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  • American Institute of Physics (AIP)  (9)
  • 1
    Electronic Resource
    Electronic Resource
    The temperature and polymerization effects on the relaxation time and conductivity, and the evolution of the localized motions (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6957-6965 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: To examine the manner in which molecular dynamics of a polymerizing liquid (stoichiometric amounts of 4,4′-diaminodicyclohexylamine and diglycidyl ether of bisphenol-A) evolves during thermal cycling from its (molecular) vitreous state to its fully polymerized vitreous state, calorimetry, and dielectric spectrometry were performed simultaneously in real time. The half-width of the relaxation spectrum of the liquid was relatively narrow and became narrower on heating. This was followed by an increase in the characteristic relaxation time and the spectrum became broader as polymerization occurred and reached completion. The dc conductivity initially increased and then decreased. The faster dynamics of the Johari–Goldstein relaxation in the fully polymerized state evolved as polymerization reached completion and the temperature increased. The dielectric polarization associated with this relaxation had a broad spectrum, whose half-width increased with decrease in the temperature. Its relaxation rate followed the Arrhenius equation with an activation energy of 63.4 kJ/mol. The temperature dependence of the faster relaxation did not change with the change in the overall configurational entropy of the liquid, a feature that substantiates the dynamic heterogeneity theories for the structure of the liquid and for the origin of the relaxation. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1311623
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  • 2
    Electronic Resource
    Electronic Resource
    Reversal in the dielectric relaxation time during polymerization: Thermal energy compensation on macromolecular growth (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 10599-10605 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The increase in the dielectric relaxation time τ0 and in the number of covalent bonds n formed during polymerization of an equimolar mixture of cyclohexylamine and a diepoxide have been studies in real time, in two experiments: (i) under isothermal conditions when the dielectric spectrum monotonically shifts to the low-frequency side and the configuration entropy decreases and (ii) on heating the polymerizing liquid at a fixed rate when the spectrum initially shifts to the low-frequency side, reverses direction at a certain temperature, and shifts towards the high-frequency side. This reversal in τ0 occurs when an increase in thermal energy begins to dominate the effects of increase in n. Consequences of the two competitive effects on the measured dielectric behavior have been discussed and relations between the change in configurational entropy, and τ0 of the ultimately formed linear chain polymer, are discussed. It is shown mathematically that for a fixed τ0 and a configurational entropy, there are an infinite number of combinations of temperature and n, some of which may be explored by altering the temperature–time profile during the macromolecular growth. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478991
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  • 3
    Electronic Resource
    Electronic Resource
    Water↔ice transformation in micron-size droplets in emulsions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3115-3120 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: To investigate the crystallization and melting behaviors of micron-size droplets in a water-emulsion, the heat capacity, Cp, has been studied by adiabatic calorimetry over the 233–273 K range. Water droplets in the emulsions began to crystallize at ca. 243 K, but the crystallization rate was slow. This was caused partly by the relatively slow coalescence of crystallized and uncrystallized droplets at that temperature, in a highly nonNewtonian viscous media of the emulsion. Crystallization occurred rapidly on heating and remained incomplete even when it occurred at 260 K. Thus a substantial amount of water droplets coexisted with ice droplets in the emulsion. The onset of crystallization shifted to lower T and the number of water droplets in the emulsion at a given T decreased as the droplets grew on thermally cycling the emulsion. Cp of emulsion increased progressively more rapidly as 273 K was approached, which is attributable to premelting of the ice droplets beginning at 260 K. Analysis of the Cp data showed that interaction between the droplets and the surfactant in the emulsion changes on crystallization. This is attributable to the decrease in the entropy at the surfactant–water interface. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479592
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  • 4
    Electronic Resource
    Electronic Resource
    Calorimetric effects of intergranular water in ice (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 4987-4990 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The heat capacity, Cp, of microcrystalline and large or single crystal ice was measured from 270 to 272.6 K and the effect of the heat input (required before measuring the increase in temperature) on the measured Cp investigated. The measured Cp of microcrystalline solids was much higher than that of large and single crystal solids and increased further with increase in the heat input. At 271.16 K, the measured Cp was independent of the heat input, but for higher temperatures, it increased nonlinearly with the heat input. These effects are caused by the increase in the volume fraction of intergranular water in impurity free ice with increase in both its temperature and the heat input. A significant fraction of the heat used for Cp measurement is consumed in melting ice at grain junctions which does not raise the temperature. Thus, the measured Cp appears to be much greater than the true value. Quantitative analysis of the data yielded results which agree reasonably well with the prediction of a theory given in J. Chem. Phys. 100, 4548 (1994). © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469548
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  • 5
    Electronic Resource
    Electronic Resource
