Chemistry Reference
In-Depth Information
1.4
1.2
1.0
0.8
0.6
0.4
0
10000
20000
30000
40000
50000
PS M
n
Fig. 5 Experimental interfacial tension between PS and PMMA 10,000 as a function of PS
M
n
at
199
C. The
solid line
represents the best fit to a
M
n
1
dependence and the
dotted line
is the fit for a
M
n
0.5
dependence [
20
]
(
z
0.54) was obtained for the system with the lowest molecular weights and
highest polydispersities (
M
w
/
M
n
¼
¼
2 for the PDMS samples), whereas the largest
value for
z
(
z
0.9) was observed for the system with the highest molecular
weights. A smaller exponent for PDMS/PBD could be explained by the occurrence
of surface segregation of the polydisperse PDMS according to molecular weight.
Surface tension data for mixtures of PDMS oligomers suggest that the lower
molecular weight species are concentrated at the surface [
176
]. Alternatively, the
PDMS/PBD system is closest to its critical point, where a
M
n
0.5
dependence of
interfacial tension has been predicted [
181
] (discussed in Sect.
3.2.4
). The interme-
diate molecular weight system of PS/PBDH shows good correspondence with the
M
n
2/3
dependence. A similar dependence for the surface tension was explained by
using a simple lattice analysis [
182
] that incorporated the contribution of the end
groups at the interface. For these moderate molecular weights, the end-group
effects are important and a
M
n
2/3
dependence might be expected.
The PS/PMMA blends, on the other hand, contain the highest molecular weight
constituents and should, thus, conform best to the limit of high molecular weights.
In this limit, the exponent
z
apparently approached unity. The fact that the estimated
exponent is 0.90 probably suggests that the asymptotic regime (the
M
n
l
behavior)
was not yet reached even for those molecular weights. The nonlinear regression
results, therefore, suggest that the exponent
z
of the molecular weight dependence
of polymer polymer interfacial tension increases as the molecular weights of the
constituents increase.
¼
