Heat of activation during gel formation

Summary The activation energies have been determined during the gel formation obtained by metathetical processes. They are found to vary widely depending upon the charge and size of the particles. It is said that due to the residual electric charge on the colloidal particles the value ofP is effected leading to an increase or decrease in the rate of gel formation, whilst the heat of activation is associated with the size of the colloidal units undergoing gelation.

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Zusammenfassung Die Aktivierungsenergie bei der Gel-Bildung für metathetische Prozesse wurden bestimmt. Es wurde gefunden, daß sie in weitem Maße von der Ladung und Größe der Teilchen abhängt. Gemäß der restlichen elektrischen Ladung an den Kolloid-Partikeln wird der Wert vonP, dem vorexponentiellen Faktor, beeinflußt, was zu einer Zunahme oder Abnahme der Geschwindigkeit der Gel-Bildung führt. Die Aktivierungswärme dagegen hängt mit der Größe der Kolloideinheiten zusammen, die gelieren.

     

Effect of substituents in the styrene ring on the dynamic mechanical relaxations of a styrene-crosslinked unsaturated polyester resin

The dynamic mechanical properties of a series of polyester resins made from a maleic/phthalic anhydride-based unsaturated polyester crosslinked with each of styrene, 4-methyl styrene, 4-ethyl styrene, 4-n-butyl styrene, 4-isopropyl styrene, tertiary butyl styrene, 4-chlorostyrene, and 3,4-dichlorostyrene were studied. The order of the α transition temperatures was as expected from that for the homopolymers, except in the case of the chlorostyrenes, for which dipolar interactions with the polyester chain may be important. The styrene bridges appeared to be involved in a steric interaction (and in the case of the chlorostyrenes, a dipolar interaction) with the β relaxing ester species. It is suggested that both the γ and γ′ relaxations involve similar interactions between the matrix and the relaxing moieties. For the 4-n-butyl styrene resin, an additional relaxation below ?170°C was observed, and is ascribed to relaxation of the n-butyl group.     

Evolution of gel structure during thermal processing of Na-geopolymer gels

The present work examines how the gel structure and phase composition of Na-geopolymers derived from metakaolin with varied Si/Al ratio evolve with exposure to temperatures up to 1000 degrees C. Gels were thermally treated and characterized using quantitative XRD, DTA, and FTIR to elucidate the changes in gel structure, phase composition, and porosity at each stage of heating. It is found that the phase stability, defined by the amount and onset temperature of crystallization, is improved at higher Si/Al ratios. Two different mechanisms of densification have been isolated by FTIR, related to viscous flow and collapse of the highly distributed pore network in the gel. Gels with low Si/Al ratio only experience viscous flow that correlates with low thermal shrinkage. Gels at a higher Si/Al ratio, which have a homogeneous microstructure composed of a highly distributed porosity, undergo both densification processes corresponding to a large extent of thermal shrinkage during densification. This work elucidates the intimate relationship between gel microstructure, chemistry, and thermal evolution of Na-geopolymer gels.     

Structural heterogeneity and dynamic mechanical relaxations of ethylene α‐olefin copolymers

Dynamic mechanical measurements are reported on two polyethylene copolymers with different types of comonomers and well‐characterized comonomer distributions. Measurements on both isotropic and oriented samples were undertaken over a wide frequency range. The mechanical anisotropy and activation energies of the α and β relaxations are consistent with the former relating to c‐shear in the crystals, and the latter to interlamellar shear. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 51–60, 1999     

