Thursday, August 09, 2007

J Phys Chem C, Macromolecules and Organometallics

J Phys. Chem. C

-J. Phys. Chem. C, 111 (21), 7605 -7611, 2007. 10.1021/jp070321y S1932-7447(07)00321-4
Pores in Nanostructured TiO2 Films. Size Distribution and Pore Permeability

Dulce Vargas-Florencia, Tomas Edvinsson, Anders Hagfeldt, and Istv n Furó*
Division of Physical Chemistry, Department of Chemistry, Royal Institute of Technology, SE-10044 Stockholm, Sweden

Abstract: Nanoporous films of crystalline anatase with intended application in dye-sensitized photovoltaic cells were investigated by NMR cryoporometry, NMR diffusiometry, electron microscopy, and X-ray diffraction. The nanoparticles from which the films were subsequently sintered were prepared in two ways, one with an acidic and one with a basic aqueous process environment and along different temperature regimes. The average morphology was similar in all films as indicated by the roughly identical <2> values where is the mean curvature of the pore surface and S/V denotes the surface-to-volume ratio. Self-diffusion of water in the pores is strongly reduced with respect to that of bulk and is influenced both by micrometer-scale obstructions to molecular displacement and by pore-size effect in pore interconnectivity. The investigated samples exhibit different transport regimes as concerning those phenomena. In this initial study performed on a limited set of samples, we found no linear correlation between particle and pore sizes. Instead, total porosity is controlled by particle-particle jamming which, together with particle size polydispersity, may also dominate the effects that lead to the observed pore size distributions for the different samples. The rich variation of structural effects and transport properties among the few prepared films call for further studies in order to find an optimal film structure.

-J. Phys. Chem. C, 111 (21), 7720 -7726, 2007. 10.1021/jp068738b S1932-7447(06)08738-3
NMR Study of Proton Sites in Group I Salts of 12-Tungstophosphoric Acid

Steven F. Dec,* George M. Jacobsen, James L. Horan, and Andrew M. Herring

Department of Chemistry and Geochemistry and Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 8040

Abstract: The Group I salts of 12-tungstophosphoric acid were studied in their limiting hydration state with use of both1H and 31P solid-state nuclear magnetic resonance (NMR). The bulk proton structures of the Li, Na, and Rb salts consist of both lone protons and water molecule protons. The K and Cs salts contain similar proton structures that reside in surface sites. Nonspinning 1H NMR results indicate that all protons in the limiting hydration state in each of the Group I salts are highly mobile at room temperature. 1H spin-lattice relaxation times obtained as a function of temperature indicate that the dominant mechanism responsible for proton relaxation in the Li, Na, and Rb salts is restricted rotation of water molecules while the K and Cs salts have proton spin-lattice relaxation that is dominated by translation of protons through surface sites of the material. Two stable Rb salt phases are crystallized when a stoichiometric amount of RbCl is added to aqueous 12-tungstophosphoric acid at room temperature.

-J. Phys. Chem. C, 111 (26), 9066 -9071, 2007. 10.1021/jp071490l S1932-7447(07)01490-2
Speciation of Silanol Groups in Precipitated Silica Nanoparticles by 1H MAS NMR Spectroscopy

Geoffrey Hartmeyer, Claire Marichal,* Bénédicte Lebeau, Séverinne Rigolet, Philippe Caullet, and Julien Hernandez

Laboratoire de Matériaux à Porosité Contrôlée, ENSCMu, Université de Haute Alsace, UMR 7016, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France, and Centre de Recherches d'Aubervilliers, RHODIA, 52 rue de la Haie Coq, 93308 Aubervilliers, France

Abstract: Speciation of the various proton species present in precipitated silica samples by 1H high-spinning frequency magic-angle spinning (MAS) NMR (25 kHz) is achieved. External standards are used for the calibration of the absolute amount of protons, the relevance of the proton spin counting being checked by comparison with the data deduced from classical thermogravimetric (TG) and 29Si MAS NMR experiments. The method also allows to follow postsynthesis modifications of silica, corresponding for instance to fluorination or deuteration treatments. In particular, a 1H resonance occurring at 1.1 ppm/TMS and corresponding to a constant proton amount whatever the applied treatment is tentatively assigned for the first time to inaccessible isolated silanol groups.

