Zachary P. Smith

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

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Publications (33)49.35 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Hydrogen and helium solubility isotherms in glassy and rubbery polymers can be correctly described by Lattice fluid models.•The solubility selectivity can be correctly predicted for glassy and rubbery polymers suitable for membrane-based gas separations.•The isosteric heat of sorption and its dependence on the penetrant concentration can be correctly predicted.
    Journal of Membrane Science 02/2015; 475:110-121. · 4.09 Impact Factor
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    ABSTRACT: Mixed-gas transport was characterized for CO2 and CH4 in HAB–6FDA polyimide and TR polymers.•Mixed-gas selectivity was generally higher than pure-gas selectivity.•CO2-induced plasticization decreased as the degree of TR conversion increased.
    Journal of Membrane Science 02/2015; 475. · 4.09 Impact Factor
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    ABSTRACT: Carbon dioxide (CO2) plasticization and sorption effects in both thick and thin films of “high free-volume” glassy perfluoropolymers were studied by monitoring CO2 permeability and by observing changes in the film thickness and refractive index with ellipsometry measurements. The film thickness, aging time, thermal history and CO2 exposure protocols have significant effect on the absolute CO2 permeability and plasticization behavior of both thick and thin films. The extent of CO2 plasticization increases as film thickness decreases and as the aging time is increased. The as-cast films showed higher plasticization compared to films which were annealed above Tg; however, the CO2 permeability of both the as-cast and annealed films continuously decreased during the depressurization step unlike other glassy polymers. In general, the various CO2 exposure protocols revealed lower CO2 plasticization for perfluoropolymers compared to other reported glassy polymers. The extent of CO2 sorption obtained from the ellipsometry measurements was found to decrease with the decrease in the excess volume and increase in the aging time for perfluoropolymers; in addition, the structural differences among the various glassy polymers resulting in different polymer-gas interactions also affects the overall sorption characteristics. The lower plasticization in perfluoropolymers compared to Matrimid was also confirmed from the smaller percent increase observed for the experimental diffusion coefficient compared to the theoretically predicted diffusion coefficient from the dual sorption-mobility model. The Langmuir sorption parameter, , and solubility at infinite dilution, S0, obtained from fitting dual sorption-mobility model to sorption data, showed an excellent linear correlation with (Tg-35) °C. The CO2 diffusivity and permeability data obtained for thin films of various glassy polymers also showed a strong correlation with free volume. The somewhat unusual behavior of thin films of AF 2400 in comparison to other glassy polymers studied to date is believed to be related to the low cohesive energy density expected of perfluorinated structures and its high free volume resulting from the bulky dioxole co-monomer.
    Polymer. 12/2014;
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    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 large. 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.
    14 AIChE Annual Meeting; 11/2014
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    ABSTRACT: A partially soluble amide-based TR polymer (PA-TR) was characterized by NMR•NMR results suggest that the PA-TR has a polybenzoxazole (PBO) structure•An imide-based TR polymer (PI-TR) with similar structure to PA-TR was prepared•Solid-state NMR suggests the PA-TR and PI-TR have similar PBO structures•Gas transport in TR polymers is affected by chemical and morphological structure
    Polymer. 11/2014;
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    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 08/2014; 463:73–81. · 4.09 Impact Factor
  • Zachary P. Smith, Benny D. Freeman
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    ABSTRACT: Größenabhängig: Graphenoxid kann in Form dünner Schichten mit Defekten und Zwischenschichtstrukturen mit Abmessungen abgeschieden werden, die zur größenbasierten Trennung von Molekülen geeignet sind und einen schnellen Transport durch diese Strukturen ermöglichen. Das Bild veranschaulicht den Weg zweier unterschiedlich großer Moleküle durch Graphenoxidschichten.
