聚甲基丙烯酸甲酯热氧化降解的化学动力学研究

使用质谱、热分析手段研究了PMMA热解反应 .结果表明 ,在氮气中 ,PMMA -CH =CH2 有两个失重阶段 ,分别对应于主链末端双键引发的断链和主链无规则断链反应 ,转折点的失重率约为 2 6 % .其中 ,第一阶段的失重速率受扩散过程控制 ,平均表观活化能E为 15 8.5kJ/mol,lnA为 2 7.6 9;第二失重阶段为 1.5级化学反应 ,平均表观活化能E为 2 14 .79kJ/mol,lnA为 4 0 .4 6 .在空气中 ,PMMA也有两个失重阶段 ,反应机理为 1级化学反应 ,转折点处的失重率约为 70 % .其中在第一失重阶段平均表观活化能E为 130 .32kJ/mol,lnA为 2 4 .81,在此阶段中 ,过氧化基团的分解反应对PMMA的失重速率有重要影响 ;在空气中第二失重阶段平均表观活化能E为 78.2 5kJ/mol,lnA为 13.97.     

Fundamental relationship between the nonequilibrium glassy state and yield stress of amorphous polymers

This paper reports the theoretical prediction and experimental verification of the connection between the yield stress of amorphous polymers and the physical aging phenomenon. The analysis reveals the existence of a fundamental relationship between the nonequilibrium glassy state and the thermally activated process controlling viscoelastic and plastic deformation. The results show that the volume relaxation and deformation kinetics share the same relaxation times, and that the activation energy for deformation below T_g is much smaller than previously mentioned in the literature. This indicates that the phenomenon of physical aging plays a very important role in the deformation and processing of polymers at low temperatures. The effect of quenching and annealing on the yield stress is described in terms of the mean energy of hole formation, the departure of volume from its equilibrium state, the distribution of hole energies, and lattice volume. The same set of molecular parameters obtained from the molecular kinetic theory of the glass transition and volume relaxation predicts the yield stress as a function of cooling rate, annealing time, temperature, and strain rate.     

Diastereoselective Aldol Addition Using Boron Trichloride or Alkoxydichloroborane

Under carefully controlled conditions, boron trichloride or alkoxydichloroborane/ethyldiisopropylamine in CH₂Cl₂ can be used to effect diastereoselective aldol additions of ethyl ketones to saturated, α, β-unsaturated, or aromatic aldehydes. The C? C bond formation takes place with relative topicity ul (‘syn,’ configuration of the aldols), in selectivities ranging from 90 to 99% ds (Tables 1–3). Mechanistic aspects of the reaction are discussed.     

Ultracompact biochemical sensor built with two-dimensional photonic crystal microcavity

We report an experimental demonstration of an ultracompact biochemical sensor based on a two-dimensional photonic crystal microcavity. The microcavity, fabricated on a silicon-on-insulator substrate, is designed to have a resonant wavelength (lambda) near 1.5 microm. The transmission spectrum of the sensor is measured with different ambient refractive indices ranging from n = 1.0 to n = 1.5. From observation of the shift in resonant wavelength, a change in ambient refractive index of delta(n) = 0.002 is readily apparent. The correspondence between absolute refractive index and resonant wavelength agrees with numerical calculation to within 4% accuracy. The evaporation of water in a 5% glycerol mixture is also used to demonstrate the capability for in situ time-resolved sensing.     

Fracture toughness determination for compact tension specimens

An analysis of determining the plane stress fracture toughness based on a beam-on-elastic foundation model for compact tension specimens (CTS) covering a wide range of a/2H and d/W ratios is presented. The solution is achieved by using the Timoshenko beam theory and Pasternak foundation with alternative formulations of the foundation modulus and the shear parameter to reflect more accurately the stress-strain distributions at the crack tip.The solution applicable to a wider range of a/2H and d/W ratios becomes desirable for practical reasons. For instance, the determination of plane-strain fracture toughness from the CTS specimens at higher a/W ratios enables the reduction of loading capacity from a testing machine which may become prohibitively high for medium strength engineering materials. Maximum fatigue crack growth data to be measured from a CTS specimen also becomes possible when the validity of fracture toughness can be ensured at the extended a/W ratios.The computed fracture toughness from the present analysis are compared with those measured experimentally and found to be satisfactory not only for high a/W ratios but also for a wide range of a/2H ratios commonly used in double-cantilever beam specimens.     

A carbon nanotube sensor for wall shear stress measurement

A novel carbon nanotube (CNT) sensor is being developed to measure the mean and fluctuating wall shear stress (WSS) in a turbulent boundary layer. The CNT WSS sensor is based on the thermal principle and featured by high spatial and temporal resolutions (in the order of nm and kHz, respectively), low power consumption (in the order of μW), and a compact fabrication process compared with traditional WSS measurement sensors. The CNT WSS-sensing element was characterized in detail before its calibration. The CNT sensor was operated under a constant temperature (CT) operation mode and an overheat ratio range of −0.15 to −0.19 and calibrated in a fully developed turbulent channel flow. It has been observed for the first time in a macroscopic flow that the sensor output power is approximately proportional to the 1/3 powered WSS, as expected for a thermal-principle-based WSS sensor, and the wall shear stress measurement is demonstrated for a low Reynolds number flow.     

Statistical approach to some ill-posed problems for linear partial differential equations

For linear partial differential equations, some inverse source problems are treated statistically based on nonparametric estimation ideas. By observing the solution in a small Gaussian white noise, the kernel type of estimators is used to estimate the unknown source function and its partial derivatives.. It is proved that such estimators are consistent as the noise intensity tends to zero. Depending on the principal part of the differential operator, the optimal asymptotic rate of convergence is ascertained within a wide class of risk functions in a minimax sense. Received: 5 May 1997 / Revised version: 18 June 1998     

Asymptotic behavior of eigenvalues and random updating schemes

For a stochastic matrix (Q _ij ^T ) _i,j=1 ^M withQ _ij ^T exp(–U(ij)/T) at the off-diagonal positions, we develop an algorithm to evaluate the asymptotic convergence rate of all eigenvalues ofQ _ij ^T asT 0 using Ventcel's optimal graphs. As an application we can compare the convergence rates of some random updating schemes used in image processing.This research was partially supported by the National Science Council, Taiwan and Air Force Office of Scientific Research Contract No. F49620 S5C 0144, and was completed while Tzuu-Shuh Chiang was visiting the Center for Stochastic Processes, Department of Statistics, University of North Carolina, Chapel Hill, NC 27599-3260, USA.     

Counting Simsun Permutations by Descents

We count in the present work simsun permutations of length n by their number of descents. Properties studied include the recurrence relation and real-rootedness of the generating function of the number of n-simsun permutations with k descents. By means of generating function arguments, we show that the descent number is equidistributed over n-simsun permutations and n-André permutations. We also compute the mean and variance of the random variable X _n taking values the descent number of random n-simsun permutations, and deduce that the distribution of descents over random simsun permutations of length n satisfies a central and a local limit theorem as n ?? +???.