David Zax - Academia.edu (original) (raw)
Papers by David Zax
arXiv: Methodology, 2016
Orthogonal Arrays allow us to test various levels of each factor and balance the different factor... more Orthogonal Arrays allow us to test various levels of each factor and balance the different factors so that we can estimate interactions as well as first order effects. There is a trade-off between how well we can sample different levels of each factor and how many interactions we are able to estimate. This paper describes one method to mitigate this trade-off. This method will allow us, with n observations, to sample n levels of each factor and minimize the correlation between the estimates of first order terms and their interactions.
Journal of the American Chemical Society, 2020
In many marine organisms, biomineralizationthe crystallization of calcium-based ionic latticesd... more In many marine organisms, biomineralizationthe crystallization of calcium-based ionic latticesdemonstrates how regulated processes optimize for diverse functions, often via incorporation of agents from the precipitation medium. We study a model system consisting of L-aspartic acid (Asp) which when added to the precipitation solution of calcium carbonate crystallizes the thermodynamically disfavored polymorph vaterite. Though vaterite is at best only kinetically stable, that stability is tunable, as vaterite grown with Asp at high concentration is both thermally and temporally stable, while vaterite grown at 10-fold lower Asp concentration, yet 2-fold less in the crystal, spontaneously transforms to calcite. Solid-state NMR shows that Asp is sparsely occluded within vaterite and calcite. CP-REDOR NMR reveals that each Asp is embedded in a perturbed occlusion shell of ∼8 disordered carbonates which bridge to the bulk. In both the as-deposited vaterites and the evolved calcite, the perturbed shell contains two sets of carbonate species distinguished by their proximity to the amine and identifiable based on 13 C chemical shifts. The embedding shell and the occluded Asp act as an integral until which minimally rearranges even as the bulk undergoes extensive reorganization. The resilience of these occlusion units suggests that large Asp-free domains drive the vaterite to calcite transformationwhich are retarded by the occlusion units, resulting in concentration-dependent lattice stability. Understanding the structure and properties of the occlusion unit, uniquely amenable to ssNMR, thus appears to be a key to explaining other macroscopic properties, such as hardness.
Electrochemical Properties of Clays
The Journal of Physical Chemistry, 1986
energy. Using the values of DT and L, we can calculate an approximate value for the mean time bet... more energy. Using the values of DT and L, we can calculate an approximate value for the mean time between jumps; we find T = These results can be interpreted in the following way: the methanol molecules jump from one site to another, across the channel (they may rotate in a restricted geometry between the jumps), and they diffuse along the channels, after a large number of jumps, with a relatively low diffusion coefficient, IO-' cm2 s-I. This value can be compared to the coefficient obtained for the diffusion of benzene in the channels of the mordenite, 2 X cm2 s-I,l1 and to the value obtained for the diffusion of ethylene in NaX, 3.1 X VI. Discussion On the time scale of IN6, Le., 10-11-10-12 s, we find that eight molecules out of ten are fixed per unit cell at 300 K. This value is in agreement with thermogravimetric results which indicate that seven methanol molecules are adsorbed per unit cell at 307 K on H-ZSM-5.,O Furthermore, our result can be related to the adsorption of ammonia on the same ~e o l i t e ;~ it was found that nine molecules per unit cell are adsorbed at 293 K. In all these studies, it appears that, at room temperature, the number of adsorbed molecules (CH,OH or NH,) is greater than that attributable to the cations or hydroxyl groups. In this work, we have shown that our results are compatible with adsorption geometries B (Figure 3b) or C (Figure 3c). The self-association of the molecules can be discarded since in that case the EISF would drop more rapidly at low Q values. We find that, in our temperature range, the methanol molecules do not react with the surface of H-ZSM-5 to produce methoxyl groups. This OCH, species would give exactly the same EISF as model 4, eq 15, but we have found that, as the temperature increases, a mobile species is progressively observed and this species corresponds to CH,OH. This is in opposition to a methoxylation of the surface since this process appears to be enhanced when the temperature is increased.,I 4 x 10-9 s. cm2 s-l.'
