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X. Wu and J. A. El-Awady
a good agreement between the CG-MD simulations and PALS measurements. The
simulations also show that the average FV radius and the standard deviation of the
FV distribution both decrease with increasing pressure. However, it should be noted
that the predicted average FV radius from the CG-MD simulations is larger than the
experimental values under all conditions. This can be attributed to the way the FV is
predicted in the simulations as compared to the experiments. The PALS experiments
measure the annihilation time of orthopositronium (o-Ps) particles created by the
reaction of positrons and electron cloud of the surrounding atoms [96], while in
the simulations the FVs are calculated by fitting the largest ellipsoids in the empty
space and reaching the surface of surrounding atoms without considering the effects
of the electron cloud. Thus, simulation predictions are systemically larger than
experiments.
Because FV holes are sensitive to temperature and pressure, it can be correlated to the polymer properties that are temperature and pressure dependent.
Aramoon et al. predicted the T g for different degrees of cross-linking and prepolymer lengths based on the calculated FV distributions obtained from CG-MD
simulations of DGEBA epoxies [64]. Figure 6a shows the density ρ and the average
radius of FV holes, , as functions of temperature for a 85% cured DGEBA/DAB
system as predicted from those simulations. The sharp turns observed in density,
ρ, and in the average FV hole radius, , both coincide with the same predicted
glass transition temperature. While conventionally T g is measured according to the
Fig. 6 (a) The density (left axis) and average FV radius (right axis) as a function of temperature
as predicted from CG-MD simulations of a DGEBA/DAB system having an initial length of 5
monomers, a degree of cross-linking of 85%, and a pressure of 0 kbar. The sharp turns observed
in the density and in the average FV hole radius curves both coincide with the same predicted
glass transition temperature. (b) Effects of prepolymer length, the degree of cross-linking, and the
average volume of free volume holes (χ) on the predicted glass transition temperature. Results
labeled by black circles and red squares are calculated using a 85% cross-linking system and
a 5-monomer system, respectively. Both systems are measured under 0 pressure. (Reprint with
permission from [64])
X. Wu and J. A. El-Awady
a good agreement between the CG-MD simulations and PALS measurements. The
simulations also show that the average FV radius and the standard deviation of the
FV distribution both decrease with increasing pressure. However, it should be noted
that the predicted average FV radius from the CG-MD simulations is larger than the
experimental values under all conditions. This can be attributed to the way the FV is
predicted in the simulations as compared to the experiments. The PALS experiments
measure the annihilation time of orthopositronium (o-Ps) particles created by the
reaction of positrons and electron cloud of the surrounding atoms [96], while in
the simulations the FVs are calculated by fitting the largest ellipsoids in the empty
space and reaching the surface of surrounding atoms without considering the effects
of the electron cloud. Thus, simulation predictions are systemically larger than
experiments.
Because FV holes are sensitive to temperature and pressure, it can be correlated to the polymer properties that are temperature and pressure dependent.
Aramoon et al. predicted the T g for different degrees of cross-linking and prepolymer lengths based on the calculated FV distributions obtained from CG-MD
simulations of DGEBA epoxies [64]. Figure 6a shows the density ρ and the average
radius of FV holes, , as functions of temperature for a 85% cured DGEBA/DAB
system as predicted from those simulations. The sharp turns observed in density,
ρ, and in the average FV hole radius, , both coincide with the same predicted
glass transition temperature. While conventionally T g is measured according to the
Fig. 6 (a) The density (left axis) and average FV radius (right axis) as a function of temperature
as predicted from CG-MD simulations of a DGEBA/DAB system having an initial length of 5
monomers, a degree of cross-linking of 85%, and a pressure of 0 kbar. The sharp turns observed
in the density and in the average FV hole radius curves both coincide with the same predicted
glass transition temperature. (b) Effects of prepolymer length, the degree of cross-linking, and the
average volume of free volume holes (χ) on the predicted glass transition temperature. Results
labeled by black circles and red squares are calculated using a 85% cross-linking system and
a 5-monomer system, respectively. Both systems are measured under 0 pressure. (Reprint with
permission from [64])