Structural rigidity and low frequency vibrational modes of long carbon tubules

We have studied the low frequency vibrational modes and the structural rigidity of long graphitic carbon tubules consisting of 100, 200, and 400 atoms. Our calculations have been performed using an empirical Keating Hamiltonian with parameters determined from first principles. We have found the beam bending mode to be one of the softest modes in these structures. The corresponding beam rigity of a bucky tube is compared to an found to exceed the highest values found in presently available materials.     

Atomic-scale contrast mechanism in atomic force microscopy

We present numerical results for the ground state and low-lying excited states of theS=1 antiferromagnetic Heisenberg chain with exchange and single-ion anisotropy for free boundary conditions and up to 16 spins. The excitation spectrum in the Néel and Haldane phases is interpreted successfully in terms of domain walls (i.e. solitons) only. We present evidence that the transition between these two phases is characterized by the condensation of the lowest soliton modes. The difference between periodic and open boundary conditions, in particular the relevance of open boundary conditions for numerical calculations on finite chains, is discussed.     

Shear modulation force microscopy study of near surface glass transition temperatures

We report results of glass transition (T(g)) measurements for polymer thin films using atomic force microscopy (AFM). The AFM mode, shear modulation force microscopy (SMFM), involves measuring the temperature-dependent shear force on a tip modulated parallel to the sample surface. Using this method we have measured the surface T(g) of thin (17-500 nm) polymer films and found that T(g) is independent of film thickness (t>17 nm), strength of substrate interactions, or even presence of substrate.     

A mathematical study of sample modulation at a membrane inlet mass spectrometer—Potential application in analysis of mixtures

An ANSI C program that simulates the diffusion profiles of sample modulation at a membrane inlet system has been developed to study the characteristics of modulated diffusion profiles. The program produces concentration profiles within the membrane and flux values at the exit side of the membrane as a function of time. Sample concentration on the inlet side can be switched between zero and an arbitrary value with a square or asymmetric cycle. Achievement of steady-state diffusion between alternations is not required. With this computer simulation, the flux profiles of analytes through a membrane inlet have been studied as a function of diffusion coefficient, modulation frequency, and concentration. The amplitude, shape, and time lag or phase angle of the flux profile are shown to be related directly to analyte concentration and diffusivity. A method that involves a set of linear equations is proposed to resolve mixtures of diffusing analytes based on differences in the time dependence of their flux profiles.     

Interfacial mobility and bonding strength in nanocomposite thin film membranes

The interfacial interaction strength and transition properties in a reverse selective thin film nanocomposite system, silica-poly[(trimethylsilyl)propyne] (SiO(x)-PTMSP), are investigated locally by heated tip atomic force microscopy. SiO(x)-PTMSP has recently been introduced as a new class of reverse selective membrane materials with extraordinarily high permeability and selectivity (reverse selectivity). Here, we examine the thermal transition properties of the polymer matrix and the debonding strength between PTMSP and silica. Transitions at 330 degrees C were identified as degradation processes. Criteria for debonding were found to include polymer viscoelastic responses, particle size, embedding depth, scan speed, and frequency of impact. Probe-particle impact forces revealed a debonding energy of 2.6 J/m(2) and an impact force transition that occurs 30 degrees C below the degradation temperature in the neat polymer, confirming the presence of enhanced polymer mobility at the SiO(x)-PTMSP interface.     

Investigations of heterogeneous ultrathin blends using lateral force microscopy

In order to study the glass transition of thin film polymer blends, high spatial resolution and temperature sensitivity is needed. In this paper, we emphasize the importance of the calibration of scanning parameters such as load and speed when measuring the glass transition temperature of polymers using lateral force microscopy. Once calibrated, this method is ideal for investigations of heterogeneous samples such as blends and co‐polymers. We present an analysis technique for lateral force imaging using a fast and stable cooling/heating stage. This approach involves mapping the friction forces over a certain area and identifying regions of different frictional properties. The difference in the average friction force can then be plotted as a function of temperature. The friction force is expected to vary around the glass transition. Therefore, the glass transition temperature can be defined as the temperature at which the difference in the average friction force undergoes a slope change. We present investigations of blends using polystyrene mixed with poly(butylmethacrylate). The transition temperatures obtained are in good agreement with the bulk values of corresponding homopolymeric films.