    Simultaneous measurements of enthalpy and heat capacity of a thermosetting polymer during the curing process (1993)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 1076-1080 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We report a new microcalorimetric technique which allows simultaneous measurements of the heat capacity and the rate of enthalpy release during the chemical reaction in a substance under quasi-isothermal conditions. The basic features of the instrument are presented together with brief experimental results on the polymerization of diglycidyl ether of bisphenol A mixed with ethylene diamine. The instrumentation is an advance for studies of both synthetic and biological formation of macromolecules, for now it becomes possible to measure the heat capacity of a time-variant system and to characterize its vitrification transition occurring at constant temperature, as the growth of the macromolecule by chemical reaction increases the sample's viscosity. The abrupt decrease observed in the heat capacity curve corresponds to the vitrification transition, which cannot be observed in the corresponding enthalpy release curve. The present calorimeter has an important use in both technological assessment and in understanding the fundamental aspects of the vitrification phenomenon.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1144127
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  • 6
    Electronic Resource
    Electronic Resource
    Time domain reflection methods for dielectric measurements to 10 GHz (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 66 (1989), S. 793-802 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Total reflection methods and instrumentation for their use are described for measurements of dielectric permittivity and loss at frequencies to 10 GHz or more. Several cell designs are shown, together with analyses of their performance. Procedures are given for correcting effects of wave propagation in the cells and residual reflections in the cells by bilinear analysis with calibrations using dielectrics of known permittivity. Representative results are presented for highly polar liquids, dilute solutions of polar molecules in nonpolar solvents, electrolyte solutions, and ionic glasses with appreciable ohmic conduction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.343499
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  • 7
    Electronic Resource
    Electronic Resource
    A differential calorimetric technique for heat capacity and thermal conductivity measurements of liquids (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 2416-2419 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Details are reported for a differential calorimetric technique to measure both the heat capacity and the thermal conductivity of liquid samples. The accuracy of the measured heat capacity is better than 0.4%, while sensitivity is about 4×10−4 J/°C. The accuracy of the measured thermal conductivity kL is about 1% if the difference between the kL value of the reference and the sample liquid is within 40%. Measurements were performed on: (a) water+t-butanol, (b) water+n-butanol, and (c) methanol+t-butanol mixtures at 25 °C. The experimental results for solute molar fraction x less than 0.2 are compared with data in the literature in the case of water+t-butanol mixtures. The heat capacity and thermal conductivity data versus x are probably the first ones published for mixtures (b) and (c).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141373
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  • 8
    Electronic Resource
    Electronic Resource
    Time domain reflectometry to study the dielectric properties of liquids: Some problems and solutions (1991)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 450-456 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: After a brief introduction we review the fundamental mathematical relations necessary to discuss the main aspects of the time domain dielectrometry performed with total reflection from an open cell. Experimental problems due to the presence of resonances in the sample cell, spurious reflections, and fringing fields are considered and possible solutions are suggested. An experimental apparatus realized to study liquids with dielectric constant values up to those of water and water solutions from 10 MHz up to 10 GHz in the temperatures range −45 °C+70 °C is also presented. The performances of some different experimental cells for liquids are compared and representative results obtained with different procedures are reported. The use of the numerical solution of a transcendental equation that takes into account for wave propagation allows us to measure the complex dielectric constant without the use of reference liquids, once the electrical parameters of the sample cell are known. In this case an accuracy better than 2% both in the real and in the imaginary parts of the dielectric constant can be obtained up to 7 GHz, as tested with chloroform. Calibration procedures using liquids of known permittivity can remove the contributions of spurious effects, so allowing us to perform measurements with the same accuracy up to 10 GHz and more on highly polar liquids too.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1142141
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  • 9
    Electronic Resource
    Electronic Resource
    Dielectric relaxation spectroscopy of reaction-controlled slowing of molecular diffusion in liquids (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 6677-6686 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dielectric relaxation spectra of a liquid in which chemical reactions lead to the growth of a macromolecule have been measured at different instants of reaction and in the kHz and MHz frequency range. The change in the complex permittivity measured for a fixed frequency, as the reaction proceeds, is phenomenologically similar to that observed on varying the frequency for a time-invariant system and obeys a stretched exponential relaxation function. The increase in the relaxation time of the liquid with reaction time is similar to the corresponding increase in the extent of reaction. As the reaction proceeds, a second relaxation, namely the α-process, separates and shifts towards the lower frequency side of the spectrum. Concomitantly, its strength decreases as does the strength of the remaining high-frequency process. The manifestation of the negative feedback between chemical reactions and molecular diffusion is remarkably similar to that observed on supercooling a molecular or polymeric liquid. A formalism given here illustrates the general aspects of this similarity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463673
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