Diffusion of linear aliphatic mono- and diesters (C
N
) having
N main chain atoms (
N=13–68) in bulk medium-density polyethylene (MDPE) has been studied under hydrostatic pressures up to 2500 bar at temperatures between 60°C and 125°C. Three triglycerides, phenyl stearate, and p-aminoazobenzene (pAAB, 80°C) as the diffusants and low-density (LDPE) and high-density (HDPE) polyethylenes as polyethylene substrate were used for comparison. Diffusion coefficient
D was determined from concentration distribution of the diffusants through stacked PE sheets as substrate. Regarding the linear esters at 90°C, the relationship
D N
holds at constant pressures. Under the atmospheric pressure, became –2.10 in accordance with de Gennes's proposal (1971)
D N
–2 as well as with the experimental results reported by Klein and Briscoe (1979) for
N larger than 30.
D's for the glycerides deviate from the relationship
D N
–2 toward the smaller values by comparison at the same
N. The exponent is pressure-dependent. It decreases with increasing pressure according to =–2.10–0.000942
P, where
P is measured by the unit of bar. Plots of ln
D vs
P for all the diffusants show linear relationships with negative slopes, from which activation volume for the diffusion
V
was calculated. At 90°C,
V
increases slowly with increasing
N and increasing
V
Ki, the intrinsic molecular volume of the diffusant, from 39.3 cm
3/mol for ethyl caprate (
C
13,
V
Ki=136 cm
3/mol) to 76.8 cm
3/mol for behenyl behenate (C
45,
V
Ki=466 cm
3/mol). Observed
V
s are explainable on the basis of the reptation mode of the chain molecule diffusion.
V
s for C
25 and C
45 are found to increase with increasing degree of crystallinity where MDPE, heat-treated MDPE, LDPE, and HDPE were used. The results obtained by varying temperature are as follows.
V
for C
45 was always found to be larger than C
25. Both decreased linearly with increasing temperature, giving two linear lines with different slopes whose extensions intersected at 132°C, the melting point of the MDPE, where the difference in
V
disappeared. The apparent activation energies
E
Ds for the diffusion of C
25 and C
45 increased linearly with increasing pressure, whose slopes are explainable according to
E
D=
E
0+
PV
[1-(
dln
V
/
dln
T)
P
].
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