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

Abstract

Sum frequency generation (SFG) spectroscopy is used to probe the surface structure of monodisperse polystyrene (PS). The amplitudes of the ν 20a and ν 2 resonances in SSP polarization are found to depend on polystyrene molecular weight between 6 and 102 kDa, whereas the amplitude of the ν 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.

Influence of Polymer Molecular Weight on Chain Conformation at the Polystyrene/Silver Interface

“…The range of possible values for θ results from the uncertainty in the ν 2 amplitude ssp over ppp ratio, is shown in Figure 7. Interestingly, the trend of decreasing tilt angle with increasing Mw resembles the trend observed previously for the PS/air interface, 24 even though the physical−chemical drivers for the different conformations are expected to be different for the two interfaces. Lastly, the second method (Figure S3) for tilt angle analysis using eq 6 and the R value (the ratio of the amplitudes of ν 20b and ν 2 in ssp polarization) qualitatively corroborates the trend of decreasing the tilt angle as Mw increases.…”

“…This mode is a strong IR and Raman mode, 47 and this is possibly the reason for its high sensitivity to orientation variations. The ν 2 mode was also reported as the most sensitive mode to Mw changes at the PS/air 24 interface.…”

“…The spectra were normalized by dividing the SFG signal by the product of the visible and IR energies. 24 A sample of PMMA that exhibits an intense SFG peak at 2955 cm −1 was used to calibrate the frequencies in the PS spectra. Two specimens per Mw were analyzed and for each specimen, at least 6 ssp and 6 ppp spectra were collected at six different spots.…”

“…Considering a molecular fixed coordinate system (a,b,c), the relationship between χ (2) and β can be described by eq 2. 24 N R R R ( , , ) ( , , ) ( , , )…”

“…Our findings highlighted the correlation between the phenyl ring orientation and the PS Mw, because of changes in the polymer free volume. 24 However, there is a lack of studies analyzing how Mw influences the phenyl ring orientation in the vicinity of highly interactive substrates, such as metals.…”