Low-temperature mechanical relaxations in polymers containing aromatic groups

Studies have been made of the secondary relaxation processes in the solid state of a number of polymers containing aromatic groups in the polymer chain. The polymers investigated include one, polystyrene, with the aromatic group in side-chain positions, and six high polymers in which phenylene rings lie in the main backbone chain. In polystyrene, wagging and torsional motions of the side chain phenyl rings give rise to a low-temperature δ-relaxation which is centered at 33°K (1.7 Hz) and which has an activation energy of about 2.3 kcal/mol. Most of the polymers with phenylene rings in the main chain exhibit a low-temperature relaxation in the temperature region from 100°–200°K. This relaxation process is centered at 159°K (0.54 Hz) in poly-p-xylylene, at 162°K (0.67 Hz) in polysulfone, and at 165°K (1.24 Hz) in poly(diancarbonate). In poly(2,6-dimethyl-p-phenylene oxide), two overlapping low-temperature relaxations are found, one in the range 125–140°K and the other near 277°K (ca. 1 Hz). The low-temperature secondary relaxation process in all of these polymers is believed to be associated with local reorientational motion of the phenylene, or substituted phenylene, rings or with combined motion of the phenylene rings and nearby chain units. For these low temperature relaxation processes, the activation energy is about 10 kcal/-mole. The temperature location of the relaxation appears to depend on the specific units to which the phenylene rings are attached and on steric and polar effects caused by substituents on the ring. In the poly-p-xylylenes the relaxation is shifted to much higher temperatures by a single Cl substitution on the ring but remains at essentially the same temperature position when dichlorosubstitution is made. The effects of water on the magnitude and temperature location of the observed low temperature relaxations, as well as the implications of the study for other polymers containing aromatic groups in their backbone chains, are discussed.     

Multiple mechanical relaxations in ethylcyclohexane above the glass transition temperature

The mechanical response of ethylcyclohexane has been investigated at ultrasonic frequencies in a large temperature range from 300 K down to the glass transition region. The results indicate the existence of a secondary relaxation not yet reported for this system. The comparison with literature data leads to a rather complex dynamic behavior. In fact, this molecular liquid exhibits three different mechanical relaxations above the glass transition temperature: a main structural process and two additional processes, both having a possible intramolecular origin.     

Dynamic mechanical and dielectric relaxations of polystyrene below the glass temperature

Dynamic mechanical and dielectric properties of various kinds of polystyrene, including bulk-polymerized, monodisperse, isotactic, and thermally degraded samples, have been measured below the glass temperature to 4°K. Five relaxation processes are found, designated β, γ, γ′, δ, and ε in order of descending temperature. The β peak (350°K at 10 kHz) is attributed to the local oscillation mode of backbone chains and the γ′ peak (180°K at 10 kHz) to rotation of phenyl groups. The δ peak (100°K at 10 kHz) is observed only in dielectric properties of the bulk-polymerized sample and is assigned to weak polar bonds, such as oxygen bonds in the chain. The δ peak (55°K at 10 kHz) which is prominent in dynamic mechanical properties is interpreted in terms of lattice defects due to a syndiotactic diad inserted between isotactic sequences in a chain or vice versa. The ε peak (ca. 25°K at 10 kHz) is first reported in the present work, but the mechanism involved is not yet clear.     

Dynamic mechanical relaxations of polyterephthalates based on trimethylene glycols

The dynamic mechanical relaxations of poly(trimethylene glycol terephthalate) (PTMT), poly(ditrimethylene glycol terephthalate) (PDTMT) and two copolymers obtained from them have been studied between ? 150 and 200°C with a dynamic viscoelastometer. The four polymers show three relaxations that are designated α, β, and γ in order of decreasing temperature. The α relaxation is considered to be the glass transition of the polymers. The β relaxation is wider and weaker than the α, as normally occurs in the polyester series. The γ relaxation takes place at temperatures below ? 100°C and is usually overlapped by the β relaxation. The influence of thermal and mechanical histories on the nature, location, and intensity of the three relaxations is studied and discussed.     

The role of mechanical phenomena during the formation of network polymers

A review of new results concerning the role of mechanical phenomena during cure of epoxy matrices has been made. The influences of various factors (temperature, nature and concentration of components, the character of adhesion interaction of matrix and filler, etc.) on the rate of structural relaxation of polymer have been examined. The problem of the mechanical degradation of network chains under the action of internal stresses has been discussed.     

Changes in the static and dynamic mechanical moduli of rubbers during ageing

Changes in the static and dynamic mechanical (complex) moduli for TMTD (filled and unfilled) and S/CBS-vulcanised (unfilled) natural rubber cylinders (height = diameter = 20 mm) were intermittently recorded as a function of time up to 1000 h at different temperatures. For the TMTD-vulcanisates, the moduli decreased with time of ageing and for the S/CBS-vulcanisate a maximum was obtained. The phase angle between stress and strain increased with increasing temperature and with time of ageing. The changes in moduli were attributed to scission and crosslinking reactions. The unfilled TMTD-vulcanisate showed no dependence on dynamic amplitude, whereas the S/CBS-vulcanisate showed a strong dependence on dynamic mechanical amplitude for ageing times longer than 300 h. Whhen log₁₀ (tan δ)_was plotted against 1/T, a straight line was obtained for temperatures between 75 and 150°C. An activation energy of 25 kJ/mole was obtained. This value is in agreement with the activation energy for the diffusion of oxygen in natural rubber.     

