Benzyl ether polymers: Crystallinity and pendant adamantyl effects

A series of benzyl ether polymers with and without pendant adamantyl groups has been synthesized. A–B homopolymers with dibenzylether and thioether connecting units were made by phase transfer reaction of 2-(4-bromomethylphenyl)-6-bromomethylbenzoxazole with KOH or Na₂S. The ether-linked material was soluble only in conc. H₂SO₄ while the thioether polymer was only soluble in a mixture of trifluoroethanol and chloroform. The parent AA–BB polyether from resorcinol and α,α′-dichloro-p-xylene (or the dibromo analog) was obtained as a very high molecular weight product which showed multiple crystalline forms depending on sample history. Solid-state ¹³C-NMR spectroscopy and x-ray diffraction patterns were obtained on two samples [each with a single (different) type of crystalline domain], a high molecular weight sample displaying both types, and on amorphous material. Incorporation of pendent adamantyl groups was also examined because of their ability to enormously modify polymer properties. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1743–1751, 1997     

Real time small-angle x-ray scattering during annealing of polymer single crystals

Experiments at the Cornell high energy synchrotron source (CHESS) have shown that it is possible to obtain 2-D small-angle x-ray scattering patterns from single crystal mats of high density polyethylene with a time resolution of 0.3 s. However, it took up to 5 s to heat the 0.1-mm-thick specimen to the annealing temperature. A logarithmic increase of long spacing was quickly established, after an induction time of <2 s at the higher annealing temperatures. At lower temperatures the original reflection remains, weakening, while a continuous scatter to smaller angles builds up. At 10–20 s annealing time a new maximum becomes clear, and then the logarithmic increase of long spacing begins. The intensity of this reflection is intially low and increases with annealing time. On cooling it decreases again. It seems that here we can directly observe two mechanisms of lamellar thickening, melting then recrystallization at short times and diffusional thickening at long times.     

The minimaximin and maximinimax theorems for eigenvalues of semibounded self-adjoint operators

A new result, namely the minimaximin theorem for eigenvalues of a semibounded self-adjoint operator whose spectrum at the bounded end consists entirely of isolated point eigenvalues of finite multiplicities, is established. A particularly simple proof of the maximinimax theorem is also given. The relevance of these results for establishing bounds on the eigenvalues of atomic hamiltonians is explained.     

Direct observation of taut tie molecules in high-strength polyethylene fibers by Raman spectroscopy

The Raman spectra of four gel-spun high-strength polyethylene fibers were recorded as a function of stress, with detailed study of the 1063 cm^?1 band. The change in the peak position of this band was linear at low stresses, but there was little change at high stresses. The center of mass of the band moved linearly with stress until fracture, and the shift per unit stress was almost the same for all the fibers studied at 4.0 ± 0.5 cm^?1/GPa. The line shape is symmetric at low stresses, and the full width at half maximum increases with stress. When plastic deformation begins the band develops a broad low intensity tail extending to 1000 cm^?1. This corresponds to stresses of up to 15 GPa, and the tail contains up to 18% of the band intensity when the maximum stress is applied. Equivalent wide-angle x-ray studies showed no such tail, so the highly stressed material is not crystalline, although it must be in the all-trans form to contribute to the Raman peak. Such load-bearing extended-chain-disordered material is described as taut tie molecules. The amount is much larger than that usually derived from mechanical modelling, it is essentially all the amorphous material if the DSC crystallinity of 85% is correct. It is thought to be interfibrillar, stressed when fibrils slide past each other in creep. On unloading the fiber, the band became more symmetrical again but did not always return to its initial position at zero stress. The peak moved to a wave number greater than its initial value on unloading and then over time relaxed to its original value. Thus tensile stresses in the disordered material, balanced by compression in the crystals, are slow to relax. Lower molecular weight fibers (M_n = 8 × 10⁵) had small anelastic effects, probably because they relaxed within the time of the experiment.     

