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  • 1
    Electronic Resource
    Electronic Resource
    Thermodynamics of isothermal vitrification by chemical reactions in a liquid (1992)
    Springer
    Il nuovo cimento della Società Italiana di Fisica 14 (1992), S. 763-766 
    Details
    ISSN: 0392-6737
    Keywords: Glass transitions ; Calorimetry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We report here first observations of the decrease in heat capacity during isothermal vitrification of a thermosetting polymer. A new experimental method has been developed for this study which allows simultaneous measurements of both the heat capacity and the enthalpy released as chemical reactions occur in quasi-isothermal conditions. During the curing the heat capacity of a thermoset first increases slightly, undergoes an abrupt decrease in a narrow range of time and thereafter slowly decreases. The abrupt decrease represents the irreversible transition of the liquid to a vitreous solid which is equivalent to the reversible glass transition appearing in a supercooled liquid when the temperature is lowered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02451722
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  • 2
    Electronic Resource
    Electronic Resource
    Simultaneous dielectric spectroscopy and calorimetry of the dynamics during a macromolecules growth (1996)
    Springer
    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1443-1448 
    Details
    ISSN: 0392-6737
    Keywords: Glass transitions ; Calorimetry ; Dielectric loss and relaxation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A method and an instrument has been developed for measuring almost simultaneously and continuously in time three different quantities: i) the heat evolved during the course of exothermic reactions, ii) the heat capacity and iii) the dielectric spectra of a liquid or solid sample. By using this instrument, we have studied the decrease in the d.c. conductivity and the increase in the relaxation time as molecules in an isothermally kept liquid chemically react to produce, irreversibly, a macromolecular structure until the liquid vitrifies and becomes rigid. The number of covalent bonds formed in the structure,n, has been calculated and the dielectric relaxation time, τ, is related ton. This τ increases progressively more rapidly withn, and decreases, of course, on increasing the temperature. A compensation between these two effects determines the dynamic behaviour observed during rate heating. The effect of temperature is found to predominate. Such observations have been made possible by means of the new instrument.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02453785
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  • 3
    Electronic Resource
    Electronic Resource
    The effect of increased crystallization on the electrical properties of nylon-12 (1993)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 31 (1993), S. 265-271 
    Details
    ISSN: 0887-6266
    Keywords: Nylon-12 ; dielectric properties ; crystallization ; calorimetry ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The dielectric properties of 30% crystalline dry Nylon-12 have been measured over the frequency range 10-105 Hz and temperature range 300-450 K, and the effect of its annealing at 423 K investigated both by dielectric measurement and differential scanning calorimetry. Annealing causes its crystallization to α phase, which increase the dc conductivity and decreases the contribution to orientation polarization, but does not alter the shape of the relaxation spectrum. The orientation polarization in Nylon-12 involves two processes, each of which occurs above the glass-transition temperature of Nylon-12, but only the spectra of the lowest temperature process could be clearly resolved. © 1993 John Wiley & Sons, Inc.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/polb.1993.090310304
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  • 4
    Electronic Resource
    Electronic Resource
    Relaxations in thermosets. XIX. Dielectric effects during curing of diglycidyl ether of bisphenol-A with a catalyst and the properties of the thermoset (1993)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 31 (1993), S. 299-311 
    Details
    ISSN: 0887-6266
    Keywords: dielectric relaxation ; curing kinetics ; thermoset ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Changes in the dielectric permittivity ε′ and loss epsiv;″ during the curing of DGEBA catalyzed by 10 mole % dimethylbenzylamine have been studied from sol to gel to glass formation regions at different temperatures from 323 to 390 K. The ε′ monotonically decreases with time of cure, and ε″ initially decreases by several orders of magnitude and then increases to reach a peak value before finally decreasing to a low value characteristic of the glassy state. The features shift to shorter times and the peak vanishes as the curing temperature is increased. The decrease of ε″ at the initial stage of cure has been analyzed in terms of dc conductivity σ0, which follows a power law, σ0 ∝ (tg-t)x, as well as a new singularity equation, σ0 ∝ exp[-B/(t0-t)] where tg, x, B, and t0 are empirical constants that vary with the curing temperature; tg is close to the time for gelation; and t0 ≥ time for vitrification. The dielectric properties of the thermoset formed after different periods of cure have been studied from 77 to 325 K. Similar studies of the thermosets formed at different temperatures have been made. Increase in the curing period decreases the heights of both the γ-and α-relaxation peaks and increases their separation, while a β-relaxation peak emerges. Isothermal curing at high temperatures decreases the height of the γ peak to a vanishingly small value and increases that of the β peak from a vanishingly small value. In both the uncured and fully cured states, there is only one sub-Tg relaxation process named γ for the uncured and β for the cured state. These results are discussed in terms of our general physical concepts of local mode motions in an amorphous matrix. © 1993 John Wiley & Sons, Inc.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/polb.1993.090310308
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  • 5
    Electronic Resource
    Electronic Resource
    Relaxations in thermosets. XXVI. Ultrasonic studies of the temperature dependence of curing kinetics of diglycidyl ether of bisphenol-A with catalyst (1994)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 32 (1994), S. 1465-1474 
    Details
    ISSN: 0887-6266
    Keywords: ultrasonics ; curing kinetics ; diglycidyl ether of bisphenol-A ; thermosets ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Longitudinal velocity and attenuation of the unreacted diglycidyl ether of bisphenol-A and dimethylbenzylamine mixture have been measured for 1 MHz ultrasonic frequency. These are combined with our earlier data [Alig et al., Acta Polymerica, Vol. 40, p. 590 (1989)], which were measured during the curing process of the thermoset, and then analyzed in terms of a process involving a negative feedback between molecular diffusion and chemical reaction. The complex plane plots of the real and imaginary components of the longitudinal modulus are fitted to a stretched exponential relation, and the structural relaxation time is calculated at different stages of curing at temperatures in the range 333-403 K. The results are compared against the calorimetric, dielectric, and viscosity measurements and it is shown that, at least for two cases, gelation time can be observed from the longitudinal modulus data. © 1994 John Wiley & Sons, Inc.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/polb.1994.090320818
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