Zero-pressure-weight-fraction Henry's constants (
) and infinite-dilution-weight-fraction activity coefficients (
) of selected aliphatic and aromatic solvents in a series of single-site
ethylene-1-octene copolymers (ss-EOs) with different branch contents were measured using inverse gas
chromatography over the temperature range from 170 to 230 °C. It was observed that measured
of
solvents with high volatility exhibited a minimum in the branch content range of 0−20 branches per
1000 backbone carbons but were independent of branch content over the range from 20 to 87, while that
of solvents with low volatility was insensitive to branch content at all. The observed behavior became
more pronounced for highly volatile solvents and propagated to solvents with medium volatility as
temperature was increased. It was found that the observed minima are mainly attributed to the relatively
more favorable interactions between the solvents and the copolymers with branch contents less than 20,
as quantified by the measured
. However, for the solvents with extremely low volatility, such
favorable changes in
in the low branch content region did not manifest themselves in the measured
as these changes were completely masked by the low volatility of the solvents even at 230 °C.
Molecular dynamics (MD) simulation results on comparable ss-EO models with branch contents from
0 to 90 suggested that the observed
behavior may originate from changes in the free volume hole size
distribution, not from changes in the specific free volume of the copolymer. In particular, the simulation
results showed that regardless of the temperature, the volume fraction of spherical free volume holes
with radii larger than 1.5 Å decreased as the branch content was increased, while the specific free volume
stayed constant.
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