Complexities associated with moisture in foamed polysiloxane composites

An understanding of mechanisms of moisture outgassing from silicones and the impact on material mechanical properties is important for compatibility and life prediction in sealed systems containing these materials. A series of thermomechanical (TMA) stress relaxation experiments have been performed to provide information on the important load bearing properties of these materials as a function of time and temperature. Two different silica reinforced foamed polysiloxane materials were tested, a peroxide cured rubber (M97) and a condensation-cured elastomer (S5370). The M97 foam showed unexpectedly complex stress relaxation profiles at temperatures around 100 °C, whereas the S5370 samples showed the expected smooth stress decay behaviour. Dried M97 foam samples show different stress relaxation behaviour to the non-dried materials. Furthermore, stress relaxation studies performed in controlled humidity environments showed that moisture has a significant accelerating influence on the underlying relaxation process. In dry regimes, a reduced stress relaxation rate was observed, with an increase in the force required to maintain a given amount of compression on the sample. To further develop our understanding of the effects of moisture, we have exposed samples to water enriched to 40% in ¹⁷O and used ¹⁷O nuclear magnetic resonance (NMR) spectroscopy to assess labelled hydrolysis reaction products. Our studies show that Si-¹⁷O-Si hydrolysis products are readily incorporated in the polymer and the degradation is enhanced by the influence of gamma radiation and/or heat. In addition, the polysiloxane foams showed different age related trends in sealed (where moisture is retained) and ventilated (open-to-air) regimes. Our observations have been explained by moisture influencing both physical and chemical degradation processes. Our findings on moisture induced changes in silicone stress relaxation rates are novel and demonstrate the importance of controlling humidity in service applications involving these materials.     

An investigation of radiation-induced structural changes in nitrile rubber

The mechanism of radiation-induced structural changes in nitrile rubber with different acrylonitrile contents were investigated by ESR, NMR, and FTIR. To investigate new structures solid-state NMR methods had to be used due to crosslinking of the irradiated rubbers, and higher probe temperatures were used to obtain better resolution. The radicals generated on the acrylonitrile groups were found to abstract hydrogen from the adjacent butadiene units resulting in the formation of allylic radicals. These allylic radicals reacted to form intermolecular crosslinks and cyclisation. Cyclisation of the butadiene units were found to occur in the initial stages of the irradiation. Radiation yields of radicals increased with acrylonitrile content from 1.42, 1.58, to 2.42 for 18, 30, and 45% acrylonitrile rubbers. The radiation yields for intermolecular crosslinking were higher in rubbers with higher acrylonitrile contents, giving G values of 17.8, 21.3, and 24.5 for 18, 30, and 45% acrylonitrile rubbers, respectively. However, the crosslink clustering was found to be less in the rubbers with a higher acrylonitrile content. © 1996 John Wiley & Sons, Inc.     

Investigation of aging behavior and mechanism of nitrile-butadiene rubber (NBR) in the accelerated thermal aging environment

Nitrile-butadiene rubber (NBR) was exposed to an accelerated thermal aging environment produced by an air-circulating oven for different time periods. NBR aging was evaluated by morphology, crosslink density, mechanical properties, chemical changes and thermal stability. The results showed that the surface damage of NBR turned severe and inhomogeneous, and the aging degree was most serious on the edge region of voids. Crosslinking reactions mainly occurred in the aging process. The tensile strength increased with increase in crosslink density up to a maximum value and thereafter decreased with further increase in crosslink density. X-ray Photoelectron Spectroscopy (XPS) and Pyrolysis Gas Chromatography-Mass Spectrometry (Py-GC/MS) analysis demonstrated that hydroxyl groups were formed and the additives migrated from inner to surface of NBR samples. In addition, the thermogravimetric analysis (TGA) indicated that the thermal stability of NBR did not significantly change in the accelerated thermal aging environment.     