Physical Review B, 1994
Nuclear-magnetic-resonance spectroscopy has been used to probe the electronic structure of the M ... more Nuclear-magnetic-resonance spectroscopy has been used to probe the electronic structure of the M cationic sites in M2Mo6Se6, M=(Li,Rb,Cs,In), a family of low-dimensional electronic conductors. Measured values of the quadrupole coupling parameter constant are compared to calculated values of the contribution from an ionic lattice. The measured values deviate significantly from calculation only for M=In, where the observed value substantially exceeds
Physical Review A, 1989
We describe a method suitable for the design of broadband propagators in two-level or pseudotwo-l... more We describe a method suitable for the design of broadband propagators in two-level or pseudotwo-level systems via amplitude-modulated irradiation. Irradiation sequences are described in terms of a Fourier expansion. The Fourier coefficients of the expansion are used to calculate the time-independent Floquet Hamiltonian which represents the time-dependent Hamiltonian. This transformation removes the difficulty of integrating the time-dependent equations of motion. The infinite Floquet matrix is approximately diagonalized by an application of perturbation theory. A set of infinite matrix operators is introduced which assists in the diagonalization. The perturbation-theory expressions are used to derive approximate expressions for the propagator. Successive terms in the perturbation expansion can be nulled by considering additional Fourier coefficients. Broadband-irradiation sequences with low mean power or low peak power are described corresponding to coherent excitation in two-level systems through the Aip angles 45', 90', 135', and 180'Ĩ 39 5725
Review of Scientific Instruments, 1986
In comparison to high field NMR, zero field techniques offer advantages in terms of spectral inte... more In comparison to high field NMR, zero field techniques offer advantages in terms of spectral interpretability in studies of polycrystalline or amorphous solids. This article describes a technique and apparatus for time-domain measurements of nuclear magnetism in the absence of applied fields (Fourier transform zero field NMR and NQR). Magnetic field cycling and high field detection are employed to enhance sensitivity. The field cycling is accomplished with an air-driven shuttle system which moves the sample between regions of high and low magnetic field, and switchable electromagnets in the low field region. Sudden field steps or pulses are used to initiate coherent nuclear spin evolution in zero field and to monitor such evolution as a function of time. Experimental results are shown and analyzed. Possible variations on the basic method are described and their relative advantages are discussed.
Molecular Physics, 1984
This article may be used for research, teaching, and private study purposes. Any substantial or s... more This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
Magnetic Resonance Imaging, 1988
Macromolecules, 2000
Spider dragline silk is Nature's high-performance protein fiber. This biomaterial ha... more Spider dragline silk is Nature's high-performance protein fiber. This biomaterial has attracted much interest from scientists in various disciplines since it has become feasible to produce spider silk proteins by means of biotechnology. This article reports on research directed ...
Macromolecules, 1995
... Disks prepared for the conductivity measurements were prepared in an inert atmosphere box by ... more ... Disks prepared for the conductivity measurements were prepared in an inert atmosphere box by casting 1.0 M PCA/PCAithium triflate solutions into circular PTFE molds of 12.5 mm diameter and 0.5 mm depth. ... 9, 1995 and ground into a fine powder in an inert atmosphere box. ...
Journal of the American Chemical Society, 1986
The librational motions of the water molecules in polycrystalline barium chlorate monohydrate hav... more The librational motions of the water molecules in polycrystalline barium chlorate monohydrate have been studied using proton and deuterium zero field NHR. In contrast to high field NHR, subtle molecular motions produce readily observable changes in the zero field spectrum. Computer simulations and application of a novel pulsed zero field technique confirm that the splitting observed in the zero field spectrum of the hydrate results from the motionally induced asymmetry of the magnetic dipole-dipole coupling tensor.. .
Journal of the American Chemical Society, 1992
Whether the ground state of 1,2,4,5-tetrakis(methylene)benzene (TMB) biradical (1) is a singlet o... more Whether the ground state of 1,2,4,5-tetrakis(methylene)benzene (TMB) biradical (1) is a singlet or a triplet is of considerable interest.'" This molecule belongs to the class of ''disjointn611
The Journal of Chemical Physics, 1984
The Journal of Chemical Physics, 1985
Methods are described and demonstrated for detecting the coherent evolution of nuclear spin obser... more Methods are described and demonstrated for detecting the coherent evolution of nuclear spin observables in zero magnetic field with the full sensitivity of high field NMR. The principle motivation is to provide a means of obtaining solid state spectra of the magnetic dipole and electric quadrupole interactions of disordered systems without the line broadening associated with random orientation with respect to the applied magnetic field. Comparison is made to previous frequency domain and high field methods. A general density operator formalism is given for the experiments where the evolution period is initiated by a sudden switching to zero field and is terminated by a sudden restoration of the field. Analytical expressions for the signals are given for a variety of simple dipolar and quadrupolar systems and numerical simulations are reported for up to six coupled spin-l/2 nuclei. Experimental results are reported or reviewed for 1H, 20, 7Li, i3C, and 27 AI nuclei in a variety of polycrystalline materials. The effects of molecular motion and bodily sample rotation are described. Various extensions of the method are discussed, including demagnetized initial conditions and correlation by two-dimensional Fourier transformation of zero field spectra with themselves or with high field spectra.
