ENVIRONMENT-INDUCED SURFACE STRUCTURAL-CHANGES OF A POLYMER - AN IN-SITU IR PLUS VISIBLE SUM-FREQUENCY SPECTROSCOPIC STUDY

IR + visible sum-frequency vibrational spectroscopy has been used to m onitor structural changes of a polymer surface in response to alterati on of environment. The polymer studied is of the polyurethane type, wi th poly(dimethylsiloxane) (PDMS) grafted on as end groups. Our data re veal that the polymer surface undergoes a significant restructuring wh en transferred from air to water. With the polymer exposed to air, the surface spectrum shows that the hydrophobic PDMS segments cover most of the surface. When immersed hr water, the PDMS component retreats fr om the surface whereas the initially ''buried'', more hydrophilic part of the polymer chain appears at the surface. The surface structural t ransformation in response to the environmental change from air to wate r takes about 25 h at 300 K. The structural change is reversible upon dehydration, but takes only 3 h. The results point to the need to char acterize the polymer surface in its working environment in order to co rrectly describe its surface properties, The consistency between the s um-frequency generation (SFG) data and the contact angle measurement i n characterizing the hydrophobicity of the polymer surface demonstrate s that SFG is a powerful spectroscopic probe to study and provide insi ght into how polymer surfaces behave at a molecular level.