Nicholas Zumbulyadis

In situ polymerization of tetraethoxysilane in polymers: chemical nature of the interactions

Abstract The formation of molecular composites by the in situ polymerization of tetraethoxysilane in various organic polymers has been studied. The results show that highly homogeneous, transparent composites can be formed for polymers such as poly (methyl methacrylate), poly (vinyl acetate), poly (vinyl pyrrolidone), and poly (N,N-dimethylacrylamide). These composites have improved mechanical properties and increased solvent resistance. Strong interactions between the SiO2 networks and these polymers were observed, and the data suggest hydrogen bonding between residual hydroxyls on the SiO2 and carbonyl groups on the polymer chains as the major source of these interactions.

Quadrupole effects in the magic‐angle‐spinning spectra of spin‐1/2 nuclei

Magic‐angle spinning fails to eliminate the effect of dipolar coupling on the spectrum of a spin‐1/2 nucleus (e.g., 13C ) coupled to a quadrupolar nucleus (e.g., 14N). A quantitative theory of this phenomenon, based on an adiabatic approximation, is presented, together with numerical simulations of spectra. The spectrum is sensitive to the sign and magnitude of the electric‐field gradient at the quadrupolar nucleus, the angles between principal axes of the dipolar and quadrupolar interaction tensors, and the orientation of the spinning axis with respect to the external magnetic field.

Selective carbon excitation and the detection of spatial heterogeneity in cross-polarization magic-angle-spinning NMR

Abstract The solid-state 13 C NMR spectra of heterogeneous materials are often quite complex owing to superposition of signals from the various domains in the sample. Here a new class of modified cross-polarization experiments is described that can be used to selectively excite carbons in different domains of an inhomogeneous solid, provided the domains differ in their proton relaxation properties. The techniques rely on bringing the 13 C spins into thermal contact with a “tailored” proton magnetization. Applications include selective observation of spectra from crystalline and amorphous domains of poly(ethylene terephthalate) and the suppression of spinner signals.

A hybrid quantum mechanical and empirical model for the prediction of isotropic 13C shielding constants of organic molecules

A method is presented that combines quantum mechanical shift calculations with empirical corrections to yield isotropic 13C nuclear magnetic resonance (NMR) shifts for organic molecules in good agreement with experiment. A comparison is made between shifts calculated using Hartree–Fock (HF), Moller–Plesset perturbation theory (MP2), and density functional theory (DFT). The absolute shifts calculated by these methods are translated into shifts relative to tetramethylsilane (TMS) using a simple empirical formula with parameters determined over a set of 37 small organic compounds. It is shown that DFT calculations using small basis sets correlate with experiments well enough that the empirical correction allows experimental shifts to be reproduced to within an RMS error of 4–5 parts per million (ppm). Carbons attached to chlorine, bromine, and iodine are treated with the same empirical corrections but with parameters of different values because of the lack of spin orbit corrections in the calculations; however, these carbons are predicted as accurately as other carbons in the data set. Two models are presented; one is applicable to very large molecules. The empirical corrections developed for these models can be used to predict shifts in a wide variety of organic molecules. One of the models is applied to a moderately sized dye molecule that contains an intramolecular hydrogen bond to demonstrate the utility of using an inexpensive quantum mechanics-based method over an empirical fragment-based method.

1H/29Si cross‐polarization dynamics in amorphous hydrogenated silicon

A modified cross‐polarization method is used to simplify the solid‐state magic‐angle spinning 29Si‐NMR spectrum of a‐Si:H. At least three magnetically distinct environments are resolved. A microscopic model for the cross‐polarization dynamics is used to assign the components of the spectrum. Contrary to expectations from solution state studies of low‐molecular‐weight silanes, the signals from 29Si bonded directly to hydrogen are shifted to a lower field compared to quaternary Si. This observation is rationalized on the basis of theoretical models and photoemission spectroscopy results published by other groups. A suggestion for the nature of shallow states is offered on the basis of the NMR results.

Applications of the direct exponential curve resolution algorithm (DECRA) to solid state nuclear magnetic resonance and mid-infrared spectra

DECRA (direct exponential curve resolution algorithm) is a fast multivariate method used to resolve spectral data with concentration profiles that are linear combinations of exponential functions. DECRA has been previously applied to a wide variety of spectroscopies. Results are presented in this paper for two new application areas: solid state nuclear magnetic resonance spectra of polymorphic crystal mixtures and mid‐infrared spectroscopy of chemical reactions. Furthermore, the paper will show the effect of the way the data set is split, which is a part of the algorithm, on the results. Copyright

29Si dynamic nuclear polarization of dehydrogenated amorphous silicon

Abstract 29Si NMR spectra were obtained on a dehydrogenated amorphous silicon sample by means of dynamic nuclear polarization (DNP). The dependence of the DNP enhancement factor, which peaked at about 40, upon the microwave frequency offset (ω - ωe) from the electron spin Larmor frequency is essentially antisymmetric about ω - ωe = 0, which corresponds to a g value of 2.0059. This observation shows that the paramagnetic centers are fixed, immobile on the time scale of electron spin larmor precession, requiring a reexamination of recently advanced models of the paramagnetic defects. Prospects for the application of DNP in the study of amorphous silicon materials are discussed.

Two-dimensional MAS NMR of abundant spins in solids

Two-dimensional incoherent mixing experiments (I) have been used to determine the proximity of protons in solution (NOESY) (2), elucidate spin dilfusion mechanisms among dilute spins in solids (3), and determine microscopic heterogeneity in multiphase systems such as mixtures (4) and polymer blends (5). In the latter two cases, the experiments were performed with abundant spins (protons). A requirement for such experiments is the resolution of the sites between which spin exchange takes place. In the case of abundant spins, the necessary resolution has been achieved either by selecting systems with inherent mobility (4) or by the application of multiple-pulse homonuclear dipolar decoupling (5). The use of magic-angle spinning (MAS) to enhance the resolution of abundant spins in conjunction with 2D NOE experiments has not been widely explored (6). This is in part because the very fast spinning speeds required have not been available until very recently. Here, we report preliminary results from combined 19F MAS/NOESY experiments on the fluorinated polyphosphazene 1 synthesized in our laboratories.

High-Mobility Doped Polymers

Hole mobilities have been measured in 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) doped in a series of segmented thermoplastic polymers. For TAPC concentrations of 25 wt%, the mobilities are as high as 3×10-3 cm2/Vs. To our knowledge, these values are a factor of 100 higher than any hole mobilities described in the literature for this dopant concentration.

A simple model for deuterium cross-polarization magic-angle spinning nuclear magnetic resonance at the interphases of amorphous materials

The structure and composition of the interphase at the boundary of two immiscible phases has long been the subject of experimental and theoretical studies in polymer science. Cross-polarization between protons and deuterons offers the potential for elucidating the composition of the interphase if one of the immiscible phases is deuterated. A prerequisite for such an analysis is the establishment of an experimental protocol for reliable spin counting in 1H-2H cross-polarization magic-angle spinning (CP-MAS) spectroscopy. In this paper we present a simple model for the quantitative analysis of deuterium CP-MAS spectra. The model will be applied to the characterization of the polystyrene-b-poly(methyl methacrylate) interphase in a subsequent publication.