The Journal of Chemical Physics, 1993
The apparent field dependence observed for some isotropic shifts in high resolution solid state N... more The apparent field dependence observed for some isotropic shifts in high resolution solid state NMR is reinvestigated using the Floquet formalism. For t3C-lH systems, second order dipolar shifts are derived that largely agree with the experimental observations made by VanderHart and his theoretical treatment while resolving some difficulties in the use of a secular approximation to handle an inherently time-dependent problem. Additional second order terms are shown to give rise to both frequency shifts and line broadening which can be significant in some spin pairs. This line broadening is found to arise from a second order coupling of the rf field with the IS dipolar interaction.
The Journal of Chemical Physics, 2000
Fluids in nanoscopic confinements possess a variety of unusual properties, and in particular, rem... more Fluids in nanoscopic confinements possess a variety of unusual properties, and in particular, remarkable dynamical heterogeneities which vary on length scales as short as a fraction of a nanometer. While the surface forces apparatus provides an experimental probe of macroscopic properties of fluids in contact with atomically smooth solid surfaces, few experimental probes are available which test the microscopic origins of these heterogeneities. In this article we describe our recent efforts to apply nuclear magnetic resonance spectroscopy to nanoscopically confined poly͑styrene͒ ͑PS͒ created by intercalation into a surface-modified fluorohectorite. A comparison between surface-sensitive cross polarization experiments with spin-echo experiments which probe the entire organic layer suggests that PS in the center of the nanopores is more mobile than the bulk at comparable temperatures, while chain segments which interact with the surface are dynamically inhibited.
Inorganic Chemistry, 2003
The novel quaternary nitride-hydride Ca(6)[Cr(2)N(6)]H was synthesized at 1000 degrees C in seale... more The novel quaternary nitride-hydride Ca(6)[Cr(2)N(6)]H was synthesized at 1000 degrees C in sealed niobium or stainless steel tubes. It crystallizes in the space group R3 (No. 148, Z = 3) with lattice constants (A) a = 9.0042(2) and c = 9.1898(3) and contains the complex anion [Cr(2)N(6)](11)(-) with a short chromium-chromium bond length of 2.26 A. To our knowledge, this is the first example of a non-nitrogen-bridged chromium-chromium dimer in an extended structure compound. Magnetic susceptibility measurements reveal the compound to be paramagnetic at room temperature and with a broad antiferromagnetic ordering centered around 55 K.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2001
Molecular Dynamics (MD) simulations are used to explore the structure and dynamics of polystyrene... more Molecular Dynamics (MD) simulations are used to explore the structure and dynamics of polystyrene confined in 2 nm slit pores, between parallel, crystalline, mica-type surfaces. The systems simulated resemble experimentally studied intercalated nanocomposites, where polystyrene is inserted between layered-silicate layers. The molecular modeling perspective complements the experimental findings and provides insight into the nature of polymers in nanoscopic confinements, especially into the molecular origins of their macroscopic behavior. Namely, a comparison between simulation and NMR studies shows a coexistence of extremely faster and much slower segmental motions than the ones found in the corresponding bulk polymer at the same temperature. The origins of these dynamical inhomogeneities are traced to the confinement induced density modulations inside the 2 nm slits. Fast relaxing phenyl and backbone moieties are found in low density regions across the film, and preferentially in the center, whereas slow relaxing moieties are concentrated in denser regions in the immediate vicinity of the confining surfaces. At the same time, the temperature dependence of the segmental relaxations suggests that the glass transition is suppressed inside the confined films, an observation confirmed by scanning calorimetry.
arXiv: Methodology, 2016
Orthogonal Arrays allow us to test various levels of each factor and balance the different factor... more Orthogonal Arrays allow us to test various levels of each factor and balance the different factors so that we can estimate interactions as well as first order effects. There is a trade-off between how well we can sample different levels of each factor and how many interactions we are able to estimate. This paper describes one method to mitigate this trade-off. This method will allow us, with n observations, to sample n levels of each factor and minimize the correlation between the estimates of first order terms and their interactions.
