Zachary P. Smith

University of Texas at Austin, Austin, Texas, United States

Are you Zachary P. Smith?

Claim your profile

Publications (15)37.4 Total impact

  • Zachary P. Smith, Benny D. Freeman
    Angewandte Chemie 07/2014;
  • Zachary P. Smith, Benny D. Freeman
    [Show abstract] [Hide abstract]
    ABSTRACT: Depending on size: Graphene oxide can be deposited into thin layers having defects and inter-layer structures with dimensions that are appropriate for separating molecules based on size differences and enable rapid transport through these structures. The picture illustrates the pathway of two differently sized molecules through the graphene oxide layers.
    Angewandte Chemie International Edition 07/2014; · 11.34 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Robust polymer membranes that are highly permeable and selective are desired for energy efficient gas separation processes. In this study, a series of rigid, bulky triptycene-based diamine monomers were designed, synthesized, and subsequently incorporated into the backbone of polyimides via polycondensation with 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) to obtain a series of polyimide membranes with high fractional free volume. These triptycene-containing polyimides with systematic variations in their chemical structure demonstrate the viability of the ‘tunable’ fractional free volume by introducing various substituents onto the polymer backbone. All the polyimides synthesized exhibited film-forming high molecular weight, high solubility, and excellent thermal properties, with glass transition temperatures ranging from 280 °C to 300 °C and thermal stability up to 500 °C. Compared to other classes of glassy polymers, these triptycene-polyimides had high combinations of permeability and selectivity, suggesting that a favorable free volume size distribution in these triptycene polyimides was induced by the unique chain packing mechanism of triptycene units. The correlation between gas transport properties and the polymer chemical structure was also investigated. Altering the size of the substituents neighboring the triptycene units provides greater opportunity to fine-tune the fractional free volume and free volume size distribution in the polymer, which in turn can change the transport properties effectively to meet various separation needs. It is expected that additional design modifications made by exploiting the chemistry versatility of the triptycene moiety and by selectively adding other components may improve these membranes to break the gas permeability–selectivity trade-off barrier.
    J. Mater. Chem. A. 07/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The permeability–selectivity upper bounds show that perfluoropolymers have uniquely different separation characteristics than hydrocarbon-based polymers. For separating helium from hydrogen, these differences are particularly dramatic. At a given helium permeability, the upper bound defined by perfluoropolymers has helium/hydrogen selectivities that are 2.5 times higher than that of the upper bound defined by hydrocarbon-based polymers. Robeson hypothesized that these differences in transport properties resulted from the unusual sorption relationships of gases in perfluoropolymers compared to hydrocarbon-based polymers, and this paper seeks to test this hypothesis experimentally. To do so, the gas permeability, sorption, and diffusion coefficients were determined at 35 °C for hydrogen and helium in a series of hydrocarbon-, silicon-, and fluorocarbon-based polymers. Highly or completely fluorinated polymers have separation characteristics above the upper-bound for helium/hydrogen separation because they maintain good diffusivity selectivities for helium over hydrogen and they have helium/hydrogen sorption selectivities much closer to unity than those of hydrocarbon-based samples. The silicon-based polymer had intermediate sorption selectivities between those of hydrocarbon-based polymers and perfluoropolymers. Comparisons of hydrogen and helium sorption data in the literature more broadly extend the conclusion that helium/hydrogen sorption selectivity is rather different in hydrocarbon and fluorocarbon-based media.
    Macromolecules 04/2014; 47(9):3170–3184. · 5.93 Impact Factor
  • Polymer. 01/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Pure gas permeability coefficients of CH4, N2, O2, CO2, and H2 are reported for thermally rearranged (TR) polymers derived from polyimides based on 3,3′-dihydroxy-4,4′-diamino-biphenyl and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (HAB-6FDA). These HAB-6FDA polymers were prepared with hydroxyl, acetate, propanoate, and pivalate groups in the ortho-position to the imide ring. Polymers with acetate ortho-position groups were synthesized via either thermal or chemical imidization. Pure gas permeability was approximately five times higher after rearrangement at 450 °C in TR polymers from polyimides with acetate, propanoate, and pivalate ortho-position groups relative to TR polymers prepared from polyimides with hydroxyl ortho-position groups. In samples with non-hydroxyl ortho-position groups, those with larger ortho-position groups had higher gas permeability for TR conversions less than roughly 60%, but permeability increased to similar values as conversion exceeded 60%. In all samples, the CO2/CH4 selectivity also approached a similar value as TR conversion increased. Despite their higher permeability, fractional free volume was not significantly higher in TR polymers from polyimides bearing non-hydroxyl ortho-position groups than in samples with hydroxyl ortho-position groups. Therefore, average free volume alone cannot explain this behavior, suggesting that free volume distribution is likely affected by these groups.
