Crosslinking study of polyethers containing pendent double bonds

This article studies the crosslinking reactions of some polyethers containing double bonds on the side chains. Thermal crosslinking takes place in all cases, but is overlapped by the degradation process. So, a catalyst has to be used to obtain the cured material and avoid degradation. The best initiator appears to be dicumyl peroxide and the maximum cross-linking degree is achieved only with 5% (w/w). Thermogravimetric analyses are used to test the quality of the cured materials. IR spectroscopy shows that crosslinking actually takes place by polymerizing pendent double bonds. Furthermore, activation energies for thermal and radical initiated crosslinking are determined by means of dynamic DSC studies. © 1995 John Wiley & Sons, Inc.     

Improved thermosets obtained from cycloaliphatic epoxy resins and γ‐butyrolactone with lanthanide triflates as initiators. I. Study of curing by differential scanning calorimetry and Fourier transform infrared

3,4‐Epoxycyclohexylmethyl 3,4‐epoxycyclohexane carboxylate was cured with different proportions of γ‐butyrolactone with lanthanum, samarium, and ytterbium triflates as catalysts. The curing was studied with differential scanning calorimetry (DSC) and Fourier transform infrared in the attenuated‐total‐reflection mode (FTIR/ATR). FTIR/ATR was used to monitor the competitive reactive processes and to quantify the evolution of the epoxide, lactone, and intermediate spiroorthoester groups. The glass‐transition temperature of the crosslinked materials was high and increased when the proportion of lactone decreased. The kinetics were studied with DSC experiments and were analyzed with isoconversional procedures. The differences in the reactivities of the systems were related to the Lewis acidity of the lanthanide salt used as the initiator. An increase in the proportion of lactone produced an increase in the reaction rate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2337‐2347, 2005     

Reduction of the shrinkage of thermosets by the cationic curing of mixtures of diglycidyl ether of bisphenol A and 6,6‐dimethyl‐(4,8‐dioxaspiro[2.5]octane‐5,7‐dione)

Ytterbium and lanthanum triflates were used as initiators to cure mixtures of diglycidyl ether of bisphenol A and 6,6‐dimethyl‐(4,8‐dioxaspiro[2.5]octane‐5,7‐dione) in several proportions. The evolution of the epoxy and 6,6‐dimethyl‐(4,8‐dioxaspiro[2.5]octane‐5,7‐dione) bands during curing and the linear ester groups in the final materials were evaluated with Fourier transform infrared in the attenuated‐total‐reflection mode. The use of a conventional cationic initiator, boron trifluoride monoethylamine, was also studied to test the advantages of lanthanide triflates. The shrinkage after curing and the thermal degradability of the materials with variations in the comonomer ratio and the initiator were evaluated and related to the chemical structure of the final network. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6869–6879, 2006     

Cationic copolymerization of cycloaliphatic epoxy resin with a spirobislactone with lanthanum triflate as initiator: I. Characterization and shrinkage

3,4‐Epoxycyclohexylmethyl 3,4‐epoxycyclohexane carboxylate (ECH) was cured with different proportions of 1,6‐dioxaspiro [4,4]nonane‐2,7‐dione (s(γ‐BL)) using lanthanum triflate as a catalyst. The shrinkage undergone during curing was monitored by means of thermomechanical analysis (TMA) in isothermal experiments. Fourier transform infrared spectroscopy in attenuated‐total‐reflection mode (FTIR/ATR) was used to study the evolution of lactone, epoxide, and intermediate spiroorthoester (SOE) groups to identify the different reactions that take place during the curing process. DSC was used to study the thermal characteristics of the curing process and to assess the glass‐transition temperature (T_g) of the cured material. The dynamic mechanical properties of the cured material were determined based on the data obtained by DMTA. An increase in the proportion of s(γ‐BL) led to a decrease in the gelation time and the shrinkage after gelation. By combining the data obtained by TMA and FTIR/ATR, it was also possible to identify the reactive processes responsible for the shrinkage. It was observed that an increase in the proportion of s(γ‐BL) also increases the speed of the curing process and modifies the structure of the material, thus giving rise to more flexible materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3421–3432, 2005     

Magnetic Thomas-Fermi-Weizsäcker model for quantum dots: A comparison with Kohn-Sham ground states