Journal of the American Chemical Society, 2020
In many marine organisms, biomineralizationthe crystallization of calcium-based ionic latticesd... more In many marine organisms, biomineralizationthe crystallization of calcium-based ionic latticesdemonstrates how regulated processes optimize for diverse functions, often via incorporation of agents from the precipitation medium. We study a model system consisting of L-aspartic acid (Asp) which when added to the precipitation solution of calcium carbonate crystallizes the thermodynamically disfavored polymorph vaterite. Though vaterite is at best only kinetically stable, that stability is tunable, as vaterite grown with Asp at high concentration is both thermally and temporally stable, while vaterite grown at 10-fold lower Asp concentration, yet 2-fold less in the crystal, spontaneously transforms to calcite. Solid-state NMR shows that Asp is sparsely occluded within vaterite and calcite. CP-REDOR NMR reveals that each Asp is embedded in a perturbed occlusion shell of ∼8 disordered carbonates which bridge to the bulk. In both the as-deposited vaterites and the evolved calcite, the perturbed shell contains two sets of carbonate species distinguished by their proximity to the amine and identifiable based on 13 C chemical shifts. The embedding shell and the occluded Asp act as an integral until which minimally rearranges even as the bulk undergoes extensive reorganization. The resilience of these occlusion units suggests that large Asp-free domains drive the vaterite to calcite transformationwhich are retarded by the occlusion units, resulting in concentration-dependent lattice stability. Understanding the structure and properties of the occlusion unit, uniquely amenable to ssNMR, thus appears to be a key to explaining other macroscopic properties, such as hardness.
Electrochemical Properties of Clays
The Journal of Physical Chemistry, 1986
energy. Using the values of DT and L, we can calculate an approximate value for the mean time bet... more energy. Using the values of DT and L, we can calculate an approximate value for the mean time between jumps; we find T = These results can be interpreted in the following way: the methanol molecules jump from one site to another, across the channel (they may rotate in a restricted geometry between the jumps), and they diffuse along the channels, after a large number of jumps, with a relatively low diffusion coefficient, IO-' cm2 s-I. This value can be compared to the coefficient obtained for the diffusion of benzene in the channels of the mordenite, 2 X cm2 s-I,l1 and to the value obtained for the diffusion of ethylene in NaX, 3.1 X VI. Discussion On the time scale of IN6, Le., 10-11-10-12 s, we find that eight molecules out of ten are fixed per unit cell at 300 K. This value is in agreement with thermogravimetric results which indicate that seven methanol molecules are adsorbed per unit cell at 307 K on H-ZSM-5.,O Furthermore, our result can be related to the adsorption of ammonia on the same ~e o l i t e ;~ it was found that nine molecules per unit cell are adsorbed at 293 K. In all these studies, it appears that, at room temperature, the number of adsorbed molecules (CH,OH or NH,) is greater than that attributable to the cations or hydroxyl groups. In this work, we have shown that our results are compatible with adsorption geometries B (Figure 3b) or C (Figure 3c). The self-association of the molecules can be discarded since in that case the EISF would drop more rapidly at low Q values. We find that, in our temperature range, the methanol molecules do not react with the surface of H-ZSM-5 to produce methoxyl groups. This OCH, species would give exactly the same EISF as model 4, eq 15, but we have found that, as the temperature increases, a mobile species is progressively observed and this species corresponds to CH,OH. This is in opposition to a methoxylation of the surface since this process appears to be enhanced when the temperature is increased.,I 4 x 10-9 s. cm2 s-l.'
