Glass transition of polymers confined to nanoporous glasses

The glassy dynamics of poly(propylene glycol) (PPG) and poly(methyl phenyl siloxane) (PMPS) confined to nanoporous glasses (pore sizes 2.5–20 nm) investigated by dielectric spectroscopy, temperature modulated DSC and neutron scattering is compared. For both systems the relaxation rates estimated from dielectric spectroscopy and temperature modulated DSC agree quantitatively indicating that both experiments sense the glass transition.For PPG the glassy dynamics in nanopores is determined by a counterbalance of an adsorption and a confinement effect where the temperature dependence of the relaxation times obeys the Vogel/Fulcher/Tammann (VFT-) equation. The former effect results from an interaction of the confined macromolecules with the internal surfaces which in general slows down the molecular dynamics. A confinement effect leads to an acceleration of the segmental dynamics compared to the bulk state and points to an inherent length scale on which the glassy dynamics takes place. The step of the specific heat capacity c_p at the glass transition vanishes at a finite length scale of 1.8 nm. This result supports further the conception that a characteristic length scale is relevant for glassy dynamics.For PMPS down to a pore size of 7.5 nm the temperature dependence of the relaxation times follows the VFT-dependence and a confinement effect is observed like for PPG. At a pore size of 5 nm this changes to an Arrhenius-like behavior with a low activation energy. At the same pore size c_p vanishes for PMPS. This points to a dramatic change in the character of molecular motions responsible for glassy dynamics and supports further the relevance of a characteristic length scale on which it takes place.Quasielastic neutron scattering experiments on PMPS reveal that the microscopic dynamics characterized by the mean square displacement depends on confinement above the glass transition. The diffusive character of the relevant molecular motions seems to disappear at a length scale of about 1.6 nm.     

Applications of novel proton-conducting polymers to hydrogen sensing

Ionic conducting polymer blends capable of supporting proton conduction from −40°C to 40 °C have been developed. These proton-conducting polymer blends have been used to fabricate hydrogen sensors that are capable of operating at room temperature without the need for an external supply of water vapor. Two sensor designs, both potentiostatic, are being investigated: the first uses a gaseous reference source, and the second a solid state palladium hydride reference. These sensors measure accurately the hydrogen concentration to better than 1%, have a response time of less than 6 s and are not adversely affected by most potential poisons.     

Unified approach to ion transport and structural relaxation in amorphous polymers and glasses

Kinetic data for structural relaxation in silver iodomolybdates at the glass transition temperature (Tg) are obtained by high-pressure differential scanning calorimetry (HP-DSC) and are compared with activation energies (EA) and volumes (VA) obtained earlier from conductivities below Tg. The results are fitted to an empirical equation, EA = MVA, and displayed in the form of a master plot of EA versus VA, an approach previously applied to strongly coupled systems, including polymer electrolytes and molten salts above their glass transition temperatures. The parameter M emerges as a localized modulus, expressive of interatomic forces within the medium, linking together EA,sigma, VA,sigma and EA,s, VA,s, the "apparent" activation parameters for ionic conductivity and structural relaxation, respectively. The VA and EA values for ion transport are much smaller than the corresponding values for structural relaxation. However, remarkably close agreement emerges between the "process parameters", Ms and Msigma, both close to 8 GPa, thus establishing a quantitative link between ion transport and structural relaxation in this highly decoupled system. A new EA versus VA master plot is constructed, which points the way to a unified approach to ion transport in polymers and glasses.     

Applicability of MNDO techniques AM1 and PM3 to ring-structured polymers

Semiempirical Hartree‐Fock techniques are widely used to study properties of long ring‐structured chains, although these types of systems were not included in the original parametrization ensembles. These techniques are very useful for an ample class of studies, and their predictive power should be tested. We present here a study of the applicability of some techniques from the NDDO family (MNDO, AM1, and PM3) to the calculation of the ground state geometries of a specific set of molecules with the ring‐structure characteristic. For this we have chosen to compare results against ab initio Restricted Hartree‐Fock 6‐31G(d,p) calculations, extended to Møller‐Plesset 2 perturbation theory for special cases. The systems investigated comprise the orthobenzoquinone (O₂C₆H₄) molecule and dimers (O₂C₆H₄)₂, as well as trimers of polyaniline, which present characteristics that extend to several systems of interest in the field of conducting polymers, such as ring structure and heterosubstitution. We focus on the torsion between rings, because this angle is known to affect strongly the electronic and optical properties of conjugated polymers. We find that AM1 is always in qualitative agreement with the ab initio results, and is thus indicated for further studies of longer, more complicated chains. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1135–1142, 2002     

