60th Annual Report on Research 2015

In the first reporting period, we identified and synthesized a conjugated microporous polymer (CMP) system for potential applications in refinery fluid separations. We focused our efforts on imine-based polymers, as imine chemistry has previously been used to synthesize microporous, covalently-linked polymers. We successfully synthesized a series of imine polymers from conjugated triamine and dialdehyde monomers. Diffraction experiments performed with synchrotron radiation from the Advanced Photon Source at Argonne National Lab showed that our conjugated polymers possess a high degree of crystallinity; in conjunction with computational modeling, the structures were determined to be composed of infinite, van der Waals-bound, two-dimensional sheets. One material in particular, formed with fused triphenylamine and benzenedithiophene (bdt) subunits and denoted “polyTB,” contains large (~2.9 nm) hexagonal pores in a honeycomb-like pattern.

To form films of polyTB, we initially focused on spin-coating monomers on a substrate and subsequently polymerizing them by heating. While this method did form polymeric material, the films did not exhibit measurable crystallinity. However, during our investigations, we discovered a method to form large-area, continuous films from the material. By allowing the synthesis solution to stand in an unsealed glass petri dish under ambient conditions, ~200 nm-thick films can be grown at the solution-air interface. Film thickness can be reduced by using a lower concentration, thermally “activated” solution: film thicknesses down to 2 nm, and still maintaining diameters spanning the reaction vessel, can be achieved with this method. The mechanism for film formation appears to be related to the hydrophobic alkyl side-chains affixed to the bdt-based monomer used to synthesize polyTB. In sealed, dry containers, films do not form; in sealed containers having atmospheres at 100% relative humidity, films form readily and rapidly. Furthermore, using bdt-based monomers without hydrophobic side-chains inhibits film formation, regardless of the synthetic conditions. We conclude from these experiments that film growth may be related to water diffusion into the synthesis solvent (DMF). As water diffuses into the solvent, it becomes slowly more polar, driving the non-polar bdt-based monomers toward the solution-air interface. The increased concentration of monomer at this interface encourages localized nucleation of polyTB at the solution surface, thereby leading to the growth of interfacial thin films.

The structure of polyTB suggested that it may be porous and hence an excellent candidate for fluid separations. However, despite multiple attempts at revealing the pores by sample drying under vacuum and with supercritical CO₂, measurements showed the material to be non-porous. Analysis by X-ray diffraction indicated that the lack of porosity was due to a loss of structural integrity – on drying, polyTB loses a significant amount of structural order. Thin films examined by grazing incidence X-ray diffraction displayed similar characteristics.

The polyTB thin films were amenable to deposition on transistor device substrates (Si/SiO₂), and evaporating gold contacts allowed interrogation of their electronic properties. While the films having thicknesses of 50-200 nm did not show electrical response, thinner films (2-30nm) displayed clear field-effect response, with device on/off ratios reaching 1000. These devices represent the first field-effect transistors from CMPs and are the first evidence of CMPs exhibiting long-range, solid-state semiconductor properties. All of the results described for polyTB were published recently in Chemical Communications.

The successful synthesis of thin, continuous polyTB films spurred us to search for methods to synthesize CMPs of having greater structural integrity and thus crystallinity and accessible porosity after drying. polyTB is formed from the condensation of amines and aldehydes to form imine bonds. Imines are susceptible to hydrolysis. Furthermore, the large size of the hexagonal rings of polyTB make the additional thermodynamic stability imparted by ring formation near-negligible (in other words, the formation of 5-member or 7-member ring defects is close in enthalpy to the formation of 6-member rings for large rings). Hence, we sought to identify chemistry that might yield a robust, and therefore permanently porous, crystalline CMP for effective membrane separations. To this end, we selected olefin metathesis chemistry, whereby a conjugated backbone made purely of carbon-carbon bonds can be formed. Using a truxene-based monomer, we attempted homopolymerization to form a 2D hexagonal hydrocarbon network. However, all tested reaction conditions to date have yielded short-chain oligomers; hence, it was not possible to isolate porous, solid-state materials. We are currently investigating ways to increase the molecular weight of polymerization products.

In addition, we are further exploring imine-based CMP formation by using truxene-based monomers, which are more rigid and less susceptible to oxidation than the triphenylamine-based monomers used to synthesize polyTB and thus may lead to CMPs with improved stability and therefore accessible porosity.

Education and Outreach: During the course of this work, we enlisted an undergraduate student who had the opportunity to learn both instrumental (UV-Vis, AFM, profilometry) and synthetic techniques. She worked on a voluntary basis for two semesters.

The postdoctoral scholar heading the project delivered a seminar based on this work at the Spring 2015 Materials Research Society Meeting in San Francisco.