Physical Review B, 1994
Nuclear-magnetic-resonance spectroscopy has been used to probe the electronic structure of the M ... more Nuclear-magnetic-resonance spectroscopy has been used to probe the electronic structure of the M cationic sites in M2Mo6Se6, M=(Li,Rb,Cs,In), a family of low-dimensional electronic conductors. Measured values of the quadrupole coupling parameter constant are compared to calculated values of the contribution from an ionic lattice. The measured values deviate significantly from calculation only for M=In, where the observed value substantially exceeds
Physical Review A, 1989
We describe a method suitable for the design of broadband propagators in two-level or pseudotwo-l... more We describe a method suitable for the design of broadband propagators in two-level or pseudotwo-level systems via amplitude-modulated irradiation. Irradiation sequences are described in terms of a Fourier expansion. The Fourier coefficients of the expansion are used to calculate the time-independent Floquet Hamiltonian which represents the time-dependent Hamiltonian. This transformation removes the difficulty of integrating the time-dependent equations of motion. The infinite Floquet matrix is approximately diagonalized by an application of perturbation theory. A set of infinite matrix operators is introduced which assists in the diagonalization. The perturbation-theory expressions are used to derive approximate expressions for the propagator. Successive terms in the perturbation expansion can be nulled by considering additional Fourier coefficients. Broadband-irradiation sequences with low mean power or low peak power are described corresponding to coherent excitation in two-level systems through the Aip angles 45', 90', 135', and 180'Ĩ 39 5725
Review of Scientific Instruments, 1986
In comparison to high field NMR, zero field techniques offer advantages in terms of spectral inte... more In comparison to high field NMR, zero field techniques offer advantages in terms of spectral interpretability in studies of polycrystalline or amorphous solids. This article describes a technique and apparatus for time-domain measurements of nuclear magnetism in the absence of applied fields (Fourier transform zero field NMR and NQR). Magnetic field cycling and high field detection are employed to enhance sensitivity. The field cycling is accomplished with an air-driven shuttle system which moves the sample between regions of high and low magnetic field, and switchable electromagnets in the low field region. Sudden field steps or pulses are used to initiate coherent nuclear spin evolution in zero field and to monitor such evolution as a function of time. Experimental results are shown and analyzed. Possible variations on the basic method are described and their relative advantages are discussed.
Molecular Physics, 1984
This article may be used for research, teaching, and private study purposes. Any substantial or s... more This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
Magnetic Resonance Imaging, 1988
Macromolecules, 2000
Spider dragline silk is Nature's high-performance protein fiber. This biomaterial ha... more Spider dragline silk is Nature's high-performance protein fiber. This biomaterial has attracted much interest from scientists in various disciplines since it has become feasible to produce spider silk proteins by means of biotechnology. This article reports on research directed ...
Macromolecules, 1995
... Disks prepared for the conductivity measurements were prepared in an inert atmosphere box by ... more ... Disks prepared for the conductivity measurements were prepared in an inert atmosphere box by casting 1.0 M PCA/PCAithium triflate solutions into circular PTFE molds of 12.5 mm diameter and 0.5 mm depth. ... 9, 1995 and ground into a fine powder in an inert atmosphere box. ...
Journal of the American Chemical Society, 1986
The librational motions of the water molecules in polycrystalline barium chlorate monohydrate hav... more The librational motions of the water molecules in polycrystalline barium chlorate monohydrate have been studied using proton and deuterium zero field NHR. In contrast to high field NHR, subtle molecular motions produce readily observable changes in the zero field spectrum. Computer simulations and application of a novel pulsed zero field technique confirm that the splitting observed in the zero field spectrum of the hydrate results from the motionally induced asymmetry of the magnetic dipole-dipole coupling tensor.. .
Journal of the American Chemical Society, 1992
Whether the ground state of 1,2,4,5-tetrakis(methylene)benzene (TMB) biradical (1) is a singlet o... more Whether the ground state of 1,2,4,5-tetrakis(methylene)benzene (TMB) biradical (1) is a singlet or a triplet is of considerable interest.'" This molecule belongs to the class of ''disjointn611
The Journal of Chemical Physics, 1984
The Journal of Chemical Physics, 1985
Methods are described and demonstrated for detecting the coherent evolution of nuclear spin obser... more Methods are described and demonstrated for detecting the coherent evolution of nuclear spin observables in zero magnetic field with the full sensitivity of high field NMR. The principle motivation is to provide a means of obtaining solid state spectra of the magnetic dipole and electric quadrupole interactions of disordered systems without the line broadening associated with random orientation with respect to the applied magnetic field. Comparison is made to previous frequency domain and high field methods. A general density operator formalism is given for the experiments where the evolution period is initiated by a sudden switching to zero field and is terminated by a sudden restoration of the field. Analytical expressions for the signals are given for a variety of simple dipolar and quadrupolar systems and numerical simulations are reported for up to six coupled spin-l/2 nuclei. Experimental results are reported or reviewed for 1H, 20, 7Li, i3C, and 27 AI nuclei in a variety of polycrystalline materials. The effects of molecular motion and bodily sample rotation are described. Various extensions of the method are discussed, including demagnetized initial conditions and correlation by two-dimensional Fourier transformation of zero field spectra with themselves or with high field spectra.