Applications of Raman-based techniques to on-site and in-vivo analysis

Raman-based technologies have proved to be excellent tools for on-site and in-vivo analysis, due to the non-invasive nature of their detection, their capability of providing structure information, their high tolerance to aqueous samples, the ultra-sensitivity of surface-enhanced Raman scattering (SERS) and resonance Raman scattering (RRS), the high spatial resolution of tip-enhanced Raman scattering (TERS), and the ultrashort spectra-acquisition time for coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS).In this review, we highlight the recent success of Raman-based technologies in various applications, including:

• (1) 

  on-site surface analysis and chemical-reaction monitoring;

• (2) 

  on-site identification of cultural objects, archeological studies and planetary science;

• (3) 

  in-vivo analysis of cells and microorganisms;

• (4) 

  in-vivo diagnosis inside human and animal bodies;

• (5) 

  in-vivo fast Raman imaging and mapping;

• (6) 

  the study of SERS processes; and,

• (7) 

  assessment of nanomaterial safety.

     

Applications and limitations of deuterium labeling methods to neutron scattering studies of polymers

Deuterium labeling techniques have been widely used to study polymer chain configuration and motion, based on the assumption that the H- and D- chains are thermodynamically identical and that the interaction parameter between them, X_HD is zero. By an appropriate choice of the degree of polymerization and temperature, both critical scattering and phase separation can be made to occur, due solely to the small but finite thermodynamic differences between protonated and deuterated molecules. These experiments helped delineate the range of validity of the assumptions which had been used hitherto in neutron scattering studies of polymers. The majority of previous experiments, carried out at modest molecular weights, have been shown to be in large part unaffected by such effects.     

Electric and dielectric properties of some gamma-irradiated cabal glasses

Electrical conductivity and dielectric properties have been studied for the glass system CaO---B₂O₃---Al₂O₃ in the temperature range 40–200°C. The substitution of 5% B₂O₃ by CaO or replacing 5% CaO by Na₂O or MgO cause a decrease in the conductivity, but the decrease obtained by soda is greater than that of magnesia. The activation energies of the tested glasses were calculated. All the glasses investigated showed a dielectric constant almost independent of temperature at fixed frequency. The effect of subjecting the glass to a constant dose of gamma-rays changes both the electrical conductivity and dielectric constant. The experimental results were discussed in relation to the specific conduction mechanism in such glasses. Also the effect of varying glass composition or temperature on the mobility or migration of current carrier was considered. The possible creation of induced defects in glass on irradiation was evaluated.     

Inorganic glasses as crosslinked polymers: The relationship between crosslink density and transformation temperature in phosphate glasses

Most substances that form stable glasses are polymers, and inorganic oxide glasses are no exception. Because most oxide glasses are highly crosslinked polymers it is not possible to measure properties in solution, and parameters such as molecular weight and molecular weight distribution which are used to characterize organic polymers are meaningless in the context of oxide glasses. Instead, the network connectivity or a measure of crosslink density must be used to characterize their structure. In silicate glasses, the network connectivity can be changed significantly only by altering the composition, but phosphate glasses of fixed composition can be prepared with variable hydroxyl contents and consequently different crosslink densities. In this paper it is shown that the relationship between the transformation temperature T(x) and crosslink density x of phosphate glasses is similar to that observed for organic polymers and can be described by an equation of the form where T(0) is the transformation temperature of the corresponding linear polymer and K is a constant which is approximately the same for three different phosphate glasses that have been studied.     

Applications of some bounds to density functionals for atoms

In this article, some applications of lower bounds to density-dependent functionals in terms of radial expectation values are performed. These diverse applications include accurate upper bounds to the exact kinetic energy and to conjecture a wide set of relationships among radial and momentum expectation values. Some open questions for the improvement of these results are remarked upon. © 1995 John Wiley & Sons, Inc.     

