Physical aging of an amorphous polyimide: Enthalpy relaxation and mechanical property changes

The physical aging behavior of an isotropic amorphous polyimide possessing a glass transition temperature of approximately 239°C was investigated for aging temperatures ranging from 174 to 224°C. Enthalpy recovery was evaluated as a function of aging time following sub‐T_g annealing in order to assess enthalpy relaxation rates, and time‐aging time superposition was employed in order to quantify mechanical aging rates from creep compliance measurements. With the exception of aging rates obtained for aging temperatures close to T_g, the enthalpy relaxation rates exhibited a significant decline with decreasing aging temperature while the creep compliance aging rates remained relatively unchanged with respect to aging temperature. Evidence suggests distinctly different relaxation time responses for enthalpy relaxation and mechanical creep changes during aging. The frequency dependence of dynamic mechanical response was probed as a function of time during isothermal aging, and failure of time‐aging time superposition was evident from the resulting data. Compared to the creep compliance testing, the dynamic mechanical analysis probed the shorter time portion of the relaxation response which involved the additional contribution of a secondary relaxation, thus leading to failure of superposition. Room temperature stress‐strain behavior was also monitored after aging at 204°C, with the result that no discernible embrittlement due to physical aging was detected despite aging‐induced increases in yield stress and modulus. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1931–1946, 1999     

Dielectric and rheological study of the molecular dynamics during the cure of an epoxy resin

Composite manufacturing is currently one of the most challenging processes for industrial lightweight applications. To date, the process conditions for polymer‐based composite manufacturing are evaluated by laboratory measurements: usually, the flow behavior and the curing of the polymer matrix material are characterized by rheology and quality assurance is performed by thermo‐physical analysis in postprocess measurements. In contrast a dielectric in‐mold sensor offers the possibility to measure the real‐time behavior of the polymer during processing. This study focuses on the correlation of simultaneous rheological and dielectric measurements on Hexcel RTM6 using a coupled setup of both techniques. For dielectric measurements a reusable in‐mold sensor was used and a calibration, taking into account the cable response, was performed. The results show good agreement with respect to glass‐transition temperature and the gel‐point. This can be understood by the fluctuation–dissipation theorem that explicitly relates molecular dynamics to the macromolecular mechanical properties under dynamic time‐dependent load. Furthermore, it was found that the dynamic viscosity can directly be related to the electrical conductivity. This proves the high potential of dielectric analysis as online‐capable technique for material characterization during composite manufacturing. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 907–913     

Structure formation of integral-asymmetric membranes of polystyrene-block-Poly(ethylene oxide)

For the first time the combination of solution casting and solvent–nonsolvent exchange (phase inversion) has been applied to generate asymmetric membranes with highly ordered hexagonally packed cylinders with perpendicular orientation composed of polystyrene-block-poly(ethylene oxide). The influence of parameters like solvent composition and evaporation time on the membrane formation is presented. The development is based on a study of the solution behavior by dynamic light scattering and the precipitation behavior of the cylinder forming diblock copolymer by turbidity measurements from different solvent and nonsolvent systems. The water flux properties, as an important membrane characteristic, show a time dependent behavior, due to swelling of the polyethylene oxide blocks. The morphologies of the membranes are imaged by scanning electron microscopy. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Polymer chain diffusion in polymer blends: A theoretical interpretation based on a memory function formalism

The diffusion of polymer chains in miscible polymer blends with large dynamic asymmetry—those where the two blend components display very different segmental mobility—is not well understood yet. In the extreme case of the blend system of poly(ethylene oxide) (PEO) and poly(methyl methacrylate)(PMMA), the diffusion coefficient of PEO chains in the blend can change by more than five orders of magnitude while the segmental time scale hardly changes with respect to that of pure PEO. This behavior is not observed in blend systems with small or moderate dynamic asymmetry as, for instance, polyisoprene/poly(vinyl ethylene) blends. These two very different behaviors can be understood and quantitatively explained in a unified way in the framework of a memory function formalism, which takes into account the effect of the collective dynamics on the chain dynamics of a tagged chain. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1239–1245     