Effect of drawing on the α relaxation of poly(vinyl alcohol)

The α relaxation of isotropic and drawn poly(vinyl alcohol) dried gel films was studied using dynamic mechanical analysis. The temperature of the relaxation T_α increased from 160°C in the isotropic gel to 220°C in a fiber drawn 19 ×. The relaxation, which is associated with the crystalline regions of the material, also decreased continuously in magnitude as drawing proceeded, although crystallinity increased. At draw ratios over 12 ×, the relaxation became difficult to resolve, and no relaxation was observed in fibers drawn over 19 ×. The melting points of the fibers increased with draw ratio, but not enough to account for the large change in T_α. Crystal thickness in the fiber direction also increased with draw ratio. An analogy is drawn to the case of polyethylene where crystal thickness has been found to control T_α. The absence of a resolvable α relaxation is one reason why it is difficult to draw poly(vinyl alcohol) gels to extremely high ratios.     

Direct imaging of hexaamine-ruthenium(III) in domain boundaries in monolayers of single-stranded DNA

We describe adsorption and identification of the binding sites of [Ru(NH3)6]3+ (RuHex) molecules in a closely packed monolayer of a 13-base ss-DNA on Au(111) electrodes by electrochemical in situ scanning tunneling microscopy (STM), cyclic voltammetry and interfacial capacitance data. In situ STM at single-molecule resolution shows that RuHex adsorbs only at the domain borders and near defects. Together with the electrochemical data that show a negative redox potential shift for RuHex adsorbed to DNA strands, this strongly suggests that RuHex binds only to the exposed phosphate groups in the DNA backbone.     

A method for the determination of speed-dependent semi-classical vector correlations from sliced image anisotropies

We present analytical expressions relating the bipolar moment β(Q)(K)(k(1)k(2)) parameters of Dixon to the measured anisotropy parameters of different pump/probe geometry sliced ion images. In the semi-classical limit, when there is no significant coherent contribution from multiple excited states to fragment angular momentum polarization, the anisotropy of the images alone is sufficient to extract the β(Q)(K)(k(1)k(2)) parameters with no need to reference relative image intensities. The analysis of sliced images is advantageous since the anisotropy can be directly obtained from the image at any radius without the need for 3D-deconvolution, which is not applicable for most pump/probe geometries. This method is therefore ideally suited for systems which result in a broad distribution of fragment velocities. The bipolar moment parameters are obtained for NO(2) dissociation at 355 nm using these equations, and are compared to the bipolar moment parameters obtained from a proven iterative fitting technique for crushed ion images. Additionally, the utility of these equations in extracting speed-dependent bipolar moments is demonstrated on the recently investigated NO(3) system.     

Poles of Zeta and Eta Functions for Perturbations¶of the Atiyah–Patodi–Singer Problem

The zeta and eta functions of a differential operator of Dirac-type on a compact n-dimensional manifold, provided with a well-posed pseudodifferential boundary condition, have been shown in [G99] to be meromorphic on ℂ with simple or double poles on the real axis. Extending results from [G99] we show how perturbations of the boundary condition of order −J affect the poles; in particular they preserve a possible regularity of zeta at 0 and a possible simple pole of eta at 0 when J≥n. This applies to perturbations of spectral boundary conditions, also when the structure is non-product and the problem is non-selfadjoint. Received: 4 October 1999 / Accepted: 7 July 2000     

Temperature dependence of the laser-initiated polymerization of a thiol-ene system

The laser-initiated polymerization of a thiol-ene photopolymerizable system was studied. The laser was operated in single- and multiple-pulse modes to generate exotherms, which were measured in a modified differential scanning calorimeter. Arrhenius plots of the polymerization, obtained by measuring polymerization exotherms at various temperatures, showed positive and negative portions. Pseudoactivation energies for the thiol-ene polymerization increased rapidly at low conversions (<50%) of the thiol and ene chromophores. The results were comparable to those obtained from photopolymerization of multifunctional acrylate monomers.