Kenneth A. Mauritz

Organic-inorganic hybrid materials: perfluorinated ionomers as sol-gel polymerization templates for inorganic alkoxides

Mauritz et al. exploited the polar/nonpolar nanophase-separated morphologies of Nafion® perfluorinated ionomer membranes, as well as a Nafion® ionomeric precursor film, as interactive templates that are capable of directing the condensation polymerizations of inorganic alkoxides and organoalkoxysilanes as well as the ultimate geometrical distribution of the inorganic oxide, or organically modified silicon oxide phases that result upon drying. This paper is a review of our extensive studies involving the in situ sol-gel reactions of the alkoxides of silicon, titanium, aluminum, zirconium and organoalkoxysilanes, as well as their mixtures and two step reactions involving these monomers. Throughout this presentation, we demonstrate how various spectroscopic (FTIR, 29Si solid state NMR, dielectric relaxation, pyrene (Py) fluorescence probe emission + UV absorption) microscopic, X-ray scattering, thermal (DSC, TGA, TGA-FTIR, DMA), mechanical tensile, and gas permeation tools were employed to interrogate the structures and properties of these heterogeneous materials over a range of dimensional scales ranging from the molecular to the macroscopic. Importantly, we established that these organic/inorganic materials are indeed structured on the scale of nanometers. Methods of tailoring the energetic environment, or polarity, within the cluster domains by the insertion of inorganic oxide or organically modified silicate nanostructures are presented. Finally, we discuss the potential for these nanocomposite membranes within a number of areas including gas and liquid separations technology as well as proton exchange membranes for fuel cell technology.

Thermal stability and flame retardancy of poly(methyl methacrylate)-clay nanocomposites

Abstract Three ammonium salts, hexadecylallyldimethyl ammonium chloride (Allyl16), hexadecylvinylbenzyldimethyl ammonium chloride (VB16) and hexadecylvinylbenzyldimethyl ammonium chloride (Bz16) were synthesized and ion exchanged onto montmorillonite. Poly(methyl methacrylate)–clay nanocomposites of all three clays were prepared by bulk polymerization and the resulting nanocomposites were characterized by X-ray diffraction and transmission electron microscopy. The clays which contain a pendant double bond are more likely to give an exfoliated material while that which contains no double bond is intercalated. The thermal stability and flame retardancy were measured by thermogravimetric analysis (TGA) and Cone Calorimetry respectively.

Nafion®/(SiO2, ORMOSIL, and dimethylsiloxane) hybrids via in situ sol–gel reactions: Characterization of fundamental properties

Nafion®/SiO2, Nafion®/[ORganically MOdified SILicate (ORMOSIL)] and Nafion®/dimethylsiloxane hybrids were created via in situ sol–gel reactions for tetraethoxysilane, diethoxydimethylsilane, and their mixtures. Differential scanning calorimetry studies showed a broad endotherm for unfilled Nafion®-H+ at Tα ≈ 215°C that shifts upward for the Q : D = 1 : 0 (mol : mol) [Q = Si(O1/2)4, D = (O1/2)2Si(CH3)2] hybrid, then shifts downward with decreasing Q : D. This endotherm likely arises from release of H2O molecules bound to ≡Si—OH groups and condensation reactions among silanol groups. The decrease in Tα is rationalized in terms of an increasing fraction of flexible D units that disrupt hydrophilic Q structures. Tm shifts to lower temperatures with decreasing Q : D, and it is suggested that main chains are restricted by side chains embedded in silicon oxide nanoparticles, but D unit insertion causes side chains to be anchored less strongly. Thermal gravimetric analysis indicates that the first mass loss step for Nafion®-H+ shifts to higher temperatures as D : Q increases; an increase in D unit fraction inhibits Q unit degradation by evolved HF. A dynamic mechanical transition at Tα may arise from side chain motions, and the increase in Tα in passing from unfilled Nafio®-H+ to the 1 : 0 hybrid is due to side chain immobilization by their entrapment in silicon oxide domains. The progression 0 : 1 1 : 2 1 : 1 2 : 1 generates increasing mechanical tensile strength and decreasing ductility; strength enhancement might be due to entanglements between ORMOSIL and pure silicon oxide phases and side chains. Liquid sorption experiments quantified the affinity of these hybrids for solvents of varying polarity. A dielectric relaxation for the 0 : 1 hybrid at about 1.5 kHz might be related to side chain mobility. A weaker relaxation in the range 104–105 Hz exists for the 0 : 1 and 1 : 1 hybrids and Nafion®-H+, but not for the 1 : 0 hybrid that exhibits the behavior e′ ∼ f−n over a broad frequency (f) range, suggestive of intercluster proton hopping.