Proton diffusion in hybrid materials of CsHSO4 and silica nanoparticles as studied by H solid-state NMR

Hybrid materials of CsHSO₄ and silica nanoparticles were prepared by mechanical milling, and hydrogen bond states and proton dynamics were studied by means of ¹H solid-state NMR. ¹H MAS NMR spectra demonstrated that three types of domains are present in the milled materials. Domain A has hydrogen bond states similar to those in the bulk compound. With respect to hydrogen bonds, domains A-II and A-III are similar to phases II and III of CsHSO₄, respectively. Protons in domain A-II undergo translational diffusion, and the diffusion is faster than in phase II of bulk CsHSO₄. Domain B is originated by mixing of CsHSO₄ with silica nanoparticles, presumably locating at the boundary region. Protons in this domain also undergo translational diffusion. The motional rate is faster than in phase II of bulk CsHSO₄ but is slower than in domain A-II. In domain C protons are contained as OH groups on the surface of silica nanoparticles. Protons are immobile in this domain.     

Characterization of the monovalent ion position and hydrogen-bond network in guanine quartets by DFT calculations of NMR parameters

Conformational stability of G-quartets found in telomeric DNA quadruplex structures requires the coordination of monovalent ions. Here, an extensive Hartree-Fock and density functional theory analysis of the energetically favored position of Li+, Na+, and K+ ions is presented. The calculations show that at quartet-quartet distances observed in DNA quadruplex structures (3.3 A), the Li+ and Na+ ions favor positions of 0.55 and 0.95 A outside the plane of the G-quartet, respectively. The larger K+ ion prefers a central position between successive G-quartets. The energy barrier separating the minima in the quartet-ion-quartet model are much smaller for the Li+ and Na+ ions compared with the K+ ion; this suggests that K+ ions will not move as freely through the central channel of the DNA quadruplex. Spin-spin coupling constants and isotropic chemical shifts in G-quartets extracted from crystal structures of K+- and Na+-coordinated DNA quadruplexes were calculated with B3LYP/6-311G(d). The results show that the sizes of the trans-hydrogen-bond couplings are influenced primarily by the hydrogen bond geometry and only slightly by the presence of the ion. The calculations show that the R(N2N7) distance of the N2-H2...N7 hydrogen bond is characterized by strong correlations to both the chemical shifts of the donor group atoms and the (h2)J(N2N7) couplings. In contrast, weaker correlations between the (h3)J(N1C6') couplings and single geometric factors related to the N1-H1...O6=C6 hydrogen bond are observed. As such, deriving geometric information on the hydrogen bond through the use of trans-hydrogen-bond couplings and chemical shifts is more complex for the N1-H1...O6=C6 hydrogen bond than for the N2-H2...N7 moiety. The computed trans-hydrogen-bond couplings are shown to correlate with the experimentally determined couplings. However, the experimental values do not show such strong geometric dependencies.     

Molecular Level Characterization of the Structure and Interactions in Peptide‐Functionalized Metal–Organic Frameworks

We use density functional theory, newly parameterized molecular dynamics simulations, and last generation ¹⁵N dynamic nuclear polarization surface enhanced solid‐state NMR spectroscopy (DNP SENS) to understand graft–host interactions and effects imposed by the metal–organic framework (MOF) host on peptide conformations in a peptide‐functionalized MOF. Focusing on two grafts typified by MIL‐68‐proline ( ‐Pro ) and MIL‐68‐glycine‐proline ( ‐Gly‐Pro ), we identified the most likely peptide conformations adopted in the functionalized hybrid frameworks. We found that hydrogen bond interactions between the graft and the surface hydroxyl groups of the MOF are essential in determining the peptides conformation(s). DNP SENS methodology shows unprecedented signal enhancements when applied to these peptide‐functionalized MOFs. The calculated chemical shifts of selected MIL‐68‐NH‐ Pro and MIL‐68‐NH‐ Gly‐Pro conformations are in a good agreement with the experimentally obtained ¹⁵N NMR signals. The study shows that the conformations of peptides when grafted in a MOF host are unlikely to be freely distributed, and conformational selection is directed by strong host–guest interactions.     

