Evaluation of the interactions between titanium dioxide nanoparticles and polystyrene chains in nanothin films

In recent years it has become evident that polymeric materials with thickness below 100 nm have physical properties that differ substantially from thicker bulk materials of the same nature. One explanation of this phenomenon concerns the chemical interactions between substrate and polymer. Our objective is to understand these interactions assuming that they affect the mobility of polymer chains. Investigation of the chain mobility of polystyrene, using physical, thermal and spectral methodologies, was performed on three types of samples: bulk PS, thin films of PS and thin films of PS with nanoparticles of TiO2. Since creep is a strong function of chain mobility and related to the glass transition, the effect of polymer size reduction and particle addition on creep behavior was observed. When compared to experimental values of Tg for bulk PS (99.2°C), the average experimental Tg of PS films decreased to 89.2°C, while films with added particles had a measured Tg of 98.4°C. Differential scanning calorimetry (DSC) measurements were performed on heating for comparison with previous creep studies, and showed similar changes in Tg. DSC physical aging studies performed to quantify the effect of the presence of nanoparticles, showed that the mobility of the polymer chains decreased by about 1--2 orders of magnitude for samples containing TiO2. To increase the magnitude of this effect stretching of the polymer films was performed near Tg and samples were compared using a polarized light microscope. Images suggest that TiO2 nanoparticles strongly interact with polymer chains and may help to increase chain orientation under directional stress by interacting with some parts of the PS molecules. Polarized FTIR spectroscopy was performed on the stretched films and analyzed by 2D correlation spectroscopy to characterize the interactions between TiO2 and polystyrene chains. Synchronous and asynchronous 2D correlation spectra led to the hypothesis that TiO2 nanoparticles interact with the aromatic ring of the side chain and hence slows down its ability to orient. This point of contact is the location of the interactions between the nanoparticles and the PS chain which results in the decrease in chain mobility.