Static and dynamic scattering behavior of regularly branched chains: A model of soft-sphere microgels

The static structure factor and the first cumulant of the time correlation function in dynamic light scattering have been calculated for a regularly branched molecule with n shells of branching generations which are connected by chains of m repeating units per chain. Ideal flexibility for the chains is assumed. The dynamic scattering functions of these soft spheres show behavior significantly different from that of a hard sphere. The theory describes very satisfactorily the static and dynamic light scattering results obtained experimentally with polyvinyl acetate microgels. The possibility of determining the glass transition temperature of latex particles is briefly discussed.     

On the role of symmetry in the ab initio hartree-fock linear-combination-of-atomic-orbitals treatment of periodic systems

The symmetry properties of the mono- and bielectronic terms contributing to the Fock matrix in the ab initio Hartree–Fock treatment of periodic systems are discussed. A computational scheme which takes full advantage of the point symmetry is presented; in this respect, it represents a generalization of the scheme proposed in Int. J. Quantum Chem. 17 , 501 (1980). Computational details and numerical examples are reported; it is shown that with respect to two of the bottlenecks of this kind of calculation, namely, computer time and backing storage required for the bielectronic integrals, it is possible to obtain saving factors as large as h and h², respectively, where h is the order of the point group. Preliminary tests are reported which indicate that the study of relatively complicated systems, like quartz or corundum (9 and 10 atoms in the unit cell, respectively) at an ab initio Hartree–Fock level is now within reach.     

Characterisation of Polyurethane Coatings Using Thermoanalytical Techniques

A two-pack polyurethane coating was analyzed using thermoanalytical techniques. The curing reaction, monitored using pressure differential calorimetry (PDSC), rheometry and dynamic mechanical analysis (DMA) shows the temperature dependency of activation energy and hence rate of curing. In-situ ATR-FTIR shows the formation of urethane linkage over time. The decomposition behavior carried out under non-isothermal mode using thermogravimetric analysis (TGA) shows bimodal behavior. The activation energies of the initial step (10% decomposition) from both iso and non-isothermal experiments are in very good agreement with each other. The use of dynamic mechanical analysis (DMA) shows the difference in glass transition behavior (T_g) and elastic modulus (E') due to the different state of cure. Also the coating exhibits a very broad loss modulus peak (E') indicating higher energy dissipation with deformation.This revised version was published online in November 2005 with corrections to the Cover Date.     

Dielectric and Viscoelastic Behavior of Poly-(n-Butyl Methacrylate) Containing Cholesteric Compounds

Dielectric and dynamic mechanical behavior of poly(n-butyl methacrylate) (PBMA) containing small concentrations of cholesteryl chloride (ChCl), cholesteryl caprylate (ChCp), and cholesteryl laurate (ChL) have been studied in the temperature range -20 to 80°C. Tan αmax (dielectric and mechanical) in all systems containing an additive shifts to lower temperature compared to that in pure PBMA. In the glassy region, the dynamic storage modulus E′ for all the PBMA plus additive systems is higher than that for pure PBMA. This behavior is similar to the β-peak suppression and the increase in tensile strength of poly(vinyl chloride) on addition of a small concentration of plasticizer. Using the WLF equation with C₁ and C₂ obtained from the data on pure PBMA, the reference temperature T_o for PBMA plus additive systems are determined for the best fit of the experimental points to the WLF curve and are in agreement with measured glass-transition temperatures. With respect to T_o of pure PBMA, the T_o of PBMA plus additive systems are lowered, and the lowering due to these additives is in the order ChL < ChCp < ChCl. From spectrophotometric studies in solutions of polymer plus additives, equilibrium constants for the 1:1 and 1:2 complexes between the polymer and the additive are evaluated. The magnitude of the equilibrium constant K₁ increases in the order ChL > ChCp > ChCl, and hence it is concluded that the extent of β-peak suppression depends on the strength of polymer-additive interaction.     