-J. Phys. Chem. C, 111 (28), 10624 -10629, 2007. 10.1021/jp0728757 S1932-7447(07)02875-0
Methane Carbonylation with CO on Sulfated Zirconia: Evidence from Solid-State NMR for the Selective Formation of Acetic Acid

Mikhail V. Luzgin, Vladimir A. Rogov, Nina S. Kotsarenko, Vera P. Shmachkova, and Alexander G. Stepanov*

Boreskov Institute of Catalysis, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia

Abstract: Using 13C and 1H solid-state NMR it has been shown, that methane can be carbonylated with carbon monoxide to give acetic acid on solid acid catalyst, sulfated zirconia. The carbonylation occurs at 473-573 K with high selectivity and essential conversion. The reaction proceeds both in the absence and in the presence of molecular oxygen. In the presence of oxygen, the catalyst can be used for the carbonylation of further portion of methane without reactivation in air. The mechanism of the reaction is discussed. The reaction observed opens up new possibilities of using sulfated zirconia-based solid catalysts for the synthesis of acetic acid from methane and carbon monoxide.


-Macromolecules, 40 (14), 4736 -4739, 2007. 10.1021/ma0700025 S0024-9297(07)00002-2

Probing Chain Interpenetration in Polymer Glasses by 1H Dipolar Filter Solid-State NMR under Fast Magic Angle Spinning

Xiaoliang Wang, Fangfang Tao, Pingchuan Sun,* Dongshan Zhou, Zhaoqun Wang, Qiang Gu, Jinlei Hu, and Gi Xue*

Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, State Key Laboratory of Co-ordination Chemistry, Nanjing University, Nanjing 210093, P. R. China, and Key Laboratory of Functional Polymer Materials, Ministry of Education, College of Chemistry, Nankai University, Tianjin 300071, P. R. China

Introduction. The concept of chain entanglement provides the basis of our current understanding of the flow behavior of polymer melts and solutions.1,2 The interpenetration process is a very important step for polymer chain condensation from the dilute solution to the aggregation state. Such a condensed multiple chain systems possess a structure where polymer chains form an entangled network, which thus establishes the mechanical property of polymers.1-3 Information about interpenetration regions would provide significant insight into intermolecular coupling and the dynamics of local segmental relaxation, which issues remain unclear for glassy polymer physics. However, compared to those in solution and in the rubber state, an intriguing question concerning the characterization of the degree of interpenetration below and around the glass transition temperature (Tg) of a polymer has still not been tackled.4-7 Atomic force microscopy, differential scanning calorimetry, dynamic mechanical thermal analysis, and scattering methods (visible light, X-ray, neutron) are frequently used to investigate chain interpenetration.8-10 One drawback of these methods is that they yield information on length scales of a few to tens of nanometers; thus, they cannot be used to directly identify the individual structure in the interpenetration regions at a molecular level. Nonradiative energy transfer (NET) fluorescence spectroscopy has been used to observe the memory of the chain interpenetration that had existed in the original solution, and many interesting conclusions have been reported.11,12 On the other hand, 13C solid-state NMR and small-angle neutron scattering (SANS) studies have cast doubt on the conclusions regarding the memory of the chain interpenetration for the freeze-dried polymer reported by NET.13-16 Developing new strategies to characterize the chain interpenetration in polymer glasses on a short length scale of 0.5 nm is still a challenge.

-Macromolecules, 40 (14), 4953 -4962, 2007. 10.1021/ma0620924 S0024-9297(06)02092-4

Investigation of Network Heterogeneities in Filled, Trimodal, Highly Functional PDMS Networks by 1H Multiple Quantum NMR

Erica Gjersing, Sarah Chinn, Jason R. Giuliani, Julie Herberg, Robert S. Maxwell,* Eric Eastwood, Dan Bowen, and Tom Stephens

Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, California 94551, Honeywell Federal Manufacturing and Technologies Plant, Kansas City, Missouri, Los Alamos National Laboratory, Los Alamos, New Mexico, and Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, California

Abstract: The segmental order and dynamics of polymer network chains in a filled, trimodal silicone foam network have been studied by static 1H multiple quantum (MQ) NMR methods to gain insight into the structure property relationships. The foam materials were synthesized with two different types of cross-links, with functionalities, , of 4 and near 60. The network chains were composed of distributions of high, low, and medium molecular weight chains. Cross-linking was accomplished by standard acid-catalyzed reactions. MQ NMR methods have detected domains with residual dipolar couplings (<>) of near 4 and 1 krad/s assigned to (a) the shorter polymer chains and chains near the multifunctional ( = 60) cross-linking sites and to (b) the longer polymer chains far from these sites. Three structural variables were systematically varied and the mechanical properties via compression and distributions of residual dipolar couplings measured in order to gain insight into the network structural motifs that contribute significantly to the composite properties. The partitioning and average values of the residual dipolar couplings for the two domains were observed to be dependent on formulation variables and provided increased insight into the network structure of these materials which are unavailable from swelling and spin-echo methods. The results of this study suggest that the domains with high cross-link density contribute significantly to the high strain modulus, while the low cross-link density domains do not. This is in agreement with theories and experimental studies on silicone bimodal networks over the last 20 years. In situ MQ-NMR of swollen samples suggests that the networks deform heterogeneously and nonaffinely. The heterogeneity of the deformation process was observed to depend on the amount of the high functionality cross-linking site PMHS. The NMR experiments shown here provide increased ability to characterize multimodal networks of typical engineering silicone foam materials and to gain significant insight into structure-property relationships.

