60th Annual Report on Research 2015

Figure 1.   TEM micrographs of porous carbon/TiO2 composites produced from 0.35 g PHEMA/0.3 g FA/0.3 g TiO2 with different molecular weight of PHEMA; (A) 20K, (B) 300K, and (C) 1000K

Interestingly, the pore size decreases as the molecular weight of PHEMA increases. The pores are resultant from the PHEMA during phase separation. From thermodynamics, the higher molecular weight PHEMA should phase separate at a lower FA conversion. These results indicate that this morphology is kinetically controlled. The morphology can be tuned by changing the PHEMA:FA composition as shown in Figure 2. As the FA content increases, the structure becomes more homogeneous with smaller pores after carbonization.

To better understand how the system impacts the structure that develops, we have performed similar experiments using ethyl acrylate (EA) as the monomer, poly(methyl methacrylate) (PMMA) as the polymer, and alumina nanoparticles. Figure 3 illustrates the range of structures that develop upon polymerization. Although it difficult to distinguish between PEA and PMMA domains in the blend (as shown in Figure 3A), the TEM micrographs of the nanocomposites show that the size of clusters constituted of Al₂O₃ nanoparticles dispersed in the PEA/PMMA blends increases as a function of Al₂O₃ nanoparticle contents. Strong interaction between particles concurrent with polymerization appears to lead to some orientation of the structure at high nanoparticle loadings. The agglomeration of Al₂O₃ nanoparticles is encouraged by the hydrogen bond formed between the surface hydroxyl groups of the nanoparticles. These surface hydroxyl groups increase the tendency to create hydrogen bonds between nanoparticles and directly result in the formation of aggregates. At less than~5 wt% of Al₂O₃ (Figure 3B, C, and D) isolated domains appear to be present in the structure, whereas the aggregated Al₂O₃ nanoparticle domains appear to be interconnected at greater than ~5 wt% of Al₂O₃ (Figure 3E, F, G, H, and I).

These changes in the morphology are manifested in changes in the mechanical properties of these nanocomposite polymer blends. Figure 4 shows the strain-stress curves of these composites. Their strain-stress behavior can be divided into ductile and quasibrittle behavior that depends on Al₂O₃ concentration into polymer matrix. The composite specimens with 0.75 - 4.87 wt% Al₂O₃ exhibit ductile behavior, whereas specimens with 9.92 - 15.32 wt% Al₂O₃ exhibit quasibrittle behavior. These differences are attributed to the dispersion state of Al₂O₃ nanoparticles into PEA/PMMA blend matrix, where the size and morphology of Al₂O₃ clusters controls the tensile stress enhancement. The large Al₂O₃ clusters cause several partial failures at relatively low strain as the polymer matrix separates along Al₂O₃ clusters. We infer that the clusters act like large soft particles during the deformation process.