Thermal diffusivity of some linear polymers

Summary Thermal diffusivity of several linear polymers and cross-linked polyester was measured as Function of temperature between 10‡ and 100 ‡C. Thermal diffusivity is lowered by reducing the interaction between chain molecules in amorphous polymers. In partially crystalline polymers, the crystallinity has a considerable effect on unsteady-state heat conduction. The variation of diffusivity with crystallinity was studied for polyethylene and polytrifluorochloroethylene.

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Zusammenfassung Die Temperaturleitf?higkeit verschiedener linearer Polymeren und vernetzter Polyester wurde als Punktion der Temperatur zwischen 10 und 100 ‡C gemessen. Die Temperaturleitf?higkeit wird durch Reduzierung der Wechselwirkung zwischen den Kettenmolekülen in amorphen Polymeren erniedrigt. In partiell kristallinen Polymeren spielt die Kristallinit?t in betr?chtlicher Weise bei der nichtstation?ren W?rmeleitung eine Rolle. Die Variation der Temperaturleitf?higkeit mit der Kristallinit?t wurde für Poly?thylen und Polytrifluorchlor?thylen untersucht.

     

Thermal degradation of some coordination polymers

Coordination polymers of Co(II), Ni(II) and Cu(II) were synthesized using two ligands, 5- benzylidene pseudothiohydantoin and 5-cinnamylidene pseudothiohydantoin. These polymers were characterized by dynamic and static TG, and their relative thermal stabilities were compared. The thermal stabilities found by dynamic and static TG were in harmony. The structure of the polymeric unit for each polymer was predicted with the help of thermoanalytical data. The data reveal one pattern for Co(II) and Cu(II), i.e. an (ML ₂ ) _n -type polymerkc unit, and another pattern for Ni(II), i.e. an (ML ₂ · 2H ₂ O)_n-type polymeric unit. The static (isothermal) TG data were analysed by use of (i) the difference equation method, and (ii) the Avrami-Erofeyev method. The activation energies of the coordination polymers were calculated, and it was observed that the chelate polymers of Ni(II) with both ligands were more stable than those of Co(II) and Cu(II). This prediction on the basis of static TG data is in agreement with the prediction based on the activation energies and initial decomposition temperatures calculated from the dynamic TG curves.

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Zusammenfassung Koordinationspolymere von Co(II), Ni(II) und Cu(II) mit 5-Benzylidenpseudothiohydantoin bzw. 5-Cinnamyliden-pseudothiohydantoin als Liganden wurden dargestellt, durch dynamische und statische TG charakterisiert und hinsichtlich der relativen thermischen Stabilität miteinander verglichen. Die durch statische und dynamische TG ermittelte thermische Stabilität stimmen überein. Ausgehend von thermoanalytischen Daten wurde die Struktur der Polymereinheit für jedes Polymer vorausgesagt. Ein Thermogrammtyp - (ML ₂ )_n - wird für Co(II) und Cu(H) und ein anderer - (ML ₂ · 2H ₂ O)_n - für Ni(II) erhalten. Die statischen (isothermen) TG-Daten wurden nach der Differenzgleichungsmethode und der Avrami-Erofeyev-Methode analysiert. Die Aktivierungsenergien der Zersetzung der Koordinationspolymere wurden berechnet, und es wurde festgestellt, daß Chelatepolymere von Ni(II) mit beiden Ligandtypen stabiler als die entsprechenden Co(II)- und Cu(II)- Komplexe sind. Diese auf Grund von statischen TG-Daten getroffene Voraussage ist in Übereinstimmung mit der, die auf aus dynamischen TG-Kurven berechneten Aktivierungsenergien und Anfangszersetzungstemperaturen beruht.

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, 5-- 5- - . . . . (ML2)_n - (ML₂ · 2H₂O)_n. - -. . , ' , . , , .

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We express our thanks to the Head of the Department of Chemistry, Nagpur University, for providing the necessary facilities. One of the authors (V. N. I.) is thankful to U. G. C., New Delhi for awarding Junior Research Fellowship.     