The Journal of Chemical Physics, 1993
The apparent field dependence observed for some isotropic shifts in high resolution solid state N... more The apparent field dependence observed for some isotropic shifts in high resolution solid state NMR is reinvestigated using the Floquet formalism. For t3C-lH systems, second order dipolar shifts are derived that largely agree with the experimental observations made by VanderHart and his theoretical treatment while resolving some difficulties in the use of a secular approximation to handle an inherently time-dependent problem. Additional second order terms are shown to give rise to both frequency shifts and line broadening which can be significant in some spin pairs. This line broadening is found to arise from a second order coupling of the rf field with the IS dipolar interaction.
The Journal of Chemical Physics, 2000
Fluids in nanoscopic confinements possess a variety of unusual properties, and in particular, rem... more Fluids in nanoscopic confinements possess a variety of unusual properties, and in particular, remarkable dynamical heterogeneities which vary on length scales as short as a fraction of a nanometer. While the surface forces apparatus provides an experimental probe of macroscopic properties of fluids in contact with atomically smooth solid surfaces, few experimental probes are available which test the microscopic origins of these heterogeneities. In this article we describe our recent efforts to apply nuclear magnetic resonance spectroscopy to nanoscopically confined poly͑styrene͒ ͑PS͒ created by intercalation into a surface-modified fluorohectorite. A comparison between surface-sensitive cross polarization experiments with spin-echo experiments which probe the entire organic layer suggests that PS in the center of the nanopores is more mobile than the bulk at comparable temperatures, while chain segments which interact with the surface are dynamically inhibited.
Inorganic Chemistry, 2003
The novel quaternary nitride-hydride Ca(6)[Cr(2)N(6)]H was synthesized at 1000 degrees C in seale... more The novel quaternary nitride-hydride Ca(6)[Cr(2)N(6)]H was synthesized at 1000 degrees C in sealed niobium or stainless steel tubes. It crystallizes in the space group R3 (No. 148, Z = 3) with lattice constants (A) a = 9.0042(2) and c = 9.1898(3) and contains the complex anion [Cr(2)N(6)](11)(-) with a short chromium-chromium bond length of 2.26 A. To our knowledge, this is the first example of a non-nitrogen-bridged chromium-chromium dimer in an extended structure compound. Magnetic susceptibility measurements reveal the compound to be paramagnetic at room temperature and with a broad antiferromagnetic ordering centered around 55 K.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2001
Molecular Dynamics (MD) simulations are used to explore the structure and dynamics of polystyrene... more Molecular Dynamics (MD) simulations are used to explore the structure and dynamics of polystyrene confined in 2 nm slit pores, between parallel, crystalline, mica-type surfaces. The systems simulated resemble experimentally studied intercalated nanocomposites, where polystyrene is inserted between layered-silicate layers. The molecular modeling perspective complements the experimental findings and provides insight into the nature of polymers in nanoscopic confinements, especially into the molecular origins of their macroscopic behavior. Namely, a comparison between simulation and NMR studies shows a coexistence of extremely faster and much slower segmental motions than the ones found in the corresponding bulk polymer at the same temperature. The origins of these dynamical inhomogeneities are traced to the confinement induced density modulations inside the 2 nm slits. Fast relaxing phenyl and backbone moieties are found in low density regions across the film, and preferentially in the center, whereas slow relaxing moieties are concentrated in denser regions in the immediate vicinity of the confining surfaces. At the same time, the temperature dependence of the segmental relaxations suggests that the glass transition is suppressed inside the confined films, an observation confirmed by scanning calorimetry.