Mechanical and chemical analysis of plasma and ultraviolet-ozone surface treatments for thermal bonding of polymeric microfluidic devices

Here we have demonstrated that radio frequency plasma and ultraviolet-ozone (UVO) surface modifications are effective treatments for enabling the thermal bonding of polymeric microfluidic chips at temperatures below the T(g) (glass transition temperature) of the polymer. The effects of UVO and plasma treatments on the surface properties of a cyclic polyolefin and polystyrene were examined with X-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM) surface roughness measurements and surface adhesion measurements with AFM force-distance data. Three-point bending tests using a dynamic mechanical analyzer (DMA) were used to characterize the bond strength of thermally sealed polymer parts and the cross-sections of the bonded microchannels were evaluated with scanning electron microscopy (SEM). The experimental results demonstrated that plasma and UVO surface treatments cause changes in the chemical and physical characteristics of the polymer surfaces, resulting in a decrease in T(g) at the surface, and thus allowing the microfluidic chips to be effectively bonded at temperatures lower than the T(g) of the bulk polymer without losing the intended channel geometry.     

Preparation and surface properties of novel low surface free energy fluorinated silane-functional polybenzoxazine films

A novel fluorinated silane-functional benzoxazine monomer is synthesized, and the structure is characterized by FTIR and (1)H NMR. Chemical bonds Si-O-Si linkage between the benzoxazine monomer and the substrate are identified through the variation of water contact angles. The low surface free energy calculation and mechanism of the benzoxazine polymer films are proved through contact angle measurement and FTIR. The film formation property and thermal stability of the polybenzoxazine are also investigated. These results clearly show that this novel polybenzoxazine can not only bond to the substrate but also possess even lower surface free energy which is 15.5 mJ/m(2). The polymer also possesses well thermal stability with a glass transition temperature of 188 °C and the 5% weight loss temperatures of 276 °C.     

Alumina/Polyimide Composite Porous Nanosolid: Dielectric Characteristics and Compressive Strength

Al₂O₃ porous nanosolid was prepared via solvothermal hot-press(SHP) method. The dielectric constant of Al₂O₃ porous nanosolid is as low as 2.34, while its compressive strength is very poor. In order to improve the compressive strength and maitain low dielectric constant, polyimide was introduced to prepare Al₂O₃/polyimide composite porous nanosolid. Compared to Al₂O₃ porous nanosolid, Al₂O₃/polyimide composite porous nanosolid possesses much higher compressive strength, which reaches its saturation value when the mass loading of polyimide is 7.75%. In addition, the in situ Fourier transformation infrared(FTIR) monitoring result reveals that Al₂O₃/polyimide composite porous nanosolid is stable up to 400℃.     

Thermodynamics of adsorption of dithiocarbamates at the hanging mercury drop

Two dimethyldithiocarbamate (DMDTC) pesticides, thiram and ziram, are adsorbed onto a Hg drop via an entropically driven process. The adsorption isotherms are described by the Frumkin equation. For both molecules, the adsorption is characterized by a nonlinear pseudosigmoid temperature dependence of the Gibbs free energy. For the temperature range of 273-313 K, DeltaGADS varies between -43.4 and -56.71 kJ/mol for thiram and -42.60 and -55.67 kJ/mol for ziram. This variation of DeltaGADS reveals that the adsorption strength is increased at higher temperatures. During the adsorption of either molecule, strong lateral interactions are developed between neighboring adsorbates, which are severely weakened as the temperature increases. A unified reaction scheme is suggested for both ziram and thiram that predicts the formation and adsorption of a surface complex, (DMDTC)2Hg. In the case of thiram, two DMDTC molecules are formed by the cleavage of the disulfide S-S bond near the Hg electrode. The thermodynamic and structural parameters reveal that there are two limiting thermodynamic regimes for the adsorbed (DMDTC)2Hg species that originate from two limiting adsorption conformations of the adsorbates on the Hg surface. A transition occurs between these two conformations at temperatures in the region of 285-295 K. This transition is accompanied by large entropic and enthalpic changes.     

Trapping-mediated dissociative chemisorption of cycloalkanes on Ru(001) and Ir(111): influence of ring strain and molecular geometry on the activation of C-C and C-H bonds.

