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FDSC-202:
Measurement of Temperature Dependence of Surface Tension on
Polystyrene via Sessile Drop and Contact Lamella Methods
The contact
angle measurements of PS at 30, 60 and 90C are obtained from
using sessile drop method with water, ethylene glycol and
diiodomethane. The surface free energy of the solid PS at
different temperatures then calculated from OWRK theory.
The surface tensions of PS melt at 120 and 150C were an
average obtained from several repeated direct measurements
using contact lamella method with the software calculation.
The contact
angles from the sessile drop method were recorded using a
movie function with software triggering to eliminate the
influence from any solvent evaporation as happened in a manual
operation for a more accurate result. The equilibrium of the
contact lamella was examined by checking the pixel number of
the width of the lamella at a certain position from the metal
probe on the top. It took about 1 hour to reach equilibrium.
The measurement was not performed until the sample temperature
reached the target temperature (+/-0.1C) for 60 minutes. Our
proprietary electrical heating chamber, TEC350, with cooling
air and inert gas paths and the controller, TC350, controlled
the temperature to an accuracy of +/- 0.1C.
All the
surface tension data have a unit of mN/m or dyne/cm. The
shaded data were obtained from contact lamella method, which
is a direct measurement based on a force balance between
gravity and surface tension (ref. 1). The unshaded data were
obtained from the contact angle measurement using sessile drop
method and OWRK theory for the surface free energy
calculation. Evidently, the surface tensions or surface
free energies of PS, either from contact lamella method or the
sessile drop method, are very consistent to show an decrease
with temperature and their values are satisfactorily close to
Wu’s results (ref. 2). The capabilities, performance and
accuracy of our Optical Contact Angle Meter, OCA20, and its
software are proved to be very reliable for different
measurements as shown in this report.
|
Temp. (° C) |
30 |
60 |
90 |
120 |
150 |
|
Surface Tension
Of PS (mN/m) |
39.72 |
37.69 |
35.84 |
33.43
+/-0.28 |
31.20
+/-0.56 |
The
literature values of the PS sample with a similar MW by Wu
(ref. 2) according to the equation of 40.7-0.072*(T-20) are
shown in the following plot (Fig. 1) to compare with the
results obtained from the FDS’ optical contact angle device,
OCA20, equipped with the high temperature chamber, TEC350.
The results from the contact lamella method performed on OCA20
+ TEC350 match very well with the literature data. The
image examples of contact lamella from the PS melt and sessile
drop on the PS surface are shown in Fig. 2&3.
| Fig.
1 |
Fig.
2 Example of contact lamella; sample PS at 150C |
 |
 |
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Fig.
3 Example of sessile drop; water drop on PS at 30C |
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Reference
1. Huh, C.
and L. E. Scriven; Shapes of Axisymmetric Fluid Interfaces of
Unbounded Extent, J. Colloid and Interface Sci., 30, 323,
1969.
2. Wu, S.;
Surface and Interfacial Tensions of Polymer Melts. II.
Poly(methyl methacrylate), Poly(n-butyl methacrylate), and
Polystyrene, J. Phys. Chem., 74 (3), 632, 1970.
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