(364e) Control Nano/Microstructure Using Photopolymerization-Induced Phase Separation (PhIPS)

Photopolymerization-induced phase separation (PhIPS) is one of the techniques to combine the thermo-mechanical and optical properties of different constituents in a single polymer. Controlling phase separation, polymer networks could be generated with enhanced physical and mechanical characteristics, including increases in material toughness, reduction of polymerization-induced shrinkage stress, and enhanced abrasion resistance compared to equivalent homogeneous polymer networks. In this study the effect of composition and polymerization rate for photocurable crosslinked systems for the creation of phase-separated polymers will be examined. Changing the composition, thermodynamic instability may allow the polymer system to become incompatible and phase separate. The polymerization kinetics are also important as cross-linked network structures may be trapped before complete phase separation occurs, affecting the morphology and domain size of polymers. A mixture composed of butyl acrylate (BA) and di-functional oxetane (DOX) was investigated at different monomer compositions and light intensities. The formation of different polymer domains was observed using atomic force microscopy and dynamic mechanical analysis. The photopolymerization kinetics for different ratios of DOX/BA were characterized using real-time Fourier transform infrared spectroscopy. BA conversions between 95-100% were reached within 2 minutes, independent of light intensity. On the other hand, DOX shows very different conversion behavior compared to those of BA. The final DOX conversions were only between 20 and 30% which can be attributed to a low propagation rate constant. Higher light intensities not only increased DOX conversion to as high as 50%, but also changed the final polymer morphology by decreasing the domain size and swapping the co-continuous phase. At low light intensity, BA soft domain was the co-continuous phase, with DOX hard domain being dispersed in the network. The mechanical properties of these polymers were very weak. However, increasing to higher light intensity, the co-continuous phase changed to DOX harder domain and the size of each domain was decreased. These changes in the morphology affected the thermo-mechanical properties of the polymers such as showing more segregated tanδ peaks, higher modulus and stress-strain values. Since light intensity affected both conversion and morphology, it was important to investigate which of these two parameters had the greatest impact on the thermomechanical properties. To obtain better control of our system, DOX conversion was brought to the same value (~70%) using UV and post thermal curing. Our investigation indicated that there were not significant changes in the polymer morphology and mechanical properties. Consequently, it is reasonable to believe that the physical properties are dependent on the initial obtained morphology from the photocuring of the samples and not to higher DOX conversions. These results demonstrate the ability to control the physical properties of phase separated polymers by changing the morphology through different light intensities.