-Macromolecules, 40 (14), 5001 -5013, 2007. 10.1021/ma062507l S0024-9297(06)02507-1

Solid-State Organization and Morphological Partitioning in Polyoxymethylene-Based Copolymers: A Solid-State NMR and WAXS Study

Cédric Lorthioir,* Françoise Lauprêtre, Karthikeyan Sharavanan, Ronald F. M. Lange, Philippe Desbois, Michel Moreau, and Jean-Pierre Vairon

Laboratoire de Recherche sur les Polymères (CNRS-UMR 7581), 2-8 rue Henri Dunant, 94320 Thiais, France; Laboratoire de Chimie des Polymères (CNRS-UMR 7610), Université Pierre et Marie Curie, Case 185 T44 E1, 4 Place Jussieu, 75252 Paris Cedex 05, France; and BASF Research, GKT/U B1, D-67056 Ludwigshafen, Germany

Abstract: The solid-state organization of copolymers based on methylene oxide (MO) units and tetramethylene oxide (T) units, with T unit contents ranging between 0.9 and 10.0 mol %, is investigated by the combined use of solid-state NMR, WAXS, and DSC experiments. In these semicrystalline copolymers, the T units can be viewed as linear defects inserted along linear poly(oxymethylene) chains. As expected, the insertion of T units induces a significant decrease of both crystallinity and crystallite size: a large part of T units is located in the amorphous domains. However, some T units can also be detected within the crystalline domains and/or the interfacial regions with the amorphous phase. More precisely, the amount of T units at both sides of the noncrystalline/crystalline interfaces seems to be much higher than the T units in the interior of the crystallites. At the lowest T unit content (0.9 mol %), the composition averaged over both interfacial and crystalline phases appears to be identical to the composition of the amorphous phase. When the comonomer content increases, the amount of T units in the interfacial and crystalline zones becomes higher and higher while the partitioning coefficient of the T units in these domains tends to a limiting value of 0.40.

-Macromolecules, 40 (15), 5411 -5419, 2007. 10.1021/ma0707786 S0024-9297(07)00778-4

Quantitative Determination of Phase Content in Multiphase Polymers by Combining Spin-Diffusion and CP-MAS NMR

L. Zhang, Z. Liu, Q. Chen,* and E. W. Hansen*

Physics Department and the State Key Laboratory of Precision Spectroscopy, East China Normal University, 3663 Northern Zhongshan Road, Shanghai 200062, People's Republic of China, and Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway

Abstract: We present a new approach to determine quantitatively the phase content of multiphase polymers by applying a spin-diffusion pulse sequence (filter) prior to cross-polarization (CP) in a 13C-CPAS experiment. The technique is exemplified for two rather different polymer systems, a high-density polyethylene (HDPE) and four styrene-isoprene diblock copolymers, all of known phase contents. In principle, the technique should be applicable to any multiphase system in which spin-diffusion between different regions/phases exists and where the magnetization of phases can selectively be filtered out

-Macromolecules, 40 (15), 5420 -5423, 2007. 10.1021/ma070790y S0024-9297(07)00790-5

Investigation of Dynamics of Poly(dimethylsilane) in the Mesophase by Solid-State 29Si NMR: Evidence for Rotator Phase

Hironori Kaji* and Fumitaka Horii

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

Abstract: The dynamics of poly(dimethylsilane) (PDMS) in the mesophase was investigated by DSC and 29Si solid-state NMR. A solid-solid transition is observed at 166 C based on DSC measurements. Below the transition temperature, typical chemical shift anisotropy (CSA) spectra are obtained by the 29Si solid-state NMR with cross-polarization (CP) preparation for the static PDMS sample. In contrast, the intensities at the isotropic chemical shift are suppressed for the CSA spectra above the transition temperature. The intensity attenuation, termed "magic angle hole", in the CSA spectra is theoretically derived for uniaxially rotating solids. Therefore, it was found that PDMS undergoes a uniaxial rotation motion above the transition temperature. Theoretically, both the direct polarization (DP) experiments without the CP process and ultraslow magic angle spinning (MAS) experiments retrieve standard CSA line shapes. Our DP and ultraslow MAS experiments confirm the theoretical consideration. The retrieved CSA spectra have axially symmetric line shapes, which also confirm the above uniaxial dynamics. On the basis of the solid-state NMR, we clearly show that the solid-solid transition at 166 C is a rigid monoclinic-mobile rotator phase transition.

