Scattered light intensity measurements of plasma treated Polydimethylsiloxane films: A measure to detect surface modification

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.