Charge Transfer Complex Role in the Formation of Chlorobenzene in the γ‐Irradiated Carbon Tetrachloride‐Benzene System

The formation of carbon tetrachloride‐benzene charge transfer complex was confirmed by UV and NMR spectrometric studies. A change in UV spectrum of benzene is observed upon addition of carbon tetrachloride. Whereas the appearance of new bands supports the formation of charge transfer complex. NMR study shows that, chemical shift of benzene pmr signal depends on the CCl₄‐C₆H₆ molar ratio. This observation is another criterion for the formation of benzene‐carbon tetrachloride charge transfer complex. Job's Continuous Variation method indicates that a 2:1 CCl₄‐C₆H₆ charge transfer complex (2:1 CTC) is formed. The association constants (K_2:1) of (2:1 CTC) was found to be 0.0197 M^?2. The maximum concentration of (2:1 CTC) was found to be in samples with 2:1 CCl₄‐C₆H₆ molar ratio (33% benzene mole). On the other hand the maximum yield of chlorobenzene was obtained, also, upon radiolysis of CCl₄‐C₆H₆ samples at a 2:1 molar ratio (33% benzene mole). Therefore, it could be concluded that (2:1 CTC) participates in the formation of chlorobenzene upon radiolysis of the benzene‐carbon tetrachloride system. This conclusion was supported by the dependence of the chlorobenzene yield of a γ‐irradiated carbon tetrachloride‐benzene system (2:1 molar ratio) on irradiation time according to a third order kinetic equation with a very good linearity (R² = 0.9977). Accordingly, the rate constant for the chlorobenzene formation under this condition was found to be ≈ 5.5 × 10^?7 L².mol^?2.h^?1. We propose a radiation chemical mechanism in which the 2:1 CTC plays a role in the formation of chlorobenzene.     

Reactions with Oleum under Harsh Conditions: Characterization of the Unique [M(S2O7)3]2− Ions (M=Si,Ge, Sn) in A2[M(S2O7)3] (A=NH4, Ag)

The reactions of group 14 tetrachlorides MCl₄ (M=Si, Ge, Sn) with oleum (65 % SO₃) at elevated temperatures lead to the unique complex ions [M(S₂O₇)₃]^2?, which show the central M atoms in coordination with three chelating S₂O₇^2? groups. The mean distances M? O within the anions increase from 175.6(2)–177.5(2) pm (M=Si) to 186.4(4)–187.7(4) pm (M=Ge) to 201.9(2)–203.5(2) pm (M=Sn). These distances are reproduced well by DFT calculations. The same calculations show an increasing positive charge for the central M atom in the row Si, Ge, Sn, which can be interpreted as the decreasing covalency of the M? O bonds. For the silicon compound (NH₄)₂[Si(S₂O₇)₃], ²⁹Si solid‐state NMR measurements have been performed, with the results showing a signal at ?215.5 ppm for (NH₄)₂[Si(S₂O₇)₃], which is in very good agreement with theoretical estimations. In addition, the vibrational modes within the [MO₆] skeleton have been monitored by Raman spectroscopy for selected examples, and are well reproduced by theory. The charge balance for the [M(S₂O₇)₃]^2? ions is achieved by monovalent A⁺ counter ions (A=NH₄, Ag), which are implemented in the syntheses in the form of their sulfates. The sizes of the A⁺ ions, that is, their coordination requirements, cause the crystallographic differences in the crystal structures, although the complex [M(S₂O₇)₃]^2? ions remain essentially unaffected with the different A⁺ ions. Furthermore, the nature of the A⁺ ions influences the thermal behavior of the compounds, which has been monitored for selected examples by thermogravimetric differential thermal analysis (DTA/TG) and XRD measurements.     

Fluorescence Studies of Solubilization and Dye Binding in Hypercoiled Poly(methacrylic Acid): A Connected Cluster Model

The compact conformation of poly(methacrylic acid) which is observed in the unneutralized polymer has been studied using the fluorescence probes 9-methyl anthracene and Rhodamine B. Steady-state and time-resolved fluorescence anisotropy measurements have been made. The results show that this conformation contains a number of binding sites which have molecular weights in the range 9,000–10,000 and which can rotate independently of the remainder of the chain.     

Characterization of carbon nanotubes and analytical methods for their determination in environmental and biological samples: A review

In the present paper, a critical overview of the most commonly used techniques for the characterization and the determination of carbon nanotubes (CNTs) is given on the basis of 170 references (2000–2014). The analytical techniques used for CNT characterization (including microscopic and diffraction, spectroscopic, thermal and separation techniques) are classified, described, and illustrated with applied examples. Furthermore, the performance of sampling procedures as well as the available methods for the determination of CNTs in real biological and environmental samples are reviewed and discussed according to their analytical characteristics. In addition, future trends and perspectives in this field of work are critically presented.