[Perfluorosulfonate ionomer]/silicate hybrid membranes via base-catalyzed in situ sol–gel processes for tetraethylorthosilicate

Base-catalyzed sol–gel reactions for tetraethylorthosilicate were conducted within Nafion® sulfonate membranes. Silicate percent mass uptake, for all counterion forms and imposed conditions, decreases with increasing pH. Environmental scanning electron microscopy-EDAX studies of the distribution of silicon across the Nafion® membrane thickness direction revealed rather uniform concentration profiles except for upturns or downturns in the near-surface regions and demonstrated that it is possible to incorporate silicate structures deep within these membranes. 29Si solid-state NMR spectroscopic studies revealed that, for Li+ and Na+ counterion forms, bonding in the silicate phase is such that there is more intra-molecular coordination at high pH. FT-IR spectroscopy uncovered all the signature peaks characteristic of silicate structures in the near-surface regions. Fingerprints of Si–O–Si groups in cyclic and linear molecular substructures, as well as SiOH ‘defects’ are present. The spectra of Li+ form hybrids reflect a silicate network that becomes increasingly more interconnected with increasing pH, in harmony with the NMR results. The relationship between the spectroscopic information, porosity of the silicate phase, and possible permselectivity based on a molecular-sieving effect, is discussed.

Asymmetric nafion/(zirconium oxide) hybrid membranes via in situ sol‐gel chemistry

Nafion-in situ sol-gel reactions were affected for Zr(OBu)4 that permeated low water content membranes unidirectionally. IR peaks reflecting ZrO2 and incomplete hydrolysis of ZrOBu groups near both surfaces were detected. Vibrations of Zr(OEt)4 detected near both sides arise from alkoxy exchange in the presence of the solvent EtOH. Unreacted alkoxy group bands are more distinct near the nonpermeated surface. An IR band for the ZnOBu group diminishes, whereas that for ZnOEt increases with increasing time near the permeated surface due to progressive alkoxy exchange. The ZrO2 band becomes more intense with time near the permeated surface. X-ray spectroscopy/scanning electron microscopy studies of Zr concentration across the membrane thickness verified compositional asymmetry. CO2 gas permeability versus upstream pressure plots are monotonically increasing, suggesting diffusion accompanied by complex plasticization.

Review and Critical Analyses of Theories of Aggregation in Ionomers

Abstract In general, ionomers are ion-containing polymers [1] which, in contrast to the traditional ion-exchange resins [2], are usually rendered insoluble through the presence of crystalline do-mains. This review will be confined to a discussion of linear organic polymers having fixed ionic sidechains of exclusively either positive or negative charges, examples of which would include the ethylene-methacrylic acid copolymer salts, sulfonated polystyrene, and Nanon perfluorosulfonate salts. More complicated systems, such as those containing zwitterion moieties, polysalts, halato-telechelics, or polyether-salt complexes, will not be of interest herein.

Nafion®/ORMOSIL nanocomposites via polymer-in situ sol–gel reactions. 1. Probe of ORMOSIL phase nanostructures by 29Si solid-state NMR spectroscopy

Abstract A series of Nafion®/ORMOSIL hybrids, generated by in situ sol–gel co-polymerizations of tetraethylorthosilicate (TEOS) and semi-organic R′nSi(OR)4−n co-monomers (SOC), were developed to generate a spectrum of nanoscale chemical environments within the Nafion® morphological template. The molecular structures of the ORMOSIL phases were analyzed by means of 29Si solid-state NMR (SSNMR) spectroscopy. A high average degree of Si atom coordination about SiO4 molecular sub-units can be achieved, but a significant number of unreacted SiOH groups on reacted Q=Si(O1/2)4 units is always present. The shifting, with relative ORMOSIL composition, of D=RR′Si(O1/2)2 or T=R″Si(O1/2)3 (R, R′ and R″ are organic moeities) peak envelopes for difunctional or trifunctional SOCs is suggested to reflect random co-condensation rather than distinct Q and D (or T) block formation. The numbers of membrane-incorporated Q and D (or T) units per fixed sulfonate group were calculated from the 29Si SSNMR spectra for those particular hybrids that had a reasonably low noise/signal aspect. Spectra for hybrids based on in situ sol–gel reactions for TEOS (no SOC present) suggest that inserted hydroquinone molecules interfere with condensation reactions between (RO)4−xSi(OH)x molecules and silanol oligomers to yield silicate structures with lower average coordination. While earlier small angle X-ray scattering studies indicated that ORMOSIL structures can be grown within the polar regions of Nafion®, the results reported here address the specific compositions of these nanoscale structures.

