  Polydimethylsiloxane (PDMS) films possess different chemical and physical
properties based on surface modification. The bond structure of pristine PDMS
films and plasma treated PDMS films differ in a particular region of silicate
bonds. We have studied the surface physical properties of pristine PDMS films
and plasma treated PDMS films through optical technique. It is already known
that plasma treated PDMS films forms very thin SiO2 layer on its surface. Due
to difference in coefficient of thermal expansion of the surface SiO2 and the
remaining bulk layer, the SiO2 layer develops cracks. These formations are
explored to characterize PDMS surface by observing intensity of scattered light
while the films are stretched. Pristine PDMS films do not show such optical
scattering. Further the intensity measurements were repeated over period of
time to monitor surface properties with time. It was observed that plasma
treated PDMS film; the scattered light intensity is linearly dependent on the
applied force for stretching. After a substantial time, scattering intensity is
reduced to the value which is almost equal to that of pristine PDMS surface. It
is proposed that basic light scattering through plasma treated PDMS occurs due
to the decrease in the width of the cracks when the PDMS film is stretched. The
change in the scattering property of plasma treated PDMS surface over time
could be attributed to healing of cracks by the migration of polymer chain
molecules from the bulk to the surface. It is also reported that with reference
to various bonds present on surface plasma treated PDMS surface regains its
original properties as that of pristine PDMS with time. Therefore the optical
technique could be employed to study surface characteristics of PDMS surface as
an alternative approach to conventional spectroscopic techniques.