End-functionalized ROMP polymers for Biomedical Applications

We present two novel allyl-based terminating agents that can be used to end-functionalize living polymer chains obtained by ring-opening metathesis polymerization (ROMP) using Grubbs' third generation catalyst. Both terminating agents can be easily synthesized and yield ROMP polymers with stable, storable activated ester groups at the chain-end. These end-functionalized ROMP polymers are attractive building blocks for advanced polymeric materials, especially in the biomedical field. Dye-labeling and surface-coupling of antimicrobially active polymers using these end-groups were demonstrated.     

Anharmonicity and the elementary event of plastic deformation of amorphous polymers and glasses

The critical displacement of an excited atom (group of atoms) corresponding to the maximum in the interatomic attraction force plays an important part in the elementary event of plastic deformation of glassy solids. As a result of considerable departure of the excited kinetic unit from the equilibrium position and the nonlinearity of the interatomic interaction force, the microdeformation in the elementary event turns out to be a function of the degree of anharmonicity (Grüneisen parameter).     

Stability and degradation of some electrically conducting polymers

The oxidative degradation reactions of polyacetylene, prepared from a soluble precursor polymer, are described and compared with those of more common polymers and of polyacetylene prepared by the conventional method. Because the band structure of the π-electron system of the polymer allows the formation of charge-transfer complexes with oxygen, the initial process in the pristine polymer is oxygen doping, with increasing conductivity. This is followed by irreversible degradation which is much faster than that of polyolefins or polydienes and faster than that of the crystalline polymer. Doping to low levels with electron acceptors removes the electrons involved in oxygen doping and the polymer becomes much more stable in air. Doping to high levels leads to new instabilities as the polymer reacts slowly with its counter-ions. Studies of polypyrrole and polythiophene show that these polymers are much more stable than polyacetylene but still undergo degradation reactions. The general features of their degradation mechanisms are discussed.     

Synthesis and characterization of some carbazole-based imine polymers

Five imine polymers, all containing 3,6-disubstituted carbazole ring in the main polymer chain, were synthesized by solution polycondensation of 3,6-diformyl (N-hexyl) carbazole with hydrazine, 1,4-diaminobenzene, 4,4′-diaminobiphenyl, 1,5-diaminonaphthalene and 3,6-(N-hexyl) diaminocarbazole. The polymers were analyzed by spectroscopic methods and compared with two imine models.     

Applications of phosphine-functionalised polymers in organic synthesis

This tutorial review deals with recent advances in the use of phosphine-functionalised polymers in organic synthesis. In the first part of the review, some recent applications of polymer-supported palladium catalysts are reviewed, particularly recyclable catalysts for C-C and C-X bond formation with aryl bromide and chloride substrates. In the second half, novel applications of phosphine-functionalised polymers as reagents, scavengers, organocatalysts and linkers in organic chemistry are presented. Emphasis is placed on the synthesis of biologically active molecules.     

New processes and designed polymers by cationic techniques

The most important recent development in cationic polymerizations is the emergence of living polymerizations leading to a variety of new potentially useful well-defined macromolecules under conventional laboratory conditions. Three requirements have to coexist for living carbocationic polymerization to occur: Controlled initiation controlled chain-transferless propagation and controlled (quasiliving) termination. The first part of this presentation will briefly discuss the road to these three key requirements. The second part will concern practical consequences and select systems. The synthesis of narrow-molecular-weight-distribution (M̄_w/M̄_n = 1.1 - 1.3) tert-chlorine telechelic polyisobutylenes over a wide molecular weight range (M̄_n from ∼1000 to ∼125, 000 g/mole) will be outlined together with recent work on aromatic olefins, e.g., styrene, tert-butylstyrene and p-chlorostyrene. These developments led to the combination of these living systems for the synthesis of block copolymers by sequential monomer addition. Tri- or higher block copolymers comprising glassy outer segments and rubbery inner segments, for example, poly(styrene-b-isobutylene-b-styrene, poly(p-chlorostyrene-b-isobutylene-b-p-chlorostyrene), have been prepared. These new thermoplastic elastomers exhibit phase-separated microstructures and an interesting combination of physical-mechanical properties.