The magnetic extension of the Thomas-Fermi-Weizs?cker kinetic energy is used within density-functional-theory to numerically obtain the ground state densities and energies of two-dimensional quantum dots. The results are thoroughly compared with the microscopic Kohn-Sham ones in order to assess the validity of the semiclassical method. Circular as well as deformed systems are considered. Received 26 October 2000 and Received in final form 14 December 2000     

Fast and stable recording of birefringence and holographic gratings in an azo-polymethacrylate using a single nanosecond light pulse

Pulsed light-induced recording in azobenzene polymers has recently been studied due to its potential use in optical storage applications. In this paper we study the photoinduced birefringence (Deltan) and holographic grating recording in an azobenzene side chain liquid-crystalline polymethacrylate irradiating with a single 4 ns light pulse at 532 nm. For some irradiation conditions, Deltan grows in less than 50 ns reaching an essentially stable value of about 10(-2). Holographic gratings have been registered using intensity and polarization patterns. Fast response and stability, similar to those of Deltan, was observed in the holographic recording process. Both light-induced anisotropy and relief contributions have been found in the case of gratings recorded using intensity patterns, relief being the dominant contribution at high recording energies. Polarization gratings have been recorded using two orthogonally circularly polarized beams. The resultant gratings showed stable efficiencies up to 0.8% (measured at 633 nm in 1-mum-thick films) and no measurable relief was observed.     

Preconditioned HSS methods for the solution of non-Hermitian positive definite linear systems and applications to the discrete convection-diffusion equation

Summary. We study the role of preconditioning strategies recently developed for coercive problems in connection with a two-step iterative method, based on the Hermitian skew-Hermitian splitting (HSS) of the coefficient matrix, proposed by Bai, Golub and Ng for the solution of nonsymmetric linear systems whose real part is coercive. As a model problem we consider Finite Differences (FD) matrix sequences {A_n(a,p)}_n discretizing the elliptic (convection-diffusion) problem with being a plurirectangle of R^d with a(x) being a uniformly positive function and p(x) denoting the Reynolds function: here for plurirectangle we mean a connected union of rectangles in d dimensions with edges parallel to the axes. More precisely, in connection with preconditioned HSS/GMRES like methods, we consider the preconditioning sequence {P_n(a)}_n, P_n(a):= D_n^1/2(a)A_n(1,0) D_n^1/2(a) where D_n(a) is the suitably scaled main diagonal of A_n(a,0). If a(x) is positive and regular enough, then the preconditioned sequence shows a strong clustering at unity so that the sequence {P_n(a)}_n turns out to be a superlinear preconditioning sequence for {A_n(a,0)}_n where A_n(a,0) represents a good approximation of Re(A_n(a,p)) namely the real part of A_n(a,p). The computational interest is due to the fact that the preconditioned HSS method has a convergence behavior depending on the spectral properties of {P_n^-1(a)Re(A_n(a,p))}_n {P_n^-1(a)A_n(a,0)}_n: therefore the solution of a linear system with coefficient matrix A_n(a,p) is reduced to computations involving diagonals and to the use of fast Poisson solvers for {A_n(1,0)}_n.Some numerical experimentations confirm the optimality of the discussed proposal and its superiority with respect to existing techniques.Mathematics Subject Classification (1991): 65F10, 65N22, 15A18, 15A12, 47B65     

Kinetic parameters and solid state mechanism of the thermal dehydration of trans[CrF(H2O)(aa′)2]K[Cr(CN)6]H2O And trans[CrF(H2O)(aa′)2]K[CrNO(CN)5]H2O (aa′=ethylenediamine or 1,3-diaminopropane)

The solid phase thermal deaquation of trans[CrF(H₂O)(aa′)₂]K[Cr(CN)₆]H₂O and trans[CrF(H₂O)(aa′)₂]K[CrNO(CN)₅]H₂O (aa′=ethylenediamine or 1,3-diaminopropane) has been investigated by means of TG measurements. The kinetic parameters (activation energy, E_a, activation entropy, ΔS^#, and frequency factor, k₀) have been determined by comparison of the isothermal and non-isothermal studies for all the principal g(α) expressions. The values found for the activation energy are low (between 80 and 110 kJ mole^?1, approximately) and permit the assignment of the deaquation-anation mechanism of the S_N1 type, involving square-pyramid activated complex and elimination of water as Frenkel defects.