We have measured the initial probabilities of dissociative chemisorption of perhydrido and perdeutero cycloalkane isotopomers on the hexagonally close-packed Ru(001) and Ir(111) single-crystalline surfaces for surface temperatures between 250 and 1100 K. Kinetic parameters (activation barrier and preexponential factor) describing the initial, rate-limiting C-H or C-C bond cleavage reactions were quantified for each cycloalkane isotopomer on each surface. Determination of the dominant initial reaction mechanism as either initial C-C or C-H bond cleavage was judged by the presence or absence of a kinetic isotope effect between the activation barriers for each cycloalkane isotopomer pair, and also by comparison with other relevant alkane activation barriers. On the Ir(111) surface, the dissociative chemisorption of cyclobutane, cyclopentane, and cyclohexane occurs via two different reaction pathways: initial C-C bond cleavage dominates on Ir(111) at high temperature (T > approximately 600 K), while at low temperature (T < approximately 400 K), initial C-H bond cleavage dominates. On the Ru(001) surface, dissociative chemisorption of cyclopentane occurs via initial C-C bond cleavage over the entire temperature range studied, whereas dissociative chemisorption of both cyclohexane and cyclooctane occurs via initial C-H bond cleavage. Comparison of the cycloalkane C-C bond activation barriers measured here with those reported previously in the literature qualitatively suggests that the difference in ring-strain energies between the initial state and the transition state for ring-opening C-C bond cleavage effectively lowers or raises the activation barrier for dissociative chemisorption via C-C bond cleavage, depending on whether the transition state is less or more strained than the initial state. Moreover, steric arguments and metal-carbon bond strength arguments have been evoked to explain the observed trend of decreasing C-H bond activation barrier with decreasing cycloalkane ring size.     

Theoretical modeling of energy redistribution and stereodynamics in CF scattering from Si(100) under grazing incidence

We have simulated CF scattering from Si(100) using the molecular dynamics method. Translational energy loss spectra are presented. The shape of the energy loss distribution as a result of internal energy release is analyzed. At the classical turning point, the internal energy of the molecule is mainly in the form of rotational energy. The strong rotational excitation results in additional molecule-surfaces interactions during the latter half of the collision. These additional collisions permit some molecules that initially gain internal energy exceeding the bond strength to ultimately survive the collision process via rotational de-excitation. The rotational motion exhibited by surviving molecules is determined by the combination of the molecular axis orientation and the local surface structure during the collision process. The rotation planes of the surviving molecules are preferentially aligned with the surface normal (cartwheel-like and propeller-like motions). In this study, propeller-like motion of the surviving molecules is predicted. The majority of surviving molecules exhibit a cartwheel-like motion. However, molecules that gain a propeller-like rotation exhibit a much better alignment of their planes-of-rotation compared with molecules exhibiting cartwheel-like motion.     

Amphiphilic siloxane phosphonate macromolecule monolayers at the air/water interface: effects of structure and temperature

A comprehensive study is reported of Langmuir-Blodgett (LB) films (spread at the air/water interface using the Langmuir balance technique) composed of surface active, nonionic, and OH-free amphiphilic siloxane phosphonate ester macromolecules. Analysis is made on three molecular structures in the form of linear polymer poly(diethylphosphono-benzyl-alphabeta-ethyl methylsiloxane) (PPEMS), cyclic oligomer methylphosphonobenzyl-alphabeta-ethyl cyclosiloxane (MPECS), and copolymer poly(PEMS-co-DMS). The surface pressure-surface area (pi -A) isotherms of homopolymer at 3-40 degrees C show a clear temperature-induced phase transition (plateaus at pit approximately 17-19 mN/m) below 10 degrees C. The magnitude of the transition substantially increases upon lowering the temperature (partial differential DeltaAt/ partial differential T approximately -0.1 nm2 unit(-1) deg(-1) and partial differential pi t / partial differential T approximately -0.25 mN m(-1) deg(-1)). The positive entropy and enthalpy gain infers that strong coupling with the subphase and excess hydration attributed to hydrogen bonding between the P=O bond and the subphase prevails at low temperatures. The cyclic oligomer MPECS forms a condensed monolayer at the air/water interface that does not display a similar transition in the experimental temperature range. The temperature sensitivity of MPECS film is observed only in the collapsed region. The nature of the interaction with the subphase is similar for MPECS and PPEMS, indicating that the size and thermal mobility are the controlling factors in these processes. The elasticity plot reveals two distinct states (above and below transition). This observation is supported by BAM images that show irregular spiral structures below 10 degrees C. The transition occurring in the copolymer at 20 degrees C is due to relaxation of the PDMS component. The two maxima shown in the elasticity plot indicate additive fractions of PPEMS and PDMS. The surface areas of these macromolecules in the relaxed (1.48 nm2/unit) and packed (0.45 nm2/unit) forms obtained by PM3 modeling agree well with the experimental data and seem to indicate that the siloxane chain is being lifted off the subphase by the hydrophobic phenylic part of the molecule.     

