Sindee L. Simon

Volume and enthalpy recovery of polystyrene

Abstract Volume and enthalpy recovery measurements were used to study the physical aging behavior of a polystyrene. Isothermal aging at temperatures near Tg was studied with aging times ranging from several minutes to several days. The data are satisfactorily fit using the Tool–Narayanswamy–Moynihan model of structural recovery. The times required to reach equilibrium are the same for the two properties at temperatures where equilibrium was achieved. Extrapolation to lower temperatures using the model indicates that the times required to reach equilibrium may be different for the two properties at these lower temperatures. The recovery data were plotted as the departure from equilibrium and then were shifted horizontally to superpose at long times. The shift factors for volume and enthalpy recovery data agree with one another above 94°C but they diverge below this temperature. The shift factor data deviates from the WLF or VTFH equation for temperatures below Tg. In addition, we find that the normalized rate of approach is the same for volume and enthalpy recovery at 97°C in the limit of a linear temperature jump, but it differs between the two properties for a nonlinear jump even though the times required to reach equilibrium are the same.

Modeling the evolution of the dynamic mechanical properties of a commercial epoxy during cure after gelation

ABSTRACT: The cure kinetics for a commercial epoxy have been established and theinfluence of the degree of cure on the glass transition determined. Time-temperatureand time-conversion superposition principles have been built into a model that success-fully predicts the development of the viscoelastic properties of the epoxy during iso-thermal cure from gelation to after vitrification.

Effect of Cation Symmetry on the Morphology and Physicochemical Properties of Imidazolium Ionic Liquids

In this paper, the morphology and bulk physical properties of 1,3-dialkylimidazolium bis{(trifluoromethane)sulfonyl}amide ([(C(N/2))(2)im][NTf(2)]) are compared to that of 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C(N-1)C(1)im][NTf(2)]) for N = 4, 6, 8, and 10. For a given pair of ionic liquids (ILs) with the same N, the ILs differ only in the symmetry of the alkyl substitution on the imidazolium ring of the cation. Small-wide-angle X-ray scattering measurements indicate that, for a given symmetric/asymmetric IL pair, the structural heterogeneities are larger in the asymmetric IL than in the symmetric IL. The correlation length of structural heterogeneities for the symmetric and asymmetric salts, however, is described by the same linear equation when plotted versus the single alkyl chain length. Symmetric ILs with N = 4 and 6 easily crystallize, whereas longer alkyl chains and asymmetry hinder crystallization. Interestingly, the glass transition temperature is found to vary inversely with the correlation length of structural heterogeneities and with the length of the longest alkyl chain. Whereas the densities for a symmetric/asymmetric IL pair with a given N are nearly the same, the viscosity of the asymmetric IL is greater than that of the symmetric IL. Also, an even-odd effect previously observed in molecular dynamics simulations is confirmed by viscosity measurements. We discuss in this paper how the structural heterogeneities and physical properties of these ILs are consistent with alkyl tail segregation.

Physical aging of a polyetherimide: Volume recovery and its comparison to creep and enthalpy measurements

Volume recovery measurements have been used to study the physical aging behavior of a polyetherimide. Isothermal aging temperatures near Tg were studied with aging times ranging up to several days. The volume decreases during physical aging and levels off at equilibrium. For comparison purposes, the data are normalized to yield the departure from equilibrium which varies from unity at very short aging times to zero when equilibrium is reached. As the aging temperature decreases, the normalized curves are shifted to longer times without a significant change in shape. Hence, the data can be reduced by aging time—temperature superposition. The temperature dependence of the shift factors used to reduce the volume recovery data and the times to reach equilibrium for the volume recovery follow the WLF equation and agree within experimental error with the values from enthalpy and creep measurements obtained in previous work. However, the approach to equilibrium for volume appears to differ from that of enthalpy, with volume recovery being faster than the enthalpy recovery at short times.

Temperature-modulated differential scanning calorimetry: theory and application

The status of temperature-modulated differential scanning calorimetry is reviewed, including current methods of data analysis, and the experimental conditions and calibrations necessary for meaningful data interpretation. The theory and application of TMDSC to absolute heat capacity measurements, the glass transition, and crystallization and melting are also reviewed and discussed.

