Handbook of Solvents - George Wypych - ChemTech - Ventech!

704 George Wypych
Surface DMS, molar %
100
99
98
97
96
95
94
93
2% DMS
1% DMS
92
0 2 4 6 8 10
Cyclohexanone content, v%
Figure 12.1.41. Surface DMS vs. cyclohexanone
concentration for two bulk contents of DMS in
polystyrene. [Data from Jiaxing Chen, J A Gardella,
Macromolecules, 31, No.26, 9328-36 (1998).]
drogen bonding solvents also affect
conductivity. 19 This phenomenon is applied
in antistatic compounds which require a
certain concentration of water to perform
their function.
Surface properties such as smoothness
and gloss are affected by the rheological
properties of coatings and solvents play
an essential role in these formulations. Surface
composition may also be affected by
solvent. In some technological processes,
the surface composition is modified by
small additions of polydimethylsiloxane,
PDMS, or its copolymers. The most well
known application of such technology has
been the protection of external surface of
the space shuttle against degradation using
PDMS as the durable polymer. Other applications
use PDMS to lower the coefficient
of friction . In both cases, it is important that
PDMS forms a very high concentrations on the surface (possibly 100%). Because of its low
surface energy, it has the intrinsic tendency to migrate to the surface consequently, surface
concentrations as high as 90% can be obtained without special effort. It is more difficult to
further increase this surface concentration. Figure 12.1.40 shows the effect of a chloroform/toluene
mixture on the surface segregation of PDMS. When the poor solvent for
PDMS (toluene) is mixed with chloroform, the decrease in the mobility of the PDMS segments
makes the cohesive migration of the polystyrene, PS, segments more efficient and the
concentration of PDMS on the surface increases as the concentration of toluene increases.
Increase in PDMS concentration further improves its concentration on the surface to almost
100%. This shows the influence of polymer-solvent interaction on surface segregation and
demonstrates a method of increasing the additive polymer concentration on the surface.
Solvents with higher boiling points also increase surface concentration of PDMS because
they extend the time of the segregation process. 20
Figure 12.1.41 shows the effect of addition of cyclohexanone to chloroform. 20 This exemplifies
yet another phenomenon which can help to increase the surface concentration of
PDMS. Cyclohexanone is more polar solvent than chloroform (Hansen parameter, δ p,is3.1
for chloroform and 6.3 for cyclohexanone). The solvation of cyclohexanone molecules will
selectively occur around PDMS segments which helps in the segregation of PDMS and in
the cohesive migration of PS. But the figure shows that PDMS concentration initially increases
then decreases with further additions of cyclohexanone. The explanation for this behavior
is in the amount of cyclohexanone required to better solvate PDMS segments. When
cyclohexanone is in excess, it also increases the solvation of PS and the effect which produces
an increased surface segregation is gradually lost.
The surface smoothness of materials is important in solvent welding. It was determined
that a smooth surface on both mating components substantially increases the strength
of the weld. 21 This finding may not be surprising since weld strength depends on close sur-