多晶粒银纳米线形变机理对温度依赖性的分子动力学研究

基于大规模分子动力学仿真,研究了包含多个晶粒的柱状银纳米线在不同温度下沿轴向拉伸形变的行为。结果表明,当温度低于200 K时,含较大晶粒的体系中位错滑移是其形变的主要机理,最大应力随温度变化不显著。当环境温度高于200 K时,晶粒的滑动逐渐成为形变的主导因素,这一特征在含更小晶粒的体系内表现更明显。同时最大应力随温度显著降低。基于上述结果,进一步讨论了温度对Hall-Petch关系的影响。     

多晶粒银纳米线形变机理对温度依赖性的分子动力学研究

基于大规模分子动力学仿真,本文研究了包含多个晶粒的柱状银纳米线在不同温度下,沿轴向拉伸形变的行为。结果表明,当温度低于200 K时,含较大晶粒的体系中位错滑移是其形变的主要机理,最大应力随温度变化不显著。当环境温度高于200 K时,晶粒的滑动逐渐成为形变的主导因素,这一特征在含更小晶的体系内表现更明显。同时最大应力随温度显著降低。基于上述结果,进一步讨论了温度对Hall-Petch关系的影响。     

Prediction of a phase transition to a hydrogen bond ordered form of ice VI

Ice VI is a hydrogen bond disordered crystal over its known region of stability. In this work, we predict that ice VI will transform into a hydrogen bond ordered phase near 108 K, and have identified the likely low-temperature phase as ferroelectric (space group Cc) with an antiferroelectric structure (space group P2(1)2(1)2(1)) close by in energy. Electronic density functional theory calculations provide input to our calculations, which are extended to cells large enough for statistical simulations by using graph invariants. A significant decrease in the configurational entropy is predicted as hydrogen bonds exhibit partial order above the transition, provided that the hydrogen bonds can equilibrate on an experimental time scale. Conversely, partial disorder is predicted at temperatures below the transition. Although some evidence for ordering of ice VI has been observed in experiments, a low-temperature proton ordered phase has not been identified experimentally.     

Gas phase chemistry in gallium nitride CVD: Theoretical determination of the Arrhenius parameters for the first Ga-C bond homolysis of trimethylgallium

Experimental evidence suggests that the energy of activation for the first homolytic Ga-C bond fission of GaMe3 of Ea = 249 kJ/mol, measured by Jacko and Price in a hot-wall tube reactor, is affected by surface catalytic effects. In this contribution, the rate constant for this crucial step in the gas-phase pyrolysis of GaMe3 has been calculated by variational transition state theory. By a basis set extrapolation on the MP2/cc-pVXZ level and a correlation correction from CCSD(T)/cc-pVDZ level, a theoretical "best estimate" for the bond energy of Delta H(289K) = 327.2 kJ/mol was derived. For the VTST calculation on the B3LYP/cc-pVDZ level, the energies were corrected to reproduce this bond energy. Partition functions of the transitional modes were approximated by a hindered rotor approximation to be valid along the whole reaction coordinate defined by the Ga-C bond length. On the basis of the canonical transition state theory, reaction rates were determined using the maxima of the free energy Delta G++. An Arrhenius-type rate law was fitted to these rate constants, yielding an apparent energy of activation of Ea = 316.7 kJ/mol. The preexponential factor A = 3.13 x 10(16) 1/s is an order of magnitude larger than the experimental results because of a larger release of entropy at the transition state as compared to that of the unknown surface catalyzed mechanism.     

Density Functional Calculations on a Double Hydrogen-bonded Dimer

Density functional theory (DFT) calculations on a double hydrogen-bonded dimer of o-hydroxybenzoic acid were carried out at the B3LYP/6-31G^* level. The optimized geometry of the dimer closely resembles that of the crystal. The calculated results show that the total energy of the dimer is much lower than the sum energies of the two monomers, and the average strength of the double hydrogen bonds is about 38.37 kJ/mol. In order to probe the origin of the interactions in the dimer, natural bond orbital analyses were performed. The thermodynamic properties of the title compound at different temperatures have also been calculated on the basis of vibrational analyses and ΔGT, the change of Gibbs free energy for the aggregation from monomer to the dimmer, is 26.47 kJ/mol at 298.15 K and 0.1MPa, implying the spontaneous process of forming the dimer. The correlation graphics of Sm^0 Hm^0 and temperatures is depicted.     