    Angewandte Chemie 07/2014;
  • Zachary P. Smith, Benny D. Freeman
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    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
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    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;
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    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 04/2014; · 3.77 Impact Factor
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    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 03/2014; 453:71–83. · 4.09 Impact Factor
  • Polymer. 01/2014;
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    ABSTRACT: Over the past several years, research on thermally rearranged (TR) polyimides has shown interesting separation properties for a number of gas separations such as CO2/CH4 and hydrogen separations. TR polymers are traditionally synthesized via a solid state reaction of polyimides that contain reactive groups ortho-functional to the polyimide diamine. The precursor polyimides can be synthesized via a number of pathways, including chemical and thermal imidization. Furthermore, polybenzoxazole structures similar to those formed from polyimides can be synthesized by polyamides. The scope of this work is address the differences in gas transport properties for H2, N2, O2, CH4, and CO2 as they relate to polybenzoxazole backbone structure, ortho-position reactive group, and synthesis of polybenzoxazoles from either polyimides or from polyamides. Three ortho-hydroxy polyimides were synthesized via thermal imidization. The diamines for synthesizing these polyimides were 2,2-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF), 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB), and the dianhydrides were 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), and 4,4'-oxydiphthalic anhydride (ODPA). From these monomers, HAB-6FDA, APAF-6FDA, and APAF-ODPA were synthesized and converted to TR polymers by heating at 450°C for 30min. Conversion under these conditions resulted in nearly complete thermal rearrangement as determined by sample mass loss. The permeation properties of the precursor polyimides and TR polymers were determined for H2, N2, O2, CH4, and CO2. Out of all of the samples considered, the APAF-6FDA polyimide and the APAF-6FDA TR polymer had the most promising transport properties out of all of the polyimides and all of the TR polymers, respectively. These samples had permeabilities near or above the 2008 Robeson upper bound for CO2/CH4 and H2/CH4separation. A critical analysis on the effect of transport properties as they relate to the ortho-position leaving group was evaluated for HAB-6FDA. An ortho-hydroxy HAB-6FDA sample was modified via an esterification reaction post-imidization. This esterification reaction added different reactive functional groups to the ortho-position of the polyimide. In addition to the ortho-hydroxy group, an acetate, propanoate, and pivalate group were added to the polyimide backbone structure. These samples were thermally rearranged to different extents of conversion, and gas permeation was tracked as these samples were converted. Samples with bulkier leaving groups showed more dramatic improvements to gas permeability than hydroxyl-functional polyimides. Finally, a polyamide sample of APAF-6FDA was prepared for gas transport properties. This sample can be converted into a polybenzoxazole via a condensation reaction at 350°C for 60min. The APAF-6FDA polyamide had consistently lower permeability than the APAF-6FDA polyimide. Furthermore, the polybenzoxazole synthesized from the polyamide had consistently lower permeability than the polybenzoxazole synthesized from the polyimide. It is believed that these differences in transport properties relate to the non-equilibrium nature of polybenzoxazoles prepared from different synthetic pathways.
    13 AIChE Annual Meeting; 11/2013
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    ABSTRACT: Results of pure and mixed-gas permeation in thermally rearranged polymers are presented. Polyimide precursor films were converted to their polybenzoxazole (PBO) analogs to varying degrees using different thermal treatment protocols. Structure/property relations in this family of polymers are described. Mixed-gas permeation results are compared against pure-gas measurements. Interestingly, CO2/CH4 mixed-gas selectivity at 35°C was 20-30% higher than the simple ratio of pure-gas permeabilities. Strong dual-mode effects were responsible for a decrease in CH4 permeability in the presence of CO2, with very little change to CO2 permeability in the mixture. Plasticization is not evident in the pure-gas CO2 permeation data.
    13 AIChE Annual Meeting; 11/2013
  • Zachary P. Smith
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    ABSTRACT: In the petrochemical industry, gas separations are traditionally accomplished using energy-intensive processes such as absorption and distillation. With proper synthesis and design, polymer membranes could provide an attractive alternative to these traditional separation techniques. However, there are three major challenges that need to be addressed to improve currently available polymer membranes. Polymers must have high permeability, high selectivity, and a resistance to plasticization. Permeability is the gas flux normalized by the film thickness and pressure driving force, and selectivity helps set the purity ratio of gas mixtures. Plasticization is the swelling of a polymer in the presence of condensable gases, which results in a decrease in gas selectivity during the operating lifetime of a polymer membrane. In 2007, thermally rearranged (TR) polyimides were first investigated for separating CO2 from CH4. These polymers showed exquisite combinations of CO2 permeability and CO2/CH4 selectivity while still maintaining a resistance to CO2 plasticization under mild conditions. Because of these composite characteristics, this study aims to elucidate the mechanism of transport through TR polymers and to determine the relationship between polymer structure and gas separation properties. TR polymers are formed via a solid state, thermally-induced decarboxylation reaction of polyimides containing reactive groups ortho-position to the polyimide diamine. Between temperatures of approximately 350°C and 450°C, these polymers are thermally reacted from fully soluble, processable polyimides to crosslinked, insoluble polybenzoxazoles. To understand the mechanism of transport for small molecules in TR polymers, the gas diffusivity and solubility was determined as a function of TR polymer conversion for H2, N2, O2, CH4, and CO2. For CO2, conversion of a TR polymer known as HAB-6FDA resulted in an over 20-fold increase in gas permeability and an over 10-fold increase in diffusivity. However, solubility only increased by a factor of approximately 2 during conversion. Therefore, TR polymers achieve their separation properties through largely increased gas diffusivity. This increase in gas diffusivity indicates that TR polymers are potentially useful for separations typically governed by diffusion selective polymers such as olefin/paraffin separation. To investigate the utility of TR polymers for these separations, several polyimide structures with varying chemical backbone functionality were synthesized and tested for ethylene/ethane and propylene/propane separation. Similar to what has been observed for other gas pairs, conversion of TR polymers to their final polybenzoxazole structure resulted in increased permeability, a minimal loss in selectivity, and an increase in plasticization resistance.
