Photodegradation and photocrosslinking of benzophenone blended polystyrene (PS) thin films were investigated primarily using Fourier
transform infrared (FT-IR) imaging, evanescent waveguide spectroscopy (WS), and quartz crystal microbalance (QCM) methods. The main
objective is to observe the changes (spectral and chemical) indicative of these competitive processes in an ultrathin polymer film. This also
serves as a model study in the application of combined spectroscopic, optical, and acoustic methods towards understanding crosslinking and
degradation phenomena within the same time frame of observation. To induce photocrosslinking, 1,12-dodecanediylbis(oxy-4,1-phenylene)]
[bis[phenylmethanone] (2BP12), a small molecule with two benzophenone groups, was blended with PS in solution, spincast onto glass and
silicon substrates, and irradiated with ultraviolet light. Photodegradation and benzophenone-mediated crosslinking were observed both
directly via functional group spectroscopies and indirectly via their effects on thin film surface properties and morphologies. Atomic force
microscopy (AFM) and QCM were used to elucidate local morphology change and mass-uptake kinetics in the presence of O2 in air,
respectively. All results correlated well with the two photoprocesses occurring simultaneously and competitively on these films with the
refractive index, thickness, and mass change differing with the presence of 2BP12. Crosslinking was observed to cause an increase in
thickness while photodegradation gave a decrease. Both processes resulted in an increase in the refractive index and mass. While various
methods have separately identified these observations, this is the first instance that the in situ chemical, optical, and structural dimensionality
of the photodegradation and photocrosslinking phenomena in a thin polymer film is correlated within the same time frame of observation.
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