[Perfluorosulfonate ionomer]/[SiO2‐TiO2] nanocomposites via polymer‐in situ sol‐gel chemistry: Sequential alkoxide procedure

In situ sol-gel chemistry was used to create inorganic/perfluoro-organic hybrids wherein titanium oxide outer regions of SiO2[1—x/4](OH)x nanoparticles, which were preformed in Nafion® membranes, were created by postreaction with tetrabutyltitanate (TBT). U-shaped Si and Ti distributions across the membrane thickness direction were determined via x-ray energy dispersive spectroscopy. Ti/Si ratio profiles are also U-shaped, indicating more Ti relative to Si in near-surface regions. IR spectroscopy verified structural bonding of TiO4 units onto SiO2 nanoparticles and indicated that alkoxide hydrolysis is not complete. Reacted silicon oxide nanophases retain the topological unconnectedness possessed by the corresponding unreacted phase. IR bands signifying molecular loops and linear fragments of Si(SINGLE BOND)O(SINGLE BOND)Si groups are seen. 29Si solid-state NMR spectroscopy indicated that, for an inorganic uptake of 16.3 wt %, the Q3 state of SiO4 is most populated although Q4 is only slightly less prominent and Q2 and Q1 are either small or absent. The silicon oxide component, although not being predominantly linear, retains a measure of uncondensed SiOH groups. Tensile stress vs. strain analyses suggested that TBT postreaction links nanoparticles, causing them to be contiguous over considerable distances. This percolative intergrowth occurs in near-surface regions generating a glassy zone.

Polyethersulfone-[silicon oxide] hybrid materials via in situ sol-gel reactions for tetra-alkoxysilanes

Polyethersulfone (PES)–[silicon oxide] hybrids were derived via sol–gel reactions for tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) in dimethylacetamide solutions of the polymer. In one scheme, water was initially present, and condensation reactions between SiOR groups competed with their reactions with —OH groups at PES chain ends. In a second scheme, water addition was delayed; TMOS molecules reacted with chain ends before competing TMOS–TMOS reactions occurred. A third study involved parallel experiments, as follows: 1) introduction of EtOH to PES–TEOS solutions for a time before water addition; and 2) reactions occurring for a time in non-EtOH-containing PES–TEOS solutions before water addition. Infrared (IR) spectroscopy uncovered signatures of Si–O–Si bridges in silicon oxide phases and PES endgroup modifications (Si–O–Ph). Composites prepared according to the latter two schemes contain more Si–O–Ph linkages than those generated via the first. Differential scanning calorimetry showed that Tg can be raised, and thermogravimetric analysis revealed how the PES thermal degradation profile can be modified via these inorganic incorporations. The schemes for late water addition produced composites having increased elongation-to-break and lowered strength relative to unfilled PES.

Asymmetric [Nafion®]/[silicon oxide] hybrid membranes via the in situ sol‐gel reaction for tetraethoxysilane

Asymmetric silicon oxide composition profiles along a direction perpendicular to the plane of Nafion® sulfonate films were created via in situ sol-gel reactions for one-sided tetraethoxysilane permeation, as verified by EDAX/ESEM. For K+ form membranes, we propose the existence of an IR spectral signature of molecular branches in addition to those characteristic of linear and cyclic fragments in the silicon oxide phase. The molecular structure of the silicon oxide phase is more interconnected than linear in K+ form membranes. For H+ form membranes, there appears to be an increasing degree of molecular linearity within the silicon oxide phase with increasing uptake. IR spectra indicate that molecular connectivity on the permeated side is lower, on the average, than that on the nonpermeated side. The inverse relationship between gas permeability and upstream pressure in steady-state helium gas transmission experiments suggests the dual-mode sorption of gases, which is in harmony with the multiphasic nature of these membranes.