ADSORPTION OF SHORT TWO-DIMENSIONAL COMPACT CHAINS

Short two-dimensional compact chains adsorbed on the attractive surface at different temperatures were investigated by using the enumeration calculation method. First we investigate the chain size and shape of adsorbed chains, such as characteristic ratios of mean-square radii of gyration 〈S^2〉x/N and 〈S^2〉y/N, shape factor 〈δ〉, and the orientation of chain bonds 〈cos^2 θ〉 to illuminate how the size and shape of adsorbed compact chains change with increasing temperatures. There are some special behaviors for the chain size and shape at low temperature, especially for strong attraction interaction. In the meantime, adsorbed compact chains have different behaviors from general adsorbed polymer chains. Some thermodynamics properties are also discussed here. Heat capacity changes non-monotonously, first increases and then reduces. The transition temperature Tc is nearly 1.0, 1.4, 2.0 and 4.2 （in the unit of To） for the case of ε = 0, -1, -2 and -4 （in the unit of kTo）, respectively. Average energy per bond increases while average Helmholtz free energy per bond decreases with increasing temperatures. From these two thermodynamics parameters we can also get another transition temperature Tc＇, and it is close to 0.7, 1.1, 1.5 and 3.4 for ε= 0, -1, -2, and -4, respectively. Therefore, Tc is greater than Tc＇ under the same condition. These investigations may provide some insights into the thermodynamics behaviors of adsorbed protein-like chains.     

N-甲硝胺异构化和分解反应机理和动力学的理论研究

The complex potential energy surface and reaction mechanisms for the unimolecular isomerization and decomposition of methyl-nitramine (CH3NHNO2) were theoretically probed at the QCISD(T)/6-311+G*//B3LYP/6-311+G* level of theory. The results demonstrated that there are four low-lying energy channels: (i) the NN bond fission pathway; (ii) a sequence of isomerization reactions via CH3NN(OH)O; (IS2a); (iii) the HONO elimination pathway; (iv) the isomerization and the dissociation reactions via CH3NHONO (IS3). The rate constants of each initial step (rate-determining step) for these channels were calculated using the canonical transition state theory. The Arrhenius expressions of the channels over the temperature range 298-2000 K are k6(T)=1014:8e-46:0=RT , k7(T)=1013:7e-42:1=RT , k8(T)=1013:6e-51:8=RT and k9(T)=1015:6e-54:3=RT s-1, respectively. The calculated overall rate constants is 6.9￡10-4 at 543 K, which is in good agreement with the experimental data. Based on the analysis of the rate constants, the dominant pathway is the isomerization reaction to form CH3NN(OH)O at low temperatures, while the NN bond fission and the isomerization reaction to produce CH3NHONO are expected to be competitive with the isomerization reaction to form CH3NN(OH)O at high temperatures.     

Alkane C-H bond activation in zeolites: evidence for direct protium exchange

The mechanism of alkane C-H bond activation in heterogeneous acid catalysis is unknown. (1)H solid-state NMR techniques have been used to simultaneously detect the reactivity of both catalyst and alkane reactant protons in a true in-situ experimental design. Specifically, the activation of isobutane C-H bonds by the solid acid zeolite HZSM-5 is directly observed, and the rate of proton transfer between the solid catalyst surface and gaseous isobutane is quantitatively measured using isotopic (1)H/(2)H exchange methods. An observable adsorption complex forms between the isobutane and the primary Bronsted acid site of ZSM-5, which leads to proton exchange between the zeolite surface and the isobutane methyl groups at temperatures (273 K) much lower than previously reported. The secondary acid site in ZSM-5 is less accessible to or less reactive with the isobutane molecule. Simultaneous detection of protium loss from the Bronsted acid site and protium gain by perdeuterated isobutane reveals a common rate constant equal to 4.1-4.6 x 10(-4) s(-1) at 298 K, but at lower temperatures, the transition between this and a much slower rate process is resolved. The measured activation energy for isobutane H/D exchange is 57 kJ/mol. In all experiments, the isobutane reagent was purified to eliminate any unsaturated impurities that might serve as initiators for carbenium-ion mechanisms, and the active catalyst was free of any organic contaminants that might serve as a source of unsaturated initiators. In total, our results are consistent with direct proton exchange between the zeolite surface and the methyl groups of isobutane.     