Dynamic mechanical and dielectric relaxations of poly(difluorobenzyl methacrylates)

This work reports the mechanical and dielectric relaxation spectra of three difluorinated phenyl isomers of poly(benzyl methacrylate), specifically, poly(2,4‐difluorobenzyl methacrylate), poly(2,5‐difluorobenzyl methacrylate) and poly(2,6‐difluorobenzyl methacrylate). The strength of the dielectric glass–rubber relaxation of the 2,6 difluorinated phenyl isomer is, respectively, nearly three and two times larger than the strengths of the 2,5 and 2,4 isomers. The 2,4 isomer presents a mechanical α peak the intensity of which is nearly two times that of the other two isomers. Both the mechanical and dielectric relaxation spectra display a subglass process, called γ relaxation, centered in the vicinity of −50 °C at 1 Hz and, in some cases, a subglass β absorption is detected at higher temperature partially masked by the glass–rubber relaxation. The mean‐square dipole moments per repeating unit, 〈μ2〉/x, measured at 25 °C in benzene solutions, are 2.5 D², 1.9 D², and 5.0 D² for poly(2,4‐difluorobenzyl methacrylate), poly(2,5‐difluorobenzyl methacrylate) and poly(2,6‐difluorobenzyl methacrylate), respectively. These results, in conjunction with Onsager type equations, permit to conclude that auto and cross‐correlation contributions to the dipolar correlation coefficient may have the same time‐dependence. On the other hand, dipole intermolecular interactions, rather than differences in the flexibility of the chains, seem to be responsible for the relatively high calorimetric glass‐transition temperature of the 2,6 diphenyl isomer, which is, respectively, nearly 36 °C and 32 °C above the T_g's of the 2,4 and 2,5 isomers. Molecular Mechanics calculations give a good account of the differences observed in the polarity of the polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2179–2188, 2000     

Ligand-induced tertiary relaxations during the T-to-R quaternary transition in hemoglobin

When human hemoglobin is encapsulated in nanoporous silica gels, tertiary and quaternary structural relaxations are dramatically slowed down, allowing the characterization of elusive reaction intermediates. In this work, the conformational and functional changes triggered by CO binding to human deoxyhemoglobin gels were determined in the absence and presence of allosteric effectors. CO rebinding kinetics to human deoxyhemoglobin gels after nanosecond laser photolysis were monitored as a function of time after CO saturation. A maximum entropy analysis of the CO rebinding kinetics shows that the T conformation slowly evolves toward R, with an associated redistribution of tertiary species. The tertiary species are characterized by different CO rebinding rates which are essentially unaffected by the protein quaternary state.     

Dynamic mechanical and dielectric relaxations in poly(monoethylphenyl itaconate)

Dynamic mechanical and dielectric relaxational behavior of poly(monoethylphenyl itaconate) at different frequencies and temperatures was studied. Three relaxation zones are found. The dynamic mechanical response is dominated by a relaxation peak at room temperature, labeled β relaxation. Two prominent shoulders labelled as γ and α relaxations are observed. Because of the overlapping of the α and γ with the β relaxation, a deconvolution method to improve the understanding of these phenomena is proposed. In spite of the complexity of the experimental spectra, the proposed deconvolution method seems to be a convenient approach to interpret the relaxational behavior of this polymer. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2749–2756, 1997     

Influence of dynamic interfacial tension on droplet formation during membrane emulsification

Membrane emulsification is a promising and relatively new technique for producing emulsions. The purpose of this study was to better understand the influence of interfacial tension on droplet formation during membrane emulsification. Droplet formation experiments were carried out with a microengineered membrane; the droplet diameter and droplet formation time were studied as a function of the surfactant concentration in the continuous phase. These experiments confirm that the interfacial tension influences the process of droplet formation; higher surfactant concentrations lead to smaller droplets and shorter droplet formation times (until 10 ms). From drop volume tensiometer experiments we can predict the interfacial tension during droplet formation. However, the strong influence of the rate of flow of the to-be-dispersed phase on the droplet size cannot be explained by the predicted values. This large influence of the oil rate of flow is clarified by the hypothesis that snap-off is rather slow in the studied regime of very fast droplet formation.