排序方式: 共有16条查询结果,搜索用时 603 毫秒
1.
ABSTRACT Copolymers of 1,5-dioxepan-2-one (DXO) and e-caprolactone (?-CL), δ-valerolactone (δ-VL) or L-lactide (LLA) have been synthesized and characterized. High molecular weight copolymers were obtained using stannous-2-ethyl hexanoate as catalyst in bulk. Reactivity ratios for the copolymerization of DXO and δ-VL were determined at 110°C as rVL=0.5 and rDXO=2.3. At high conversion, depolymerization of δ-VL occurred, resulting in lower molecular weight and variations in the copolymer composition. Physical properties, such as crystallinity and melting temperature of the DXO-copolymers proved to be strongly dependent on the choice of comonomer and on the molar composition of the copolymers. DXO appears to be incorporated into the poly-?-caprolactone (PCL) crystals and to some extent into the poly-δ-valerolactone (PVL) crystals, resulting in a more gradual decrease in crystallinity with increasing amount of DXO. 相似文献
2.
RD Harris WS Baker Van Stipdonk MJ RM Crooks EA Schweikert 《Rapid communications in mass spectrometry : RCM》1999,13(14):1374-1380
A suite of keV polyatomic or 'cluster' projectiles was used to bombard unoxidized and oxidized self-assembled monolayer surfaces. Negative secondary ion yields, collected at the limit of single ion impacts, were measured and compared for both molecular and fragment ions. In contrast to targets that are orders of magnitude thicker than the penetration range of the primary ions, secondary ion yields from polyatomic projectile impacts on self-assembled monolayers show little to no enhancement when compared with monatomic projectiles at the same velocity. This unusual trend is most likely due to the structural arrangement and bonding characteristics of the monolayer molecules with the Au(111). Copyright 1999 John Wiley & Sons, Ltd. 相似文献
3.
A Mukherjee M Dasgupta DJ Hinde CR Morton AC Berriman RD Butt JO Newton H Timmers 《Pramana》2001,57(1):195-198
Fusion cross-sections for the 7Li + 12C reaction have been measured at energies above the Coulomb barrier by the direct detection of evaporation residues. The heavy
evaporation residues with energies below 3 MeV could not be separated out from the α-particles in the spectrum and hence their
contribution was estimated using statistical model calculations. The present work indicates that suppression of fusion cross-sections
due to the breakup of 7Li may not be significant for 7Li + 12C reaction at energies around the barrier. 相似文献
4.
5.
6.
Burrell KH Ejima S Schissel DP Brooks NH Callis RW Carlstrom TN Colleraine AP DeBoo JC Fukumoto H Groebner RJ Hill DN Hong R Hosogane N Jackson GL Jahns GL Janeschitz G Kellman AG Kim J Lao LL Lee P Lohr JM Luxon JL Mahdavi MA Moeller CP Ohyabu N Osborne TH Overskei D Petersen PI Petrie TW Phillips JC Prater R Scoville JT Seraydarian RP Shimada M Sleaford BW Snider RT Stambaugh RD Stav RD St John HE Stockdale R Strait EJ Taylor T Tooker JF Yamaguchi S 《Physical review letters》1987,59(13):1432-1435
7.
LANGLEY J. K. 《中国科学 数学(英文版)》2010,(3)
We determine all real meromorphic functions f in the plane such that f has finitely many zeros, the poles of f have bounded multiplicities, and f and F have finitely many non-real zeros, where F is a linear differential polynomial given by F = f (k) +Σk-1j=0ajf(j) , in which k≥2 and the coefficients aj are real numbers with a0≠0. 相似文献
8.
Nonlinear elastic effects play an important role in the dynamics of microelectromechanical systems (MEMS). A Duffing oscillator
is widely used as an archetypical model of mechanical resonators with nonlinear elastic behavior. In contrast, nonlinear dissipation
effects in micromechanical oscillators are often overlooked. In this work, we consider a doubly clamped micromechanical beam
oscillator, which exhibits nonlinearity in both elastic and dissipative properties. The dynamics of the oscillator is measured
in both frequency and time domains and compared to theoretical predictions based on a Duffing-like model with nonlinear dissipation.
We especially focus on the behavior of the system near bifurcation points. The results show that nonlinear dissipation can
have a significant impact on the dynamics of micromechanical systems. To account for the results, we have developed a continuous
model of a geometrically nonlinear beam-string with a linear Voigt–Kelvin viscoelastic constitutive law, which shows a relation
between linear and nonlinear damping. However, the experimental results suggest that this model alone cannot fully account
for all the experimentally observed nonlinear dissipation, and that additional nonlinear dissipative processes exist in our
devices. 相似文献
9.
The nonlinear dynamical behavior of a micromechanical resonator acting as one of the mirrors in an optical resonance cavity is investigated. The mechanical motion is coupled to the optical power circulating inside the cavity both directly through the radiation pressure and indirectly through heating that gives rise to a frequency shift in the mechanical resonance and to thermal deformation. The energy stored in the optical cavity is assumed to follow the mirror displacement without any lag. In contrast, a finite thermal relaxation rate introduces retardation effects into the mechanical equation of motion through temperature dependent terms. Using a combined harmonic balance and averaging technique, slow envelope evolution equations are derived. In the limit of small mechanical vibrations, the micromechanical system can be described as a nonlinear Duffing-like oscillator. Coupling to the optical cavity is shown to introduce corrections to the linear dissipation, the nonlinear dissipation and the nonlinear elastic constants of the micromechanical mirror. The magnitude and the sign of these corrections depend on the exact position of the mirror and on the optical power incident on the cavity. In particular, the effective linear dissipation can become negative, causing self-excited mechanical oscillations to occur as a result of either a subcritical or supercritical Hopf bifurcation. The full slow envelope evolution equations are used to derive the amplitudes and the corresponding oscillation frequencies of different limit cycles, and the bifurcation behavior is analyzed in detail. Finally, the theoretical results are compared to numerical simulations using realistic values of various physical parameters, showing a very good correspondence. 相似文献
10.
Baskin I Zaitsev S Lipson D Gilad R Keren K Ben-Yoseph G Sivan U 《Physical review letters》2006,97(20):208103
A novel algorithmic approach to the synthesis of fairly long DNA molecules with nonrecurring sequences is demonstrated. The scheme exploits chemical embodiment of shift registers (SR) to execute algorithms similar to those used to generate pseudorandom numbers on a computer. Single stranded DNA molecules guide the synthesis of double stranded DNA according to the SR truth table. The SR logic facilitates an exponentially smaller synthesis effort compared with all other strategies. A redundancy based scheme, similar to those used in communication, is utilized to suppress synthesis errors. 相似文献