    Journal of Membrane Science 01/2014; 463:73–81. · 4.09 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Prior analyses of the upper bound of permselectivity versus permeability, both theoretical and empirical, have assumed that this relationship is a consequence of the dependence of gas diffusion coefficients on the molecular diameter of the gases of interest. The solubility selectivity has been assumed to be invariant with permeability (and free volume). However, a few literature sources note that the solubility coefficient for specific families of glassy polymers correlate with free volume. A large database of permeability, diffusivity and solubility coefficients for glassy polymers was compiled to investigate this hypothesis. A critical analysis of the data demonstrates a modest solubility selectivity contribution to permselectivity as a function of free volume and, thus, permeability. The solubility selectivity (Si/Sj) generally decreases with increasing permeability (and free volume) when the diameter of gas j is larger than that of gas i. This empirical trend is likely a consequence of larger gas molecules having less access than smaller molecules to sorption sites as the polymer packing density increases and free volume decreases. The diffusion data permit determination of a diffusivity upper bound, which is modestly different from the permeability-based upper bound relationship. The diffusion data analysis allows a determination of a new set of gas diameters more appropriate for gas diffusion in polymers than prior correlations.
    Journal of Membrane Science 01/2014; 453:71–83. · 4.09 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Hydrogen sorption between −20 °C and 70 °C and at pressures up to 60 bara was determined for a polyimide and corresponding thermally rearranged (TR) polymers prepared from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). Hydrogen sorption increased by a factor of approximately 2.6 between the polyimide precursor and the most highly converted TR polymer. This relative increase in sorption was similar to that observed for other non-polar light gases such as N2, O2, and CH4, but less than that observed for CO2. Additionally, H2 sorption was measured for other polymers commonly studied in the membrane literature, including AF 2400, Matrimid®, polysulfone, and poly(dimethylsiloxane). Among the glassy polymers tested, polysulfone had the lowest H2 sorption, and the HAB-6FDA TR polymer had the highest H2 sorption. A slight dual-mode curvature was observed for H2 sorption in several of the glassy materials, and it was most pronounced at low temperatures and for the TR polymers. Enthalpies of sorption were also determined. The most exothermic enthalpy of sorption occurred in Matrimid®, and a slightly endothermic enthalpy of sorption was observed in rubbery poly(dimethylsiloxane). Comparisons between gravimetric and volumetric sorption showed similar results.
    Polymer. 05/2013; 54(12):3026–3037.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Soluble aromatic polyimides containing ortho-positioned hydroxy groups were synthesized as precursors for thermal rearrangement (TR) to polybenzoxazoles (PBOs). Fully imidized polyimides with high-molecular-weights were afforded via a ‘one-pot’ solution imidization technique (i.e., ester-acid method). The poly(hydroxyimide)s were designed to vary in their glass transition temperatures (Tg) by carefully selecting dianhydride–bisaminophenol combinations to introduce various levels of chain rigidity. TR conversion (imide-to-benzoxazole conversion) occurred in solid-state films only under inert atmosphere and over a temperature range of 300–450 °C, depending on the chemical structure (chain rigidity) of precursors. The effect of the precursor Tg on TR conversion was studied using TGA, DSC, FTIR and gel fraction measurements. The TR conversion temperature of imide-to-benzoxazole rearrangement strongly depended on the precursor Tg. Thus, for example, the feasible TR temperature was successfully reduced by 100 °C by lowering the precursor Tg by using a bisphenol A type dianhydride in the polymer synthesis. Gas permeation properties of representative TR systems are also reported. The TR process significantly increased gas permeabilities while maintaining good selectivities. By correlating the TR conversion degree with gas transport properties, there appears to be an optimal TR conversion degree that can maximize both gas permeability and selectivity. Systematic studies on TR polymers derived from low Tg precursors were suggested to further explore this correlation.
    J. Mater. Chem. A. 04/2013; 1(19):6063-6072.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Over the past three decades, polymeric gas separation membranes have become widely used for a variety of industrial gas separations applications. This review presents the fundamental scientific principles underpinning the operation of polymers for gas separations, including the solution-diffusion model and various structure/property relations, describes membrane fabrication technology, describes polymers believed to be used commercially for gas separations, and discusses some challenges associated with membrane materials development. A description of new classes of polymers being considered for gas separations, largely to overcome existing challenges or access applications that are not yet practiced commercially, is also provided. Some classes of polymers discussed in this review that have been the focus of much recent work include thermally rearranged (TR) polymers, polymers of intrinsic microporosity (PIMs), room-temperature ionic liquids (RTILs), perfluoropolymers, and high-performance polyimides.
