carbonates is exclusively initiated by alcoholate ions and not by carboxylate anions. Since butyllithium is a suitable Initiator for the ring‐opening anionic polymerization of cyclic carbonates, living vinyl polymers may be used as initiators as well to obtain the respective blockcopolymers. Instead of living vinyl polymers obtained in the conventional manner, using carbanionic Initiators, also living polymers obtained by group transfer polymerization may be used as initiators for the ring‐opening polymerization of cyclic carbonates; the mechanism for the latter polymerization, however, then is rather a non‐metallic anionic one than a group transfer mechanism.
Anionic ring‐opening polymerization of cyclotetrasilanes
Authors: Matyjaszewski Krzysztof 
Published: March 2011

AbstractVarious cyclopolysilanes are discussed as potential monomers for the ring‐opening polymerization in terms of reactivity and thermodynamic feasibility. Strained and polymerizable 1,2,3,4‐tetramethyl‐1,2,3,4‐tetraphenylcyclotetrasilanes (1) are prepared from octaphenylcyclotetrasilane by substitution of four phenyl groups with triflate groups using triflic acid, and followed by displacement of triflate groups by methyl substituents using methylmagnesium bromide. Anionic ring‐opening polymerization of a mixture of stereoisomers of (1) provides poly(methylphenylsilylene) with molecular weights up to M=100 000. Anionic polymerization of 1 was initiated by butyllithium and silylpotassium initiators with and without cryptands. Strong solvent effects on polymerization and subsequent cyclization were observed.
The reactivity‐selectivity principle in polymerization. The case of polymerization of ϵ‐caprolactone
Authors: Penczek Stanislaw , Duda Andrzej , Slomkowski Stanislaw 
Published: March 2011

AbstractAnionic and pseudoanionic polymerizations of ϵ‐caprolactone (CL) have been compared in terms of the principle relating reactivity with selectivity (reactivity‐selectivity principle, RSP). RSP can be formulated as follows: there are two reagents X and Y; reagent X reacts faster with substrates A and B than reagent Y does (i.e. kxa>kya and kxb>kyb, where kxa, kya, kxb and kyb are the respective rate constants); moreover, reactions with A are faster than reactions with B (i.e. kxa>kxb and kya>kyb); finally, if RSP is operative the inequality kya/kyb>kxa/kxb holds. Comparison of the anionic and pseudoanionic polymerizations of CL reveals that this system conforms to the RSP. Propagation on the …‐O−Na+ ion pairs proceeds much faster than on the …−OAIR2 covalent species (kp∓ = 1,7 l/mol · s vs kpc = 3,0 · 10−2 l/mol · s, THF solvent, 20°C). Propagation is accompanied by transesterification and cyclization. Formation of the cyclic dimer (with the rate constant kb (2)) was taken as a measure of an extent of these “side” reactions. Following selectivity coefficients ß = kp/kb(2) have been measured (in l/mol): 1, 55 · 103 (…−O−Na+), 4, 6 · 104 (…‐OAl(C2H5)2), and 7,7 · 104 (…−OAl(i‐C4H9)2). Thus, kpc/kbc(2)>kp∓/kb∓(2). Therefore formation of the linear, high molar mass polymer is more pronounced in the system with the less reactive active centers.
Studies on the mechanism of amine‐catalyzed cyclic oligomeric carbonate formation
Authors: Brunelle Daniel J. , Boden Eugene P. 
Published: March 2011

AbstractA mechanism for the selective formation of cyclic oligomeric carbonates via triethylamine‐catalyzed, pseudo‐high dilution hydrolysis/‐condensation of bisphenol A‐bischloroformate is proposed. The mechanism is based on results of model and cyclization reactions using amines of various structure. Intermediacy of an acyl ammonium salt is invoked for both hydrolysis and condensation reactions. An oligomeric carbonate with a phenoxide on one end and an acyl ammonium salt on the other end is proposed as the immediate precursor to a cyclic oligomer.
Application of fluorescence quenching techniques to the study of emulsion polymerization and latex characterization
Authors: Winnik Mitchell A. , Xu Huilin , Satguru R. 
Published: March 2011

AbstractWe investigate two aspects of emulsion polymerization through fluorescence studies of models of those processes. The first is the uptake of monomer by seed latex using poly(butyl methacrylate) (PBMA) seeds labeled with phenanthrene (Phe) groups. Nitroalkanes serve as models for the monomer. Data analysis gives the partition coefficients and the diffusion coefficients (D) of monomer inside the latex (D = 6 × 10−8cm2s−1). The second aspect involves the generation of core‐shell structures in poly(methyl methacrylate) (PMMA) latex where the Phe‐labeled seed is treated, during the second stage polymerization, with additional MMA plus a polymerizable anthracene (An) derivative. The extent of Phe to An energy transfer in the final latex measures the amount of mixing of added monomer during the second stage polymerization.
Influence of components and interface conditions on mechanical properties of composites and blends
Authors: Pegoraro M. , Di Landro L. 
Published: March 2011