Denaturation of Drew-Dickerson DNA in a high salt concentration medium: molecular dynamics simulations

We have performed molecular dynamics simulation on B-DNA duplex (CGCGAATTGCGC) at different temperatures. The DNA was immerged in a salt-water medium with 1 M NaCl concentration to investigate salt effect on the denaturation process. At each temperature, configurational entropy is estimated using the covariance matrix of atom-positional fluctuations, from which the melting temperature (T(m)) was found to be 349 K. The calculated configuration entropy for different bases shows that the melting process involves more peeling (including fraying from the ends) conformations, and therefore the untwisting of the duplex and peeling states form the transition state of the denaturation process. There is a narrow minor groove in the AATT sequence that becomes wider by increasing temperature which disappears at high temperatures, especially above the melting temperature. We have also calculated the fraction of denatured base pairs, f-curve, from which T(m) was found to be 340 K, close to experimental value of 341 K. We found that DNA at high salt concentrations has few hydrogen bonds even at temperatures higher than the T(m). Our calculations show the fact that adding salt leads to increase of T(m) and stabilization of DNA.     

Inverted-sandwich-type and open-lantern-type dinuclear transition metal complexes: theoretical study of chemical bonds by electronic stress tensor

We study the electronic structure of two types of transition metal complexes, the inverted-sandwich-type and open-lantern-type, by the electronic stress tensor. In particular, the bond order $b_\varepsilon$ measured by the energy density which is defined from the electronic stress tensor is studied and compared with the conventional MO-based bond order. We also examine the patterns found in the largest eigenvalue of the stress tensor and corresponding eigenvector field, the ??spindle structure?? and ??pseudo-spindle structure??. As for the inverted-sandwich-type complex, our bond order $b_\varepsilon$ calculation shows that relative strength of the metal-benzene bond among V, Cr, and Mn complexes is V?>?Cr?>?Mn, which is consistent with the MO-based bond order. As for the open-lantern-type complex, we find that our energy density-based bond order can properly describe the relative strength of Cr?CCr and Mo?CMo bonds by the surface integration of the energy density over the ??Lagrange surface?? which can take into account the spatial extent of the orbitals.     

Unimolecular dissociation of protonated trans-1,4-diphenyl-2-butene-1,4-dione in the gas phase: rearrangement versus simple cleavage

Fragmentation mechanisms of trans-1,4-diphenyl-2-butene-1,4-dione were studied using a variety of mass spectrometric techniques. The major fragmentation pathways occur by various rearrangements by loss of H(2)O, CO, H(2)O and CO, and CO(2). The other fragmentation pathways via simple alpha cleavages were also observed but accounted for the minor dissociation channels in both a two-dimensional (2-D) linear ion trap and a quadrupole time-of-flight (Q-TOF) mass spectrometer. The elimination of CO(2) (rather than CH(3)CHO or C(3)H(8)), which was confirmed by an exact mass measurement using the Q-TOF instrument, represented a major fragmentation pathway in the 2-D linear ion trap mass spectrometer. However, the elimination of H(2)O and CO becomes more competitive in the beam-type Q-TOF instrument. The loss of CO is observed in both the MS(2) experiment of m/z 237 and the MS(3) experiment of m/z 219 but via the different transition states. The data suggest that the olefinic double bond in protonated trans-1,4-diphenyl-2-butene-1,4-dione plays a key role in stabilizing the rearrangement transition states and increasing the bond dissociation (cleavage) energy to give favorable rearrangement fragmentation pathways.     

Thermal decomposition of 2-butanol as a potential nonfossil fuel: a computational study

The thermochemistry and kinetics of the pyrolysis of 2-butanol have been conducted using ab initio methods (CBS-QB3 and CCSD(T)) and density functional theory (DFT). The enthalpies of formation and bond dissociation energies of some alcohols including 2-butanol and its derived radicals have been calculated. A variety of simple and complex dissociations have been examined. The results indicated that dehydration to 1- and 2-butene through four-center transition states is the most dominant channel at low to moderate temperatures (T ≤ 700 K), where formation of butenes is kinetically and thermodynamically more favorable than other complex and simple bond scission reactions. Although the C-C bond fission channels require more energy than needed for some complex decomposition reactions, the former pathways predominate at higher temperatures (T ≥ 800 K) due to the higher values of the pre-exponential factors. The progress of the complex decomposition reactions has been followed through intrinsic reaction coordinate (IRC) calculations to understand the mechanism of transformation of 2-butanol to different products.     

Water assisted thermal segregation

Solid state purification generally requires efficient diffusion mechanisms in order to allow impurity migration towards the sample surface, from which it can be removed by a suitable mean. Since solid state diffusion just becomes efficient near the melting point, generally high working temperatures are required, resulting in expensive, energy consuming processes. The addition of small amounts of a common liquid solvent of both matrix and impurity results, even at low temperatures, in effective diffusion mechanisms the thermodynamical aspects of which are discussed in this work. Thermal cycling enhances the efficiency of the described process. Its concerns industrial and analytical applications.