Adam F. Gross

Hierarchical carbon foams with independently tunable mesopore and macropore size distributions.

Hierarchical carbon foams with independently tunable mesopore and macropore size distributions were formed in a high internal phase emulsion (HIPE) template. The HIPE consists of an internal oil phase that controls the macropore dimensions and an aqueous resorcinol-formaldehyde precursor solution external phase that directs the mesopore size distribution. Once the emulsion is formed, the precursor solution is cured, fluid elements are extracted from the monolith via solvent exchange, and then the sample is pyrolyzed to create a hierarchical open-cell foam consisting of macropores with mesoporous carbon xerogel walls. Both mesopore and macropore size distributions may be independently tuned by changing the synthesis parameters. These samples have a peak in the mesopore size distribution that may be tuned to between 5 and 8 nm and macropore average diameters that may be tuned to between 0.7 and 2.1 microm. Furthermore, the 0.7 and 2.1 microm average diameter macropores have 0.18 and 0.53 microm diameter macropore windows between adjacent pores, respectively. Pore volumes up to 5.26 cm(3)/g and electrical conductivities as high as 0.34 S/cm are observed after 1200 degrees C carbonization of the framework. These foams may have potential applications as 3-D current collectors in batteries and as fuel cell catalyst supports.

Directional energy migration in an oriented nanometer-scale host/guest composite : semiconducting polymers threaded into mesoporous silica

Abstract In this paper, we show that the semiconducting polymer poly[2-methoxy-5-(2 ′ -ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) can be incorporated into the channels of an aligned mesoporous silica host. Polarized fluorescence spectroscopy is used to show that more than 80% of the polymer in the composite is aligned by incorporation into the host. Time-resolved transient absorption spectroscopy further indicates that the incorporated chains are isolated from each other, while the unincorporated polymer is aggregated, probably at grain boundaries or surfaces. Control of the fraction of polymer inside versus outside the pores can be achieved by selective oxidation of the unincorporated polymer. Because of the unique nanoscale geometry of this material and the existence of multiple environments, excitations in this composite are funneled from outside the pores down into the aligned, isolated polymer chains inside the pores. Time-resolved stimulated emission spectroscopy is used to follow this process by monitoring the increase in luminesce polarization with time. The results show that control of polymer morphology through host/guest chemistry can be used to direct the motion of excitations and thus to deliver energy to specific regions of a material.

The role of silica chemistry in controlling phase transitions in silica/surfactant composite materials

Real time in-situ X-ray diffraction is used to examine kinetic barriers between metastable phases in silica/surfactant composites. These materials are synthesized in a P6mm hexagonal structure using a range of base concentrations and then heated under a linear ramp in hydrothermal conditions. The composites are observed to first anneal into a more ordered hexagonal structure and then to undergo a hexagonal to lamellar phase transition. Activation energies for the phase transition and for annealing are calculated using non-isothermal kinetics. Activation energies for both processes correlate with the synthesis conditions. Materials made at the lowest pH show an activation energy of 163 ± 3 kJ/mol while materials made at the highest pH show an activation energy of only 106 ± 3 kJ/mol for the hexagonal to lamellar phase transition. This result can be explained with the postulate that materials synthesized at lower pH have a more condensed framework thus requiring hydrolysis of more siloxane linkages for the material to rearrange. The annealing process shows the opposite dependence on pH: materials synthesized at the highest pH have a higher activation energy (E a = 80 ± 16 kJ/mol) than materials synthesized at the lowest pH (E a = 58 ± 12 kJ/mol). This trend implies that the activation energy for annealing is related to silica condensation, rather than hydrolysis. The predicted affects of silica chemistry on annealing and the phase transformation are corroborated by 29 Si MAS-NMR. This relationship between synthetic parameters, silica chemistry, and the stability of silica/surfactant nanostructured composites is explored in this work.