Characterization of the molecular deformation behavior of glassy epoxy resins by rheo-optical FTIR spectroscopy

Rheo-optical FTIR spectroscopy was used for the first time to monitor molecular orientation phenomena in highly crosslinked epoxies. After studying the orientation behavior of epoxy/amine networks during uniaxial deformation above their glass transition temperature in a preceding article, this article deals with the rheo-optical characterization of the deformation process of those epoxy systems below T_g. The orientation behavior is influenced by the different molecular structure of the constituents and the free volume entrapped in the resins. Yield strain and tensile modulus are correlated with the slope of the orientation function. The orientation function was found to show an abrupt change of its slope in the yield point region. This phenomenon is discussed with respect to the mechanism of plastic deformation. © 1996 John Wiley & Sons, Inc.     

Conformational Analysis of Sulfur-Containing Heterocycles,Part I. Synthesis and Structural Determination of Diastereomerically Pure 4,6-Bis-(phenoxymethyl)-1,2,5-trithiepanes

Some diastereomerically pure 4,6-bis-(phenoxymethyl)-l,2,5-trithiepanes were synthesized and unambiguously assigned. Their conformational properties and dynamic behavior were investigated by various NMR spectroscopic methods and quantum-chemical calculations at the HF/6-31G* level. The ground states of these compounds proved to be twist-chairs. A ring interconversion can occur in the meso-isomers as well as in the (±)-isomers. This interconversion can be described as a simultaneous inversion of the disulfide bridge. In the case of the meso-isomers, both ground states are mirror images of each other and the transition state is a highly symmetrical chair. The barrier heights of interconversion were determined to be in the range of 50 kJ/mol by variable-temperature NMR measurements. The ground states as well as the transition state of the (±)-isomers were found to be nonsymmetrical. However, those dynamic processes that are fast with respect to the NMR time scale lead to averaged NMR spectra at room temperature. A further dynamic process found through the quantum-chemical calculations is a flapping of the meth-ylene groups of the rings. The energy barrier of this flapping was calculated to be very small (< 20 kJ/mol) and could not be observed by low-temperature NMR measurements.     

Effects of micellar structure on solubilization ofn-octane andn-octanol

The solubilization ofn-octane andn-octanol in nonionic micelles of octaethylene glycoln-alkyl ethers was investigated by means of various techniques including solubilization, fluorescence, and dynamic light-scattering measurement. With respect to the effect of alkyl chain length of the surfactants, different solubilization behavior was observed betweenn-octane andn-octanol. That is to say, it was shown that the solubilization ofn-octane increases with an increase in the alkyl chain length, while that ofn-octanol decreases. An interpretation of these solubilization mechanisms is explained from the standpoint of the volume of hydrophobic and hydrophilic regions in a micelle.     

Constrained optimization in cartesian coordinates

Modifications are made to a previously published algorithm for constrained optimization in Cartesian coordinates (J. Comp. Chem., 13 , 240, 1992) to incorporate both fixed and dummy atoms. Standard distance and angle constraints can now be specified with respect to dummy atoms, greatly extending the range of constraints that can be handled. Fixed atoms can be eliminated from the optimization space and so there is no need to calculate their gradients resulting in potentially significant savings of CPU time in ab initio computations. Several examples illustrate the range and versatility of the modified algorithm. © John Wiley & Sons, Inc.     

