Dependence of the Surface Structure of Polystyrene on Chain Molecular Weight Investigated by Sum Frequency Generation Spectroscopy

Sum frequency generation (SFG) spectroscopy is used to probe the surface structure of monodisperse polystyrene (PS). The amplitudes of the nu(20a) and nu(2) resonances in SSP polarization are found to depend on polystyrene molecular weight between 6 and 102 kDa, whereas the amplitude of the nu(20b) resonance is invariant, which all together indicates a reorientation of the phenyl ring with different chain lengths. The measured resonant amplitudes are consistent with the C-2 axis of the phenyl ring lying flat in the surface plane at the lowest molecular weights and tilting toward the surface normal at the highest molecular weights. Such reorientation is supported by the observation that the SFG intensity increases with polymer molecular weight in SSP polarization, but the spectrum exhibits no significant difference when PPP polarization is used (where the letters indicate the polarization of the SFG, visible and infrared beams). It is clear from these results that molecular weight can influence the surface structure of polystyrene in ways that are important to surface tension and surface segregation of smaller chains. Such understanding is key to providing a fundamental picture of the relationship between polymer molecular weight distribution, local surface structure, and macroscopic properties.