AbstractFiller‐matrix adhesion, particularly in case of thermoplastic matrices, is influenced by the presence of residual stresses due to the cooling of the material from the processing temperature to the final temperature (usually room temperature). A model for the evaluation of thermal stresses in fibre composites at the fibre level is presented which, starting from relationships valid in case of purely elastic components, extends the analysis to account for the matrix viscoelasticity. A Master Curve is needed for this purpose.Thermal stresses around a single fibre are thus evaluated as a function of the final temperature and of the cooling rate of the composite. Fibre matrix adhesion at different temperatures was measured by the single fibre composite method applied to polyetherimide matrix/carbon fibre composites and the results are related to the calculated residual stresses.The relationship between filler‐matrix adhesion efficiency and mechanical (tensile and impact) strength of spherical particle filled composites is also discussed.
Chemical approaches to oriented protein immobilization
Authors: Voivodov Kamen , Chan Wai‐Han , Scouten William 
Published: March 2011

AbstractA procedure has been developed for converting 1,1′‐carbonyldiimidazole‐modified agarose to activated agarose capable of coupling protein in an oriented fashion. This procedure is based on replacing the imidazole groups with orthonitrophenyl derivatives containing ionic and/or hydrophobic groups. The degree of orientation of immobilized chymotrypsin and trypsin was estimated by comparing the activity of both proteases toward low‐ and high‐molecular‐weight substrates. The differences in these activities were greater in the case of enzymes immobilized on agarose gel containing hydrophobic activating agents such as 4‐hydroxy‐3‐nitrobenzophenone and 2‐nitro‐4‐benzylphenol than in those immobilized on gels containing ionic activating agents.
Structure elucidation and NMR assignments of two new triterpenoids from the stems of Paragonia pyramidata (Bignoniaceae)
Authors: Wang Xiao‐Ling , Hay Anne‐Emmanuelle , Matheeussen An , Gupta Mahabir P. , Hostettmann Kurt 
Published: March 2011

AbstractPhytochemical investigation of dichloromethane (DCM) extract from the stems of Paragonia pyramidata var. pyramidata L. Rich. (Bur.) resulted in the isolation and characterization of two new triterpenoids 3β,19β‐dihydroxylup‐12, 20(29)‐diene‐28‐oic acid (1) and 3β,19β‐dihydroxylup‐12‐en‐28‐oic acid (2), three known triterpenoids lupeol (3), spinosic acid A (4) and oleanolic acid (5), together with four known steroids (20R)‐22E‐24‐ethylcholesta‐4,22‐dien‐3‐one (6), (20R)‐24‐ethylcholest‐4‐en‐3‐one (7), stigmasterol (8) and β‐sitosterol (9). HREIMS, GC–MS and NMR experiments including HSQC, HMBC, COSY and NOESY were used for the determination of the structures and NMR spectral assignments. This is the first report about the chemical constituents for this plant. Copyright © 2011 John Wiley & Sons, Ltd.
Studies of some monocyclic and bicyclic arylacetonitriles
Authors: Manimekalai A. , Balamurugan A. 
Published: March 2011

AbstractThe high‐resolution 1H, 13C, 1H1H COSY and 1H13C COSY NMR spectra have been recorded in CDCl3 for arylacetonitriles 1–12 and analyzed. The arylacetonitriles 3–7 exist in two isomeric forms E (methyl group is anti to cyano group) and Z (the methyl group is syn to cyano group) in solution. Normal chair conformation with equatorial orientations of phenyl rings at C‐2 and C‐6 for monocyclic nitriles 1 and 2, epimeric chair structure EC (axial configuration of methyl group at C‐3) for both the E and Z isomers of arylacetonitrile derivatives (3–7) and a distorted boat form, B3, for the N‐acylacetonitrile derivatives (8–10) have been proposed based on NMR data. The bicyclic nitriles 11 and 12 exist in twin chair conformations in solution. DFT calculations and chemical shifts also support these conformations. Geometry optimizations for 1–12 were carried out according to density functional theory using B3LYP/6‐31G(d,p) basis set and for 1 and 8 the theoretical geometrical parameters have been compared with those of single crystal measurements. Copyright © 2011 John Wiley & Sons, Ltd.
1H, 13C and 15N NMR spectral assignments of adenosine derivatives with different amino substituents at C6‐position
Authors: Casati Silvana , Manzocchi Ada , Ottria Roberta , Ciuffreda Pierangela 
Published: March 2011

AbstractThe complete 1H, 13C and 15N NMR signals assignment of adenosine derivatives differently substituted at C6‐position was achieved using one‐ and two‐dimensional experiments (gs‐COSY, gs‐NOESY, gs‐HSQC and gs‐HMBC). Copyright © 2011 John Wiley & Sons, Ltd.
TROMBONE: T1‐relaxation‐oblivious mapping of transmit radio‐frequency field (B1) for MRI at high magnetic fields
Authors: Fleysher Roman , Fleysher Lazar , Inglese Matilde , Sodickson Daniel 
Published: March 2011

AbstractFast, 3D radio‐frequency transmit field (B1) mapping is important for parallel transmission, spatially selective pulse design and quantitative MRI applications. It has been shown that actual flip angle imaging—two interleaved spoiled gradient recalled echo images acquired in steady state with two very short time delays (TR1, TR2)—is an attractive method of B1 mapping. Herein, we describe the TROMBONE method that efficiently integrates actual flip angle imaging with EPI imaging, alleviates very short TR requirement of actual flip angle imaging and through their synergy yields up to 16 times higher precision in B1 estimation in the same experimental time. High precision of TROMBONE can be traded for faster scans. The map of B1 reconstructed from the ratio of intensities of two images is insensitive to longitudinal relaxation time (T1) in the physiologically relevant range. A table of the optimal acquisition protocol parameters for various target experimental conditions is provided. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.