    Polymer 01/2013; 54(18):4729–4761. · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Aromatic polyimides bearing various ortho-functional groups (i.e., acetate group and pivalic acetate group) were prepared via acetylation of a poly(hydroxyimide) containing ortho-positioned hydroxy groups using acetic anhydride or pivalic anhydride. The completeness of acetylation was confirmed by 1H NMR and FTIR. Chemically derivatized polyimides were used as precursors for an imide-to-benzoxazole thermal rearrangement (TR) process. The influence of various ortho-functionalities on the TR process and gas transport properties of the resulting TR polymers was studied. Differing from the –OH groups in a poly(hydroxyimide), the acetate groups of acetylated polyimide precursors degrade at elevated temperatures, and the degradation process interplays with imide-to-benzoxazole conversion. The acidic degradation product, as detected by 1H NMR, is suspected to have some catalytic effect on the TR process, which along with the protecting function of the acetate groups, resulted in a lower onset TR conversion temperature, the ability to conduct the TR process in air, and a higher TR conversion level. Gas permeation properties greatly depend on the ortho-functionality of polyimide precursors as well. The precursor films containing larger functional groups are much more permeable with comparable gas selectivities. Similarly, the resulting TR polymers formed from polyimides with larger leaving groups also showed much higher gas permeabilities despite similar degrees of TR conversion. The incorporation of bulkier functional groups in the TR precursors provides an effective way to significantly improve the gas transport performance, particularly the gas permeabilities of both the polyimide precursors and the resulting TR polymers.
    J. Mater. Chem. A. 12/2012; 1(2):262-272.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Within a polymer thin film, free volume elements have a wide range of size and topology. This broad range of free volume element sizes determines the ability for a polymer to perform molecular separations. Herein, six permeable thermally rearranged (TR) polymers and their precursors were studied. Using atomistic models, cavity size (free volume) distributions determined by a combination of molecular dynamics and Monte Carlo methods were consistent with experimental observation that TR polymers are more permeable than their precursors. The cavity size distributions determined by simulation were also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy. The diffusion, solubility and permeation of gases in TR polymers and their precursors were also simulated at 308 K, with results that agree qualitatively with experimental data.
    Polymer. 01/2011; 52(10):2244-2254.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Pure gas permeability coefficients of a series of copolymers based on polar, hydrophilic poly(ethylene glycol) dimethacrylate, n = 14 (PEGDMA) and siloxane-based co-monomer, [methyl bis(trimethylsiloxy)silyl] propyl glycerol methacrylate (SiGMA) are reported. SiGMA is miscible with PEGDMA and able to form homogeneous films. SiGMA contains a bulky siloxane-based end group, which acts to increase permeability, and an –OH pendant group, which increases miscibility with polar co-monomers, such as PEGDMA. As the SiGMA content in these copolymers increases to 53 vol%, CO2 permeability increases from 95 to 255 barrer, while CO2/N2 and CO2/H2 pure gas selectivities decrease from 58 to 20 and 6.4 to 3.2, respectively. At the same time, fractional free volume of the copolymer increases from 0.118 to 0.140. Comparisons to a similar copolymer system are made to rationalize the permeability and selectivity trends of this series of copolymers.
    Polymer. 01/2010; 51(24):5734-5743.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The solubilities of H2, N2, O2, CH4, and CO2 were determined over a range of pressures at 35 °C in a glassy, amorphous, ortho-functional polyimide prepared from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). The HAB-6FDA polyimide was partially converted to its corresponding thermally rearranged (TR) polymer by thermal treatments at different times and temperatures. At 10 atm, solubility coefficients of H2, N2, O2, CH4, and CO2 increased by a factor of approximately two between the polyimide and the most highly converted TR polymer. Correlations between solubility and penetrant condensability were in good agreement with such correlations in other fluorinated polymers. Dual-mode sorption model parameters were determined from the sorption isotherms. The affinity constant, Henry’s law solubility, and Langmuir capacity constant increased with gas condensability, and increases in the Langmuir capacity constant were observed as TR polymer conversion increased. Comparisons were made between the solubility selectivity of CO2/CH4, O2/N2, CH4/N2, and CO2/N2 with HAB-6FDA, its corresponding TR polymers, and with other polymers in the literature. Qualitatively, a decrease in solubility selectivity for gas pairs including CO2 correlates with imide and acetate loss during conversion.
    Journal of Membrane Science s 415–416:558–567. · 4.09 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: HAB-6FDA polyimide was synthesized from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) by a two-step polycondensation method with chemical imidization. This polyimide was used as the precursor to prepare thermally rearranged (TR) polymers. The rearrangement reaction was performed at temperatures from 350 to 450 °C. Based on mass loss during the TR process, TR conversion levels as high as 76% were observed. CO2 permeability increased from 12 Barrer in the precursor polyimide (HAB-6FDA) to 410 Barrer in the TR polymer prepared at 450 °C, while pure gas CO2/CH4 selectivity decreased from 42 to 24. Both diffusivity and solubility increased following the rearrangement process, but the change in gas diffusivity was the largest contribution to the increase in permeability. From the polyimide to the sample heated at 450 for 30 min, CO2 diffusivity increased by approximately a factor of 20 while solubility increased by a factor of 1.7. Similar changes were observed for other gases. Fractional free volume increased from 15% in the polyimide precursor to 20% in the TR polymer rearranged at 450 °C. The HAB-6FDA TR polymers exhibited higher CO2 permeability than other classes of polymers with similar free volume, suggesting a free volume distribution in these TR polymers that favors high permeability.
    Journal of Membrane Science s 409–410:232–241. · 4.09 Impact Factor