Enthalpy recovery, creep and creep–recovery measurements during physical aging of amorphous selenium

The physical aging behavior of amorphous selenium has been investigated using differential scanning calorimetry and conventional and interrupted creep experiments. As a result of physical aging, enthalpy decreases and the creep and recovery curves shift to longer times. The times required to reach equilibrium for enthalpy recovery and creep appear to have different temperature dependences resulting in enthalpy reaching equilibrium before creep at aging temperatures a few degrees below the nominal glass temperature. In the nominal glass transformation range, however, the times required to reach equilibrium are approximately the same. A general picture of aging behavior has emerged from our data on selenium coupled with past work on polyetherimide and polystyrene.

Low-k organosilicate films prepared by tetravinyltetramethylcyclotetrasiloxane

Low-k films with k of 2.5–2.9 were deposited under different conditions of pressures and temperatures using a plasma-enhanced chemical vapor deposition (PECVD) system. These films were prepared using a new liquid precursor, tetravinyltetramethylcyclotetrasiloxane (TVTMCTS) and H2 carrier gas. The rf power was kept as low as possible to maintain the original ring structure in the films. The as-deposited films were annealed and the dielectric and optical properties were investigated. Identification of the absorption bands in the IR spectra for as-deposited films reveals a broadband around 950–1200 cm−1 arising from the Si–O stretching mode of the ring (1065 cm−1) and chain structure (1000 cm−1), respectively; a band at 750–900 cm−1 due to Si–O bending (790 cm−1); Si–CH3 rocking mode (760 cm−1); a sharp band centered at 1260 cm−1 due to a Si–CH3 bending mode; and a broadband at 2800–3000 cm−1 due to the CH group. A comparison of the IR spectra of the PECVD film and TVTMCTS liquid reveals that vinyl vibration...

Chain length dependence of the thermodynamic properties of linear and cyclic alkanes and polymers

The specific heat capacity was measured with step-scan differential scanning calorimetry for linear alkanes from pentane (C(5)H(12)) to nonadecane (C(19)H(40)), for several cyclic alkanes, for linear and cyclic polyethylenes, and for a linear and a cyclic polystyrene. For the linear alkanes, the specific heat capacity in the equilibrium liquid state decreases as chain length increases; above a carbon number N of 10 (decane) the specific heat asymptotes to a constant value. For the cyclic alkanes, the heat capacity in the equilibrium liquid state is lower than that of the corresponding linear chains and increases with increasing chain length. At high enough molecular weights, the heat capacities of cyclic and linear molecules are expected to be equal, and this is found to be the case for the polyethylenes and polystyrenes studied. In addition, the thermal properties of the solid-liquid and the solid-solid transitions are examined for the linear and cyclic alkanes; solid-solid transitions are observed only in the odd-numbered alkanes. The thermal expansion coefficients and the specific volumes of the linear and cyclic alkanes are also calculated from literature data and compared with the trends in the specific heats.

Interpretation of the dynamic heat capacity observed in glass-forming liquids

Slow structural relaxations can complicate the interpretation of thermodynamic measurements on glass-forming liquids. Here we demonstrate using model calculations that structural recovery can lead to an apparent frequency-dependent heat capacity in ac calorimetry experiments. The model is shown to describe the complex heat capacity data reported in the literature for glycerol and poly(vinyl acetate). Importantly, the model does not invoke a complex heat capacity; rather, only static heat capacities are used. The analysis further suggests that ac calorimetry should provide a powerful way of testing models of structural recovery.

Trimerization of monocyanate ester in nanopores.

The effects of nanoconfinement on the reaction kinetics and properties of a monocyanate ester and the resulting cyanurate trimer are studied using differential scanning calorimetry (DSC). On the basis of both dynamic heating scans and isothermal reaction studies, the reaction rate is found to increase with decreasing nanopore size without a change in reaction mechanism. Both the monocyanate ester reactant and cyanurate product show reduced glass transition temperatures (T(g)s) as compared to the bulk; the T(g) depression increases with conversion and is more pronounced for the fully reacted product, suggesting that molecular stiffness influences the magnitude of nanoconfinement effects. Our results are consistent with the accelerated reaction and the T(g) depression found previously for the nanoconfined difunctional cyanate ester, supporting the supposition that intracyclization is not the origin of these effects.