Reports: ND753739-ND7: Nanostructured and Nanoporous Composites from Nanoparticle Jamming during Polymerization Induced Phase Separation

Bryan D. Vogt, University of Akron

This work focuses on understanding how nanoparticles impact the structure and properties of polymer blends formed through polymerization induced phase separation. In these cases, the nature of the nanoparticle and the relative incompatibility of the polymers drives the morphology from a thermodynamic perspective, while the mobility of the swollen polymers during phase separation can kinetically arrest the structure. Figure 1 illustrates the composite structures obtained from polymerization of furfuryl alcohol (FA) with TiO2 nanoparticles and poly(hydroxyethylmethacrylate) (PHEMA) dissolved. In order to easily resolve the structure, the polymerized furfuryl alcohol is carbonized at elevated temperature, which eliminate the PHEMA
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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.

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Figure 2.  TEM micrographs of porous carbon/TiO2 composites produced from 0.35 g PHEMA/0.3 g TiO2/different amount of FA; (A) 0.2, (B) 0.3, (C) 0.5, and (D) 0.7 g.

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).

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Figure 3.  TEM images of 1.5 g PEA/0.5 g PMMA/Al2O3 composites fabricated using different amount of Al2O3 nanoparticles; (A) 0, (B) 0.75, (C) 1.65, (D) 4.87, (E) 8.35, (F) 9.92, (G) 10.03, (H) 12.27, and (I) 15.32 wt%.

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.

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Figure 4.  Strain-stress curves of 1.5 g PEA/0.5 g PMMA/Al2O3 composites as a function of Al2O3 nanoparticle contents in the composites.

Future work over the next year will focus on control of the nanoparticle surface chemistry and lower molecular weight polymers to push the morphology from the kinetically controlled as reported here to thermodynamically controlled.