-Macromolecules, 40 (16), 5776 -5786, 2007. 10.1021/ma070485c S0024-9297(07)00485-8

Various Types of Hydrogen Bonds, Their Temperature Dependence and Water-Polymer Interaction in Hydrated Poly(Acrylic Acid) as Revealed by 1H Solid-State NMR Spectroscopy

Baohui Li, Lu Xu, Qiang Wu, Tiehong Chen, Pingchuan Sun,* Qinghua Jin, Datong Ding, Xiaoliang Wang, Gi Xue,* and An-Chang Shi

Key Laboratory of Functional Polymer Materials, Ministry of Education, College of Chemistry and College of Physics, Nankai University, Tianjin, 300071, P. R. China, Department of Polymer Science and Engineering, The School of Chemistry and Chemical Engineering, State Key Laboratory of Co-ordination Chemistry, Nanjing University, Nanjing 210093, P. R. China, and Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada

Abstract: Various types of hydrogen bonds, their temperature dependence and water-polymer interaction in hydrated poly(acrylic acid) (PAA) were systematically investigated using 1H CRAMPS solid-state NMR techniques in the temperature range from 25 to 110 C. The 1H CRAMPS NMR methods are based on a recently developed continuous phase modulation technique for 1H-1H homonuclear dipolar decoupling. The 1H CRAMPS experiments revealed four types of protons in hydrated PAA which are assigned to protons from the mutually hydrogen-bonded COOH groups (1), from the free COOH groups (2), from the COOH groups bounded with water or from water bounded with COOH groups which are undergoing fast chemical exchange mutually (3), and from main chain groups (4), respectively. Furthermore, we proposed double-quantum filtered and dipolar filtered 1H CRAMPS experiments to further assign the protons according to their dipolar coupling strength. In addition, high-resolution spin echo 1H CRAMPS experiments were further employed to accurately determine the chemical shift of these protons. These NMR techniques were also used to elucidate the molecular mobility of the different groups. It was found that dehydration in PAA promotes the formation of hydrogen bonds between COOH groups. Variable-temperature 1H CRAMPS experiments demonstrated that the dissociation of the hydrogen bonds between COOH groups occurs dramatically at lower temperature in hydrated PAA and slowly over a wide range of temperature in dehydrated PAA. It was also found that the dehydration of water bounded with COOH groups in hydrated PAA occurs significantly at high temperature. The NMR results were compared with previous work using DSC and other techniques. Besides undergoing fast chemical exchange, the adsorbed water was also demonstrated in proximity with the free COOH groups and far from the hydrogen bonds between COOH groups by using two-dimensional 1H-1H spin-exchange NMR experiments.

-Macromolecules, 40 (16), 5787 -5790, 2007. 10.1021/ma070793a S0024-9297(07)00793-0

Cross-Link Density of a Dispersed Rubber Measured by 129Xe Chemical Shift
Wallace O. Parker, Jr.,* Angelo Ferrando, Dino Ferri, and Valentino Canepari

Physical Chemistry, Refining & Marketing Division, Eni S.p.A., via Maritano 26, 20097 San Donato Milanese, Italy, and Polymer Research Center "C. Buonerba", Polimeri Europa, via Taliercio 14, 46100 Mantova, Italy

Abstract: 129Xe NMR chemical shifts of adsorbed xenon (natural isotopic abundance) were used to probe the free volume in a cross-linked rubber, polybutadiene (PB), alone and occluded within high impact polystyrene (HIPS). Shift dependency on cross-link density, or Nm (average number of monomer units between chemical cross-links) determined by the classical solvent swelling method, was detailed for high-cis-1,4-PB. A decrease in Nm from 262 to 4 caused the shift to increase 4 ppm. A curve inflection, evident at 218 ppm (and Nm = 30), corresponded to a free volume with an average spherical diameter of 0.49 nm using the empirical (shift-pore size) relation known for zeolites. This diameter is essentially equal to that of the PB chain (0.48 nm) at its Tg (glass transition temperature). Tg plotted against Nm showed the same inflection (at Nm = 25), corroborating a regime with more severe interchain contacts when the free volume diameter is inferior to the chain diameter. The empirical relation between 129Xe shift and Nm observed for pure PB was used to estimate the cross-link density of PB dispersed in HIPS as a function of cure time. Cross-linkage was greatly slowed after 8 h of curing, when Nm estimated by NMR spectroscopy was 10.

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