Time–temperature-dependent behavior of a substituted poly(paraphenylene): Tensile,creep, and dynamic mechanical properties in the glassy state

The linear viscoelastic behavior of a poly(paraphenylene) with a benzoyl substituent has been examined using tensile, dynamic mechanical, and creep experiments. This amorphous polymer was shown to have a tensile modulus of 1–1.5 Msi, nearly twice that of most common engineering thermoplastics. The relaxation behavior, which is similar to that of common thermoplastics, can be described by the WLF equation. Outstanding creep resistance was observed at low temperatures, with rubbery-like behavior being exhibited as the temperature approached T_g. Physical aging was shown to interact with long-term creep, rendering time–temperature superposition invalid for predicting the long-term properties. The effect of physical aging on the creep behavior was characterized by the shift rate μ. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 70: 2971–2979, 1998     

Rheological and relaxation properties of MQ copolymers

The rheological properties of MQ copolymer melts are investigated under steady-state shear flow and dynamic oscillatory shear within a wide temperature range. The MQ copolymers are highly branched polycyclic compounds (densely crosslinked nanosized networks) incapable of further intermolecular interactions. The samples have identical chemical compositions, but their detailed molecular structures are different. The polymers under consideration show Newtonian behavior in a wide shear-stress range. The values of viscosity vary considerably with the molecular structure of the copolymers. The generalized frequency dependence of complex dynamic modulus components is constructed with the use of the temperature-frequency superposition method. In a first approximation, the viscoelastic behavior of the materials is satisfactorily described by the Maxwell model with a single relaxation time. In this respect, the studied materials are similar to micellar colloids. The relaxation spectra of the copolymers distinguished by narrow distributions are calculated.     

Some properties of aqueous xanthan gels formed under the action of aluminium(III) ions

Aqueous xanthan solutions form gels when aluminium salts are added and the solutions are heated above 45 °C. The gelation process was followed by dynamic light scattering. Characterization was based on the heterodyne and nonergodic approaches. Both techniques gave the same fast relaxation times, but for the slow motion much larger values were found in the heterodyne than in the nonergodic approach. The heterodyne fraction 1-X was found to correlate closely with the plateau height of the time correlation function (TCF) at large delay times in the nonergodic experiments. Three methods of gel point determination are demonstrated: (i) onset of heterodyne/nonergodic behavior, (ii) observation of a sharp maximum for the fast relaxation time at the gel point, (iii) observation of power-law behavior of the TCF. The statistics of nonergodic fluctuations were examined and evaluated. The potential of this procedure for detailed structure evaluation of inhomogeneities in the gel is emphasized.     

Studies on Separation of Liquid‐Liquid Dispersions: Diesel‐Water System in Batch Settler

In this article, batch settling of liquid‐liquid dispersion in a vertical batch settler was comprehensively studied. The experiment results of the investigation on liquid‐liquid batch settler phase separation were compared with a well predefined physical model proposed by Jeelani and Hartland [1998, Ind. Eng. Chem. Res. 37: 547–554] using a semi theoretical approach. The effect of initial dispersion height, initial hold‐up volume, settler diameter, and mixing time on separation of batch liquid‐liquid dispersions were experimentally investigated in terms of variation in the final settling time and coalescing interfaces with time. From the present work it was found that, final separation time varied as polynomial function of degree 2 with respect to initial dispersion height and was constant with respect to initial hold‐up volume. Final separation time also varied according to the completeness of dispersion achieved with respect to the mixing time. The experimental data obtained showed a good correlation with the theoretically predicated values. Results allow the use of experimental procedure with the mathematical model as a tool in monitoring the dispersion behavior in commercial units on industrial importance.     

Relaxations in thermosets. X. Analysis of dipolar relaxations in DGEBA-based thermosets during isothermal cure

Dielectric properties measured during isothermal curing of DGEBA-based thermosets using a mixture of aromatic amines as curing agent are analyzed. The evolution of the dielectric features of thermosets during curing and after a time when their dc conductivity has reached a negligibly small value are phenomenologically similar to the dielectric features of physically and chemically stable dipolar liquids and solids observed with increasing frequency or decreasing temperature. This equivalence is a consequence of the invariance of the dynamic behavior of dielectric susceptibility with respect to either the frequency of measurement or the relaxation time of the substance and demonstrates that crosslinking of a thermoset causes its relaxation time to increase monotonically. It is shown that the stretched exponential relaxation function formalism satisfactorily describes the dielectric results and that the value of its distribution parameter initially decreases and, after gelation, reaches a constant value, which we denote γ, in the latter part of the cure. The value of the curing parameter, γ, which lies between 0.2 and 0.4, monotonically decreases with increasing curing temperature and tends to a limiting value characteristic of a thermoset at higher temperatures. This is in contrast with the increase found in the corresponding representation in the Kohlrausch-Williams-Watts parameter β with increasing temperature. The curing time dependence of the dipolar relaxation time ι has been determined and found to have the shape of an elongated S, with a well-defined point of inflexion, as ι increases during the cure, from a value characteristic of a liquid to an ultimate value characteristic of a glass.     