    13 AIChE Annual Meeting; 11/2013
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    ABSTRACT: Thermally rearranged (TR) polymers are polybenzoxazoles synthesized via a decarboxylation reaction of polyimides that contain leaving groups ortho-position to the diamine. These polymers have shown interesting separation properties for a number of gas separations such as CO2/CH4 separation and hydrogen separation. The aim of this study is to investigate the effect of polyimide precursor structure on the transport characteristics for polyimides and TR polymers. Additionally, we seek to elucidate the effect of precursor and polybenzoxazole synthesis route on TR polymer transport properties. Two diamine and two dianhydride monomers were used to synthesize a series of polyimides. The monomers consisted of 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB), 2,2-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF), 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), and 4,4'-oxydiphthalic anhydride (ODPA). From these monomers, hydroxyl –functional polyimides HAB-6FDA, APAF-6FDA, and APAF-ODPA were synthesized. These samples were characterized using 1H and 13C solution NMR, TGA coupled with mass spectrometry, FT-IR, density measurements, and DSC. Additionally, several samples were characterized by positron annihilation lifetime spectroscopy and solid state 13C NMR. Samples were thermally reacted at 450°C for 30min to form their analog TR polymer structure. Gas permeability of H2, N2, O2, CH4, and CO2 were determined up to 50 bar to compare the effect of polyimide and TR polymer structure gas transport properties. In general, conversion of these samples caused changes in permeability and selectivity that parallel the upper bound for CO2/CH4, O2/N2, N2/CH4, and H2/CH4 separation. Incorporation of the fluorinated monomers (i.e., APAF and 6FDA) increased gas permeability while only slightly decreasing gas selectivity. Two comparisons of synthesis route were examined for this study. First, the effect of the polyimide synthesis route was compared by synthesizing HAB-6FDA from thermal, chemical, and solid state imidization. These studies showed that samples synthesized from thermal and solid state imidization (i.e., samples of identical chemical structure) showed similar transport properties; however, samples synthesized via chemical imidization, which contain acetate instead of hydroxyl leaving groups, had higher gas permeability. Second, the effect of TR polymer synthesis route was compared by testing transport properties of chemically similar polybenzoxazoles prepared from unique precursors. For this comparison, a TR polymer prepared from the APAF-6FDA polyimide was compared to a chemically similar polybenzoxazole prepared via the solid state condensation reaction of an APAF-6FDA polyamide. Permeability was consistently higher in the polyimide than in the polyamide for all gases. Moreover, conversion of the polyimide resulted in a greater increase in gas permeability than conversion of the polyamide.
    North American Membrane Society Meeting 2013; 06/2013
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    ABSTRACT: Thermally rearranged (TR) polymers are polybenzoxazoles synthesized via a decarboxylation reaction of polyimides that contain leaving groups ortho-position to the diamine. These polymers have shown interesting separation properties for a number of gas separations such as CO2/CH4 separation and hydrogen separation. The aim of this study is to investigate the effect of polyimide precursor structure on the transport characteristics for polyimides and TR polymers. Additionally, we seek to elucidate the effect of precursor and polybenzoxazole synthesis route on TR polymer transport properties. Two diamine and two dianhydride monomers were used to synthesize a series of polyimides. The monomers consisted of 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB), 2,2-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF), 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), and 4,4'-oxydiphthalic anhydride (ODPA). From these monomers, hydroxyl –functional polyimides HAB-6FDA, APAF-6FDA, and APAF-ODPA were synthesized. These samples were characterized using 1H and 13C solution NMR, TGA coupled with mass spectrometry, FT-IR, density measurements, and DSC. Additionally, several samples were characterized by positron annihilation lifetime spectroscopy and solid state 13C NMR. Samples were thermally reacted at 450°C for 30min to form their analog TR polymer structure. Gas permeability of H2, N2, O2, CH4, and CO2 were determined up to 50 bar to compare the effect of polyimide and TR polymer structure gas transport properties. In general, conversion of these samples caused changes in permeability and selectivity that parallel the upper bound for CO2/CH4, O2/N2, N2/CH4, and H2/CH4 separation. Incorporation of the fluorinated monomers (i.e., APAF and 6FDA) increased gas permeability while only slightly decreasing gas selectivity. Two comparisons of synthesis route were examined for this study. First, the effect of the polyimide synthesis route was compared by synthesizing HAB-6FDA from thermal, chemical, and solid state imidization. These studies showed that samples synthesized from thermal and solid state imidization (i.e., samples of identical chemical structure) showed similar transport properties; however, samples synthesized via chemical imidization, which contain acetate instead of hydroxyl leaving groups, had higher gas permeability. Second, the effect of TR polymer synthesis route was compared by testing transport properties of chemically similar polybenzoxazoles prepared from unique precursors. For this comparison, a TR polymer prepared from the APAF-6FDA polyimide was compared to a chemically similar polybenzoxazole prepared via the solid state condensation reaction of an APAF-6FDA polyamide. Permeability was consistently higher in the polyimide than in the polyamide for all gases. Moreover, conversion of the polyimide resulted in a greater increase in gas permeability than conversion of the polyamide.
    North American Membrane Society Meeting 2013; 06/2013
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    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.
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    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.