Dynamic mechanical and dielectric relaxations of poly(difluorobenzyl methacrylates)

This work reports the mechanical and dielectric relaxation spectra of three difluorinated phenyl isomers of poly(benzyl methacrylate), specifically, poly(2,4‐difluorobenzyl methacrylate), poly(2,5‐difluorobenzyl methacrylate) and poly(2,6‐difluorobenzyl methacrylate). The strength of the dielectric glass–rubber relaxation of the 2,6 difluorinated phenyl isomer is, respectively, nearly three and two times larger than the strengths of the 2,5 and 2,4 isomers. The 2,4 isomer presents a mechanical α peak the intensity of which is nearly two times that of the other two isomers. Both the mechanical and dielectric relaxation spectra display a subglass process, called γ relaxation, centered in the vicinity of −50 °C at 1 Hz and, in some cases, a subglass β absorption is detected at higher temperature partially masked by the glass–rubber relaxation. The mean‐square dipole moments per repeating unit, 〈μ2〉/x, measured at 25 °C in benzene solutions, are 2.5 D², 1.9 D², and 5.0 D² for poly(2,4‐difluorobenzyl methacrylate), poly(2,5‐difluorobenzyl methacrylate) and poly(2,6‐difluorobenzyl methacrylate), respectively. These results, in conjunction with Onsager type equations, permit to conclude that auto and cross‐correlation contributions to the dipolar correlation coefficient may have the same time‐dependence. On the other hand, dipole intermolecular interactions, rather than differences in the flexibility of the chains, seem to be responsible for the relatively high calorimetric glass‐transition temperature of the 2,6 diphenyl isomer, which is, respectively, nearly 36 °C and 32 °C above the T_g's of the 2,4 and 2,5 isomers. Molecular Mechanics calculations give a good account of the differences observed in the polarity of the polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2179–2188, 2000     

Water sorption by poly(tetrahydrofurfuril methacrylate)'s

Polymers consisting of poly(heterocyclic methacrylate)s are considered as potential materials for clinical applications such as drug delivery and cartilage repair. Much of the success of these systems has been attributed to the complex nature of their water sorption properties. Dielectric permittivity is very sensitive to water sorption. Dielectric relaxation spectroscopy studies have been carried out on two heterocyclic poly(methacrylate)s: poly(tetrahydrofurfuryl methacrylate) (PTHFMA) and poly(3‐methyl tetrahydrofurfuryl methacrylate) (P3MTHFMA). The isochrones representing the dielectric losses show in both cases high conductivity at low frequencies and high temperatures. In PTHFMA two conductive processes are observed, which can be associated to the existence of two types of water sorption. These effects have been analyzed and were removed from the dielectric spectra by using classical empirical equations. Both polymers show ostensible α‐relaxation centered in the vicinity of 350 K at 10⁰ Hz. This relaxation was analyzed by means of the empirical Havriliak‐Negami equation. Reminiscent β‐relaxation could also exist. Both polymers present well defined γ and δ subglass absorptions at approximately 120 K, 160 K for PTHFMA and 125 K, 163 K for P3MTHFMA, at 10⁰ Hz, associated to local intramolecular relaxations in side groups. These relaxations were analyzed using semiempirical symmetric model. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 109–120, 2008     

Resonant Raman scattering by N-related local modes in AlGaAs/InGaAsN multiquantum wells

We report resonant Raman scattering and secondary ion mass spectrometry measurements on InGaAsN/AlGaAs multiquantum wells grown by plasma-assisted molecular beam epitaxy. The appearance of a strong TO band at resonance with nitrogen (N)-related electronic levels has been observed. The N-induced vibration mode at 470 cm⁻¹ changes in intensity and shape with increasing N and In content. A new vibration mode has been observed at 320 cm⁻¹, whose intensity scales with the N concentration. This mode is not present in InGaAsN films, so it is linked to the presence of Al. Its frequency is close to the B₁ silent mode of wurtzite GaN. It is attributed to the formation of GaN pairs, near the MQW interfaces as a consequence of the preferential Al–N bonding.     

Job scheduling, cooperation, and control

This paper studies one machine job scheduling situations where clients can have more than one job to be processed and where a job can be of interest for different players. Corresponding cooperative games are introduced and a result on balancedness is provided.     

Contrast reversal and shape changes of atomic adsorbates measured with scanning tunneling microscopy

Systematic, quantitative comparisons between scanning tunneling microscopy (STM) experiments and first principles simulations of O(2 x 2)/Ru(0001) have been performed. The shape of the atomic adsorbates in the images depends strongly on the tunneling resistance and changes reversibly from circular (high resistance) to triangular (low resistance). In addition, after adsorption of oxygen on the STM tip we observe a contrast reversal on the surface, confirmed by extensive numerical simulations.     

Molecular and lamellar orientation in polyethylene thin films

Summary A combined wide (WA) and small angle X-ray diffraction (SAXS) study of melt compressed low density PE samples into the form of very thin films is reported. The WAXD patterns show an uniaxialb axis orientation normal to the film surface which can be interpreted in terms of a row structure in the plane of the film. The analysis of SAXS data indicates, in addition, a preferential orientation of bundles of stacked lamellae parallel to the film surface separated by longitudinal microvoids.With 3 figures     

Influence of the interband transitions on the absorption spectrum of CsBr crystals containing colloids

The contribution of interband transitions to the optical absorption of Cs colloids in a CsBr crystal has been calculated by introducing the expression of Wilson and Butcher for the imaginary part of the dielectric constant of cesium into the standard Mie theory. The resulting absorption bands are smaller and much broader than those corresponding to the free-electron model. The dependence of the calculated band half-width on colloid radius is compared with experimental data. The agreement obtained is considerably better than that corresponding to the sole Drude contribution     

Hardness of nitric acid treated polyethylene followed by recrystallization

The hardness variation of melt crystallized polyethylene as a consequence of controlled fuming nitric exposure has been investigated using the microindentation technique. This study complements previous results obtained using other reagents (H₂SO₄, ClHSO₃). After HNO₃ exposure the microhardness of polyethylene decreases very rapidly, instead of increasing after the first hours of treatment. The hardness decrease is correlated to the volume fraction of interlamellar microvoids arising through selective acid digestion. For longer treatment times (t>40 h) the fragility of the material increases and the sample collapses under the indenter. The hardening of the degraded material after recrystallization from the melt is followed as a function of treatment time. The results are discussed in the light of the molecular mechanisms involved. Comparison of the experimental data with hardness calculations for ideal PE lamellar structures and chain extended dicarboxylic crystals implies that the major contribution to hardening is due to electron dense groups attachment at the surface of a mixed lamellar structure.