An ESR study of radical reactions of C60 and C70

The results of ESR spectroscopic studies of radical reactions of fullerenes are presented. Reactivities of radicals of various chemical natures with respect to C₆₀ and C₇₀, delocalization of the unpaired electron in monofullerenyl radicals, and their reactivity are considered. The examples of dynamic effects in the ESR spectra of fullerenyl radicals, associated with the hindered rotation of the attached radicals, are presented. Characteristic features of the structures of the spin adducts resulting from polyaddition of free radicals to fullerenes and of fullerenyl radicals containing ²-bonded metaIloco mplexes are discussed.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 10, pp. 2396–2406, October, 1996.     

Convergence problem in ab initio crystal orbital calculations

The various computational procedures for treating the problem of lattice summations in ab initio crystal orbital calculations are briefly reviewed and the cutoff problem is discussed. Several numerical examples are presented for demonstrating the fast convergence of the total energy per unit cell with respect to the inclusion of further neighbors' interactions, in case reasonable cutoff procedures are applied. The importance of electrostatic considerations for the interpretation of long range effects is stressed.     

The Art of Titration. From Classical End Points to Modern Differential and Dynamic Analysis

Titration represents the quantitative determination of a chemical change in response to the variation in concentration of a standrad. Information on reaction parameters is usually obtained from an analysis of the shape of the titration curve. In this paper the derivatives of titration functions with respect to concentration variables and with respect to equilibrium parameters are compared with one another. The latter can be determined directly from amplitudes and time constants of the dynamic responses to equilibrium perturbations, brought about by fast alterations of temperature, pressure or electirc field. Three general situations have to be distinguished: 1. If the interaction is very strong (i.e. the complex stability is high: K → ∞), the (average) number of sample praticles can be counted directly by means of their reaction with a calibrated number of standard particles.—2. If, on the contrary, the interaction is weak (i.e. the complex stability is low), the sample will react quantitatively only in the presence of a large excess of standard; the half-way point of which provides a direct measure of the binding constant.—3. Only for moderate interactions can individual curves characteristic of the binding strength be observed. The limiting cases 1 and 2 yield titration curves which reflect only the general class to which the system belongs. The dynamical procedures are fairly insensitive in case 1, which represents the classical end-point situation and is most suitable for quantitative anlaysis, i.e. for a determination of sample concentrations. For case 2, the classical and dynamical techniques yield comparable information: mass-action parameters are obtained from the extrema of the curves, which occur only in the presence of an excess of the standard. In case 3 the dynamical treatment provides a more direct access to the equilibrium constants, and also to the enthalpies or volumes of reaction as well as to the rate constants. The advantage of the dynamic method is due to the fact that in the two derivatives of the titration function d T_i/d ln q and d T_i/d ln p (where q is the titration variable, i.e. the ratio of standard and sample concentration, and p the mass-action parameter, i.e. a reduced binding constant) the terms resulting from a differentiation with respect to p are more closely related to the reaction parameters than those following from a variation of q. The dynamic analysis is generalized for applications to multiple-step titration processes, where it allows for the measurement fo equilibrium and rate parameters of individual steps. A uniform representation utilizing trigonometric functions was chosen which expresses clearly the singualr character of the end-point. The relations are summarized intabular form. They provide in conjunction with the illustrations the basis for a comparative discussion.